Raúl G. Enríquez

Total assignment of 13C and 1H spectra of three isomeric triterpenol derivatives by 2D NMR: an investigation of the potential utility of 1H chemical shifts in structural investigations of complex natural products

It is shown that a recently proposed indirect heteronuclear shift-correlated pulse sequence, XCORFE, can be used to unambiguously assign the 13C spectra of three isomeric (C30H50O) triterpenols: taraxasterol (1a), pseudo-taraxasterol (2) and lupeol (3). This sequence gives excellent resolution combined with sensitivity far in excess of that given by 13C-13C connectivity experiments. Direct heteronuclear shift-correlated spectra are used to totally assign 1H spectra for 1a, 2, 3 and taraxasteryl acetate (1b). 1H chemical shifts are mainly sensitive to local environment and often show values which are characteristic of a particular environment. Knowledge of 1H chemical shifts and splitting patterns also places useful constraints on assignment of 13C chemical shifts for 13C1Hn units. It is strongly recommended that natural products chemists routinely use 2D NMR to assign 1H chemical shifts of complex organic derivatives in order to build up a data bank of 1H spectral data.

Assignment of 1H and 13C spectra and investigation of hindered side‐chain rotation in lupeol derivatives

Complete 1H and 13C spectral assignments are reported for lupeol (1a) and two derivatives where the C‐30 methyl group is replaced by CH2OH (1b) and HC O (1c). Compound 1c shows conformationally dependent substituent effects on 1H chemical shifts. It also shows line broadening of some 13C signals at 25 °C, suggesting hindered rotation of the side‐chain group. This is confirmed by low‐temperature spectra which show splitting of broadened peaks into pairs in a ca 2 : 1 area ratio. The free energy of activation of hindered rotation is estimated as 13.5 kcal mol−1. By contrast, 1a shows no evidence of hindered rotation down to −40 °C although NOE data suggest the presence of two conformers. Spartan molecular mechanics calculations confirm the presence of two stable conformers for 1a and 1c but overestimate the rotational barrier in 1a. The additional barrier in 1c probably reflects loss of conjugative stabilization during rotation. Copyright

Comparison of13C Resolution and Sensitivity of HSQC and HMQC Sequences and Application of HSQC-Based Sequences to the Total1H and13C Spectral Assignment of Clionasterol

It is demonstrated that HSQC gives considerably better 13C resolution and sensitivity than HMQC for CH2 groups of natural products, particularly when combined with linear prediction. Similarly, coupled HSQC spectra provide a useful method for the determination of 1H multiplet structure and consequent assignment of individual CH2 protons as axial or equatorial in fused cyclohexane rings. These and related techniques are used to assign 1H and 13C spectra of the marine sterol clionasterol.

Hypotensive and vasorelaxant effects of the procyanidin fraction from Guazuma ulmifolia bark in normotensive and hypertensive rats

THE AIM OF THE STUDY was to investigate the in vivo and in vitro cardiovascular activity of a procyanidin fraction (PCF) obtained from acetone extract of Guazuma ulmifolia bark which has traditionally been used as an antihypertensive agent. RESULTS 10 mg/kg PCF doses orally administered to sugar-fed hypertensive rats decreased both the systolic arterial pressure and the heart rate, whereas the same doses intravenously administered induced arterial hypotension which was attenuated by NG-nitro-L-arginine methylester (L-NAME 31 mg/kg) pretreatment. In these experiments we employed carbachol as a positive control test. The PCF reduced the contraction induced by norepinephrine (1x10(-7) M) in isolated aortic rings of normotensive (IC50=35.3+/-12.4 ng/mL) and sugar-fed hypertensive (IC50=101.3+/-57.2 ng/mL) rats. This relaxant activity was inhibited by either vascular endothelium removal or L-NAME (30 microM) pretreatment, while indomethacin (10 microM) or atropine (10 microM) had no effect. Preliminary analysis of the PCF by HPLC-DAD-MS and FAB+ mass spectrometry allowed the detection of the main components such as the complex of procyanidin oligomers consisting mainly of tetramers and trimers. CONCLUSIONS Guazuma ulmifolia bark possesses long-lasting antihypertensive and vasorelaxing properties linked to the endothelium related factors, where nitric oxide is involved.

Investigation of the Advantages and Limitations of Forward Linear Prediction for Processing 2D Data Sets

Although the potential advantages of f1 forward linear prediction for the processing of multi‐dimensional NMR spectra are well established, this method is surprisingly little used for 2D spectra used for organic structure determination. A detailed investigation of the advantages and limitations of f1 forward linear prediction for this purpose is reported. This is a reliable technique which is particularly useful for 1H‐detected 13C–1H shift correlation spectra, allowing up to 16‐fold linear prediction of the 13C axis of HSQC spectra. In general, the use of linear prediction allows one to obtain comparable 2D spectra in one quarter of the time or double the sensitivity in a comparable time relative to similar spectra without linear prediction. The one exception is the absolute value COSY spectrum, where linear prediction beyond a factor of two gives poor results. Linear prediction is generally superior to zero filling as a time‐saving technique, although the difference between the two approaches disappears as the f1 data point resolution approaches the natural linewidth. By contrast, f2 forward linear prediction is not recommended.

A pulse sequence which provides rapid, routine 1H13C shift-correlated spectra

Abstract A pulse sequence is evaluated which gives heteronuclear shift-correlated 2D spectra with full 1H1H decoupling by using a fixed 1H evolution time, T, in combination with incrementally stepped 1H and 13C refocusing pulses. Due to the nature of the sequence, T is restricted to one of two possible values: 0.012 ± 0.002 s and 0.020 ± 0.002 s. Using either of these values, the sequence gives excellent sensitivity and adequate 1H resolution making it feasible to obtain very rapid, routine shift-correlated spectra, even with very limited amounts of sample.

Pharmacology of Casimiroa edulis; III. Relaxant and contractile effects in rat aortic rings

The relaxant and contractile effects of an aqueous extract of the seeds of the hypotensive plant Casimiroa edulis were investigated in rat aortic rings. The extract inhibited contractions elicited by noradrenaline, serotonin and prostaglandin F2 alpha, but did not affect responses to KCl. Inhibition did not require the presence of intact vascular endothelium and was not affected by histamine antagonists. In this preparation, the extract also elicited concentration-related contractions which were more marked in the absence of endothelium, were not blocked by histamine antagonists, and were completely suppressed by alpha-adrenergic blockade. It was concluded that the relaxant effect of the extract is not exerted through release of an endothelial relaxing factor nor through blockade of calcium channels or of specific smooth muscle receptors, and does not involve histaminergic mechanisms. The contractile effect is modulated by vascular endothelium and is alpha-adrenergic in nature.

Pharmacology of Casimiroa edulis IV. Hypotensive effects of compounds isolated from methanolic extracts in rats and guinea pigs

Bioassay-directed fractionation of the methanolic extract of seeds of Casimiroa edulis led to the isolation of seven constituents with cardiovascular activity, namely the new compound synephrine acetonide and the known compounds N-monomethylhistamine, N,N-dimethylhistamine, proline, N-methylproline, gamma-aminobutyric acid and casimiroedine. In anesthetized rats, both histamine derivatives produced transient hypotension mediated via H1-histaminergic receptors and in the case of N,N-dimethylhistamine, via nitric oxide release. Synephrine acetonide produced transient hypertension and tachycardia, mediated via alpha- and alpha- and beta-adrenergic receptores, respectively. The chromatographic zone containing N-methyproline, proline and gamma-aminobutyric acid elicited marked and prolonged hypotension. Finally, casimiroedine did not modify the blood pressure of anesthetized rats, but lowered it persistently in anesthetized guinea pigs. It was concluded that hypotension produced by C. edulis is due to several active components. The immediate effect can be attributed to the histamine derivatives acting on H1-receptors. More prolonged hypotension would be produced by the mixture of amino acids through an unknown mechanism, as well as by casimiroedine, possibly by activation of H3-receptors. Hypotension is partially offset by synephrine acetonide through adrenergic mechanisms.

1) H and (13) C NMR characterization of new cycloartane triterpenes from Mangifera indica.

From the stem bark of Mangifera indica, seven cycloartane‐type secondary metabolites were isolated. Compound 1 has been isolated for the first time from M. indica, whereas compounds 2 (2a and 2b, as an epimeric mixture), 3, and 4 are new triterpenoid‐type cycloartanes. Unambiguous 13C and 1H NMR assignments for these compounds and the known compounds mangiferonic acid (compound 5), isomangiferolic acid (compound 6), ambolic acid (compound 7), and friedelin (compound 8) are reported; the latter because full NMR data for these compounds are not available in the literature. Copyright

Room-temperature (1H, 13C) and variable-temperature (1H) NMR studies on spinosin

Spinosin, a C‐glycoside flavonoid, was isolated from the plant Desmodium tortuosum (Sw.) DC. The 1H and 13C NMR data acquired at room temperature exhibited doubling of signals, suggesting the presence of two rotamers in solution. Variable‐temperature 1H NMR experiments confirmed this hypothesis. T‐ROESY experiments in conjunction with theoretical (MM2) calculations supported the proposal that the two rotamers interchange via rotation about the C‐6— C‐1′′ bond. The spectral data were completely assigned using 1H– 1H COSY, HMQC, HSQC and HMBC experiments. Copyright