Enantioselective preparation of 3,4,5-trisubstituted 4,5-dihydroisoxazoles and their stereoselective elaboration of 5-side chain

The magnesium ion-induced nitrile oxide cycloaddition to chiral α-silylallyl alcohols was examined. Treatment of chiral allyl alcohol with nitrile oxide in the presence of magnesium cation resulted in the smooth progress of the 1,3-dipolar cycloaddition to give optically active 4,5-dihydroisoxazoles in good yields. The present procedure serves as a good method for the preparation of multi-substituted 4,5-dihydroisoxazoles with high enantiomeric excess. When exposed to tetrabutylammonium fluoride (TBAF), the cycloadducts were converted into 4-substituted-5,6-dihydro-4H-[1,2]-oxazines in good yields without loss of optical purity. Acylsilanes, obtained by the oxidation of the cycloadduct, underwent stereoselective allylation reaction induced by Lewis acid to accomplish the elongation of C5-side chain in a stereoselective manner.