ONE-STEP SYNTHESIS AND HIGHLY REGIOSELECTIVE 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 a nd isocyanates, was described. This highly convergent synthetic strate gy has been successfully used for the preparation of novel (Z)-5-ethyl idene-4-methylene analogs 4 in fair yields. Both dienes 1 and 4 underg o efficient addition of symmetric dienophiles in thermal Diels-Alder r eactions, 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 catalys ts (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 theo ry by ab initio calculations. For dienes 4, addition of MVK gave the e ndo isomer as the major product. Dimerization of dienes 4 was also hig hly regio-, chemo-, and stereoselective, giving only isomer 17. This r eaction furnished a second product, which corresponded to dienes 18 ob tained by the [1,5] sigmatropic rearrangement of 4. The structure of t he dienes and main products was established by NMR experiments and X-r ay diffraction analysis.