Base-Catalyzed Endo-Mode Cyclization of Allenes: Easy Preparation of Five- to Nine-Membered Oxacycles

A reliable and efficient procedure for constructing five- to eight-membered oxacycles has been developed. Allenes with both a phosphoryl group and a suitable delta-hydroxyalkyl side chain at the C1-position underwent an endo mode ring-closing reaction to give five- to seven-membered oxacycles. Changing the phosphoryl group to a phosphono functionality facilitated the preparation of eight-membered congeners. Introduction of a cis double bond to the alkyl side chain of the starting allenes made possible the easy formation of medium-sized oxacycles, such as the dihydrooxocin and tetrahydrooxonin frameworks, regardless of the electron-withdrawing group (POPh2, PO(OEt)2, SOPh, and SO2Ph) at the C1-position.