Azapentalenes, aromatic synthesis

In the first part of this review we deal with the concept of heteroaromaticity and consider the aromaticity of heteropentalenes, particularly azapentalenes. Methods of synthesis, chemical reactivity, and spectroscopic properties are dealt with, and the review ends with a brief survey of their industrial uses and biological activity. 

A wide variety of methods for the synthesis of aromatic azapentalenes have been reported, and, to rationalize these, we shall deal with compounds of types A, B, and C (Section II,C,3) separately and classify synthetic routes to each type according to the number of new bonds formed in the reaction (Scheme 5). 

Aromatic azapentalenes have not been found naturally, though an imidazo[4,5-d]imidazole derivative has been implicated in the prebiotic synthesis of purines130 74 (see also Section III,A,l,d). Saturated derivatives occur fairly widely the Senecio alkaloids contain the reduced pyrrolol l,2-a]pyrrole (pyrrolizidine) skeleton,487 and the alkaloid withasomnine is a derivative of pyrrolo[ l,2-6]pyrazole.374,488 The mitomycin antibiotics mentioned earlier in this review (Sections III,B,l,f and III,B,5) contain the pyrrolol l,2-a]indole ring,166,331 and the recently reported fungal metabolite sporidesmin is a saturated derivative of pyrrolo[2,3-6]indole.489... 

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