Synthetic Studies on the Starfish Alkaloid Imbricatine. Construction of an ent-Imbricatine Framework.

A chiral synthetic route to the amino esters 5 and 6, which contain the fundamental framework of ent-imbricatine (ent-3), has been developed as a prelude to the total synthesis of the starfish alkaloid imbricatine (3). The route started from the sulfur-containing L-phenylalanine derivative 7 and proceeded through key steps such as cyclization of the amide 8 without racemization, reduction to the 1,3-cis-tetrahydroisoquinoline 9, and introduction of a chiral α-amino acid moiety into the chloride 18 bv the bis-lactim ether method.