Enantioselective synthesis of the C-14 to C-5 cyclopentane segment of jatrophane diterpenes
摘要:
The enantioselective synthesis of the C-14 to C-5 cyclopentane segment of jatrophane diterpenes is reported. An Evans aldol addition, a Horner-Wadsworth-Emmons olefination and a thermal intramolecular carbonylene reaction of an alpha-keto ester served as key C/C-connecting transformations. (C) 2003 Elsevier Ltd. All rights reserved.
Reactions between acyclic (E)-allylic acetates and arylboronic acids in the presence of a palladium catalyst prepared from Pd(OAc)(2), phenanthroline (or bipyridine), and AgSbF(6) (1:1.2:1) proceeded with excellent gamma-selectivity to afford allyl-aryl coupling products with E-configuration. The reactions of alpha-chiral allylic acetates took place with excellent alpha-to-gamma chirality transfer
germanium(II) iodide with potassium metal, was found to promote the Reformatskyreaction effectively under mild conditions. In the presence of activated germanium metal, the reactions of alpha-bromo ketones 2a and 2b and alpha-bromo imides 2e and 2f with benzaldehyde (1a) proceeded smoothly to give the corresponding beta-hydroxy carbonyl compounds 3a, 3b, 3e and 3f, respectively, in good yields and with
Asymmetric aldol reactions: a novel model for switching between chelation- and non-chelation-controlled aldol reactions
作者:Tu Hsin Yan、Chang Wei Tan、Hui Chun Lee、Hua Chi Lo、Tzung Yu Huang
DOI:10.1021/ja00060a010
日期:1993.4
A new camphor-based N-propionyloxazolidinethione provides remarkable levels of asymmetric induction for both chelation-and non-chelation-controlled aldol processes. While the aldol condensation of the derived di-n-butylboryl enolate with various aldehydes affords the 'Evans'' syn aldol with excellent diastereoselectivity, the chlorotitanium enolate gives the ''non-Evans' syn aldol expected from chelation control. Most noteworthy is the observation that the sense of facial selectivity from the chlorotitanium enolate of propionyloxazolidinethione is opposite to that obtained from propionyloxazolidinone. This important finding illustrates the importance of increased chelating potential of the enolate ligand, ring thiocarbonyl, in maximizing the aldol pi-facial discrimination. Final nondestructive chiral auxiliary removal via hydroperoxide-assisted hydrolysis and subsequent esterification provides enantiomerically pure methoxy-carbonyl aldols.
Synthetic Studies toward Jatrophane Diterpenes from <i>Euphorbia characias</i>. Enantioselective Synthesis of (−)-15-<i>O</i>-Acetyl-3-<i>O</i>-propionyl-17-norcharaciol
作者:Hannes Helmboldt、Martin Hiersemann
DOI:10.1021/jo802581g
日期:2009.2.20
The enantioselective synthesis of (+)-17-norcharaciol is described. An uncatalyzed intramolecular carbonyl-ene reaction and a ring-closing metathesis were used as key C/C-connecting transformations to assemble the trans-bicyclo[10.3.0]pentadecane norditerpenoid core. We also report the evolution of our synthetic strategy toward the fully substituted characiol skeleton and the experiences from this venture.
Enantioselective synthesis of anti-.alpha.-methyl-.beta.-hydroxy esters through titanium tetrachloride-mediated aldol condensation