Alexei V. Buevich

LR-HSQMBC: a sensitive NMR technique to probe very long-range heteronuclear coupling pathways.

HMBC is one of the most often used and vital NMR experiments for the structure elucidation of organic and inorganic molecules. We have developed a new, high sensitivity NMR pulse sequence that overcomes the typical (2,3)JCH limitation of HMBC by extending the visualization of long-range correlation data to 4-, 5-, and even 6-bond long-range (n)JCH heteronuclear couplings. This technique should prove to be an effective experiment to complement HMBC for probing the structure of proton-deficient molecules. The LR-HSQMBC NMR experiment can, in effect, extend the range of HMBC to provide data similar to that afforded by 1,n-ADEQUATE even in sample-limited situations. This is accomplished by optimizing responses for very small (n)JCH coupings as opposed to relying on the markedly less sensitive detection of long-range coupled (13)C-(13)C homonuclear pairs at natural abundance. DFT calculations were employed to determine whether the very long-range correlations observed for cervinomycin A2 were reasonable on the basis of the calculated long-range couplings.

Homodecoupled 1,1‐ and 1,n‐ADEQUATE: Pivotal NMR Experiments for the Structure Revision of Cryptospirolepine

Cryptospirolepine is the most structurally complex alkaloid discovered and characterized thus far from any Cryptolepis specie. Characterization of several degradants of the original, sealed NMR sample a decade after the initial report called the validity of the originally proposed structure in question. We now report the development of improved, homodecoupled variants of the 1,1- and 1,n-ADEQUATE (HD-ADEQUATE) NMR experiments; utilization of these techniques was critical to successfully resolving long-standing structural questions associated with crytospirolepine.

Experimental and theoretical investigation of 1J(CC) and (n)J(CC) coupling constants in strychnine.

A relatively unexplored and unexploited means of establishing molecular structure, stereochemistry, and probing vicinal bond angles is through the use of long-range (13)C-(13)C coupling constants. The measurement of these multifunctional, diagnostic (3)J(CC) couplings has not been reported on sample amounts viable for the practicing organic chemist. A generalized protocol for the measurement of (1)J(CC) and (3)J(CC) couplings using a 4.6 mg sample of strychnine as a model compound is described, and the utility of DFT calculations for the prediction of these useful molecular descriptors and the congruence of the calculated and experimental data is demonstrated.

Synthesis and SAR Studies of Fused Oxadiazines as γ-Secretase Modulators for Treatment of Alzheimer's Disease

Fused oxadiazines (3) were discovered as selective and orally bioavailable γ-secretase modulators (GSMs) based on the structural framework of oxadiazoline GSMs. Although structurally related, initial modifications showed that structure-activity relationships (SARs) did not translate from the oxadiazoline to the oxadiazine series. Subsequent SAR studies on modifications at the C3 and C4 positions of the fused oxadiazine core helped to identify GSMs such as compounds 8r and 8s that were highly efficacious in vitro and in vivo in a number of animal models with highly desirable physical and pharmacological properties. Further improvements of in vitro activity and selectivity were achieved by the preparation of fused morpholine oxadiazines. The shift in specificity of APP cleavage rather than a reduction in overall γ-secretase activity and the lack of changes in substrate accumulation and Notch processing as observed in the animal studies of compound 8s confirm that the oxadiazine series of compounds are potent GSMs.

Iminoheterocycles as γ-secretase modulators

The synthesis of a novel series of iminoheterocycles and their structure-activity relationship (SAR) as modulators of gamma-secretase activity will be detailed. Encouraging SAR generated from a monocyclic core led to a structurally unique bicyclic core. Selected compounds exhibit good potency as gamma-secretase modulators, excellent rat pharmacokinetics, and lowering of Abeta42 levels in various in vivo models.

Quantum Chemical Calculations of 1JCC Coupling Constants for the Stereochemical Determination of Organic Compounds

Quantum chemical calculations of one-bond carbon-carbon coupling constants are demonstrated as potential probes for the configurational assignment of organic molecules. The stereochemical analysis of strychnine and its possible stereoisomers is presented as proof of concept.

Coniothyrione: anatomy of a structure revision.

Coniothyrione is a xanthone‐derived antibiotic reported several years ago by researchers at Merck & Co. Inc. Revision of the position of the chloro substitution was recently proposed on the basis of empirical reinterpretation of the carbon chemical shift data and a hypothetical biosynthetic argument without the acquisition of any new spectral data to support the postulated change in substituent location. The originally published HMBC data lead to an equivocal assignment of the structure and do not provide a solid basis of support for either structure. Neural network 13C chemical shift calculations and density functional theory calculations also led to undifferentiated structures. Definitive confirmation of the structure of coniothyrione based on the acquisition and interpretation of 1,1‐ADEQUATE and inverted 1JCC 1,n‐ADEQUATE data is now reported. Copyright

Synergistic Combination of CASE Algorithms and DFT Chemical Shift Predictions: A Powerful Approach for Structure Elucidation, Verification, and Revision

Structure elucidation of complex natural products and new organic compounds remains a challenging problem. To support this endeavor, CASE (computer-assisted structure elucidation) expert systems were developed. These systems are capable of generating a set of all possible structures consistent with an ensemble of 2D NMR data followed by selection of the most probable structure on the basis of empirical NMR chemical shift prediction. However, in some cases, empirical chemical shift prediction is incapable of distinguishing the correct structure. Herein, we demonstrate for the first time that the combination of CASE and density functional theory (DFT) methods for NMR chemical shift prediction allows the determination of the correct structure even in difficult situations. An expert system, ACD/Structure Elucidator, was used for the CASE analysis. This approach has been tested on three challenging natural products: aquatolide, coniothyrione, and chiral epoxyroussoenone. This work has demonstrated that the proposed synergistic approach is an unbiased, reliable, and very efficient structure verification and de novo structure elucidation method that can be applied to difficult structural problems when other experimental methods would be difficult or impossible to use.

Turning Spiroketals Inside Out: A Rearrangement Triggered by an Enol Ether Epoxidation

Spiroketals organize small molecule structures into well-defined, three-dimensional configurations that make them good ligands of proteins. We recently discovered a tandem cycloisomerization–dimerization reaction of alkynyl hemiketals that delivered polycyclic, enol-ether-containing spiroketals. Here we describe rearrangements of those compounds, triggered by epoxidation of their enol ethers that completely remodel their structures, essentially turning them “inside out”. Due to the high level of substitution on the carbon skeletons of the substrates and products, characterization resorted to X-ray crystallography and advanced computation and NMR techniques to solve the structures of representative compounds. In particular, a new proton-detected ADEQUATE NMR experiment (1,1-HD-ADEQUATE) enabled the unequivocal assignment of the carbon skeleton of one of the new compounds. Solution of the structures of the representative compounds allowed for the assignment of product structures for the other compounds in two separate series. Both the rearrangement and the methods used for structural determination of the products are valuable tools for the preparation of characterization of new small molecule compounds.

Novel Quinoline-Based P2-P4 Macrocyclic Derivatives As Pan-Genotypic HCV NS3/4a Protease Inhibitors.

We have previously reported the discovery of our P2-P4 macrocyclic HCV NS3/4a protease inhibitor MK-5172, which in combination with the NS5a inhibitor MK-8742 recently received a breakthrough therapy designation from the US FDA for treatment of chronic HCV infection. Our goal for the next generation NS3/4a inhibitor was to achieve pan-genotypic activity while retaining the pharmacokinetic profile of MK-5172. One of the areas for follow-up investigation involved replacement of the quinoxaline moiety in MK-5172 with a quinoline and studying the effect of substitution at 4-position of the quinoline. The rationale for this effort was based on molecular modeling, which indicated that such modifications would improve interactions with the S2 subsite, in particular with D79. We wish to report herein the discovery of highly potent inhibitors with pan-genotypic activity and an improved profile over MK-5172, especially against gt-3a and A156 mutants.