Heterocycles in organic synthesis: thiazoles and triazoles as exemplar cases of synthetic auxiliaries

This Perspective article illustrates the key role of thiazole and triazole in the work carried out in the author's laboratory over three decades and deals with the synthesis of carbohydrate-based bioactive molecules. The first part reports on the development of synthetic strategies exploiting the use of various thiazole-based reagents and the ready conversion of thiazole into the formyl group. After describing the chain elongation of monosaccharides into higher-carbon homologues, the synthesis of target natural and non-natural carbohydrates, or their ultimate precursors, is presented. These include some sphingoids, neuraminic and destomic acids, lincosamine, various 3-deoxy-2-ulosonic acids (KDO, KDN, iso-Neu4Ac), iminosugars (nojirimycin, mannojirimycin, galactostatin) and homoazasugars. Also prepared were the disaccharide subunit of bleomycin A(2) and the side-chain of taxol and taxotere.(R) The use of 1,2,3-triazole is discussed in the second part of the paper. The service of this heterocycle that is easily formed by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is considered in light of its use as a robust linker (a sort of keystone) of complex and diverse molecular architectures. Thus, the assembly of triazole-linked glycosyl amino acids, non-natural nucleotides, 1,6-oligomannosides, sialoside clusters on calixarene platfom via CuAAC is described and the biological relevance of these compounds is discussed in brief.