Stereoselective Functionalization of Dithiolanes and Dithianes: The First
Example of an Axial Trapping

Abstract

2-Silyl-1,3-dithiolanes and dithianes can be easily functionalized under fluoride ion conditions. The functionalization occurs stereoselectively, the products bearing the incoming groups in the position formerly occupied by the silyl moiety. These results lead to the first example of dithiolane and dithiane axial trapping.

Key words

dithiolanes - dithianes - stereoselectivity - organosilanes - fluoride ion

References

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8

A solution of methoxymethyl trimethylsilane (400 µL, 2.57 mmol) in CCl₄ (5 mL), was treated dropwise with a solution of bromine (132 µL, 2.57 mmol) in CCl₄ (4 mL). The mixture was stirred until it became colorless. A solution of dithiol 1 (315 µL, 2.57 mmol) in CH₂Cl₂ (5 mL) was then added and the mixture was stirred overnight. After washing with water, the organic layer was dried (Na₂SO₄). Eva-poration of the solvent gave 525 mg (2.55 mmol) of crude product 3 (99%), which was purified by chromatography on silica gel (hexanes/EtOAc 200:1), to afford the pure isomers 3a and 3b. 3a: ¹H NMR(ppm): δ = 0.14 (s, 9 H), 1.25-1.28 (m, 6 H), 3.50-3.64 (m, 2 H), 3.69 (s, 1 H). ¹³C NMR(ppm): δ = -2.5, 16.5, 36.9, 53.2. MS: m/z (%) = 206 (5) [M⁺], 150 (11), 135 (32), 73 (100), 59 (16), 55 (11). Calcd for C₈H₁₈S₂Si: C, 46.54; H, 8.79. Found: C, 46.20; H, 8.90. 3b: ¹H NMR(ppm): δ = 0.16 (s, 9 H), 1.31 (d, 6 H, J = 6.6 Hz), 3.62-3.75 (m, 2 H), 3.81 (s, 1 H). ¹³C NMR(ppm): δ = -2.6, 15.5, 33.7, 53.4. MS: m/z (%) = 206 (30) [M⁺], 163 (25), 150 (71), 135 (100), 73 (89), 59 (56). Calcd for C₈H₁₈S₂Si: C, 46.54; H, 8.79. Found: C, 46.32; H, 9.06.

9

Typical Procedure : A solution of 2-trimethylsilyl-4,5-dimethyl-1,3-dithiolane 3a (134 mg, 0.65 mmol), and benzaldehyde (132 µL, 1.3 mmol) in anhyd DMF (3 mL) was stirred for 30 min on activated molecular sieves 4 Å, then CsF (99 mg 0.65 mmol) was added and stirred at r.t. overnight. Then, the solution was extracted with diethyl ether, washed with water, dried (Na₂SO₄) and evaporated. TLC of the crude mixture (8:1 hexane/EtOAc) gave 90 mg (0.37 mmol, 57%) of 4a. ¹H NMR(ppm): δ = 1.32-1.39 (m, 6 H), 2.85 (bs, 1 H), 3.62-3.84 (m, 2 H), 4.73 (dd, 1 H, J = 7.0 Hz, 1.6 Hz), 4.86 (d, 1 H, J = 7.0 Hz), 7.32-7.44 (m, 5 H). ¹³C NMR(ppm): δ = 16.6, 17.1, 52.4, 53.5, 60.7, 77.3, 126.4, 128.3, 128.4, 141.2. MS: m/z (%) = 240 (0.08) [M⁺], 222 (5), 166 (5), 135 (15), 134 (34), 133 (100), 121 (6), 107 (8), 89 (11), 77 (25), 59 (16), 55 (22). Calcd for C₁₂H₁₆OS₂: C, 59.96; H, 6.71. Found: C, 59.57; H, 6.75. Following the same procedure, starting from 100 mg (0.48 mmol) of 3b were obtained 62 mg (0.26 mmol, 54%) of 4b. ¹H NMR(ppm): δ = 1.29 (d, 3 H, J = 6.6 Hz), 1.31 (d, 3 H, J = 6.6 Hz), 3.67-3.86 (m, 2 H), 4.64 (d, 1 H, J = 7.0 Hz), 4.73 (d, 1 H, J = 7.0 Hz), 7.28-7.44 (m, 5 H). ¹³C NMR(ppm): δ = 15.8, 16.5, 52.1, 52.7, 59.4, 77.4, 126.8, 128.2, 128.3, 140.6. MS: m/z (%) = 222 (21) [M⁺ - 18), 166 (16), 134 (100), 121 (22), 107 (35), 105 (23), 91 (24), 89 (33), 79 (67), 77 (96), 59 (40), 55 (31). Calcd for C₁₂H₁₆OS₂: C, 59.96; H, 6.71. Found: C, 59.72; H, 6.93.

22

meso-4,6-Dimethyl-1,3-dithiane 10 (57 mg, 0.39 mmol) dissolved under an inert atmosphere in anhyd THF was cooled to -78 °C and treated with butyllithium (244 µL,
1.6 M solution, 0.39 mmol), stirred 2 h, then cannulated into a -78 °C cooled solution of Me₃SiCl (51 mg, 0.46 mL, 0.47 mmol) and stirred for an additional hour. After warming to
-20 °C, the solution was diluted with ether and a solution of NH₄Cl, the organic layer separated and evaporated to afford 73 mg of crude product 11a (85%). ¹H NMR(ppm): δ = 0.18 (s, 9 H), 1.22 (d, 6 H, J = 6.6 Hz), 1.15-1.25 (m, 1 H, partially overlapped with the peak at 1.22 ppm), 2.09 (dt,
1 H, J = 13.6 Hz, J = 2.4 Hz), 2.69-2.86 (m, 2 H), 3.64 (s,
1 H). ¹³C NMR(ppm): δ = -2.5, 22.3, 36.3, 40.9, 44.6. MS: m/z (%) = 220 (7) [M ⁺ ], 147 (8), 115 (100), 73 (52), 69 (10), 59 (13). Calcd for C₉H₂₀S₂Si: C, 49.03; H, 9.14. Found: C, 48.80; H, 9.00.

23

11b: Yield 69%. ¹H NMR(ppm): δ = 0.17 (s, 9 H), 1.23 (d,
6 H, J = 7.0 Hz), 1.20-1.36 (m, 1 H), 1.96-2.09 (m, 1 H), 2.79-2.99 (m, 2 H), 3.20 (s, 1 H). ¹³C NMR(ppm): δ = -1.1, 23.1, 28.8, 36.2, 42.7. MS: m/z (%) = 220 (6) [M ⁺ ], 147 (11), 115 (100), 73 (77), 69 (13), 59 (19). Calcd for C₈H₁₈S₂Si: C, 46.54; H, 8.79. Found: C, 48.85; H, 9.08.

24

12a: Yield 72%. ¹H NMR(ppm): δ = 1.23 (d, 3 H, J = 7.0 Hz), 1.26 (d, 3 H, J = 6.6 Hz), 1.27-1.34 (m, 1 H, partially overlapped with the peak at 1.26 ppm), 2.07 (dt, 1 H, J = 13.8 Hz, J = 2.4 Hz), 2.74-2.94 (m, 2 H), 4.49 (d, 1 H, J = 6.2 Hz), 4.93 (d, 1 H, J = 6.2 Hz), 7.31-7.52 (m, 5 H). ¹³C NMR(ppm): δ = 21.6, 21.7, 39.7, 40.1, 43.9, 57.3, 76.0, 126.4 128.3, 128.4, 140.1. MS: m/z (%) = 254 (1) [M ⁺ ], 199 (2), 147 (100), 105 (6), 91 (13), 77 (15), 69 (23). Calcd for C₁₃H₁₈OS₂: C, 61.37; H, 7.13. Found: C, 61.05; H, 7.20.

25

12b: Yield 63%. ¹H NMR(ppm): δ = 1.18 (d, 3 H, J = 7.0 Hz), 1.26 (d, 3 H, J = 6.6 Hz), 1.25-1.43 (m, 1 H), 2.12 (dtd, 1 H, J = 13.6 Hz, J = 2.6 Hz, J = 1.2 Hz), 3.00-3.19 (m, 2 H), 3.75 (d, 1 H, J = 9.8 Hz), 4.80 (d, 1 H, J = 9.8 Hz), 7.31-7.44 (m, 5 H). ¹³C NMR(ppm): δ = 21.7, 33.7, 34.3, 43.5, 53.0, 73.8, 127.3, 128.1, 128.2, 140.7. MS: m/z (%) = 254 (0.04) [M ⁺ ], 236 (1), 147 (100), 134 (5), 105 (11), 77 (21, 69 (36), 59 (15). Calcd for C₁₃H₁₈OS₂: C, 61.37; H, 7.13. Found: C, 61.00; H, 7.34.