Study on the Synthesis of Dihydroflavonol Compounds

From the roots of Campylotropis hirtella (Franch.) Schindl., many natural dihydroflavonol compounds have been extracted, and they possess a structurally novel feature of chiral tertiary alcohol groups. However, the content of the compounds in the plant is extremely low. At this point, to satisfy the needs of further experimental research on these compounds, much effort has been invested in the chemical synthesis of the core structure of 3-hydroxy-3-phenylchroman-4-one. This study aimed to explore a novel method for the synthesis of dihydroflavonol (3), and its application in the production of a natural product (14). The method started with commercially available materials, and provided a foundation for total synthesis of natural compounds of dihydroflavonol.

Introduction the extract of C. hirtella root, from which a dihydroflavonol compound with a chiral center of tertiary alcohol (►Fig. 1, compound 1) was isolated and identified, which was very similar to the structure of a substance discovered by Zhang et al in 2010 (►Fig. 1, compound 2). 5,6 The core structure of the two compounds was 3-hydroxy-3-phenylchroman-4-one, and this inspires us to perform a case study to learn how to apply tools to the synthesis of dihydroflavonol derivatives.
A methanol extract of Uraria crinita afforded 3-(2,4-dihydroxyphenyl)-3,5,7-trihydroxychroman-4-one (14), which is a new 3-hydroxylisoflavanone, and exhibited weak or no cytotoxic activity against several human cancer cell lines (KB, HepG2, Lu, and MCF7). 7 However, the natural source of 14 is very few and limited, and further pharmacological investigation of the compound remains with much challenges. To help the exploration of the compound and enrich the chemical library for biological activity, it is highly enviable to explore a novel synthesis route that utilizes the common structure, 3-hydroxy-3-phenylchroman-4-one, to achieve a series of dihydroflavonol compounds.

Synthesis of Key Intermediate and Core Structure
The core structures of compounds 1 and 2 are 3-hydroxybenzopyranone as shown in ►Scheme 1, and the total synthesis route was designed as shown in ►Scheme 2, in which 2,4,6-trihydroxybenzaldehyde (5) and 1-(2,4-dihydroxyphenyl)ethan-1-one (9) are commercially available and used as the starting materials.
Compound 4 was a key intermediate. The key step of the synthesis of the compound is protecting its aldehyde group to avoid intramolecular aldol condensation reaction. Besides, as polyphenolic compounds, the phenol hydroxyl groups should also be protected, thus, the benzyl group was selected as a protecting group according to reported studies. 8,9 Intermediate 4 was synthesized from two fragments. Briefly, phenolic hydroxyl groups of 5 were selectively protected under alkaline conditions to give product 6 a yield of 73.8 %. However, it is difficult to directly protect the aldehyde group of compound 6, and this is because of the intramolecular hydrogen bonding between the aldehyde group and the ortho-phenolic hydroxyl group. Therefore, we mask the ortho-position of the phenolic hydroxyl group with acetyl protection to give compound 7, followed by the protection of its aldehyde group to obtain compound 8. Our initial study suggested that the benzyl groups of 7 were unstable under acidic conditions or high temperature, thus, a mild reaction condition was selected with tetrabutylammonium tribromide as catalyst to catalyze 7 and 1,3-propylene glycol to form an acetal that was hydrolyzed under alkaline conditions to obtain product 8. Mechanically, alcohol may accelerate the formation of a hemiacetal, which is further facilitated by HBr generated in the media. On the other hand, HBr also protonates triethyl orthoformate to produce an oxonium species, which in turn reacts with an intermediate hemiacetal to give the desired acetal. 10 Another fragment focused on the formation of aryl iodine. 11 Compound 9 was used as a raw material to generate compound 12. It is worth noting that introducing aryl iodide was innovative in the designed synthetic route, because it provides the possibility of adding side chains in the total synthesis of natural products. Then, K 2 CO 3 -promoted addition of the phenolic hydroxyl group in compound 12 gives intermediate 4 with a two-step yield of 44.8%. Our data suggested that when the aldehyde group of 8 reacted with 12 under alkaline conditions, intramolecular aldol condensation could be avoided. However, aldol condensation could occur under both acidic and alkaline conditions. Hydrolyzing acetals under acidic conditions led to the formation of some aldol condensation products.

Intramolecular Benzoin Condensation and Synthesis of Compound 14
Compound 4 is involved in the most important step in the total synthesis, namely intramolecular cross-benzoin reaction, to construct the core structure of dihydroflavonol and the chiral center with tertiary alcohol (►Scheme 3). The catalyst and base of the reaction were screened. Unfortunately, no target product was observed when the catalysts N-heterocyclic carbenes A-D were choosen (►Fig. 2) 12,13 and the bases, such as Et 3 N, DBU, Cs 2 CO 3 , and K 2 CO 3 were selected (entries 1-7, 10, 12; ►Table 1). Under these conditions, the intramolecular aldol condensation reaction was the main competitive reaction, and catalyst C and D were very effective at inducing intramolecular cross-benzoin reactions, producing the required core nucleus in an ideal yield. The reasons behind may be deduced as follows. First, the existence of the electron-withdrawing aryl substituent in triazole catalysts C and D can form nucleophilic intermediates only under weak basic conditions. Second, triazole catalysts have inherent basicity that can also catalyze aldol condensation reactions. The amount of base can be precisely controlled by using a weaker base than the catalyst, which helps to reduce the side reaction of aldol condensation. 14 Catalyst C (15 mol%), Et 3 N (10 mol%), and the solvent toluene were selected as the optimal conditions (►Table 1, entry 9) with the maximum yield of compound 3 being 40%.
Under a H 2 atmosphere, Pd/C-catalytic hydrogenation of 3 afforded the target compound 14. In the process, 3 was deprotected and aryl iodine was also removed (►Scheme 4). Our data confirmed the structure of compound 14 as a dihydroflavonol compound, which was consistent with a reported study. 7 Compound 14 had an optical rotation value of À10.498, and an enantiomeric excess (ee) value of 46%. The (S)-enantiomers were predominant. 15 Compound 14 contains four exposed phenolic hydroxyl groups, but it is stable at room temperature. However, in other methods of de-protection, we found that the tertiary alcohol structure would dehydrate under strong acidic conditions.

Conclusion
The total synthesis of natural compounds is one of the main driving forces for the development of organic chemistry, which has not only theoretical significance for the development of organic chemistry, but also important value for the development of natural medicine and clinical medicine. At the same time, pharmaceutical and organic chemists were attracted by high efficiency and low-toxic metabolites of flavonoids. In this study, starting from simple materials, we obtained the key structure of the three products by synthesis and designing experimental methods, which provided the basis for future total synthesis of natural compounds of dihydroflavonol.
In summary, we successfully synthesized a representative compound 14 and explored a new method to synthesize dihydroflavonol compounds, which provides a useful reference for future studies on the total synthesis of dihydroflavonol compounds.

Experimental Section
General Unless otherwise noted, reagents were commercially available (obtained from Shanghai Titan Scientific Co., Ltd., Bide Pharmatech Ltd., Shanghai Suyuanchem Ltd., etc.) and are analytically pure. NMR data were obtained using a Bruker ascend 400 instrument (Bruker BioSpin AG) at 400 MHz for 1 H and 101 Hz for 13 C in CDCl 3 (Chloroform-d) or MeOD (Methanol-d). The chemical shifts are reported in d ppm relative to tetramethylsilane. The molecular weight and purity of a compound were determined by Waters Corporation separations module LC/MS e2695.

Experimental Process
The synthesis steps of the key intermediate 4, the core compound 3, and the target product 14 are described as follows.