1.2.3.4- Oxatriazoles synthesis

The synthesis of mesoionic oxatriazoles was reported as early as 1896 <1896CB2161>, but the structure was first proved in 1964 <64JCS906>. The original method of preparation is still in use, but alternative procedures have been developed. Activity in the field peaked in 1979 and in the period... 

While the neutral 1,2,3,4-oxatriazoles (1) still await synthesis, some of their properties have been predicted by theoretical calculations. AMI calculations combined with a principal component analysis loading data from other related heteroaromatics have been used to estimate geometric characteristics, aromaticity, energy of formation, and N chemical shifts <90JPR885>. The oxatriazoles (1) and (7) and the 1,2,3,5-thiatriazoles, which also have not been prepared, are calculated to be in the group with the lowest classical and magnetic aromaticity. 

Analogous routes cannot be successfully employed for the synthesis of meso-ionic 3-aryl-l,2,3,4-oxatriazol-5-ones (271). Thus, nitrosation of 1-arylsemicarbazides yields arylazocarbonamides with loss of nitroxyl. Meso-ionic 3-aryl-1,2,3,4-oxatriazol-5-ones are formed by... 

Treatment of meso-ionic l,2,3,4-oxatriazole-5-thiones (286) (Section VII, I, 3) with boiling ethanolic ammonia yields the isomers 297.200 These belong to a new class of meso-ionic heterocycle, which by O-alkylation with triethyloxonium tetrafluoroborate yield the salts 298.200 These are useful intermediates for the synthesis of a number of novel types of meso-ionic 1,2,3,4-thiatriazoles (299,207 300,200 and 301).212... 

This chapter covers the literature from mid-1995 to mid-2007. Recent advances include the preparation of the first A2-l,2,3,4-oxatriazolines 7 by intramolecular [3+2] cycloaddition (Section 6.08.9), synthesis of mesoionic derivatives 4 using bromonitroformaldehyde iV-arylhydrazones as the starting materials (Section 6.08.9), and detailed studies of the biologically active mesoionic oxatriazoles in the context of their nitric oxide donating capabilities (Section... 

The synthesis of mesoionic 1,2,3,4-oxatriazole 31 from compound 30 using w-chloroperbenzoic acid (MCPBA) has been reported for the first time (Equation 3) <1997MRC124>. Mesoionic 1,2,3,4-oxatriazole 30 is converted electrochemically to isomer 32, albeit on a small scale and in modest yield (Equation 4) <1997MRC124>. 

Methods of synthesis of the known 1,2,3,4-oxatriazoles <1979J(P1)732, 1979J(P1)736> and the very rare 1,2,3,5-oxatriazoles have recently been compiled <2003HOU823>. In this section, the examples that have appeared since the last survey of the area <1996CHEC-II(4)679> are presented. 3-Alkyl-anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides such as compound 35 are formed from semicarbazides after nitrosation (Scheme 4) <1999FA316>. 

Since the last survey <1996CHEC-II(4)679>, there have been no examples of synthesis of 1,2,3,4-oxatriazoles or 1,2,3,5-oxatriazoles by transformation of another ring. 

Direct, reliable methods for synthesis of mesoionic 1,2,3,4-oxatriazoles 4-6 are well established. All rely on the transformation of acyclic starting materials. Thus, 3-alkyl-anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides 4 (R1 = alkyl) are readily obtained by nitrosation of semicarbazides followed by cyclization (Section 6.08.9). The 3-aryl-substituted analogues 4 (R1 = aryl) are prepared either by cyclization of arylazonitromethanes or by nitrosation and cyclization of arylhydrazonomethanesulfonate salts <1984CHEC(4)579, 1996CHEC-II(4)679>. 

Oxatriazole derivatives are usually prepared by the cyclization of A-nitrosohydrazines as illustrated by the synthesis of 3-alkyl-1,2,3,4-oxatriazolium-5-olates 764 from semicarbazides via nitrosation (Scheme 330) . 

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