Free radical cyclizations in alkaloid total synthesis: (+/-)-21-oxogelsemine and (+/-)-gelsemine

Total syntheses of (+/-)-21-oxogelsemine (3) and (+/-)-gelsemine (1) are described. Free radical cyclizations play important roles in constructing the tricyclic core (40 --> 41) and oxindole (93 --> 96) substructures of the target alkaloids, and an isomerization-cyclization strategy (101 --> 102) was used to complete construction of the gelsemine cage. Observations made along the way include intramolecular 1,4- and 1,6-hydrogen atom transfers (64 --> 65 and 40 --> 47) and a free radical cyclization-fragmentation sequence (85 --> 86 + 87). A retroaldol-aldol strategy for adjusting oxindole stereochemistry in gelsemine-like structures is also described (72 --> 74 and 86 --> 87).