ON THE INTRAMOLECULAR 1,4-DIPOLAR CYCLOADDITION REACTION OF THIAZINIUM BETAINES FOR THE CONSTRUCTION OF AZA-HETEROCYCLIC, DIAZA-HETEROCYCLIC, AND POLYAZA-HETEROCYCLIC RING-SYSTEMS

A series of bicyclic anhydro-4-hydroxy-2-oxo-1,3-thiazinium hydroxides containing tethered pi-systems are easily prepared from the reaction of thiolactams with 1,3-bielectrophiles. These cross-conjugated hetero aromatic betaines undergo regio- and stereospecific 1,4-dipolar cycloa ddition in good yield to produce cycloadducts containing a carbonyl su lfide bridge which can be induced to lose COS on further heating. Two of the cycloadducts were characterized by single crystal X-ray determi nations. Control of ring size in the final product of the cycloadditio n can be achieved by variation of the dipolarophilic chain length. Ent ry to the [6,6,5]- and [6,6,6]-pyridone ring systems was possible usin g phenylalkynyl-substituted thioamides. Intramolecular 1,4-dipolar cyc loaddition of a thiazinium betaine dipole also occurs across an indoly l pi-bond. With only one substituent group in the 9-position of the bi cylic betaine, the reaction takes an entirely different course unless a highly activated pi-bond is incorporated into the tether. The prefer red reaction with modestly activated pi-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.