The first synthesis of a vitamin B-12 A-B-semicorrin 44 is described. Both A and B rings were prepared from the same precursor, enamide 40, which is obtained by a sequence of sigmatropic rearrangements and a biomimetic oxylactonization. The coupling has been achieved via Eschenmoser sulfide contraction. A-B-semicorrin 44 allows a new approach toward vitamin B-12 and other uroporphinoids.