Aldol reactions Zinc chloride

Conducting the aldol reaction at temperatures below —78 "C increases the diastereoselectivity, but at the cost of reduced yields45. Transmetalation of the lithium enolate 2 a by treatment with diethylaluminum chloride generated an enolate species that provided high yields of aldol products, however, the diastereoselectivity was as low as that of the lithium species45. Pre treatment of the lithium enolate 2a with tin(II) chloride, zinc(II) chloride, or boron trifluoridc suppressed the aldol reaction and the starting iron-acyl complex was recovered. 

Scheme 7.5 gives some examples of the Reformatsky reaction. Zinc enolates prepared from a-haloketones can be used as nucleophiles in mixed aldol condensations (see Section 2.1.3). Entry 7 is an example. This type of reaction can be conducted in the presence of the Lewis acid diethylaluminum chloride, in which case addition occurs at -20° C.171... 

In the presence of zinc chloride, stereoselective aldol reactions can be carried out. The aldol reaction with the lithium enolate of /-butyl malonate and various a-alkoxy aldehydes gave anti-l,2-diols in high yields, and 2-trityloxypropanal yielded the syn-l,2-diol under the same conditions.633 Stoichiometric amounts of zinc chloride contribute to the formation of aminoni-tropyridines by direct amination of nitropyridines with methoxyamine under basic conditions.634 Zinc chloride can also be used as a radical initiator.635... 

The zinc chloride-mediated tandem Mukaiyama aldol-lactonization reaction of aldehydes 21 and thiopyridylketene acetals 22 gave mainly the trans isomer 23. However, if the catalyst is stannic chloride and the reaction is carried out at -78 °C, then the cyclization is highly diastereoselective and yields the cis-isomer 24 <990L1197>. 

Organoaluminum reagents, 202 1,1,1-Trifluoroacetone, 323 Trityllithium, 338 Zinc chloride, 349 Stereoselective aldol reactions With boron enolates Boron trichloride, 43 Chlorodimethoxyborane, 73 9-(Phenylseleno)-9-borabicyclo-[3.3.1]nonane, 245 With silyl enol ethers... 

Disubstituted furans are available from a,/3-unsaturated enones in a two-step sequence. At first, conjugate addition of a cuprate generates an enolate, which undergoes an aldol reaction with (tetrahydropyranyloxy)acetalde-hyde under zinc chloride catalysis (Scheme 19) <20000 L4095>. Treatment of the reaction product with acid affords the disuhstituted furans in good yields. 

Mechanistically, the cycloaddition reaction is rather complex. Depending on the catalyst or solvent used and the reaction substrates, pericyclic and/or Mukaiyama aldol-like pathways may be involved.43 The pericyclic mechanism, generally favored by zinc chloride and the lanthanide catalysts, tends to produce adducts having the cis relative stereochemistry at C-5,6. It is assumed that chelation of the aldehyde with the Lewis acid occurs in an anti fashion and that the steric bulk of R is less than that of the Lewis acid-solvent complex L [Eq. (11)], thus favoring a Diels-Alder transition state with R endo. 

When acetaldehyde is treated with a dilute solution of an alkali or certain salts, such as sodium acetate and zinc chloride, a polymer is formed, which consists of two molecules of aldehyde in combination with each other. This substance, called aldol is a liquid which boils without decomposition under diminished pressure, and readily undergoes further polymerization. Aldol shows the reactions which are characteristic of aldehydes and can be converted by oxidation into jS-hydroxybutyric acid. These facts are evidence that aldol is the aldehyde of this acid. The union of the aldehyde molecules to form aldol is, therefore, best represented by the equation,—... 

The solution produced from the reaction of enones and dimethylcuprate undergoes aldol condensation with acetaldehyde in the presence of zinc chloride producing the )8-methyl a-(l-hydroxyethyl) ketones in acceptable yields (Heng and Smith, 1975). 

Aldol Reactions.—A new highly effective aldol synthesis employs an aluminium enolate derived regiospecifically from the appropriate a-halogeno-ketone using zinc and dialkylaluminium chloride (Scheme 18). ... 

Acylation, Alkylation, and Aldolization (Acyl Species-+ a-, P-, or a/fi-Functionalized Acyl Product) Alkylation reactions of sodium enolates of various lV-acyl-a-methyltoluene-2,a-sultams with selected (both activated and nonactivated ) alkyl iodides and bromides proceed with good C(a)-re stereocontrol (90-99% de). Analogous acylations with various acid chlorides can also be performed, giving p-keto products (97-99% de). Selective reduction of these latter products with Zinc Borohydride (chelate controlled, 82.6-98.2% de) or N-Selectride (nonchelate controlled, 95.8-99.6% de) can provide syn- and anft-aldol derivatives, respectively. ... 

Unexpectedly, the trienyl silyl ketene acetal (122) reacts with p-dimethylaminobenzoyl chloride and zinc bromide at the a-position to give the ester (123) as the only isolable product.It is not known at this time whether such a-selectivity is general for other aldol-type reactions of (122) or limited to acylations. ... 

Aldol synthesis. A new synthesis of 3-hydroxy ketones and esters involves regiospecific conversion of an a-bromo ketone or ester into an aluminum enolate by a coupled reaction with diethylaluminum chloride and zinc activated with copper(I) bromide in THF at —20°. This enolate adds to carbonyl compounds to give, after work-up, j3-hydroxy ketones (equation I). 

The reductive coupling of bis(enones) is another reaction class that may involve alkene-derived metallacyclopentanes. Treatment of a symmetrical bis(enone) 89 tvith bis(r] -cy-cloocta-l,5-diene)nickel(0) (2) and butyllithium/zinc(II) chloride leads to coupling of the two enone -carbons followed by an intramolecular aldol addition to afford bicyclo[3.3.0]-octanols 90 (Scheme Sterically hindered enones have also been shovm to partic-... 

The ethyl derivatives of aluminum, cadmium, magnesium, and zinc yield highly crystalline polymers. Organometallic eomplexes such as magnesium diethyl cobalt chloride, which coordinate strongly with the polymer anion and the monomer, produce white crystalline PMVK. PMVK obtained by alkoxides, sodium naphthalene, and -butyllithium are intensively colored because of a partially occurring aldol condensation [292]. The mechanism of these reactions has been studied intensively. It is assumed that... 

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