摘要:
Enantiomerically enriched (1 S,4R)-2-oxa-3-azabicyclo[2.2.1]hept-5-ene of formula wherein PG 1 is an amino-protective group, are prepared from cyclopentadiene via hetero-Diels-Alder cycloaddition with protected 1- C -nitroso-β-D-ribofuranosyl halides of formula wherein X is a halogen atom selected from fluorine, chlorine, bromine and iodine, PG 2 is a hydroxyl-protective group and PG 3 is a 1,2-diol-protective group.