Synergic effect of vicinal stereocenters in [3+2] cycloadditions of carbohydrate azadipolarophiles and mesoionic dipoles: Origin of diastereofacial selectivity

The intermolecular [3 + 2] cycloaddition of carbohydrate-derived 1,2-diaza- 1,3-butadienes and 1,3-thiazolium-4-olates provides a conceptual basis for the problem of diastereofacial preference in the acyclic series of unsatura ted sugars. Experimental results employing a side chain of D-arabino config uration have shown the stereodifferentiation exerted by the first stereogen ic center that renders the Re,Re face of the acyclic sugar-chain azadiene e ligible for cycloaddition (J. Org. Chem. 2000, 65, 5089). The results of th e present work, now utilizing an alternative framework of D-lyxo configurat ion, evidence the discriminating power of the second stereogenic carbon, wh ich induces the preferential approach to the Re,Si face of the heterocyclic dipole. This scheme of face selectivity is also grounded in theoretical ca lculations at a semiempirical level. In addition to dihydrothiophenes, whic h are the expected products of the [3 + 2] cycloaddition, bicyclic systems based on dihydrothieno[2,3-c]piperidine skeleton can also be obtained.