General Route to 4a-Methylhydrofluorene Diterpenoids: Total Syntheses of (±)-Taiwaniaquinones D and H, (±)-Taiwaniaquinol B, (±)-Dichroanal B, and (±)-Dichroanone

A general and convergent route for the synthesis of the 4a-methylhydrofluorene diterpenoids has been established through a common hexahydrofluorenone intermediate (10) obtained via Pd(0)-catalyzed reductive cyclization of a substituted 2-(2-bromobenzyl) methylene cyclohexane (13). The strategy has been successfully utilized for the synthesis of (+/-)-taiwaniaquinones D (3) and H (5), (+/-)-taiwaniaquinol B (1), (+/-)-dichroanal B (7), and (+/-)-dichroanone (8).