Preparation of N-protected allylic amines and α-methylene-β-amino acids from vinylalumination/Baylis-Hillman products via tandem SN2′ substitution-Overman rearrangement

S_N2′ reaction on the acetates obtained from vinylalumination or Baylis-Hillman products, followed by in situ reduction afforded allylic alcohols. Upon conversion to trichloroacetimidates and [3,3]-sigmatropic rearrangement, the corresponding N-protected β-substituted allylic amines were obtained in good yields. Utilization of hydroxy group as the nucleophile furnished allylic hydroxy esters, which were converted to protected α-methylene-β-amino acids via Overman rearrangement.     

Stereoselective Addition of Methyl Acrylate to α-Amino Aldehydes

Variously N-protected α-aminoaldehydes exhibit reasonable diastereoselectivity in the Baylis-Hillman coupling¹ with methyl acrylate to provide either primarily syn or anti α-methylene-β-hydroxy-γ-amino esters.     

N-acyl-2-methylene-β-alanine methyl esters from the sponge Fasciospongia cavernosa

The isolation and structural determination of a series of N-acylated 2-methylene-β-alanine methyl esters, a new naturally-occurring β-amino acid, from the sponge Fasciospongia cavernosa, is reported.     

Novel one-pot asymmetric cascade approach toward densely substituted enantioenriched α-methylene-γ-lactams

A novel one-pot asymmetric synthesis of densely substituted α-methylene-γ-lactams in favor of cis-isomer involving Zn-promoted aza-Barbier-type reaction has been well established. A wide array of α-methylene-γ-lactams were obtained in high yields with excellent diastereomeric ratios from 98:2 to >99:1 and good enantioselectivities. The synthetic potential of such α-methylene-γ-lactams was demonstrated by successfully oxidizing the phenol moiety to quinone in quantitative yield.     

Synthesis of cis-3,4-diaryl α-methylene-γ-butyrolactams via sonochemical Barbier-type reaction

A series of cis-3,4-diaryl α-methylene-γ-butyrolactams were synthesized by the addition reaction of 3-phenylallyl bromide with N-tosyl aldimine via sonochemical Barbier-type reaction condition and then followed by the in situ intramolecular amidation. The cis-3,4-diaryl α-methylene-γ-butyrolactam was obtained as the sole regio- and stereoisomeric product when N-tosyl aldimine was used as the substrate whereas the monoaryl α-methylene-γ-butyrolactam was also generated when N-phenyl aldimine was used.     

A highly stereoselective total synthesis of (±)-isoalantolactone

(±)-Isoalantolactone (1) has been synthesized by a highly stereoselective route from the versatile bicyclic ketone, trans-5-methylene-9-methyl-2-decalone 2. Three major points in the synthesis involve (a) introduction of the acetic acid side chain; (b) stereoselective reduction of the ketone function yielding cis-lactone 6; and (c) introduction of the α-methylene group onto the butyrolactone ring.     

Cyclisation of α-chiral aminyl radicals

α-Chiral aminyl radicals have been generated from sulfenamides of α-amino acid esters and α-phenylethylamine using Bu₃SnH. The aminyl radicals of α-amino acid esters undergo 5-exo-trig cyclisation reactions onto side chain alkenes to yield proline analogues with reasonable diastereoselectivity. Preliminary studies show urethanyl radicals generated from sulfenamides of alkenyl urethane derivatives of α-amino acid esters and α-phenylethylamine undergo 5-exo-rig cyclisations to providing a protocol for the radical amination of unactivated alkenes. The α-ester of the amino acid or the urethane groups impart electrophilic behaviour to the aminyl radicals and facilitates cyclisation onto alkenes.     

Imine allylation using 2-alkoxycarbonyl allylboronates as an expedient three-component reaction to polysubstituted α-exo-methylene-γ-lactams

α-exo-Methylene-γ-lactams are key structural units in a wide variety of biologically active natural products. A concise route to the formation of polysubstituted α-exo-methylene-γ-lactams is described. In this three-component reaction, an imine is formed from an aldehyde and ammonia in situ, and is subsequently allylated through the use of a 2-alkoxycarbonyl allylboronate. Due to the ester functionality, the addition intermediate subsequently undergoes in situ cyclization to form the observed α-exo-methylene-γ-lactam products. This route allows access to highly substituted α-exo-methylene-γ-lactams in moderate to excellent yields.     

Formation of cinnoline derivatives by a gold(I)-catalyzed hydroarylation of N-propargyl-N′-arylhydrazines

A study concerning the gold(I)-catalyzed hydroarylation of N-propargyl-N′-arylhydrazinecarboxylic acid methyl esters is described. The use of the gold complex [XPhosAu(NCCH₃)SbF₆] as the catalyst in refluxing nitromethane allows the generally rapid and efficient synthesis of a range of functionalized 4-exo-methylene-1,2-dihydrocinnolines.     

Enzymatic enantioselective reduction of α-ketoesters by a thermostable 7α-hydroxysteroid dehydrogenase from Bacteroides fragilis

A thermostable 7α-hydroxysteroid dehydrogenase (7-HSDH) from Bacteroides fragilis ATCC 25285 was cloned and over-expressed in E. coli, and its substrate specificity and stereoselectivity toward reduction of various ketones were examined. This alcohol dehydrogenase was active toward a series of aromatic and bulky aliphatic α-ketoesters. The substituents at the phenyl ring of aromatic α-ketoesters greatly affected the activity, but their effects on enantioselectivity were minimal. The synthetic application of this enzyme was then demonstrated through the preparation of a few α-hydroxy carboxylic acid esters of pharmaceutical interest.     

A General Synthesis of α-Substituted Acrylic Esters

During a study of synthetic approaches to various naturally-occurring sesquiterpenes lactones, we developed a new synthesis of the α-methylene-γ-butyrolactone group.¹ This new sequence suggested that it might be applicable to the general synthesis of α-methylenecarbonyl compounds.² This report describes the application to a general synthesis of α-substituted acrylic esters.     

Proline derivatives incorporating hydrophobic long-chain derived from natural and synthetic fatty acids

The α-hydrophobic long chain-α-amino esters are prepared by α-hydroxylation of a series of fatty acid esters [derived from oleic acid (OA), linoleic acid (LA), arachidonic acid (ARA) and docosahexaenoic acid (DHA)] followed by Mitsunobu reaction and hydrazinolysis of the phthalimide. These amino esters are mixed with aldehydes and electrophilic alkenes to give very good chemical yields and diastereoselectivities of prolinate derivatives incorporating a hydrophobic long chain at the α-position. This multicomponent 1,3-dipolar cycloaddition (1,3-DC) takes place at room temperature. The synthesis of the homologue hydrophobic chain of OA is performed by its oxidation to aldehyde/racemic N-tert-butylsulfinyl imine/Neff reaction. Final 1,3-DC with benzaldehyde and N-methylmaleimide affords homologue prolinate derivative in good yield.     

Asymmetric syntheses of the N-terminal α-hydroxy-β-amino acid components of microginins 612, 646 and 680

The asymmetric syntheses of the N-terminal α-hydroxy-β-amino acid components of microginins 612, 646 and 680 are reported. Conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide to the requisite (E)-α,β-unsaturated ester followed by in situ enolate oxidation with (?)-(camphorsulfonyl)oxaziridne (CSO) gave the corresponding anti-α-hydroxy-β-amino esters. Sequential Swern oxidation followed by diastereoselective reduction gave the corresponding syn-α-hydroxy-β-amino esters. Subsequent N-debenzylation (i.e., hydrogenolysis for microginin 612, and NaBrO₃-mediated oxidative N-debenzylation for microginins 646 and 680) followed by acid catalysed ester hydrolysis gave the corresponding syn-α-hydroxy-β-amino acids, the N-terminal components of microginins 612, 646 and 680, in good yield. An analogous strategy for elaboration of the enantiopure anti-α-hydroxy-β-amino esters facilitated the asymmetric synthesis of the corresponding C(2)-epimeric α-hydroxy-β-amino acids.     

Development of general methods for the synthesis of new substituted allyl bromides as promising alkenylating agents

A general procedure has been developed for the synthesis of hitherto unknown substituted allyl bromides. The procedure includes preparation of the corresponding α,β-unsaturated carboxylic acid esters from accessible ketones according to the Horner-Emmons reaction, reduction of these esters with diisobutylaluminum hydride to allylic alcohols, and substitution of the hydroxy group by bromine by the action of PBr₃. The E,Z isomer ratio of the synthesized unsaturated compounds ranges from 3: 1 to 4: 1.     

Organophosphorus Reagents in Organocatalysis: Synthesis of Optically Active α-Methylene-δ-lactones and δ-Lactams

In this paper we describe new asymmetric, catalytic strategies for the synthesis of biologically important α-methylene-δ-lactones and δ-lactams. The elaborated protocols utilize iminium-ion-mediated Michael addition of trimethyl phosphonoacetate to α,β-unsaturated aldehydes catalyzed by (S)-(-)-α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether as the key step. Enantiomerically enriched Michael adducts are employed in three different reaction pathways. Transformation into α-methylene-δ-lactones is realized by a sequence of reactions involving chemoselective reduction of the aldehyde, followed by a trifluoroacetic acid (TFA)-mediated cyclization and Horner-Wadsworth-Emmons olefination of formaldehyde. On the other hand, indolo[2,?3-a]quinolizine-framework-containing products can be accessed when enantiomerically enriched Michael adducts are employed in a Pictet-Spengler reaction with tryptamine, followed by Horner-Wadsworth-Emmons olefination. Finally, reductive amination of the Michael adducts by using methylamine and Horner-Wadsworth-Emmons olefination of formaldehyde is demonstrated to give α-methylene-δ-lactams. The developed strategies can be realized without the purification of intermediates, thus greatly increasing their practicality.     

α-(1H-Imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters as prodrugs of carboxylic acid containing drugs

Synthesis of α-(1H-imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters have been reported in 2-3 simple steps. α-(1H-Imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters were found to be chemically labile and thus serve as novel prodrugs of carboxylic acids.     

Enzymic resolution of an α-methylene-γ-lactonic ester

The possibility of using enzymes for the kinetic resolution of an α-methylene-γ-lactone was explored. The probe molecule, ethyl 2-methyl-4-methylene-tetrahydro-5-oxo-2-furancarboxylate was successfully resolved with Porcine pancreatic lipase and Candida rugosa lipase, in the presence of a cosolvent, leading to both enantiomers of the corresponding acid with high enantiomeric excesses.     

A novel synthesis, including asymmetric synthesis, of α-quaternary α-amino acid methyl esters from ketones via sulfinyloxiranes

Sulfinyloxiranes were synthesized from ketones and chloromethyl p-tolyl sulfoxide in two steps in almost quantitative yields. The sulfinyloxiranes were treated with NaN₃ in the presence of NH₄Cl to afford α-azido aldehydes, which were oxidized with iodine in the presence of KOH in methanol to give α-azido methyl esters in good overall yields. Catalytic hydrogenation of the α-azido esters afforded α-quaternary α-amino acid methyl esters in quantitative yields. Starting from β-tetralone and optically pure (R)-chloromethyl p-tolyl sulfoxide, an asymmetric synthesis of optically pure (R)-(+)-methyl 2-aminotetraline-2-carboxylate was realized in good overall yields.     

Stereochemical control in microbial reduction. Part 31: Reduction of alkyl 2-oxo-4-arylbutyrates by baker's yeast under selected reaction conditions

Treatment of baker's yeast with phenacyl chloride in an aqueous–organic solvent has been proven to be an effective method of inhibiting the enzymes that afford (S)-enantiomers of α-hydroxy esters in the reduction of α-keto esters. The procedure is effective for the whole-cell system to produce the (R)-product with high chemical yield and high enantiomeric excess.     

Studies on indium-mediated additions to lactones: synthesis of 2-deoxy-2-substituted-3-ulosonic acids

The indium-mediated Reformatsky reaction of a mannose-derived lactone with α,α-disubstituted-α-bromo esters yielded the corresponding ulosonic acid esters as mixtures of anomers. In contrast, the reaction with α-monosubstituted-α-bromo esters is highly stereoselective and afforded a single anomer of the corresponding (2S)-ulosonic acid esters. A mechanistic proposal for the reaction and an explanation of its outcome is discussed. The indium-mediated Reformatsky reaction of the mannose-derived lactone with 2-bromo-lactones was also achieved and the products obtained were consistent with those of our proposed mechanism in all cases. Moreover, indium-mediated allylation of the model sugar lactone was also investigated.