Isoxazolinyl Spironucleosides by Nitrile Oxide Cycloadditions to 7-Methylenepyrrolo[1 ,2-cJpyrimidin-1 (5H}-one

1,3-Dipolar cycloadditions of TBDPS substituted 6,7-dihydro-5-hydroxy-3-methoxy-7-methylenepyrrolo[1,2-c]pyrimidin-1(5H)-one with methoxycarbonyl- and cyanonitrile oxide proceed with complete regioselectivity, the approach of the dipole taking place predominantly from the less sterically hindered side of the dipolarophile. The isoxazolinyl spironucleoside, bearing a primary hydroxymethyl group in C-3 position of the isoxazolinyl ring, was prepared in two steps from the major isoxazoline. The deprotection of cyanosubstituted spiroisoxazoline furnished the α,β-unsaturated oxime.