Asymmetric synthesis of (S)-(−)-tetrahydropalmatine and (S)-(−)-canadine via a sulfinyl-directed Pictet–Spengler cyclization

(S)-(−)-Tetrahydropalmatine 2 and (S)-(−)-canadine 4 were synthesized in three steps from (S)-6, in 33% and 34% overall yield, respectively. Thus, condensation of the (S)-(E)-sulfinylimines 10 and 11 with the carbanion derived from (S)-6 gave the tetrahydroisoquinolines 12 and 13, respectively, which upon TFA induced N-desulfinylation, and subsequent microwave assisted Pictet–Spengler cyclization effected both cyclization and C-desulfinylation producing (S)-(−)-tetrahydropalmatine 2 and (S)-(−)-canadine 4 in optically pure form.