A cyclic impurity associated with vitamins was structurally elucidated through comprehensive 2D NMR analysis. Detailed interpretation of ¹H–¹H COSY, HSQC, and HMBC spectra enabled unambiguous assignment of proton–carbon connectivity and long-range correlations, confirming intramolecular cyclization within the corrin framework.
Key HMBC correlations established the linkage responsible for ring closure, while HSQC data supported accurate proton and carbon assignments across the modified side chain. The combined 2D NMR dataset allowed differentiation from closely related linear impurities and ruled out alternative structural hypotheses.
This study demonstrates the critical role of advanced NMR techniques in:

• Impurity structure elucidation
• Degradation pathway understanding
• Analytical support for complex vitamin derivatives