GLYCOSYLIDENE CARBENES .12. A NEW SYNTHESIS AND SOME REACTIONS OF SPIROOXIRANES

The diazirine 1, upon thermolysis or photolysis in either acetone or cyclohexanone, at different concentrations, yielded the spiro epoxides 2 and 3, and 4 and 5, respectively (Scheme 1). Yields of 2 and 3 depended both on the temperature and the concentration, and correlated inversely with the yield of the major byproduct, the enol-derived glycoside 6. Other by-products were the benzyloxyglycal 7 and the lactone azines 8. ZnCl2-Promoted methanolysis of 2 under mild conditions yielded a mixture of the ulosides 9 and 10 (1.2:1); similarly, 4 yielded 11 and 12 (1.8:1, Scheme 2). More strongly acidic conditions converted 11 into 12, evidencing that ZnCl2-promoted methanolysis proceeds under kinetic control, which is rationalized. The diazirine 13, upon thermolysis or photolysis in either acetone or cyclohexanone, yielded the alpha-D-configurated spiro epoxides 14 and 16, and the alpha-D-configurated dihydrooxazoles 15 and 17, respectively (Scheme 3), which are either formed by ring-opening of beta-D-epoxides, or by competitive interception of the initially formed, hypothetical addition products of the intermediate carbene to the ketones. The glycosylidene carbenes, derived from 1 or 13 are not very reactive towards ketones, yields are good only when sterically unhindered ketones are used in large excess.