Synthetic Studies on Enantioselective Total Synthesis of Cyathane Diterpenoids: Cyrneines A and B, Glaucopine C, and (+)-Allocyathin B2

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The details for the synthetic studies on enantioselective total synthesis of cyathane diterpenoids cyrneine A ( 1 ) and B ( 2 ), glaucopine C ( 3 ), and (+)-allocyathin B 2 are presented. We established a mild Suzuki coupling for heavily substituted nonactivated cyclopentenyl triflates using a phosphinamide-derived palladacycle as precatalyst and a chelation-controlled highly regioselective Friedel–Crafts cyclization. The utilization of these two key reactions enabled a rapid construction of the 5-6-6 tricyclic skeleton. In the middle stage of the synthesis, a Birch reductive methylation, a modified Wolff–Kishner–Huang reduction, and a carbenoid-mediated ring expansion were employed as the key reactions to furnish the 5-6-7 tricyclic core bearing two antiorientated all-carbon quaternary stereocenters at the C 6 and C 9 ring junctions. By applying these key transformations, a more efficient total synthesis of cyrneine A and allocyathin B 2, and the first total synthesis of cyrneine B and glaucopine C, were accomplished through a collective manner. The late-stage conversions involving a base-mediated double bond migration and a double bond migration/aerobic γ-CH oxidation cascade for the stereoselective synthesis of cyrneine B and glaucopine C were interesting.