Concise,efficient and highly selective asymmetric synthesis of (+)-(3S,4R)-cisapride

A concise asymmetric synthesis of the gastroprokinetic agent (+)-(3S,4R)-cisapride {(+)-(3S,4R)-N(1)-[3′-(4″-fluorophenoxy)propyl]-3-methoxy-4-(2″′-methoxy-4″′-amino-5″′-chlorobenzamido)piperidine} from commercially available starting materials has been developed. The key step of this synthesis employs the diastereoselective conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 5-[N-3′-(4″-fluorophenoxy)propyl-N-allylamino]pent-2-enoate and in situ enolate oxidation with (?)-camphorsulfonyloxaziridine to set the (3S,4R)-configuration found within the piperidine ring of the product. This synthesis proceeds in 9 steps from commercially available 1-(4′-fluorophenoxy)-3-bromopropane with an overall yield of 19%.     

Stereoselective total synthesis of almorexant

A highly stereoselective synthesis of almorexant has been achieved using (R)-tert-butanesulfinamide as a chiral source. The chiral tetrahydroisoquinoline core was constructed through allylation of chiral N-sulfinyl imine followed by ring closure of the secondary amide with a tethered halide. The chiral α-phenyl amide was introduced by means of S_N2 substitution of (S)-methyl 2-phenyl-2-(tosyloxy)acetate with chiral tetrahydroisoquinoline.     

Synthesis of pseudopeptidic (S)-6-amino-5-oxo-1,4-diazepines and (S)-3-benzyl-2-oxo-1,4-benzodiazepines by an Ugi 4CC Staudinger/aza-Wittig sequence

Sequential Ugi reaction between p-substituted arylglyoxals, alkylamines, cyclohexyl isocyanide and 3-azido-(S)-2-(tert-butoxycarbonylamino)propanoic acid, followed by a Staudinger/aza-Wittig cyclization in the presence of triphenylphosphine, gave rise to enantiomerically pure N-cyclohexyl 4-alkyl-2-aryl-5-oxo-(S)-6-(tert-butoxycarbonylamino)-4,5,6,7-tetrahydro-1H-1,4-diazepine-3-carboxamides, that can be useful for new drug design. By the same sequence, p-substituted benzaldehydes, 2-aminobenzophenone, cyclohexyl isocyanide and (S)-3-phenyl-2-azidopropionic acid gave rise to N-cyclohexyl 2-((S)-3-benzyl-2-oxo-5-phenyl-2,3-dihydro-1H-benzo[e][1,4]diazepin-1-yl)-(R/S)-2-arylacetamides.     

Stereoselective synthesis of protected (2S,3S)-N-methyl-5-hydroxyisoleucine, a component of halipeptins

The stereocontrolled synthesis of the protected (2S,3S)-N-methyl-5-hydroxyisoleucine, a component of halipeptins A and B with potent anti-inflammatory activity, has been achieved. The key steps include (i) installation of a double bond to bicyclic lactam 4 using N-tert-butyl phenylsulfinimidoyl chloride, (ii) highly exo-selective Michael reaction with lithium dimethylcuprate in the presence of chlorotrimethylsilane, and (iii) Ru-catalyzed oxidative deprotection of N,O-benzylidene acetal to the acid anhydride.     

Enantioselective synthesis of (1S,2S)-1,2-di-tert-butyl and (1R,2R)-1,2-di-(1-adamantyl)ethylenediamines

An enantioselective synthesis of sterically congested 1,2-di-tert-butyl and 1,2-di-(1-adamantyl)ethylenediamines has been developed. Thus, diastereomerically pure trans-1-apocamphanecarbonyl-4,5-dimethoxy-2-imidazolidinones 6 and 7 were successfully prepared by optical resolution of (±)-trans-4,5-dimethoxy-2-imidazolidinone using apocamphanecarbonyl chloride (MAC-Cl) followed by stereospecific and stepwise substitution of the dimethoxyl groups using tert-butyl or 1-adamantyl cuprates to provide (4S,5S)-4,5-di-tert-butyl and (4R,5R)-4,5-di-(1-adamantyl)-2-imidazolidinones 12 and 15, respectively. Furthermore, N-acetyl 4,5-di-tert-butyl and 4,5-di-(1-adamantyl)-2-imidazolidinones 16a,b were enantioselectively deacetylated using a catalytic oxazaborolidine system to provide enantiopure 1-p-tolylsulfonyl-4,5-di-tert-butyl-2-imidazolidinones 12 and 19 and 1-p-tolylsulfonyl-4,5-di-(1-adamantyl)-2-imidazolidinones 18 and 20, respectively. Finally, N-p-tolylsulfonyl-2-imidazolidinones 12 and 15 were treated with 30 equiv of Ba(OH)₂·8H₂O to achieve ring cleavage and to provide (1S,2S)-1,2-di-tert-butylethylenediamine 3 and (1R,2R)-1,2-di-(1-adamantyl)ethylenediamine 4.     

Reaction of sulfene with heterocyclic N,N-disubstituted α-aminomethvlene ketones V. Synthesis of 1,2-oxatliiino[5.6-c] pyridine and 1,2-oxathiino[5,6-c] quinoline derivatives

Reaction of sulfene with N,N-disubstituted 3-aminomethylene-1-(methyl, methylphenyl, phenyl)-4-piprridones and 3-aminomethylene-2,3-dihydro-1-plumy 1–4(1H) quinolones gave N,N-disubstituted 4-amino-3,4,5,6,7.8-hexahydro-6-(methyl, methylphenyl, phenyl)-1,2-oxathiino-[5,6-c] pyridine 2,2-dioxides and 4-amino-6-phenyl-3,4,5,6-tetrahydro-1,2-oxathiino[5,6-c]quinoline 2,2-dioxides, respectively, whereas N,N-disubstituted 3-aminomethylene-2,3-dihydro-1-methyl-4(1H) quinolones did not react. Slow air oxidation in the cold of intermediates 2,3-dihydro-3-hydroymethyIene-1-(methyl, phenyl)-4(1H) quinolones gave the corresponding 1-substituted 1,4-dihydro-4-oxo-3-quinolinecarboxyaldehydes.     

Base dependent purported synthesis of congested 2-aminobenzylamines and tetrahydro-2-oxoquinazolines from 2-pyranones

An expeditious and concise synthesis of highly congested 2-amino-3-aminomethyl-5-methylsulfanyl/-sec-aminobiphenyl-4-carbonitrile 4 and 1-tert-butoxycarbonyl-6-sec-amino-8-aryl-5-cyano-2-oxo-1,2,3,4-tetrahydroquinazoline 5 has been delineated through base catalyzed ring transformation of 6-aryl-4-methylsulfanyl/-sec-amino-2H-pyran-2-one-3-carbonitrile 1 with 1,3-bis(tert-butoxycarbonylamino)-2-propanone 2, followed by TFA catalyzed hydrolysis of the intermediate [3-tert-butoxycarbonylaminomethyl-4-cyano-5-methylsulfanyl/-sec-aminobiphenyl-2-yl]carbamic acid tert-butyl ester 3 in moderate yield. The mechanism of formation of 5 has been established through isolation and transformation of the intermediate 3 to the 1-tert-butoxycarbonyl-6-sec-amino-8-aryl-5-cyano-2-oxo-1,2,3,4-tetrahydroquinazoline.     

Synthesis of chiral β-3-amino-2-hydroxyalkanoates and 3-alkyl-3-hydroxy-β-lactams by double asymmetric induction

Reactions of chiral (2S)-enolates of dioxolan-4-ones, derived from lactic, mandelic, and phenyllactic acids, with aliphatic (S_S)- and (S_R)-tert-butylsulfinyl aldimines afforded conformationally restrained C2-disubstituted N,O-orthogonally protected 3-amino-2-hydroxyalkanoates in the form of N-sulfinyl protected 1′-aminodioxolan-4-ones. The product distribution showed that there is significant kinetic selectivity, due to the presence of ‘matched’ and ‘mismatched’ components, between the (S)- or (R)-tert-butylsulfinyl aldimines and the (2S)-enolates of the 1,3-dioxolan-4-ones. Selective methoxide-induced removal of the acetal group of the N-sulfinyl-1′-aminodioxolanones yielded the corresponding N-sulfinyl protected methyl alkanoates. In addition, the selective acid-induced removal of the sulfinyl group of the N-sulfinyl-1′-aminodioxolanones provided the corresponding N-unprotected 1′-aminodioxolanones, whose base-induced cyclization afforded the corresponding β-lactams.     

Synthesis of tricyclic isoindoles and thiazolo[3,2-c][1,3]benzoxazines

The thermolysis of (3R,9bS)-5-oxo-2,3,5,9b-tetrahydrothiazolo[2,3-a]isoindole-3-carboxylic acids in Ac₂O led to novel 3-methylene-2,5-dioxo-3H,9bH-oxazolo[2,3-a]isoindoles and chiral (9bS)-5-oxo-2,3,5,9b-tetrahydrothiazolo[2,3-a]isoindoles were obtained on FVP. Starting from l-cysteine methyl ester (3R,10bR)-5-oxo-2,3-dihydro-10bH-[1.3]thiazolo[3,2-c][1,3]benzoxazines were obtained as single stereoisomers. The thermolysis of (3R,10bR)-5-oxo-2,3-dihydro-10bH-[1.3]thiazolo[3,2-c][1,3]benzoxazine-3-carboxylic acid in Ac₂O gave 5-acetyl-2-phenyl-2,3-dihydrothiazole. The structures of methyl (3R,9bS)-5-oxo-2,3,5,9b-tetrahydrothiazolo[2,3-a]isoindole-3-carboxylate 1a and methyl (2R,4R)-N-chlorocarbonyl-2-(2-hydroxyphenyl)thiazolidine-4-carboxylate 9 were determined by X-ray crystallography.     

Catalytic enantioselective synthesis of atropisomeric 2-aryl-4-quinolinone derivatives with an N–C chiral axis

In the presence of an (R)-MOP-Pd₂(dba)₃ catalyst, the reaction of ortho-tert-butylaniline with 2-bromophenyl arylethynyl ketone proceeded via a tandem amination (1,4-addition of aniline to an ynone and subsequent intramolecular Buchwald–Hartwig amination) to afford axially chiral N-(2-tert-butylphenyl)-2-aryl-4-quinolinone derivatives with moderate enantioselectivity (up to 72% ee).     

Reactions of 3-(polyfluoroacyl)chromones with hydroxylamine. The first synthesis of 3-cyano-2-(polyfluoroalkyl)chromones

Reaction of 3-(polyfluoroacyl)chromones with hydroxylamine proceeds via nucleophilic 1,4-addition followed by opening of the pyrone ring and subsequent cyclization to 4-(polyfluoroalkyl)-4H-chromeno[3,4-d]isoxazol-4-ols in good yields. On treatment with trifluoroacetic acid, the isoxazole ring of this annulated heterocyclic system opens to give 3-cyano-2-(polyfluoroalkyl)chromones. Reaction of 3-(polyfluoroacyl)chromones with hydroxylamine hydrochloride occurs only at the carbonyl carbon atom connected to the R^F group to give the corresponding oximes in low yields.     

Structural revision of shatavarins I and IV, the major components from the roots of Asparagus racemosus

The two major steroidal saponins from the roots of Asparagus racemosus were isolated by RP-HPLC and their structure determined by extensive NMR studies. Their structures did not match those reported previously for shatavarins I and IV and were found to be 3-O-{[β-d-glucopyranosyl(1→2)][α-l-rhamnopyranosyl(1→4)]-β-d-glucopyranosyl}-26-O-(β-d-glucopyranosyl)-(25S)-5β-furostan-3β,22α,26-triol and 3-O-{[β-d-glucopyranosyl(1→2)][α-l-rhamnopyranosyl(1→4)]-β-d-glucopyranosyl}-(25S)-5β-spirostan-3β-ol.     

Boron trifluoride-induced reactions of phenylglycidyl ethers: a convenient synthesis of enantiopure, stereodefined fluorohydrins

Ring-opening hydrofluorination of enantiomerically pure (2S,3S)-3-arylglycidyl ethers (aryl = phenyl, 4-trifluoromethylphenyl) by boron trifluoride-diethyl ether under mild conditions provides β-fluoro alcohols in good yield in a stereospecific manner with complete regiocontrol.     

N-Alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole-containing organic dyes for efficient dye-sensitized solar cells

Two new organic sensitizers, 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid and 2-cyano-3-(6-(4-(diphenylamino)phenyl)-4-(4-(hexyloxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)acrylic acid, consisting of electron donating (triphenylamine) and electron accepting (cyanoacrylic acid) functionalities linked by two different rigidified π-spacers, N-alkyl- and N-aryl-dithieno[3,2-b:2′,3′-d]pyrrole, were designed, synthesized and applied for dye-sensitized solar cells, respectively. The materials were successfully synthesized through Knoevenagel condensation reactions. Ultraviolet–visible absorption spectra revealed that the use of either of rigidified π-spacer resulted in similar charge transfer transition, however, enhanced spectral response was observed when compared with an oligothiophene analogue. In terms of their photovoltaic performance, new dyes outperformed the reference bithiophene sensitizer when tested with nitrile-based and ionic liquid-based electrolytes.     

Stereoselective synthesis of both enantiomers of trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid using a chiral pool approach and their incorporation in dipeptides

The stereoselective synthesis of (1R,2R)- and (1S,2S)-trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid has been accomplished in six steps starting from (2S)- and (2R)-β-benzyl N-(tert-butoxycarbonyl)aspartate, respectively. The key-step in the reaction sequence is a stereoselective base-induced ring closure with a good trans diastereoselectivity. These novel trans-β-ACC derivatives could be incorporated in dipeptides employing a standard peptide coupling technique.     

Silicon-carbon unsaturated compounds. 73. Photolysis of cis- and trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane in the presence of tert-butyl alcohol and acetone

Irradiation of cis-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane (1a) in the presence of tert-butyl alcohol in hexane with a low-pressure mercury lamp bearing a Vycor filter proceeded with high stereospecificity to give cis-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2a), in 33% isolated yield, together with a 15% yield of 1-[(tert-butoxy)methylphenylsilyl]-4-(methylphenylsilyl)butane (3). The photolysis of trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane (1b) with tert-butyl alcohol under the same conditions gave stereospecifically trans-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2b) in 41% isolated yield, along with a 12% yield of 3. Similar photolysis of 1a and 1b with tert-butyl alcohol-d₁ produced 2a and 2b, respectively, in addition to 1-[(tert-butoxy)(monodeuteriomethyl)(phenyl)silyl]-4-(methylphenylsilyl)butane. When 1a and 1b were photolyzed with acetone in a hexane solution, cis- and trans-2,3-benzo-1-isopropoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (4a and 4b) were obtained in 25% and 23% isolated yield. In both photolyses, 1-(hydroxymethylphenylsilyl)-4-(methylphenylsilyl)butane (5) was also isolated in 4% and 5% yield, respectively. The photolysis of 1a with acetone-d₆ under the same conditions gave 4a-d₆ and 5-d₁ in 18% and 4% yields.     

Reactions of 1,4-bis(tetrazole)benzenes: formation of long chain alkyl halides

The reactions of 1,4-bis[2-(tributylstannyl)tetrazol-5-yl]benzene with α,ω-dibromoalkanes were carried out in order to synthesise pendant alkyl halide derivatives of the parent bis-tetrazole. This led to the formation of several alkyl halide derivatives, substituted variously at N1 or N2 on the tetrazole ring. The crystal structures of 1,4-bis[(2-(4-bromobutyl)tetrazol-5-yl)]benzene (2-N,2-N′), 1,4-bis[(2-(4-bromobutyl)tetrazol-5-yl)]benzene (1-N,2-N′) and 1,4-bis[(2-(8-bromooctyl)tetrazol-5-yl)]benzene (2-N,2-N′) are reported. Further discussion involves the structure of 1,4-bis[2-(6-bromohexyl)-2H-tetrazol-5-yl]benzene (2-N,2-N′) previously reported.     

Stereoselective radical cyclizations of N-(2-halobenzoyl)-cyclic ketene-N,X(X=O, S)-acetals

2-Alkyloxazolines and 2-alkylthiazolines react with 2-halobenzoyl chlorides to form N-(2-halobenzoyl)-cyclic ketene-N,O-acetals and N-(2-halobenzoyl)-cyclic ketene-N,S-acetals in excellent yields, respectively. These ketene acetals readily undergo stereocontrolled aryl radical cyclizations to afford the central six-membered rings of substituted-2,3,10,10α-tetrahydrooxazolo[3,2-b]isoquinolin-5-ones and their 2,3,10,10α-tetrahydrothiazolo[3,2-b]isoquinolin-5-one analogs. The tertiary N,O- and N,S-radicals formed upon aryl radical reaction at the ketene-N,X(X=O, S)-acetal double bond appear to have reasonable stability. The stereoselectivity in hydrogen abstractions by these intermediate radicals from both Bu₃SnH and (Me₃Si)₃SiH was investigated. The N,S-heterocyclic fused ring products may have potential medical value.     

Substituent effect on the stereoselectivity of acylation of racemic heterocyclic amines with N-phthaloyl-3-aryl-(S)-alanyl chlorides

The acylative kinetic resolution of racemic 2-methyl-1,2,3,4-tetrahydroquinoline and 2,3-dihydro-3-methyl-4H-1,4-benzoxazine using acyl chlorides of N-phthaloyl-(S)-phenylalanine, N-phthaloyl-3-(4-nitrophenyl)-(S)-alanine and N-phthaloyl-O-methyl-(S)-tyrosine as chiral resolving agents has been carried out. It is shown that the effectiveness of an acylative kinetic resolution depends on the electronic effects of substituents in the phenyl fragment of the acylating agent and increases as the electron-donating properties of the para-substituent (OMe > H > NO₂) in phenyl fragment of N-phthaloyl-3-aryl-(S)-alanyl chlorides increase; conducting the process at a reduced temperature also contributes to an enhancement of the kinetic resolution.     

Synthesis and conformational properties of model dipeptides containing novel axially chiral α,β-didehydroamino acids at the (i+1) position of a β-turn conformation

A series of model dipeptides containing some novel axially chiral α,β-didehydroamino acids at the (i+1) position has been synthesised by reaction of the corresponding 4-(4-alkylcyclohexylidene)-2-phenyl-1,3-oxazol-5(4H)-one with (S)-phenylalanine cyclohexylamide. The conformations of two dipeptides in the crystal state have been studied by X-ray diffraction crystallographic analysis. The backbone torsion angles suggest that both peptides adopt similar type-II′ β-turn conformations. NMR spectroscopy has revealed that relatively rigid β-turn structures also persist in solution and that the absolute configurations of the axially chiral α,β-didehydroamino acids do not significantly influence the conformation of the peptide chain. Both heterochiral and homochiral dipeptides are found to accommodate the same βII′-turn conformation. Axially chiral α,β-didehydroamino acids (R_a)- and (S_a)-4-methyl-, 4-phenyl- and (4-tert-butylcyclohexylidene)glycine can be considered as elongated structural analogues of alanine, phenylglycine and tert-leucine of R and S configuration since, in these chiral α,β-didehydroamino acids, the methyl, phenyl and tert-butyl groups are located about 4.3 Å away from the peptide backbone in which they are incorporated.