Total syntheses of natural products containing α-substituted α-amino acid structures from aldohexoses using overman rearrangement as the key reaction

Recently, natural products containing highly functionalized α-substituted α-amino acid structures have been isolated and reported to show interesting biological properties such as neurotrophic, immunosuppressive, antifungal, and other activities. Efficient synthesis of these compounds, which are expected to be potent lead compounds for new drugs, is an important issue. In this article, development of the novel methodology for the synthesis of natural products in this class; rearrangement of allylic trichloroacetimidates (Overman rearrangement) on sugar scaffolds followed by further transformation of the rearranged products into highly functionalized α-substituted α-amino acid structures by use of the residual functionalities on sugars, is described. Overman rearrangement of imidates derived from aldohexofuranoses generated the tetrasubstituted carbon with nitrogen in a stereoselective manner. The rearranged products were efficiently converted into heterocyclic and acyclic natural products by conventional reactions. Based on this methodology, total syntheses of lactacystin, myriocin, and sphingofungin E starting from aldohexofuranoses have been achieved. Utilization of Overman rearrangement in natural product synthesis is also shortly reviewed.