Unprecedented Ring Contractions in Cytochalasans: Formation of [10]cytochalasans and a cyclohepta[4,5]benz[1,2‐ d ]isoindole derivative

A series of transformations of cytochalasin D ( 1 ; zygosporin A) was carried out. After treatment of the diol 7 with mesyl chloride and pyridine, and subsequent chromatography on silica gel, the 17‐acetyl derivatives 12 and 13 , the first members of the hitherto unknown [10]cytochalasans, were obtained. The alcohol 8 was converted to the chlorosulfinate 18 under the same conditions. Replacement of pyridine by triethylamine in the mesylation reaction led to the cyclohepta[4,5]benz[1,2‐ d ]isoindole derivative 20 , also representing a novel ring system.