Versatile Photochemical Reactivity of Diverse Substituted 2‐, 4‐ and 5‐(o‐Vinylstyryl)oxazoles

Abstract

To study the effects of the position of the hexatrienyl moiety on the oxazole ring, novel substituted cis/trans‐2/4/5‐(2‐vinylstyryl)oxazoles have been synthesized. These novel compounds were prepared by Wittig reaction from the diphosphonium salt of α,α′‐o‐xylenedibromide, formaldehyde and the corresponding 2‐methyl‐4‐, 4‐methyl‐2‐, 2‐pheny‐5‐ and 4‐methyl‐5‐oxazolecarbaldehyde, respectively. Aldehydes were synthesized by using several synthetic approaches. By applying intramolecular photocycloaddition, 2‐methyl‐4‐(2‐vinylstyryl)oxazole afforded, as major product, fused oxazoline‐benzobicyclo[3.2.1]octene with small quantities of 4‐(1,2‐dihydronaphthalen‐2‐yl)‐2‐methyloxazole, as electrocyclization product. Upon irradiation of 4‐methyl‐5‐(2‐vinylstyryl)oxazole, endo‐ and exo‐benzobicyclo[2.1.1]hexene products were formed by [2+2] cycloaddition; this was the first instance of the 1,4‐closure to the bicyclo[2.1.1]hexene skeleton in the 2‐, 4‐, and 5‐oxazole‐stilbene derivatives studied so far. Derivatives 2‐phenyl‐5‐ and 4‐methyl‐2‐(2‐vinylstyryl)oxazole did not react and gave only high‐weight molecular products but were crucial as a comparison in the overall mechanistic study. We have found that, depending on the position of the hexatrienyl moiety in the oxazole ring, as well as on the position of the methyl/phenyl substituents, these new vinylstyryl‐2/4/5‐oxazole derivatives show diverse photochemical behavior.