First Ring Contraction-Desulfurization of 1-(Arylcarbonyl)pyrido[2,1-c]-1,4-thiazines to 1-(Arylcarbonyl)indolizines and Its Application to 3-Arylthieno[3,2-a]indolizine Synthesis

Abstract

Dedicated to Prof. Dr. Ekkehard Winterfeldt on the occasion of his 75 th birthday.The ring contraction-desulfurization route from transient 1-(arylcarbonyl)-pyrido[2,1-c]-1,4-thiazine intermediates, generated in situ from the treatment of the corresponding pyridinium salts with a base and then a dehydrogenation agent, to 1-(arylcarbonyl)indolizines was first observed. By using this route three 1-arylcarbonyl-6,8-dimethylindolizines having the protected 2-thiol group were prepared and their transformation to 2-acyl-3-arylthieno[3,2-a]indolizine derivatives were performed in good yields.We previously developed a novel and convenient synthetic method for various functionalized indolizine derivatives. 2-6We also disclosed that this method involves transient pyrido 2-4However, we could not explain the reason why such a difference in the reactivity

Two Reaction Modes of Pyridinium 1,4-Zwitterionic Thiolates with Sulfenes: Synthesis of 3H-1,2-Dithiole 2,2-Dioxides, 1,9a-Dihydropyrido[2,1-c][1,4]thiazines, and Indolizines

“…Based on our experimental results and previous reports, , the plausible mechanisms are outlined in Scheme . Initially, sulfene A is generated from alkanesulfonyl chloride 2 in the presence of a base .…”

“…We finally identified the new substrate to be indolizine 4b via XRD analysis, which is an analogue of 4a (eq b, Scheme ). We speculated that 4a and 4b were generated via a same tandem reaction sequence via aerobic oxidation/sulfur extrusion . The direct formation of 4a in the reaction system is due to the inherent instability of pyridothiazine 3o , which was more susceptible to aerobic oxidation/sulfur extrusion, compared with compounds of 3a – 3n .…”