The Chemistry of Galanthamine. Classical Synthetic Methods and Factor: 2.841 Comprehensive Study on Its Analogues

1450 Send Orders for Reprints to [email protected]

Mini-Reviews in Medicinal Chemistry, 2016 , 16, 1450-1461 REVIEW ARTICLE

ISSN: 1389-5575 eISSN: 1875-5607

Impact The Chemistry of Galanthamine. Classical Synthetic Methods and Factor: 2.841 Comprehensive Study on its Analogues

BENTHAM SCIENCE

Péter Keglevich, Csaba Szántay and László Hazai*

Department of Organic Chemistry and Technology, University of Technology and Economics, Budapest, Hungary, H-1111 Budapest, Gellért tér 4. Hungary

Abstract: Galanthamine as an Amaryllidaceae alkaloid has an important role in the treatment of Alzheimer’s disease. Some efforts were made to elaborate the total A R T I C L E H I S T O R Y synthesis, and hundreds of its derivatives were prepared to find a more effective molecule with advantageous properties. Moreover, almost every part of the Received: September 22, 2015 Revised: January 07, 2016 tetracycle was changed; in members of the rings, in the nature and position of the Accepted: February 30, 2016 heteroatoms, and ring-opened analogues were also synthesized. In this review the

DOI: basic synthetic works and the most important derivatives and analogues are 10.2174/13895575166661603211145 overviewed. 56 L. Hazai

Keywords: Acetylcholinesterase inhibition, Alzheimer’s disease, butyrylcholinesterase inhibitor, galanthamine, hexahydrobenzofurobenzazepine, structural analogues.

1. INTRODUCTION galanthamine review [3] 3.97 μM is given for IC50, while in a letter published in 2008, however, this value was 1.82 μM 1.1. Galanthamine and Related Amaryllidaceae Alkaloids [11]. While sanguinine (3) is more effective in the AChE (-)-Galanthamine (1), (-)-lycoramine (2), (-)-sanguinine inhibitory activity [11], in fact, galanthamine (1) has come to (3) and their precursor (-)-narwedine (4) (Fig. 1) belong to the front in the treatment of Alzheimer’s disease, because the the family of Amaryllidaceae alkaloids [1]. Among the physiological activity is unfavourably influenced by the Amaryllidaceae alkaloids the acetylcholinesterase (AcChE) phenolic hydroxyl group in the aromatic ring of sanguinine inhibitory activity can be connected to the structures (3). containing galanthamine or galanthamine-like skeleton. Galanthamine (1) is used in the form of hydrobromide Furthermore, it was pointed out, that galanthamine (1) has salt (Nivalin, Reminyl, Razadyne) as a medicine in several butyrylcholinesterase (BuChE) activity as well [2]. countries, e.g. in Austria, in Sweden, or in England. The Nevertheless, galanthamine (1) has an important role in the clinical investigations in connection with galanthamine (1) modulation of nicotine receptors in the brain. are in the last phase all over Europe and also in the United The chemistry, biological properties [3, 4] and States. The important effect of galanthamine (1) in the biosynthesis [5] of galanthamine (1) were reviewed. course of the treatment of Alzheimer’s disease proves to Attempts in developing biotechnology as an alternative retard the progression of the trouble. This compound has a process for galanthamine production [6-8] and in silico unique positive influence to the cognitive activities (learning, screening with molecular docking [9] on its derivatives as remembering, etc.), to the everyday actions and to the potential cholinesterase inhibitors were also investigated. psychological confusions and disturbances.

For the IC50 values of the AChE inhibitory effect of Galanthamine (1) has been discovered and isolated from galanthamine (1) there are data in the literature between 1 Galanthus woronowii, but for commercial needs it is and 5 μM, respectively, e.g. in a work [10] from 2006 obtained synthetically or by extraction from Narcissus spp. presenting natural compounds with acetylcholinesterase and Leucojum spp. [5]. The clinical indication is getting inhibitory activity, IC50 1.07 μM can be found. In a more and more topical, the increasing needs, the high cost and the restricted supply of the material justify the efforts of *Address correspondence to this author at the Department of Organic different research laboratories to elaborate efficient industrial Chemistry and Technology, University of Technology and Economics, synthetic procedures, as well as to synthesize new Budapest, Hungary, H-1111 Budapest, Gellért tér 4. Hungary; Tel: +36-1- derivatives with modification of the structure of 463-2208; Fax: +36-1-463-3297; E-mail: [email protected] galanthamine (1).

OH OH OH O 6 5 c 7 4a 8a 8 10 4 O b N CH3 O N CH3 O N CH3 O N CH3 9 d 11 a 12 H CO 3 1 H CO HO H CO 3 2 3 3 (-)-1 (-)-2 (-)-3 (-)-4 galanthamine lycoramine sanguinine narwedine Fig. (1). The most important Amarylidaceae alkaloids.

ring using L-Selectride, (-)-galantamine (1) could be 2. SYNTHESIS AND STRUCTURAL MODIFICA- isolated. TIONS OF GALANTHAMINE AND GALANTHA- MINE DERIVATIVES Node and co-workers [14] also used the method of biomimetic phenolic oxidative coupling. They synthesized 2.1. Synthesis of Galanthamine (-)-galanthamine (1) by means of a remote asymmetric In the course of the synthesis of galanthamine (1) most of induction where conformation of the seven-membered the known synthetic methods involved the biomimetic azepine ring after the coupling reaction was restricted by intramolecular phenolic oxidation to form the quaterner forming a fused chiral imidazolidinon ring. carbon atom resulting in the tetracycle of the galanthamine Among the new synthetic methods for galanthamine (1), skeleton. In developing this procedure the work of Barton the use of Heck-reaction is a very important and elegant [12] played an important role. The next synthesis can be procedure. In the course of the enantioselective synthesis carried out in kilogram scale and presented here [13] as a elaborated by Trost and his research group [15, 16], the classic based on its results. azepine ring was closured after the formation of the a-b-c In the first step, 3,4-dimethoxybenzaldehyde (5) was tricycle containing the benzofurane skeleton obtained by brominated (Scheme 1), then the demethylation reaction of Heck reaction. The starting materials were 2- the bromo-substituted aldehyde (6) resulted in the 6- bromoisovanilline (13) and a cyclohexenol ester (14) bromoisovanilline (7). After condensation of 7 with substituted by trichloroethoxycarbonyl group. The reaction tyramine, Schiff-base (8) was obtained. The reduction of 8 between 13 and 14 in the presence of a palladium catalyst and formylation of saturated amine (9) gave the N-formyl and a chiral ligand (15) resulted in the aryl ether (16) derivative (10). (Scheme 3). The oxidative cyclisation of compound 10 resulted in the Then, the aldehyde function was protected with bromo-formyl-narwedine (11) (Scheme 2). The keto group triethylorthoformate and the ester group was reduced by of 11 was protected with propylene glycol as a ketal, and DIBALH to the corresponding alcohol (17). The unsaturated after reduction of compound 12 racemic narwedine (4) was nitrile (18) was prepared by the modified Mitsunobu obtained. Second-order asymmetric transformation of reaction, and the aldehyde was deprotected. Heck reaction of racemic narwedine (4) in the presence of catalytic amount of compound 18 resulted in the benzofurane (19) which was (-)-narwedine (4) gave the corresponding (-)-narwedine ((-)- oxidized with selenium dioxide under vigorous reaction 4). After reduction of the carbonyl group of the cyclohexene conditions, and in this reaction the 20 hydroxy compound

OCH3 OCH3 OH H3CO H3CO H3CO Br2 cc. H2SO4 tyramine MeOH CHO CHO CHO Br Br 5 6 7

OH OH OH

H3CO NaBH4 H3CO HCO2C2H5 H3CO N NH N CHO Br Br Br 8 9 10 Scheme (1). Preparation of intermediates for a classical galanthamine synthesis. 1452 Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 Keglevich et al.

OH CH3 O O O

OH propane-1,2-diol 1. LiAlH4 H3CO K3[Fe(CN)6] O N CHO O N CHO 2. HCl N CHO H3CO Br Br H3CO Br

10 11 12

O O OH

EtOH/NEt3 L-Selectride O N CH3 O N CH3 O N CH3 cat. (-)-narwedine

H3CO H3CO H3CO 4; racemic narwedine (-)-4 (-)-1 Scheme (2). The cyclisation step and second-order asymmetric transformation.

OH H3CO Br η3 15, [ -C3H5PdCl2]2 O CO CH + TrocO 2 3 Et N, CH Cl , rt H3CO Br CHO CO2CH3 3 2 2 CHO 13 14 16 Ph Ph O O Troc: 2,2,2-trichloroethoxycarbonyl NH HN η3 [ -C3H5PdCl2]2: allylpalladium chloride dimer 15: PPh2 Ph2P Scheme (3). The Trost’s synthetic method.

1. CH(OCH3)3 p-TsOH, MeOH 16 O 1. Ph3P, acetone cyanohydrin, O 2. DIBAL-H, OH CN H3CO Br DIAD, Et2O H3CO Br toluene, -78oC 2. TsOH, THF, H2O CH(OCH3)2 CHO 17 18 OH H H

O SeO2, NaH2PO4, O Pd(OAc)2, dppp, o H3CO 1,4-dioxane, 150 C, H3CO o CN Ag2CO3, toluene, 107 C CN sealed tube, quartz sand CHO CHO 19 20 OH

H DIAD: diisopropyl azodicarboxilate one-pot reaction: O N CH 3 dppp: 1,3-bis(diphenylphosphino)propane 1. CH3-NH2, MeOH 2. DIBAL-H, then NaH2PO4 H3CO 3. NaCNBH3 (-)-1 Scheme (4). Enantioselective synthesis using Mitsunobu and Heck reaction. The Chemistry of Galanthamine Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 1453 was obtained, in which the configuration of the hydroxy A derivative containing one carbon carbon double bond group was the same as in the galanthamine. After all, (28) was obtained by the oxidation of cyclohexanone (26) galanthamine (1) was obtained in a one-pot reaction as using o-iodoxybenzoic acid; however, in this way, the presented in Scheme 4. Reaction with methylamine in second C=C double bond could not be introduced into the methanol, reduction with DIBALH, and then reaction with cyclohexane ring. The α,β-unsaturated ketone (28) was sodium cyanoborohydride resulted in the product. transformed to the corresponding dienone (30) by oxidation with selenium dioxide. Here, it has to be mentioned that in the literature there are further enantioselective syntheses, in which not only Heck Our synthetic method was extended to derivatives reaction, but Mitsunobu coupling is also often used [17]. containing a methoxy group in the aromatic ring and after preparing the methoxy-substituted spirocyclohexanone (27), Our previous results on the synthesis of galanthamine (1) the latter was oxidized also with o-iodoxybenzoic acid and were published more than a decade ago [18]. The synthetic the expected key intermediate (29) was obtained. The strategy was using Grewe cyclisation for the preparation of derivative containing a methoxy group was then spiro-substituted benzo[c]azepines (e.g. 24), from which was demethylated and cyclized in one step in methanesulfonic formed galanthamine (1) (Scheme 5). acid in the presence of racemic methionine to result in the In the first step the substituted amine (21) was subjected desired hexahydrobenzofurobenzazepine tetracycle (31) to Birch reduction. The major product was the enol ether characteristic for Amaryllidaceae alkaloids containing the (22a), in addition, the over reduced cyclohexene derivative galanthamine skeleton [20]. (23) was also isolated. After demethylation of compound Considering that further reactions of the tetracycle (31) 22a, the corresponding β,γ-unsaturated cyclohexenone leading to the corresponding galanthamine analogue were derivative (22b) was obtained, which failed to react under unsuccessful, reactions of compound 29 were investigated Grewe conditions. Grewe cyclisation of the cyclohexene (Scheme 7), which was chosen as the starting material. derivative without a keto group (23) carried out at room Firstly, the protection of the ketone group of compound 29 temperature in a 1:1 mixture of cc. sulfuric acid and cc. was carried out. Ketal 32 was prepared using ethyleneglycole phosphoric acid led, however, to the desired spiro-derivative in refluxing benzene using collidinium p-toluenesulfonate (24). The by-product alcohol (25) formed by hydratation (CPTS) as the catalyst. Methylation of ketal 32 was reaction could also be transformed to the desired azepine accomplished in refluxing acetone with methyl iodide in the derivative. In this reaction it is very important that it is the presence of anhydrous potassium carbonate. Reduction of first case that a 7-membered ring was formed by Grewe the carbonyl groups of imide 33 was performed using cyclisation. lithium aluminium hydride in refluxing THF. Deprotection Hereafter, our research group was successful of the ketone group of ketal 34 in hydrochloric acid solution synthesizing the [2]benzazepinone skeleton (26) containing a was unsuccessful. Eventually, the deprotection reaction, O- spiro-cyclohexanone ring [19] starting from 2-tetralone and demethylation and cyclisation were carried out in a one-pot using simple reaction steps and inexpensive reagents reaction in methanesulfonic acid, in the presence of racemic (Scheme 6). methionine at room temperature, similarly as described

H3CO H3CO

R R R Li/liquid NH3 + N N N -60oC -55oC

H CO 3 H3CO H3CO OH OH OH 21R= H, Me, CHO 22a 23 X b

g R R HO N R N cc. H SO 2 4 + N cc. H3PO4

H3CO H CO 3 OH H3CO OH OH 23 24 25 22b; X= O cc. H2SO4 23; X= H2 cc. H3PO4 Scheme (5). Preparation of the [2]benzazepinone skeleton by Grewe cyclisation. 1454 Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 Keglevich et al.

O O O

O O

IBX SeO2 NH NH NH R DMSO, fluorobenzene R R O O O

30; R= H 26 R=H 28; R=H 27; R= OMe 29; R= OMe IBX: 2-iodoxybenzoic acid

O O

O O MsOH NH O NH H3CO D,L-methionine O O 29 31 Scheme (6). Synthesis the tetracycle of galanthamine.

O O O O O O O O HOCH2CH2OH MeI NH NH N CH3 H3CO benzene H3CO H CO acetone 3 CPTS O O O K2CO3 29 32 33

O O O

LiAlH 4 MsOH N CH H CO 3 O N CH3 THF 3 D,L-methionine

34 35 (racemic demethoxylycoraminone) Scheme (7). Preparation of racemic demethoxylycoraminone. earlier [20]. In this reaction, racemic demethoxy- galanthamine (1), there are more functional groups, for lycoraminone (35), as one of the most important key instance, hydroxy, (after O- and/or N-demethylation) intermediates for the synthesis of different galanthamine- phenolic hydroxy and NH group, however, a lot of analogues type Amaryllidaceae alkaloids was obtained [21]. could be synthesized by changing the ring moiety of the molecule. 2.2. Synthesis of Substituted Galanthamine Derivatives For the substitution of the nitrogen atom of galanthamine In 2014, 238 publications were found in the Chemical (1), the azepine nitrogen was N-demethylated using the non Abstract in connection with galanthamine (1), from these, 60 classical Polonovski reaction [23]. In the course of this works deal with synthetic aspects, e.g. Magnus and co- reaction, the galanthamine-N-oxide (36) was reacted with workers [22] who presented a synthetic method in which not hydrated ferrous sulfate in methanol to provide only galanthamine (1), but codeine was also synthesized norgalanthamine (37) (Fig. 2). from a common starting intermediate. These data and the The number of galanthamine (1) derivatives substituted high interest gave the reason to review and to present the on the nitrogen atom and/or on the hydroxy group can be derivatives which were prepared by substitution reactions of estimated to be more hundreds; most of them are presented galanthamine (1), and in a better way with modifications of in different patents, which are not cited in this review. the tetracyclic ring in different ways. In the skeleton of The Chemistry of Galanthamine Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 1455

OH OH OH

CH3 O N CH3 O N O NH O

H3CO H3CO H3CO 37 1 36 galanthamine; galanthamine N-oxide norgalanthamine μ IC50 3.97 M

O OH OH O N H

N O N O N O N CH3

H3CO O H3CO H3CO 38 39 40 galanthamine N-butylcarbamate; N-allylnorgalanthamine; μ IC 5.62 nM μ IC50 10.9 M 50 IC50 0.18 M Fig. (2). Simple galanthamine derivatives with important biological activity.

Nevertheless, two relatively simple derivatives are presented dihydrogenphosphate, etc. to obtain phosphorylated derivatives here, the galanthamine N-butylcarbamate (38) [24] and 41 (Fig. 3). These type of compounds unlike most compound 39 containing a long chain substituent on the galanthamine derivatives showed promising antimicrobial azepine nitrogen atom [25]. Both compounds have important and antioxidant activities. acetylcholinesterase inhibitory activity. The iminium salts of galanthamine are shown in Fig. (4). Galanthamine (1) has been isolated since 1960 in great The new derivatives (42, 43, 44) were planned on the basis amounts from the leaves of snow-flake Leucojum aestivum of the results of crystallographic investigation of the active belonging to the Amaryllidaceae family. Recently, the center and the catalytic site of the acetylcholinesterase mother liquor of the industrial procedure was investigated, enzyme together with docking studies of galanthamine (1) and more than 20 alkaloids, the N-allylnorgalanthamine (40) Galanthamine derivatives (42, 43, 44) may have a dual having excellent AChE inhibitory effect and other similar interaction with the active site [27, 28, 30]. The long side structures were isolated [11]. chains were formed with simple reactions on the phenolic oxygen and the azepine nitrogen atom after demethylation. The phosphorylated analogues of galanthamine were This type of bis-functional molecules showed substantially synthesized from galanthamine (1) as the starting material higher AChE inhibitory activity than galanthamine (1). [26] by reaction with bis(2-chloroethyl)phosphoramidic dichloride and with 4-nitrophenyl phosphorodichloridate. The iminium salt of galanthamine (45) was prepared from galanthamine (1) by reaction with N-bromosuccinimide. This The further reactions were achieved with various derivative showed not only important acetylcholinesterase compounds like 2-aminoethanol, ethyleneglycol, 2- activity [29], but it could be used as an intermediate in aminoethanethiol, 2-hydroxyethanethiol, monopotassium reactions with nucleophiles (Scheme 8) resulting in new

O O OH OH P R R1: O P OH ; O ; N ; O R1 OH H

OH NH NH S ; S 2 ; N 2 O N CH3 H

H3CO Cl 41 R: N ; O NO2 Cl Fig. (3). Phosphorylated galanthamine derivatives. 1456 Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 Keglevich et al.

OH OH

Br Br

O N O O N CH3

O N O H3CO N O

O 42 43 IC 203 nM IC50 11 nM 50 OH OH Br

O N Br

O N CH3 OCH3 H3CO

H3CO N O 45

IC50 266 nM 44 OH IC50 12 nM Fig. (4). The iminium salts of galanthamine.

R1 R2 R1 R2 R1 R2 Br KCN or NBS MeMgBr O N CH3 O N CH3 O N CH3

R4 H3CO R3 H3CO R3 H3CO R3 46 47 48 R1, R2= H, OH R = H, Me 3 R4= CN, Me Scheme (8). Reactions of galanthamine iminium salt.

galanthamine derivatives with further substituents. In the OH reaction of iminium salt (47) with potassium cyanide or with R= CH2 N ; 49a Grignard reagent, derivatives 48 were obtained containing a 3 nitrile or a methyl substituent, respectively, in the azepine O N R ring. O O S Novel bis-functional galanthamine derivatives were CH N 49b prepared by coupling galanthamine (1) with a piperidine ring H CO 2 3 6 or with saccharin (Fig. 5). The appropriate N-N connection O was achieved by means of a long alkyl chain (49). The 49 AChE inhibitory activity of both derivatives were higher, as compared to that of galanthamine (1); the molecule Fig. (5). Galanthamine containing a piperidine ring and hybride containing a piperidine ring (49a) had a value of 0.35 μM for with saccharin. IC50 measured on recombinant human brain, while the hybride molecule with saccharin (49b) showed an IC50 of 0.05 μM. The synthesis of these derivatives was simple, the Galanthamine containing a fluoro atom in position 1 of corresponding chloroalkylpiperidine and the the aromatic ring (see Fig. 1) was also synthesized [32]. bromoalkylsaccharin were reacted as alkylating agents with Diazotation of 1-aminogalanthamine was unsuccessful, so norgalanthamine (37). The crystal structure of the complexes that the fluoro atom was already built in the structure to the were also determined and confirmed the dual interaction desired position in the early steps of the synthesis. [31]. The Chemistry of Galanthamine Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 1457

The key intermedier (50), which is similar to compound prepared as an intermediate of galanthamine analogues with 10 (see Scheme 1 and 2), but containes a fluoro atom, was opened azepine ring [39] (see compound 69, Fig. 8), and a subjected to the generally used intramolecular phenolic medium value was obtained for its AChE inhibitory effect. oxidative coupling resulting in the planned tetracycle (51), Galanthamine derivative containing an eight membered ring from which the fluorinated narwedine (52) was obtained instead of azepine (60) did not proved to be a potent after the corresponding reaction steps (Scheme 9). Racemic inhibitor of acytelcholinesterase enzyme either [40]. fluorogalanthamine (53) was prepared by the reduction of The furane ring of galanthamine (1) was also modified, fluoronarwedine (52) with L-Selectride. Fluorinated but only one such derivative is known in the literature [41]. galanthamines, ((-)-54 and (+)-54) were isolated by means of In this compound, there is a methylated nitrogen atom (61) chiral preparative column chromatography. In the instead of the oxygen atom in the five membered ring. The corresponding publication [32] there were no biological data provided. cited patent also includes the structures containing sulfur atom, sulfoxide and sulfonyl groups in place of the oxygen The procedure presented in Scheme 9 can already be atom of the furane ring, however, example was given only regarded as the classical synthesis of galanthamine (1) and for the aza analogues (61). its aromatic analogues. The 1-methylgalanthamine [33] for Compounds containing a nitrogen atom in the aromatic SAR studies was synthesized with the same method. ring (Fig. 7) are almost entirely different than galanthamine. Nevertheless, the exact pyridine analogue of galanthamine 2.3. Modifications on the Rings of Galanthamine does not exist, and the furane ring is also lacking from these Those compounds in which the nitrogen atom is located structures (62, 63, 64). Probably compound 64 can be in another position than in the galanthamine (55, 56) can be considered as the pyridine analogue of narwedine. Despite of considered further galanthamine analogues (Fig. 6). These the fact that these compounds cannot be strictly considered compounds contain [1]benzazepine (55) or [3]benzazepine as galanthamine analogues, after all derivatives 62-64 (56) ring [34]. showed significant acetylcholinesterase inhibitory activity [42, 43], although it proved to be lower than that of However, the new ring systems, did not bring the galanthamine (1). expectations, as were proved to be biologically ineffective. In derivative 57, there is a carbocycle instead of the azepine In the course of developing of our synthetic strategy to ring with a carboxamide substituent [35, 36] or with an synthesize galanthamine [44], the reactions of spirocycle 29 amine group with different configurations, respectively, and tetracycle 31 were investigated (Scheme 10). In containing a phenylethylamine motif. On the way of the the course of O-demethylation−ring closure reaction of synthesis of these compounds presented by the Austrian the N-methylated intermediate (65), an unexpected research group [37], the formerly elaborated [13] phenolic rearrangement took place resulting in a new type of oxidation was used for the preparation of the key cyclopentanoisoquinoline (66). In that case, when the ring intermediate. closure reaction occured before methylation, in the methylation of 31 beside the desired major product (67) The sulfur analogue bearing a sulfonyl group on the almost the same cyclopentanoisoquinoline (68) was obtained nitrogen atom in the azepine ring (58), practically did not as a side product. In our work [44], the mechanisms for show any activity in the course of the acetylcholinesterase formation of the two anomalous derivatives were also given. inhibitory investigations [38]. Oxazocine derivative (59) is

OH O O

OH O N CH3 H3CO K3[Fe(CN)6] O N CHO 1. 1,2-propanediol

N 2. LiAlH4 CHO 3. HCl H3CO F H CO F F 3 52 50 51 OH OH OH

L-Selectride separation on O N CH O N CH3 + O N CH3 3 chiral column

H3CO F H3CO F H3CO F (-)-54 (+)-54 53 Scheme (9). Synthesis of fluoro substituted galanthamine.

1458 Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 Keglevich et al.

OH OH OH

CH3 N O R O O N

CH3 H3CO H3CO H3CO 57 55 56 R= -CONH2, (R) and (S)-NH2

OH OH HO OH

CH3 CH3 N N O H C N N CH O SO2 O 3 3 O

H3CO H CO H3CO H3CO 3 61 58 59 60 Fig. (6). Galanthamine analogues modified on the azepine ring.

O H3C X N (CH2)n HN O O N CH3 N CH3 N CH3 N N N X

Cl Cl Cl

CH3 CH3 CH3

n=1,2 X= CO, CH2 X= CO, CH2 62 63 64 Fig. (7). Derivatives containing pyridine ring instead of benzene.

O O HO O 1. MsOH O O H D.L-methionine, rt CH3 MeI, K2CO3 O N NH N CH H CO H CO 3 H CO 3 acetone. reflux 3 3 2. H O O O O 2

29 65 66

O O O H3CO O MeI, K2CO3 O H acetone. reflux NH O O NH O N CH3 + H3CO trace of H2O O O O

31 67 68 Scheme (10). Preparation of cyclopentanoisoquinolines from the tricyclic and tetracylic galanthamine intermediates.

The Chemistry of Galanthamine Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 1459

OH O OH N ONO2 O CO2C2H5 O X N O O O NH N CH CH 3 R 3

H3CO OCH3 H3CO H3CO 69 70 71 72 R= CH Cl, CH OH, CHO X= O, NH μ 2 2 IC50 150 M

Fig. (8). Ring opened galanthamine derivatives.

Galanthamine derivatives, where one of the rings was modelling studies can be regarded as structural analogues of in opened form (Fig. 8) were also investigated. The galanthamine (1) and lycoramine (2). Although some acetylcholinesterase inhibitory activity of the derivative with differences can be observed between to two structures, e.g. an opened d azepine ring (69) is much lower than that the lack of the furane ring and the six or seven membered of galanthamine (1) [39]. Derivatives with opened azepine ring, the chair conformation of the spiro ring determined by ring containing a nitrate function (70) showed (together molecular mechanics and AM1 calculations is very similar to with other effects) a potent inhibition towards that of natural products 1 and 2. Despite of these structural acetylcholinesterase, and compound 70 (X=O) revealed a similarities, the two new derivatives did not showed significant spatial memory improving effect in vivo [45]. anticholinergic activities [48]. In derivative 71 there is an alkyl side chain with an oxo In the course of cyclopropanation reaction of group instead of the spirocyclohexane ring. It is already a galanthamine (1) with diazomethane (Scheme 12) in the very far analogue of galanthamine [46], yet it has the 70% of presence of palladium(II) acetate or copper(I) bromide as the the AChE inhibitory activity of galanthamine (1). However, catalyst, the desired cyclopropanated product (77) could not in the case of galanthamine prepared without its b furane be isolated. In the reaction, methylene insertion in the ring (72), a rather weak acetylcholinesterase inhibitory aromatic ring was observed, and compound 78 was obtained activity could be observed [47]. and oxidative ring opening was also detected as a minor route [49, 50]. Spirocyclohexylisoquinolines (75 and 76) were prepared from the product of a photochemical ring closure reaction Galanthamine-based hybride molecules were synthesized (Scheme 11). From spirocyclohexenylisoquinolinone (74) from galanthamine and di- and tripeptides (Fig. 9), via epoxidation of the double bond in 3,4-position followed containing glycine, alanine, phenylalanine and valine with an by lithium aluminum hydride reduction derivatives 75 and o-dichlorophenyl ring [51]. These compounds (80) showed 76 were obtained. These molecules based on molecular very high inhibitory effect towards acetylcholinesterase,

4 HO H 3

H3CO ν h H3CO OH N N H CO H3CO CH3 3 CH3 O O N N CH CH3 73 74 3 76 75 Scheme (11). Spiro substituted isoquinolines as galanthamine analogues.

O R O O O H H H H N N Cl N N Cl O N O N H H O O Cl Cl

O O N CH3 N CH3

H3CO H3CO

80 81 R= benzyl, isobutyl, isopropyl

Fig. (9). Galanthamin and tripeptide hybride molecules. 1460 Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 Keglevich et al.

O N CH3 OH

H3CO

diazomethane 77 O N CH3 rt, catalyst OH OH H3CO CH 1 3 O N CHO O N CH3 + CHO 7 H3CO H3CO

78 79

Scheme (12). Reaction of galanthamine with diazomethane.

butyrylcholinesterase and γ-secretase. Surprisingly, the [7] Georgiev, V.; Ivanov, I.; Berkov, S.; Ilieva, M.; Georgiev, M.; alanylvalylglycyl hybride molecule (81) as antiproliferative Gocheva, T.; Pavlov, A. Galanthamine production by Leucojum aestivum L. shoot culture in a modified bubble column bioreactor peptide ester inhibits the cell growth rate of cultured 3T3 with internal sections. Eng. Life Sci., 2012, 12, 534-543. mouse embryonic fibroplast cells [52]. [8] Georgiev, V.; Ivanov, I.; Berkov, S.; Pavlov, A. Temporary immersion systems for Amaryllidaceae alkaloids biosynthesis by 3. CONCLUSION Pancratium maritimum L. shoot culture. J. Plant Biochem. Biotechnol., 2014, 23, 389-398. The known analogues of galanthamine practically [9] Atanasova, M.; Yordanov, N.; Dimitrov, I.; Berkov, S.; exhaust all of the possibilities for modification and changing Doytchinova, I. Molecular docking study on galanthamine derivatives as cholinesterase inhibitors. Mol. Inform., 2015, 34, the structure of galanthamine. Considering the biological 394-403. effects and the pharmacological properties [53], after all, in [10] Houghton, P. J.; Ren, Y.; Howes, M.-J. Acetylcholinesterase the therapy galanthamine itself remained the molecule that is inhibitors from plants and fungi. Nat. Prod. Rep., 2006, 23, 181- used more and more widely in treatment of Altzheimer’s 199. [11] Berkov, S.; Codina, C.; Viladomat, F.; Bastida, J. N-Alkylated disease. In spite of this fact synthetic chemistry was enriched galanthamine derivatives: potent acetylcholinesterase inhibitors considerably by the efforts performed for the syntheses of from Leucojum aestivum. Bioorg. Med. Chem. Lett., 2008, 18, the new derivatives. 2263-2266. [12] Barton, D. H. R.; Kirby, G. W. Phenol oxidation and biosynthesis. Part V. The synthesis of galanthamine. J. Chem. Soc., 1962, 806- CONFLICT OF INTEREST 817.

The author(s) confirm that this article content has no [13] Czollner, L.; Frantsist, W.; Küenburg, B.; Hedenig, U.; Fröhlich, J.; Jordis, U. New kilogram-synthesis of the anti-Alzheimer drug (-)- conflict of interest. galanthamine. Tetrahed. Lett., 1998, 39, 2087-2088. [14] Node, M.; Kodama, S.; Hamashima, Y.; Katoh, T.; Nishide, K.; ACKNOWLEDGEMENTS Kajimoto, T. Biomimetic synthesis of (±)-galanthamine and asymmetric synthesis of (-)-galanthamine using remote asymmetric Declared none. induction. Chem. Pharm. Bull., 2006, 54, 1662-1679. [15] Trost, B. M.; Toste, F. D. Enantioselective total synthesis of (-)- galanthamine. J. Am. Chem. Soc., 2000, 122, 11262-11263. REFERENCES [16] Trost, B. M.; Tang, W. An efficient enantioselectives synthesis of (-)-galanthamine. Angew. Chem. Intl. Ed., 2002, 41, 2795-2797. [1] Lewis, J. R. Amaryllidaceae alkaloids. Nat. Prod. Rep., 1997, 14, 303-308. [17] Satcharoen, V.; McLean, N. J.; Kemp, S. C.; Camp, N. P.; Brown, R. C. D. Stereocontrolled synthesis of (-)-galanthamine. Org. Lett., [2] Walsh, R.; Rockwood, K.; Martin, E.; Darvesh, S. Synergistic inhibition of butyrylcholinesterase by galantamine and citalopram. 2007, 9, 1867-1869. [18] Lukács, A.; Szabó, L.; Hazai, L.; Szántay, Cs., jr. ; Mák, M.; Biochim. Biophys. Acta, 2011, 1810, 1230-1235. [3] Marco-Contelles, J.; do Carmo Carreiras, M.; Rodríguez, C.; Gorka, Á.; Szántay, Cs. Seven-membered ring formation through Grewe-cyclization. Tetrahedron, 2001, 57, 5843-5850. Villarroya, M.; Garcia, A. G. Synthesis and pharmacology of galanthamine. Chem. Rev., 2006, 106, 116-133. [19] Gorka, Á.; Hazai, L.; Szántay, Cs., jr., Háda, V.; Szabó, L.; Szántay, Cs. Synthesis of spiro-substituted benzo[c]azepinones. [4] Marco, L.; do Carmo Carreiras, M. Galanthamine, a natural product for the treatment of Alzheimer’s disease. Rec. Pat. CNS Drug Heterocycles, 2005, 65, 1359-1371. [20] Herke, K.; Hazai, L.; Hudák, M. Sz.; Ábrahám, J.; Sánta, Zs.; Discov., 2006, 1, 105-111. [5] Berkov, S.; Ivanov, I.; Georgiev, V.; Codina, C.; Pavlov, A. Háda, V.; Szántay, Cs., Jr.; Szántay, Cs. Synthesis of the tetracyclic skeleton of the galanthamine-type Amaryllidaceae alkaloids. Galanthamine biosynthesis in plant in vitro systems. Eng. Life. Sci., 2014, 14, 643-650. ARKIVOC, 2009, (xi), 235-246. [21] Herke, K.; Hazai, L.; Dubrovay, Zs.; Háda, V.; Sánta, Zs.; Szántay, [6] Ivanov, I.; Georgiev, V.; Berkov, S.; Pavlov, A. Alkaloid patterns in Leucojum aestivum shoot culture cultivated at temporary Cs., jr.; Kalaus, Gy.; Szántay, Cs. Synthesis of demethoxylycoraminone. Heterocycles, 2011, 83, 581-589. immersion conditions. J. Plant. Physiol., 2012, 169, 206-211. The Chemistry of Galanthamine Mini-Reviews in Medicinal Chemistry, 2016, Vol. 16, No. 18 1461

[22] Magnus, P.; Sane, N.; Fauber, B. P.; Lynch, V. Concise syntheses [38] Treu, M.; J.; Jordis, U.; Mereiter, K. 12H-[2]-Benzothiepino of (-)-galanthamine and (±)-codeine via intramolecular alkylation [6,5a,5-bc]benzofuran: Synthesis of a Sulfur-Analog of of a phenol derivative. J. Am. Chem. Soc., 2009, 131, 16045-16047. Galanthamine. Heterocycles, 2001, 55, 1727-1735. [23] Mary, A.; Renko, D. Z.; Guillou, C.; Thal, C. Selective N- [39] Herlem, D.; Martin, M.-T.; Thal, C.; Guillou, C. Synthesis and demethylation of galanthamine to norgalanthamine via a non structure-activity realtionships of open D-ring galanthamine classical Polonovski reaction. Tetrahed. Lett., 1997, 38, 5151-5152. analogues. Biorg. Med. Chem. Lett., 2003, 13, 2389-2391. [24] Han, S. Y.; Sweeney, J. E.; Bachman, E. S.; Schweiger, E. J.; [40] Lan, P.; Jackson, C. J.; Banwell, M. G.; Willis, A. C. Synthesis of a Forloni, G.; Coyle, J. T.; Davis, B. M.; Jouillé, M. M. Chemical D-ring isomer of galanthamine via a radical-based Smiles and pharmacological characterization of galanthamine, an rearrangement reaction. J. Org. Chem., 2014, 79, 6759-6764. acetylcholinesterase inhibitor, and its derivatives. A potential [41] Guillou, C.; Bernard, C.; Gras, J.-L. Total synthesis of application in Alzheimer’s disease? Eur. J. Med. Chem., 1992, 27, galanthamine, analogues and derivatives thereof. WO 2002/102803; 673-687. Chem. Abstr., 2002, 138, 39446. [25] Ping, J.; Rong, S.; Jing, Z.; Liang, F.; Quiaojun, H.; Bo, Y.; [42] Vanlaer, S.; De Borggraeve, W. M.; Compernolle, F. Synthesis of Yongzhou, H. Design, synthesis and evaluation of galanthamine spirocyclic pyridoazepines as analogues of galanthamine by derivatives as acetylcholinesterase inhibitors. Eur. J. Med. Chem., nucleophilic aromatic substitution of 3-substituted 2- 2009, 44, 772-784. chloropyridines. Eur. J. Org. Chem., 2007, 4995-4998. [26] Rao, V. K.; Rao, A. J.; Reddy, S. S.; Raju, C. N.; Rao, P. V.; Gosh, [43] Vanlaer, S.; De Borggraeve, W. M.; Voet, A.; Gielens, C.; De S. K. Synthesis, spectral characterization and biological evaluation Maeyer, M.; Compernolle, F. Spirocyclic pyridoazepine analogues of phosphorylated derivatives of galanthamine. Eur. J. Med. Chem., of galanthamine: synthesis, modelling studies and evaluation as 2010, 45, 203-209. inhibitors of acetylcholinesterase. Eur. J. Org. Chem., 2008, 2571- [27] Mary, A.; Renko, D. Z.; Guillou, C.; Thal, C. Potent 2581. acetylcholinesterase inhibitors: design, synthesis, and structure- [44] Herke, K.; Hazai, L.; Sánta, Zs.; Dubrovay, Zs.; Háda, V.; Szántay, activity relationships of bis-interacting ligands in the galanthamine Cs., jr.; Kalaus, Gy.; Szántay, Cs. An unexpected rearrangement of series. Bioorg. Med. Chem., 1998, 6, 1835-1850. the benzofurobenzazepine skeleton of galanthamine-type alkaloids. [28] Guillou, C.; Mary, A.; Renko, D. Z.; Gras, E.; Thal, C. Potent Tetrahed. Lett., 2010, 51, 6932-6934. acetylcholinesterase inhibitors: design, synthesis, and structure- [45] Fang, L.; Fang, X.; Gou, S.; Lupp, A.; Lenhardt, I.; Sun, Y.; activity relationships of alkylene linked bis-galanthamine and Huang, Z.; Chen, Y.; Zhang, Y.; Fleck, C. Design, synthesis and galanthamine-galanthaminium salts. Bioorg. Med. Chem. Lett., biological evaluation of D-ring opened galantamine analogs as 2000, 10, 637-639. multifunctional anti-Alzheimer agents. Eur. J. Med. Chem., 2014, [29] Hametner, C.; Hemetsberger, M.; Treu, M.; Mereiter, K.; Jordis, 76, 376-386. U.; Fröhlich, J. Addition of nucleophiles to immonium [46] Chandrasekhar, S.; Basu, D.; Sailu, M.; Kotamraju, S. Novel galanthamines. Eur. J. Org. Chem., 2005, 2, 404-409. synthetic route to the tricyclic core of (±)-galanthamine. Tetrahed. [30] Greenblatt, H. M.; Guillou, C.; Guénard, D.; Argaman, A.; Botti, Lett., 2009, 50, 4882-4884. S.; Badet, B.; Thal, C.; Silman, I.; Sussman, J. L. The complex of [47] Liang, P.-H.; Hsin, L.-W.; Pong, S.-L.; Hsu, C.-H.; Cheng, C.-Y. the bivalent derivatives of galanthamine with Torpedo Synthesis of galanthamine analogs as acetylcholinesterase accetylcholinesterase displays drastic deformation of the active-site inhibitors via intramolecular Heck cyclization. J. Chin. Chem. Soc., gorge: implications for structure-based drug design. J. Am. Chem. 2003, 50, 449-456. Soc., 2004, 126, 15405-15411. [48] Missoum, A.; Sinibaldi, M.-E.; Vallée-Goyet, D.; Gramain, J.-C. [31] Bartolucci, C.; Haller, L. A.; Jordis, U.; Fels, G.; Lamda, D. Photochemical synthesis of spirocyclohexylisoquinolines, Probing Torpedo californica acetylcholinesterase catalytic gorge analogues of (±)-galanthamine and (±)-lycoramine. Synth. with two novel bis-functional galanthamine derivatives. J. Med. Commun., 1997, 27, 453-466. Chem., 2010, 53, 745-751. [49] Keglevich, P.; Kovács, P.; Hazai, L.; Sánta, Zs.; Dubrovay, Zs.; [32] Knesl, P.; Yousefi, B. H.; Mereiter, K.; Jordis, U. Synthesis of (-)- Háda, V.; Szántay, Cs., jr.; Kalaus, Gy.; Szántay, Cs. A new and (+)-8-fluoro-galanthamine. Tetrahed. Lett., 2006, 47, 5701- derivative of galanthamine: methylene insertion into the aromatic 5703. ring in place of cyclopropanation. Heterocycles, 2012, 84, 1171- [33] Hemetsberger, M.; Treu, M.; Jordis, U.; Mereiter, K.; Hametner, 1178. C.; Fröhlich, J. 1-Methyl-galanthamine derivatives. Monatsch. [50] Keglevich, P.; Hazai, L.; Kalaus, Gy.; Szántay, Cs. Chemie, 2004, 135, 1275-1287. Cyclopropanation of some alkaloids. Period. Polytech., Chem.. [34] Lewin, A. H.; Szewczyk, J.; Wilson, J. W.; Carroll, F. I. Eng., 2015, 59, 3-15. Galanthamine analogs: 6H-benzofuro[3a,3,2-e,f][1]benzazepine [51] Vezenkov, L.; Sevalle, J.; Danalev, D.; Ivanov, T.; Bakalova, A.; and 6H-benzofuro[3a,3,2-e,f][3]benzazepine. Tetrahedron, 2005, Georgieva, M.; Checler, F. Galantamine-based hybrid molecules 61, 7144-7152. with acetylcholinesterase, butyrylcholinesterase and γ-secretase [35] Treu, M.; Fröhlich, J.; Jordis, U. Carbocyclic galanthamine inhibition activity. Curr. Alzheimer Res., 2012, 9, 600-605. analogues: construction of the novel 6H-benzo[a]cyclohepta[hi] [52] Tsvetkova, D. D.; Obreshkova, D. P.; Petkova, V. B.; Atanasov, P. benzofurane ring system. Mendeleev Commun., 2002, 12, 52-53. Y.; Malik, R.; Siddiq, S.; Hadjieva, B.; Dimitrov, M. V. [36] Treu, M.; J.; Jordis, U. 4a,5,9,10,11,12-Hexahydro-6H- Investigation of antiproliferative activity of galantamine peptide benzo[a]cyclohepta[hi]benzofuran – synthesis of unnatural GAL-VAL against 3T3cell lines. World J. Pharm. Pharmaceut. galanthamine analogs. Molecules, 2002, 7, 374-381. Sci., 2014, 3, 10-19. [37] Treu, M.; Mereiter, K.; Hametner, C.; Fröhlich, J.; Jordis, U. J. [53] Maelicke, A.; Samochoki, M.; Jostock, R.; Fehrenbacher, A.; Carbocyclic galanthamine analogs: incorporating the Ludwig, J.; Albuquerque, E. X.; Zerlin, M. Allosteric sensitization phenethylamine motif. Heterocycl. Chem., 2002, 39, 1167-1171. of nicotinic receptors by galantamine, a new treatment strategy for Alzheimer’s disease. Biol. Psychiatry, 2001, 49, 279-288.

DISCLAIMER: The above article has been published in Epub (ahead of print) on the basis of the materials provided by the author. The Editorial Department reserves the right to make minor modifications for further improvement of the manuscript.

PMID: 26996619