Preparation of new nitrogen-bridged heterocycles. 59. Syntheses and intramolecular interactions of 3-(acylmethylthio)- and 3-[(3-ethoxycarbonyl-2-propenyl)thio]thieno[3,4-b]indolizine derivatives

Various thieno[3,4-b]indolizine derivatives having an acylmethylthio or (3-ethoxycarbonyl-2-propenyl)thio group at the 3-position which could not be obtained by a conventional method were prepared by a new procedure using cyanoethyl group as a protecting group and their intramolecular arene-pi interactions were investigated. In the (1)H-NMR spectra of these thieno[3,4-b]indolizines, the low-field shifts (delta 0.10-0.33 ppm) for the 5-protons were observed in comparison with those of 3-(methylthio)thieno[3,4-b]indolizines as a standard. The UV spectra also exhibited a characteristic absorption band at 425-445 nm attributable to the arene-pi interaction but their intensities were generally lower than those of 3-(arylmethylthio)thieno[3,4-b]indolizines. In their X-ray analyses, the anti conformation for 3-(acylmethylthio)thieno[3,4-b]indolizines and the gauche one for the 3-(3-ethoxycarbonyl-2-propenyl)thio derivatives were exhibited.