Chameleon Reactivity of the Allene Bond of 4‐Vinylidene‐2‐oxazolidinone: Novel Through‐Space Conjugative Nucleophilic Addition of Electron‐Rich Alkenes and Hetero‐Nucleophiles

The CαCβ double bond of allene carbamates 1 serves as an electron acceptor similar to the double bond of conjugated enones by means of a through‐space interaction with the NSO 2 bond; the carbamate double bond is thus subject to nucleophilic addition for a wide variety of nucleophiles, which proceeds under mild conditions by heating at 70–100 °C. Depending on the kind of nucleophiles, 1 displays three different reaction modes: 1) Typically enol ethers and allylsilanes promote 1,3‐sulfonyl migration of 1 and undergo the inverse electron demand Diels–Alder reaction with the 1‐aza‐1,3‐butadiene intermediates II thus formed to furnish bicyclic 2‐alkoxy‐5‐sulfonyltetrahydropyridines 2 and 2‐silylmethyl‐5‐sulfonyltetrahydropyridines 3 , respectively, with high regio‐ and stereoselectivity and retention of configuration of the double bonds of these electron‐rich alkenes; 2) silanes (R n SiH 4− n , n =1–3) and thiols deliver the hydride and the thiolate at the Cβ carbon and promote the 1,3‐sulfonyl migration, followed by protonation of the thus‐formed carbamate anion ( Z )‐ III to provide, for example, ( Z )‐ 4 a and ( Z )‐ 4 j , respectively; 3) alcohols simply add to the CαCβ double bond and provide ( E )‐ 6 . Usually, the reaction with alcohols is accompanied by the second pathway, giving rise to, for example, ( Z )‐ 4 b in addition to ( E )‐ 6 b . Phenol engages in the third pathway and provides ( E )‐ 6 g exclusively. Heteroaromatics, such as furans and benzofurans follow the first pathway, however, in a different regioselectivity from enol ethers and allylsilanes, delivering the oxygen atom at the 3‐position of 5‐sulfonyltetrahydropyridines ( 2 g and 2 h ). Indoles, on the other hand, show a dichotomy, equally enjoying the first and the third pathways and provide mixtures of ( E )‐ 7 and ( E )‐ 8 , respectively.