Synthesis of Naphthoxazoles by Photocyclization of 4‐/5‐(Phenylethenyl)oxazoles

Promising test results on the biological activity of our previously described naphtho[1,2-d]oxazoles and heterobenz[1,2-d]oxazoles, obtained by the photochemical cyclization of 5-phenylethenyl- and 5-heteroarylethenyloxazoles, prompted us to continue with the photochemical synthesis of substituted naphtho[1,2-d]oxazoles, and to extend the photochemical cyclization to the synthesis of naphtho/heterobenz[2,1-d]oxazoles from 4-(aryl/heteroarylethenyl)oxazoles. The required p- and o-phenyl-substituted 5-arylethenyloxazoles were prepared from the corresponding ,-unsaturated aldehydes and the TosMIC reagent (tosylmethyl isocyanide) by the Van Leusen reaction. The substituted 4-(aryl/heteroarylethenyl)oxazoles were prepared by the Wittig reaction starting from various phosphonium salts and 2-H/methyl-4-oxazolecarbaldehydes.