Functionalisation reactions of 2,5-diphenyl-1,3,4-oxadiazoles bearing a terminal ethynyl or butadiynyl substituent: X-ray crystal structures of the products.
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 7 salt 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 pi-systems of all four molecules adopt predominantly planar conformations. A comparison of bond lengths in the structures of 5 and 11 reveals extended pi-conjugation in the latter.
Kreher
David
D
Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
Batsanov
Andrei S
AS
Wang
Changsheng
C
Bryce
Martin R
MR