Tunable Cyclization Strategy for the Synthesis of Zizaene-, <i>allo</i>-Cedrane-, <i>seco</i>-Kaurane-, and <i>seco</i>-Atesane-Type Skeletons

A versatile Lewis acid-mediated cyclization strategy has been developed for selectively establishing zizaene-, allo-cedrane-, seco-kaurane-, and seco-atesane-type skeletons. The zizaene- and seco-atesane-type skeletons can be obtained in a cascade manner, which involves Diels–Alder reaction of cyclic enones with bis-silyloxy dienes and carbocyclization of yne–enolates through Lewis acid dependent 5- or 6-exo-dig modes. This cyclization strategy was also employed for the core synthesis of tashironin.