Acetals mercaptals, cyclic

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

Starting from hydroxybenzaldehydes, several workers prepared independently phenylcarbamates the phenyl group of which contains cyclic acetal (1,3-dioxolane) and cyclic mercaptal (1,3-dithiolane) groups (Nikles et al., 1966 Nikles, 1969 Durden and Weiden, 1969 Bachmann and Legge, 1968). Of the former, 2-(l,3-dioxolan-2-yl)-phenyl N-methylcarbamate, dioxocarb (22), and of the latter, 2-(l,3-dithiolan-2-yl)-phenyl N-methylcarbamate, (23) attained practical importance. 

Mercuric acetate Ketones from cyclic mercaptals... 

Boron fiuorideJacetic acid Cyclic mercaptals from acetals... 

Protection of aldehyde groups as cyclic acetals s. 13, 733 Methylene from keto groups via cyclic mercaptals... 

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