(Hemi)synthesis of ribazole-2',3'-cyclophosphate: a key building block in cobamide chemistry

This study presents a quick and effective method for producing α-ribazole-2',3'-cyclophosphate from vitamin B₁₂. Using an optimized microwave-assisted cyanolysis, α-ribazole-2',3'-cyclophosphate was obtained in a high isolated yield of 90% after purification by silica column chromatography. This method was compared to a revised total synthesis route where α-ribazole was produced in six steps with a 34% overall yield. This was followed by an optimized microwave-assisted cyclophosphorylation step that afforded α-ribazole-2',3'-cyclophosphate in 42% yield. Both synthetic strategies were monitored by UHPLC-MS and UHPLC-fluorescence to ensure the confirmation and quantification conversions as well as to optimize the synthesis conditions. Finally, spectroscopic studies and pH-dependent analyses have shown that the protonation of the benzimidazole moiety (pK_a values) is virtually unaffected by structural variations. Furthermore, the emission properties of α-ribazole and its cyclophosphate analogue have been found to be comparable, which confirms the validity of the analytical approaches used to monitor and quantify them.