One-Step Synthesis and Highly Regio- and Stereoselective Diels−Alder Cycloadditions of Novel exo-2-Oxazolidinone Dienes

An improved synthesis of exo-heterocyclic dienes like N-substituted 4,5-dimethylene 2-oxazolidinones 1, by a one-step method from diacetyl and isocyanates, was described. This highly convergent synthetic strategy has been successfully used for the preparation of novel (Z)-5-ethylidene-4-methylene analogs 4 in fair yields. Both dienes 1 and 4 undergo efficient addition of symmetric dienophiles in thermal Diels−Alder reactions, inasmuch as they react stereo- and regioselectively with the unsymmetric olefins methyl vinyl ketone (MVK) and methyl propiolate. This regioselectivity was greatly improved by using Lewis acid catalysts (TiCl4, AlCl3). The nitrogen atom of the 2-oxazolidinone ring seems to control the orientation of the dienophile approach. These results have been rationalized in terms of the frontier molecular orbital theory by ab initio calculations. For dienes 4, addition of MVK gave the endo isomer as the major product. Dimerization of dienes 4 was also highly regio-, chemo-, and stereoselective, giving only isomer 17. This reaction furnished a second product, which corresponded to dienes 18 obtained by the [1,5] sigmatropic rearrangement of 4. The structure of the dienes and main products was established by NMR experiments and X-ray diffraction analysis.