A Useful and Catalytic Method for Protection of Carbonyl Compounds into the Corresponding 1,3-Oxathiolanes and Deprotection to the Parent Carbonyl Compounds [1]

Abstract

A wide variety of carbonyl compounds 1 can be easily protected to the corresponding 1,3-oxathiolanes 2 in good yields in the presence of catalytic amount of perchloric acid in dry CH₂Cl₂ at 0-5 °C. On the other hand, various 1,3-oxathiolanes 2 can be selectively deprotected to the parent carbonyl compounds 1 in very good yields by H₂MoO₄·H₂O-H₂O₂ catalyzed oxidation of ammonium bromide in the presence of perchloric acid in CH₂Cl₂-H₂O solvent system. Mild reaction condition, high selectivity, efficient and relatively good yields are some of the major advantages of the procedure.

Key words

protection - 1,3-oxathiolanes - O,S-acetals - deprotection - molybdic acid - hydrogen peroxide - ammonium bromide - perchloric acid

1

Future address for author correspondence: Department of Chemistry, Visva-Bharati, Santiniketan, West Bengal, 731 235, India.

References

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1

Future address for author correspondence: Department of Chemistry, Visva-Bharati, Santiniketan, West Bengal, 731 235, India.

13

A typical protection procedure: To a solution of compound 1a (0.680 g, 5 mmol) in dry CH₂Cl₂ (10 mL) were added 2-mercaptoethanol (0.4 mL, 5.5 mmol) and 70% perchloric acid (30 µL, 0.5 mmol) and the resulting solution was stirred at 0-5 °C. The reaction was over within 20 min as monitored by TLC. On completion, a saturated solution of sodium bicarbonate (2 drops) was added to neutralize the reaction mixture and the reaction mixture was extracted with CH₂Cl₂ (2 × 15 mL). The organic layer was separated, washed with water, and dried over anhydrous Na₂SO₄. The solvent is removed in vacuo to give crude residue, which is purified through a column of silica gel. The product (0.688 g, 70%) was obtained as a colorless gummy liquid.

14

Spectroscopic data for compound 2g: ¹H NMR (300 MHz, CDCl₃): δ 1.85-2.09 (m, 6 H, cyclohexyl CH₂-), 3.16-3.32 (m, 2 H, -SCH₂-), 3.87-3.95 (m, 1 H, -OCH₂-), 4.48-4.54 (m, 1 H, -OCH₂-), 4.79-4.91 (m, 1 H, OCH-), 5.83-5.99 (m, 3 H, olefinic H, -O-CH-S), 6.89 (d, 2 H, J = 8.5 Hz, ArH), 7.39 (d, 2 H, J = 8.5 Hz, ArH). Anal. Calcd. For C₁₅H₁₈O₂S: C 68.67, H 6.91. Found C 68.52, H 6.88.

15

A typical deprotection procedure: To a stirred solution of molybdic acid (0.018 g, 0.1 mmol) in water (1.0 mL), were added 30% hydrogen peroxide solution (1 mL, 9 mmol) and 70% perchloric acid (0.3 mmol, 18 µL) at ice-bath temperature and stirring was continued. After 20 min, ammonium bromide (0.112 g, 1.14 mmol) was added in portion and color changed immediately to deep yellow from light pale yellow. Then, the substrate 2-[p-methoxyphenyl]-1,3-oxathiolane(1a) (0.196 g, 1.0 mmol) dissolved in CH₂Cl₂ (5 mL) was added to the above solution. The reaction was completed within a 2.5 h as monitored by TLC. The reaction mixture was finally extracted with CH₂Cl₂ (2 × 10 mL) and the organic layers were dried over anhydrous Na₂SO₄. The organic phase was concentrated in vacuo to give the crude product, which was finally purified by column chromatography on silica gel (eluent: hexane-EtOAc, 1:1). The pure product p-meth-oxybenzaldehyde(2a) was obtained 0.125 g (92%).

16

Spectroscopic data for compound 1g: ¹H NMR (300 MHz, CDCl₃): δ 1.87-2.17 (m, 6 H, -cyclohexyl CH₂-), 4.92 (bs, 1 H, -OCH-), 5.84-5.87 (m, 1 H, -CH=CH), 6.01-6.04 (m, 1 H, CH=CH), 7.01 (d, 2 H, J = 8.5 Hz, ArH), 7.82 (d, 2 H, J = 8.6 Hz, ArH), 9.87 (s, 1 H, -CHO) Anal. Calcd. For C₁₃H₁₄O₂: C 77.20, H 6.98. Found C 77.01, H 6.94.