Stereocontrolled Synthesis of a Nonracemic Vitamin B12 A−B-Semicorrin

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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.