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
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.