Stereoselective Synthesis of 4,5-Diethylidene-Oxazolidinones as New Dienes inDiels-Alder Reactions

The N-substituted isomeric (4Z,5Z)- and (4E,5Z)-4,5-diethylideneoxazolidin-2-ones 5 and 6 were synthesized, the latter being favored during the one-step process from the α-diketone 1c and different isocyanates. The steric interaction between the N-substituent and the Me group attached to the exocyclic diene moiety plays a decisive role in controlling the observed stereoselectivity, as suggested by the calculated free energies of the two isomers. Both dienes undergo efficient additions to symmetric dienophiles in thermal Diels-Alder reactions to yield the adducts 11 and 13, respectively. These molecules displayed interesting C−H⋅⋅⋅π, and C−H⋅⋅⋅X (X=O, Cl) interactions according to their X-ray crystal structures. Isomers 6 suffered highly stereo- and regioselective additions with nonsymmetrical dienophiles such as methyl vinyl ketone or methyl propiolate. Steric interactions, promoted by the inward-pointing Me group in 6, seem to explain such selectivity. These results have also been rationalized by ab initio calculations in terms of the FMO theory.