Features on the Way to the Synthesis of 1-Benzoyl-2-Phenyl-3a, 6a-Diazapentalene and 1-Pivaloyl-2-Tert-Butyl-3a,6a-Diazapentalene

Abstract

Recently discovered 1,3a,6a-triazapentalene systems are among the new fluorophores whose compact and biocompatible structures are suitable for a variety of biological applications. The wavelengths of 1,3a,6atriazapentalenes vary depending on the nature of the substituents, allowing for the easy synthesis of yellow and red fluorescent reagents for labeling biomolecules. The prototype of the 1,3a,6a-triazapentalene system (without one aza group) is 3a,6a-diazapentalene, which, originally called pyrazolo[1,2-a]pyrazole, which may also exhibit fluorescent activity. However, their research has been limited to a few papers reporting simple and universal principles for synthesizing the 3a,6a-diazapentalene system, one of which involves double alkylation of pyrazole with α-halocarbonyl compounds and treatment of the resulting products with a base. In this work, all stages of the previously performed synthesis of 1-benzoyl-2-phenyl-3a,6a-diazapentalene by the reaction of N-acylalkylation of pyrazole with α-bromoketones through the stages of formation of pyrazolium cation salts are investigated. Based on the studied data, the studied synthesis conditions were first applied by us in stepwise reactions leading to the possible formation of a tert-butyl derivative of 3a,6a-diazapentalene. As a result of the studies, a new, previously undescribed adduct of 3a,6a-diazapentalene, a bicyclic aldol, was obtained. The structure of the substances of the stepwise synthesis was characterized by NMR, IR spectroscopy and mass spectrometry. The mass fragmentation of intermediate N-alkylacylpyrazoles was also considered in detail.