“Pure by NMR”?

A reference scale of cucurbit[7]uril binding affinities

The accurate determination of ultra-high binding affinities in supramolecular host-guest chemistry is a challenging endeavour because direct binding titrations are generally limited to affinities <10⁶ M^-1 due to sensitivity constraints of common titration methods. To determine higher affinities, competitive titrations are usually performed, in which one compound with a well established binding affinity serves as a reference. Herein, we propose a reference scale for such competitive titrations with the host cucurbit[7]uril (CB7) comprising binding affinities in the range from 10³ to 10¹⁵ M^-1. The suggested reference compounds are commercially available and will aid in the future determination of CB7 binding affinities for stimuli-responsive host-guest systems.

Sustainable Hydrocarbon Oligomer Solvent Systems for Sequestration of Trace Organics from Water

Low-viscosity poly(α-olefin)s (PAOs) either alone or with functional hydrocarbon oligomer cosolvents are nontoxic, nonvolatile, recyclable solvent systems that effectively and efficiently sequester trace amounts of nonpolar organic compounds such as benzene and halogenated organics from water. More polar compounds including perfluorooctanoic acid and nitrobenzene or water-miscible compounds such as THF and triethylamine can also be sequestered if the PAO phase contains an H-bonding PAO-anchored cosolvent.

Insight into old and new pure shift nuclear magnetic resonance methods for enantiodiscrimination

Enantiodiscrimination and their quantification using nuclear magnetic resonance (NMR) spectroscopy has always been a subject of great interest. Proton is the nucleus of choice for enantiodiscrimination due to its high sensitivity and ubiquitous presence in nature. Despite its advantages, enantiodiscrimination suffers from extensive signal splitting by the proton-proton scalar couplings, which give complex multiplets that spread over a frequency range of some tens of hertz. These multiplets often overlap, further complicating interpretation of the spectra and quantifications. In the present review, we discuss some of the recent developments in the pure shift ¹ H NMR based methods for enantiomer resolution and enantiodiscrimination. We also compare various pure shift methods used for enantiodiscrimination and measurement of enantiomeric excess, considering the fact that conventional ¹ H NMR fails to provide any detailed insight.

Diastereoselective Ammonium-Directed Epoxidation in the Asymmetric Syntheses of Dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2

Epoxidations (40% aq HBF₄ then m-CPBA) of racemic cis-2-( N-benzylamino)cyclohex-3-en-1-ol and racemic cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol proceed with very high levels of diastereoselectivity (>95:5 dr). The latter is in direct contrast to the epoxidation of the corresponding trans-diastereoisomer (which proceeds with essentially no selectivity), showing that the relative configuration of the substrate dramatically influences the diastereoselectivity in these instances. Meanwhile, epoxidations of enantiopure (1 R,2 S,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[( N-α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N-benzylamino)cyclohex-3-en-1-ol] proceed with complete diastereoselectivity (>95:5 dr) under the same conditions, showing that neither the presence of the α-methyl group nor the relative configuration of the α-methylbenzyl stereocenter have an effect upon the established level of diastereoslectivity in these cases. In contrast, epoxidations of enantiopure (1 R,2 S,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol and (1 S,2 R,α R)-2-[ N-benzyl- N-(α-methylbenzyl)amino]cyclohex-3-en-1-ol [surrogates for the enantiomers of cis-2-( N, N-dibenzylamino)cyclohex-3-en-1-ol] proceed with lower diastereoselectivity (∼70:30 dr). Thus, the presence of the α-methyl group has a detrimental effect on the established level of diastereoselectivity in these cases (although again the relative configuration of the α-methylbenzyl stereocenter is unimportant). The diastereoselective epoxidation pathway is used to enable the asymmetric syntheses of six hitherto unknown, enantiopure dihydroconduramines (+)-C-2, (-)-C-2, (+)-D-2, (+)-E-2, (+)-F-2, and (-)-F-2 (>99% ee in each case).

Formal synthesis of cis-solamin: acid-catalyzed one-step construction of 2,5-disubstituted tetrahydrofuran

A divergent strategy has been used for the concise and efficient enantioselective formal synthesis of Annonaceous acetogenin cis-solamin. Our synthetic strategy comprises concise preparation of the diepoxyester via an 11-membered silaketal constructed by ring-closing metathesis after the dimerization of chiral epoxides, and uses an acid-catalyzed tandem intramolecular SN2-like reaction to construct the threo-cis-threo configuration of the tetrahydrofuran-diol moiety.

Stereodivergent Olefination of Enantioenriched Boronic Esters

A stereodivergent coupling reaction between vinyl halides and boronic esters is described. This coupling process proceeds without a transition-metal catalyst, instead proceeding by electrophilic selenation or iodination of a vinyl boronate complex followed by stereospecific syn or anti elimination. Chiral, nonracemic boronic esters could be coupled with complete enantiospecificity. The process enables the highly stereoselective synthesis of either the E or Z alkene from a single isomer of a vinyl coupling partner.

NMR-based approach to the analysis of radiopharmaceuticals: radiochemical purity, specific activity, and radioactive concentration values by proton and tritium NMR spectroscopy

Compounds containing tritium are widely used across the drug discovery and development landscape. These materials are widely utilized because they can be efficiently synthesized and produced at high specific activity. Results from internally calibrated (3)H and (1)H nuclear magnetic resonance (NMR) spectroscopy suggests that at least in some cases, this calibrated approach could supplement or potentially replace radio-high-performance liquid chromatography for radiochemical purity, dilution and scintillation counting for the measurement of radioactivity per volume, and liquid chromatography/mass spectrometry analysis for the determination of specific activity. In summary, the NMR-derived values agreed with those from the standard approaches to within 1% to 9% for solution count and specific activity. Additionally, the NMR-derived values for radiochemical purity deviated by less than 5%. A benefit of this method is that these values may be calculated at the same time that (3)H NMR analysis provides the location and distribution of tritium atoms within the molecule. Presented and discussed here is the application of this method, advantages and disadvantages of the approach, and a rationale for utilizing internally calibrated (1)H and (3)H NMR spectroscopy for specific activity, radioactive concentration, and radiochemical purity whenever acquiring (3)H NMR for tritium location.

Conformational features of secondary N-cyclopropyl amides

NMR studies in conjunction with ab initio calculations revealed unexpected conformational behavior of N-cyclopropylacetamide (1). This secondary amide displays 16-19% E-rotamer (cis) around the carbonyl-nitrogen bond in apolar solvents, in contrast to other aliphatic secondary acetamides in which significant E-rotamer populations are rare due to steric contacts between the substituents on the amide bond. In addition, 1 adopts an ortho conformation around the N-cPr bond instead of the anti conformation generally preferred by secondary acetamides. This distinct conformational behavior was also observed for other secondary N-cyclopropyl amides.

Synthesis of aromatic sphingosine analogues by diastereoselective amination of enantioenriched trans-γ,δ-unsaturated β-hydroxyesters

An effective route to N-Boc-protected aromatic sphingosine analogues is accomplished. The strategy is based on the diastereoselective amination of enantioenriched trans-γ,δ-unsaturated β-hydroxyesters to establish anti,N-Boc-α-hydrazino-β-hydroxyesters. Nonreductive E1cB elimination is essential for the successful N-N bond cleavage of hydrazine while preserving the trans double bond. Either the (3R,2S) and (3S,2R) enantiomer of N-Boc-protected sphingosine analogues has been synthesized in five steps with excellent optical purity with ∼99% ee and >99% de.

From arylamine N-acetyltransferase to folate-dependent acetyl CoA hydrolase: impact of folic acid on the activity of (HUMAN)NAT1 and its homologue (MOUSE)NAT2

Acetyl Coenzyme A-dependent N-, O- and N,O-acetylation of aromatic amines and hydrazines by arylamine N-acetyltransferases is well characterised. Here, we describe experiments demonstrating that human arylamine N-acetyltransferase Type 1 and its murine homologue (Type 2) can also catalyse the direct hydrolysis of acetyl Coenzyme A in the presence of folate. This folate-dependent activity is exclusive to these two isoforms; no acetyl Coenzyme A hydrolysis was found when murine arylamine N-acetyltransferase Type 1 or recombinant bacterial arylamine N-acetyltransferases were incubated with folate. Proton nuclear magnetic resonance spectroscopy allowed chemical modifications occurring during the catalytic reaction to be analysed in real time, revealing that the disappearance of acetyl CH₃ from acetyl Coenzyme A occurred concomitantly with the appearance of a CH₃ peak corresponding to that of free acetate and suggesting that folate is not acetylated during the reaction. We propose that folate is a cofactor for this reaction and suggest it as an endogenous function of this widespread enzyme. Furthermore, in silico docking of folate within the active site of human arylamine N-acetyltransferase Type 1 suggests that folate may bind at the enzyme’s active site, and facilitate acetyl Coenzyme A hydrolysis. The evidence presented in this paper adds to our growing understanding of the endogenous roles of human arylamine N-acetyltransferase Type 1 and its mouse homologue and expands the catalytic repertoire of these enzymes, demonstrating that they are by no means just xenobiotic metabolising enzymes but probably also play an important role in cellular metabolism. These data, together with the characterisation of a naphthoquinone inhibitor of folate-dependent acetyl Coenzyme A hydrolysis by human arylamine N-acetyltransferase Type 1/murine arylamine N-acetyltransferase Type 2, open up a range of future avenues of exploration, both for elucidating the developmental role of these enzymes and for improving chemotherapeutic approaches to pathological conditions including estrogen receptor-positive breast cancer.

Determining and reporting purity of organic molecules: why qNMR

Although NMR has been routinely used to determine/estimate relative number of protons for structure elucidation, it has been rarely used to determine and report the purity of organic compounds. Through this paper, we want to emphasize on routine use of quantitative NMR (qNMR) for this purpose. The results of qNMR can be routinely considered as documentation of purity much like other established methods (HPLC, elemental analysis and differential scanning calorimetry). qNMR is a fast, easy, accurate and non-destructive alternate to speed up the whole analytical process and serves the purpose of both identification and purity determination of compounds using single technique.

Synthesis of the Prelog-Djerassi lactone via an asymmetric hydroformylation/crotylation tandem sequence

A synthesis of the Prelog-Djerassi lactone [(+)-1] has been accomplished in three isolations and 57% overall yield from the known vinyl ortho ester 2. A Rh(I)-catalyzed asymmetric hydroformylation/crotylation tandem sequence has been developed and used to set the C2-C4 stereochemistry. A Rh(I)-catalyzed asymmetric hydrogenation was employed to set the C6 sterechemistry, resulting in an unusually short and efficient enantioselective synthesis of this touchstone molecule from achiral starting material.

Quantitative 1H NMR. Development and potential of an analytical method: an update

Covering the literature from mid-2004 until the end of 2011, this review continues a previous literature overview on quantitative (1)H NMR (qHNMR) methodology and its applications in the analysis of natural products. Among the foremost advantages of qHNMR is its accurate function with external calibration, the lack of any requirement for identical reference materials, a high precision and accuracy when properly validated, and an ability to quantitate multiple analytes simultaneously. As a result of the inclusion of over 170 new references, this updated review summarizes a wealth of detailed experiential evidence and newly developed methodology that supports qHNMR as a valuable and unbiased analytical tool for natural product and other areas of research.

Highly (E)-selective Wadsworth-Emmons reactions promoted by methylmagnesium bromide

An experimentally simple protocol for the very highly (E)-selective Wadsworth-Emmons reaction [(E):(Z) selectivities in excess of 180:1 in some cases] of a range of straight-chain and branched aliphatic, substituted aromatic, and base-sensitive aldehydes via reaction with an alkyl diethylphosphonoacetate and MeMgBr is reported.

Carbon dioxide as a carbonylating agent in the synthesis of 2-oxazolidinones, 2-oxazinones, and cyclic ureas: scope and limitations

Carbon dioxide can be used as a convenient carbonylating agent in the synthesis of 2-oxazolidinones, 2-oxazinones, and cyclic ureas. The transient carbamate anion generated by treating a primary or secondary amine group in basic media can be activated with phosphorylating agents such as Diphenylphosphoryl azide (DPPA) and Diphenyl chlorophosphate (DPPCl) but also with other types of electrophiles such as SOCl(2), TsCl, or AcCl. The intramolecular trapping of the activated carbamate by a hydroxyl group leads to the formation of 2-oxazolidinones or 2-oxazinones in good to excellent yields. This methodology was successfully applied to the synthesis of cyclic ureas up to 7-membered rings from the corresponding diamines.

Highly stereoselective metal-free catalytic reduction of imines: an easy entry to enantiomerically pure amines and natural and unnatural alpha-amino esters

A highly efficient catalytic stereoselective ketimine reduction is described. The combination of an inexpensive chiral organocatalyst, easily prepared in a single step, and of a very cheap removable chiral auxiliary allowed us to obtain enantiomerically pure amino compounds. The methodology allowed synthesis of chiral secondary and primary amines and natural and unnatural amino esters in high yields often with total control of the absolute stereochemistry.

NMR quantitation of natural products at the nanomole scale

We describe a simple and accurate method for quantitation by solvent 13C-satellites (QSCS) of very small amounts of natural products using microprobe NMR spectroscopy. The method takes advantage of integration of 13C satellite peaks of deuterated solvents, in particular CDCl3, that have favorable intensities for measurements of samples in NMR microcoils and microprobe tubes in the 1-200 nanomole range.