Chiral capillary electrophoresis-mass spectrometry of tetrahydroisoquinoline-derived neurotoxins: observation of complex stereoisomerism

Previous studies have shown that certain 1,2,3,4-tetrahydroisoquinoline derivatives (TIQs) are neurotoxins inducing Parkinsonism. Further, individual enantiomers of these toxins such as (R/S)-N-methylsalsolinol ((R/S)-NMSal) possess distinct neurotoxicological properties. In this work, a chiral capillary electrophoresis (CE) method with electrospray ionization-tandem mass spectrometric (ESI-MS/MS) detection was developed for the quantification of TIQ enantiomers. Enantioseparation was achieved with sulfated β-cyclodextrin (sulfated β-CD) as chiral selector. To avoid any potential contamination of MS ionization source by the non-volatile chiral selector, partial filling technique was deployed in the CE separation. TIQ derivatives, including (R/S)-6,7-dihydroxy-1-methy-TIQ (salsolinol, Sal), (R/S)-1-benzyl-TIQ (BTIQ), and (R/S)-NMSal, were base-line resolved with resolution values (R) ranging from 3 (for Sal) to 4.5 (for BTIQ), which were much better than those reported previously by HPLC methods. ESI-MS/MS detection of the resolved TIQ enantiomers was specific and sensitive (LOD=1.2 μM for Sal enantiomers). The proposed chiral CE-MS/MS method was used to study in vitro formation of (R/S)-NMSal. It was found that NMSal was formed from the incubation of epinine (a dopamine metabolite) with acetaldehyde (a metabolite of alcohol). More interestingly, four isomers of NMSal were separated and detected in the incubation solution. They were identified as (R)-e.e-NMSal, (R)-e.a-NMSal, (S)-e.e-NMSal, and (S)-e.a-NMSal. This was the first lab evidence that this Parkinsonian neurotoxin exists in multiple isomeric forms.     

A chiral synthesis of four stereoisomers of 1,3-dimethyl-1,2,3,4-tetrahydroisoquinoline, an inducer of Parkinson-like syndrome

Four stereoisomers of 1,3-dimethyl-1,2,3,4-tetrahydroisoquinoline, an inducer of Parkinson-like syndrome, were synthesized by applying a new method of 1,2,3,4-tetrahydroisoquinoline (TIQ) synthesis utilizing the Pummerer reaction as a key step. The chiral centers at C-1 and C-3 were constructed by two routes starting from alaninol (3) and 1-phenylethylamine (4) as a chiral source. Enantiomerically pure 1,3-dimethyl-TIQs (1R,3S)-(2) [corrected], (1S,3R)-(ent-2) [corrected], (1S,3S)-(1) [corrected], and (1R,3R)-(ent-1) [corrected] were prepared in a stereochemically unambiguous manner from 3 in 11 steps (route I) and from 4 in 6 steps (route II). The conformations of tetrahydroisoquinoline ring in 1-methyl, 3-methyl, and 1,3-dimethyl-TIQs were discussed on the basis of their CD, 1H-NMR spectra, and steric energies.     

Capillary and nano-liquid chromatography-tandem mass spectrometry for the quantification of small molecules in microdialysis samples: comparison with microbore dimensions

Enhanced sensitivity is a well known benefit of miniaturised LC-electrospray (ESI)-MS/MS methods. The suitability of miniaturised LC-MS/MS for quantification of small molecules in dialysates was investigated using the anti-epileptic drug oxcarbazepine, its active metabolite, 10,11-dihydro-10-hydroxycarbamazepine, and the internal standard for microdialysis probe calibration, 2-methyl-5H-dibenz(b,f)azepine-5-carboxamide, as test compounds. ESI-MS detection is sensitive to matrix effects. Therefore, dialysate matrix effects were investigated by comparing the responses of standards made in water, Ringer's solution (salt solution used as perfusion fluid) and blank dialysate matrix. Due to the occurrence of ion suppression or enhancement, direct injection of dialysis samples onto the analytical column could not be applied for quantification of small molecules in dialysis samples. Column switching was necessary for desalting and preconcentration of the dialysates. However, this approach was not able to completely eliminate salt effects when the injection volume exceeded 1 microL. No differences in response between Ringer's solution and dialysate matrix were detected at capillary and nano-dimensions. Calibration standards should be prepared with Ringer's solution instead of water for quantitative analysis of microdialysates. A microbore, capillary and nano-LC-ESI-MS/MS method were compared in terms of method feasibility, linearity, sensitivity, accuracy and precision. Downscaling to capillary and nano-dimensions resulted in a gain in detection sensitivity of 5 and 50, respectively. Miniaturised LC-MS/MS was found to be fit for quantification of small molecules in dialysates with acceptable accuracy and method precision.     

Synthesis and biological activity of 1-(aryloxy)-3-(6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinolin-2-yl)propan-2-ols

Treatment of 1-(4-chlorobenzylamino)-2-methylpropan-2-ol with concentrated sulfuric acid at 0°C gave 6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinoline which reacted with (aryloxymethyl)oxiranes to afford new propan-2-ol derivatives of the tetrahydroisoquinoline series, 1-(aryloxy)-3-(6-chloro-4,4-dimethyl-1,2,3,4-tetrahydroisoquinolin-2-yl)propan-2-ols. Some of the synthesized compounds or their hydrochlorides showed moderate adrenergic blocking and sympatholytic activities.     

Endogenous synthesis of N-methylsalsolinol, an analogue of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, in rat brain during in vivo microdialysis with salsolinol, as demonstrated by gas chromatography-mass spectrometry.

N-Methylsalsolinol, an analogue of 1,2,3,6-tetrahydropyridine, is present in the brains of patients with Parkinson's disease. To determine the metabolic pathway for the synthesis of N-Methylsalsolinol in the brain, salsolinol was perfused through the striatum or the substantia nigra of the rat brain by in vivo microdialysis. N-Methylsalsolinol was detected in the brain dialysate samples during microdialysis with salsolinol using gas chromatography-mass spectrometry with selected-ion monitoring. These results demonstrate that endogenous N-methylation of salsolinol into N-methylsalsolinol occurs in the brain in vivo.     

Enhancing capillary liquid chromatography/tandem mass spectrometry of biogenic amines by pre-column derivatization with 7-fluoro-4-nitrobenzoxadiazole

This paper describes a capillary liquid chromatography/tandem mass spectrometry (LC/MS/MS) determination of biogenic amines enhanced by pre-column derivatization with 7-fluoro-4-nitrobenzoxadiazole (NBD-F). Biogenic amines including tryptamine, N-methylsalsolinol, histamine, and agmatine were studied. The biogenic NBD-amine derivatives could be quantitatively enriched in-line on 20 x 0.25 mm capillary columns packed in-house with 5 microm C(8) silica particles. In an electrospray ionization (ESI) source these derivatives were ionized effectively, and collision-induced dissociation (CID) produced predominant characteristic ions allowing sensitive MS/MS detection. Agmatine, a potential neurotransmitter/modulator, was taken as a reference compound to study the analytical figures of merit of the procedure. The detection limit of agmatine was estimated to be 0.6 ng/mL (signal-to-noise (S/N) = 3). A linear calibration curve in the range 15-1000 ng/mL agmatine with an r value of 0.9997 was obtained. Tissue samples of rat brain, stomach, and intestine were analyzed. Minimum sample pre-treatment was needed. Each analysis was accomplished within ca. 12 min. The concentration of agmatine was found to be 0.246, 3.31, and 0.058 microg/g wet tissue in the brain, stomach, and intestine, respectively.     

Studies on Debenzylation and Photolysis of 1-Benzyl- and 1-(β-Phenethyl)-1,2,3,4-tetrahydroisoquinolines

Debenzylation of 1-(3-benzyloxybenzyl)-1,2,3,4-tetrahydroisoquinolines 1 , 6 , 7 with hydrochloric acid and ethanol gave the corresponding phenolic isoquinolines 2 , 8 , 9 and tetrahydroprotoberberines 4 , 12 , 13 . Compounds 2 , 8 , 9 on photolysis also gave, besides the expected noraporphines 3 , 10 , 11 , the tetrahydroprotoberberines 4 , 12 , 13 [1–4] (Schemes 1 and 2). 6-Benzyloxy-1-(5-benzyloxy-2-bromo-benzyl)-1,2,3,4-tetrahydroisoquinoline (27a) containing no methoxy or methylenedioxy groups either in ring A or C does not give protoberberine during debenzylation; but 28 , the debenzylation product of 27a , on photolysis gives both the noraporphine 29 and the tetrahydroprotoberberine 30 (Scheme 6), proving that during debenzylation of 1-(3-benzyloxybenzyl)-1,2,3,4-tetrahydroisoquinolines containing additional methoxy or methylenedioxy groups, the necessary formaldehyde comes from the latter groups. During photolysis both the methoxy groups (methylenedioxy groups) and the C(3) atom of the tetrahydroisoquinoline moiety provide the formaldehyde. Veratrole under debenzylation and photolytic conditions and tetrahydroisoquinoline under the latter condition also give rise to formaldehyde (Schemes 8 and 10). The novel bromohomoprotoberberine 43 along with 42 was formed during debenzylation of the 1-phenethyl-1,2,3,4-tetrahydroisoquinoline 41 . Photolysis of 42 yielded the novel nor-homoaporphine 44 , in addition to 43 ; the latter was debrominated to give the homoberbine 45 .     

Isolation and X-ray crystal structure of tetrahydroisoquinoline alkaloids from Calycotome villosa Subsp. intermedias

Two tetrahydroisoquinoline alkaloids were extracted from the alkaloid fraction of a methanol extract of the seeds of Calycotome Villosa Subsp. intermedia. Their structures were established as (R)-1-hydroxymethyl-7-8-dimethoxy-1,2,3,4-tetrahydro- isoquinoline (1) and (S)-7-hydroxymethyl-2-3-dimethoxy-7,8,9,10-tetrahydroisoquinoline chloride (2) by spectroscopic techniques and X-ray diffraction analysis.     

The preparation and oxidative dimerisation of 2-acetyl-7-hydroxy-1,2,3,4-tetrahydroisoquinoline. A new approach to tetrahydroisoquinoline synthesis

The title compound has been prepared from 1,2,3,4-tetrahydroisoquinoline via successive nitration, acet-ylation, reduction and diazotisation. Earlier conflicting reports on the nitration of tetrahydroisoquinoline have been clarified. A better synthetic route, based upon reduction of the corresponding isocarbostyril, has been devised and should be applicable to the obtention of a wide range of tetrahydroisoquinolines. The structure of the neutral product of oxidative dimerisation of 2-acetyl-7-hydroxy-1,2,3,4-tetrahydroisoquinoline has been established.     

Sensitive and simultaneous determination of HIV protease inhibitors in rat biological samples by liquid chromatography-mass spectrometry

A sensitive and simultaneous liquid chromatographic-mass spectrometric (LC/MS) method for the determination of current four HIV protease inhibitors (PIs), indinavir (IDV), saquinavir (SQV), nelfinavir (NFV) and amprenavir (APV) in rat plasma and liver dialysate by a microdialysis method was described. An isocratic LC/MS method in combination with atmospheric pressure chemical ionization was developed for the determination of these four PIs in biological samples in the same run. The analytes including an internal standard were extracted from 100 microL of plasma or 150 microL of liver dialysate samples by salting-out with 100 microL of ice-cold 2 M K(3)PO(4) followed by ether extraction. The separation of analytes was carried out on a reversed-phase semi-micro column using 50% of acetonitrile containing 1% acetic acid as mobile phase at a flow rate of 0.2mL/min(-1). The separation was completed within 5 min. Precision, recovery and limits of detection indicated that the method was suitable for the quantitative determination of these PIs in rat plasma or liver dialysate. This simple, sensitive and highly specific LC/MS method is suitable for pharmacokinetic studies and therapeutic drug monitoring in AIDS patients who receive double protease therapy.     

Azines of the Isoquinoline Series: III. Reactions of Substituted 1-Hydrazono-3,4-dihydroisoquinolines with Dicarbonyl Compounds

3,3-Dimethyl-1-hydrazono-1,2,3,4-tetrahydroisoquinoline derivatives react with benzil in 1 : 1 or 2 : 1 ratio to give the corresponding benzil monohydrazones. Diacetyl under the same conditions forms dihydrazones. Enolizing 1,3-dicarbonyl compounds form with 3,3-dimethyl-1-hydrazono-1,2,3,4-tetrahydroisoquinoline derivatives mixtures of mono- and dihydrazones. Dehydracetic acid forms monohydrazones at the exochain carbonyl group.     

Studies on the syntheses of heterocyclic compounds. Part CCCXXIX. Syntheses of i-spiroisoquinoline derivatives by phenolic cyclization

Phenolic cyclization of 2-(3-hydroxyphenyl)-2-methylethylamine (XIIIa) and 2-(3-hydroxyphenyl)phenethylamine (XIIIb) with various carbonyl compounds afforded eight types of corresponding 1-spirocycloalkano- and 1-spiroheterocycloalkano-1,2,3,4-tetrahydroisoquinoline derivatives (1-VIII) and 1,1-disubstituted-1, 2,3,4-tetrahydroisoquinoline derivative (IX). The acetyl derivatives of VI and IX and the benzoyl derivatives of III and V were also prepared. In addition, a synthetic method for obtaining the starting phenethylamines was examined.     

Synthesis and pharmacological evaluation of 2-(3-piperidyl)-1,2,3,4-tetrahydroisoquinoline derivatives as specific bradycardic agents

Novel 1,2,3,4-tetrahydroisoquinoline derivatives bearing directly a cyclic amine at the 2-position were prepared and examined for their bradycardic activities in isolated right atria and in anesthetized rats. The structure-activity relationships (SAR) study revealed that the 2-(3-piperidyl)-1,2,3,4-tetrahydroisoquinoline skeleton is essential for the appearance of potent in vitro activity, and that the presence of at least one methoxy group at the 6- or 7-position of the 1,2,3,4-tetrahydroisoquinoline ring is important to exert potent in vitro activity. In vivo tests of selected compounds demonstrated that 2-(1-benzyl-3-piperidyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (6c) exhibited potent bradycardic activity with negligible influence on mean blood pressure in rats, although its potency is a half of that of Zatebradine.     

Electrochemical Oxidations of Central Nervous System Indoleamines,Catecholamines and Alkaloids: Potential Significance into Neurodegenerative Diseases

The serotonergic neurotransmitter 5-hydroxytryptamine (5-HT), the catecholaminergic neurotransmittcr dopaminc (DA) and various tetrahydroisoquinoline (TIQ) and tetrahydro-β-carboline (THβC) alkaloids are all easily oxidized compounds. Aberrant oxidative transformations of 5-HT and other central indoles might be involved in neurodegenerative Alzheimer's Disease (AD). Changes in the oxidation chemistry of DA appear to be fundamental in substantia nigra neurons in Parkinson's Disease (PD). Various TIQ and THβC alkaloids are elevated in the brain as a result of ethanol drinking. Recent studies into the electrochemical oxidation chemistry of 5-HT, DA and various TIQ and THβC alkaloids have been reviewed. The potential roles of the oxidation chemistry of these compounds in neurodegenerative AD, PD and alcoholism are discussed.     

Synthesis and pharmacological evaluation of 1-isopropyl-1,2,3,4-tetrahydroisoquinoline derivatives as novel antihypertensive agents

A series of 1-isopropyl-1,2,3,4-tetrahydroisoquinoline derivatives were synthesized and their bradycardic activities were evaluated in isolated guinea pig right atria. Structure-activity relationship studies revealed that the introduction of an appropriate substituent and its position on the 1,2,3,4-tetrahydroisoquinoline ring are essential for potent in vitro activity. Furthermore, the tether between the piperidyl moiety and the terminal aromatic ring is important for potent antihypertensive activity. Oral administration of 6-fluoro-1-isopropyl-2-{[1-(2-phenylethyl)piperidin-4-yl]carbonyl}-1,2,3,4-tetrahydroisoquinoline (3b) to spontaneously hypertensive rats (SHR) elicited antihypertensive effects without inducing reflex tachycardia, which is often caused by traditional L-type Ca2? channel blockers.     

Quantification of salsolinol enantiomers by stable isotope dilution liquid chromatography with tandem mass spectrometric detection

Salsolinol, 1‐methyl‐6,7‐dihydroxy‐2,3,4,5‐tetrahydroisoquinoline (SAL), is a precursor of a Parkinsonian neurotoxin, N‐methysalsolinol (N‐methyl‐SAL). Previous studies have shown that individual enantiomers of N‐methyl‐SAL possess distinct neurotoxicological properties. In this work, a chiral high‐performance liquid chromatography (HPLC) method with electrospray ionization tandem mass spectrometric (ESI‐MS/MS) detection was developed for the quantification of (R/S)‐SAL enantiomers. Enantioseparation was achieved on a β‐cyclodextrin‐bonded silica gel column, and the resolved enantiomers were detected by ESI‐MS/MS operated in positive ion mode. The ESI collision‐induced dissociation (CID) mass spectrum of SAL was studied together with that of its deuterium‐labeled analog (i.e. salsolinol‐α,α,α,1‐d₄, SAL‐d₄) so that the fragmentation pathways could be elucidated. Further, using SAL‐d₄ as internal standard in HPLC/MS/MS analysis of SAL improved significantly assay accuracy and reliability. Determination of (R/S)‐SAL enantiomers present in food samples such as dried banana chips was demonstrated. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of new 2-substituted 6,7-dimethoxy-1-(methylcarbamoyl)-4-spirocyclopentane-1,2,3,4-tetrahydroisoquinolines

New 2-(heterylmethyl) derivatives were synthesized from 6,7-dimethoxy-4-spirocyclopentane-1,2,3,4-tetrahydroisoquinoline-1-carboxylic acid N-methylamide and heterylmethyl chlorides. The reactions of 2-chloroacetyl-substituted tetrahydroisoquinoline with versatile secondary amines and heterylthiols afforded the corresponding 2-aminoacetyl and 2-(heterylsulfanylacetyl) derivatives of tetrahydroisoquinoline series.     

An effective method for the preparation of mono N-alkyl derivatives of 1,1′-bis(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline)

The condensation of 1,1′-bis(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline) with alkyl, aralkyl and aryl aldehydes, but not ketones, in ethanol or chloroform provides useful cyclic aminal [8-substituted 5,6,10,11,15b,15c-hexahydro-2,3,13,14-tetramethoxy-8H-imidazo[5,1-a:4,3-a′]diisoquinoline] intermediates that when subsequently treated with sodium cyanoborohydride in ethanol, followed by the addition of 2 M hydrochloric acid, gave monosubstituted N-alkyl 1,1′-bis(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline) derivatives in very high yields. The rates of the initial condensation with four different aldehydes were measured, and the entire sequence was successfully applied in one example to a ‘one-pot’ process; this signals a versatile route to differentially N-substituted 1,1′-bis(1,2,3,4-tetrahydroisoquinoline) derivatives.     

Complete assignments 1H and 13C NMR spectral data of four anabaseine derivatives

The anabaseine derivatives 6-methoxy-7-hydroxy-1-(pyridin-3-yl)-3,4-dihydroisoquinoline, 6,7-dimethoxy-1-(pyridin-3-yl)-1,2,3,4-tetrahydroisoquinoline and 6,7-dimethoxy-1-(piperidin-3-yl)-1,2,3,4-tetrahydroisoquino- line were prepared either by demethylation with HBr or by reduction with different reagents, NaBH4 and H2/PtO2 from 6,7-dimethoxy-1-(pyridin-3-yl)-3,4-dihydroisoquinoline, as starting material. The structures have been fully assigned by the combination of one- and two-dimensional experiments.     

Synthesis of novel isoquinoline derivatives as potential CNS-agents

A series of 4-aminomethyl-1,2,3,4-tetrahydroisoquinoline derivatives were prepared as potential CNS-agents acting via amino-acid neurotransmitter systems. The compounds were synthesized from 1,2,3,4-tetra-hydro-1-oxoisoquinoline-4-carboxylic acids obtained by dipolar cycloaddition reactions of imines with homo-phthalic anhydride. Among the compounds tested 5c and 5m showed sub-micromolar affinity for the NMDA receptor and represent a structurally novel class of ligand for this site.