Abstract
X-ray crystal structures have been determined and compared for fluorescent 1,3,4,6-tetraazapentalenes that are syn and anti disubstituted with pyrimidinone rings or with a combination of pyrimidinone and pyridine rings. Syn-disubstituted tetraazapentalenes were synthesized by a three-step sequence from 1-ethylcytosine or O-protected cytidine (also O-protected adenosine), chloroketene diethyl acetal, and either 2-aminopyridine or another cytosine or cytidine unit. The final step was an oxidative cyclization by means of an I III compound. Anti-disubstituted tetraazapentalenes bearing terminal pyrimidinone rings or pyrimidinone and pyridine rings were obtained from the corresponding syn compounds by treatment with ammonia or sodium methoxide in methanol. The anti tetracyclic ring system based on two cytidine units attached to the 1,3,4,6-tetraazapentalene central moiety provides a covalently linked cross section that is a model for base pairing in a double-helical polynucleotide having parallel rather than antiparallel strands
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