2- -3-alkenal 2-lithio-1,3-dithiane

Halogenation of 106 with triphenylphosphine, iodine, and imidazole provided the iodo derivative 109. On treatment with lithium aluminum hydride, 109 was converted into two endocyclic alkenes, 110 and di-O-isopro-pylidenecyclohexanetetrol, in the ratio of 2 1. Oxidation of 110 with dimethyl sulfoxide - oxalyl chloride afforded the enone 111.1,4-Addition of ethyl 2-lithio-l,3-dithiane-2-carboxylate provided compound 112. Reduction of 112 with lithium aluminum hydride, and shortening of the side-chain, gave compound 113, which was converted into 114 by deprotection. ... 

Thermolysis of 2-diazo-l,3-dithiane, prepared in situ from the reaction of 2-lithio-2-trimethylsilyl-l,3-dithiane and tosyl azide, occurs already below 0°C. The resulting carbene dimerizes efficiently even in the presence of alkenes and alkynes to give bis(l,3-dithianylidene) in 78% yield (Scheme 41) <1997T9269>. 

The dibromobutene 149 could be monosubstituted using 2-lithio-l,3-dithiane as a carboxylic acid precursor, followed by substitution of the remaining bromide with sodium azide as the amine precursor (Scheme 32). The azide was reduce under Staudinger conditions and acy-lated. After separation of the EIZ isomers, further functional group transformations gave the dimethyl-substituted Gly-Gly alkene isostere 152J123 ... 

A variation of this method has also been developed for cases in which reactions with (1) give poor results (eq 6). Thus the carbonyl compound is treated with 2-lithio-l,3-dithiane followed by TMSCl to generate the silyl ether. Subsequent addition of a second equivalent of n-butyllithium effects alkenation, affording the thioketene acetal in good yield. ... 

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