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 α,β‐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.