Ketene dithioacetal derivative preparation

The pivotal step in this sequence is an electrophilic substitution on indole. Although the use of l,3-dithian-2-yl carbanions is well documented, it has been shown only recently that 1,3-dithian-2-yl carbenium ions can be used in a Priedel-Crafts type reaction. This was accomplished initially using 2-methoxy-l,3-dithiane [1,3-Dithiane, 2-methoxy-] or 2-metlioxy-l,3-dithiolane [1,3-Dithiolane, 2-methoxy-] and titanium tetrachloride [Titanate(l —), tetrachloro-] as the Lewis acid catalyst.9 2-Substituted lysergic acid derivatives and 3-substituted indoles have been prepared under these conditions, but the method is limited in scope by the difficulties of preparing substituted 2-methoxy-1,3-dithianes. l,3-Dithian-2-yl carbenium ions have also been prepared by protonation of ketene dithioacetals with trifluoroacetic acid,10 but this reaction cannot be used to introduce 1,3-dithiane moieties into indole. 

One-pot reactions between the Meldrum s acid derivatives 464 and anilines give the enaminones 465 and, with increased temperature, 4-quinolones 466 in 60-90% overall yields. Scheme 127 (87S482). A similar strategy is used for the preparation of polyfunctionalized quinolones 469, for which the ketene dithioacetal 467 reacts with anilines to give enaminones 468 followed by ring closure (90JHC1217). 

Keten dithioacetals are versatile intermediates for ketone synthesis. Their preparation by the condensation of ketones with the lithium derivatives of bis(phenylthio)methylboronates (25) and from dithians have been reported... 

Horton and coworkers have described an enol thioacetal-forming, elimination procedure starting from 2,3 4,5-di-0-isopropylidene-n-xylose diphenyl dithioacetal (246) leading to 2-deoxy-4,5-0-isopropylidene-D-tfireo-pent-l-enose diphenyl dithioacetal (247), which is, formally, a carbohydrate ketene derivative. Compound 247, obtained in a yield of 62%, is of potential interest for the preparation of various rare sugars. Treatment with acid converts it into derivatives of heteroaromatic compounds (furans and pyrans). 

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