Synthesis of enantiomerically pure perhydrofuro[3,4-b]pyrans and perhydrofuro[3,4-b]furans

Olefinic diols, prepared from (R)-(+)-2,2-dimethyl-1,3-dioxolane-4-carboxaldehyde via olefination and hydrolysis, were converted into enantiomerically pure hydroxy substituted tetrahydrofuran derivatives by cyclization using N-phenylselenophthalimide and BF₃. The PhSe group in the C-4 position of these tetrahydrofurans was then substituted by an allyl group using allyltributylstannane in the presence of AIBN. The selenium promoted cyclizations of the allyl tetrahydrofurans in which the OH and the allyl groups are trans to each other formed the enantiopure perhydrofuro[3,4-b]pyrans, while the cyclization of the allyl tetrahydrofurans in which the OH and the allyl groups are cis gave rise to the perhydrofuro[3,4-b]furans. These bicyclic products were finally deselenenylated with triphenyltin hydride and AIBN.     

Ring-closure reactions through intramolecular displacement of the phenylselenonyl group by nitrogen nucleophiles: a new stereospecific synthesis of N-tosyl and N-benzoyl-1,3-oxazolidin-2-ones from beta-hydroxyalkyl phenyl selenides

A new and convenient method for the stereospecific synthesis of variously substituted 1,3-oxazolidin-2-ones from the easily available beta-hydroxyalkyl phenyl selenides is presented. After transformation into the N-tosyl or N-benzoyl carbamates, the selenides were oxidized to the corresponding selenones. The key step of the process is the ring-closure reaction, which occurs by stereospecific intramolecular nucleophilic substitution of the selenone group by the nitrogen atom of the carbamate. Enantiomerically pure 1,3-oxazolidin-2-one derivatives can be easily prepared by using enantiomerically pure beta-hydroxyalkyl phenyl selenides as starting materials.     

Organoselenium mediated asymmetric cyclizations. Synthesis of enantiomerically pure 1,6-dioxaspiro[4.4]nonanes

The asymmetric cyclization of 1-hydroxyoct-7-en-4-one, promoted by camphorselenenyl tetrafluoroborate, generated from camphor diselenide and silver tetrafluoroborate in dichloromethane at room temperature, afforded a mixture of two diastereoisomeric E- and two diastereoisomeric Z-2-[(camphorseleno)methyl]-1,6-dioxaspiro[4.4]nonanes. These were separated by medium pressure liquid chromatography and then deselenenylated with triphenyltin hydride and AIBN to give enantiomerically pure 2-methyl-1,6-dioxaspiro[4.4]nonanes. The camphorseleno group was also substituted by an allyl function using allyltributyltin in the presence of AIBN.     

A convergent synthesis of enantiopure bicyclic scaffolds through multicomponent Ugi reaction

An efficient and convergent Ugi synthesis of enantiomerically pure N-acyl-2,5-disubstituted pyrrolidines was coupled with an appropriate secondary transformation to give two series of bicyclic derivatives, namely hexahydro pyrrolo-oxazocinediones and -diazepinediones.     

First asymmetric syntheses of 6-substituted nipecotic acid derivatives

Various nipecotic acid derivatives are known to be potent GABA uptake inhibitors thus being useful in the treatment of a number of neurological and psychological disorders. In this paper, the first asymmetric syntheses of 6-substituted nipecotic acid derivatives are presented. The synthetic strategy was designed to provide access to a large variety of enantiomerically pure 6-substituted nipecotic acid derivatives. The synthesis starts from the chiral N-acyldihydropyridines 15 and 16 obtained via asymmetric electrophilic α-amidoalkylation reaction of a chiral N-acylpyridinium ion. These were utilized for the preparation of enantiomerically pure 6-(4,4-diphenylbutyl)nipecotic acids and 6-(4,4-diphenylbutenyl)nipecotic acids in a multistep synthesis, including the removal of the dimethylphenylsilyl blocking group from the dihydropyridine ring, the reduction of the dihydropyridine heterocycle, a Horner-Wittig reaction and the removal of the chiral auxiliary. The obtained target molecules, however, showed only negligible affinity to the GAT-1- and GAT-3 transport proteins.     

Adventures in sulfur-nitrogen chemistry

This account reviews our efforts over the past 37 years to understand the chemistry of a select group of sulfur-nitrogen compounds including sulfinimines (N-sulfinyl imines) and N-sulfonyloxaziridines. Our early exploration of the thermal properties of sulfenamides, a class of sulfur-nitrogen compounds about which little was known, resulted in a new procedure, the silver-assisted method, for the construction of sulfenimines (N-sulfenyl imines). Selective oxidations of these compounds resulted in the production of N-sulfinyl imines (sulfinimines) and N-sulfonyloxaziridines. N-Sulfonyloxaziridines turned out to be a new class of aprotic neutral oxidizing reagents. Enantiomerically pure examples afford high ee values in the oxidation of enolates to alpha-hydroxy carbonyl compounds and in the oxidation of sulfides and selenides to sulfoxides and selenoxides. Additions of organometallic reagents to enantiomerically pure sulfinimines provide the best and most versatile method for the asymmetric construction of the carbon-nitrogen stereocenters found in many biologically active compounds. Sulfinimine-derived chiral building blocks provide efficient access to many classes of nitrogen heterocycles including aziridines, 2H-azirines, pyrrolidines, and piperidines.     

Synthesis of enantiomerically pure 1,4-dioxanes from alkenes promoted by organoselenium reagents

An alkene can be converted into two enantiomerically pure diastereomeric selenoethers through a regio- and stereospecific anti addition reaction mediated by N-(phenylseleno)phthalimide in the presence of an enantiomerically pure 1,2-diol. This alkoxyselenylation reaction can be employed as the key step to allow the synthesis of several isomeric tetrasubstituted 1,4-dioxanes in enantiomerically pure form.     

Highly functionalised organolithium and organoboron reagents for the preparation of enantiomerically pure α-amino acids

Homochiral, highly functionalised organolithium reagents derived from l-serine have been generated and reacted with electrophiles. The novel enantiomerically pure adducts thus obtained were then converted, through β-amino alcohols, into novel non-proteinogenic α-amino acids. The methodology also made available a novel boronic acid which was then employed as a Suzuki cross-coupling partner, elaborating a new pathway to phenylalanine analogues.     

Preparation of new chiral pyridine–phosphine ligands and their Pd-catalyzed asymmetric allylic alkylations

Enantiomerically pure 2-(diphenylphosphino)methyl-N-[1-(2-pyridinyl)ethyl]pyrrolidines 1 and 2 have been prepared by the stereospecific substitution of enantiomerically pure 1-(2-pyridinyl)ethyl methanesulfonate 6 with enantiomerically pure 2-(diphenylphosphino)methylpyrrolidine. Asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate 11 with dimethyl malonate sodium salt using the (S,S)-ligand 1 affords the (R)-product 12 with up to 86% e.e. in good yield.     

Oxidative C-C bond cleavage of N-alkoxycarbonylated cyclic amines by sodium nitrite in trifluoroacetic acid

Oxidative carbon-carbon bond cleavage of N-alkoxycarbonylated cyclic amines was accomplished by NaNO₂ in TFA to afford ω-amino carboxylic acid in high yield. Optically active 3-hydroxypiperidine derivatives and 3-pipecolinate were converted to enantiomerically pure (R)-4-amino-3-hydroxybutanoic acid (GABOB) and (S)-2-pyrrolidone-4-carboxylate, respectively.     

Syntheses of Enantiomerically Pure 2‐Substituted Pyrrolidin‐3‐ones via Lithiated Alkoxyallenes – An Auxiliary‐Based Synthesis of both Enantiomers of the Antibiotic Anisomycin

The hydrochlorides of both enantiomers of the antibiotic anisomycin were prepared starting with the ‘diacetone‐fructose’‐substituted allene 1 and the N‐Boc‐protected imine precursor 2a . Addition of an excess of lithiated 1 to 2a provided a 2 : 1 mixture 3a of diastereoisomers, which were cyclized to 4a under base promotion (Scheme 2). The two diastereoisomers of 4a were separated and converted into enantiomerically pure pyrrolidin‐3‐ones (2R)‐ 5a and (2S)‐ 5a . A similar sequence yielded the N‐Tos‐protected compounds (2R)‐ 5b and (2S)‐ 5b . Compounds 5a were converted into silyl enol ethers 6 and by subsequent regio‐ and stereoselective hydroboration into pyrrolidine derivatives 7 (Scheme 3). Straightforward functional‐group transformations led to the hydrochlorides 9 of anisomycin (Scheme 3). The (2R) series provided the hydrochloride (2R)‐ 9 of the natural occurring enantiomer, whereas the (2S) series furnished the antipode (2S)‐ 9 . The overall sequence to the natural product involved ten steps with eight purified intermediates and afforded an overall yield of 8%. Our stereochemically divergent approach to this type of hydroxylated pyrrolidines is highly flexible and should easily allow preparation of many analogues.     

An efficient and practical synthesis of BINAM derivatives by diastereoselective [3,3]-rearrangement

This work describes an efficient, practical, and diastereoselective synthesis of enantiomerically pure BINAM derivatives by utilizing the acid-catalyzed [3,3]-sigmatropic rearrangement of readily accessible N-(−)-menthoxycarbonyl-diaryl hydrazines as a key step. The yield of the rearrangement is very high and two diastereomers thus obtained are easily separable.     

Synthesis of chiral 1-(imidazol-2-yl)alkanamines using neomenthanethiol as a chiral auxiliary

Diastereomeric N-(imidazol-2-ylmethylidene)neomenthane-3-sulfinamides were obtained from enantiomerically pure neomenthanethiol in three steps. The compounds obtained added Grignard reagents at the C=N bond. After separation of diastereomers, an acid resolution gave enantiomerically pure primary 1-(imidazol-2-yl)alkanamines. In this scheme, the neomenthane fragment played the role of a chiral auxiliary.     

Unprecedented formation of benzo[d][1,2,3,6]oxatriazocine derivatives via diazo-oxygen bond formation and synthesis of enantiomerically pure 1-alkyl benzotriazole derivatives

A series of amino acid-derived enantiomerically pure substituted benzo[d][1,2,3,6]oxatriazocine derivatives and 1-alkyl substituted benzotriazoles has been prepared by the diazotization of amino acid-derived benzo-fused alicycles. The first unprecedented diazo-oxygen bond formation in acidic medium led to an entirely new kind of substituted benzo[d][1,2,3,6]oxatriazocine heterocycles.     

Diastereoselective In and Zn-mediated allylation of pyrazol-4-yl derived (R)-tert-butanesulfinyl imines: synthesis of enantiomerically pure 6-(pyrazol-4-yl)-piperidin-2,4-diones

The In and Zn-mediated allylation of substituted pyrazol-4-yl derived (R)-N-tert-butanesulfinyl imines proceeds with high diastereoselectivity depending on the conditions and additives (up to 99.4% de). Thus, the synthesized diastereomeric homoallylsulfinamides were isolated and characterized as pure diastereomers, which were then converted into the corresponding pyrazol-4-yl-derived N-Boc-homoallylamines via consecutive treatment with HCl and Boc₂O. The latter were then subjected to a sequence of reactions: cyclobromocarbamation with NBS and enolate–isocyanate rearrangement with tBuOK to give novel enantiomerically pure (S)-6-(pyrazol-4-yl)-piperidine-2,4-diones in 88–96% yield. The absolute configurations of the allylation products were assigned by X-ray single crystal analysis of the intermediate bromomethylurethanes.     

Synthesis of enantiomerically pure α-amino-β-hydroxy-cyclobutanone derivatives and their transformations into polyfunctional three- and five-membered ring compounds

Ketenes readily cycloadded to (R)-tert-butyldihydrooxazole 2a-d to yield enantiomerically pure bicyclic cyclobutanones. The cycloadditions proceeded with unusual regiochemistry giving predominantly or exclusively protected α-amino-β-hydroxycyclobutanone derivatives. The adducts could be converted into a variety of interesting enantiopure intermediates equipped with many functional groups: α-amino-β-hydroxy cyclopropane carboxylic acid derivatives, α-amino-β-hydroxy succinic acid derivatives, α-amino-β-hydroxy lactones and lactams derivatives.     

Multicomponent Hantzsch cyclocondensation as a route to highly functionalized 2- and 4-dihydropyridylalanines, 2- and 4-pyridylalanines, and their N-oxides: preparation via a polymer-assisted solution-phase approach

An improved and efficient entry to highly functionalized β-(2-pyridyl)- and β-(4-pyridyl)alanines and the corresponding 1,4-dihydro and N-oxide derivatives has been developed by one-pot thermal Hantzsch-type cyclocondensation of aldehyde-ketoester-enamine systems in which one of the reagents (aldehyde or ketoester) was carrying the unmasked but protected chiral glycinyl moiety. Thus coupling N-Boc-O-benzyl aspartate β-aldehyde, acetoacetate and aminocrotonate esters afforded tetrasubstituted β-(4-dihydropyridyl)alanines (75% yield). One of these products was almost quantitatively transformed into the β-(4-pyridyl)alanine derivative which in turn was oxidized to the corresponding N-oxide. Each of these enantiomerically pure (Mosher's amide analysis) heterocyclic α-amino acids was incorporated into a tripeptide by coupling with (S)-phenylalanine. In a similar way tetrasubstituted β-(2-dihydropyridyl)alanine, β-(2-pyridyl)alanine and β-(1-oxido-2-pyridyl)alanine were prepared via Hantzsch cyclocondensation reaction using benzaldehyde, aminocrotonate, and acetoacetate carrying the N-Boc-O-benzyl glycinate moiety. It was shown that the work up of the reaction mixtures derived from the cyclocondensation and oxidation reactions can be carried out by the use of polymer supported reagents and sequestrants thus allowing the isolation of the products in high purity without any chromatography.     

Synthesis of highly substituted pyrrolidines via palladium catalysed formal [2+3] cycloaddition of 5-vinyloxazolidin-2-ones to activated alkenes

Glycine-derived N-tosyl-5,5-divinyloxazolidin-2-one 10 undergoes a palladium catalysed decarboxylative ring-opening cyclization with strongly electron deficient alkylidenemalonate derivatives to give highly substituted pyrrolidines 14 containing two contiguous quaternary centres.     

Controlling stereoselectivity by enzymatic and chemical means to access enantiomerically pure (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinoline derivatives

A chemoenzymatic strategy for the synthesis of enantiomerically pure novel alkaloids (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinolines is presented. The key steps are the biocatalytic stereoselective reductive amination of substituted 1-phenylpropan-2-one derivatives to yield chiral amines employing microbial ω-transaminases, and the diastereoselective reduction of a Bischler–Napieralski imine intermediate by catalytic hydrogenation in the presence of palladium on charcoal, leading exclusively to the desired cis-isomer.     

Non-destructive Removal of the Bornanesultam Auxiliary in α-Substituted N-Acylbornane-10,2-sultanis under Mild Conditions: An efficient synthesis of enantiomerically pure ketones and aldehydes

α-Substituted N-acylbornane-10, 2 -sultams 6, 9 , and 10 can be converted into enantiomerically pure ketones 5. 13 , and 14 , respectively, via a two-step procedure involving a known mercaptolysis reaction followed by an [Fe(acac)₃]-mediated coupling of the resulting S-benzyl thioesters with Grignard reagents. Furthermore, enantiomerically pure aldehydes 23 can be obtained from α-substituted N-acylbornane-10,2-sultams 6 via a one-step reduction with (i-Bu)₂AIH. No epimerization at the α-chiral center is observed during the cleavage reaction whereby the chiral auxiliary, bornane-10,2-sultam 1 or ent- l , was recovered. By using this methodology, several natural products or precursors thereof can be prepared.