Use of the p-tolylsulfinyl group as a chiral inductor in stereoselective [4+3] cycloaddition reactions: preparation of enantiopure polysubstituted 8-oxabicyclo[3.2.1]oct-6-en-3-one systems having up to five stereocenters

The use of the p-tolylsulfinyl group as a chiral inductor in stereoselective [4+3] cycloaddition reactions has made possible the preparation of enantiopure polysubstituted 8-oxabicyclo[3.2.1]oct-6-en-3-one systems having up to five stereocenters. The presence in the furan diene of a susbtituent at the 3-position bearing an atom with lone-pair electrons propitiates the coordination of a metallic atom by the sulfoxide and the R "ligand" group. This chelation effect, together with the stereoelectronic demand of the R and sulfoxide groups, considerably decreases the degree of conformational freedom of the diene moiety in the cyclic transition state leading to the observed stereoselectivity. By choosing appropriate substituents in the furan ring, it is possible to obtain 100:0 cis-trans and 100:0 endo-exo diastereoselectivities. Up to 100% π-facial diastereoselectivity (enantioselectivity after removal of the sulfoxide group) could be reached. Yields lie within the range of 50-80%, depending on the furan substrate. However, if the furan diene is fitted with deactivating electron-withdrawing substituents, low conversions were observed under certain reaction conditions, but always with excellent-to-complete chemo- and stereoselectivities. The conversion and yield increase on introduction of an electron-donating substituent at the C-3 of the furan ring.