Polyphosphoric acid trimethylsilyl ester promoted intramolecular acylation of an olefin by a carboxylic acid: convenient construction of C-18-functionalized delta14-hecogenin acetate.
[reaction: see text]. Polyphosphoric acid trimethylsilyl ester (PPSE)-promoted intramolecular Friedel-Crafts reactions on a nonaromatic carboxylic acid system have been investigated. Studies led to the synthesis of C-18 functionalized steroidal compounds 5 and 9a-d with strict retention of the spiroketals. Isomerization of spiroketal 9e was studied.
