Preparation, Structure, and Unique Thermal [2+2], [4+2], and [3+2] Cycloaddition Reactions of 4Vinylideneoxazolidin‐2‐one

The terminal allene C α C β bonds of 4vinylidene‐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 (70–100 °C) and provide 3substituted ( Z )‐methylenecyclobutenes 6 , 3substituted methylenecyclobutanes 7 and 8 , and 3vinylmethylenecyclobutanes 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 [(π 2s +π 2s ) allene + π 2s ] Hückel 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 α double bonds and give spiro compounds 10 and 11 , respectively. The bent allene bonds (173–176°) and the unique reactivity associated with 2 are attributed to a low‐lying LUMO (C α C β ) that is substantiated by a through‐space σ*(N‐SO 2 )–π*(C α C β ) orbital interaction.