Regio- and stereoselective behavior of l-arabinal-derived vinyl epoxide in nucleophilic addition reactions. Comparison with conformationally restricted d-galactal-derived analogs

The regio- and stereoselectivity of the addition reactions of O-, C-, N-, and S-nucleophiles to l-arabinal-derived vinyl epoxide 2, the simplest non-conformationally restricted glycal-derived vinyl epoxide, has been examined and compared with the corresponding, conformationally restricted d-galactal-derived analogs 1β and 1β-Me. Results indicated that the 1,4-/1,2-regioselectivity ratio and the related syn-1,4-/anti-1,2-stereoselectivity observed in glycal-derived vinyl oxiranes is independent of the presence of substituents on the six-membered unsaturated ring, and the absence of conformational freedom: it depends only on the ability of the nucleophile to give a coordination process with the oxirane oxygen in the form of a hydrogen bond or through a coordinating cation.