Synthesis and anticonvulsant properties of 1,2,3,4-tetrahydroisoquinolin-1-ones

Following our previous molecular modeling studies we herein report the synthesis of substituted 1,2,3,4-tetrahydroisoquinolinone-4-carboxylic acids in an attempt to obtain new anticonvulsants acting as negative modulators of AMPA-type glutamate receptor. The evaluation of their pharmacological effects demonstrated that some derivatives were able to prevent audiogenic induced seizures in DBA/2 mice


Introduction
L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian central nervous system.Glu interacts with two different classes of receptors, ionotropic receptors (iGluRs) and metabotropic receptors (mGluRs).iGluRs are ligand-gated cationic channels which cause neuronal depolarization by generating fast excitatory postsynaptic potentials, while mGluRs are coupled to G proteins and have a modulatory role in neurotransmission.
Using PD00735 as template, we planned and performed the synthesis of 1,2,3,4tetrahydroisoquinolinone-4-carboxylic acids and corresponding ethyl esters to obtain new noncompetitive antagonists with higher biological efficacy.Structural and stereochemical features as well as the results of preliminary pharmacological screening of compounds synthesized are also herein reported.

Results and Discussion
][30][31] The products 9 possess two asymmetric centres (C-3 and C-4) and therefore could exist as cisor transdiastereoisomers.When we treated the suitable imine 7a-k with homophthalic anhydride 8a-k in chloroform at room temperature, only the cis diastereoisomer was generally recovered.On the contrary, in some cases using more drastic conditions (i.e boiling toluene), the reaction mixture afforded trans-diasteroisomer.As previously reported 30 this latter can be recovered by isomerization of cis-isomer on heating in acetic acid (e.g.9e).
Only trans-diastereoisomers have been isolated during the preparation of ethyl esters 10a-f and 10j-k by treatment with diethylsulfate in alcaline medium.
The stereochemical configurations of the substituents at C-3 and C-4 were established on the basis of coupling constants observed in proton NMR spectrum for the two protons at C3 and C4.In particular, the cis relative configuration was established from the two doublets with a coupling constant of ~ 6 Hz observed close to 4 ppm and 5.5 ppm, while in the corresponding trans diastereoisomers the H 3 and H 4 hydrogens appear as two singlets.
cis and/or trans isomers

Scheme 1
Although the stereochemical control of reaction has been studied and reported by different authors, the exact mechanism of the cyclocondensation process is far to be clarified; it is known that both the reaction conditions (solvent polarity, temperature, etc.) and the nature of reagents employed influences the stereochemistry; in fact, the electronic and steric effects determine the stereochemical outcome of the reaction of Schiff bases 7 with homophthalic anhydride 8.In particular, the bulk ot the N-substituent of the imine has an effect on the ratios of the E-and Zisomer and thus on the ratios of the diastereoisomers formed. 29In our case the presence of a methylene group between the nitrogen atom and R 2 reduces the steric hindrance thus preferentially affording cis-isomers.
The anticonvulsant effects of some selected 1,2,3,4-tetrahydroisoquinolinones 9-10 were evaluated after intraperitoneal (ip) administration against audiogenic seizures in DBA/2 mice, which are considered an excellent animal model for generalized epilepsy and for screening new anticonvulsant drugs. 32The results of anticonvulsant test were reported in Table 1 and compared with those of well known noncompetitive AMPA receptor antagonists such as GYKI 52466 (1), talampanel (2), CFM-2 (3) as well as compound PD00735 (9f).As shown in Table 1 the preliminary pharmacological screening pointed out that among this class of compounds only 9d and 10d showed significant activity.Moreover, the evaluation of the ED 50 values of 10f suggested that the presence of the ethyl carboxylate moiety negatively influences the anticonvulsant properties.
In conclusion new 1,2,3,4-tetrahydroisoquinoline-4-carboxylic acids and carboxylates were synthesized and the different diastereoisomers were separated and characterized.Some of them showed anticonvulsant properties.General procedure for the synthesis of 1,2,3,4-tetrahydroisoquinolinone-4-carboxylic acid derivatives.Method A. Homophthalic anhydride (8) (0.81g , 5 mmol ) was added to a chloroform (30 mL) solution of the imines 7 (5 mmol).The mixture was stirred at room temperature for 2 h.When a white solid had formed, the solution was filtered off, the crude product washed with chloroform and dried to give compounds 9 as cis (9a, 9d, 9e, 9h) or trans diastereoisomer (9j) or as a mixture of both diasteroisomers (9f and 9g).For compounds 9b, 9c, 9i, 9k the reaction mixture was evaporated under reduced pressure leaving a dark oil, that by crystallization from ethyl acetate gave the pure products as cis isomers.Method B. The suitable imine 7b, 7c, 7e, 7g, or 7k (5 mmol) was added dropwise for 30 minutes to a hot and stirred solution of 8 (0.81g , 5mmol ) in dry toluene (30 mL).The reaction mixture was then refluxed for 15 minutes and left overnight.The solid was filtered off, the filtrate was extracted twice with 0.5% aqueous sodium hydroxide and the alkaline solutions were acidified with HCl, extracted three times with ethyl acetate.The combined organic layers were dried over Na 2 SO 4 and evaporated under reduced pressure leaving an oil that by crystallization from ethyl acetate furnished compounds 9b-c, 9e, 9g, or 9k as trans isomers.Method C. The cis isomer of compound 9e was refluxed in glacial acetic acid for 8h, then was poured into water and the solid obtained was crystallized from ethyl acetate to give corresponding trans isomer.

Figure 2 .
Figure 2. Chemical structure of PD00735 and its alignment into pharmacophore hypothesis.As shown in Figure2, the four-feature pharmacopore model is well-mapped by PD00735: i) the carboxylic acid function overlaps with the hydrogen bond acceptor site; ii) the benzene-fused ring is positioned over the aromatic feature; iii) the benzyl and phenyl substituents on the tetrahydroisoquinolinone skeleton correspond to the two hydrophobic areas.
1H-NMR spectra were measured with a Varian Gemini 300 spectrometer; chemical shifts are expressed in (ppm) relative to TMS as internal © ARKAT USA, Inc standard and coupling constants (J) in Hz.All exchangeable protons were confirmed by addition of D 2 O.