Diheteropentalene systems

1 Synthesis by construction of the second heterocyciic ring onto an existing heterocycle 

This chapter begins where the earlier chapter in CHEC-II(1996) 1996CHEC-II(7)1 finished and extends the literature coverage to the end of 2006. Some earlier references have been included to cover material absent from the second edition. 

The 5 5 fused heterocyclic systems containing one heteroatom in each ring are represented by the general structures (1) (4), wherein X and Y may be the same or different heteroatoms and represent O, NR, S, Se, and very rarely Te. 

There are four possible modes of 5 5 fusion of the simple five-membered heterocycles leading to structures (l —(4). Compounds of type (4) can only be represented by 1,3-dipolar, diradical, or hypervalent structures and only S- or Se-substituted derivatives are known. In the case of X = Y = O or N, some reduced derivatives have been reported. 

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Diheteropentalene systems are very popular substrates for the investigation of cycioaddition reactions due to their diene character. 

The l,3,3,3a,8a-tetrahydro-8-oxa-2-thia- 205, l,3,3,3a,8a-tetrahydro-2,8-dioxa- 206, and 3,3a,8,8a-tetrahydro-12/-2-thia-8-aza-cyclopenta[ ]indene-2,2-dioxide 207 were prepared by Michael addition or radical cyclization from the corresponding sulfones 202-204 and brominated-dehydrobrominated (in the case of 207, after tosylation) to produce 1,5-diheteropentalene systems 208-210, respectively <1997TL5315>. 

Keywords Diheteropentalene systems, 5, 5-Fused heterocycles aromaticity, 5, 5-Fused heterocycles reactivity, Ring syntheses... 

The most frequent methods for the preparation of 1,5-diheteropentalene systems involve construction of the second heterocyclic ring onto an existing heterocycle. Three new syntheses of iso-condensed heteroaromatic pyrroles and their derivatives... 

These 107i-systems are isoelectronic with the pentalene dianion and have been of some theoretical interest. 1,5-Diheteropentalene systems are very popular substrates for the investigation of inter- and intramolecular cycloaddition reactions due to their diene character. Such cycloaddition processes allow for a rapid entry into complex polyheterocyclic rings and makes these compounds potentially useful for natural product synthesis. 

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