41515-58-6Relevant articles and documents
Enantioselective biomimetic cyclization of isoprenoids using Lewis acid-assisted chiral Bronsted acids: Abnormal claisen rearrangements and successive cyclizations
Nakamura, Shingo,Ishihara, Kazuaki,Yamamoto, Hisashi
, p. 8131 - 8140 (2007/10/03)
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 °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.
