Asymmetric Synthesis of the Functionalized A/E-Ring Fragment of C 18 -Diterpenoid Alkaloids.

A new asymmetric synthesis of the A/E-ring fragment of C 18 -diterpenoid alkaloids is described. The crucial contiguous stereogenic centers at C4, C5, and C11 were established through an asymmetric Michael addition/allylation sequence. The unique azabicyclo[3.3.1]nonane motif (A/E rings) was assembled by employing ring-closing metathesis and Mitsunobu reaction as key strategies.