Α-keto Acetals Research Articles

The Synthesis of α-Keto Acetals from Terminal Alkynes and Alcohols via Synergistic Interaction of Organoselenium Catalysis and Electrochemical Oxidation.

Herein, an unprecedented electrochemical approach for the synthesis of α-keto acetals has been established from readily available terminal alkynes and alcohols. By merging the electrochemical and organoselenium-catalyzed processes, the desired products are obtained at room temperature in the absence of basic or metallic additives, with carbonyl and acetal motifs incorporated simultaneously across the triple bonds in a single operation.

Direct asymmetric reductive amination of α-keto acetals: a platform for synthesizing diverse α-functionalized amines.

We report an efficient and straightforward method to synthesize enantio-enriched N-unprotected α-amino acetals via ruthenium-catalyzed direct asymmetric reductive amination. The α-amino acetal products are versatile and valuable platform molecules that can be converted to the corresponding α-amino acids, amino alcohols, and other derivatives by convenient transformations.

Organocatalytic asymmetric biomimetic transamination of α-keto acetals to chiral α-amino acetals

This paper describes a chiral base-catalyzed asymmetric biomimetic transamination of α-keto acetals. A wide variety of α-amino acetals containing various functional groups can be synthesized in 50–85% yield and 82–86% ee.

Enantioselective hydrogenation of α-keto acetals with cinchona modified Pt catalyst

The enantioselective hydrogenation of a variety of α-keto acetals to the corresponding α-hydroxy acetals with Pt catalysts modified with cinchonidine derivatives is described with ees up to 97% and high reaction rates, and the influence of the substrate structure, the modifier and the reaction conditions (catalyst, solvent, temperature, pressure, modifier concentration) was investigated in some detail.

Catalytic enantioselective reactions. Part 16. Oxazaborolidine-catalyzed asymmetric borane reduction of α-keto acetals

Asymmetric reductions of α-keto acetals using various oxazaborolidines and borane reagents as catalyst and the hydride source, respectively, were compared. The reduction catalyzed by Corey’s CBS reagents with N-phenylamine–borane reagents provided α-hydroxy acetals with very high enantioselectivities for most aromatic analogues.

Electrosynthesis of α-keto acetals by a selenium-catalyzed transformation of aryl methyl ketones

An indirect electrochemical pathway exists for the conversion of aryl methyl ketones to their corresponding α-keto acetals. Cyclic voltammetry demonstrates that diphenyl diselenide can act as an electron mediator in the conversion because it has the lowest oxidation potential in the reaction mixture. Diphenyl diselenide, upon oxidation at the anode, forms an electrophilic phenylselenenylating reagent which can then react with the aryl methyl ketone in the presence of methanol to form the desired α-keto acetal. Moreover, the diphenyl diselenide can be used in catalytic amounts because it can be recycled by oxidizing the phenylselenenyl anion which results from the formation of the α-keto acetal. Current efficiencies up to 64% were obtained without optmization. Low yields can be attributed to the diffusion of reactant, catalyst, and product through the diaphragm of the divided cell.

Highly Diastereoselective Additions to Chiral α-Keto Acetals

A general synthesis of (4R*,5R*)-2-acyl-4,5-diphenyl-1,3-dioxolanes and the addition of alkylmagnesium bromides to these compounds is described. The addition reactions occur with high diastereoselectivity (typical selectivity ca. 90:10). An example addition reaction carried out on an optically active acetal proves the sense of asymmetric induction for addition of organomagnesium reagents to these α-keto acetals. An explanation of the mechanism of asymmetric induction is given.

Unusual Variant of the Favorsky Rearrangement. Formation of α-keto acetals

Unusual Variant of the Favorsky Rearrangement. Formation of α-keto acetals

Asymmetric reduction of α-keto acetals with potassium 9- O-(1,2-isopropylidene-5-deoxy-D-xylofuranosyl)-9-boratabicyclo[3.3.1]nonane. Enantioselective synthesis of α-hydroxy acetals with high optical purities

Asymmetric reduction of α-keto acetals with a chiral borohydride, potassiun 9- O-(1,2-isopropylidene-5-deoxy-α- D-xylofuranosyl)-9-boratabicyclo[3.3.1]nonane in THF at −78 °C provided the corresponding α-hydroxy acetals with 87 – 99 % ee.

Selenium-mediated conversion of alkynes into .alpha.-dicarbonyl compounds

The reaction of terminal and internal alkynes with diphenyl diselenide and ammonium peroxydisulfate in methanol proceeds smoothly to give α-keto acetals and α-keto ketals, respectively. This one-pot procedure is suggested to proceed through the initial formation of phenylselenenyl sulfate, a strong electrophilic reagent which effects the methoxyselenenylation of the alkynes. The addition products thus formed suffer methoxyde-selenylation giving the observed products and regenerating the phenylselenenylating agent. In some cases the reaction can be carried out using only catalytic amounts of diphenyl diselenide. The same reaction carried out in the presence of water or of ethylene glycol gives the unprotected or the diprotected α-dicarbonyl compounds, respectively

Asymmetric synthesis using chiral acetals. Studies on the nucleophilic addition of organometallics to chiral .ALPHA.-keto acetals in open-chain systems.

Nucleophilic addition of organometallic reagents (Grignard reagents and organolithium reagents) to three chiral α-keto acetals (1a-c) in open-chain systems was studied. The reactions of the chiral a-keto acetals (1a, 1b) having a chiral auxiliary as a ketone equivalent with Grignard reagents proceeded in a highly diastereoselective manner (> 94% diastereomeric excess (de)). As an application of the reaction, the syntheses of the key intermediates (6, 8) for (R) -and (S) -mevalolactone were achieved from the product obtained by the reaction of 1a and vinylmagnesium bromide. On the other hand, in the reactions of the chiral α-keto acetal (1c) having a chiral auxiliary as an aldehyde equivalent, lower stereoselectivity was observed even when Grignard reagents were used as addends. The difference of stereoselectivity among the three chiral α-keto acetals (1a-c) is discussed.

Asymmetric synthesis using chiral acetals: Studies on the nucleophilic addition of organometallics to chiral x-keto acetals in cyclic systems.

Four chiral cyclic a-keto acetals (3a-d) were prepared through transacetalization of the α-hydroxydimethyl acetals [1 (n = 1, n = 2)] with (-) - (2S, 3S) -1, 4-dimethoxy-2, 3-butanediol or (-) - (2R, 3R) -2, 3-butanediol and reacted with organometallic reagents (Grignard reagents and organo-lithium reagents). The reactions of the α-keto acetals (3a, 3b) derived from 1 and (-) - (2S, 3S) -1, 4-dimethoxy-2, 3-butanediol with Grignard reagents proceeded in a highly diastereoselective manner.