Stereoselective synthesis of conjugated all-trans-tetraenes. Application to the synthesis of β-parinaric acid methyl ester

in this paper is reported the stereoselective synthesis of all-trans-tetraenes by reductive elimination of 1,8-dibenzoate-2,4,6-trienes with sodium amalgam. The method was applied to the syntheses of 4E, 6E, 8E, 10E-heptatetraene and β—parinaric acid methyl ester.     

A new access to β-methyl substituted secondary alcohols. Application to the synthesis of 4-methylheptan-3-ol

All four stereoisomers of 4-methylheptan-3-ol were synthesized by a five-step route starting from 4-thianones. Key steps in the synthesis include: (a) reduction of 3-propyl-4-thianone to yield an easily separable isomeric mixture of cis- and trans-3-propyl-4-thianols; and (b) a highly efficient resolution of the particular cis/trans-isomers through a chromatographic separation of their respective esters with (S)-chlorofluoroacetic acid. Subsequent hydrolysis and desulfurization gave the required compounds in 18% overall yield. All the stereoisomers are obtained with purities better than 90%.     

Synthesis of β,β′-diamino acids from α-amino acid-derived β-lactams by ring opening with nucleophiles. Utilization in the synthesis of peptidomimetics

Ring opening of protected 3-aminoalkyl-substituted azetidin-2-ones with O-, N- or S-nucleophiles led to β,β′-diaminocarboxylic esters, amides and thioesters, respectively. The reaction outcome is improved by the addition of catalytic amounts of sodium azide. Utilization of a glycine derivative with unprotected amino function as nucleophile was possible. When bulkier amino acid esters were used, the intermediate acid azide underwent a Curtius rearrangement. The isocynates formed were trapped as the corresponding urea derivatives. Reduction of β-lactam's amide moiety led to diaminoalcohols.     

Synthesis of 4,4-dialkoxy-3-piperidinols. Application to the synthesis of γ-acetate dehydropipecolinonitrile

The synthesis of 4,4-dialkoxy-3-piperidinols 7 was carried out by the α-bromination of piperidin-4-one 5 with N-bromosuccinimide in acetic acid and alkoxide ion-mediated α,α-dialkoxyhydroxylation. Under acidic condition, trimethyl orthoformate-mediated reaction of compound 7a yielded aminodienylester 8 in the presence of Ph₃PCHCO₂Et. The γ-acetate dehydropipecolinonitrile 4 was also synthesized via boron trifluoride etherate-promoted addition of compound 8 with trimethylsilyl cyanide and N-bromosuccinimide and selective hydrogenation.     

Synthesis of β-silyl ketones via reaction of trimethylsilyllthium with unsaturated hydrazones. Application to the synthesis of brevicomin

Trimethylsilyllithium undergoes conjugate addition to dimethylhydrazones of α,β-unsaturated ketones. The resulting metalloenamine intermediates can be alkylated, and the resulting hydrazone products cleaved to give α-substituted-β-silyl ketones. These reactions, together with a silicon-directed Baeyer-Villiger reaction, have been applied to the synthesis of brevicomin.     

Totally stereocontrolled synthesis of α,β-diamino acids by addition of Grignard reagents to nitrones derived from l-serine

The asymmetric synthesis of protected (2R,3S)- and (2R,3R)-3-substituted 2,3-α-amino acids is reported. The key step in the synthesis of these compounds is the diastereoselective addition of Grignard reagents to α-amino nitrones derived from l-serine. Total stereocontrol of the addition step is achieved by changing the protecting groups in the starting material. The predominant selectivity in each case can be reasonably interpreted in terms of steric effects of the substituents.     

Catalyzed synthesis of β-amino acids esters

Russian Journal of Organic Chemistry -     

Enantioselective synthesis of β-hydroxy carboxylic acids: direct conversion of β-oxocarboxylic acids to enantiomerically enriched β-hydroxy carboxylic acids via neighboring group control

β-Oxocarboxylic acids can be reduced to the corresponding β-hydroxy carboxylic acids employing DIP-Cl^™ as a reducing agent. The β-carboxylic substituent exerts a remarkable neighboring group effect on the reduction. The reaction presumably proceeds in an intramolecular fashion through a `rigid' bicyclic transition state assembly, which produces enantioselectivities approaching 99%.     

The asymmetric synthesis of β-lactam antibiotics-v. Application of chiral α,β-epoxyimines in ketene-imine cycloaddition reactions leading to homochiral 3-aminoazetidinones.

Imines derived from chiral α,β-epoxyaldehydes have been shown to be useful chiral glyoxal imine synthons in the ketene-imine cycloaddition reaction. This process, which proceeds with high levels of reaction diastereoselection, affords enantiomerically pure cis-substituted 3-amino-4-alkylazetidinones in good yields.     

α-Bromo-β,γ-unsaturated ketenes for the synthesis of α-benzylamino-β,γ-unsaturated acids

The synthesis of α-benzylamino-β,γ-unsaturated acids has been developed starting from α-bromo-α,β-unsaturated chlorides. Via treatment of the acyl chlorides with (R)-pantolactone in the presence of TEA, the in situ formation of the deconjugated ketenes and their direct transformation into chiral esters was performed. The substitution of bromine with benzylamine, followed by acid hydrolysis, allowed to us obtain enantiomerically enriched α-benzylamino-β,γ-unsaturated acids.     

A new enantioselective synthesis of β-amino acids

Enantioselective hydrogenation of some α,β-unsaturated nitriles and their corresponding methyl esters bearing a phthalimidomethyl substituent at the α-carbon using Rh-DuPHOS catalysts afforded β-amino acid precursors with modest e.e.s of up to 48%. Hydrogenation of the α,β-unsaturated methyl esters using a Ru-BINAP catalyst gave higher e.e.s of up to 84%. Method development for the determination of the enantiomeric excesses of these derivatives using chiral HPLC is also reported.     

Asymmetric synthesis of β-fluoroaryl-β-amino acids

The conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide to a range of β-fluoroaryl-α,β-unsaturated esters gave the corresponding β-amino esters with high diastereoselectivity and in good isolated yields. Sequential treatment of the resultant β-fluoroaryl-β-amino esters under optimised hydrogenolysis conditions, followed by ester hydrolysis with 2.0 M aq HCl, provided access to a range of β-fluoroaryl-β-amino acids in good yield.     

Synergistic chiral ion pair catalysts for asymmetric catalytic synthesis of quaternary α,β-diamino acids

The combination of a chiral phosphate anion with a silver ion has been demonstrated as a powerful and synergistic ion pair catalyst for the aza-Mannich reaction. A series of valuable quaternary α,β-diamino acid derivatives was obtained in high yield, and with excellent diastereo- (up to 25:1 dr) and enantioselectivity (up to 99% ee). The adducts can be smoothly transformed into the corresponding protected chiral quaternary α,β-diamino acids by a one-pot hydrolysis reaction.     

Application of the addition of triorganozincates to N-(tert-butanesulfinyl)imines to the enantioselective synthesis of α-amino acids

Highly enantiomerically enriched N-protected α-amino acids can be easily prepared from optically pure N-(tert-butanesulfinyl)imines by a four-step sequence involving: diastereoselective addition of a triorganozincate to the imine, removal of the sulfinyl group, benzoylation of the nitrogen atom of the obtained primary amine and oxidation of one of the substituents on the carbon atom α to the nitrogen. Using the same configuration in the sulfinyl chiral auxiliary, amino acids with the (R) or the (S) configuration can be prepared by choosing the proper combination of imine and organozincate. α,α-Disubstituted α-amino esters with high optical purity can also be prepared by the diastereoselective addition of trialkylzincates to α-imino esters.     

Efficient synthesis of functionalized 1,3-dihydroisobenzofurans from salicylaldehydes:Application to the synthesis of escitalopram

An efficient synthesis of substituted 1,3-dihydroisobenzofurans is developed. In this novel route, oaroylbenzaldehydes, as key intermediates, can be obtained by lead tetraacetate oxidation of Naroylhydrazones of salicylaldehydes. The mild and general strategy enables the synthesis of various substituted 1,3-dihydroisobenzofurans in high yields. Moreover, this method can be applied to efficiently synthesize escitalopram.     

Efficient three-step sequence for the deamination of α-aminoesters. Application to the synthesis of CysLT1 antagonists

A practical and efficient three-step sequence for the deamination of α-aminoesters is reported. This method is based on the NaBH₄-mediated selective reduction of α-diazoesters to α-hydrazonoesters and has been successfully applied to the deamination of several representative α-aminoesters including some l-cysteine ethyl ester derivatives, key intermediates in the synthesis of a series of CysLT1 antagonists.     

Enantioselective synthesis of β-amino acids. Part 10: Preparation of novel α,α- and β,β-disubstituted β-amino acids from (S)-asparagine

Perhydropyrimidinone (S)-1 is alkylated with very high diastereoselectivity to give trans products (2S,5R)-3, (2S,5R)–4 and (2S,5R)-5. Dialkylation of (S)-1 also proceeds with complete stereoselectivity to afford adducts (2S,5R)-6, (2S,5S)-6, (2S,5R)-7 and (2S,5S)-7. Hydrolysis (6N HCl, 100°C) of monoalkylated derivative (2S,5R)-3 gives enantiopure α-substituted β-amino acid (R)-8. Hydrolysis of dialkylated adducts 6 and 7 affords enantiopure α,α-disubstituted β-amino acids (R)- or (S)-9 and (R)- or (S)-10. Related iminoester (2S,6S)-2 is alkylated with complete diastereoselectivity to give products (2S,6S)-11–13 whose hydrolysis under relatively mild conditions (2N CF₃CO₂H, CH₃OH, 100°C) affords enantiopure N-benzoylated β,β-disubstituted β-amino acid esters (S)-14–16, with intact double bonds in the olefinic substituents.     

Samarium iodide-mediated Reformatsky reactions for the stereoselective preparation of β-hydroxy-γ-amino acids: synthesis of isostatine and dolaisoleucine

The synthesis of β-hydroxy-γ-amino acids via SmI(2)-mediated Reformatsky reactions of α-chloroacetyloxazolidinones with aminoaldehydes is reported. Diastereoselective coupling is demonstrated to depend on the absolute configuration of the Evans chiral auxiliary employed in the reaction, allowing erythro or threo products to be obtained selectively. The potential utility of the methodology is exemplified by the facile synthesis of biologically relevant N-Boc-isostatine (2b) and N-Boc-dolaisoleucine (3c).