Asymmetric Synthesis of (S)-(−)-Xylopinine. Use of the Sulfinyl Group as an Ipso Director in Aromatic SE

Optically pure (S)-(−)-xylopinine 2 was prepared in three steps in 52% overall yield. Thus, condensation of the carbanion derived from (S)-4 with the (S)-(E)-sulfinylimine 5 gave a 2:1 mixture of tetrahydroisoquinolines 6a and 6b, differing only in configuration at sulfur. N-Desulfinylation of this mixture gave the diastereomeric sulfoxides which, without separation, were converted into (S)-(−)-xylopinine (2) with loss of the sulfinyl moieties under Pictet–Spengler conditions. This unprecedented ipso electrophilic substitution of a sulfinyl group may have synthetic implications beyond that described in this work.