Asymmetric Synthesis of AB Rings in ent-Kaurene Carbon Framework

The total synthesis of ent-kaurene natural products of Xerophilusin I, Neolaxiflorin L, Eriocalyxin B, 15-epi- Enmelol completed by diversity-oriented synthesis strategy. In this paper, the regioselectivity of Horner-Wadsworth-Emmons reaction was optimized, and a single Z configuration was transformed from Z/E mixture under acidic condition. Based on the conversion of a single configuration under mild acidic conditions, a mechanism for hydrogen bond-assisted Michael addition/retro-Michael elimination is proposed. The asymmetric Diels-Alder reaction with silylenol ether dienyl 3 and β-ketoate 1 as dienophile was developed. Using Cu(OTf)₂ as Lewis acid and oxazoline as chiral ligand, AB-ring-skeleton was constructed with 65% yield and >19:1 dr value.