Diastereoselective Vinyl Addition to Chiral Hydrazones via Tandem Thiyl Radical Addition and Silicon-Tethered Cyclization
作者:Gregory K. Friestad、Sara E. Massari
DOI:10.1021/ol0067991
日期:2000.12.1
A diastereoselective method for addition of a vinyl group to alpha-hydroxy hydrazones under neutral tin-freeradicalcyclizationconditions, leading to substituted vinylglycinols, is presented. Tandem thiyl radical addition/cyclization upon a silicon-tethered vinyl group followed by treatment with potassium fluoride accomplishes a one-pot neutral vinyl addition process to afford acyclic allylic anti-hydrazino
Tandem thiyl radical addition and cyclization of chiral hydrazones using a silicon-tethered alkyne
作者:Gregory K. Friestad、Tao Jiang、Gina M. Fioroni
DOI:10.1016/s0957-4166(03)00540-8
日期:2003.10
A diastereoselective method for addition of a trans-2-(phenylthio)vinyl group to α-hydroxy hydrazones is presented. An ethynyl group, tethered to α-hydroxy hydrazones via a silicon tether, undergoes thiylradicaladdition and cyclization under neutral tin-free conditions. In the same pot, desilylation with potassium fluoride or tetrabutylammonium fluoride affords (E)-vinylsulfides. The one-pot process
A Silicon Tether Approach for Diastereocontrol in Radical Addition to Chiral Hydrazones
作者:Gregory K. Friestad
DOI:10.1021/ol991059h
日期:1999.11.1
A radical carbon-carbon bond construction approach to chiral a branched amines is presented. Stereocontrolled radical addition to chiral hydrazones can be achieved by virtue of conformational constraints imposed during cyclizations using a temporary silicon connection. Oxidative removal of the tether completes the hydroxymethylation process to afford anti-2-hydrazino-1,3-diols in good yield. The 1,2-induction increases with increasing A values of the appended groups, consistent with prediction by the Beckwith-Houk model for stereocontrol in 5-hexenyl radical cyclizations.
Enhanced reactivity in radical cyclizations of hydrazones using the silicon-tethered 1-bromovinyl group
作者:Gregory K. Friestad、Tao Jiang、Alex K. Mathies
DOI:10.1016/j.tet.2007.03.017
日期:2007.5
A silicon-tethered 1-bromovinyl group was shown to function as a radical precursor for tin-mediated vinyl additions to chiral alpha- or beta-hydroxyhydrazone. In contrast to related thiyl-mediated methods, these vinyl bromides were not limited to the 5-exo cyclization mode. A series of Si-tethered 5-exo and 6-exo cyclizations formed the corresponding five- and six-membered exo-methylene- substituted oxasilacycles. Treatment with fluoride cleaved the Si-C and Si-O bonds to afford the corresponding allylic hydrazines. Diastereoselectivities ranged from 2:1 to 25:1 (anti:syn) for the 5-exo cyclizations, depending on the size of the exocyclic substituent, but 6-exo cyclization was not diastereoselective. A variant involving Tamao oxidation of the exo-methylene oxasilacyclopentane intermediate afforded a methyl ketone, a net process corresponding to addition of a radical acyl anion equivalent. (C) 2007 Elsevier Ltd. All rights reserved.