Boris Weiss-López

Use of long-range C-H (nJ n>3) heteronuclear multiple bond connectivity in the assignment of the 13C NMR spectra of complex organic molecules

The structural elucidation of complex organic molecules relies heavily on the application of proton detected heteronuclear NMR. Among these techniques, the HMBC NMR experiment is probably the most useful 2D NMR method The HMBC (C-H) experiment allows the assignment of structural fragments through correlations between protons and carbons separated by more than one bond, usually two or three bonds (JCH and JCH) via 1H,13C-coupling constants. It is also possible to obtain valuable information through longer correlations, JCH n>3, performing several HMBC experiments with different long-range delays and using a deeper threshold in the contour plot. There have been several attempts to improve the results of the HMBC experiment, mainly focused on the question of optimization of the longrange delay, ∆2. The D-HMBC, 3D-HMBC, CT-HMBC, ACCORD-HMBC, IMPEACH-MBC and CIGARHMBC experiments which provide much better experimental access to sample long-range couplings are briefly discussed. These long-range correlations have proven to be crucial in the structure elucidation of molecules with proton deficient skeleton. INTRODUCTION concert with Heteronuclear Multiple Quantum Coherence, HMQC [7], has proven to be extremely useful for the total structure elucidation and NMR spectral assignments of numerous natural products and complex organic molecules. At the beginning, the sensitivity of HMBC was low as compared with HMQC; however, this characteristic has improved significantly with the introduction of pulsed field gradients [8-10] into these experiments in the early 1990 s. It allowed the receiver gain of the spectrometer to be significantly raised, since the unwanted coherences were already filtered off in the probe head. Moreover, the addition of pulsed field gradients into NMR pulse sequences yields spectra with fewer artifacts and decreases the data collection time, because the selection of the desired coherence pathways occurs without extensive phase cycling. Today, the gradient modification of the HMBC sequence [11] has become a routine standard, accessible to most operators of NMR instruments capable to generate pulsed gradients. In this review we will focus our attention only on Heteronuclear Multiple Bond Correlation (HMBC) [1-3] and modifications of it. Other types of heteronuclear correlation experiments will not be treated here, since HMBC is the most widely used experiment to observe 13C-1H long range couplings. The structural elucidation of complex organic molecules relies heavily on the application of proton detected heteronuclear NMR. Among these techniques, the HMBC NMR experiment is probably the most useful 2D NMR method [4], since it detects 13C-1H long range couplings using inverse detection of the 1H signal, the most sensitive NMR nucleus. Inverse detection techniques also present a considerably higher sensitivity when compared to older 2D experiments [5]. The sensitivity is particularly good when the 1H signals to be observed appear as sharp lines. The HMBC experiment gives a wealth of structural and assignment information through long-range correlation signals for C,H spin pairs, that can span quaternary carbons or heteronuclei, providing a way to link structural fragments together. Therefore, it can be efficiently used to elucidate the molecular skeleton. Two reviews on this topic were published about ten years ago [5,6]. The use of HMBC in Generally, the HMBC (C-H) experiment is described as a technique that allows the assignment of structural fragments through correlations between protons and carbons that are separated by more than one bond, usually through two or three bonds (JCH and JCH) [11,12] via 1H,13C-coupling constants, despite the early observation of a crucial four bond C-H correlation in the HMBC spectrum of antibiotic distamycin A [5], The valuable information that can be obtained through these correlations (JCH n>3 ), is generally discarded because the relative intensities of the resonances are directly related to the magnitude of the coupling constants. Therefore, for JCH n>3 the cross peaks show a low intensities. This characteristic has been used as a criterion to *Address correspondence to this author at the Departamento de Quimica Organica y Fisicoquimica, Facultad de Ciencias Quimicas y Farmaceuticas, Universidad de Chile, Casilla 233, Santiago 1, Chile; Tel.: +56-2-6782865; Fax: +56-2-6782868; e-mail: raraya@ciq.uchile.cl 254 Araya-Maturana et al. discard signals in a computational method of analysis of 2D NMR spectra [13]. have values from 1 to 25 Hz. In practice, a delay shorter than the theoretical value, 65 to 100 ms, is employed to avoid the decay of the 1H magnetization during this delay [15]. Usually, the experimental settings of these parameters in the HMBC experiment are the result of a compromise: when the spin coupling constant is about 8 Hz, the ∆2 delay in the sequence is about 60 ms, allowing an optimum transfer for correlations. EXPERIMENTAL DETAILS When the signals appear as broad lines due to complex splittings, HMBC suffers from a considerable lack of sensitivity. As a consequence, the detection of cross peaks becomes difficult. This problem arises when the power mode data processing causes the cancellation of antiphase signal components. The situation is made worse when the separation of these components is small and when their signals are broad. The efficiency of the HMBC technique is affected also by spectrometer instabilities resulting in t1noises ridges, which also originate from protons bound to 12C. This problem is solved by the application of pulsed field gradients [8-10] leading to much better results in a fraction of the time, since the unwanted coherences are already filtered off in the probe head. The pulse sequence of the HMBC experiment is shown in Fig. (1) [11]. Running the experiment under these conditions, several important couplings could give rise to only very small correlation signals, or they may be completely lost. Furthermore, the direct translation of the connectivities observed in the HMBC spectrum into the bonding network, may be hampered by the fact that JCH and 3J correlations cannot be distinguished [13]. Different methods have been developed to solve both problems mentioned before. In particular, a method to distinguish between JCH and JCH correlations obtained in HMBC experiments has been described some years ago. The experiment is known as 1,1-ADEQUATE and yields only two bond 1H-13C connectivities in H-C-C moieties, allowing differentiation of HMBC from two and three 1H-13C bond connectivities [12, 15]. An additional advantage of this method is that it permits to observe correlations that are missing in the HMBC experiment. A second problem associated with inverse protondetected heteronuclear shift correlation experiments is the lack of resolution in the indirectly detected dimension (F1). For a given spectral width, an increase in F1 resolution requires an increase in the number of t1 increments. Generally, not all spectral regions are crowded enough to need such a treatment. F1 restricted 2D maps can be a great help to ensure a proper spectral analysis [14]. Theoretically, in the HMBC pulse sequence the optimum choice for the first delay is calculated from the expression ∆1 = 1/(2 1J(C,H)). Generally, the 1J(C,H) coupling constants span a range of values from 130 to 160 Hz. The 1J(C,H) filter delay in the pulse sequence currently implemented, is calculated by entering an average value of JCH, usually 140 or 125 Hz, depending on whether there are aromatic or alkene groups, respectively, giving a value of ∆1 = 3.6 or 4.0 ms. In the same manner, the optimum value of the second delay is calculated from ∆2 = 1/(2 nJ(C,H)), where nJ(C,H) is the long range coupling constant. However a drawback of the experiment is due to the range of values of 2J (C,H) and 3J (C,H) spin coupling constants, which can In general, the observed correlations in different HMBC experiments depend on the value the long-range delay, obtained from the individual long-range C-H coupling constant responsible for creating the heteronuclear multiple quantum coherence. Usually, the long-range delay is optimized for a value between 5 and 10 Hz for 1H-13C longrange correlation experiments. The choice is generally made on an arbitrary basis rather than from a knowledge of the actual value of the couplings. On this basis, a first approximation to observe more long-range correlations is to perform several HMBC experiments with different long-range delays and using a deeper threshold in the contour plot. Each one of the spectra obtained in this series of experiments, will show different long-range correlations, according to the value of nJ(C,H). Actually, different delay times will enhance the proper signals and the rest may not be detected. This technique, optimized for small couplings, was employed to observe two and four-bond 1H-13C correlations and unambiguously assign the 13C NMR signals of several ∆ 1 ∆ 2 p1 t1/2 t1/2 aq p2 1H

Mitochondria: A Promising Target for Anticancer Alkaloids

A great number of alkaloids exhibit high potential in cancer research. Some of them are anticancer drugs with well-defined clinical uses, exerting their action on microtubules dynamics or DNA replication and topology. On the other hand, mitochondria have been recognized as an essential organelle in the establishment of tumor characteristics, especially the resistance to cell death, high proliferative capacity and adaptation to unfavorable cellular environment. Interestingly, many alkaloids exert their anticancer activities affecting selectively some functions of the tumor mitochondria by 1) modulating OXPHOS and ADP/ATP transport, 2) increasing ROS levels and mitochondrial potential dissipation by crosstalk between endoplasmic reticulum (ER) and mitochondria, 3) inducing mitochondria-dependent apoptosis and autophagy, 4) inhibiting mitochondrial metabolic pathways and 5) by alteration of the morphology and biogenesis of this organelle. These antecedents show the relevance of developing research about the effects of alkaloids on functions controlled by tumor mitochondria, offering an attractive target for the design of new alkaloid derivatives, considering organelle- specific delivery strategies. This review describes mitochondria as a central component in the anticancer action of a set of alkaloids, in a way to illustrate the importance of this organelle in medicinal chemistry.

Regioselectivity in the Diels-Alder reaction of 8,8-dimethylnaphthalene-1,4,5(8H)-trione with 2,4-hexadien-1-ol

Abstract The Diels-Alder reactions of 8,8-dimethylnaphtalene-1,4,5(8 H )-trione with 2,4-hexadien-1-ol and its O -acetyl derivative were investigated in different solvents. The regiochemistry of the cycloaddition of the hexadienol was determined through chemical correlation of one of the products. The solvent effect on the regioselectivity and endo/exo selectivity of this reaction is attributed to intermolecular hydrogen bonding between the hydroxyl group of the diene and the carbonyl oxygen atoms at C-4 and C-5 of the quinone in the transition state. The possible transition states have been modelled by AM1 calculations in order to better interpret these experimental results.

Improved Selective Reduction of 3-Formylchromones Using Basic Alumina and 2-Propanol

Abstract Treatment of formylchromones, dissolved in 2-propanol with basic alumina at 75°C, selectively reduces the formyl group with good yields without any activation process of the alumina.

Intramolecular hydrogen bond in biologically active o-carbonyl hydroquinones.

Intramolecular hydrogen bonds (IHBs) play a central role in the molecular structure, chemical reactivity and interactions of biologically active molecules. Here, we study the IHBs of seven related o-carbonyl hydroquinones and one structurally-related aromatic lactone, some of which have shown anticancer and antioxidant activity. Experimental NMR data were correlated with theoretical calculations at the DFT and ab initio levels. Natural bond orbital (NBO) and molecular electrostatic potential (MEP) calculations were used to study the electronic characteristics of these IHB. As expected, our results show that NBO calculations are better than MEP to describe the strength of the IHBs. NBO energies (∆Eij(2)) show that the main contributions to energy stabilization correspond to LP→σ* interactions for IHBs, O1…O2-H2 and the delocalization LP→π* for O2-C2 = Cα(β). For the O1…O2-H2 interaction, the values of ∆Eij(2) can be attributed to the difference in the overlap ability between orbitals i and j (Fij), instead of the energy difference between them. The large energy for the LP O2→π* C2 = Cα(β) interaction in the compounds 9-Hydroxy-5-oxo-4,8, 8-trimethyl-l,9(8H)-anthracenecarbolactone (VIII) and 9,10-dihydroxy-4,4-dimethylanthracen-1(4H)-one (VII) (55.49 and 60.70 kcal/mol, respectively) when compared with the remaining molecules (all less than 50 kcal/mol), suggests that the IHBs in VIII and VII are strongly resonance assisted.

Surface-enhanced Raman scattering activity of negatively charged bio-analytes from a modified silver colloid

ABSTRACT A modification to the traditional synthesis of reduced silver metal nanoparticles with hydroxylamine hydrochloride is proposed. The new colloid, obtained by decreasing the concentration of hydroxide and chloride ions, is stable in a large pH range. The low negative surface charge density of the nanoparticles allowed to obtain surface-enhanced Raman scattering (SERS) spectral activity of bio-analytes containing negatively charged groups in aqueous solution. The new colloidal system, characterized by electronic spectroscopy, atomic force microscopy, and zeta potential measurements, was used in the SERS study of the bio-analytes aspartic acid, four oligopeptides, one polypeptide and a probe, the organic dye fluorescein.

NMR assignment in regioisomeric hydroquinones

A set of regioisomeric pairs of tricyclic hydroquinones, analogues of antitumor 9,10‐dihydroxy‐4,4‐dimethyl‐5,8‐dihydroanthracen‐1(4H)‐one (1) and other derivatives, were synthesized and their regiochemistry and NMR spectra assigned by using 1H‐detected one‐bond (CH) HMQC and long‐range CH HMBC, in good agreement with theoretical O3LYP/Alhrichs‐pVTZ calculations. The 5‐hydroxymethyl derivatives (11, 15, 19) showed a 3JH, H coupling constant of methylene protons evidencing the presence of a seven‐membered intramolecular hydrogen bonded ring, not observed for the 8‐hydroxymethyl isomers. Copyright

Long-range correlations ( n j C,H n > 3 ) in the HMBC spectra of 3-(4-oxo-4H-chromen-3-YL)-acrylic acid ethyl esters

A systematic study about the long-range correlations observed in the HMBC spectra of a series of cis and trans 3-(4-oxo-4H-chromen-3-yl)-acrylic acid ethyl esters, and 5-hydroxy derivatives, was carried out. The results were compared with those observed in IMPEACH-MBC and CIGARHMBC spectra. An uncommon 5JC,H between the methyl protons and the Ca to the carbonyl group was observed in these systems as well as in ethyl chrotonate. The complete assignments of the 1H and 13C NMR spectra are provided.

Infrared spectra of nitrostyrene derivatives

Abstract The IR spectra of a series of 6 phenylnitroethenes (PNE) and 6 phenylnitropropenes (PNP) were assigned, based on a normal coordinates calculation performed on styrene and styrene-d 8 . Some frequencies were sensitive to the electronic properties of the substituents, and others to the substitution on Cβ. AM1 minimum energy conformations and rotational barriers around the ClCα bond were calculated. According to these calculations, PNE are planar and PNP display an angle of 45° between the ethylenic and aromatic planes. AM1 underestimates the height of the electronic barriers, which is however modulated by the electron donor properties of the substituent on the ring. A correlation between the calculated electronic barrier and the CC ethylenic stretching frequency was observed.

Determinants of Anti-Cancer Effect of Mitochondrial Electron Transport Chain Inhibitors: Bioenergetic Profile and Metabolic Flexibility of Cancer Cells

Recent evidence highlights that energy requirements of cancer cells vary greatly from normal cells and they exhibit different metabolic phenotypes with variable participation of both glycolysis and oxidative phosphorylation (OXPHOS). Interestingly, mitochondrial electron transport chain (ETC) has been identified as an essential component in bioenergetics, biosynthesis and redox control during proliferation and metastasis of cancer cells. This dependence converts ETC of cancer cells in a promising target to design small molecules with anti-cancer actions. Several small molecules have been described as ETC inhibitors with different consequences on mitochondrial bioenergetics, viability and proliferation of cancer cells, when the substrate availability is controlled to favor either the glycolytic or OXPHOS pathway. These ETC inhibitors can be grouped as 1) inhibitors of a respiratory complex (e.g. rotenoids, vanilloids, alkaloids, biguanides and polyphenols), 2) inhibitors of several respiratory complexes (e.g. capsaicin, ME-344 and epigallocatechin-3 gallate) and 3) inhibitors of ETC activity (e.g. elesclomol and VLX600). Although pharmacological ETC inhibition may produce cell death and a decrease of proliferation of cancer cells, factors such as degree of inhibition of ETC activity by small molecules, bioenergetic profile and metabolic flexibility of different cancer types or subpopulations of cells in a particular cancer type, can affect the impact of the anti-cancer actions. Particularly interesting are the adaptive mechanisms induced by ETC inhibition, such as induction of glutamine-dependent reductive carboxylation, which may offer a strategy to sensitize cancer cells to inhibitors of glutamine metabolism.