Versatile photochemical reactivity of diverse substituted 2-, 4- and 5-(o-vinylstyryl)oxazoles

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 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 several synthetic approaches. By 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 and 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 do not react and give only high weight molecular products but are 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 the position of the methyl/phenyl substituents these new vinylstyryl-2/4/5-oxazole derivatives show diverse photochemical behavior.

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