The acid hydrolysis of natural vitamin B12 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 d-acid derivatives 1 and 2, respectively, since functionalisation at these positions of the corrin ring generally keeps the affinity for vitamin B12 transport proteins intact. Although the authenticity of 1 and 2 seemed evident from 1H 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 peptide synthetic methods gave the vitamin B12 derivatives 4 and 5, respectively. The structures of 4 and 5 could be characterised by X-ray analysis, which unambiguously confirmed the presence of the b- and d-acid in 1 and 2, respectively. The reaction of 4 and 5 with the complex [Re(OH2)3(CO)3]+ in water gave the corresponding vitamin B12 derivatives 8 and 9, and the aqueous reaction of 4 and 5 with the complex [99mTc(OH2)3(CO)3]+ afforded the 99mTc homologues 10 and 11, respectively.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)