Functionalisation reactions of 2,5-diphenyl-1,3,4-oxadiazoles bearing a terminal ethynyl or butadiynyl substituent: X-ray crystal structures of the products

Abstract

2-(4-tert-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 and the butadiyne analogue 2 reacted with triethyl orthoformate in the presence of zinc iodide to give the acetal derivatives 3 and 4 which were hydrolysed with Amberlyst-15 in acetone-water to afford the alkynylaldehyde derivatives 5 and 6 in high yields. The reaction of 4,5-bis(methoxycarbonyl)-2-tributylphosphonium-1,3-dithiole tetrafluoroborate salt 7 with 5 (nBuLi, THF) gave the Wittig product 2-(3-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}prop-2-ynylidene)-1,3-dithiole-4,5-dimethyl dicarboxylate 11 (33% yield) whereas other attempted Wittig and Horner–Wadsworth–Emmons reactions led to the unexpected loss of the aldehyde group from compounds 5 and 6 to give 1 and 2, respectively. The X-ray crystal structures of compounds 3, 4, 5 and 11 are reported: the π-systems of all four molecules adopt predominantly planar conformations. A comparison of bond lengths in the structures of 5 and 11 reveals extended π-conjugation in the latter.