The Assignment of Absolute Configuration by NMR

The assignment of absolute configuration of cyanohydrins by NMR

Derivatization of aldehyde cyanohydrins as (R)- and (S)-MPA esters and comparison of the corresponding 1H- and 13C-NMR spectra allows the assignment of their absolute configuration.

Synthesis of Strained Tricyclic β-Lactams by Intramolecular [2+2] Cycloaddition Reactions of 2-Azetidinone-Tethered Enallenols: Control of Regioselectivity by Selective Alkene Substitution

A convenient metal-free methodology for the preparation of structurally novel, strained tricyclic beta-lactams containing a cyclobutane ring has been developed. The first examples accounting for the intramolecular [2+2] cycloaddition reactions in beta-lactams have been achieved by the thermolysis of 2-azetidinone-tethered enallenols, which have been prepared in aqueous media by regio- and diastereoselective indium-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes. Notably, the regioselectivity of the cycloaddition can be tuned in the allene component just by a subtle variation in the substitution pattern of the alkene component.

Preparation of enantiopure Wieland–Miescher ketone and derivatives by the MαNP acid method. Substituent effect on the HPLC separation

Enantiopure Wieland-Miescher ketone (4, W-M ketone) and derivatives were prepared by the enantioresolution with 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid 1). Various racemic derivatives of 4 were esterified with acid (S)-(+)-1 yielding diastereomeric MalphaNP esters, which were separated by HPLC on silica gel. It was clarified that the HPLC separation of diastereomers depended on the substituent of the derivatives, leading to the working hypothesis that MalphaNP acid esters of alcohols with less polar and more bulky aliphatic substituents are more effectively separated. The best separation was obtained in the case of tert-butyldimethylsilyl (TBDMS) ether derivative (12a/12b): separation factor alpha=1.80, and resolution factor, Rs=1.30. The (1)H NMR spectra of separated MalphaNP esters showed anomalously large magnetic anisotropy effects, from which their absolute configurations were determined. Solvolysis or reduction of the separated MalphaNP esters yielded alcohols, which were converted to enantiopure W-M ketones 4. The results thus provided another route for preparation of enantiopure ketones (8aR)-(-)-4 and (8aS)-(+)-4.

Assignment of absolute configuration of cyclic secondary amines by NMR techniques using Mosher's method: a general procedure exemplified with (-)-isoanabasine

A general procedure to determine the absolute configuration of cyclic secondary amines with Mosher's NMR method is demonstrated, with assignment of absolute configuration of isoanabasine as an example. Each Mosher amide can adopt two stable conformations (named rotamers) caused by hindered rotation around amide C--N bond. Via a three-step structural analysis of four rotamers, the absolute configuration of (-)-isoanabasine is deduced to be (R) on the basis of Newman projections, which makes it easy to understand and clarify the application of Mosher's method to cyclic secondary amines. Furthermore, it was observed that there was an unexpected ratio of rotamers of Mosher amide derived from (R)-isoanabasine and (R)-Mosher acid. This phenomenon implied that it is necessary to distinguish the predominant rotamer from the minor one prior to determining the absolute configuration while using this technique.

Ligand-mediated enantioselective addition of lithium carbazolates to aldehydes

The enantioselective synthesis of acyclic pyrrole, indole and other N-carbazole carbinols via ligand-mediated addition of lithium carbazolates to aldehydes, together with studies into their catalytic enantioselective synthesis using substoichiometric base and ligand, are reported. The subsequent exploitation of the resulting stereocentre as a controlling element in 1,3-syn- and anti-selective reduction of beta-ketones and elaboration to homoallylic alcohols is also described.

Solvent effects on the conformational distribution and optical rotation of γ-methyl paraconic acids and esters

A computational investigation of the optical rotatory power of cis and trans 2-methyl-5-oxo-tetrahydro-3-furancarboxylic acids and the corresponding methyl and ethyl esters is presented. Solvent effects on both the conformational space and the rotatory power are analyzed by comparing results obtained in vacuo with those computed--using the Polarizable Continuum Model--in methanol. A comparison with experimental observations for the optical rotatory power of the title compounds in methanol is also carried out, in a few cases also for several wavelengths. Agreement between theory and experiment is in all cases excellent, in particular when solvent effects are included both in the geometry optimization and in the calculation of the OR, thus confirming the validity of the computational procedure adopted, even for this challenging family of floppy molecules.

Multiple Component Approaches to C-Glycosyl β-Amino Acids by Complementary One-Pot Mannich-Type and Reformatsky-Type Reactions

The development of new methods for the preparation of C-glycosyl beta-amino acid libraries with chemical and stereochemical diversity levels was investigated and the results are described herein. Two complementary one-pot three-component Mannich-type and Reformatsky-type synthetic strategies have been developed for the construction of chiral 3-amino propanoate fragments (eventually bis-substituted at C-2) directly linked to the anomeric carbon of pyranose and furanose residues. Both methods involved as the initial step the coupling of a sugar aldehyde to p-methoxybenzylamine but differed in the nucleophile (a d(2) synthon equivalent) which was successively added: a ketene silyl acetal (Mannich route) or a bromozinc enolate (Reformatsky route). Individual C-glycosyl beta-amino esters were isolated as single 3R diastereoisomers in fair to excellent yield (60-90%) and their structure assigned by NMR spectroscopy (Riguera protocol) supported by X-ray crystallography. A tentative explanation of the observed stereochemical outcome based on transition-state models is provided. A preliminary study on the synthesis of alpha,alpha-difluoro C-glycosyl beta-amino acids via a more traditional Reformatsky route is also reported.

The Prediction of the Absolute Stereochemistry of Primary and Secondary 1,2-Diols by1H NMR Spectroscopy: Principles and Applications

The absolute configuration of 1,2-diols formed by a primary and a secondary (chiral) hydroxyl group can be deduced by comparison of the 1H NMR spectra of the corresponding (R)- and bis-(S)-MPA esters (MPA = methoxyphenylacetic acid). This method involves the use of the chemical shifts of substituents L1/L2 attached to the secondary (chiral) carbon, and of the hydrogen atom linked to the chiral center (C alpha-H) as diagnostic signals. Theoretical (AM1, HF, and B3 LYP calculations) and experimental data (dynamic and low-temperature NMR spectroscopy, studies on deuterated derivatives, constant coupling analysis, circular dichroism (CD) spectra, and NMR studies with a number of diols of known absolute configuration) prove that the signs of the delta delta(RS) obtained for those signals correlate with the absolute configuration of the diol. A graphical model for the reliable assignment of the absolute configuration of a 1,2-diol by comparison of the NMR spectra of its bis-(R)- and bis-(S)-MPA esters is presented.

Synthesis of (R)- and (S)-3,3,3-trifluoro-2-phenylpropanoyl isocyanates and their use as reactive analogues of Mosher's acid

The title compounds have been prepared from the respective 3,3,3-trifluoro-2-methoxy-2-phenylpropanoic acids (MTPA) by a three-step synthesis with MTPA chloride and MTPA amide as reaction intermediates. The requested compounds were obtained in high chemical yields without any change in optical purity during the preparation. To ascertain the usefulness of this auxiliary agent in the chiral analysis, isomeric 3,3,3-trifluoro-2-phenylpropanoyl isocyanates were subjected in NMR tubes to noncatalyzed reactions with 16 different commercially available chiral alcohols. The steric arrangement of all diastereomers prepared correlated well with their NMR spectral nonequivalence data (Deltadelta), thus demonstrating the regular sign distribution of Deltadelta of particular groups according to the devised molecular model. The usefulness of the novel derivatization is discussed.

(S)-2-[(R)-Fluoro(phenyl)methyl]oxirane: A General Reagent for Determining the ee of α-Chiral Amines

(S)-2-[(R)-Fluoro(phenyl)methyl]oxirane is a new, synthetic, yet enantiopure, chiral resolution reagent, readily obtained from enantiopure (2S,3S)-phenylglycidol, that reacts with a variety of alpha-chiral primary and secondary amines in a straightforward manner through a regioselective ring-opening. Diastereomeric products are easily identified and quantified by (19)F, (1)H, and (13)C NMR and by HPLC, which makes this fluorinated compound a most versatile reagent for the analysis of scalemic mixtures of amines. [reaction: see text]

A micromethod for the stereochemical analysis of fatty acid desaturase-mediated sulfoxidation reactions

The stereochemistry of fatty acid desaturase-mediated sulfoxidation can be evaluated at micromolar concentrations of analyte using (19)F NMR in combination with a chiral shift reagent: (S)-(+)-MPAA.

Novel Carbonyl Bromoallylation/Heck Reaction Sequence. Stereocontrolled Access to Bicyclic β-Lactams

[reaction: see text] The reaction of 2,3-dibromopropene with racemic as well as enantiopure 4-oxoazetidine-2-carbaldehydes 1 in aqueous media was promoted by tin in the presence of several additives to afford the corresponding bromohomoallyl alcohols 2 in high diastereoselectivities. However, indium or zinc were unable to promote the bromoallylation reaction of aldehydes 1 under similar Barbier-type conditions. Vinyl bromides 2 bearing an extra alkene tether were used for the preparation of differently sized, fused bicyclic beta-lactams of nonconventional structure via Heck cyclization.

Total structural assignment and absolute configuration of terreinol by 13C and 1H NMR

The carbon-carbon connectivity of terreinol, a new metabolite isolated from Aspergillus terreus, and its previous (13)C assignments were confirmed by a two-dimensional INADEQUATE experiment using a few milligrams of the compound with natural (13)C abundance. The carbon-carbon correlations were determined by computational analysis (with >99% probability) of this experiment. Additionally, the absolute configuration of terreinol was achieved indirectly via its corresponding secondary alcohol by the modified Mosher method allied to conformational analysis. The shielding effect of the phenyl group of methoxytrifluoromethylphenylacetic acid (MTPA) on the substituents of the carbonylic centre gave a fully regular Deltadelta(SR) sign distribution, allowing reliable assignment of the R configuration for terreinol.

Bioactive constituents of Artemisia monosperma

During a study on the chemistry and biological activity of Kuwaiti plants, new metabolites including 4,6-dihydroxy-3-[3'-methyl-2'-butenyl]-5-[4''-hydroxy-3''-methyl-2''-butenyl]-cinnamic acid (1), the 3R,8R stereoisomer of the C17 polyacetylene dehydrofalcarindiol (2) and a C10 polyacetylene glucoside (3) were characterised by spectroscopic means. Additionally, the previously characterised natural products 1,3R,8R-trihydroxydec-9-en-4,6-yne (4), spathulenol (5) and eriodyctiol-7-methyl ether (6) were also isolated. Compounds 2, 3, and 4 were evaluated for their ability to inhibit the enzyme 12-lipoxygenase and 3 and 4 showed moderate activity at 30 microg/ml. Compound 2 was evaluated against a panel of colorectal and breast cancer cell lines and IC50 values ranged from 5.8 to 37.6 microg/ml. Against a panel of fast-growing mycobacteria and a standard ATCC strain of Staphylococcus aureus, compound 6 exhibited minimum inhibitory concentrations in the range of 64-128 microg/ml.

Assignment of absolute configuration of 3-hydroxy ?-lactams by the NMR ?mix and shake? method

The absolute configurations of several 3-hydroxy beta-lactams were assigned by the NMR "mix and shake" methodology developed by Riguera and co-workers. The results from the NMR study correlated perfectly with the absolute configurations obtained from X-ray crystallographic structure analyses and chiral-phase HPLC data.

Synthesis of enantiopure phthalides including 3-butylphthalide, a fragrance component of celery oil, and determination of their absolute configurations

Enantiopure phthalides 2 and 5-8 were synthesized via enantioresolution of the corresponding alcohols with a chiral auxiliary of camphorsultam dichlorophthalic acid, (1S,2R,4R)-(-)-CSDP acid 3, followed by solvolysis with KOH in MeOH and the catalytic oxidation of chiral glycols with iridium complex 28. The absolute configurations of phthalides 2 and 5-8 were determined by applying the (1)H-NMR anisotropy method of MalphaNP acid (4), 2-methoxy-2-(1-naphthyl)propionic acid, to the chiral synthetic precursory alcohols. In the case of 3-phenylphthalide (R)-(-)-7, the absolute configuration determined by the (1)H-NMR anisotropy method using MalphaNP acid 4 agreed with that by the X-ray crystallographic method. By applying these methods, 3-butylphthalide (S)-(-)-2, a fragrance component of essential oil of celery, has been synthesized in an enantiopure form, and its absolute configuration was unambiguously determined.

Diastereoselective Ni(0)-catalyzed carbocyclization to chiral vinylic sulfoxide

Diastereoselective Ni(0)-catalyzed carbocyclization of enone to chiral vinylic sulfoxide has been accomplished, wherein two stereogenic centers were constructed simultaneously to give cis- and trans-disubstituted cyclopentanes from (E)- and (Z)-vinylic sulfoxides, respectively.

Absolute configuration of amino alcohols by 1H-NMR

A general NMR spectroscopy protocol for determination of absolute configuration of 1,2-amino alcohols, that allows differentiation of the four possible stereoisomers by analysis of the 1H NMR spectra of their bis-MPA derivatives, is described.

Application of Mosher's method for absolute configuration assignment and resolution of 2-hydroxypyrrolizidinones

Derivatization of racemic 2-hydroxypyrrolizidinones with S-(+)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride (MTPA-Cl) was successfully used for enantiomer separation and absolute configuration assignment.

Chiral derivatives of 2-(1-naphthyl)-2-phenylacetic acid

The spectral properties of diastereomeric esters and amides (1b-20b), derived from optically pure 2-(1-naphthyl)-2-phenylacetic acids (1-NPA), were systematically investigated. It was found that all compounds prepared exhibit the NMR spectral nonequivalence (Deltadelta) with regular sign distribution of particular groups according to the predicted model. Further, the analysis of data revealed that the phenyl ring is responsible for a shielding effect (upfield shift) instead of a naphthyl one. This conclusion is supported by the crystallographic analysis showing the almost ap-arrangement of the acid methine hydrogen atom and carbonyl group. In this arrangement, the phenyl ring faces toward the ester part of the molecule while the naphthyl one is orthogonal to the phenyl plane. Therefore, the mutual position of phenyl and alkyl groups with respect to the central molecule co-planarity thus determines the chemical shifts of the alcohol/amine substituents. The relative magnitude of the Deltadelta corresponds to those of Mosher's derivatives.

Diastereoselective [2+2]-Cycloaddition Reactions of Unsymmetrical Cyclic Ketenes with Imines: Synthesis of Modified Prolines and Theoretical Study of the Reaction Mechanism

The synthesis of enantiomerically pure modified proline derivatives was achieved by using spiro beta-lactams as starting material that were prepared in turn by the [2+2]-cycloaddition of unsymmetrical cyclic ketenes with optically active imines. A theoretical study of the [2+2]-cycloaddition reaction, using density-functional methods, gave insights on the origin of the observed stereoselectivity of the Staudinger reaction. The spiro beta-lactams were transformed in the N-Boc derivatives and subjected to nucleophilic ring opening, affording the corresponding enantiomerically pure modified proline derivatives, isolated as orthogonally protected compounds.

Stereochemical studies of chiral resolving agents, M9PP and H9PP acids

The stereoselective Grignard reaction of (1R,2S,5R)-(-)-2-isopropyl-5-methylcyclohexyl pyruvate (menthyl pyruvate) with 9-phenanthrylmagnesium bromide yielded diastereomeric hydroxy-esters, where intramolecular OH em leader O=C hydrogen bond was observed in IR and (1)H NMR spectra. The alkaline hydrolysis of the major product gave (+)-2-hydroxy-2-(9-phenanthryl)propionic acid (H9PP acid (3)), whose absolute configuration was assigned as S based on the chemical correlation with (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl ester of (S)-2-methoxy-2-(9-phenanthryl)propionic acid (M9PP acid (2)); the absolute configuration of 2 had been previously established by X-ray crystallography. The enantioresolution of (+/-)-6-methyl-5-hepten-2-ol, sulcatol, an insect pheromone, was carried out using (S)-(+)-M9PP acid 2.

M?NP acid, a powerful chiral molecular tool for preparation of enantiopure alcohols by resolution and determination of their absolute configurations by the1H NMR anisotropy method

A novel methodology using a chiral molecular tool of MalphaNP acid (1), 2-methoxy-2-(1-naphthyl)propionic acid, useful for preparation of enantiopure secondary alcohols and determination of their absolute configurations by the (1)H NMR anisotropy method was developed; racemic MalphaNP acid (1) was enantioresolved with (-)-menthol, and the enantiopure MalphaNP acid (S)-(+)-(1) obtained was allowed to react with racemic alcohol, yielding a mixture of diastereomeric esters, which was clearly separated by HPLC on silica gel. By applying the sector rule of (1)H NMR anisotropy effect, the absolute configuration of the first-eluted MalphaNP ester was unambiguously determined. Solvolysis or reduction of the first-eluted MalphaNP esters yielded enantiopure alcohols.

Determination of the absolute configuration ofAnisotomeirregular diterpenes: Application of CD and NMR methods

Several Anisotome diterpene derivatives were synthesized in an attempt to obtain a crystalline compound for X-ray analysis. Although we were unable to obtain a suitable crystal, the absolute configuration of the irregular diterpene skeleton was determined using two other techniques: a circular dichroism (CD) protocol based on a tetraarylporphyrin molecular tweezer that allowed prediction of the absolute stereochemistry on a microscale level, and a method employing differences in NMR shifts from derivatization of the naturally occurring acid 1 with enantiomers of a phenylglycine methyl ester (PGME) chiral anisotropic reagent. The excellent agreement between the CD and NMR methods led to the assignment of a 2S-absolute configuration for anisotomenoic acid 1.