Synthesis and highly selective Diels–Alder cycloadditions of the new dienes N-substituted 2,3,5,6-tetrahydrobenzoxazol-2-ones

The synthesis of the new dienes N-substituted 2,3,4,5-tetrahydrobenzoxazol-2-ones is described, through a one-step convergent process from 1,2-cyclohexanedione () and the corresponding isocyanates . The presence of electron-donor substituents in the aryl ring of the isocyanate gave rise to the exclusive formation of the captodative olefins . Diene proved to be reactive and stereoselective in Diels–Alder additions with a cyclic olefin. The reaction with acetylenic dienophiles yielded the 2,3-dihydrobenzoxazol-2-ones and , as the products of sequential [4+2] cycloaddition and retro-Diels–Alder reactions. Methyl vinyl ketone () underwent regio- and stereoselective tandem Diels–Alder and Michael additions to give propellane . Evidence of an endo π-pyramidalization of the central double bond of adduct would rationalize the exo stereoselection in the formation of . The regioselectivity in these reactions has been rationalized in terms of FMO theory by ab initio calculations.