Generation of Alkyl Radicals from 1-Oxidoalkylidenechromium(0) Complexes by Oxidation with Manganese(III) 2-Pyridinecarboxylate and Their Reactions with Olefins
作者:Koichi Narasaka、Hidehiro Sakurai
DOI:10.1246/cl.1993.1269
日期:1993.7
Tetramethylammonium pentacarbonyl(1-oxidoalkylidene)chromium(0) complexes, prepared from the corresponding carbanion and hexacarbonylchromium(0), are oxidized with manganese(III) 2-pyridinecarboxylate to generate carbon-centered radicals which react with various olefins giving the intermolecular addition products.
The total synthesis of (-)-tetrahydrolipstatin utilizing two approaches is described. In the first, L-malic acid was used as a chiral template to obtain enantiomerically pure (R)-3-(benzyloxy)-tetradecanal (11) which was chain-extended using 1-(trimethylsilyl)-2-nonene and a Lewis acid. This advanced intermediate was further elaborated to the target compound in good overall yield. The second approach utilized lauraldehyde as a starting material and capitalizes on an asymmetric allylboronation (91 % ee). The product could be obtained enantiomerically pure by conversion to the (R)-acetoxymandelate ester and hydrolysis. Oxidative cleavage of the terminal double bond led to 11 which was further extended using 1,3- and 1,2-asymmetric induction based on existing neighboring chirality. The synthesis of tetrahydrolipstatin using the second approach comprises seven steps from 11 and proceeds in 38 % overall yield.