Ene Reactions of Nitrosocarbonyl Intermediates with Trisubstituted Cycloalkenes: "Cis Effect" and Steric and Conformational Factors Drive the Selectivity.

Nitrosocarbonyl intermediates, generated at room temperature by oxidation of nitrile oxides, undergo clean ene reactions with trisubstituted cycloalkenes. Nitrosocarbonyl benzene follows a Markovnikov orientation and abstracts preferentially the twix hydrogens over the lone ones. With the more sterically demanding nitrosocarbonyl mesitylene in the presence of 5- and 6-membered ring olefins, the Markovnikov directing effect is relieved, and twix and lone abstractions are observed. Endocyclic allylic hydrogens on the more congested side of the alkene are exclusively abstracted (the "cis effect") resembling the singlet oxygen behavior. The balance between steric and conformational factors, as well as the acylnitroso generation conditions, dictates the regioselectivity in some cases. Larger ring olefins undergo selective twix allylic hydrogen abstraction. The photochemical generation of nitrosocarbonyl is totally selective according to the Markovnikov orientation. The synthetic utility of the ene compounds is also accounted.