Synthesis and Biological Profile of Substituted Azetidinyl Carbox­amides, A Novel Class of Herbicidal Acyl-ACP Thioesterase Inhibitors

Dedicated to Klemens Minn on the occasion of his 70^th birthday

Abstract

Utilizing scaffold hopping and bioisosteric replacement strategies, we explored new azetidinyl carboxamide inhibitors of the plant-specific enzyme acyl-ACP thioesterase (FAT). Amongst the investigated compounds we identified new structural motifs that showed promising target affinity coupled with good in vivo efficacy against commercially important weed species. We further studied the structure-activity relationship (SAR) of the novel azetidinyl pyrazole carboxamide scaffold which showed promise as a new type of FAT inhibiting herbicides. Accordingly, a focused synthetic approach towards azetidinyl carboxamides was explored.

Key words

herbicide - acyl-ACP thioesterase - azetidine - pyrazole carboxamide - structure-activity relationship - bioisosteres

Global agriculture is currently facing unprecedented pressure from the effects of climate change,[1] as well as from the continued burden exhibited by invasive pests and diseases.[2] Amongst the invasive pest classes, weed infestations apply a significant strain on worldwide food production by preventing the growth of healthy crops via a variety of mechanisms.[3] With the ever-increasing world population, it is critical that weed populations are effectively controlled to maximize crop yields. Herbicides represent a vital tool for farmers to control weeds, but present solutions are facing a high level of scrutiny as agricultural practices are pressured to achieve optimum levels of resource efficiency, safety, and sustainability.[4] Furthermore, they face various challenges such as the emergence and growth of resistant weed populations.[5] Hence, it is crucial that crop protection researchers act quickly to supply farmers with innovative solutions that tip the balance back in the fight against resistant weed species.[6] The discovery of new and commercially viable plant-specific modes of action within the timeframe needed to exert a substantial impact to control resistant weeds is a highly demanding task. Given the surging cost of bringing new herbicides to the market and the rather low success rate in screening for strongly active compounds, it has been suggested recently that exploring and modifying pre-existing active ingredients via isostere and scaffold-hopping concepts could serve as a fruitful starting point to inspire discovery approaches in the field of herbicides and plant health research.[7] [8] Our group has successfully utilized this creative chemical approach previously, demonstrating resistance-breaking inhibitors of protoporphyrinogen oxidase (PPO, HRAC group 14), whose genesis employed isostere and scaffold-hopping methodologies.[9,10]

To this effect, we have investigated several plant-specific herbicidal modes of action with particular emphasis on structural diversity of small molecule ligands. Whilst individual inhibitors of acyl-acyl carrier protein (ACP) thioesterases are structurally quite simple,[11] there is an eye-catching diversity as to the functional groups within this class of small molecule inhibitors. Cinmethylin (1), an isostere of the naturally occurring monoterpene 1,4-cineole and acting as a potent pre-emergence herbicide, was firstly reported by Shell in 1981 and commercialized in 1989 in rice crops (Figure [1]). It has achieved a renewed recognition for efficiently controlling grass weeds in pre-emergence applications in cereals, particularly in wheat and barley.[12] [13] [14] Concurrent with its use in the field for over three decades, the mode of action of cinmethylin (1) remained unknown until 2018, when Campe et al. determined acyl-acyl carrier protein (ACP) thioesterases as its target, belonging to the protein family of fatty acid thioesterases (FAT, HRAC group 30).[13] Furthermore, they built on their report of cinmethylin (1) by assigning FAT as the herbicidal MoA of the previously unassigned herbicides methiozolin (2) and oxaziclomefone (3).[13] [15] Whilst oxaziclomefone (3) bears a gem-dimethylbenzylamide moiety, cinmethylin (1) and methiozolin (2) both carry a diether motif possessing an ortho-substituted benzyl ring. Our initial investigations of FAT-inhibiting scaffolds resulted in the discovery of several heterobicyclic lead structures exhibiting promising pre-emergence control of grass weeds, e.g. [1,3]thiazolo[4,5-b]pyridine 4a,[16] 2,3-dihydro[1,3]thiazolo[4,5-b]pyridine 4b,[17] and 1-oxa-2,7-diazaspiro[4.4]non-2-en-6-one 5a and 2,7-diazaspiro[4.4]nonane-1,6-dione 5b (Figure [1]).[18]

Figure 1 Scaffold-hopping inspired design approach towards the new azetidinyl pyrazole carboxamides 10 and 11a starting from known fatty acid thioesterase-inhibitors and fungicidal or nematicidal SDH-inhibitors

Remarkably, the FAT-inhibitors 1–5b do not contain any moieties that are affected by current PFAS (per- and polyfluoroalkyl substances) definitions. Inspired by the strong herbicidal activity of carboxamide-based FAT-inhibitor 3 and spirocyclic lactam-based lead structures 5a and 5b, we wanted to expand the structural scope of FAT inhibitors further by exploring new heterocyclic carboxamides using scaffold hopping strategies.

Through a high throughput screening cascade of agrochemical compounds from other indications, it became apparent that pyrazole carboxamides 9a and 9b had a high potential to serve as promising herbicidal lead structures. 9a, for example, emerged initially from a fungicidal research programme aiming at identifying new pyrazole-based fungicidal succinate dehydrogenase (SDH) inhibitors. Inspired by established SDH inhibitors bixafen (6a) and pydiflumetofen (6b), a research group at Bayer identified the new N-cyclopropyl pyrazole carboxamide isoflucypram (6c).[19] [20] [21] Within the in-depth optimization work leading to 6c, pyrrolidine carboxamide 9a was prepared in order to explore cyclic secondary amide motifs. This compound displayed good fungicidal activity paired with considerable control of weeds in initial test systems. Subsequently, a shift of substituents in the pyrazole moiety from the CHF₂ group of 9a (essential for strong fungicidal effects) to the CH₂OCH₃ group of 9b afforded significantly improved herbicidal effects with a concurrent drop in fungicidal efficacy. Furthermore, 9b showed remarkably strong herbicidal activity in glasshouse trials against grassy weeds with promising hints at crop selectivity in cereals.[22]

Recently, nematicidal SDH-inhibitors bearing aminocyclobutane motifs were identified, e.g., cyclobutyl nicotinamide cyclobutriflam (7a) and related pyrazole carboxamide 7b.[23] [24] Enlightened by these examples of successful scaffold hopping, and by the identification of azetidinyl carboxamide-base GPR43-antagonist 8 within a medicinal chemistry project, we tried to replace the pyrrolidine moiety in 9a and 9b with corresponding azetidines.[25] Whilst 3-(difluoromethyl)-5-fluoro-1-methyl-1H-pyrazol-4-yl-substituted azetidinyl carboxamide 10 showed good initial fungicidal activity against septoria leaf spot (SEPTTR) paired with moderate control of some grass weeds, the corresponding 3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl-substituted azetidinyl carboxamide 11a afforded good initial control of grass weeds at several application rates. Remarkably, both prototype compounds exhibited significant inhibition of the plant-specific enzyme acyl-ACP thioesterase [pI₅₀(FAT@LEMPA) 10 (5.0) and 11a (5.6)]. Encouraged by these promising initial findings, we developed analogous azetidinyl carboxamides 11b–n and isomers 12a and 12b (Table [1]), then investigated the in vitro and in vivo activities of these molecules.

Synthesis of 10, 11a, and further analogues began with a well-precedented preparation of pyrazolecarboxylic acids from appropriately substituted β-keto esters (Scheme [1]).[26] After condensation of these starting materials with DMF-DMA to give 13a–c, treatment with methylhydrazine in refluxing ethanol reliably gave the desired pyrazoles. In our hands, steric bulk on the substrate impaired regioselectivity in the reaction such that 14b and 14c were obtained in near equal amounts with the regioisomeric pyrazole as a byproduct, while 14a was the major product of the reaction (61% yield). Ester hydrolysis to give the acids 15a–c furnished the pyrazole portion of the target compounds.

Scheme 1 Synthetic pathway utilized for synthesis of pyrazole acids 15a–c

As a foundation for future SAR development, we sought a robust synthetic route toward the azetidine component of the target compounds that would allow for simple modification of the phenyl ring if desired. Adapting a preparation for similar benzyl azetidines,[27] the commercially available acid 16 was converted into the Weinreb amide derivative 17 before introduction of the phenyl moiety of 18a–c via the aryllithium reagent formed in situ (Scheme [2]).

Scheme 2 Synthetic pathway utilized for the synthesis of 22a–c

The now-superfluous carbonyl of intermediate 18 was then removed through a 3-step sequence beginning with reduction to give the alcohol 19a–c, followed by a Barton–McCombie deoxygenation through O-thiocarbamates 20a–c to deliver the desired benzyl azetidines 21a–c. Simple BOC group deprotection yielded the free azetidines 22a–c, which could then be coupled with the pyrazoles prepared previously to give target compounds 10 and 11a–n (Scheme [3]). Selected prototypes were separated via chiral HPLC to obtain enantiomerically pure azetidinyl carboxamides.

Scheme 3 General synthesis of pyrazole carboxamides 11a–n

Additionally, to confirm that the selected pyrazole regioisomer was indeed the most effective against FAT as was observed previously with similar substrates,[22] we prepared the isomeric pyrazole 23 through an acid-catalyzed condensation of methylhydrazine with the same starting material 13a (Scheme [4]). This compound was then hydrolyzed to 24 and coupled with benzylazetidines 22a,b to give 12a and 12b.

Table 1 Pre-emergence In Vivo Efficacy Screening of Azetidinyl Carboxamides 10, 11a–n and Isomeric 12a,b Against Selected Weeds and Binding Affinity to Acyl-ACP Thioesterase (FAT) from LEMPA

Compounda						In vitro activity	ALOMYb,c		ECHCGb,c		POAANb,c		SETVIb,c		LOLRIb,c
	R1	R2	R3	R4	Q	pI50 ± SE LEMPA	1280 g/ha	320 g/ha	1280 g/ha	320 g/ha	1280 g/ha	320 g/ha	1280 g/ha	320 g/ha	1280 g/ha	320 g/ha
10	CH3	F	H	F	CHF2	5.0 ± 0.11	5	1	5	5	5	5	4	3	5	1
rac-11a	CH3	F	H	H	CH2OMe	5.6 ± 0.08	2	–	5	3	5	1	4	1	–	–
(+)-11a	CH3	F	H	H	CH2OMe	< 4.0	–	–	3	1	3	1	–	–	–	–
(–)-11a	CH3	F	H	H	CH2OMe	5.7 ± 0.07	5	2	5	3	5	4	5	3	4	1
rac-11b	CH3	Cl	H	H	CH2OMe	6.0 ± 0.08	3	–	5	3	5	5	1	–	1	–
(+)-11b	CH3	Cl	H	H	CH2OMe	4.7 ± 0.12	–	–	4	–	4	–	–	–	–	–
(–)-11b	CH3	Cl	H	H	CH2OMe	6.1 ± 0.04	5	2	5	4	5	5	4	1	3	1
11c	CH3	F	Cl	H	CH2OMe	6.1 ± 0.07	3	–	5	3	5	1	4	2	3	1
11d	CH3	F	H	Cl	CH2OMe	4.4 ± 0.14	–	–	5	1	5	5	1	–	1	–
11e	CH3	F	H	H	tetrahydrofuran-2-yl	4.8 ± 0.09	–	–	5	2	5	1	4	1	–	–
11f	CH3	F	Cl	H	tetrahydrofuran-2-yl	5.1 ± 0.07	–	–	5	3	5	3	5	2	–	–
11g	CH3	F	H	H	CH2OPh	< 4.0	–	–	–	–	1	–	1	–	–	–
11h	CH3	F	H	H	CH2OiBu	4.5 ± 0.12	–	–	4	3	4	3	4	1	3	1
11i	CH3	F	H	H	CH2OCH2CHF2	5.7 ± 0.05	2	1	5	5	5	5	5	4	5	1
11j	CH3	Cl	H	H	CH2OCH2CHF2	6.2 ± 0.05	3	2	5	5	5	5	5	4	4	1
11k	CH3	F	Cl	H	CH2OCH2CHF2	6.1 ± 0.08	5	5	5	5	5	5	5	4	5	1
11l	CH3	F	H	H	CH2OEt	5.4 ± 0.05	2	–	5	5	5	4	5	4	5	1
11m	CH3	Cl	H	H	CH2OEt	5.8 ± 0.07	3	1	5	5	5	5	5	5	5	1
11n	CH3	F	Cl	H	CH2OEt	6.0 ± 0.08	3	1	5	5	5	5	5	4	5	5
12a	CH3	F	H	H	CH2OMe	5.0 ± 0.09	1	–	2	–	2	–	1	–	–	–
12b	CH3	Cl	H	H	CH2OMe	5.2 ± 0.05	3	1	2	–	2	–	1	–	–	–
cinmethylin (1)d						6.8 ± 0.09	5	5	5	5	5	4	5	4	5	1
methiozolin (2)d						7.1 ± 0.05	5	4	5	1	5	5	5	5	5	1

^{a i}Bu = 2-methylprop-1-yl.

^b n = 10, i.e., ten monocotyledonous weed seeds were grown per pot.

^c Efficacy values are given based on a rating scale by final visual experts’ assessments of greenmass, e.g. 5 ≥ 90% inhibition, 4 = 70–89% inhibition, 3 = 50–69% inhibition, 2 = 40–49% inhibition, 1 = 21–39% inhibition and ‘−’ ≤ 20% inhibition; cinmethylin (1) and methiozolin (2) were used as comparative internal standard. Weed species: Alopecurus myosuroides (ALOMY), Echinochloa crus-galli (ECHCG), Poa annua (POAAN), Setaria viridis (SETVI), Lolium rigidum (LOLRI). They afforded stronger effects against the tested weed spectrum than their corresponding (+)-enantiomers along with a significantly higher target affinity [pI₅₀ range 5.7–6.1 (–) vs <4.0 to 5.0 (+)]. This is in line with earlier results observed for spiro-isoxazolinyl lactam 5a and spirocyclic bis-lactam 5b,[18] whereby the stereochemistry has a significant impact on target affinity and in vivo efficacy for this new class of azetidinyl carboxamides.

^d For structures see Figure [1].

Scheme 4 Synthetic pathway utilized for synthesis of ‘inverse’ pyrazole acids 24

With all target compounds in hand, we turned to investigating their herbicidal efficacy. In order to develop a thorough SAR, all prepared azetidinyl carboxamides (10, 11a–n, and 12a,b) were tested for target affinity in dedicated in vitro tests, as well as for herbicidal effects in vivo upon foliar pre-emergence application on plants (Table [1]). Based on our experience with thiazolopyridine- and 2,3-dihydro[1,3]thiazolo[4,5-b]pyridine-based FAT inhibitors,[16] [17] five representative grass weeds (ALOMY, ECHCG, LOLRI, POAAN, SETVI) were selected as model plants to assess initial pre-emergence activity using dose rates of 1280 and 320 g/ha. Similarly, in vitro tests were carried out using the FAT A enzyme isolated from duckweed (Lemna paucicostata = LEMPA/Lp) as used previously. As outlined in Table [1], commercial FAT-inhibitors cinmethylin (1) and methiozolin (2) proved to be suitable reference compounds as they showed good and broad control of all grass weeds, albeit afforded incomplete control of commercially important grass weed LOLRI (rigid or wymmera ryegrass) at the lower dosage of 320 g/ha and insufficient control of broadleaf weeds, e.g., MATIN. A particular emphasis was placed upon investigating pre-emergence efficacy as this application type remains important for cereal crops and the development of innovative new solutions, particularly with respect to PFAS-free lead structures (PFAS = per- and polyfluoroalkyl substances).

Firstly, we investigated the aforementioned prototype azetidinyl carboxamides 10 and rac-11a which contained 2,6-difluoro and 2-fluorobenzyl substituents, including enantiomerically pure samples (+)-11a and (–)-11a. By preserving at least one ortho-fluoro substituent like in the parent FAT-inhibitor methiozolin (2), we could more critically assess the impact of the azetidinyl pyrazole carboxamide moiety on in vivo and in vitro efficacy. Significant receptor affinity to acyl-ACP thioesterase (FAT) from LEMPA was observed for all four samples in question (10, 11a all isomers, Table [1]), including considerable differences in target affinity between the separated enantiomers. Interestingly, changing the substituents of the benzyl moiety in azetidinyl carboxamides to that of 11b and 11c did not have a huge impact on the target affinities. 2-Chlorobenzyl-substituted compounds rac-11b, (–)-11b and their closely related racemic analogue 11c (2-fluoro-6-chlorobenzyl) proved to be comparably effective and afforded pI₅₀ values in the range of 6.0–6.1. Although these target affinities were lower than those of cinmethylin (1; 6.8) and methiozolin (2; 7.1), the in vivo activities of 11a–c were competitive with (–)-11a and showed efficacy against all tested weeds, including ALOMY (black twitch or slender foxtail) and LOLRI that are difficult to control.

It is worth noting that enantiomers with negative optical rotation, i.e., (–)-11a and (–)-11b, proved consistently to be the main carrier of the observed activity.

Secondly, we evaluated the impact of structural changes in the pyrazole part of the azetidinyl carboxamides. Introducing an additional chloro substituent in the CH₂OCH₃ substituted pyrazole moiety (11d) afforded weaker target affinity (pI₅₀ 4.4) paired with significantly weaker in vivo efficacy, i.e., lacking sufficient control of commercially important weeds ALOMY, LOLRI, and SETVI (Table [1]). Likewise, replacing the CH₂OCH₃ group in initial lead structure 11a by a cyclic tetrahydrofuryl group in 11e and 11f afforded moderate target affinity (pI₅₀ 4.8 and 5.1), albeit with insufficient control of ALOMY and LOLRI. Whilst the sterically less-demanding CH₂OEt substituent in 11l was well tolerated affording control of ECHCG, POAAN, SETVI, and LOLRI paired with good target affinity (pI₅₀ 5.4, Table [1]), the bulkier CH₂OPh (11g, pI₅₀ < 4.0) and CH₂OⁱBu (11h, pI₅₀ 4.5) substituents showed considerably weaker or no control of the tested weeds in line with lower in vitro activity. Prompted by the target affinities of 11l and of the initial CF₂H-containing hit structure 10, we introduced the CH₂OCH₂CHF₂ group in target compounds 11i–k. All three analogues exhibited good target affinities paired with broad control of the tested grass weeds. Modifying the substituents in the benzyl moiety had an additional beneficial impact on both target affinity and in vivo efficacy. The 2-fluoro-6-chlorobenzyl-substituted azetidinyl pyrazole carboxamide (11k, pI₅₀ 6.1, Table [1]) showed full control of challenging grass weed ALOMY paired with good control of ECHCG, POAAN, and SETVI, but with insufficient control of LOLRI at the lower application rate. Closely related CH₂OCH₂CHF₂ substituted analogues 11j (2-chlorobenzyl, pI₅₀ 6.2) and 11i (2-fluorobenzyl, pI₅₀ 5.7) afforded broad, but slightly weaker control of the tested grass weeds. Significantly, azetidinyl pyrazole carboxamide 11n bearing the combination of 2-fluoro-6-chlorobenzyl and CH₂OEt substituents showed full control of most grass weeds including LOLRI at both dose rates, albeit lacking control of ALOMY at the lower dose rate.

Figure 2 Pre-emergence efficacy [%] of azetidinyl pyrazole carboxamide based FAT-inhibitors 11i, 11k, and 11n and standards cinmethylin (1) and methiozolin (2) against selected warm and cold season monocotyledon weeds in corn and wheat at application rates of 320 and 80 g/ha in advanced greenhouse trials, e.g., DIGSA, ECHCG, and LOLRI (replicates: 10 plants per pot)

However, while many different structural modifications were well-tolerated in our SAR-study thus far, isomeric pyrazoles 12a and 12b exhibited considerably weaker control of the tested grass weeds than their parent analogues 11a and 11b. These results together paint a clear picture of the chemical features within this class of herbicide candidates that can be freely modified to potentially improve their effectiveness (e.g., phenyl substituents), and those that are necessary to maintain herbicidal efficacy (e.g., azetidine stereochemistry and pyrazole regioisomers).

In order to gain further insights into the biological profile of the new azetidinyl pyrazole carboxamide based FAT-inhibitors, we selected three novel representatives (11i, 11k, and 11n) with promising initial screening results in Table [1] for advanced greenhouse tests (more replicates, larger pots, lower application rates) with a particular focus on their efficacy against commercially relevant grass weeds and crop selectivity profiles. We thus tested against warm-season weeds [crabgrass (DIGSA), barnyard grass (ECHCG), and green bristlegrass (SETVI)], cold-season weeds [black-grass (ALOMY, also known as black twitch or slender foxtail) and ryegrass (LOLRI)], as well as corn (ZEAMX) and wheat (TRZAS) as crops (Figure [2]). Whilst all three selected azetidinyl pyrazole carboxamides 11i, 11k, and 11n matched the efficacy of standards 1a and 2 against ECHCG (above 90% at both application rates), target compound 11k controlled warm-season grass weed DIGSA slightly better than cinmethylin (1) alongside with no measurable crop damage in corn. Remarkably, application of the other target compounds 11i and 11n also resulted in low crop damage in corn and wheat, whereas cinmethylin (1) exhibited moderate crop damage in corn and high damage in wheat.

Whilst azetidinyl carboxamides 11i and 11n showed insufficient control of ALOMY, 11k afforded competitive control of ALOMY, clearly exceeding the efficacy of methiozolin (2) (Figure [2]) but still slightly weaker than cinmethylin (1). Furthermore, azetidinyl carboxamide 11n afforded good control of LOLRI at both application rates, superior to the results obtained for both standards. Given the need to reduce the impact of herbicides on the environment, both most promising azetidinyl carboxamides 11k and 11n will be employed in further chemical elaboration studies aiming to reduce their application rates and improve their initial crop selectivity even further.

Hence, the azetidinyl pyrazole carboxamides 11a–n represent a new class of herbicidal lead structures combining pre-emergence control of commercially relevant grass weeds with low crop damage. In our view, the results of the SAR study outlined herein emphasize that chemical innovation available by exploiting isostere and scaffold-hopping concepts, in line with addressing unusual structural features, is a useful tool to broaden the structural scope of modern agrochemical research. Furthermore, it is a valuable approach to meet important sustainability goals, including demanding environmental safety goals and avoiding unwanted structural features such as PFAS (per- and polyfluoroalkyl substances).

3-(Difluoromethyl)-5-fluoro-1-methyl-1H-pyrazole-4-carboxylic acid required to synthesize 10 was purchased from a commercial supplier (Princeton Biomolecular Research, Inc.). The 1-methylpyrazole-4-carboxylic acids required to synthesize 11e,f, 11i–k, and 11l–n were purchased from commercial suppliers (Princeton Biomolecular Research, Inc. and Enamine). 5-Chloro-3-(methoxymethyl)-1-methyl-1H-pyrazole-4-carboxylic acid required for the synthesis of 11d was purchased from a commercial supplier (UkrOrgSynthesis Ltd.).

Synthetic Procedures for the Preparation of Substituted Azetidinyl Carboxamides 11a–n and Their Isomers 12a,b

Synthesis of Pyrazinylcarboxylic Acids; General Procedures

Synthesis of 13a–c

The β-keto ethyl ester (12.2 mmol) and DMF-DMA (1.74 g, 14.6 mmol) were added successively to toluene (10 mL) and the mixture was heated at reflux for 18 h. After cooling to rt, the solvent was removed under reduced pressure to yield the ethyl 2-[(dimethylamino)methylene]-3-oxobutanoate 13a–c (80–91% yield).

Synthesis of 14a–c

The ethyl 2-[(dimethylamino)methylene]-3-oxobutanoate 13a–c (13.4 mmol) and methylhydrazine sulfate (2.32 g, 16.1 mmol) were added successively to EtOH (50 mL) and the mixture was heated at reflux for 5 h. After cooling to rt, the solvent was removed under reduced pressure. The crude mixture was diluted with water and extracted with EtOAc (3 × 100 mL). The combined organic layers were washed with brine and dried (Na₂SO₄), and the solvent was removed in vacuo. The product was purified via column chromatography (heptane/EtOAc = 100:0 to 0:100) to afford methyl 1-methylpyrazole-4-carboxylate 14a–c (28–61% yield).

Synthesis of 15a–c

The methyl 1-methylpyrazole-4-carboxylate 14a–c (35.0 mmol) was suspended in water (17 mL) and NaOH (6.52 g, 163 mmol) was added. The mixture was heated at reflux for 4 h. After cooling to rt, the pH was adjusted to 4–5 (HCl), and the aqueous mixture was extracted with DCM (3 × 50 mL). The combined organic layers were washed with brine and dried (Na₂SO₄), and the solvent was removed under reduced pressure to yield 1-methylpyrazole-4-carboxylic acid 15a–c (90–94% yield) as a white solid.

Synthesis of Benzyl Azetidines; General Procedures

tert-Butyl 2-[Methoxy(methyl)carbamoyl]azetidine-1-carboxylate (17)

1-(tert-Butoxycarbonyl)azetidine-2-carboxylic acid (16; 300 g, 1.49 mol), methoxy(methyl)amine (109 g, 1.79 mol) and T₃P (50% solution in THF, 2.66 mL, 4.47 mol) were added successively to a solution of DIPEA (963 g, 7.46 mol) in DCM (3 L). The mixture was stirred under N₂ at 25 °C for 16 h and then quenched with aq NH₄Cl soln. The aqueous layer was extracted with EtOAc (3 × 800 mL), and then the combined organic layers were washed with brine, dried (Na₂SO₄), and solvent removed under reduced pressure to obtain 17 as a yellow oil (330 g, 91% yield).

Substituted tert-Butyl 2-Benzoylazetidine-1-carboxylates 18a–c

To a flame-dried round-bottomed flask equipped with a magnetic stir bar and addition funnel under N₂ was added the substituted bromobenzene (286 mmol) and dry THF (600 mL). The solution was cooled to –78 °C. n-BuLi (157 mL, 2.0 M solution in cyclohexane, 314 mmol) was added dropwise, then the mixture was stirred at –78 °C for 2 h, followed by the addition of 17 (83.8 g, 343 mmol) also at this temperature. The mixture was stirred for 16 h and allowed to warm to r.t. throughout. The reaction was quenched with aq NH₄Cl soln and diluted further with water. The aqueous layer was extracted with EtOAc (3 × 300 mL) and the combined organic layers were washed with brine and dried (Na₂SO₄) and the solvent was removed under reduced pressure. The crude residue was purified by column chromatography (petroleum ether/EtOAc = 100:0 to 90:10) to afford 18a–c (55–65% yield).

Substituted tert-Butyl 2-[Hydroxy(phenyl)methyl]azetidine-1-carboxylates 19a–c

A solution of the substituted tert-butyl 2-benzoylazetidine-1-carboxylate 18a–c (107 mmol) in dry MeOH (300 mL) was cooled to 0 °C under argon atmosphere. NaBH₄ (4.86 g, 129 mmol) was added carefully portionwise to the stirred solution under a constant argon stream. Stirring was continued at 0 °C for 1 h. The mixture was quenched with aq NH₄Cl soln, and the aqueous layer was extracted with EtOAc (3 × 300 mL). The combined organic layers were washed with brine, dried (Na₂SO₄), and concentrated under reduced pressure to obtain 19a–c as yellow oils (63–72%).

Substituted tert-Butyl 2-[Imidazole-1-carbothioyloxy(phenyl)methyl]azetidine-1-carboxylates 20a–c

A mixture of 19a–c (71.1 mmol), 1,1′-thiocarbonyldiimidazole (19.0 g, 107 mmol), and DMAP (869 mg, 7.11 mmol) in dry THF (200 mL) was stirred at 70 °C for 16 h under a N₂ atmosphere. When no more starting material was detected (LCMS), the reaction was cooled to r.t., quenched with water, and extracted with EtOAc. The combined organic layers were dried (MgSO₄), filtered, concentrated under reduced pressure, and purified via flash chromatography to afford 20a–c (74–82% yield) as off-white solids.

Substituted tert-Butyl 2-Benzylazetidine-1-carboxylates 21a–c

A mixture of 20–c (56.2 mmol), SnHBu₃ (49.1 g, 169 mmol), and AIBN (1.85 g, 11.2 mmol) in dry toluene (100 mL) was stirred at 80 °C under argon for 5 h. After complete conversion of the starting materials (LCMS) the mixture was cooled to r.t., quenched with water, and extracted thoroughly with EtOAc. The combined organic layers were dried (MgSO₄), filtered, concentrated under reduced pressure, and purified via flash chromatography to afford 21a–c (76–81% yield).

Substituted 2-Benzylazetidines 22a–c

To a solution of 21a–c (45.2 mmol) in DCM (100 mL) was added TFA (25 mL) slowly. The reaction was stirred at r.t. for 16 h, concentrated under vacuum, and purified by column chromatography to afford 22a–c as off-white solids (TFA-salts, 88–92% yield).

Amide Couplings; General Procedures

The corresponding pyrazolecarboxylic acid (0.650 mmol) and Et₃N (0.265 mL, 1.9 mmol) were successively dissolved in DCM (5 mL) and stirred at rt for 10 min. Then T₃P (50% solution in THF, 0.500 mL, 0.840 mmol) was added, followed by the azetidine (0. 680 mmol) and the mixture was stirred at r.t. until all starting material was consumed (LCMS). The mixture was diluted with water (50 mL) and additional DCM (50 mL) and the phases separated. The combined organic layers were dried (MgSO₄), filtered, concentrated under reduced pressure, and purified via flash chromatography to give the amide.

Procedure for Preparing ‘Inverse’ Pyrazole Isomers

Synthesis of 23

A solution of methylhydrazine (1.19 mL, 22.6 mmol) in MeCN (50 mL) and AcOH (5.00 mL, 87.0 mmol) was cooled to 0 °C before 13a (5.00 g, 17.4 mmol) was added. The mixture was heated to 80 °C and stirred for 5 h. The mixture was cooled to rt, diluted with EtOAc, and washed carefully with 1 M NaOH soln. The organic layer was dried (MgSO₄), filtered, concentrated under reduced pressure, and purified via flash chromatography to give 23 (1.45 g, 45%).

Synthesis of 24

Pyrazole 23 (1.45 g, 7.87 mmol) was dissolved in MeOH (15 mL) and water (2 mL), then NaOH (66.8 mg, 1.67 mmol) was added and the mixture stirred at r.t. for 5 h. The mixture was evaporated and the crude material redissolved in water (30 mL) and washed with EtOAc (2 × 20 mL). The aqueous layer was acidified with 1 M HCl and extracted with DCM (3 × 20 mL). The combined organic layers were dried (MgSO₄), filtered, concentrated under reduced pressure, and purified via flash chromatography to give 24 (300 mg, 22%). This material was used to prepare 12a and 12b via amide coupling described above (36% and 45% yield, respectively).

[3-(Difluoromethyl)-5-fluoro-1-methyl-1H-pyrazol-4-yl][(2R/S)-2-(2-fluorobenzyl)azetidin-1-yl]methanone (10)

¹H NMR (600 MHz, CDCl₃): δ = 7.23 (br d, J = 5.0 Hz, 2 H), 7.17–6.88 (m, 3 H), 4.84 (br s, 1 H), 4.08–3.90 (m, 2 H), 3.79 (br s, 3 H), 3.41–3.02 (m, 2 H), 2.36 (br s, 1 H), 2.04 (br s, 1 H), 1.67 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 171.2 (br s), 161.4 (br d, J = 242.0 Hz), 150.0 (br d, J = 288.8 Hz), 131.9 (br s), 128.4 (br s), 124.1 (d, J = 3.3 Hz), 123.9–123.3 (m), 115.3 (d, J = 22.9 Hz), 109.3 (br t, J = 237.6 Hz), 98.0 (br s), 61.2 (br s), 48.4 (br s), 34.7 (br s), 32.5 (br s), 20.7 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –114.55 (br dd, J = 307.3, 54.5 Hz, 1 F), –117.41 (br s, 1 F), –120.43 (br dd, J = 307.2, 53.1 Hz, 1 F), –124.22 (br s, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₆N₃OF₄: 342.1229; found: 342.1229.

[(2R/S)-2-(2-Fluorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (11a)

¹H NMR (600 MHz, CDCl₃): δ = 7.54–7.38 (m, 1 H), 7.26–7.19 (m, 2 H), 7.10–7.03 (m, 2 H), 4.83 (d, J = 12.4 Hz, 1 H), 4.81–4.76 (m, 1 H), 4.76–4.73 (m, 1 H), 4.07–3.96 (m, 2 H), 3.90 (s, 3 H), 3.50 (s, 3 H), 3.41–3.33 (m, 1 H), 3.12 (br s, 1 H), 2.32 (br s, 1 H), 2.07–1.98 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.6 (br s), 161.4 (d, J = 244.4 Hz), 151.2 (s), 132.0 (br d, J = 4.1 Hz), 130.5 (s), 129.3–127.6 (m), 124.0–123.8 (m), 124.1 (br d, J = 2.7 Hz), 115.3 (d, J = 22.6 Hz), 113.1 (br s), 67.4 (s), 61.3 (br s), 58.9 (s), 49.6 (br s), 39.2 (s), 32.8 (br s), 21.1 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.4 (br s, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂F: 318.1618; found: 318.1608.

[(+)-2-(2-Fluorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone ((+)-11a)

Racemic [2-(2-fluorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone rac-11a (86 mg) was separated into its enantiomers on a chiral preparative SFC system using a S10-Chiralpak IA-3 column (Gradient Chir2_C5_IA_B3_90CO2_MeOH_CD) to afford (–)-11a (Ent-1, 41 mg, t _R = 1.80 min) and (+)-11a (Ent-2, 43 mg, t _R = 2.08 min) as colorless waxy solids.

¹H NMR (600 MHz, CDCl₃): δ = 7.46 (m, 1 H), 7.23 (m, 2 H), 7.07 (m, 2 H), 4.83 (d, J = 12.4 Hz, 1 H), 4.79 (m, 1 H), 4.75 (d, J = 12.4 Hz, 1 H), 4.01 (m, 2 H), 3.90 (s, 3 H), 3.50 (s, 3 H), 3.36 (br dd, J = 14.2, 2.9 Hz, 1 H), 3.12 (m, 1 H), 2.31 (m, 1 H), 2.03 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (br s), 161.4 (d, J = 244.4 Hz), 151.1 (s), 131.9 (br s), 130.5 (s), 128.3 (br s), 124.1 (br s), 123.9 (br s), 115.2 (d, J = 22.3 Hz), 113.1 (br s), 67.4 (s), 61.1 (br s), 58.9 (s), 49.6 (br s), 39.1 (s), 32.7 (br s), 29.7 (s), 21.1 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.41 (br s, 1 F).

Optical rotation: [α]²⁰ +116.67.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂F: 318.1618; found: 318.1620.

[(–)-2-(2-Fluorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone ((–)-11a)

¹H NMR (600 MHz, CDCl₃): δ = 7.45 (br s, 1 H), 7.23 (m, 2 H), 7.07 (m, 2 H), 4.83 (d, J = 12.4 Hz, 1 H), 4.79 (m, 1 H), 4.75 (d, J = 12.4 Hz, 1 H), 4.02 (m, 2 H), 3.91 (s, 3 H), 3.50 (s, 3 H), 3.36 (m, 1 H), 3.13 (br d, J = 3.0 Hz, 1 H), 2.32 (m, 1 H), 2.04 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.6 (br s), 161.4 (d, J = 244.4 Hz), 151.1 (s), 131.9 (br s), 130.5 (s), 128.3 (br s), 124.1 (br s), 115.3 (d, J = 22.6 Hz), 113.1 (br s), 67.4 (s), 61.2 (br s), 58.9 (s), 49.7 (br s), 39.1 (s), 32.7 (br s), 21.1 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.4 (s).

Optical rotation: [α]²⁰ –118.93.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂F: 318.1618; found: 318.1616.

[(2R/S)-2-(2-Chlorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (11b)

¹H NMR (600 MHz, CDCl₃): δ = 7.47 (br s, 1 H), 7.41–7.33 (m, 1 H), 7.25 (br d, J = 3.4 Hz, 1 H), 7.22–7.15 (m, 2 H), 4.89–4.84 (m, 1 H), 4.82 (br d, J = 12.4 Hz, 1 H), 4.79–4.70 (m, 1 H), 4.09–3.98 (m, 2 H), 3.90 (s, 3 H), 3.55–3.44 (m, 4 H), 3.20 (br s, 1 H), 2.36–2.25 (m, 1 H), 2.08 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.6 (br s), 151.0 (br s), 134.9 (br s), 134.4 (br s), 131.5 (s), 130.5 (s), 129.4 (s), 127.9 (s), 126.7 (s), 113.0 (br s), 67.3 (s), 61.1 (br s), 58.8 (s), 49.1 (br s), 39.0 (s), 36.9 (br s), 20.9 (s).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂Cl: 334.1322; found: 334.1324.

[(+)-2-(2-Chlorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone ((+)-11b)

Racemic [2-(2-chlorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone rac-11b (146 mg) was separated into its enantiomers on a chiral preparative SFC system using a S10-Chiralpak IA-3 column (Gradient Chir2_C5_IA_B3_90CO2_MeOH_CD) to afford (–)-11b (Ent-1, 68 mg, t _R = 1.86 min) and (+)-11a (Ent-2, 65 mg, t _R = 2.12 min) as colorless waxy solids.

¹H NMR (600 MHz, CDCl₃): δ = 7.50 (m, 1 H), 7.37 (m, 1 H), 7.26 (br s, 1 H), 7.19 (m, 2 H), 4.87 (m, 1 H), 4.83 (d, J = 12.5 Hz, 1 H), 4.75 (d, J = 12.3 Hz, 1 H), 4.04 (m, 2 H), 3.90 (s, 3 H), 3.50 (s, 3 H), 3.20 (br s, 1 H), 2.32 (m, 1 H), 2.08 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (br s), 151.1 (br s), 135.0 (br s), 134.5 (s), 131.62 (s), 130.61 (br s), 129.5 (s), 128.0 (s), 126.8 (s), 113.2 (br s), 67.4 (s), 61.3 (br s), 58.9 (s), 49.3 (br s), 39.2 (s), 37.0 (br s), 21.0 (s).

Optical rotation: [α]²⁰ +115.35.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂Cl: 334.1322; found: 334.1321.

[(–)-2-(2-Chlorobenzyl)azetidin-1-yl][3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone ((–)-11b)

¹H NMR (600 MHz, CDCl₃): δ = 7.48 (br s, 1 H), 7.38 (m, 1 H), 7.26 (br s, 1 H), 7.19 (m, 2 H), 4.87 (m, 1 H), 4.83 (d, J = 12.3 Hz, 1 H), 4.75 (d, J = 12.4 Hz, 1 H), 4.04 (br dd, J = 8.4, 6.6 Hz, 2 H), 3.90 (s, 3 H), 3.50 (s, 3 H), 2.32 (m, 1 H), 2.08 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (br s), 151.2 (br s), 135.0 (br s), 134.5 (s), 131.6 (s), 130.7 (br s), 129.5 (s), 128.0 (s), 126.8 (s), 113.3 (br s), 67.4 (s), 61.3 (br s), 58.9 (s), 49.2 (br s), 39.2 (s), 37.1 (br s), 21.0 (s).

Optical rotation: [α]²⁰ –109.98.

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂Cl: 334.1322; found: 334.1324.

[(2R/S)-2-(2-Chloro-6-fluorobenzyl)azetidin-1-yl][3-(methoxy­methyl)-1-methyl-1H-pyrazol-4-yl]methanone (11c)

¹H NMR (600 MHz, CDCl₃): δ = 7.52 (br s, 1 H), 7.20–7.14 (m, 2 H), 7.00–6.95 (m, 1 H), 4.85 (s, 1 H), 4.82 (d, J = 12.5 Hz, 1 H), 4.76–4.71 (m, 1 H), 4.21 (td, J = 8.7, 6.3 Hz, 1 H), 4.04 (td, J = 8.8, 6.0 Hz, 1 H), 3.90 (s, 3 H), 3.60–3.42 (m, 4 H), 3.28 (br dd, J = 12.8, 9.3 Hz, 1 H), 2.37–2.24 (m, 1 H), 2.15–2.06 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (br s), 161.9 (d, J = 247.4 Hz), 151.1 (br s), 135.8 (br d, J = 3.0 Hz), 130.7 (br s), 128.5 (br d, J = 9.5 Hz), 125.4 (d, J = 3.3 Hz), 123.0–123.8 (m), 114.0 (d, J = 23.4 Hz), 113.2 (br s), 67.4 (s), 60.2 (br s), 58.9 (s), 49.4 (br s), 39.2 (s), 31.4 (s), 21.2 (d, J = 1.1 Hz).

¹⁹F NMR (565 MHz, CDCl₃): δ = –112.8 to –111.4 (m, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₀N₃O₂FCl: 352.1228; found: 352.1228.

[5-Chloro-3-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl][(2R/S)-2-(2-fluorobenzyl)azetidin-1-yl]methanone (11d)

¹H NMR (600 MHz, CDCl₃): δ = 7.36–7.28 (m, 1 H), 7.22 (br d, J = 5.7 Hz, 1 H), 7.13–6.99 (m, 2 H), 4.87–4.80 (m, 1 H), 4.58 (d, J = 11.9 Hz, 1 H), 4.44 (d, J = 11.9 Hz, 1 H), 3.93 (br d, J = 12.4 Hz, 1 H), 3.82 (s, 2 H), 3.86–3.73 (m, 1 H), 3.42 (s, 3 H), 3.30 (s, 1 H), 3.23–3.12 (m, 1 H), 2.31 (br s, 1 H), 2.01 (s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.4 (br s), 161.4 (br dd, J = 242.9, 2.6 Hz), 148.5 (br d, J = 1.1 Hz), 133.7–131.3 (m), 128.4 (br d, J = 7.9 Hz), 126.3 (br s), 124.1 (d, J = 3.3 Hz), 123.9 (br s), 115.3 (d, J = 22.3 Hz), 112.1 (br s), 66.8 (s), 61.0 (br s), 58.7 (s), 48.5 (br s), 36.4 (s), 32.6 (br s), 20.7 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.0 to –117.6 (m, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₀N₃O₂FCl: 352.1228; found: 352.1232.

[(2R/S)-2-(2-Fluorobenzyl)azetidin-1-yl]{1-methyl-3-[(2SR)-tetrahydrofuran-2-yl]-1H-pyrazol-4-yl}methanone (11e)

¹H NMR (600 MHz, CDCl₃): δ = 7.42 (m, 1 H), 7.22 (m, 2 H), 7.06 (m, 2 H), 5.42 (br t, J = 6.9 Hz, 1 H), 4.78 (m, 1 H), 4.14 (m, 1 H), 4.00 (m, 2 H), 3.89 (d, J = 2.5 Hz, 4 H), 3.35 (m, 1 H), 3.13 (m, 1 H), 2.42 (m, 1 H), 2.32 (m, 1 H), 2.03 (m, 4 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.9 (br s), 161.4 (d, J = 244.7 Hz), 154.8 (br d, J = 22.6 Hz), 132.0 (br s), 131.9 (br s), 130.5 (d, J = 35.7 Hz), 128.3 (br s), 124.1 (br s), 115.2 (br d, J = 22.3 Hz), 112.9 (br s), 74.7 (s), 74.5 (br s), 68.58 (s), 68.56 (s), 61.1 (br s), 49.6 (br s), 39.2 (s), 39.1 (s), 32.3 (s), 32.2 (br s), 29.7 (s), 25.89 (s), 25.87 (s), 21.0 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.41 (s, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₉H₂₃N₃O₂F: 344.1774; found: 344.1772.

[(2R/S)-2-(2-Chloro-6-fluorobenzyl)azetidin-1-yl]{1-methyl-3-[(2SR)-tetrahydrofuran-2-yl]-1H-pyrazol-4-yl}methanone (11f)

¹H NMR (600 MHz, CDCl₃): δ = 7.48 (br d, J = 16.4 Hz, 1 H), 7.21–7.11 (m, 2 H), 6.97 (t, J = 8.7 Hz, 1 H), 5.40 (br t, J = 6.9 Hz, 1 H), 4.84 (br s, 1 H), 4.19–4.09 (m, 2 H), 4.07–3.97 (m, 1 H), 3.94–3.84 (m, 4 H), 3.60–3.40 (m, 1 H), 3.28 (br dd, J = 12.6, 9.0 Hz, 1 H), 2.46–2.33 (m, 1 H), 2.33–2.26 (m, 1 H), 2.11–1.93 (m, 4 H).

¹³C NMR (151 MHz, CDCl₃): δ = 165.0 (br s), 163.5–160.5 (m), 154.7 (br d, J = 26.2 Hz), 135.8 (br s), 131.4–130.3 (m), 128.4 (br d, J = 9.3 Hz), 125.4 (d, J = 3.3 Hz), 124.0–123.1 (m), 114.0 (br d, J = 23.4 Hz), 113.5–112.2 (m), 74.5 (br d, J = 27.5 Hz), 68.6 (s), 60.2 (br s), 49.4 (br s), 39.2 (s), 33.0–32.1 (m), 31.4 (br s), 25.9 (s), 21.1 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –113.0 to –111.1 (m, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₉H₂₂N₃O₂FCl: 378.1385; found: 378.1389.

[(2R/S)-2-(2-Fluorobenzyl)azetidin-1-yl][1-methyl-3-(phenoxy­methyl)-1H-pyrazol-4-yl]methanone (11g)

¹H NMR (600 MHz, CDCl₃): δ = 7.57 (br s, 1 H), 7.30 (m, 2 H), 7.23 (m, 1 H), 7.06 (m, 4 H), 6.99 (t, J = 7.4 Hz, 1 H), 5.64 (d, J = 12.5 Hz, 1 H), 5.46 (br d, J = 12.3 Hz, 1 H), 4.84 (br s, 1 H), 4.08 (br t, J = 7.2 Hz, 2 H), 3.98 (s, 3 H), 3.38 (br s, 1 H), 3.19 (br d, J = 1.5 Hz, 1 H), 2.34 (br d, J = 1.1 Hz, 1 H), 2.05 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.8 (br s), 161.4 (d, J = 244.7 Hz), 157.8 (s), 140.2 (br s), 137.3 (br s), 131.9 (br s), 129.5 (s), 128.3 (br d, J = 6.0 Hz), 124.1 (br d, J = 3.3 Hz), 121.5 (s), 116.3 (s), 115.3 (br d, J = 22.3 Hz), 114.9 (br s), 114.8 (s), 61.2 (br s), 58.7 (s), 50.0 (br s), 37.6 (s), 32.6 (br s), 21.1 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.3 (s).

HRMS (ESI): m/z [M + H]⁺ calcd for C₂₂H₂₃N₃O₂F: 380.1774; found: 380.1775.

[(2R/S)-2-(2-Fluorobenzyl)azetidin-1-yl][3-(isobutoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (11h)

¹H NMR (600 MHz, CDCl₃): δ = 7.44 (m, 1 H), 7.23 (m, 2 H), 7.07 (m, 2 H), 4.83 (d, J = 12.0 Hz, 1 H), 4.79 (m, 1 H), 4.73 (d, J = 12.2 Hz, 1 H), 4.01 (m, 2 H), 3.90 (s, 3 H), 3.36 (m, J = 6.9, 0.7 Hz, 3 H), 3.35 (m, 3 H), 2.31 (m, 1 H), 2.03 (m, 1 H), 1.97 (spt, J = 6.7 Hz, 1 H), 0.93 (d, J = 1.8 Hz, 3 H), 0.92 (d, J = 1.8 Hz, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (s), 161.4 (d, J = 244.7 Hz), 151.1 (br s), 132.0 (br s), 130.5 (s), 128.3 (br s), 124.1 (br s), 115.3 (d, J = 22.6 Hz), 113.4 (s), 78.2 (s), 65.4 (s), 61.0 (br s), 49.7 (br s), 39.1 (s), 32.7 (br s), 28.2 (s), 21.0 (br s), 19.43 (s), 19.42 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.40 (br s, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₂₀H₂₇N₃O₂F: 360.2087; found: 360.2083.

{3-[(2,2-Difluoroethoxy)methyl]-1-methyl-1H-pyrazol-4-yl}[(2R/S)-2-(2-fluorobenzyl)azetidin-1-yl]methanone (11i)

¹H NMR (600 MHz, CDCl₃): δ = 7.51–7.41 (m, 1 H), 7.25–7.20 (m, 2 H), 7.10–7.03 (m, 2 H), 6.06–5.85 (m, 1 H), 4.98 (d, J = 12.2 Hz, 1 H), 4.92–4.85 (m, 1 H), 4.79 (br s, 1 H), 4.07–3.97 (m, 2 H), 3.91 (s, 3 H), 3.86–3.79 (m, 2 H), 3.35 (br d, J = 12.6 Hz, 1 H), 3.13 (br s, 1 H), 2.34 (br s, 1 H), 2.04 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.3 (br s), 161.4 (d, J = 244.4 Hz), 150.2 (s), 131.9 (d, J = 4.9 Hz), 130.6 (s), 128.4 (br d, J = 7.9 Hz), 124.2 (d, J = 3.5 Hz), 123.9 (br d, J = 16.3 Hz), 115.3 (d, J = 22.6 Hz), 114.6 (br t, J = 241.4 Hz), 113.6 (s), 70.1 (t, J = 28.1 Hz), 66.3 (s), 61.5 (br d, J = 2.2 Hz), 49.2 (br s), 39.3 (s), 33.0 (br s), 21.1 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.4 (br s, 1 F), –124.7 (t, J = 13.8 Hz, 1 F), –124.8 (t, J = 13.9 Hz, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₁N₃O₂F₃: 368.1586; found: 368.1589.

[(2R/S)-2-(2-Chlorobenzyl)azetidin-1-yl]{3-[(2,2-difluoroethoxy)methyl]-1-methyl-1H-pyrazol-4-yl}methanone (11j)

¹H NMR (600 MHz, CDCl₃): δ = 7.48 (br s, 1 H), 7.41–7.33 (m, 1 H), 7.24 (br d, J = 2.1 Hz, 1 H), 7.23–7.16 (m, 2 H), 6.06–5.85 (m, 1 H), 4.97 (br d, J = 12.2 Hz, 1 H), 4.92–4.82 (m, 2 H), 4.09–3.99 (m, 2 H), 3.90 (s, 3 H), 3.82 (td, J = 13.9, 4.2 Hz, 2 H), 3.50 (br s, 1 H), 3.20 (br s, 1 H), 2.33 (br s, 1 H), 2.14–2.00 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.5 (br s), 150.1 (br s), 135.0 (br s), 134.5 (s), 131.6 (s), 130.7 (s), 129.6 (s), 128.1 (s), 126.9 (s), 114.6 (br t, J = 240.9 Hz), 113.6 (br s), 70.1 (t, J = 28.1 Hz), 66.3 (s), 61.3 (br s), 49.1 (br s), 39.2 (s), 37.1 (br s), 21.0 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –124.7 (br t, J = 13.8 Hz, 1 F), –124.8 (br t, J = 13.8 Hz, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₁N₃O₂F₂Cl: 384.1290; found: 384.1279.

[(2R/S)-2-(2-Chloro-6-fluorobenzyl)azetidin-1-yl]{3-[(2,2-difluoroethoxy)methyl]-1-methyl-1H-pyrazol-4-yl}methanone (11k)

¹H NMR (600 MHz, CDCl₃): δ = 7.52 (br s, 1 H), 7.20–7.14 (m, 2 H), 7.01–6.95 (m, 1 H), 6.05–5.84 (m, 1 H), 4.99–4.91 (m, 1 H), 4.91–4.82 (m, 2 H), 4.20 (td, J = 8.8, 6.3 Hz, 1 H), 4.04 (td, J = 8.9, 5.9 Hz, 1 H), 3.90 (s, 3 H), 3.81 (tdd, J = 13.9, 13.9, 4.3, 1.3 Hz, 2 H), 3.64–3.40 (m, 1 H), 3.33–3.23 (m, 1 H), 2.33 (br s, 1 H), 2.14–2.07 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.5 (br s), 161.9 (d, J = 247.7 Hz), 150.0 (s), 135.7 (br d, J = 5.7 Hz), 130.7 (s), 128.5 (br d, J = 9.8 Hz), 125.4 (d, J = 3.3 Hz), 123.4 (br d, J = 18.5 Hz), 114.6 (br t, J = 247.4 Hz), 114.0 (d, J = 23.4 Hz), 113.7 (s), 70.0 (t, J = 28.2 Hz), 66.3 (s), 60.4 (br s), 48.8 (br s), 39.2 (s), 31.4 (s), 21.2 (d, J = 1.1 Hz).

¹⁹F NMR (565 MHz, CDCl₃): δ = –111.6 to –112.2 (m, 1 F), –124.8 (t, J = 13.9 Hz, 1 F), –124.9 (t, J = 13.9 Hz, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₀N₃O₂F₃Cl: 402.1196; found: 402.1197.

[3-(Ethoxymethyl)-1-methyl-1H-pyrazol-4-yl][(2R/S)-2-(2-fluorobenzyl)azetidin-1-yl]methanone (11l)

¹H NMR (600 MHz, CDCl₃): δ = 7.53–7.36 (m, 1 H), 7.25–7.19 (m, 2 H), 7.10–7.03 (m, 2 H), 4.88–4.83 (m, 1 H), 4.82–4.75 (m, 2 H), 4.05–3.97 (m, 2 H), 3.90 (s, 3 H), 3.68 (q, J = 7.0 Hz, 2 H), 3.35 (br d, J = 12.6 Hz, 1 H), 3.20–3.03 (m, 1 H), 2.37–2.25 (m, 1 H), 2.03 (br s, 1 H), 1.28 (t, J = 7.0 Hz, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.7 (br d, J = 1.1 Hz), 161.4 (d, J = 244.7 Hz), 151.3 (s), 132.0 (d, J = 4.6 Hz), 130.6 (s), 128.4 (br d, J = 7.9 Hz), 124.0 (br d, J = 16.1 Hz), 124.1 (d, J = 3.3 Hz), 115.3 (d, J = 22.3 Hz), 113.2 (br s), 66.6 (s), 65.3 (s), 61.4 (br s), 49.1 (br s), 39.2 (s), 33.0 (br s), 21.1 (br s), 15.2 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.4 (br d, J = 4.2 Hz, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₃N₃O₂F: 332.1774; found: 332.1775.

[(2R/S)-2-(2-Chlorobenzyl)azetidin-1-yl][3-(ethoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (11m)

¹H NMR (600 MHz, CDCl₃): δ = 7.50–7.41 (m, 1 H), 7.41–7.33 (m, 1 H), 7.26–7.21 (m, 1 H), 7.21–7.16 (m, 2 H), 4.89–4.82 (m, 2 H), 4.82–4.75 (m, 1 H), 4.09–3.97 (m, 2 H), 3.89 (s, 3 H), 3.67 (q, J = 7.0 Hz, 2 H), 3.57–3.44 (m, 1 H), 3.20 (s, 1 H), 2.37–2.25 (m, 1 H), 2.07 (s, 1 H), 1.27 (t, J = 7.0 Hz, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.9 (br d, J = 2.2 Hz), 151.2 (br s), 135.0 (br s), 134.5 (br d, J = 1.6 Hz), 131.6 (s), 130.5 (s), 129.6 (s), 128.0 (br s), 126.8 (br s), 113.2 (br s), 66.6 (s), 65.3 (s), 61.1 (br s), 49.9 (br s), 39.2 (s), 36.7 (br s), 21.0 (br s), 15.2 (s).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₃N₃O₂Cl: 348.1479; found: 348.1475.

[(2R/S)-2-(2-Chloro-6-fluorobenzyl)azetidin-1-yl][3-(ethoxy­methyl)-1-methyl-1H-pyrazol-4-yl]methanone (11n)

¹H NMR (600 MHz, CDCl₃): δ = 7.51 (br s, 1 H), 7.20–7.13 (m, 2 H), 7.00–6.95 (m, 1 H), 4.84 (br d, J = 12.4 Hz, 2 H), 4.81–4.74 (m, 1 H), 4.20 (td, J = 8.7, 6.3 Hz, 1 H), 4.04 (td, J = 8.9, 6.0 Hz, 1 H), 3.90 (s, 3 H), 3.66 (q, J = 7.0 Hz, 2 H), 3.61–3.48 (m, 1 H), 3.32–3.24 (m, 1 H), 2.31 (br s, 1 H), 2.15–2.04 (m, 1 H), 1.27 (t, J = 7.0 Hz, 3 H).

¹³C NMR (151 MHz, CDCl₃): δ = 164.9 (br s), 161.9 (d, J = 247.4 Hz), 151.1 (br s), 135.8 (br d, J = 2.5 Hz), 130.6 (s), 128.8–127.9 (m), 125.4 (br d, J = 2.7 Hz), 123.8–123.2 (m), 114.0 (br d, J = 23.4 Hz), 113.2 (br s), 66.5 (s), 65.3 (s), 59.8 (br s), 49.7 (br d, J = 3.0 Hz), 39.2 (s), 31.4 (br s), 21.2 (d, J = 0.8 Hz), 15.2 (s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –111.1 to –112.9 (m, 1 F).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₈H₂₂N₃O₂FCl: 366.1385; found: 366.1385.

[(2R/S)-2-(2-Fluorobenzyl)azetidin-1-yl][5-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (12a)

¹H NMR (600 MHz, CDCl₃): δ = 7.50 (m, 1 H), 7.23 (m, 2 H), 7.06 (m, 2 H), 4.95 (br d, J = 12.4 Hz, 1 H), 4.82 (m, J = 12.4 Hz, 2 H), 4.08 (br s, 2 H), 3.92 (s, 3 H), 3.41 (s, 3 H), 3.37 (m, 1 H), 3.16 (br s, 1 H), 2.32 (br s, 1 H), 2.03 (m, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 165.0 (br s), 161.4 (d, J = 244.7 Hz), 141.2 (s), 137.2 (br s), 131.9 (br s), 128.3 (br s), 124.1 (br s), 123.8 (br s), 115.3 (br d, J = 22.6 Hz), 114.6 (br s), 62.5 (s), 61.0 (br s), 58.2 (s), 50.3 (br s), 37.1 (s), 32.6 (br s), 21.1 (br s).

¹⁹F NMR (565 MHz, CDCl₃): δ = –117.3 (s).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂F: 318.1618; found: 318.1625.

[(2R/S)-2-(2-Chlorobenzyl)azetidin-1-yl][5-(methoxymethyl)-1-methyl-1H-pyrazol-4-yl]methanone (12b)

¹H NMR (600 MHz, CDCl₃): δ = 7.53 (br s, 1 H), 7.38 (br s, 1 H), 7.25 (m, 1 H), 7.19 (m, 2 H), 4.88 (m, 3 H), 4.13 (m, 2 H), 3.92 (s, 3 H), 3.54 (br s, 1 H), 3.41 (s, 3 H), 3.24 (br s, 1 H), 2.32 (br s, 1 H), 2.09 (br s, 1 H).

¹³C NMR (151 MHz, CDCl₃): δ = 165.1 (br s), 141.3 (br s), 137.2 (br s), 135.0 (br s), 134.6 (s), 131.5 (s), 129.6 (s), 128.0 (br s), 126.8 (s), 116.3 (s), 114.7 (br s), 62.5 (s), 61.0 (br s), 58.2 (s), 50.2 (br s), 37.1 (s), 36.8 (br s), 21.0 (s).

HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₂₁N₃O₂Cl: 334.1322; found: 334.1322.

Biochemistry and Biology

The detailed descriptions of the biochemical and biological test procedures can be found in the supplementary information and in references 16–18 as these tests were carried out in the same manner. The crop and weed species outlined herein and used in the corresponding glasshouse tests were the following species (listed in alphabetic order): Alopecurus myosuroides (ALOMY), Digitaria sanguinalis (DIGSA), Echinochloa crus-galli (ECHCG), Lolium rigidum (LOLRI), Matricaria inodora (MATIN), Poa annua (POAAN), Setaria viridis (SETVI), Triticum aestivum (TRZAS), Zea mays (ZEAMX).

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We would like to thank Susanne Ries, Gudrun Fey, Andreas Uhl, Peter Zöllner and Martin Annau for valuable analytical support.

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Publikationsverlauf

Eingereicht: 08. September 2025

Angenommen nach Revision: 13. Oktober 2025

Artikel online veröffentlicht:
10. November 2025