Total Synthesis of Phomazarin

Abstract

A concise total synthesis of phomazarin ( 1 ) is detailed enlisting a heterocyclic azadiene inverse electron demand Diels−Alder reaction (1,2,4-triazine → pyridine) for preparation of the fully substituted and appropriately functionalized pyridine C-ring. Thus, [4 + 2] cycloaddition (85%) of triethyl 1,2,4-triazine-3,5,6-tricarboxylate ( 2 ) with trimethoxyethylene ( 3 ) followed by conversion of the cycloadduct 11 to the cyclic anhydride 13 provided the phomazarin C-ring with the three carboxylates suitably differentiated. Linkage of the A- and C-rings through selective nucleophilic addition of the aryllithium reagent 9 to the least hindered anhydride carbonyl of 13 followed by Friedel−Crafts closure of the B-ring provided the fully functionalized phomazarin skeleton. The successful structural correlation of synthetic 1 with natural material and its derivatives confirmed the latest structural assignment for the natural product.