Trading N and O. Part 4: Asymmetric synthesis of syn-β-substituted-α-amino acids

A total of nine enantiopure syn-β-substituted-α-amino acids have been synthesised, comprising both syn-β-hydroxy-α-amino acids and syn-β-fluoro-α-amino acids. The key step in the synthetic strategy towards these syn-β-substituted-α-amino acids involves a stereospecific rearrangement, which proceeds via the intermediacy of the corresponding aziridinium ions. The requisite enantiopure syn-α-hydroxy-β-amino esters were prepared via asymmetric aminohydroxylation of the corresponding α,β-unsaturated esters followed by epimerisation of the resultant anti-α-hydroxy-β-amino esters at the C(2)-position. Subsequent activation of the α-hydroxy moiety as a leaving group followed by displacement by the β-amino substituent gave the corresponding aziridinium species. Regioselective in situ ring-opening of the aziridinium intermediates with either water or fluoride gave the corresponding syn-β-hydroxy-α-amino ester or syn-β-fluoro-α-amino ester, respectively, and N-deprotection and ester hydrolysis afforded the target syn-β-substituted-α-amino acids as single diastereoisomers in good overall yield.     

Practical method for synthesis of 2,3-disubstituted indole derivatives promoted by β-(benzotriazol-1-yl)allylic O-stannyl ketyl radicals

Treatment of β-aryl-β-(benzotriazol-1-yl)-α-primary alkyl (or aryl)-α,β-unsaturated ketones 1 with n-Bu₃SnH (4 equiv) in a catalytic amount of AIBN in PhH at reflux afforded 3-alkyl (or aryl)-2-arylindoles 8 in good yields. However, when tert-butyl group is bonded at α-position, 3-acyl (or aroyl)-2-arylindoles 9 were obtained as major products along with phenanthridines 5 as minor products.     

Diastereoselective conjugate addition of 1-(α,β-unsaturated acyl)hydantoin with nucleophiles

A diastereomeric conjugate addition of dialkylaluminum chloride to 1-(α,β-unsaturated acyl)hydantoin provided the corresponding alkyl adduct with inducted chirality in the β-position. Treatment of 1-(α,β-unsaturated acyl)hydantoin with Gilman reagent in the presence of Lewis acid also gave the same product. In this reaction, diethylaluminum chloride was the most effective Lewis acid and the absolute configuration of the major adduct at the β-position of acyl group depended on the kinds of existing metals.     

Selective 17α-Alkylation of Cortienic Acid Derivatives in a Solid-Liquid Biphasic System

11β,17α-Dihydroxy-3-oxoandrost-4-ene 17β-carboxylic acid (cortienic acid), or its methyl ester, are efficiently and selectively alkylated at the 17α-position using methyl iodide in a solid-liquid biphasic system consisting of potassium hydroxide and dimethyl sulfoxide. When a higher alkyl halide homolog is used, it is necessary to protect the α-enone function.     

Assembling of medium/long chain-based β-arylated unnatural amino acid derivatives via the Pd(II)-catalyzed sp3 β-C-H arylation and a short route for rolipram-type derivatives

In this paper, we report the assembling of libraries of β-arylated short/medium/long chain-based non-α-amino acid (aminoalkanoic acid) derivatives via the Pd(II)-catalyzed, bidentate directing group 8-aminoquinoline-aided sp³ β-C-H activation/arylation method. Short/medium chain-based unnatural amino acid derivatives containing an aryl group at the β-position are promising small molecules with therapeutic properties. Thus, it is necessary to enrich the libraries of short/medium/long chain-based unnatural amino acid derivatives containing an aryl group at the β-position. Considering the importance of β-arylated short/medium/long chain-based non-α-amino acid derivatives, an inclusive attention was paid to explore the Pd(II)-catalyzed sp³ β-C-H arylation of short/medium/long chain-based non-α-amino acids. Representative synthetic transformations including a short route for the assembling of rolipram and related compounds and 3-arylated GABA derivatives such as, baclofen, phenibut and tolibut were shown using selected β-C-H arylated non-α-amino acid derivatives.     

Synthesis of β-amino-α-trifluoromethyl-α-amino acids exhibiting intramolecular interaction of CF3 with NHβ

We prepared β-amino-α-trifluoromethyl-α-amino acids through ring-opening reaction of N-tosyl-2-trifluoromethyl-2-ethoxycarbonylaziridine with aromatic and benzylic amines, and investigated the intramolecular interaction between the trifluoromethyl (CF₃) group at the α-position and the NH group at the β-position (NH_β). NMR, UV/vis, and circular dichroism measurements indicated that the conformation of these compounds is fixed by intramolecular interaction of CF₃ with NH_β to form a six-membered ring.     

A novel synthetic method for γ-acetoxy-(E)-α,β-unsaturated esters by the palladium catalyzed regio- and stereoselective acetoxylation of β,γ-unsaturated esters

A new synthetic method for γ-acetoxy-α,β-unsaturated esters by the acetoxylation of β,γ-unsaturated esters catalyzed by PdCl₂ in the presence of KOAc and pentyl nitrite in acetic acid is presented. The reaction takes place at γ-position of the esters regioselectively with double bond migration to α,β-position. The E configuration of the double bond was confirmed by NMR analysis. Preliminary investigation to synthesize pyrenophorin and pyrenophorol using this reaction is described.     

Synthesis of γ-N-acylamino-β-keto esters and ethyl 5-oxazoleacetates via Ritter reaction and hydration of γ-hydroxy-α,β-alkynoic esters

The classical Ritter reaction on γ-hydroxy-α,β-alkynoic esters produced γ-N-acylamino-β-keto esters or ethyl 5-oxazoleacetates using alkyl or aryl nitriles, respectively. The γ-N-acylamino-β-keto esters resulting from alkyl nitriles are useful intermediates in the synthesis of a variety of building blocks. We also show that these can be converted into ethyl 5-oxazoleacetates using an additional step involving POCl₃.     

Trading N and O: asymmetric syntheses of β-hydroxy-α-amino acids via α-hydroxy-β-amino esters

Both diastereoisomers of 2-amino-3-hydroxybutanoic acid and 2-amino-3-hydroxy-3-phenylpropanoic acid have been prepared from enantiopure α-hydroxy-β-amino esters via the intermediacy of the corresponding cis- and trans-aziridines. Aminohydroxylation of two α,β-unsaturated esters produced enantiopure 2,3-anti-α-hydroxy-β-amino esters in >99:1 dr. Subsequent epimerisation at the C(2)-position via a sequential oxidation/diastereoselective reduction protocol gave the corresponding enantiopure 2,3-syn-α-hydroxy-β-amino esters in >99:1 dr. These syn- and anti-substrates were then converted into the corresponding N-Boc protected cis- and trans-aziridines, respectively, via a three step reaction sequence: (i) hydrogenolysis and in situ N-Boc protection; (ii) OH-activation; and (iii) aziridine formation. Subsequent regioselective ring-opening of the C(3)-methyl-aziridines with Cl₃CCO₂H proceeded with inversion of configuration to give the corresponding 2-amino-3-trichloroacetate esters, whereas the analogous reaction with the C(3)-phenyl-aziridines resulted in rearrangement to the corresponding oxazolidin-2-ones with retention of configuration. In each case, hydrolysis of the products from these ring-opening reactions produced the corresponding enantiopure β-hydroxy-α-amino acids as single diastereoisomers.     

A highly efficient michael addition of indoles to α,β-unsaturated electron-deficient compounds in acidic SDS micellar media

The Michael addition of indoles to α,β-unsaturated electron deficient compounds was catalyzed efficiently at room temperature in acidic micellar solution of sodium dodecyl sulfate (SDS). The substitution on the indole nucleus occurred exclusively at the 3-position in good to excellent yields, and no N-alkylation products were observed.     

Synthesis of natural ecdysteroids and structural analogs by chemical transformations of available phytoecdysteroids

Selective transformations of available phytoecdysteroids to access minor ecdysteroids and some new their structural analogs are reviewed. Among them are the one-step transformation of ecdysteroids into 2-dehydro-3-epi-ecdysteroids by oxidation with ozone in pyridine, and the catalytic hydrogenation of ecdysteroids, including conjugated diene ones under basic conditions. 7,8α-Dihydro- and 7,8α-dihydro-14α-deoxyecdysteroids were synthesized by these methods. 9α,14α-Epoxyecdysteroids with the oxetane cycle in the C ring were obtained for the first time. The acid-catalyzed rearrangement of 9α,14α-epoxyecdysteroids gave 9α,13α-epoxy derivatives with the 18-methyl group migrated to the 14β-position.     

Synthesis of δ-cyclohexyl- and δ,δ-alkylene-α,α-dicarbonyl-substituted dienes and study of their valence isomerization

β-Cyclohexylacrolein, β-cyclohexylmethacrolein, or α-cycloalkylidenalkanals were condensed with methyl acetoacetate or dimethyl malonate to give the δ-cyclohexyl- and δ,δ-alkylene-substituted α,α-dicarbonyl-containing α,β∶γ,δ-dienes. The structures of the reaction products were studied using¹H NMR,¹³C NMR, and UV spectroscopy. The diene keto esters bearing no substituents at the γ-position were shown to be in fact three-component equilibrium mixtures comprised ofE- andZ-isomers of the diene (at the α,β bond) and a corresponding 2H-pyran. On the other hand, for keto esters with a Me group at the γ-position the equilibrium is shifted entirely to the 2H-pyrans. In contrast with the keto esters, dienic diesters exist only in the open form.     

N-(α-Amidoalkyl)benzotriazole-mediated synthesis of β′-amido β-diketones: a general synthetic protocol for N-[β-(3,5-di and 1,3,5-trisubstituted pyrazol-4-yl)alkyl] amides

Various β′-amido-β-diketones were first synthesized with N-(α-amidoalkyl)benzotriazole-mediated amidoalkylation of 1,3-diketones in moderate yields. These intermediates undergo rapid condensation with hydrazines to give the corresponding N-[β-(3,5-di and 1,3,5-trisubstituted pyrazol-4-yl)alkyl]amides.     

Conjugate addition of grignard reagents to N-(α,β-unsaturated)acylpyrazoles. Diastereoselective β-alkylation using 3-phenyl-l-menthopyrazole

The conjugate additions of N-(α,β-unsaturated)acylpyrazoles were carried out by the treatment with Grignard reagents in the presence of cuprous halides. The reaction of 2-(α,β-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3a-h occurred in higher chemical yields and with asymmetric inductions on β-position, where the addition of magnesium bromide as a Lewis acid influenced to the yields and the diastereoselectivities. In the case of α-methylated 2-(α,β-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3i-n , the excellent asymmetric induction on the α-position was also observed through the diastereofacial protonation.     

An efficient synthetic method for Z-fluoroalkene dipeptide isosteres: Application to the synthesis of the dipeptide isostere of Sta-Ala

Reaction of γ,γ-difluoro-α,β-enoates having a δ-hydroxyl group with trialkylaluminum (R₃Al) was found to be promoted by CuI·2LiCl and to proceed in SN2′ manner giving rise to the α-alkylated (Z)-γ-fluoro-β,γ-enoates, while reductive defluorination of γ,γ-difluoro-α,β-enoates with Me₂CuLi followed by reaction with alkyl halides provided the corresponding (Z)-α-alkylated products in high yields. The latter reaction was applied to the preparation of the dipeptide (Z)-fluoroalkene isostere of Sta-Ala, which is the central dipeptide unit in Pepstatin, a natural inhibitor of aspartate proteases.     

Stereoselective synthesis of the 3,6-branched Fuzi α-glucans up to 15-mer via a one-pot and convergent glycosylation strategy

A family of the 3,6-branched Fuzi α-glucans including the pentasaccharide repeating unit as well as its di- and trimers were efficiently achieved via a one-pot and convergent glycosylation strategy. All the protected α-glucans up to 15-mer were assembled with high yields and excellent α-stereoselectivity, which was secured by the synergistic α-directing effects of the TolSCl/AgOTf promotion system and the steric β-facial shielding of bulky saccharide residues linked at the 6-O-position of glucosyl donors. Moreover, the 3,6-branched architecture of glycosyl donor was revealed to be more favorable for the α-selective glucosidation of primary hydroxyl group, especially in the case of large oligosaccharide acceptor. The structurally well-defined synthetic α-glucans would be useful for various biological studies.     

Thermolyse et catalyse acide d'azido-steroïdes—6. comparaison avec les reactions de photolyse

Elimination of nitrogen concerted with migration of the alkyl group overlapping the p_y orbital of the N_α atom of the chromophore N_αN_βN_γ is involved during photochemistry and thermolysis of steroidal azides. Acid catalysed decomposition was explained by alkyl migration to nitrogen, in a manner anti to N₂ elimination. Electronic and steric considerations support these different mechanisms.     

A convenient method for the preparation of chiral phosphonoacetamides and their Horner–Wadsworth–Emmons reaction

Chiral phosphonoacetamides bearing (S)-(α-methylbenzyl)benzylamine, (S,S)-bis(α-methylbenzyl)amine, l-phenylglycine methyl ester and l-phenylglycinol were easily prepared in good yield by means of the Michaelis–Arbuzov reaction of chiral bromoacetamides obtained in quantitative yield, with trimethyl phosphite, which under Horner–Wadsworth–Emmons conditions with several aryl and alkyl aldehydes under Masamune–Roush procedure using LiCl and DBU in THF or toluene gave the corresponding chiral α,β-unsaturated amides. The present procedure is a convenient and efficient methodology for the preparation of phosphonoacetamides and chiral α,β-unsaturated amides in high E-selectivity.     

An expeditious synthesis of β-pyrimidyl-α,β-didehydro-α-amino acid derivatives and pyrano[2,3-d]pyrimidines using microwave-assisted conditions

An expeditious transformation of 5-acyl-2H-pyran-2-ones with various amidines as 1,3-binucleophiles into isomerically pure (E)-α,β-didehydro-α-amino acid derivatives (DDAAD) containing the 5-pyrimidyl moiety at the β-position is described. The method was performed in ethanolic (or ethanol/water) solutions in the presence of Na₂CO₃ as a nontoxic base and under microwave-assisted conditions. When starting from the 5-ethoxycarbonyl-2H-pyran-2-one derivative and in the presence of DBU as a base the corresponding pyrano[2,3-d]pyrimidines were prepared.     

(E)-Selective Horner-Wadsworth-Emmons reaction of aryl alkyl ketones with bis(2,2,2-trifluoroethyl)phosphonoacetic acid

The stereoselective Horner-Wadsworth-Emmons reaction of aryl alkyl ketones with bis(2,2,2-trifluoroethyl)phosphonoacetic acid utilizing lithium hexamethyldisilazide in DMF afforded (E)-α,β-unsaturated carboxylic acids as the major products.