From Amino Acids To Dihydrofurans: Functionalized Allenes in Modern Organic Synthesis

In this account, recent accomplishments in the field of target-oriented synthesis involving allenes are summarized. Allenic α-amino acid derivatives 9, which are of interest as possible vitamin B6 decarboxylase inhibitors, were prepared by 1,6-addition of the cyano-Gilman reagent t-Bu2CuLi·LiCN to 2-amino-substituted enynoates 8, and selective deprotection at either the amino or the ester group was realized. 2,5-Dihydrofurans 18 were obtained by cyclization of the corresponding α-hydroxyallenes; for this step, new methods (treatment with hydrogen chloride gas or acidic ion exchange resin; gold(III)-chloride catalysis) were developed. The 2-hydroxy-3,4-dienoates 14 were obtained by diastereoselective oxidation of titanium enolates formed from 3,4-dienoates 12 with di­methyl dioxirane (DMDO), whereas hydroxyallenes 16 were prepared by copper-mediated SN2′-substitution of propargylic epoxides 15.