Asymmetric cyclization of substituted 4-pentenals (Ia,b,c; IIa,b,c,d) by Rh(I) with chiral ligands was tested, and substrate (IId) with a bulkier substituent at C4 underwent more stereoselective cyclization by Rh(I)-((+)-DIPMC) to yield substituted cyclepentanone (IIID). The Rh(I) with (+)-DIPMC was found to proceed in a different manner from the Wilkinson-catalyzed cyclization.
An approach to an asymmetric synthesis of stemofoline
作者:Eric J. Thomas、Clare F. Vickers
DOI:10.1016/j.tetasy.2009.03.002
日期:2009.5
A stereoselective Mannich reaction between an (S)-tert-butylsulfinimine and methyl (S)-4-benzyloxy-3-methylbutanoate followed by treatment with acid and N-protection was used to prepare methyl (2R,3S)-2-[(S)-2-benzyloxy-1-methylethyl]-3-tert-butoxycarbonylamino-6-methylenedecanoate. This was taken through to methyl (4R,5S)-4-[(S)-2-benzyloxy-1-methylethyl]-5-tert-butoxycarbonylamino-3,8-dioxododecanoate which on treatment with trifluoroacetic acid cyclised stereoselectively to give (1R,2S,4R,5S)-4-[(S)-2-benzyloxy-1-methylethyl]-1-butyl-2-methoxycarbonyl-8-tert-butoxycarbonyl-3-oxo-8-azabicyclo[3.2.1]octane, a potential precursor of stemofoline. Reduction and N-deprotection of this ketone gave (1R,2S,3R,4R,5S)-4-[(S)-2-benzyloxy-1-methylethyl]-1-butyl-2-methoxycarbonyl-8-azabicyclo[3.2.1]octan-3-ol the structure of which was confirmed by X-ray diffraction. (C) 2009 Elsevier Ltd. All rights reserved.
Asymmetric cyclization reactions by Rh(I) with chiral ligands
The exo-selective C-H cycloaddition of imidazoles to 1,1-disubstitutedalkenes has been achieved for the first time by using half-sandwich scandium catalysts. A wide range of imidazole compounds bearing various 1,1-disubstituted aliphatic alkenes, styrenes, dienes and enynes have been selectively converted in high yields to the corresponding bicyclic imidazole derivatives bearing -all-carbon-substituted