Abstract
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.