Naturally-occurring Acetylenic Compounds and Derivatives. Regio- and Stereocontrolled Chloroformylation of 2-(1-propynyl)thiophene and 2-phenyl-5-(1-propynyl)thiophene

2-Phenyl-5-(1-propynyl)thiophene (1), isolated from Coreopsis grandiflora, and 2-(1-propynyl)thiophene (5), an immediate precursor in the synthesis of junipal (2), were synthesized in high yield by a Pd-catalyzed reaction between propyne and 2-iodo-5-phenylthiophene (4) or 2-iodothiophene (6), respectively. Reaction of 5 with the Vilsmeier reagent derived from POCl3 and N-methylformanilide (MFA) afforded a mixture from which it was possible to isolate (E)-3-chloro-2-methyl-3-(2-thienyl)acrolein (10) in 37.7% yield. The structure and stereochemistry of 10 was unequivocally established by X-ray diffraction of a single crystal of the 2,4-dinitrophenylhydrazone of 10. GLC analysis showed that 10 was contaminated by ca 7% with an isomer to which, on the basis of 1H-NMR and mass spectra, the structure of (Z)-3-chloro-2-methyl-3-(2-thienyl)acrolein (11) was attributed.