A Through-Space Interaction of 3-Benzylthiothieno[3,4-b]indolizine Derivatives Combined by a Highly Flexible Spacer

Abstract

Some ethyl 1-benzoyl-3-[(un)substituted benzylthio]thieno- [3,4-b]indolizine-9-carboxylates were prepared and their conformations were investigated.In the solid state and the chloroform solution they had stacked structures in which the phenyl ring and the pyridine moiety of theino [3,4-b]indolizine ring are partly overlapped.It is well known that two aromatic rings separated by the appropriate spacer are attracted by the π-π o r ar en e-ar ene i nt er act i o n, 2 an d t he m o l ecu l es h avi n g suc h f un cti on ar e p ot ent i al can di dates f or a mo lecular switc h and a chi ral aux ili ar y. 3 We have r ecent ly r epo rt ed an interesting stacked structure in bis[1-cyano-9-ethoxycarbonylthieno[3,4-b]-indolizin-3-yl] disulfides 4 and the unusual proximity of the two indolizine rings in 1,2-bis[[(2-indolizinyl)thio]methyl]benzene derivatives, 1 and proposed the contribution of the unique ionic structure of the indolizine ring as a driving force for this through-space interaction.Furthermore, our recent development of a novel constructive method of a thieno[3,4-b]indolizine skeleton 5 prompted us to i nvestigate th e th rough-s pace i nter act ion for various t ypes of th ieno [3,4-b ]ind oli zin e derivatives.The conformational analyses for some model molecules using Dreiding models showed that the phenyl ring and the pyridine moiety of the thieno [3,4-b]indolizine ring in the title compounds can considerably approach in the gauche form (G) in the relation to the exocyclic sulfide linkage, though they are largely remote in the anti one (A), as shown in Figure 1.In this