SmI_2-mediated radical cross-couplings of α-hydroxylated aza-hemiacetals and N, S -acetals with α,β-unsaturated compounds: Asymmetric synthesis of (+)-hyacinthacine A_2, (-)-uniflorine A, and (+)-7- epi -casuarine

摘要

The SmI_2-mediated radical coupling reactions of β-hydroxylated pyrrolidine/piperidine aza-hemiacetals 8 and 9 and N,S-acetals 6 and 33 with α,β-unsaturated compounds are described. This method allows a rapid access to β-hydroxylated pyrrolidines, piperidines, pyrrolizidinones, and indolizidinones. Starting from N,S-acetal 33 and via a common intermediate 27, the alkaloids hyacinthacine A2 (2), uniflorine A (3, 6-epi-casuarine), and the unnatural epimer 7-epi-casuarine (37) have been synthesized in four and five steps with overall yields of 34%, 16%, and 13%, respectively. The radical mechanism of the coupling reactions has been confirmed by controlled experiments, which also allowed deducing the anionic mechanism in the coupling between N,S-acetal 6 and carbonyl compounds. This demonstrates that the mechanisms of these SmI2-mediated reactions are switchable from Barbier-type anionic to radical by cooperative action of BF_3?OEt_2 and t-BuOH.