Access to Isolable Azomethine Ylides by Photochemical Transformation of 2,3-Dihydroxisoxazoles

摘要

Photochemical excitation of the 2,3-dihydroisoxazoles 19-21 resulted in the formation of the stable azomethine ylides 22, 25, and 28, representing the first isolable examples of such species bearing stabilizing groups only at one end of the 1,3-dipecies bearing stabilizing groups only at one end of the 1,3-dipole. The UV and NMR spectroscopic data of the photoproducts clearly indicate that the iminium and the anionic parts of the azomethine ylide systems are not planar. This conclusion is unambiguously confirmed by crystal structure analysis of 25b, in which a twist angle of 73 deg was determined between the two polar moieties. In the case of 25d the rotation barrier around the central CN bond amounts to 16 kcal/mol at 333 K. In line with the unusual stability of the ylides is their low reactivity against dipolarophiles. Only the highly reactive N-methyltriazolidinedione gives formation of the products 33a, 33a-c, and 41, but these are the result not of an initial 1,3-dipolar cycloaddition but rather of a formal [4+2] addition with involvement of 34a, 36a-c, and 41 as un-stable primary products. On heating in refluxing toluene the azomethine ylides are transformed into annulated pyrrole systems (42/43, 46/47, 48/49). In contrast to other examples the direct thermal transformation of the annulated 2,3-dihy-droisoxazoles into pyrroles is not successful, but the reaction can be achieved by simultaneous thermal and photochemical activation, as shown for the conversion of 19 into the pyrroles 42/43.