Diethylzinc-Mediated Cross-Coupling Reactions between Dibromoketones and Monobromo Carbonyl Compounds

A novel route to synthesize 1,4-dicarbonyl compounds is described. α,α-Dibromoketones generate zinc enolates through a diethylzinc-mediated halogen-metal exchange and react with α-bromocarbonyl compounds to furnish 1,4-dicarbonyl compounds via a second generation of zinc enolates. This cross-coupling reaction is enabled by the chemoselective formation of zinc enolates from α,α-dibromoketones in the presence of α-bromocarbonyl compounds. Chiral 1,4-dicarbonyl compounds can be obtained via the enantioselective bromination of aldehydes using a chiral secondary amine catalyst and a subsequent cross-coupling reaction between the resulting chiral α-bromoaldehydes and α,α-dibromoacetophenones.     

Stereoselective kinetic protonation of chiral γ-lactone enolates

When lithium enolates generated from five chiral α, γ-disubstituted γ-lactones are treated with proton sources, formation of the α, γ-syn epimers always predominates over the a,y-anti epimers in ratios of 10.2–2.9:1. Higher syn/anti ratios are obtained via silyl enolates in some cases. Chirality discrimination by chiral proton source is also observed in particular cases.     

Asymmetric synthesis of α,β-diamino acid derivatives with an aziridine-, azetidine- and γ-lactone-skeleton via Mannich-type additions across α-chloro-N-sulfinylimines

The efficient asymmetric synthesis of new chiral γ-chloro-α,β-diamino acid derivatives via highly diastereoselective Mannich-type reactions of N-(diphenylmethylene) glycine esters across a chiral α-chloro-N-p-toluenesulfinylimine was developed. The influence of the base, LDA or LiHMDS, used for the formation of the glycine enolates, was of great importance for the anti-/syn-diastereoselectivity of the Mannich-type reaction. The γ-chloro-α,β-diamino acid derivatives proved to be excellent building blocks for ring closure towards optically pure anti- and syn-β,γ-aziridino-α-amino esters, and subsequent ring transformation into trans-3-aminoazetidine-2-carboxylic acid derivatives and α,β-diamino-γ-butyrolactones.     

α-Alkylation of Amino Acids without Racemization. Preparation of Either (S)- or (R)-α-Methyldopa from (S)-Alanine

Enantiomerically pure cis- and trans-5-alkyl-1-benzoyl-2-(tert-butyl)-3-methylimidazolidin-4-ones ( 1, 2, 11, 15, 16 ) and trans-2-(tert-butyl)-3-methyl-5-phenylimidazolidin-4-one ( 20 ), readily available from (S)-alanine, (S)-valine, (S)-methionine, and (R)-phenylglycine are deprotonated to chiral enolates (cf. 3, 4, 12, 21 ). Diastereoselective alkylation of these enolates to 5,5-dialkyl- or 5-alkyl-5-arylimidazolidinones ( 5, 6, 9, 10, 13a-d, 17, 18, 22 ) and hydrolysis give α-alkyl-α-amino acids such as (R)- and (S)-α-methyldopa ( 7 and 8a , resp.), (S)-α-methylvaline ( 14 ), and (R)-α-methyl-methionine ( 19 ). The configuration of the products is proved by chemical correlation and by NOE ¹H-NMR measurements (see 23, 24 ). In the overall process, a simple, enantiomerically pure α-amino acid can be α-alkylated with retention or with inversion of configuration through pivaladehyde acetal derivatives. Since no chiral auxiliary is required, the process is coined ‘self-reproduction of a center of chirality’. The method is compared with other α-alkylations of amino acids occurring without racemization. The importance of enantiomerically pure, α-branched α-amino acids as synthetic intermediates and for the preparation of biologically active compounds is discussed.     

Stereoselective Aldol Reactions with α-Unsubstituted Chiral Enolates

The aldol reaction is among the most important methods of forming carbon-carbon bonds. The addition of an enolate to an aldehyde leads to the formation of at least one chiral center. In the case of α-substituted enolates it has to a large extent been possible to control the product stereochemistry, while the aldol reaction of α-unsubstituted chiral enolates was for many years a “problem child” for synthetic chemists because of its insufficient stereoselectivity. Progress in this area has only been made in the last few years using either new chiral auxiliaries or alternatives to the aldol reaction.     

New strategy for the stereoselective synthesis of fluorinated beta-amino acids

Racemic and chiral nonracemic alpha-substituted and alpha-unsubstituted beta-fluoroalkyl beta-amino acid derivatives 6 and 9 have been synthesized in two steps starting from fluorinated imidoyl chlorides 1 and ester enolates. This approach is based on the chemical reduction of previously obtained gamma-fluorinated beta-enamino esters 4 by using ZnI(2)/NaBH(4) in a nonchelated aprotic medium (dry CH(2)Cl(2)) as the reducing agent. A metal-chelated six-membered model has been suggested to explain the stereochemical outcome of the reduction reaction. The process takes place with high yields and with moderate to good diastereoselectivity. The best results related to diastereoselective reduction of chiral beta-enamino esters 4 were provided by the use of (-)-8-phenylmenthol as a chiral auxiliary.     

Diastereoselective alkylation of chiral 2-imidazolidinone glycolates: asymmetric synthesis of α-hydroxy carboxylic acids

Sodium enolates of chiral 2-imidazolidinone glycolates reacted with alkyl halides to produce α-alkylated products with high diastereoselectivities, which were readily removed by simple alkaline hydrolysis and were converted to the protected α-hydroxy carboxylic acids. The new stereogenic center was assigned the (R)-configuration by comparison with known compounds.     

Synthesis of α-CF3-substituted carbonyl compounds with relative and absolute stereocontrol using electrophilic CF3-transfer reagents

Evans-type chiral lithium imide enolates undergo diastereoselective α-trifluoromethylation with a hypervalent iodine-CF(3) reagent with up to 91% combined isolated yield and 97:3 dr. The resulting isolated diastereopure products can be further transformed into valuable products without racemization.     

Stereoselektive Alkylierung an C(α) von Serin,Glycerinsäure,Threonin und Weinsäure über heterocyclische Enolate mit exocyclischer Doppelbindung

Steroselective Alkylation at C(α) of Serine, Clyceric Acid, Threonine, and Tartaric Acid Involving Heterocyclic Enolates with Evocyelic Double Bonds The chiral, non-racemic title acids are converted to methyl dioxolane-(cf. 13 ), oxazoline-( 4 ) and oxazolidinecarboxylates (cf. 9 ). Deprotonation by Li(i-Pr) ₂N at dry-ice temperature gives solutions of the lithium enolates A–D With exocyclic enolate double bonds. These are stable crough with respect to β-elimination (Scheme 1) to be alkylated with or without cosolvents such as HMPA or DMPU The products are formed in good to excellent yields and, with the exception of the tartrate-derived acetonlde (see Scheme 2), with diastereoselectivities above 90%. While the tartrate-and threonine-derived enolates ( A and B , resp.) are chiral due to the second stereogenic center of the precursors, the serine- and glyceric-acid-derived enolates ( A and B , resp.) are chiral due to the second sterogenic center of the precursors, the serine-nd glyceric-acid-derived enolates are non-racemic due to a tert butyl-substituted (pivalaldehyde-derived) acetal center ( C and D , resp.). The products of alkylation can be hydrolyzed to give α-branched tartaric acid (Scheme 2), allothreonine (Scheme 3), serine (Scheme 4), and glyceric-acid derivatives (Scheme 5) with quaternary stereogenic centers. The configurations of the products are determined by NOE-NMR measurements and by chemical correlation. These show that the dioxolane-derived enolates A and D are alkylated preferentially from that face of the ring which is already substituted (‘syn’-attack), while the dihydrooxazol-and oxazolidine-derived enolates B and C are alkylated from the opposite face (‘anti’-attack). The ‘syn’-attack is postulated to arise from strong folding of the heterocyclic ring due to electronic repulsion between the enolate π-system and non-bonding electron pairs on the heteroatoms (see Scheme 6).     

Diastereoselective protonation of enolates of chiral Schiff bases

Diastereoselective protonation of potassium enolates of chiral Schiff bases prepared from racemic α-amino esters and 2-hydroxypinan-3-one afforded, after mild cleavage of the imine function, optically active α-amino esters.     

Primary amine/CSA ion pair: a powerful catalytic system for the asymmetric enamine catalysis

A novel ion pair catalyst containing a chiral counteranion can be readily derived by simply mixing cinchona alkaloid-derived diamine with chiral camphorsulfonic acid (CSA). A mixture of 9-amino(9-deoxy)epi-quinine 8 and (-)-CSA was found to be the best catalyst with matching chirality, enabling the direct amination of α-branched aldehydes to proceed in quantitative yields and with nearly perfect enantioselectivities. A 0.5 mol % catalyst loading was sufficient to catalyze the reaction, and a gram scale enantioselective synthesis of biologically important α-methyl phenylglycine has been successfully demonstrated.     

A novel method for asymmetric synthesis of both enantiomers of α-substituted carboxylic acid derivatives from optically active 1-chlorovinyl p-tolyl sulfoxides and lithium ester enolates with 1,4-chiral induction from the sulfur chiral center

Treatment of optically active 1-chlorovinyl p-tolyl sulfoxides, which were synthesized from symmetrical ketones and (R)-(−)-chloromethyl p-tolyl sulfoxide in three steps, with lithium enolate of carboxylic acid tert-butyl esters gave optically active adducts having a substituent at the α-position with high 1,4-chiral induction from the sulfur chiral center in high yields. The adducts were converted to optically active esters and carboxylic acids having a chiral center at the α-position. When this addition reaction was carried out with the ester enolate generated from excess carboxylic acid tert-butyl ester with LDA in the presence of HMPA, the diastereomer of the adduct was obtained. By using the two reaction conditions for the generation of the ester enolates, a new method for asymmetric synthesis of both enantiomers of carboxylic acid derivatives having a substituent at the α-position from the one chiral source, (R)-(−)-chloromethyl p-tolyl sulfoxide, was realized.     

Recent advances in the use of chiral aldimines in asymmetric synthesis

Chiral N-(tert-butyl)sulfinyl aldimines easily prepared from commercially available compounds have been used as starting materials for the following processes: (i) hydrogen transfer, (ii) addition of zincates, (iii) In-promoted allylation, and (iv) addition of zinc enolates. In all cases, the final desulfinylation to yield the expected chiral α-substituted primary amines was easily performed with hydrochloric acid in an organic solvent. This methodology has been successfully applied to the preparation of chiral natural products such as alkaloids and amino acids.     

Aldol addition of lithium and boron enolates of 1,3-dioxan-5-ones to aldehydes. A new entry into monosaccharide derivatives

Methods allowing control of stereoselectivity in aldol reactions of enolates derived from 1,3-dioxan-5-ones (4) are described. Boron enolates, generated in situ, react with benzaldehyde to give the corresponding anti aldol selectively (the anti:syn ratio of up to 96:4) and in high yield. Lithium enolates give high anti selectivity only with aldehydes branched at the alpha-position. Enantioselective deprotonation of C(S) symmetrical dioxanones (e.g., 4b) can be accomplished efficiently, with enantiomeric excess of up to 90%, with chiral lithium amide bases of general structure PhCH(Me)N(Li)R (9, 10) if the R group is sufficiently bulky (e.g, R = adamantyl) or is fluorinated (e.g., R = CH2CF3). Dioxanone boron and lithium enolates react readily with glyceraldehyde derivatives (19), yielding protected ketohexoses (20 and 21).     

A diastereo- and enantioselective Michael addition of chiral amide enolates to α, β-unsaturated esters a stereoelective synthesis of (+)-dehydroiridodiol and (−)-isodehydroiridodiol

(+)-Dehydroiridodiol and (−)-isodehydroiridodiol were synthesized stereoselectively using the diastereo- and enantioselective Michael addition of chiral amide enolates to α, β-unsaturated esters.     

Efficient asymmetric protonation of enolates with readily accessible chiral α-sulfinyl alcohols

The efficient asymmetric protonation of lithium enolates of 2-alkylcycloalkanones (87–96% ee) with readily accessible chiral α-sulfinyl alcohols is described. Optimal stereoselection is achieved for each lithium enolate at a different reaction temperature in the range −40 to −100°C.     

Role of pseudoephedrine as chiral auxiliary in the "acetate-type" aldol reaction with chiral aldehydes; asymmetric synthesis of highly functionalized chiral building blocks

We have studied in depth the aldol reaction between acetamide enolates and chiral α-heterosubstituted aldehydes using pseudoephedrine as chiral auxiliary under double stereodifferentiation conditions, showing that high diastereoselectivities can only be achieved under the matched combination of reagents and provided that the α-heteroatom-containing substituent of the chiral aldehyde is conveniently protected. Moreover, the obtained highly functionalized aldols have been employed as very useful starting materials for the stereocontrolled preparation of other interesting compounds and chiral building blocks such as pyrrolidines, indolizidines, and densely functionalized β-hydroxy and β-amino ketones using simple and high-yielding methodologies.     

Novel methods for the facile construction of 3,3-disubstituted and 3, 3-spiro-2H,4H-benzo[e]1,2-thiazine-1,1-diones: synthesis of (11S,12R, 14R)-2-fluoro-14-methyl-11-(methylethyl)spiro[4H-benzo[e]- 1, 2-thiazine-3,2'-cyclohexane]-1,1-dione, an agent for the electrophilic asymmetric fluorination of aryl ketone enolates

Novel methods for the facile construction of 3,3-disubstituted and 3, 3-spiro 2H,4H-benzo[e][1,2]thiazine-1,1-diones 8a-h are described. o-Methyl lithiation of N-Boc-o-toluenesulfonamide 6 followed by reaction with a variety of ketones gave the corresponding carbinol sulfonamides 7a-g, which underwent cyclization under acidic (methanesulfonic acid) or neutral (NaI/TMSCl/MeCN) conditions to afford the sultams 8a-h in high yields. The chiral spiro sultams 8g, h were subjected to FClO(3) fluorination to give the N-fluorosultams 11a,b, respectively, which were tested for electrophilic asymmetric fluorination of aryl ketone enolates. As a result, the N-fluorosultam 11a exhibited modest asymmetric inducing abilities with the highest ee, reaching 70% for enantioselective fluorination of the lithium enolate of 2-methyl-1-tetralone.     

Catalytic asymmetric protonation of lithium enolates using amino acid derivatives as chiral proton sources

[reaction: see text] Asymmetric protonation of lithium enolates was examined using commercially available amino acid derivatives as chiral proton sources. Among the amino acid derivatives tested, Nbeta-l-aspartyl-l-phenylalanine methyl ester was found to cause significant asymmetric induction in the protonation of lithium enolates. The enantiomeric excess (up to 88% ee) of the products obtained in the presence of a catalytic amount of the chiral proton source was higher than those obtained in the stoichiometric reaction.     

Efficient asymmetric synthesis of novel gastrin receptor antagonist AG-041R via highly stereoselective alkylation of oxindole enolates

An efficient method for asymmetric synthesis of the potent Gastrin/CCK-B receptor antagonist AG-041R was developed. Core oxindole stereochemistry was established by asymmetric alkylation of oxindole enolates with bromoacetic acid esters, using l-menthol as a chiral auxiliary. The key alkylation reaction of the oxindole enolates generated tetrasubstituted chiral intermediates with high diastereoselectivity. The stereoselective alkylation reactions are described in detail.