The hydrogen bond directing effect in nitrile oxide cycloadditions to allylic substituted cyclopentenes.

A quantitative evaluation of the H-bond directing effect on the stereoselectivity in the cycloaddition of nitrile oxides to 2-cyclopenten-1-ol and allylic cyclopentenyl amides is reported. In apolar solvents the H-bond directing effect promotes a high syn stereoselectivity while H-bond acceptor solvents divert the reactions to the anti face of the dipolarophile. Taft's b parameter gives a good description of the solvent effect on the H-bond directing effect. The persistence of some syn stereoselectivity even in good H-bond acceptor solvents points out the existence of some residual hydrogen bond direction. The syn stereoselectivity in the presence of M(II) salts was also investigated and the results discussed in the light of the potential application of these scaffolds in nucleoside synthesis.