Preparation of new nitrogen-bridged heterocycles. 58. Syntheses and intramolecular arene-pi interactions of 3-(allylthio)- and 3-(propargylthio)thieno[3,4-b]indolizine derivatives

Various thieno indolizine derivatives having an allylthio or propargylthio group at the 3-position were prepared and their intramolecular arene-pi interactions were investigated. Their 1H-NMR spectra showed significant low-field shifts (delta 0.10-0.34 ppm) to the 5-proton on the thieno indolizine ring, and this effect was the reverse to that observed in 3-(arylmethylthio)thieno indolizines. However, their UV spectra exhibited a characteristic absorption band due to the arene-pi interaction near 430 nm and these values were almost similar to those for arene-arene interaction of 3-arylmethylthio derivatives though their molar extinction coefficients were largely varied by the 3-substituents. Furthermore, both types of gauche conformations in which the intramolecular arene-pi interactions are possible in one form and impossible in the other were confirmed by X-ray analyses of some compounds.