Islam MD. Rafiqul
|$B3X0LO@J8BjL\(B||Novel Conversion of 6$B#H(B-1,3,5-Oxachalcogenazines into Five-Membered Heterocyclic Compounds Having Nitrogen and Chalcogen Atoms
$B!!(BRecently, the enormous increase in synthesis of heterocyclic compounds containing chalcogen and nitrogen atoms has been leading to develop new methods. In this way, generation and synthetic application of 1,3-chalcogenaza-1,3-butadienes and their oxidized variants would be versatile building blocks because of their heterocumulene-like structure and potential reactivities. Within this earnest demand, only limited findings have been realized because of the preparative difficulties of their precursors and lack of suitable trapping methods for these highly reactive species. In the course of studies of highly-reactive chalcogenocarbonyl functionalities bearing $B&P(B-conjugation systems, several novel methods were developed for the conversion of 6H-1,3,5-oxachacogenazines into five-membered heterocyclic compounds.
The following methods for the synthesis of five-membered heterocyclic compounds have been developed:
Preparation of 6H-1,3,5-Oxachalcogenazines and S-Oxides
6H-1,3,5-oxachalcogenazines 1 (X=S) and 2 (X=Se) were prepared as single stereoisomers by treating a thio- or selenoamide with an aldehyde and BF3.OEt2 according to the reported procedure. Oxidation of 1 by mCPBA gave the corresponding 6H-1,3,5-oxathiazine S-oxides 3 as single epimers in high yields.
$B!!(BSynthesis of 5H-1,2,4-Oxathiazoles 4 through Thermal Cycloreversion of 6H-1,3,5-Oxathiazine S-Oxides 3
$B!!(BHeating of a CHCl3 solution of 3 afforded hitherto unknown 5H-1,2,4-oxathiazoles 4 quantitatively. When a CDCl3 solution of 3a (R1=Ph, R2=t-Bu) was kept standing at 25 0C in an NMR tube, a mixture of 4 and pivalaldehyde was formed in a 1:1 molar ratio, suggesting that thermal cycloreversion of 3 would generate A, and which might subsequently cause facile ring closure to afford 4 (Scheme 1).
Compounds 4 were efficiently converted into 1,2,4-thidiazoles 5 through plausible pathway involving acid-induced hydrolytic fragmentation of 4 and the subsequent self-condensation of intermediacy, thioamide S-oxide. Novel Conversion of 6H-1,3,5-Oxathiazine S-oxides 3 into 3H-1,2,4-Dithiazoles 6 by the treatment with LR When 3 was heated with Lawesson's reagent (LR) at toluene refluxing temperature, 3H-1,2,4-dithiazoles 6 were afforded in moderate to high yields. An NMR monitoring experiment of 3a with LR in CDCl3 at 25 0C was observed quantitative formation of pivalaldehyde along with a trace amount of 6a. This result suggests that the reaction of 3a with LR might initially form intermediacy B, and the subsequent thermal cycloreversion of B would form C and which finally give 6 by eliminating PSOAnis moiety (Scheme 2). On the other hand, when 3 was treated with LR in the presence of EtOH, deoxygenation products 1 were formed.
Synthesis of 3H-1,2,4-Dithiazoles, 3H-1,2,4-Thiaselenazoles and 3H-1,2,4-Diselenazoles
In conclusion, a novel method was developed for the synthesis of 5H-1,2,4-oxathiazoles 4 through thermal cycloreversion of 6H-1,3,5-oxathiazine S-oxides 3. A novel conversion of 3 into 6 by the treatment of LR was realized. A wide range of 5-membered 1,2,4-dichalcogenazoles 6-10 were also synthesized by the reaction of in situ generated 1,3-chalcogenaza-1,3-butadienes with elemental chalcogen.