Disconnection at b and d

Based on biogenetic cyclization concept, Corey achieved a one-step cyclization of the A and B rings (Figure 1.1) onto a preformed C ring (Tet. Lett., 2455 (1972)). The key for success in this scheme is the prior formation of bond ‘d’ followed by the formation of the ‘b’ bond. Note the role of a cyclopropyl ketone that orchestrated the development of a carbonium ion followed by an incipient carbanion to complete the A and B rings in this order.

Figure 1.1