Ketones cyclic thioacetal formation

H"], HSCH2CH2SH, (-H2O) Cyclic thioacetal formation Ethylene thioglycol can be used to convert an aldehyde or ketone into a cyclic thioacetal. 

Cyclic Acetals Formation of an acetal using a diol as the alcohol gives a cyclic acetal Cyclic acetals often have more favorable equilibrium constants, since there is a smaller entropy loss when two molecules (a ketone and a diol) condense than when three molecules (a ketone and two molecules of an alcohol) condense. Ethylene glycol is often used to make cyclic acetals its acetals are called ethylene acetals (or ethylene ketals). Dithiane (Section 18-8) and its alkylated derivatives are examples of cyclic thioacetals (sulfur acetals). 

Thiols are the sulfur analogs of alcohols (Section 15.11). The sulfur atom of a thiol is a better nucleophile than the oxygen atom of an alcohol. Thus, thiols react with aldehydes or ketones to form thioacetals or thioketals by a mechanism similar to that described for acetals and ketals. These sulfur derivatives form in high yield because the equdibrium constant for thioacetal formation is much greater than that for acetal formation. We use Lewis acids such as BFj or ZnCl2 rather than protic acids to catalyze the formation of the thioacetal. Both 1,2-ethanedithiol and 1,3-propanedithiol are used to form cyclic thioacetals and thioketals. 

Sulphur dioxide has been found to be a convenient catalyst for the formation of both cyclic and acyclic thioacetals. Nitromethyl ketones undergo reductive thioacetalization with ethanethiol and aluminium chloride, giving the thioacetal of the methyl ketone. ... 

Formation of a secondary alcohol may be achieved by addition of a Grignard reagent to an aldehyde. Choose a deprotection method that will release the aldehydic carbonyl function but will leave the thioacetal protected cyclic ketone part untouched. 

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