On the Intramolecular 1,4-Dipolar Cycloaddition Reaction of Thiazinium Betaines for the Construction of Aza-, Diaza-, and Polyaza-Heterocyclic Ring Systems

A series of bicyclic anhydro-4-hydroxy-2-oxo-1,3-thiazinium hydroxides containing tethered π-systems are easily prepared from the reaction of thiolactams with 1,3-bielectrophiles. These cross-conjugated heteroaromatic betaines undergo regio- and stereospecific 1,4-dipolar cycloaddition in good yield to produce cycloadducts containing a carbonyl sulfide bridge which can be induced to lose COS on further heating. Two of the cycloadducts were characterized by single crystal X-ray determinations. Control of ring size in the final product of the cycloaddition can be achieved by variation of the dipolarophilic chain length. Entry to the [6,6,5]- and [6,6,6]pyridone ring systems was possible using phenylalkynyl-substituted thioamides. Intramolecular 1,4-dipolar cycloaddition of a thiazinium betaine dipole also occurs across an indolyl π-bond. With only one substituent group in the 9-position of the bicyclic betaine, the reaction takes an entirely different course unless a highly activated π-bond is incorporated into the tether. The preferred reaction with modestly activated π-systems corresponds to loss of the activated proton to produce an S,N-ketene acetal. When a ketene S,S-acetal group was incorporated onto the side chain, the 1,4-dipolar cycloaddition reaction was facilitated relative to proton loss.

intramolecular - dipolar-cycloaddition - 1,4-dipole - heteroaromatic - cross-conjugated - betaine - 1,3-thiazinium hydroxide - S,S-ketene acetal - thioamide - cyclization