Synthesis of 1-substituted tetrahydroisoquinolines by lithiation and electrophilic quenching guided by in situ IR and NMR spectroscopy and application to the synthesis of salsolidine, carnegine and laudanosine.

The lithiation of N-tert-butoxycarbonyl (N-Boc)-1,2,3,4-tetrahydroisoquinoline was optimized by in situ IR (ReactIR) spectroscopy. Optimum conditions were found by using n-butyllithium in THF at -50 °C for less than 5 min. The intermediate organolithium was quenched with electrophiles to give 1-substituted 1,2,3,4-tetrahydroisoquinolines. Monitoring the lithiation by IR or NMR spectroscopy showed that one rotamer reacts quickly and the barrier to rotation of the Boc group was determined by variable-temperature NMR spectroscopy and found to be about 60.8 kJ mol(-1), equating to a half-life for rotation of approximately 30 s at -50 °C. The use of (-)-sparteine as a ligand led to low levels of enantioselectivity after electrophilic quenching and the "poor man's Hoffmann test" indicated that the organolithium was configurationally unstable. The chemistry was applied to N-Boc-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline and led to the efficient synthesis of the racemic alkaloids salsolidine, carnegine, norlaudanosine and laudanosine.

[Asymmetric reactions based on activation and structure control of molecule--asymmetric reaction of lithiated nucleophiles].

The methodology we developed relies on an external chiral coordinating reagent that forms a deaggregated chelate complex with organolithium reagents. Under the positive control of a chiral dimethyl ether of stilbenediol 4, an asymmetric conjugate addition reaction of organolithium reagents with unsaturated imines and esters proceeded successfully to yield the corresponding addition products with reasonably high stereoselectivity. The sense of stereochemistry is predictable based on a coordination model. The methodology has been extended to a catalytic asymmetric 1,2-addition reaction of organolithium reagents with imines. An enantiotopic group differentiating the opening of cyclohexene oxide with organolithium was also mediated by a chiral ligand. The asymmetric Horner-Wadsworth-Emmons reaction of phosphonates and Peterson reaction of alpha-silylester with 4-substituted cyclohexanone were another successful extension of the methodology. A three-component reagent of lithium ester enolate, lithium amide, and chiral diether reacts with imines to afford beta-lactam with reasonably high enantioselectivity. Tridentate aminoether ligands were also shown to affect the catalytic asymmetric addition of lithium ester enoaltes to imines, giving beta-lactams with high enantioselectivity. Asymmetric conjugate addition of lithium amide to enoates was mediated by a chiral diether ligand to give the beta-aminoester with high yield and enatioselectivity. The methodology has been successfully applied to an asymmetric synthesis of biologically potent compounds. Dihydrexidine, a promising anti-Parkinsonism candidate, and salsolidine, a representative isoquinoline alkaloid, have been synthesized using asymmetric addition reactions of organolithium reagents as the key steps.

Simultaneous gas chromatographic-mass spectrometric determination of dopamine, norsalsolinol and salsolinol enantiomers in brain samples of a large human collective.

Using a solid-phase extraction procedure, an enantioselective derivatization and a gas chromatographic-mass spectrometric method, the levels of dopamine (DA) and of the dopamine-derived tetrahydroisoquinoline alkaloids (R)/(S)-salsolinol (SAL) and norsalsolinol (NorSAL) were determined in human brain samples. A complex pre-analytical synthesis of reference substances as well as deuterated internal standards allowed the standardized and reproducible analysis. In this study, to our best knowledge for the first time, the regional distribution of (R)-SAL and (S)-SAL, as well as NorSAL is examined systematically in a large collective of human brain samples obtained by autopsy. The material comprises 91 brains and 8 standardized specimens in each case. Anatomical concentration differences and no ubiquitous occurence were encountered. Significant amounts of (R)-SAL, (S)-SAL and NorSAL were only found in dopamine-rich areas of the basal ganglia, whereas in other regions of the brain no tetrahydroisoquinolines were detected. These findings suggest that the concentration of the substrate dopamine may determine the alkaloid level during in vivo formation. In our opinion, non-enzymatic formation of SAL via the Pictet-Spengler reaction reveals both the SAL enantiomers. An additional enzymatic synthesis of only (R)-SAL could explain the predominant occurrence of this enantiomer. Especially in the nucleus caudatus, the concentrations of DA, SAL and NorSAL decreased significantly with rising age, which may be consistent with apoptotic effects of ageing. Our data can serve as reference for other studies in humans concerning the etiology of alcoholism or other neurodegenerative diseases with the involvement of tetrahydroisoquinolines.

Oxidation of N-methyl(R)salsolinol: involvement to neurotoxicity and neuroprotection by endogenous catechol isoquinolines.

1(R), 2(N)-Dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, N-methyl(R)salsolinol, is a potent dopaminergic neurotoxin to induce parkinsonism in rats. The cytotoxicity of N-methyl(R)salsolinol proved to be ascribed to its oxidation into cytotoxic 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion with generation of hydroxyl radical. The isoquinolinium ion caused massive necrosis in the striatum, whereas N-methyl(R)salsolinol depleted selectively dopaminergic neurons in the substantia nigra without necrotic tissue reaction. N-Methyl(R)salsolinol induced DNA damage to human neuroblastoma SH-SY5Y cells, which could be prevented by anti-oxidants and cycloheximide. These results suggest that oxidative stress through oxidation of N-methyl(R)salsolinol induces apoptotic cell death. On the other hand, (R)salsolinol proved to scavenge hydroxyl radical produced by oxidation of dopamine. The neurotoxicity and neuroprotection of catechol isoquinolines may be ascribed to their oxidation and scavenging of radicals.