The acid hydrolysis of natural vitamin B 12 yields several products in which the acetamide or propionamide side chains on the corrin framework are converted into the corresponding acids. These acids can be derivatised with further functionalities. We have separated in particular the b-and the dacid derivatives 1 and 2, respectively, since functionalisation at these positions of the corrin ring generally keeps the affinity for vitamin B 12 transport proteins intact. Although the authenticity of 1 and 2 seemed evident from 1 H NMR investigations, it has not been supported by X-ray structure analysis. The coupling of ethyl N-(3-aminopropyl)-N-(pyridin-2-ylmethyl)glycinate (3) to the carboxylate groups in 1 and 2 by