[2+3] CYCLOADDITIONS OF ENANTIOMERICALLY PURE OXAZOLINE N-OXIDES - AN ALTERNATIVE TO THE ASYMMETRIC ALDOL CONDENSATION

A series of camphor-derived oxazoline N-oxides were obtained by condensation of the appropriate orthoesters with (+)-3-(hydroxyamino)borneol (8). These unstable dipoles were used without isolation in various [2 + 3] asymmetric cyclocondensations. The regio- and diastereoselectivities of these reactions were good to excellent. After functional group transformations, adducts were cleaved by a two--step sequence (i.e., oxidation followed by acidic hydrolysis) giving beta-hydroxy ketones and (+)-3-(hydroxyimino)borneol (19). Asymmetric synthesis of (+)-1,7-dioxaspiro[5.5]-undecan-4-ol, a pheromone of Dacus oleae, was achieved as an application of this new method.