Sulfur and Phosphorus Heterocyclic Compounds in Nature

The antibiotic leinamycin isolated in 1989 from a culture broth of a Streptomyces variety has a disulfide group in a 5-membered ring and thus is a 1,2-dithiole derivative. As found in stmcture 3.65, one sulfur is in the sulfide state and the other in the sulfoxide state. It has another novel feature, an 18-membered nitrogen heterocycle (in a lactam stmcture), as well as a thiazole ting. It is of considerable 

Until 2009, no cyclic structures containing only phosphorus and carbon have been found in nature, even though this is a large and important class of the huge field of organophosphoms chemistry. 

PRINCIPLES OF SYNTHESIS OF AROMATIC HETEROCYCLES BY INTRAMOLECULAR CYCLIZATION 

The synthesis of heterocyclic systems is a major activity of academic research and of the chemical industry, especially in pharmaceutical and agrochemical research and development. The organic chemists of the nineteenth century devised some of the synthetic methods that we still use today, but of course many new methods were discovered later, and in the current chemical literature there are continual reports of new methods. The body of literature on the synthesis of heterocycles is enormous and weU covered in the comprehensive reference works listed in the Appendix of Chapter 1. Here, we will only describe some of the more versatile common methods that produce unsaturated rings of five and six members, for such rings constitute major backbones on which more complicated stmctures can be constructed. 

The early chemists recognized that fully unsaturated rings of five and six members were generally stable and possessed some properties 

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