ABSTRACT
In systems which contain more than two hetero atoms in the same ring we find the trends in properties, which this book has described, taken to further extremes. In particular, the additional hetero atoms, in both six- and five-membered systems, lead to a suppression of electrophilic substitution and a slowing of electrophilic addition to nitrogen. Analogues of the pyrimidine nucleosides have been extensively studied: 5-Azacytidine is antileukemic and Ribavirin, an antiviral agent, is used in the treatment of lung infections in infants. 1,2,3-Triazoles are generally prepared by the cycloaddition of an alkyne with an azide, but the hazardous nature of some alkyl azides limits the method in these cases. Tetrazoles are usually prepared by the reaction of an azide with a nitrile, or an activated amide; tri-n-butyltin azide and trimethylsilyl azide are more convenient and safer reagents than azide anion in some cases. 1,2,3-Thiadiazoles are prepared by reaction of a hydrazone, containing an acidic methylene group, with thionyl chloride.
