Dithiolanes carbonyl group protection

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Open chain and cyclic thioacetals 1,3-Dithiolane and 1,3-dithiane derivatives are versatile intermediates in the synthesis and interconversion of monocarbonyl and 1,2-dicarbonyl compounds. Protection of carbonyl groups as their open-chain and cyclic thioacetals is an important method in the synthesis of organic molecules. Thioacetals are stable... 

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

Removal of ethylenethioketal protecting group.1 A variety of 1,3-dithiolanes (ethylenethioketals) react smoothly with chloramine-T in aqueous methanol or ethanol to give the corresponding carbonyl compounds in good to excellent yield. For example, treatment of (1) with chloramine-T in aqueous methanol/ethanol yields fluorenone (2)... 

The ability to convert a dithianyl or dithiolanyl carbon to methylene by treatment with Raney nickel enables methylene protection a to a carbonyl. For example, the less enolizable a-methylene of a cycloalkanone can be alkylated by the sequence (a) dithiolanylation of the more enolizable methylene (b) enolization and alkylation at the remaining methylene (c) reductive removal of the dithiolane group (eq 8). ... 

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