Diastereoselective synthesis of benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles via cycloaddition reaction of benzothiazolium salts with 3-nitrochromenes

Abstract

The triethylamine mediated 1,3-dipolar cycloaddition reaction of 2-phenacyl- or 2-alkoxycarbonylmethylbenzothiazolium bromides with 3-nitrochromenes in ethanol at room temperature afforded functionalized tetrahydrobenzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles in 83–95% yields and with high diastereoselectivity. The corresponding dehydrogenated benzo[d]chromeno[3′,4′:3,4]pyrrolo[2,1-b]thiazoles were also easily obtained by sequential oxidation with DDQ. The stereochemistry of the polycyclic compounds was clearly elucidated by analysis of NMR spectroscopy results and determination of single crystal X-ray structures. The reaction is believed to proceed via endo-[3 + 2] cycloaddition of the in situ generated anti-form ylides to the cyclic dipolarophiles.