Scheme 5. aConditions for cyclization of azaenyne: Rh2(S-TCPTTL)4 (2 mol%), n-hexane, rt for 48 h or monitored by TLC until the starting material disappeared; conditions for oxidative chirality transfer: DDQ (2 equiv.), DCE, −20 °C for 48 h or monitored by TLC until the starting material disappeared. b45 °C. cDDQ (5 equiv.). dRoom temperature. eAfter one recrystallization.
