Chiral Diels–Alder reaction between cyclopentadiene and nitroso derivatives: thermal isomerisation/racemisation of the adducts

Cyclopentadiene nitroso adducts 1a and ent-1a were synthesised in good yields and good enantiomeric excess from chiral chloro-nitroso derivatives 4 and 5 in the d-mannose and d-ribose series, respectively. The thermal racemisation of these adducts occurred below room temperature. Some other chiral Diels–Alder nitroso adducts were prepared in the d-mandelic, l-prolinol and d-O-methylprolinol series and their thermal isomerisation was specified according to the N-substitution. A simple synthesis of the chiral amido-cyclopentenol (+)-2b, an important precursor for biological interesting compounds, is presented from ent-1a.     

New chiral morpholine-pyrrolidine ligands affecting asymmetric selectivity in copper catalyzed Henry reaction

In this study, several chiral diamine (35, 36, 37 and 39), triamine (24 and 25), diaminol (38, 42 and 43) and triaminol ligands (29 and 30) having chiral morpholine and pyrrolidine moieties conjugated constitutes, were prepared from commercially available (S)-2-amino-2-phenylacetic acid and (S)-proline. These ligands were complexed with copper (II) salt in situ and applied to asymmetric Henry reaction. Asymmetric addition of various structurally divergent aldehydes and nitromethane in the presence of 5?mol% of chiral ligand with CuCl₂ furnished corresponding β-nitro alcohol in good yields (up to 55–79%) with good enantioselectivity (up to 89%).     

Study on enantiomerically pure 2-substituted N,N-dialkyl-1-naphthamides: resolution,absolute stereochemistry,and application to desymmetrization of cyclic meso anhydrides

The axially chiral 2-substituted N,N-diisopropyl-1-naphthamides 1 and 2 were resolved by HPLC over a chiral stationary phase to provide enantiomerically pure atropisomers. The absolute stereochemistry of (−)-syn-1 was determined by X-ray crystallographic analysis of the corresponding (1S)-camphanic acid ester derivative. Desymmetrization of cyclic meso anhydrides 5a and 5b using (−)-syn-1 gave a single diastereomer in good yield.     

Boron enolates of a hydantoin chiral auxiliary derived from l-phenylalanine: a versatile tool for asymmetric aldol reactions

The aldol reactions of boron enolates derived from a hydantoin chiral auxiliary derived from l-phenylalanine occur in good yields with high syn diastereoselectivity. Aldol adduct 4a is readily cleaved by hydrolysis to afford (2S,3S)-3-hydroxy-2-methyl-3-phenylpropionic acid 5a in good yield and in almost enantiomerically pure form.     

Chiral thiophosphoramide catalyzed asymmetric aryl transfer reactions for the synthesis of functional diarylmethanols

In this investigation, chiral thiophosphoramide 3d was easily prepared from chiral (1R,2R)-1,2-diphenylethylenediamine and then applied as an efficient chiral ligand in the catalytic asymmetric arylation reactions of various aromatic aldehydes. The corresponding diarylmethanol products were produced with good to excellent yields (up to 98%) and enantioselectivities (up to 94%). The recovery of chiral ligand 3d could be as high as 96%.     

Asymmetric zinc-Reformatsky reaction of Evans chiral imide with acetophenones and its application to the stereoselective synthesis of triazole antifungal agents

The Ni(acac)₂ catalytic ZnEt₂-mediated asymmetric Reformatsky-type reaction of Evans chiral imide with various acetophenones was studied. The chiral imido zinc enolate, which was formed through the metal–halogen exchange reaction of chiral α-bromopropionyl-2-oxazolidinones 2 with diethyl zinc under the catalysis of Ni(acac)₂, performed the asymmetric zinc-Reformatsky reaction with activated α-haloacetophenones 3 to give the chiral β-hydroxyamide 4 in good yields and high ratios of syn-(2R,3R)-isomers (up to >97%). This new asymmetric synthesis technology affords a practical method to synthesize the versatile chiral building block 5 for triazole antifungal agents, such as Voriconazole, Ravuconazole, TAK-187, and RO-0094815.     

Axially chiral C2-symmetric N-heterocyclic carbene (NHC) palladium complexes-catalyzed asymmetric arylation of aldehydes with arylboronic acids

Chiral C₂-symmetric N-heterocyclic carbene (NHC) palladium diaquo complexes 5a–c and the chiral C₂-symmetric NHC-palladium complexes 5d and 5e prepared from (R)-BINAM or H₈-(R)-BINAM could be used as the catalysts for the enantioselective arylation of arylaldehydes with arylboronic acids in which NHC-Pd complex 5a was found to be fairly effective in this reaction to give the corresponding adducts in moderate enantioselectivities along with moderate to good yields.     

Highly enantioselective synthesis and potential biological activity of chiral novel nucleoside analogues containing adenine and naturally phenol derivatives

This paper described an efficient synthetic strategy for chiral acyclic nucleoside analogues containing both the phenoxy components of some bioactive natural compounds and a heterocyclic base. The phenoxy components with adenine moiety were incorporated into the chiral acyclic nucleoside analogues through two key synthetic tactics. Chiron 5-(R)-menthyloxy-2(5H)-furanone 5 was obtained in good yield from the cheap starting material furfural via a valuable synthetic route. The asymmetric Michael addition of 5 with adenine and the subsequent reduction reaction afforded the key chiral intermediate, 2-(R)-(9′-adeninyl)-1,4-butanediol 8. The absolute configuration of 8 was established by X-ray crystallography. The intermolecular dehydration reaction between 2-(9′-adeninyl)-1,4-butanediol 8 and phenoxy components 9 on treatment with diethyl azodicarboxylate and triphenylphosphine was carried out to give the chiral acyclic nucleoside analogues 1a-1e. The regioselectivity of the reaction was established by NMR methods, especially through ¹³C NMR shifts and NOE effect observed in the target molecule 1c, as well as by HMBC/HMQC experiments. The target compounds were tested for inhibition of cytopathogenicity against different cancer cells and exhibited potential anticancer activity.     

Self-assembly of enantiopure and racemic 2,2′,6,6′-tetrasubstituted biaryl tectons into hydrogen bonded chiral squares and infinite chiral square layers

Both chiral and achiral hydrogen bonding o-,o-,o′-,o′-tetrasubstituted biaryls often exhibit unique self-assembling properties giving rise to chiral square cyclotetramers. The individual chiral squares self-assemble further with the formation of infinite layers which are either achiral or are chiral (homochiral). A priori analysis reveals various hydrogen bonding modes which may take part in the self-assembly, differing each from the other by structure and symmetry of the resulting squares and layers. Results of the theoretical analysis are compared with the experimental findings obtained from X-ray crystallographic analysis of three novel and two already known isosteric biaryl tectons, (S)-2a, (RS)-2a, 3a, (S)-5 and (RS)-5, respectively.     

Novel chiral dithioethers derived from l-tartaric acid

The synthesis of a new family of systematically modified chiral dithioethers to be used as ligands is described. Phenylthioether derivative 5 and fluorine-containing dithioether ligands 6–8 and 13–15 were prepared by direct reaction of phenylthiol and o-, m- or p-fluorophenylthiol with two different ditriflate derivatives based on the l-tartaric skeleton. The chiral ditriflate 12 containing a dioxolane moiety was reacted with ethane- and propanedithiol, producing cyclic dithioethers 16 and 17, respectively, in good yields (≈50%). The analogous ditriflate 4 with benzyl ether protecting groups, having a skeleton without restricted rotation, gave the thiolane 9 as the main product.     

A flexible enantioselective approach to 3,4-dihydroxyprolinol derivatives by SmI2-mediated reductive coupling of chiral nitrone with ketones/aldehydes

A flexible enantioselective approach to polyhydroxylated prolinol derivatives was described, which is based on the samarium diiodide-mediated reductive coupling of the chiral nitrone (3S,4R)-8, derived from d-isoascorbic acid with aldehydes/ketones. Thereby, polyhydroxyprolinol derivatives 9a–e and 9h–j were obtained from aromatic ketones and aliphatic aldehydes in good to excellent yields of 65–91%. These reductive hydroxyalkylations are highly diastereoselective in establishing the C-4 stereogenic center. By this way, the asymmetric syntheses of (?)-8a-epi-swainsonine (4) and (?)-8,8a-di-epi-swainsonine (5) have been achieved.     

Stereoselective alkylation of C2-symmetric chiral N-phthaloylglycinamides in the preparation of enantiopure α-amino acids

The novel, chiral glycinamides (S,S)-3 and (S,S)-4 were prepared in good yields from C₂-symmetric chiral amines (S,S)-1 and (S,S)-2, respectively. Enolate formation and addition to methyl iodide and benzyl bromide proceeded in good yield and high diastereoselectivity, especially in the presence of LiCl or DMPU. Removal of the phthaloyl protecting group with hydrazine, followed by hydrolysis with 6N HCl, converted the benzylated product (S,S,S)-7 to enantiopure (S)-phenylalanine.     

Dual chiral silver catalyst in the synthetic approach to the core of hepatitis C virus inhibitor GSK 625433 using enantioselective 1,3-dipolar cycloaddition of azomethine ylides and electrophilic alkenes

The asymmetric 1,3-dipolar cycloaddition of an imino ester 5 with tert-butyl acrylate is catalyzed by a dual chiral silver(I) complex formed from a chiral phosphoramidite 14 and the chiral silver(I) binolphosphate (R)-17. This reaction is selected to achieve the synthesis of enantiomerically enriched key structures to access the third generation of GSK HCV inhibitors. The scope of this dual chiral catalytic system is analyzed by employing different imino esters and dipolarophiles, and also compared with the same cycloaddition reactions performed with the chiral phosphoramidite 14·AgClO₄ complex.     

An efficient synthesis of chiral homophenylalanine derivatives via enantioselective hydrogenation

Chiral homophenylalanine derivatives were synthesized via enantioselective hydrogenation of 5a and 5b catalyzed by rhodium complexes bearing chiral phosphine and phosphinite legands. Enantiomeric excesses up to 96.2% were achieved when S-spiroOP(S-1) was used as a chiral ligand under 500 psi of H₂ pressure in acetone.     

(S)-(−)-α-Methylbenzylamine as an efficient chiral auxiliary in enantiodivergent synthesis of both enantiomers of N-acetylcalycotomine

(S)-(−)-α-Methylbenzylamine 2 was used as a chiral auxiliary in the enantiodivergent synthesis of simple isoquinoline alkaloids. The prochiral imine moiety in compound 4 was reduced with different reagents, giving diastereomeric amines 5a or 5b, which subsequently were transformed to either (S)-(−)-N-acetylcalycotomine 6 or (R)-(+)-N-acetylcalycotomine ent-6 in good enantiomeric excess. ¹⁹F NMR of its Mosher's acid ester was used to establish the purities of final compounds.     

Two approaches to the enantioselective synthesis of (4R)-(−)-4-hydroxymethyl-4-thiobutyro-1,4-lactone

Enantiomerically pure (4R)-4-hydroxymethyl-4-thiobutyro-1,4-lactone [(5R)-dihydro-5-(hydroxymethyl)-2(3H)-thiophenone (12)] and derivatives were synthesized by two enantiospecific sequences employing d-ribono-1,4-lactone (1) and l-glutamic acid (6) as chiral templates. The key step in the first approach was the SmI₂-promoted 2,3-deoxygenation of a 4-thio-l-lyxono-1,4-lactone derivative, prepared from 1. The other strategy, which starts from 6, involves the (5S)-dihydro-5-(p-tolylsulfonyloxymethyl)-2-(3H)-furanone (8) as chiral precursor. This was converted into a 4,5-thiirane derivative via the corresponding 4,5-epoxide. Regioselective opening of the thiirane ring by acetate followed by O-deacetylation gave 12 (40% overall yield from 8).     

Enantioselective synthesis of β-amino acids. Part 9: Preparation of enantiopure α,α-disubstituted β-amino acids from 1-benzoyl-2(S)-tert-butyl-3-methylperhydropyrimidin-4-one,

Double alkylation of enantiopure N,N-acetal pyrimidinone (S)-1, a masked chiral derivative of β-alanine prepared from (S)-asparagine, proceeds with high stereoselectivity to give C(5) disubstituted adducts (2S,5R)-6, (2S,5S)-6, (2S,5R)-7, and (2S,5S)-7. Acid hydrolysis of these derivatives affords enantiopure α,α-dialkylated β-amino acids (R)-8, (S)-8, (R)-9, and (S)-9 in very good yields.     

First enantioselective reductive amination of α-fluoroenones

From α-fluoroenones 2, a synthesis of (E) ketone oxime O-alkyl ethers 5 is reported with good to excellent yields. Then the first enantioselective reduction of these ketimines, via oxazaborolidine, is described with moderate to good enantiomeric excesses, leading to valuable chiral fluoroallylic amines 1.     

Enantioselective addition of diethylzinc to aldehydes catalyzed by γ-amino alcohols derived from (+)- and (−)-α-pinene

Primary, secondary and tertiary γ-amino alcohols 4, 5, 7 and 9 and 1,3-diamine 6 were synthesized from (+)- and (−)-α-pinene 1 via chiral N-Boc β-amino ester 3a and carboxamide 3b. The amino alcohols and diamine obtained were applied as chiral catalysts in the enantioselective addition of diethylzinc to aromatic aldehydes, resulting in chiral 1-aryl-1-propanols. The first evidence of the substituent-dependent enantioselectivity of 1,3-amino alcohol catalysts was observed, and the phenomenon interpreted by using molecular modelling at the ab initio level.     

Synthesis of new chiral N-arylsulfonyl-1,3-oxazolidin-2-ones from α-amino acids

A variety of new chiral N-arylsulfonyl-1,3-oxazolidin-2-ones were prepared in three steps starting from (d)- and (l)-amino acid. N-Arylsulfonyl amino alcohols, derived from amino acids, were carbonylated with the bis-(trichloromethyl) carbonate (BTC), in the presence of triethylamine, to provide optically pure N-phenylsulfonyloxazolidin-2-ones 3a–f, N-naphthylsulfonyloxazolidin-2-ones 3g–j and N-tosylsulfonyloxazolidin-2-ones 3k–p in good yields.