Preparation, structure, and unique thermal [2+2], [4+2], and [3+2] cycloaddition reactions of 4-vinylideneoxazolidin-2-one

The terminal allene C-alpha=C-beta bonds of 4-vinylidene-2-oxazolidinone (2) readily undergo [2 + 2] cycloaddition with a wide variety of terminal alkynes, alkenes, and 1,3-dienes irrespective of their electronic nature under strictly thermal activation conditions (70-100degreesC) and provide 3-substituted (Z)-methylenecyclobutenes 6, 3-substituted methylenecyclobutanes 7 and 8, and 3-vinylmethylenecyclobutanes 9, respectively, in good to excellent yields. Alkenes react with 2 with complete retention of configuration. The [2 + 2] cycloaddition is concluded to proceed via a concerted [(pi(2s) + pi(2s))(allene) + pi(2s)] Huckel transition state on the basis of experimental evidences and quantum mechanical methods. Some highly polarized enones and nitrile oxide, on the other hand, react with 2 selectively at the internal C-4=C-a double bonds and give spiro compounds 10 and 11, respectively. The bent allene bonds (173-176degrees) and the unique reactivity associated with 2 are attributed to a low-lying LUMO (C-a=C-beta) that is substantiated by a through-space sigma*(N-SO2)-pi*(C-a=C-beta) orbital interaction.