title: Substrate-Controlled Asymmetric Total Syntheses of Microcladallenes A, B, and C Based on the Proposed Structures.
abstract: Substrate-controlled asymmetric total syntheses of (+)-microcladallenes A, B, and C have been accomplished based on the proposed structures. The syntheses of microcladallenes A and B confirmed the structures and absolute configurations of both natural products. However, the synthesis of microcladallene C, which includes seven stereogenic centers and an (R)-bromoallene in its compact C15 framework, brought the realization that its proposed structure must be revised. The introduction of C12-bromine into these natural products with retention of configuration relied on TiBr4 -mediated nucleophile-assisting leaving group brominations, the stereochemical outcome of which could be attributed, at least in part, to an oxonium or halonium ion formation-fragmentation sequence through intricate neighboring group participation. In addition, the pivotal β-oriented vicinal cis-dichloride function in microcladallene C was elaborated through a novel tandem Cl2 -induced electrophilic cyclization/imidate chlorination process. The positive rotations of these natural products with an (R)-bromoallene constitute exceptions to Lowe's rule for reasons yet to be determined.
authors.affiliation: College of Pharmacy, Seoul National University, Seoul 151-742 (Korea).
authors.forename: Te-Ik
authors.initials: TI
authors.lastname: Sohn
authors.affiliation: College of Pharmacy, Seoul National University, Seoul 151-742 (Korea). deukjoon@snu.ac.kr.
authors.forename: Deukjoon
authors.initials: D
authors.lastname: Kim
authors.affiliation: Chemical Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA (UK).
authors.forename: Robert S
authors.initials: RS
authors.lastname: Paton
mesh_terms: D001688:Biological Products / Q000138:chemical synthesis* / Q000737:chemistry; D008968:Molecular Conformation; D015394:Molecular Structure; D012717:Sesquiterpenes / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: D001688:Biological Products; D012717:Sesquiterpenes; C464752:microcladallene A; C523111:microcladallene B; C000608090:microcladallene C
keywords: asymmetric synthesis; microcladallenes; natural products; synthesis design; total synthesis