Biomedical subjects

Karl A Scheidt

Publications and source records attributed to Karl A Scheidt.

At least 19 recordsLinked to original sources

PubMed2007

Tandem oxidation of allylic and benzylic alcohols to esters catalyzed by N-heterocyclic carbenes.

N-heterocyclic carbenes catalyze the oxidation of allylic, propargylic, and benzylic alcohols to esters with manganese(IV) oxide in excellent yields. A variety of ester derivatives can be synthesized, including protected carboxylates. This one-pot tandem oxidation represents the first organocatalytic oxidation of alcohols to esters. Saturated esters can also be accessed from aldehydes using this method. Through the utilization of a chiral catalyst, the acyl-heteroazolium intermediate becomes a chiral acylating agent, which can desymmetrize meso-1,2-diols. [reaction: see text].

Allyl Compounds↗

PubMed2006

Synthesis of tertiary beta-hydroxy amides by enolate additions to acylsilanes.

The synthesis of tertiary beta-hydroxy amides from acylsilanes, acetamides, and electrophiles is described. The addition of amide enolates to acylsilanes generates beta-silyloxy homoenolate reactivity by undergoing a 1,2-Brook rearrangement. These unique nucleophiles formed in situ can then undergo smooth addition to alkyl halides, aldehydes, and ketones. Enolates derived from amides are crucial for the success of this process since ketone enolates suffer from internal return of the beta-carbanion onto the carbonyl carbon. The use of optically active amide enolates delivers beta-hydroxy amide products with good levels of diastereoselectivity (>/=10:1).

Acylation↗

PubMed2006

Stereoselective Lewis acid-catalyzed alpha-acylvinyl additions.

Silyloxyallenes derived from alpha-hydroxypropargylsilanes undergo efficient addition to aldehydes with catalytic amounts of Lewis acids. The allenes are accessed from the corresponding propargylsilanes in a base-catalyzed 1,2-Brook rearrangement/SE2' process. Enantioenriched propargylsilanes are synthesized by a new zinc-promoted addition of alkynes to acylsilanes in up to 74% ee. This work demonstrates that conversion to the silyloxyallenes occurs with minimal erosion in optical activity. The use of a chiral chromium(III) Lewis acid effects the catalytic asymmetric addition of racemic silyloxyallenes to aromatic aldehydes in up to 92% ee. The overall reaction is broad in scope and accommodates a wide variety of aromatic and aliphatic substituents on both the propargylsilane and aldehyde.

Journal Article↗

PubMed2006

Thiazolium-catalyzed additions of acylsilanes: a general strategy for acyl anion addition reactions.

A strategy utilizing N-heterocyclic carbenes (NHCs) derived from thiazolium salts has been developed for the generation of carbonyl anions from acylsilanes. Synthetically useful 1,4-diketones and N-phosphinoyl-alpha-aminoketones have been prepared in good to excellent yields via NHC-catalyzed additions of acylsilanes to the corresponding alpha,beta-unsaturated systems and N-phosphinoylimines. These organocatalytic reactions are air- and water-tolerant methods to execute robust carbonyl anion addition reactions. Additionally, polysubstituted aromatic furans and pyrroles have been efficiently synthesized in a one-pot process using this carbonyl anion methodology. The addition of alcohols to the reaction renders the process catalytic in thiazolium salt. In an effort to synthesize a potential intermediate along the proposed reaction pathway, silylated thiazolium carbinols have been identified to provide good yields of carbonyl anion addition products when subjected to the standard reaction conditions in the presence of suitable electrophiles.

Journal Article↗

PubMed2006

Direct nucleophilic acylation of nitroalkenes promoted by a fluoride anion/thiourea combination.

The direct nucleophilic acylation of nitroalkenes is reported utilizing a fluoride-initiated rearrangement of protected thiazolium carbinols. The key combination of fluoride anion and thiourea accesses carbonyl anion reactivity without the use of amine or amide bases typically employed in the generation of these Umpolung nucleophiles. The mild reaction conditions used to generate the reactive carbonyl anion species in situ allow for conjugate additions in good yield to sensitive nitroalkene electrophiles. The process is tolerant of a variety of thiazolium carbinols and nitroalkene substrates and can be rendered enantioselective and diastereoselective by the addition of a chiral thiourea derived from quinine.

Journal Article↗

PubMed2006

Hydroacylation of activated ketones catalyzed by N-heterocyclic carbenes.

N-heterocyclic carbenes derived from triazolium salts are effective catalysts between 10 and 15 mol % for the hydroacylation of activated ketones. The reducing equivalent is generated via the interaction of a nucleophilic carbene species and an aromatic aldehyde. The subsequent alcohol product can undergo an acylation event with the resulting acyl heteroazolium intermediate formed in situ between the NHC and the aldehyde. This unprecedented multiple bond-forming reaction can accommodate aromatic aldehydes as the hydride source and various electron-deficient ketones. Preliminary mechanistic evidence indicates that the reduction and acylation steps are sequential operations. The intramolecular variant of this organocatalytic reaction affords benzofuranones in good yield.

Journal Article↗

PubMed2006

Enantioselective MSPV reduction of ketimines using 2-propanol and (BINOL)Al(III).

[reaction: see text] A highly enantioselective Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction of N-phosphinoyl ketimines by (BINOL)Al(III)/2-propanol is reported. Yields ranging between 79 and 85% with high enantiomeric excesses (93-98%) are observed for a wide range of structurally diverse ketimines. A [2.0.4] bicyclic chelation model is proposed to account for this high selectivity.

Journal Article↗

PubMed2005

Catalytic conjugate additions of carbonyl anions under neutral aqueous conditions.

The conjugate addition of carbonyl anions catalyzed by thiazolium salts that is fully operative under neutral aqueous conditions has been accomplished. The combination of alpha-keto carboxylates and thiazolium-derived zwitterions produces reactive carbonyl anions in a buffered protic environment that readily undergo conjugate additions to substituted alpha,beta-unsaturated 2-acyl imidazoles. The scope of the reaction has been examined and found to accommodate various alpha-keto carboxylates and beta-aryl substituted unsaturated 2-acyl imidazoles. The optimal precatalyst for this process is the commercially available thiazolium salt 5, a simple analogue of thiamin diphosphate. In this process, no benzoin products from carbonyl anion dimerization are observed. The corresponding 1,4-dicarbonyl compounds can be efficiently converted into esters and amides by way of activation of the N-methylimidazole ring via alkylation.

Alkylation↗

PubMed2005

Lewis base-catalyzed additions of alkynes using trialkoxysilylalkynes.

[reaction: see text]. The Lewis base-catalyzed additions of alkynyl nucleophiles to aldehydes, ketones, and imines is described. Mechanistic studies strongly indicate that the use of new triethoxysilylalkynes facilitates access of a reactive hypervalent silicate intermediate. This activated carbon nucleophile subsequently undergoes rapid addition to carbonyl compounds and imines, thus affording the secondary and tertiary propargyl systems in moderate to high yield.

Journal Article↗

PubMed2005

Stereoselective synthesis of alpha-silylamines by the direct addition of silyl anions to activated imines.

[reaction: see text] A highly efficient stereoselective synthesis of unusual alpha-silylamines via a direct silyl anion addition reaction is reported. This approach is convergent and avoids any problematic aza-Brook shifts of the anionic intermediates. The use of enantiopure tert-butanesulfinyl imines as the electrophiles affords exceedingly high levels of diastereocontrol for the newly formed stereogenic carbon.

Journal Article↗

PubMed2005

Stereoselective synthesis of tetrahydropyran-4-ones from dioxinones catalyzed by scandium(III) triflate.

[reaction: see text] A scandium triflate catalyzed, diastereoselective cyclization between aldehydes and beta-hydroxy dioxinones has been discovered. This process capitalizes on the untapped nucleophilicity of the embedded enol ether within the dioxinone core. The bicyclic compounds from the resulting cyclization can be isolated, or alternatively, alkoxide nucleophiles can be directly added. This in situ addition fragments the dioxinone rings and delivers the 3-carboxy-substituted tetrahydropyran-4-ones in good yields with high levels of diastereoselectivity.

Journal Article↗

PubMed2005

Conversion of alpha,beta-unsaturated aldehydes into saturated esters: an Umpolung reaction catalyzed by nucleophilic carbenes.

N-Heterocyclic carbenes derived from benzimidazolium salts are effective catalysts for generating homoenolate species from alpha,beta-unsaturated aldehydes. These nucleophilic intermediates can be protonated, and the resulting activated carbonyl unit is trapped with an alcohol nucleophile, thereby promoting a highly efficient conversion of an alpha,beta-unsaturated aldehyde into a saturated ester. A kinetic resolution of secondary alcohols can be achieved using chiral imidazoylidene catalysts. [reaction: see text]

Journal Article↗

PubMed2005

Copper-promoted N-arylations of cyclic imides within six-membered rings: a facile route to arylene-based organic materials.

[reaction: see text] Cyclic imides within six-membered rings are shown to undergo efficient N-arylation using various arylboronic esters mediated by copper(II) acetate in the presence of an amine base and oxygen atmosphere with gentle heating. Until now, the synthesis of N-arylated cyclic imides having six-membered rings was restricted largely to strongly heating anilines in the presence of anhydrides. This reaction is applicable to the synthesis of new organic materials based on arylene imide and bis(imide) dyes, such as perylene-3,4:9,10-bis(dicarboximide)s.

Journal Article↗

PubMed2004

Catalytic additions of acylsilanes to imines: an acyl anion strategy for the direct synthesis of alpha-amino ketones.

The addition of acylsilanes to imines catalyzed by neutral carbenes (or zwitterions) generated in situ from readily available thiazolium salts is described. The general reaction successfully utilizes acylsilanes as carbonyl anion precursors and is tolerant of a range of structural diversity on the acylsilane or imine electrophile. The overall reaction utilizes easily available precursors and directly accesses protected alpha-amino ketones in the correct oxidation state.

Journal Article↗

PubMed2004

Efficient synthesis of acylsilanes using morpholine amides.

[reaction: see text] A general synthesis of acylsilanes from the corresponding morpholine amides and silyllithium species is described. The use of morpholine amides is economical and prevents over-addition by the silyl nucleophile. The procedure cleanly affords acylsilanes in good yields and circumvents the use of stoichiometric copper(I) cyanide typically employed to synthesize these compounds from acid chlorides.

Journal Article↗

PubMed2004

Catalytic multicomponent synthesis of highly substituted pyrroles utilizing a one-pot sila-Stetter/Paal-Knorr strategy.

[reaction: see text] A multicomponent synthesis of highly substituted pyrroles catalyzed by thiazolium salts has been disclosed. The reaction employs an acyl anion conjugate addition reaction of acylsilanes (sila-Stetter) and unsaturated ketones to generate 1,4-dicarbonyl compounds in situ. The subsequent addition of various amines promotes a Paal-Knorr reaction, affording the desired pyrrole nucleus in an efficient one-pot process.

Journal Article↗

PubMed2004

The thiazolium-catalyzed Sila-Stetter reaction: conjugate addition of acylsilanes to unsaturated esters and ketones.

A new acyl anion addition reaction between acylsilanes and alpha,beta-unsaturated conjugate acceptors promoted by a nucleophilic organic catalyst has been disclosed. The 1,4-dicarbonyl products produced in this reaction are highly useful synthons. Neutral carbenes (or zwitterions) generated in situ from commercial thiazolium salts are used as effective catalysts for the reaction which is in contrast to established anionic catalysts typically employed to promote the required Brook rearrangement (1,2-silyl shift from carbon to oxygen) involved in the reported reaction. This process successfully utilizes acylsilanes as tunable acyl anion progenitors and is tolerant of a wide range of structural diversity on the acylsilane or the conjugate acceptor.

Journal Article↗

PubMed2004

Copper(I)-catalyzed disilylation of alkylidene malonates employing a Lewis base activation strategy.

The disilylation of activated alpha,alpha-unsaturated esters has been accomplished by utilizing symmetrical disilanes and catalytic quantities of copper(I) salts in the presence of Lewis bases such as DMF. This new methodology with various alkylidene malonates is moderate to high yielding and affords alpha-silyl diesters, compounds that previously have not been readily accessible, that can be further manipulated to cleanly afford useful alpha-substituted alpha-hydroxy esters.

Journal Article↗