Reaction of dopamine with D-glyceraldehyde under biomimetic conditions: stereoselective formation of tetrahydroisoquinolines and rate-accelerating effects of transition metal ions

Reactions of d-glucose with phenolic amino acids: further insights into the competition between Maillard and Pictet–Spengler condensation pathways

The reactions of 5-S-cysteinyldopa, L-alpha-methyldopa and DL-m-tyrosine with D-glucose were investigated at 90 degrees C in phosphate buffer at pH ranging from 5.0 to 9.0. Whereas gave mainly the double Maillard condensation product N,N'-bis(1''-deoxy-D-fructos-1''-yl)-5-S-cysteinyldopa, as an inseparable mixture of beta-D-fructopyranosyl and alpha,beta-D-fructofuranosyl derivatives, 2 and 3 gave both Maillard and Pictet-Spengler products, although to different extents and with different regio- and stereochemistry. A peculiar pattern of reactivity was displayed by which gave, besides the Maillard product and the expected 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline C-1 diastereoisomeric pairs, the unprecedented 7,8-dihydroxy-1,2,3,4-tetrahydroisoquinoline derivative via the ortho cyclization pathway. Pictet-Spengler cyclization of 2 and 3 proceeded with Felkin-Anh-type asymmetric induction, favouring the 1R isomer throughout the pH range 5.0-9.0. These results, which highlight the first example of carbohydrate-derived 7,8-dihydroxytetrahydroisoquinoline, provide new insights into the factors governing competition between Maillard and Pictet-Spengler condensation pathways.

Tetrahydrobiisoquinoline Derivatives by Reaction of Dopamine with Glyoxal: A Novel Potential Degenerative Pathway of Catecholamines under Oxidative Stress Conditions

In 0.1 M phosphate buffer, pH 7.4, dopamine reacts with glyoxal, a cytotoxic and genotoxic alpha-oxoaldehyde produced by oxidative degradation of carbohydrates, to give three main products, two of which could be isolated and identified as the isomeric tetrahydrobiisoquinolines 1 and 2 by extensive two-dimensional NMR and mass spectrometric analysis. Time course studies indicated that 1 is the first intermediate in the process and changes slowly to 2 via an unstable species that escaped all efforts at isolation and structural identification. Products 1 and 2 were detected also among the species formed by the interaction of dopamine with oxidized carbohydrates, such as glucose, ribose, and fructose. Mechanistic evidence suggests that the formation of 1 proceeds by an unusual reaction pathway involving intramolecular cyclization of a double Schiff base intermediate followed by glyoxal-induced oxidation of the resulting octahydrobiisoquinoline intermediate (4). Subsequent conversion of 1 to 2 would involve a complex redox mechanism depending on an initial oxidation step. Product 2 was only poorly toxic to PC12 cells, whereas its methylated derivative 3 was as toxic as salsolinol, an established neurotoxin. Overall, these results throw light on a novel pathway of dopamine modification of potential relevance to the mechanisms underlying neurodegenerative changes in Parkinson's disease and other disorders characterized by a prooxidant state.

An unusual decarboxylative Maillard reaction between L-DOPA and D-glucose under biomimetic conditions: factors governing competition with Pictet-Spengler condensation

In 0.1 M phosphate buffer at pH 7.4 and 37 degrees C, the tyrosine metabolite L-3,4-dihydroxyphenylalanine (L-DOPA) reacts smoothly with D-glucose to afford, besides diastereoisomeric tetrahydroisoquinolines 1 and 2 by Pictet-Spengler condensation, a main product shown to be the unexpected decarboxylated Amadori compound N-(1-deoxy-D-fructos-1-yl)-dopamine (3). Under similar conditions, dopamine gave only tetrahydroisoquinoline products 4 and 5, whereas L-tyrosine gave exclusively the typical Amadori compound 6. Fe(3+) and Cu(2+) ions, which accumulate in relatively high levels in parkinsonian substantia nigra, both inhibited the formation of 3. Cu(2+) ions also inhibited the formation of 1 and 2 to a similar degree, whereas Fe(3+) ions increased the yields of 1 and 2. Apparently, the formation of 3 would not be compatible with a simple decarboxylation of the initial Schiff base adduct, but would rather involve the decarboxylative decomposition of a putative oxazolidine-5-one intermediate assisted by the catechol ring. These results report the first decarboxylative Maillard reaction between an amino acid and a carbohydrate under biomimetic conditions and highlight the critical role of transition metal ions in the competition with Pictet-Spengler condensation.

Preparation and characterisation of oligosaccharides produced by nitrous acid depolymerisation of chitosans

Two chitosans with widely different chemical composition (fraction of N-acetylated units (F(A))<0.001 and F(A)=0.59), were degraded by nitrous acid, to obtain the reactive 2,5-anhydro-D-mannose- (M-) unit at the new reducing end. The fully N-acetylated and fully N-deacetylated oligomers were separated by size-exclusion chromatography. Both the chemical structure and purity were studied by one- and two-dimensional 1H and 13C NMR methods. The fully N-acetylated oligomers were found to be stable, whereas the N-deacetylated oligomers reacted intermolecularly by a Schiff base reaction between the 2-amino group on the N-deacetylated units and the M-units, facilitating the cleavage of the glycosidic bond next to the M-unit and the formation of 5-hydroxymethylfurfural (HMF).

Pictet–Spengler condensation of the antiparkinsonian drug l-DOPA with d-glyceraldehyde. Opposite kinetic effects of Fe3+ and Cu2+ ions and possible implications for the origin of therapeutic side effects

In 0.05 M phosphate buffer, pH 7.4, and at 37 degrees C. L-DOPA, a widely used antiparkinsonian drug, reacted smoothly with D-glyceraldehyde to afford diastereoisomeric (1R, 1'S,3S)-3-carboxy-1-(1',2'-dihydroxyethyl)-6,7-dihydroxy-1,2,3,4- tetrahydroisoquinoline (1) and (1S,1'5S,3S)-3-carboxy-1-(1',2'-dihydroxyethyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (2) in an approx. 3:2 ratio. The prevalent formation of 1 over 2 reflects stereoselective cyclisation of a transient Schiff base in accord with the Felkin-Anh model. Fe3+ ions, present at relatively high levels in parkinsonian brains, markedly accelerated formation of 1 and 2, whereas Cu2+ decreased the reaction rate, due apparently to different sites of chelate formation between L-DOPA and the metal ions. Both metal ions markedly decreased the stereoselectivity of the reaction. Product 1 exhibited chelating properties toward metal ions comparable or stronger than those of L-DOPA. These results throw new light on the effects of transition metal ions on the Pictet-Spengler reaction and suggest a possible role of tetrahydroisoquinoline products from L-DOPA and carbohydrate metabolites in the severe side effects of the drug.

A novel octahydropyridobenzothiazepine metabolite in human urine: biomimetic formation from the melanogen 5-S-cysteinyldopa and formaldehyde via a peculiar sulfur-controlled double Pictet-Spengler condensation

HPLC evidence is reported demonstrating the occurrence in some human urine samples of a novel catecholic metabolite, (3R,7S)-3, 7-dicarboxy-10,11-dihydroxy-2,3,4,5,6,7,8,9-octahydropyrido[ 4,3-g][1, 4]benzothiazepine (2). The compound was shown to arise by a double Pictet-Spengler condensation of the urinary melanogen 5-S-cysteinyldopa (1) with formaldehyde, in which regioselective formation of the six-membered ring ortho to the activating hydroxyl group lends assistance to the subsequent closure of the seven-membered 1,4-thiazepine moiety. Under physiologically relevant conditions, i.e., in 0.1 M phosphate buffer pH 7.4 and at 37 degrees C, the 7,8-tetrahydroisoquinoline 5 was the sole detectable intermediate in the formation of 2. N-Acetylcysteinyldopa (4) reacted likewise with formaldehyde to give the 7, 8-dihydroxytetrahydroisoquinoline 6. The anomalous regiochemistry underlying formation of 5 and 6 was rationalized with the aid of AM1/PM3 calculations on the model alkylthiocatechol 10, predicting a higher HOMO-controlled reactivity on the position ortho rather than para to the activating hydroxyl group. The potential of the reported chemistry as a convenient synthetic access to the 2,3,4, 5-tetrahydro[1,4]benzothiazepine ring system is suggested by the efficient conversion of the cysteinylcatechol 3 to 8 in the presence of formaldehyde.