In this study, a photoredox-catalyzed sulfanylation strategy was developed and optimized, resulting in the successful synthesis of eleven 3-sulfanyl-substituted (aza)quinolizinone products. The key precursors, 4-oxo-4H-quinolizin-3- and 4-oxo-4H-pyrido[1,2-a]pyrimidin-3-diazonium salts, were prepared following well-established literature protocols. These diazonium intermediates underwent photochemical reactions with diphenyl disulfide, dimethyl disulfide, or N,N'-bis[(tert-butoxy)carbonyl]-L-cystine dimethyl ester, leading to the desired sulfanylated products. Additionally, analogous reactions were carried out using thiols, yielding the same products as those obtained with disulfides. While both approaches proved effective, the disulfide-based approach demonstrated superior efficiency, yielding products with higher overall yields. Further, thermal conditions were explored for reactions with certain thiols, where it was observed that thermal activation favoured better yields compared to the photochemical conditions. Based on the successful sulfanylation of diazonium salts with cystine and cysteine, the method was extended to include the reaction of 4-oxo-4H-pyrido[1,2-a]pyrimidin-3-diazonium tetrafluoroborate with the protein BSA.
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