Polyene cyclizations offer rapid entry into terpenoid ring systems. Although enantioselective cyclizations of (E)‐polyenes to form trans‐decalin ring systems are well precedented, highly enantioselective cyclizations of (Z)‐polyenes to form the corresponding cis‐decalins have not been reported. Here, we describe the first application of iminium catalysis to the initiation of polyene cyclizations. Ethyl 1,2‐diazepane‐1‐carboxylate catalyzes the cyclization of polyenes bearing enal initiators. Moreover, chiral bicyclic hydrazides catalyze the cyclizations of (Z)‐polyene substrates to form cis‐decalins with enantioselectivities of up to 97:3 er. DFT calculations suggest the catalysts promote the reaction by stabilizing positive charge as it develops during the bicyclization.

The first total synthesis of the abietaquinone methide diterpenoid (−)-3-oxoisotaxodione is reported. The key enabling step is the use of a chiral bicyclic hydrazide as an organocatalyst for the enantioselective polyene cyclization of a (Z)-polyene substrate to form the cis-decalin core of the natural product. The α-oxo-paraquinone methide unit is formed by a two-step oxidation from a phenol, enabling an efficient synthesis of the natural product.

Polyene cyclizations offer rapid entry into terpenoid ring systems. Although enantioselective cyclizations of (E)‐polyenes to form trans‐decalin ring systems are well precedented, highly enantioselective cyclizations of (Z)‐polyenes to form the corresponding cis‐decalins have not been reported. Here, we describe the first application of iminium catalysis to the initiation of polyene cyclizations. Ethyl 1,2‐diazepane‐1‐carboxylate catalyzes the cyclization of polyenes bearing enal initiators. Moreover, chiral bicyclic hydrazides catalyze the cyclizations of (Z)‐polyene substrates to form cis‐decalins with enantioselectivities of up to 97:3 er. DFT calculations suggest the catalysts promote the reaction by stabilizing positive charge as it develops during the bicyclization.