Alkaloids from alkaloids: total synthesis of (+/-)-7a-epi-hyacinthacine A(1) from Z-protected tropenone via Baeyer-Villiger oxidation

摘要

Baeyer-Villiger oxidations of several tropane derivatives have been investigated. Whereas tropenones 15a-c underwent exclusive epoxidation to 21a-c, the corresponding 6-oxotropane derivative 28 yielded the desired lactone 29. Baeyer-Villiger oxidation was also possible for the O-isopropylidene-protected diols 32a,b. The resulting lactones 33a,b were employed in the total synthesis of (+/-)-7a-epi-hyacinthacine A(1) (7a-epi-7) via an intramolecular nucleophilic alkyllithium addition to a carbamate as the key lactamization step. The target compound was prepared from tropenone 15b in 10 steps and 14% overall yield. Enzymatic resolution of pyrrolidine (+/-)-36 provided a formal total synthesis to both enantiomers of 7.