One Heteroatom in Each Ring

Systems with Two Heteroatoms 1. One Heteroatom in Each Ring 

Compounds of the general formula 69 are prepared by cycloaddition of N-methyl- or A(-arylmaleimides with arylidene imines of AAs and in the presence of an aromatic aldehyde. Stabilized azomethine ylides are formed as intermediates, which then afford the cycloadducts. Several isomers are formed, and the influence of various metal salts and solvents was investigated (87BCJ4067 88T557). Similar transformations have been performed with A-ailyl glycine esters (91TL1359). 

Condensation of A-protected pyrrole-2-carboxaldehyde with ethyl a-amino-/3, -diethoxypropionate followed by reduction and cyclization in the presence of TiCU afforded the pyrrolo[2,3-c]pyridine derivative 70 in excellent yield (93T8139). 

Derivatives of the 2,5-diazabicyclo[2.2.1]heptaine ring system (bridged piperazines) have been prepared from 4-hydroxyprolines. In a multistep transformation from tra j-4-hydroxy-L-proline (the last step was cyclization with benzylamine) a mixture of diastereoisomers 71 was obtained and separated [92H(34)241]. In a similar manner, the methyl and oxo analogs were obtained [67AJC1493 92H(34)679]. The commercially available N- 

Cbz-4-hydroxy-L-proline was used in the preparation of 72 (the Geissman-Waiss lactone), which is a synthon for necines (82H23). This compound was prepared in eight steps (62JOC139). The alkaloids retronecine, platy-necine, and croalbinecine were prepared from the lactone (83H1331). 

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The aim of this chapter is to present the chemistry of five-five fused aromatic heterocyclic systems containing one heteroatom in each ring, represented by (la)-(ld), wherein X and Y may be the same or different heteroatoms and represent O, N, S and Se. 

Abstract This chapter is devoted to recent progress in the chemistry of the 5 5 fused heterocyclic systems. There are four possible modes of 5 5 fusions of the simple five-membered heterocycles leading to four structures containing one heteroatom in each ring. The heteroatoms may be the same or different and may be O, NH, S, Se, Te, P, As, or Sb. The fully conjugated hetero analogs of pentalene dianion have a central C-C bond and are isoelectronic with the 10-7t-electron pentalene dianion. The scope of the chapter is outlined with a survey of various structural types and nomenclature of the parent compounds and their derivatives. New synthetic procedures and synthetic applications of title compounds are presented. This review has concentrated on the new developments achieved from 1997 to September 2007. 

Reports of bicyclic systems with one nitrogen atom containing two additional heteroatoms in each ring are scant in the literature. Fourteen such ring systems have been reported previously 1-14 <1996CHEC-II(8)733>. The literature examples available during the last decade include 1,2,4-triazino[3,4-Z ][ l,3,4]thiadiazine-4,7-dione 15, [l,2,4]tri-azino[3,4-/d [ 1,3,4]thiadiazinc 16, l,9a-dihydro-8//-l, 2,4-triazino-[3,4-Z ] [1,3,4] thiadiazine-4-oncs 17, [l,3,5]tria-... 

By far, the greatest interest in 5-5 heterocyclic systems containing two heteroatoms in each ring has been in the field of supramolecular chemistry and in the preparation of molecular receptors of various types. One of the most widely used heterocyclic backbones for these receptors has been glycoluril 5 and its derivatives. 

Compounds with one heteroatom in each heterocyclic ring... 

Compounds with one heteroatom in each heterocyclic ring 10.23.9.1.1(1) Synthesis of the heterocyclic ring... 

The heterocycles (la)-(ld) are heteropentalenes they have a central C—C bond, and are isoelectronic with the 107r pentalene dianion (2). They are formally derived by the fusion of two of the well-known simple heterocycles including furan, pyrrole, thiophene and selenophene. Considering the two rings as rings A and B, this entire group of five-five fused heterocycles with one heteroatom in each can be conveniently referred to as A,B-diheteropentalenes. 

This chapter covers bicyclic ring systems with one bridgehead N atom and with one extra heteroatom in each ring, and their benzo-fused derivatives. Of the 78 theoretically possible bicyclic systems, 33 are known, as are 53 benzo-fused derivatives. 

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