Reaction of esters of 2-arylcyclo-propanecarboxylic acids with nitrous acid. Synthesis of aryl-substituted 3-ethoxycarbonyl-4,5-dihydroisoxazoles and 3-ethoxycarbonylisoxazoles

Esters of 2-arylcyclopropanecarboxylic acids react with nitrous acid generated in situ with regioselective insertion of the nitrosyl cation into the cyclopropane ring. Depending on the substrate/nitrosylating agent ratio, the reaction proceeds with the formation of either aryl-substituted 3-ethoxycarbonyl-4,5-dihydroisoxazoles or the corresponding isoxazoles. The nature and position of the substituents in the aromatic ring of the starting 2-arylcyclopropanecarboxylic acid esters affect the reaction rate but have no effect on the regioselectivity of the attack by the nitrosyl cation on the three-membered ring. A dependence of the reactivity of isomeric substrates on their stereochemistry and position of the nitro group in the aromatic ring is noted for 2- and 4-nitrophenyl derivatives of esters of cis- and trans-2-arylcyclopropanecarboxylic acids.