Reactions of aromatic ketones with 3‐mercapto‐1,2‐propanediol. Synthesis of cis‐ and trans‐2‐alkyl‐2‐aryl‐(1,3‐oxathiolane‐5‐methanols and 1,3‐dioxolane‐4‐methanethiols)

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Abstract
Aromatic ketones react with 3‐mercapto‐1,2‐propanediol (1) in refluxing benzene under the catalytic influence of a sulfonic acid and with azeotropic removal of water to yield a mixture comprised predominantly of cis‐ and trans‐2‐alkyl‐2‐aryl‐1,3‐oxathiolane‐5‐methanols 7, accompanied by lesser amounts of cis‐ and trans‐2‐alkyl‐2‐aryl‐1,3‐dioxolane‐4‐methanethiols 8 (up to 30%). It was discovered that 8 is the kinetic product and is isomerized by 4‐toluenesulfonic acid in hot benzene to the thermodynamically more stable 7. Under these conditions, ortho‐ and α‐substituted aromatic ketones tend to produce more of 8, which can be attributed to steric hindrance encountered by the thiol as it attacks the ketone. Ketalizations of 1‐aryl‐2‐(1H‐imidazol‐1‐yl)‐1‐ as well as 1‐aryl‐2‐(1,H‐1,2,4‐triazol‐1‐yl)‐1‐ethanones by 1 fail under these conditions, even after 24 hours of reflux in toluene. However, 1‐(4‐chlorophenyl)‐3‐(1H‐imidazol‐1‐yl)‐1‐propanone and 1‐(4‐bromophenyl)‐4‐(1H‐imidazol‐1‐yl)‐1‐butanone are ketalized by 1 as expected. Interestingly, the reaction of 2‐bromo‐4′‐chloroace‐tophenone with 1 produces 1‐(4‐chlorophenyl)‐2,8‐dioxa‐6‐thiabicyclo[3.2.1]octane. Characterization of all isomers and separation of some diastereomers is described. Nuclear Overhauser enhancement experiments are utilized to establish the stereochemistry of 1,3‐oxathiolanes.

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