Total Synthesis, Stereochemical Reassignment, and Absolute Configuration of Chlorofusin
journal contributionposted on 15.08.2007 by Sang Yeul Lee, Ryan C. Clark, Dale L. Boger
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Chlorofusin is a recently isolated, naturally occurring inhibitor of p53−MDM2 complex formation whose structure is composed of a densely functionalized azaphilone-derived chromophore linked through the terminal amine of ornithine to a nine-residue cyclic peptide. Herein we report full details of the total synthesis of chlorofusin, resulting in the assignment of the absolute stereochemistry and reassignment of the relative stereochemistry of the complex chromophore. Condensation of each enantiomer of an azaphilone chromophore precursor with the Nδ-amine of a protected ornithine-threonine dipeptide, followed by a one-step oxidative spirocyclization of the most reactive olefin provided all eight diastereomers of the fully elaborated chromophore-dipeptide conjugate. Comparison of the spectroscopic properties for these eight compounds with simpler models and with that reported for the natural product allowed the full assignment of the (4R,8S,9R)-stereochemistry of the chlorofusin chromophore. The desired, but stereochemically reassigned, diastereomer of the dipeptide conjugate was incorporated in a convergent total synthesis of chlorofusin confirming both the relative stereochemical reassignment and the establishment of its absolute stereochemistry. In addition to natural chlorofusin, the (4R,8R,9R)-diastereomer proposed by Williams as well as the (4R,8S,9S) and (4R,8R,9S)-diastereomers of chlorofusin were also prepared as key analogues of the natural product, and the noncorrelation of their spectral data with that reported for natural chlorofusin further supports the new structural assignment and maintains the diagnostic spectroscopic distinctions observed with the eight diastereomers of the chromophore ornithine-threonine dipeptide conjugates. The reassignment of the relative stereochemistry for the chlorofusin chromophore is still, but less obviously, consistent with the experimental NOEs observed by Williams in the original structural assignment work, and the assignment of its absolute configuration is opposite that recently disclosed by Yao.