A unified strategy for accessing the core structure of cyathane diterpenoids has been developed, enabling the formal synthesis of (-)-erinacine B. The key feature includes an organocatalyzed asymmetric intramolecular vinylogous aldol reaction for convergently building up the 5-6-6 tricyclic ring system. This strategy also highlights a hydroxyl-directed cyclopropanation/ring opening sequence to stereoselectively set up 1,4-anti and -cis angular-methyl quaternary carbon centers.