Stereoselectivity of intramolecular cyclisations of nitrones derived from 3-oxahept-6-enals

Intramolecular [1,3]-dipolar cycloadditions of nitrones 33, derived from 3-oxahept-6-enals 7, substituted at either the 4- or 5-position and prepared using a variety of approaches, give good to excellent yields of the cycloadducts 34-37, with good levels of stereoselectivity, especially when the substituent is adjacent to the alkene. The cyclisations appear to proceed via well-defined transition state conformations which should be of predictive value.