Ring Opening Reactions and Rearrangements

Reagents i. HCl or HBr ii.H2S04, H2O iii. CI2 or Br2 iv. PhOH, R=H only v.AcOH, glacial vi Se02, C6H4CI2 

Treatment of a 3-aminotriazolopyridine with acid gave the imidazopyridine 242 (81T1787), also obtained from the 3-nitro derivative by catalytic reduction (83AHC79). Quaternary salts derived from compound 2, when treated with tri-ethylamine and subsequently heated give 2-pyridylcyanamides 243 or 2-(oxazol-l-yl)pyridines 244 depending on the alkyl group (86H(24)2563) the ylides are presumably intermediates (see also Section IV.I). 

Of derivatives of compound 3, the zwitterions 245 react with DMAD if magnesium bromide is added to give aminopyridine derivatives (00H(53)265). Compounds such as 102 are opened by acid to give quaternary pyridinium salts (92LA885). There are various interconversions of 2-substituted triazolopyridin-3-ones into 1-substituted zwitterions. An example is the reaction of 246 (83BCSJ2969) the reverse conversion is also reported. 

Pyrolyses of Nl- or N3-substituted derivatives of compounds 4 and 5 have continued to find use as routes to azacarbazoles, although the yields are often indifferent and there are no recent examples. The photochemical reactions are dealt with in Section IV.G. Pyrolysis media are paraffin (P) or PPA, and examples of products are compounds 247 (P, cytostatic) (83MI2), 248 (P) (84MI1), and 249 (from a 1-substituted derivative) (86MI2). Indications of diradical intermediates are provided by the thermolysis of compound 250 (P) (83MI2) where one product is a dimer. 

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The first step is an electrocyclic ring-opening reaction, and the second step is a [1,7] sigmatropic rearrangement. This sequence of steps not only occurs in our bodies but is also utilized commercially to enrich milk with vitamin D3. All milk sold in the United States is irradiated with UV light, which converts the 7-dehydrocholesterol present in the milk into vitamin D3. 

Figure 15.29 The Cope rearrangement and Dewar benzene ring-opening reaction...

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