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Substrate‐Controlled Asymmetric Total Syntheses of Microcladallenes A, B, and C Based on the Proposed Structures
Author(s) -
Sohn Teik,
Kim Deukjoon,
Paton Robert S.
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201502592
Subject(s) - oxonium ion , stereocenter , chemistry , nucleophile , electrophile , stereochemistry , total synthesis , vicinal , walden inversion , enantioselective synthesis , combinatorial chemistry , ion , organic chemistry , catalysis
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 C 15 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 TiBr 4 ‐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 Cl 2 ‐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.