Dithio ketals

The racemic dilithio species 69 was prepared from l-(l,3-dithian-2-yl)pro-pan-l-ol and added to nitrone 62 to provide hydroxylamine 70, which was then subjected to Cope-House pericychc cyclization, installing the methyl substituent at the C-5 position (Scheme 14). The resulting pyrrolizidine AT-oxides 71-72, obtained as a 1 1 mixture in a moderate 55% yield, were submitted to subsequent N—O bond reductive cleavage, providing the desired pyrrolizidines 73-74. A two-step sequence involving hydrolysis of the dithio-ketal and then reduction of the ketone provided the partially protected hya-cinthacine core as a single isomer in 33% yield. Subsequent removal of the... 

The main interest in dithio-ketals and -acetals has been in connection with their hydrogenolysis by Raney nickel to give the corresponding hydrocarbons, as an alternative to the Clemmensen or Wolff-Kischner reduction on the parent carbonyl compound. This subject has been thoroughly reviewed [90]. Hydrogenolysis of thioketals has also been effected by hydrazine [91, 92, 93] and by alkali metals in liquid ammonia [94]. 

The regeneration of carbonyl compounds from dithioacetals and dithio ketals is done best with reagents that activate the sulfur as a leaving group and facilitate hydrolysis. Among the reagents which have been found effective are nitrous acid (X = NO ), t-butyl hypochlorite (X = Cr), and cupric salts (X =... 

Ethylthio)-4-(phenylthio)cyclopentene (trad. Cyclopent-3-en-l-one ethyl phenyl dithio-ketal)... 

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. 

Diseleno acetals and ketals are cleaved more rapidly than their dithio counterparts a methyl derivative is cleaved more rapidly than a phenyl derivative. Methyl iodide or ozone converts diseleno acetals and ketals to vinyl selenides. ... 

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