Dithioketals deprotection

While chloro-mesylates 331 served as 2-hydrocarbylpenem precursors, the 2-alkylthio-, 2-oxy-, and 2-aminopenems were prepared from 4-chloroazetidin-ones 348a c, bearing at nitrogen a ketene dithioketal, hemithioketal, or aminothioketal moiety. Owing to the instability of the free SH form, S-protected precursors had to be used, whose deprotection could be accompanied by cyclization. Thioesters (e.g. 360,364) or thiolactones (351) were convenient forms of protection. 

Dithioketals such as 32 have also been deprotected to give substituted ketones, such as 33, using a mixture of Dowex 50W (1000% w/w) in propanone and water (eq 26). However, these reactions proceeded In low to moderate yield, presumably due to the reduced basicity of the dithioketal motif compared to that of ketals. 

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