Thioureas in the Synthesis of Heterocycles

The chapter presents the heterocyclic systems prepared from thioureas in order of increasing ring size and in order of increasing numbers of heteroatoms. The heteroatorns have been arranged in the following sequence: Nitrogen, sulfur, oxygen, and other elements. Heterocycles prepared from selenoureas and selenopseudoureas are presented together with their sulfur analogs. Heteropolycycles are discussed in a separate section and are grouped according to general ring type, rather than strictly by ring size. Thioureas are useful in the preparation of two four-membered ring systems: Thietanes and 1,3-thiazetidin-4-ones. The chapter also discusses five-membered rings, six-membered rings, and seven-membered rings. C1yclohcxanones have been employed ill reactions with thioureas to give quinazolines with varying degrees of saturation. A simple preparation of otherwise difficulty prepared heterocyclic ring systems is the cyclization of derivatives of type, resulting from reaction of carbethoxy isotliiocyanate with 2-aminoazaheterocycles to give bridgehead-nitrogen heterobicyclic, s-triazinonethiones.