Thieme Chemistry Journal Awardees - Where Are They Now?Diastereoselective Tandem Iodocarbonate Cyclization of 1,5-Enynes

Abstract

A diastereoselective, metal-free tandem iodocarbonatecyclization of 1,5-enyne, emulating the reactivity of gold catalysis is described. The normal selectivity of iodonium reagents for alkene is reversed favoring alkyne activation in the presence of aryl alkynes.

Keywords

enynes - iodocyclization - IBr - tandem reactions - diastereoselectivity

References and Notes

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9

Key spectral feature of monoalkene carbonation products 3 is as follows: Chemical shift (δ) of CH₂I peak is ca. 3.5 ppm (^¹H NMR), and 5-15 ppm (^¹³C NMR) depending on the diastereomer. Proton and carbon correlation of these resonances is apparent in HSQC spectra of compounds 3.

11

The structural and stereochemical identity of the iodocarbonate products 4 were confirmed by the conversion of 4f/4g into 7f/7g (Scheme  [²] ), respectively, by the radical deiodination as below. Spectra of 7f/7g matched those of the respective products obtained in Au(I) catalysis.^6c For 4o and 4q, relative stereochemistry was based on 1D-NOE spectra.

Scheme 2

12

The assignment of iodobromination products 5 was based on the following experiments: i) LRMS (CI) fragmentation data containing ⁷⁹Br/^8¹Br isotope, ii) 1D-NOE spectra of 5g, iii) conversion of 5o into 4o′ (diastereomer of 4o) by Ag(I)-promoted carbonation, iv) conversion of 5o into 6o by E₂ elimination (NaOMe), and v) the reaction of 2n with ICl into a mixture of 4n and iodochlorinated product 7n (Scheme  [³] ), corresponding to 5n.

Scheme 3

15

For a hydride addition on an alkyne: Strozier R. W., Caramella, P.; Houk, K. N. J. Am. Chem. Soc. 1979, 79, 1340; for addition of a weaker nucleophile (olefin in this case) having a later TS, preferential attack of alkyne should be even more pronounced.

16

However, in the formation of 4h, 4k, and 4n/5n, diastereomeric products having trans-1,2-vicinal relationship between the two methyl bearing centers were not observed. This observation indicates some degree of concertedness of C-C and C-O bond formations (or less likely, stereoelectronically driven nucleophilic trapping of the cationic center).

17

Typical Procedure for the IBr-Promoted Tandem Cyclization To a solution of 2f (57.3 mg, 0.200 mmol) in CH₂Cl₂ (1 mL) at -78 ˚C was added dropwise a solution of IBr (82.7 mg, 0.400 mmol) in CH₂Cl₂ (1 mL). The mixture was kept stirred at -78 ˚C for 20 min, then aq sat. Na₂S₂O₃ (4 mL) was added at once. The mixture was allowed to warm to r.t. with stirring. Layers were separated, and the aqueous layer was extracted with CH₂Cl₂ (3 × 4 mL). The combined organic layers were dried (MgSO₄), evaporated, and the residue was purified by silica gel chromatography (EtOAc-hexane = 1:6) to yield 57.5 mg (81%) of 4f as a white solid (mp 154-156 ˚C).
Compound 4f: IR (neat): 2916, 1800, 1436, 1350, 1185, 1057 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.45-7.28 (m, 3 H), 7.28-7.15 (m, 2 H), 4.60 (t, J = 3.0 Hz, 1 H), 3.32 (dd, J = 2.2, 16.9 Hz, 1 H), 2.97 (dd, J = 2.6, 17.0 Hz, 1 H), 2.93 (d, J = 15.8 Hz, 1 H), 2.52 (d, J = 15.8 Hz, 1 H), 1.60 (s, 3 H). ^¹³C NMR (100 MHz, CDCl₃): d = 154.5, 144.5, 144.0, 129.1, 128.6, 128.2, 88.7, 83.6, 82.0, 44.7, 42.7, 26.7. HRMS: m/z calcd for C₁₄H₁₃INaO₃ [M + Na]⁺: 378.9807; found: 378.9809.
Compound 4g: white solid; mp 114-115 ˚C. IR (neat): 2921, 1790, 1506, 1246, 1052 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.18 (d, J = 8.8 Hz, 2 H), 6.89 (d, J = 8.5 Hz, 2 H), 4.58 (t, J = 2.9 Hz, 1 H), 3.82 (s, 3 H), 3.31 (dd, J = 2.2, 16.9 Hz, 1 H), 2.96 (dd, J = 2.5, 16.9 Hz, 1 H), 2.92 (d, J = 15.8 Hz, 1 H), 2.51 (d, J = 15.8 Hz, 1 H), 1.60 (s, 3 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 159.8, 154.5, 144.0, 136.3, 129.6, 114.3, 88.1, 83.7, 82.0, 55.9, 44.8, 42.9, 26.7. Anal Calcd for C₁₅H₁₅IO₄: C, 46.65; H, 3.92. Found: C, 45.60; H, 4.10. LRMS (CI⁺): m/z calcd for C₁₅H₁₆IO₄ [M⁺ + H]: 387; found: 387 (100) [M⁺ + H], 343 (6) [M⁺ + H - CO₂], 325 (67) [M⁺ + H - CO₂ - H₂O].
Compound 5g: colorless liquid. IR (neat): 2968, 2926, 1738, 1506, 1279, 1246, 1156, 1085 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.10 (d, J = 8.4 Hz, 2 H), 6.89 (d, J = 8.8 Hz, 2 H), 5.00 (t, J = 4.7 Hz, 1 H), 3.82 (s, 3 H), 3.44 (d br, J = 18.7 Hz, 1 H), 3.07 (d of ABq, J = 7.9 Hz, 1 H), 3.00 (d of ABq, J = 7.9 Hz, 1 H), 2.94 (d br, J = 18.7 Hz, 1 H), 1.81 (s, 3 H), 1.52 (s, 9 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 159.6, 153.3, 141.2, 137.9, 129.7, 114.3, 93.1, 83.6, 61.4, 55.9, 47.7, 45.7, 28.8, 28.4. LRMS (CI⁺): m/z calcd for C₁₉H₂₅ ⁷⁹BrIO₄ [M⁺ + H]: 523; found: 523 (24) [M⁺ + H, ⁷⁹Br], 525 (24) [M⁺ + H, ^8¹Br], 467 (10) [M⁺ + H - C₄H₈, ⁷⁹Br], 469 (8) [M⁺ + H - C₄H₈, ^8¹Br], 443 (6) [M⁺ - ⁷⁹Br], 405 (37) [M⁺ + H - C₄H₈ - CO₂ - H₂O, ⁷⁹Br], 407 (36) [M⁺ + H - C₄H₈ - CO₂ - H₂O, ^8¹Br].
Compound 4n: white solid; mp 105-107 ˚C. IR (NaCl): 2916, 2850, 1790, 1601, 1511, 1350, 1246, 1057 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.12 (d, J = 8.8 Hz, 2 H), 6.90 (d, J = 8.4 Hz, 2 H), 4.55 (t, J = 3.7 Hz, 1 H), 3.82 (s, 3 H), 3.24-3.16 (m, 2 H), 2.95 (q, J = 7.3 Hz, 1 H), 1.58 (s, 3 H), 1.07 (d, J = 7.3 Hz, 3 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 159.8, 154.6, 149.6, 136.7, 130.0, 129.8, 114.4, 89.0, 86.0, 81.9, 55.9, 46.3, 43.8, 25.0, 13.4. ES-HRMS: m/z calcd for C₁₆H₁₇IO₄Na [M + Na]⁺: 423.0069; found: 423.0068.
Compound 5n: pale yellow liquid. IR (NaCl): 2916, 2930, 1743, 1601, 1506, 1365, 1279, 1242, 1156 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 7.16 (d, J = 8.4 Hz, 2 H), 6.91 (d, J = 8.8 Hz, 2 H), 5.49 (dd, J = 3.3, 7.7 Hz, 1 H), 4.03 (q, J = 6.9 Hz, 1 H), 3.82 (s, 3 H), 3.37 (dd, J = 7.7, 17.3 Hz, 1 H), 2.77 (dd, J = 3.3, 17.2 Hz, 1 H), 1.70 (d, J = 7.0 Hz, 3 H), 1.51 (s, 9 H), 1.14 (s, 3 H). ^¹³C NMR (100 MHz, CDCl₃):
δ = 159.9, 153.6, 152.8, 131.2, 129.5, 114.3, 95.2, 83.1, 78.3, 60.4, 57.4, 55.8, 50.7, 28.4, 21.8, 18.7. LRMS (CI⁺): m/z calcd for C₂₀H₂₆ ⁷⁹BrIO₄ [M⁺ + H]: 537; found: 537 (22) [M⁺ + H, ⁷⁹Br], 539 (22) [M⁺ + H, ^8¹Br], 481 (26) [M⁺ + H - C₄H₈, ⁷⁹Br], 483 (24) [M⁺ + H - C₄H₈, ^8¹Br], 419 (52) [M⁺ - BocO, ⁷⁹Br], 421 (51) [M⁺ - BocO, ^8¹Br], 401 (100) [M⁺ - Br - C₄H₈, ⁷⁹Br].