Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 122, Issue 34, Pages 8131-8140Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja001165a
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Syntheses of polycyclic isoprenoids have been achieved by several groups; however, no general 'biomimetic method has yet been reported. In this paper we describe the biomimetic cyclization of simple isoprenoids to polycyclic isoprenoids using Lewis acid-assisted chiral Bronsted acids, chiral LBAs. This is the first example of a proton-induced enantioselective ene cyclization in synthetic chemistry. Geranyl phenyl ethers, o-geranylphenols, and geranylacetone derivatives were directly cyclized at -78 degrees C in the presence of (R)-binaphthol derivatives and tin tetrachloride. During the cyclization, [1,3] abnormal Claisen rearrangement often took place. The enantioselectivities were up to 90% ee. Compounds bearing a farnesyl group could also be cyclized under the same conditions to give the natural products (-)-Ambrox and (-)-chromazonarol. These chiral LBAs recognize a trisubstituted terminal olefin enantiotopically and generate site-selective carbocations on the substrates. The absolute stereochemistry of the cyclization is discussed with model studies using DFT calculations on the B3LYP/LANL2DZ level.
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