Synthesis of optically active pyrrolizidinediols: (+)-Heliotridine

Article abstract of DOI:10.1021/ja00349a051

A practical synthesis of the unsaturated pyrrolizidinediol (+)-heliotridine (5) is reported, starting from the readily available (S)-malic acid. The azabicyclo[3.3.0]octane ring system has been constructed via a stereoselective acyliminium ion cyclization directed by an acetoxy substituent. A ketene dithioacetal substituent in the cyclization precursor serves both as an efficient cationic cyclization terminator and later as a means of controlling double bound migration regioselectively into the correct position in the pyrrolizidine skeleton.