Study On The Synthesis Of Antidepressant Compound -CTN986 And Its Analogue

Quercetin 3-O-β-D-apiofuranosyI-( 1→2)-[ α-L-rhamnopyranosyl-(1→6) ]- β-D-glucopyranoside 1 (code name CTN986), a new flavonol triglycoside isolated from glandless cottonseeds in our lab, showed specific 5HT_1A receptor binding activity in radio ligand competition experiment and notable antidepressant effect in forced swimming experiment on mice. It seems to be a potential antidepressant agent. Since the content of 1 in cottonseeds was limited, it is difficult to isolate 1 from cottonseeds in large scale. In order to provide sufficient 1 for further pharmacological studies, there is the necessity to develop an effective synthetic method for large-scale preparation of this compound.In the molecule of 1, there are multiple hydroxyl groups of similar chemical reactivity, both in the aglycon structure and in the glycosyl moiety. In order to create glycosyl bonds specifically at the 3-OH of quercetin and at the 2-OH and 6-OH of the glucose fragment, selective protection of the other hydroxyl groups is one of the major challenges in the synthesis of this compound. In the glycosylation reactions, two conformational isomers, α- or β-isomer, can be formed. The stereoselectivity of the glycosylation reactions is the second challenge. Furthermore, the synthesis of 1 includes the construction of three glycosyl linkages, often complicated by low yields of the needed products and the production of unwanted stereoisomers. Considering these facts, our working strategy is to study at first the synthetic methods to resolve the problems mentioned above, and then to explore the complete synthetic route, actually, methods for stereo selective glycosylation of the 3-OH of quercetin and the 2- and 6-OH of glucose have been individually studied, and 1 and an analogue of it were synthesized thereafter.In the methodological study, hirsutrin 3 and hyperin 4 were synthesized by phase transfer catalysis (PTC). The problems of selective protection of the aglycon and stereo selective glycosilation of the 3-OH group of the quecertin have been resolved. First, commercially available rutin was converted into 5, 7, 3', 4'-tera-0-benzoyI- quercetin 5 by benzoylation and hydrolysis. Unfortunately, coupling 5 with 2, 3, 4, 6-tetra-Oa-D-glucopyranosyl bromide 7 did not give the targeted product under phase-transfer-catalysis. Then rutin was converted into 7, 3', 4'-tri-Obenzylquercetin 6, a new intermediate, via benzylation and hydrolysis. Under the same conditions, 5 reacted successfully with 7 or 2, 3, 4, 6-tetra-O-a-D-galactopyranosyl bromide 8 afforded corresponding protected glycosides 7, 3', 4'-tri-O-benzylquercetin 3-0-2, 3, 4, 6 - tera - O- acetyl-/? - D -glucopyranoside 9 and 7, 3', 4'-tri-Obenzylquercetin 3-0-2, 3, 4, 6- tera- O - acetyl - /? - D -galactopyranoside 10 respectively. Debenzylation of 9 and 10 by catalytic hydrogenation followed by removal of all acyl protection groups using catalytic amount of NaOMe in MeOH and final purification provided the targeted molecules 3 and 4. These results indicated that 7, 3', 4'-tri-0-benzyl- quercetin can be used as a versatile key intermediate for the synthesis of 3-0-substituted quercetin derivatives, and phase-transfer-catalyzed glycosylation of quercetin 3-OH is very efficient in diluted aqueous K2CO3 and CHCI3 using TBAB as phase-transfer-catalyst.Two strategies have been tried for the synthesis of 1. In the first strategy, a convergent synthetic strategy, the trisaccharide moiety was synthesized at first, and then coupled with the protected aglycon. In the second strategy, namely a simpie linear, starting with the protected aglycon, the three monosaccharids were linked to the major fragment one by another. In the practice, quercetin 3-0-/?-D-glucopyranosyl -(1—*2)-[/?-D-glucopyranosyl- (1—>6)]-/?-D-glucopyranoside 2, an analogue of CTN986, were synthesized by the second strategy.By the first synthetic strategy, 2 - O - (2, 3, 4, 6 - tetra -O- benzoyl - /? - D -glucopyranosyl)-3, 4, 6-tri-O-acetyl-a-D-glucopyranosyl bromide 14 was prepared from glucose through conventional glycosylation and protecting groupmanipulation in 11 steps. Unfortunately, coupling 6 with 14 under PTC conditions also failed to afford the desired product. Possibly the large size of the oligosaccharide moiety caused spatial hindrance that makes the glycosylation reaction more difficult. After the unsuccessful experience with the first strategy, the simple linear strategy was adopted for the synthesis of 2. Glucose was converted into 2, 4, 6-tri-0-acetyl-3-(3-benzoyl -a-D-glucopyranosyl bromide 18 in 4 steps. Coupling 18 with 6 in 0.15 M aqueous K2CO3-CHCI3 system in the presence of TBAB at 50 °C gave the desired 7, 3', 4'-tri-(9-benzyl-quercetin 3-O-2, 4, 6-tri-0-acetyl-3-(9-benzoyl-/?-D -glucopyranoside 29 in 90% purified yield. Treatment of 29 with benzyl bromide in the present of K2CO3 in DMF, followed by deacetylation, led to the partially protected 5,7,3',4'-tera-0-benzyl-quercetin-3-0-benzoyl-/?-D-glucopyranoside 17. Regioselec- tive glycosylation of 17 with 2, 3, 4, 6- tetra-0-benzoyl-oc-D-glucopyranosyl bromide 12 in anhydrous CH2CI2 in the presence of AgOTf at -30 °C provided 1—>6 linked glycoside 31. Under similar conditions, regioselective coupling of 12 with diol 31 provided mainly the desired 2-0-glycosylated product 5, 7, 3', 4'-tera-Obenzyl-quercetin-3-0-2,3,4,6- tetra -O-benzoyl-/?-D-glucopyranosyl-(l—^2)-[2,3,4,6-tetra-O-benzoyl-y9-D-glucopyranosyl-( l^"*6)]-3-O-benzoyl-/?-D-gluco-pyranoside 32. Finally, removal of all acyl and benzyl protection groups in usual ways furnished the target compound 2.Two synthetic routes have been tried for the synthesis of apiose, a monosaccharide moiety of 1 and not available commercially. 2,3-di-Oisopropylidene-/?-D -apiofuranose 38, a protected form of apiose, has been reached from D-mannose in 4 steps. The synthesis of CTN986 is ongoing.In summary , during our experiments, three final compounds (2, 3 and 4) along with five intermediates (5, 6,14,37 and 38), useful for the synthesis of 3-O-substituted quercetin or oligosaccharides, were obtained. 7,3',4'-tri-0-benzyl- quercetin 6, a versatile key assembling unit for the synthesis of 3-0-substituted quercetin, was conveniently prepared from rutin. On this basis, compound 2, a flavonol 0-triglycoside with typical structural pattern of a branched trisaccharide moiety, was synthesized for...