Chiral [η6-Arene-Cr(CO)3] Complexes as Synthetic Building Blocks: A Short Enantioselective Total Synthesis of (+)-Ptilocaulin
作者:Kurt Schellhaas、Hans-Günther Schmalz、Jan W. Bats
DOI:10.1002/(sici)1521-3765(199801)4:1<57::aid-chem57>3.0.co;2-h
日期:1998.1
An enantioselective total synthesis of the marine natural product (+)-ptilocaulin is described. The synthesis starts from [anisole-Cr(CO)(3)], which is converted to [2-trimethylsilylanisole - Cr(Co)(3)] (greater than or equal to 99 % ee) by enantioselective deprotonation/silyLation and recrystallization. After attachment of a 2-butenyl side-chain, nucleophile addition (2-lithio-1,3-dithiane) followed by treatment with chloratrimethylsilane and hydrolysis leads to (5S,6S)-6-((E)-but-2-enyl)-5-[1,3]-dithian-2-yl-2-trimethylsilylcyclohex-2-enone with complete chirality transfer. This compound, which was characterized by X-ray crystallography, is transformed into (5 xi,6S,7aS)-5-butpl-6-methyl-1,2,5,6,7,7a-hexahydro-inden-4-one, a ptilocaulin precursor known from the literature, by a 4-step sequence consisting of diastereoselective 1,4-addition, ultrasound-assisted desulfurization/hydrogenation (Raney Ni) and aldol cyclization. The target molecule was prepared in both racemic and optically active form. The X-ray crystal structure of rac-ptilocaulin nitrate shows flat ribbons of homochiral units (parallel double chains) connected by an interesting pattern of hydrogen bonds between the guanidinium and the nitrate ions. A different mode of hydrogen bonding resulting in the formation of helical monochains was found in the solid-state structure of (+)-ptilocaulin co-crystallized with about 29 % of its C-3a epimer.