1,2,3-Thiadiazolo pyridine

The comparative ionization potentials of various triazolopyridines and [l,2,3]-thiadiazolo-pyridines versus benzotriazole or benzothiadiazole were determined by electron-impact mass spectrometry (Table 2) <74CS222>. Since their ionization potentials are similar to the benzo-fused analogues, it infers that they have comparable aromaticity profiles. The same technique shows that the [l,2,3]oxadiazolopyridines (9a,b) occur exclusively as the ozy/zo-quinoid-like pyridodiazo-oxide tautomers (Equation (1)). 

This type of method has been used to prepare l,2,4-thiadiazolo[2,3- ]pyridine derivatives. The oxidative hetero-cyclization is exemplified by the formation of compound 86 from thioacetamide 85 using nitrosobenzene (Equation 24) <2004S2975>. 

Boron substituents in the [l,3,2]diazaborolo[l,5- ]pyridine derivative 109 were studied. This compound was obtained via reduction of its precursor 108 with sodium amalgam (Scheme 27). The bromide attached to the boron atom was further displaced with various halide, hydride, sulfur, and carbon nucleophiles <2001JCD378>. [l,2,5]thiadiazolo[2,3- ]pyridine derivative 110 was deprotected (R = Cbz to R=H) by classical hydrogenolysis <2002AGE3866>. 

Oxidative ring closure on to a ring nitrogen atom occurs readily as in the formation of the [ 1,2,4]thiadiazolo[2,3-a]pyridine (78) from the 2-pyridylthiourea (77) (75JHC1191). Similarly, bromine oxidation of 2-thiazolylthiourea (79) gives the 2-aminothiazolo[3,2-/ ] [l,2,4]thiadiazolylium bromide (80) (71JPR1148). 

Two modifications of the well-known benzothiazole preparation have been employed to prepare unusual heteropoly cycles. Konig et al.ils treated l-thiocarbamoyl-l,2,3,4-tetrahydroquinoline (236) with bromine in chloroform to give the thiazolo[3,4,5-J,i]quinoline derivative 237. In a process which requires disruption of the resonance stabilization of the pyridine ring, Harris416 reported that treatment of l-(2-pyridyl)-2-thiourcas with sulfuryl chloride or with bromine gives the hydrohalide salts of 2-imino-2//-[l,2,4]thiadiazolo[2,3-a]pyridines (238). 

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