(+)-樟脑亚甲胺α位负碳离子的不对称氧化偶联反应研究

本文研究了(+)-樟脑亚甲胺α位负碳离子的氧化偶联反应, 所得偶联产物经高效液相色谱分析,测定了苏式赤式比,苏式异构体的de.值由^1HNMR测定为20-95%,对影响偶联反应的诸因素:如氧化偶联剂.溶剂.碱等进行了初探.     

Stereochemically general approach to adjacent bis(tetrahydrofuran) cores of annonaceous acetogenins

A series of six 2,5-disubstituted adjacent bis(tetrahydrofuran) stereoisomers with trans/erythro/cis, trans/threo/trans, or cis/threo/cis relative stereochemistry have been synthesized from known dihydroxycyclooctenes via ring opening/cross metathesis and Pd(0)-mediated asymmetric double cycloetherification. The stereochemistry of four of these isomers has been found in the biologically active annonaceous acetogenin natural products. [reaction: see text].     

Comparative Studies of Cathodically-Promoted and Base-Catalyzed Michael Addition Reactions of Levoglucosenone

Regioselective Michael addition of nitro and heterocyclic compounds to levoglucosenone, 1, is effectively catalyzed by amines and also by cathodic electrolysis. In comparison to the base-catalyzed reaction, it was found that under electrochemical conditions the reaction proceeds under milder conditions and with higher yields. Cathodically-initiated Michael addition of thiols to levoglucosenone using small currents produces the previously unknown threo addition product in several instances. The normal erythro isomer, identified as the kinetic product, tends to be formed when large currents are used. In contrast, slow, low current electrolyses promote equilibration of the two forms so that erythro can be converted to threo by the retro reaction and readdition. Addition of 2-naphthalenethiol to (R)-(+)-apoverbenone is also reported.     

Vinylogous Mannich reactions: some theoretical studies on the origins of diastereoselectivity

[reaction: see text] Ab initio calculations at the RHF/3-21G level were used to investigate the limiting transition states in the addition of 2-methoxyfuran to a pyrrolinium ion. Four stationary points were found on the potential energy surface with relative energies of threo Diels-Alder, 0.0 kcal/mol; erythro open, 0.9 kcal/mol; erythro Diels-Alder, 1.3 kcal/mol; and threo open, 1.8 kcal/mol.     

Effects of magnesium salts and amines on the stereoselectivity in the imine aldol reaction

In the imine aldol reactions of 1 with aromatic aldehydes using magnesium salts in the presence of amines, the threo/erythro ratios of products increased in the order Mg(ClO4)2>MgI2>MgBr2>MgCl2>Mg(OTf)2 and N,N,N',N'-tetramethylethylenediamine (TMEDA)>Et3N. This increase in the threo/erythro ratios of products was estimated to be caused by a retro-imine aldol reaction under thermodynamic control.     

Heterosubstituted allylic carbanion based stereocontrol,regio- and stereo- selective reaction of O and S substituted allylic carbanions with aldehydes

Regio- and diastereoselective reaction of oxygen and sulfur substituted allylic carbanions with aldehydes is described. Either threo or erythro isomer can be obtained predominantly or exclusively by merely choosing an additive.     

Remarkable change of the diastereoselection in the dye-sensitized ene hydroperoxidation of chiral alkenes by zeolite confinement

[reaction: see text] The ene reaction of singlet oxygen with chiral trisubstituted alkenes bearing an alkyl and a phenyl group at the stereogenic center is erythro diastereoselective in solution and threo diastereoselective if carried out within zeolite Na-Y. The change of the diastereoselection trend by zeolite confinement is attributed to a synergism of steric effects and cation-pi interactions.     

Threo-selective aldol condensations of lithium enolates in the presence of trialkylboranes

The reaction of preformed lithium enolates in the presence of trialkylboranes, such as Et₂B and n-Bu₃B, with aldehydes leads to product mixtures rich in the more stable threo aldol.     

Diastereoselective self-condensation of dihydroxyfumaric Acid in water: potential route to sugars

Jack of all trades: Water-soluble salts of DHF underwent self-condensation to afford the threo diastereomer of pentulosonic acid, through differing reaction pathways contingent on the metal salt used. This transformation exemplifies the diverging roles of DHF as a nucleophile (a synthon for α-hydroxyacetyl anion) and an electrophile (an α-carboxyglycolaldehyde equivalent).     

Stereoselective synthesis of pamamycin-607

A macrodiolide antibiotic pamamycin-607 was synthesized by joining two hydroxy acid components. Three cis-2, 5-disubstituted tetrahydrofuran rings in the molecule were stereoselectively prepared by radical cyclization reactions of beta-alkoxyvinyl ketone intermediates and a beta-alkoxymethacrylate substrate. The key step of the synthesis is characterized by the predominant threo product formation in the radical cyclization reaction of a beta-alkoxymethacrylate intermediate.     

Chlorinated long-chain fatty acids. Their properties and reactions—VI : Kinetics and mechanisms of base-catalyzed dehydrochlorination of sodium threo,threo-9,10,12,13-tetrachlorooctadecanoate

The base-catalyzed dehydrochlorination of sodium threo,threo-9,10,12,13-tetra-chlorooctadecanoate has been carried out in aqueous ethylene glycol solutions. During the first reaction step two chlorine atoms are eliminated nearly simultaneously as hydrogen chloride whereas the third and fourth chlorine atoms are released separately. The relative rates of these three reaction steps at 130°C are 497,41 and 1, respectively. The possible reaction mechanisms have been discussed in light of the kinetic results and product analyses which showed that the dehydrochlorination results mainly in conjugated diene-yne systems.     

Stereoselective syntheses of rolliniastatin 1, rollimembrin, and membranacin

A radical cyclization of beta-alkoxyvinyl sulfoxides-Pummerer rearrangement-allylation protocol was successfully applied to the synthesis of the threo/cis/threo/cis/erythro bis-oxolane moiety in rolliniastatin 1 (1), rollimembrin (2), and membranacin (3).     

Chromium(II) mediated threo selective synthesis of homoallyl alcohols

Remarkable threo selectivity is observed in the reaction of aldehydes with $\text{trans}$- and $\text{cis}$-1-bromo-2-butene with the aid of chromium(II) salts. Solvent effect as well as the synthetic application is discussed.     

STERIC CONTROL OF THE P-CHIRAL GROUP IN GRIGNARD ADDITION TO THE CARBONYL GROUP OF α - KETOPHOSPHINE OXIDE

Abstract

1,2-Asymmetric inductions in Grignard addition to the carbonyl group of α-keto P-chiral phosphine oxides have been investigated. The favoured reaction product erythro-, 6, is thermodynamically less stable than the threo isomer. This seems to be the first stereochemical evidence of the reversible character of reaction addition of sec-phosphine oxides to the carbonyl group.     

Crotylzirconium derivatives as a new reagent for the threo selective synthesis of β-methylhomoallyl alcohols

Crotylzirconium derivatives, prepared from the reaction of crotylmagnesium chloride or crotyllithium with bis (cyclopentadienyl) zirconium dichloride, undergo a rapid reaction with aldehydes to afford the threo product predominantly. The ratio of Cram/anti-Cram product via this reagent is ca. $\text{75}\text{25}$ ~ $\text{60}\text{40}$.     

用^1H NMR法研究以CpTiCl3-LiAlH4为还原剂的pinacol反应的立体选择性

Corey 等人曾以CpTiCl_3-LiAlH_4(cp≡■)为还原剂成功地进行了一些Pinacol反应,并对该法做了较深入的研究,但对醛类化合物分子间反应产物的立体选择没有做详尽的讨论。这样,存一些要求有一定空间构型的邻二仲醇化合物的合成中,可否利用该方法就难以确定。为此,我们研究了一些醛类化合物     

Enantioselective synthesis of erythro-4-deoxyglycals as scaffolds for target- and diversity-oriented synthesis: new insights into glycal reactivity

An efficient, enantioselective synthesis of erythro-4-deoxyglycals has been developed using asymmetric aldehyde allylation and tungsten-catalyzed alkynol endo-cycloisomerization as the key steps. These versatile synthetic scaffolds have been elaborated to a variety of products using stereoselective transformations that are complementary to those available using the corresponding threo glycals. This work has provided valuable insights into the relationships between glycal structure and reactivity. In addition, a new diene-forming side reaction during tungsten-catalyzed alkynol cycloisomerization has been discovered.     

REACTION OF LITHIUM DERIVATIVE OF DIETHYL PHENYLMETHANE-PHOSPHONATE WITH KETONES

Abstract

The reaction of the lithium derivative of diethyl ester of phenylmethanephosphonic acid (1-Li) with alkanones, cycloalkanones, alkylaryl and diarylketones 2s-b is studied at -70°C in THF. The corresponding adducts-diethyl esters of l-phenyl-2,2-dialkyl(phenyl)-2-hydroxyethanephosphonic acids 3s-h are isolated, their yields being usually higher at short reaction time. The olefination of 3-Li as well as of 3 (both by thermolysis or in acidic media) proceeds in low degree, while in the case of 3-Na the yields of alkenes 4 are good. The relative configurations of 3b. 31 and 3g are determined by IR and NMR-spectra, as well as by their stereospecific olefination. “Threo”-stereoselectivity of the addition stage of the reaction of 1-Li with 2b, 21 and 2g is observed, the “threo”/“erythro” ratio remaining independent on the reaction time.     

Samarium iodide-mediated Reformatsky reactions for the stereoselective preparation of β-hydroxy-γ-amino acids: synthesis of isostatine and dolaisoleucine

The synthesis of β-hydroxy-γ-amino acids via SmI(2)-mediated Reformatsky reactions of α-chloroacetyloxazolidinones with aminoaldehydes is reported. Diastereoselective coupling is demonstrated to depend on the absolute configuration of the Evans chiral auxiliary employed in the reaction, allowing erythro or threo products to be obtained selectively. The potential utility of the methodology is exemplified by the facile synthesis of biologically relevant N-Boc-isostatine (2b) and N-Boc-dolaisoleucine (3c).