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

Abstract ( 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.