Chapter 1 Synthesis of (±)-Baclofen and Analogs (Homobaclofen, PCPGABA, Phaclofen, Saclofen)

Chapter 1 Synthesis of (±)-Baclofen and analogs (Homobaclofen, PCPGABA, Phaclofen, Saclofen).

Chapter 1: Synthesis of (±)-Baclofen and analogs

Section-I: Synthesis of (±)-Baclofen Section-II: Synthesis of (±)-Homobaclofen Section-III: Synthesis of (±)-PCPGABA Section-IV: Synthesis of (±)-Phaclofen and (±)-Saclofen

Section-I: Synthesis of (±)-Baclofen 1.0.1 Introduction 1.0.2 GABA The brain and the spinal cord are two main components of the central nervous system. The central nervous system (CNS) integrates information received from different body parts, coordinates and influences the activity of all parts of the body. The nervous system is made up of individual nerve cells called neurons through which nerve signals are transmitted as an electrical impulse. When a nerve impulse reaches to the end of the neuron it can jump over to the next cell using chemical messengers called neurotransmitters. Glutamate and GABA (Figure-1) are the most abundant neurotransmitters in the central nervous system, and especially in the cerebral cortex, where thinking occurs and sensations are interpreted. Glutamate acts as an excitatory neurotransmitter while GABA does the opposite.

Figure-1 In 1950, Eugene Roberts and Jorge Awapara independently discovered that very large amounts of GABA was present in the mammalian central nervous system, 1 mg per gram, while undetectable in other tissues. Ernst Florey in 1953 showed that GABA was the unknown compound which inhibited the crayfish stretch receptor when applied exogenously.1 γ-Amino butyric acid 1 (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system (CNS) and synthesized by decarboxylation of glutamate using the enzyme glutamic acid decarboxylase (GAD). This enzyme converts

L-glutamate to GABA and CO2, using pyridoxal phosphate as a cofactor (Figure-2).

Figure-2 GABAs can be grouped into two broad classes: First class consists of ligand-gated ion channels or (GABAA and GABAC) (Table-1) while second class consists of G-protein-

2 coupled or GABAB receptors (Table-2) based on electrophysiological and binding studies.3

Table-1: Ligand-gated ion channel receptors Function type Ligand Ion channel

Glutamate (non-NMDA class receptors) Na+/ K+ Excitatory Glutamate (NMDA class receptors) Na+/ K+ and Ca2+ Acetylcholine (nicotinic receptor) Na+/ K+ Serotonin (5HT3 class receptors) Na+/ K+

Inhibitory GABAA/ C and glycine receptors Cl¯

Table-2: Some Neurotransmitter G-protein-coupled receptors

Glutamate receptors (metabotropic receptors) GABAB receptors Acetylcholine (muscarinic receptors) Dopamine receptors Epinephrine, norepinephrine Histamine receptors Serotonin (5HT1, 5HT2, 5HT4 receptors

1.0.2.1 Ligand-gated ion channel receptors Muscimol 2, ibotenic acid4,5 3, thiomuscimol 4, dihydromuscimol6 5, isoguvacine 6 and isonipecotic acid 7 are the GABAA agonist and used to design the different classes of GABA analogues7 (Figure-3).

Figure-3: GABAA agonists. Bicuculline8 8 and its quaternized analogue bicuculline methyl chloride9 (BMC)

9 are GABAA antagonists and have played a key role in studies on GABAA receptors. 10 Iso-THAZ 10, is a moderately potent GABAA antagonist. Gabazine 11, and its 11 derivatives 12, 13 showed very potent and selective GABAA antagonist effects.

Pentylenetetrazole 14, and Picrotoxinin 15 (Figure-4) are non competitive GABAA antagonist.12

Figure-4: GABAA antagonists.

4,5,6,7-Tetrahydroisoxazolo[5,4]-3-pyridinol (THIP)13 16, Imidazole-4-acetic acid14 (IAA) 17 and Piperidine-4-sulphonic acid15 (PAS) 18 shows the characteristics of 16 partial GABAA agonists. The non fused THIP analogue 5-(4-piperidyl)-3-isoxazolol (4-PIOL) 19 was about 200 times less potent as an agonist than isoguvacine 6 and its response was competitively antagonized by bicuculline methyl chloride 9. The 3- isothiazolol analogue of 19, Thio-4-PIOL 20, was approximately equally active with 19 at GABAA binding sites, whereas the unsaturated analogue of 20, DH-Thio-4-PIOL 21, was significantly more efficacious.17 Compounds 22 and 23 also showed a potent 18 GABAA antagonist activity (Figure-5).

Figure-5: Partial GABAA agonists. Benzodiazepines (BDZs), barbiturates and steroids etc. (Figure-6) possesses a large number of binding sites for drugs and primary mechanism involves allosteric modulation of the GABAA receptor complex. Some examples are shown in Figure-6.

Figure-6: Benzodiaxepines

GABAC receptors are non-GABAA, non-GABAB (NANB) ionotropic GABA 19 receptors and they are bicuculline and baclofen-insensitive receptors. GABAC receptors share several agonists with GABAA receptors but the sensitivity of GABAC 20 receptors to GABA is much higher than that of GABAA receptors. The conformationally restricted GABA analogues, namely cis-4-amino-crotonic acid (CACA) 28 (fig.-7) and cis-2-aminomethyl-cyclopropane-carboxylic acid (CAMP) 29 (fig.-7) are GABA-like neuronal depressants which are not sensitive to 9 and they bind to a class of GABA receptor sites which do not recognize isoguvacine 6 or (R)- Baclofen.21 IAA 17 has recently been shown to be an antagonist at the retinal GABA 22 receptors, probably of the GABAC type. Also, 1,2,5,6-tetrahydro-4-pyridinylmethyl- phosphinic acid (TPMPA) 30 (fig.-7), a methyl-phosphonic acid derivative of 6, is a weak antagonist at GABAA receptors and a weak agonist at GABAB receptors, but 23 shows a highly potent antagonist effect at GABAC receptors.

Figure-7

1.0.2.2 G-Protein-Coupled Receptors/GABAB Receptors G-protein-coupled receptors are cell-surface receptors containing seven transmembrane α-helical regions with N-terminal segment on the exoplamic face and their C-terminal segment on the cytosolic face of the plasma membrane. Ligand binding to these receptors activates their associated trimeric signal transducing G protein, which in turn activates or inhibits an effectors enzyme that generates an intracellular specific second messenger or modulates an ion channel, causing a change in membrane potential.24 This large receptor super family has been divided based on their homology into three families: family A (rhodopsin receptor-like), family B (secretin receptor-like) and family C (metabotropic glutamate receptor-like, calcium sensing receptor, GABAB receptors and several pheromone receptors).25

GABAB receptors (GABABR) are metabotropic transmembrane receptors for gamma-amino butyric acid (GABA) that are linked via G-proteins to potassium 26 channels. The GABAB receptor is a G-protein coupled receptor (GPCR) that associates with a subset of G-proteins, that in turn regulate specific ion channels and trigger cAMP cascades. GABAB agonists inhibit basal and forskolin-stimulated neuronal adenylate cyclase in brain slices through a G protein-dependant mechanism that results in a 27 reduced level of intracellular cAMP. In addition, activation of GABAB receptors decreases Ca2+ conductance and increases K+ conductance in neuronal membranes.28 The importance of GABA in the central and peripheral nervous systems prompted the researchers to design an analogue which unlike GABA itself could readily access to the brain. 4-Amino-3-phenyl-butanoic acid 31 inhibits spinal reflexes after oral administration. Baclofen 32 emerged as a possible GABA-mimetic which could administered orally and activate GABAB receptors in a stereoselective manner. Many analogs of Baclofen are synthesized and tested for pharmacological properties. 4-amino-3-benzo[b]-2-furanyl- butanoic acid 33 and 4-amino3-(5-methoxy-benzo[b]-2-furanyl)-butanoic acid 34 29 showed affinity for the GABAB receptors. 4-amino-3-(7-methyl-benzo[b]-2-furanyl)- butanoic acid 35, 4-amino-3-(5-methyl2-thienyl)-butanoic acid 36, 4-amino-3-(5-

19 chloro-2-thienyl)-butanoic acid 37, and 4-amino-3-(2′,4′-dichloro-phenyl)butanoic acid

38 showed affinity for GABAB receptors. The conformationally restricted analogues of Baclofen, namely 1-(aminomethyl)-5-chloro-2,3-dihydro-1H-indene-1-acetic acid 39 and (1R,2S)-2-aminomethyl-2-(4′-chloro-phenyl)-cyclopropane-carboxylic acid 40 30 were surprisingly found inactive as GABAB ligands in the binding assay. Phosphinic acid derivatives 3-aminopropyl-phosphinic acid (3-APPA, CGP27492) 41 and its methyl homologue 3-aminopropyl-methyl-phosphinic acid (3-APMPA, CGP35024) 42 are more potent than the other active isomers of baclofen. The fluorine atoms of 3- aminopropyl-(difluromethyl)-phosphinic acid (CGP47656) 43 possess high affinity to

GABAB receptors with partial agonist activity. Compound 44 show moderate activity while compound 45 show about one-third the activity of racemic Baclofen.31 In 1991, Kristiansen and Fjalland reported that the (R)-4-amino-3-hydroxybutanoic acid (3-OH-

GABA) 46 acts as a GABAB agonist at ileal GABAB receptors. (R)-5-amino-3-(4′- chloro-phenyl)-pentanoic acid 47 and (S)-5-amino-3-(4′-chloro-phenyl)-pentanoic acid 48 show somewhat less activity than Baclofen32 (Figure-8).

Figure-8

The development of selective GABAB receptor antagonists with increasing receptor affinity and potency play an important role in establishing the significance and structure of the GABAB receptor. Some GABAB receptor antagonists are shown in Figure-9.

Figure-9: GABAB receptor antagonists.

Allosteric modulators may delay dissociation by stabilizing the bound-agonist state or act as subtype-selective enhancers of agonist binding and promote receptor G protein coupling. 2,6-di-tert-butyl-4-(3hydroxy-2,2-dimethyl-propyl)-phenol

(CGP7930) 59 was identified as a positive modulator for GABAB receptor. N-(3,3- diphenyl-propyl)-α-methyl-benzylamine (fendiline) 60, N-(3,3-diphenyl-propyl)-α- methyl-3methoxy-benzylamine (F551) 61, and N-(3,3-diphenyl-propyl)-α-methyl- phenylethylamine (prenylamine) 62 are some another examples of allosteric modulators (Figure-10).

Figure-10: Allostteric modulators.

GABA protects pancreatic islet cells against apoptosis and exerts anti- inflammatory effects, and also inhibited the activation of NF-kB in both islet cells and lymphocytes.33 GABA is involved in the control of many physiological mechanisms, including the regulation of prolactin secretion and other hormones, including the growth hormone.34 Furthermore, GABA plays a role in the regulation of cardiovascular functions, such as blood pressure, heart rate and is involved in the sensation of pain and anxiety.35 GABA is not transported efficiently into the brain from the bloodstream because of its low lipophilicity. The disfunctioning of the central GABA system is responsible for the development and outbreak of epilepsy, Huntington’s and Parkinson’s diseases, and other psychiatric disorders, such as anxiety and pain which causes extensive research in the synthesis and clinical application of GABA derivatives.36

Figure-11 shows some GABAB derivatives synthesized and tested for pharmaceutical activity by different scientists. All these derivatives have been commercialized in their racemic form although the enantiomers show divergent biological activities. Phenibut37 63 (Figure-11) is a psychotropic drug. Rolipram 64 (Figure-11) is used in the treatment of depression and as an anti-inflammatory agent. Pregabalin 68 (Figure-11) is the active ingredient in the drug Lyrica, which is used for the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Due to the therapeutic potential of GABA derivatives, synthetic protocols of excessive amount have been developed. GABAs deficiency is associated with several neurological and psychiatric disorders, which has triggered extensive research in the synthesis and clinical application of GABA derivatives.38

Figure-11: GABAB derivatives

1.1 Baclofen Baclofen is an analog of GABA, but unlike GABA, it can cross the blood brain barrier. Baclofen is the reference selective agonist for the GABA receptor, and racemic baclofen is widely used as an antispastic agent. The enantiomers (Figure 12) of this compound differ in their pharmacodynamic and toxicological properties; the (-) enantiomer is much more active but also more toxic than the (+) enantiomer. The R configuration was assigned to the more active enantiomer on the basis of X-ray crystallography.39

Figure-12: Baclofen isomers

Baclofen is the one most important lipophilic derivative of GABA acts as an inhibitory neurotransmitter in central nervous system40 was synthesized by Swiss chemist Heinrich Keberle for the first time in 1962. The excitatory effect of active compounds such as benzodiazepine, barbiturates, etc. can be reduced with the help of Baclofne.41 Baclofen is also one of the most promising drugs in the control and treatment of the paroxysmal pain of trigeminal neuralgia42 as well as spasticity of spine without influencing the sedation.43 Although biological activity resides exclusively in R-enantiomers, Baclofen has been commercialized in its racemic form.44 Baclofen shows a significant increment in gastric acid secretion through the activation of central cholinergic mechanisms in rats. Baclofen is a potent and selective agonist for bicuculline-insensitive GABAB receptors and is used clinically as an antispastic and muscle relaxant agent.45

Although several speciﬁc agonists or antagonists at GABAA receptor sites have been developed, 3-(4-chlorophenyl)-4-aminobutyric acid (Baclofen, 32) is the only 46 clinically useful selective GABAB agonists. It is used in the treatment of paroxysmal pain of trigeminal neuralgia as well as spasticity of spinal, a serious disease

24 characterized by an increase in muscle tone usually perceived as muscle tightness or achiness in limbs.47 For further studies in neurochemistry, pharmacology, medicinal chemistry, as well as electrophysiology, it is necessary to synthesize analogs of GABA and study them. Despite the great effort made to optimize the synthesis of a particular GABA analogue, a simple, general, and sustainable protocol to obtain all possible derivatives is still lacking. So we decided to design a simple, general and sustainable protocol for the synthesis of GABA analogs and homologues and here we developed the same. Here, we report a new methodology for the preparation of g-amino butyric acid (GABA) derivatives.

1.1.1 Review of literature Literature search revealed that there are several reports available on the synthesis of (±)-Baclofen 32 as well as enantioselective synthesis of R and/or S form of Baclofen, following representation of the literature review is organized as given belows.

Fernando Coelho (1997)48 Fernando et al. synthesized Baclofen using [2+2] cycloaddition reaction between 4-chlorostyrene 73 and trichloro acetyl chloride to produce 2,2-dichloro- 3-(4- chlorophenyl)- cyclobutanone 74. Reductive dechlorination of cycloadduct in acetic acid/Zn dust furnished cyclobutanone derivative 75 which on ring expantion with Beckmann rearrangement using N-hydroxylamine-O-sulphonic acid gave γ-lactam 76. The acid hydrolysis of lactam 76 furnished (±)-Baclofen 32 as a white solid in 70% yield (Scheme-1).

Scheme-1: Reagent and conditions: (a) Cl3CCOCl, Zn-Cu, POCl3, ether, reflux, 20 h, 82%; (b) Zn/CH3CO2H, 70 °C, 14 h, 93%; (c) H2NOSO3H, HCO2H, 98%, reflux, 10h, 43%; (d) HCl, reflux, 12h, 70%.

Alcindo A. dos Santos’s approach (2001)49

Alcindo et al. introduced a synthon equivalent to a carboxymethyl anion to enones and nitroalkenes, through a 1,4-Michael addition reaction of 2,4,4-trimethyl-2- oxazoline cyanocuprate to the commercially available p-chloro-β-nitrostyrene 77, afforded γ-nitro- oxazoline 78 which proved to be an interesting methodology for the synthesis of (±)-Baclofen. Acid hydrolysis of 78 gave γ-nitro ester 79 which on reduction provides γ-lactam 76 for acid hydrolysis to produce (±)-Baclofen (Scheme-2).

Scheme-2: Reagent and conditions: (a) THF, -78 ºC, argon, 40 min, 76%; (b)

EtOH/H2SO4, reflux 52h, 84%; (c) H2, Raney-Ni, EtOH, rt, 24h; (d) reflux, o-xylene, 1h, 75% ; (e) HCl (6 mol L-1), reflux, 6h 82%.

Marcos Jose S. Carpes and Carlos Roque D. Correia (2002)50 Marcos et al. used Heck arylation of N-Boc-3-pyrroline 81 with 4- Chlorophenyl- diazonium tetraﬂuoroborate 82 using palladium acetate under phosphine free conditions to gave tert-butyl 4-(4-chlorophenyl)-2-hydroxypyrrolidine-1- carboxylate 83 which was oxidized to the desired γ-lactam 84. Acidic hydrolysis of the γ-lactam 84 produced (±)-Baclofen (Scheme-3).

Scheme-3: Reagent and conditions: (a) Pd(OAc)2 (2 mol%), H2O/CH3CN (1:1) 75%; (b) PCC, DCM, 90%; (c) 6N HCl, reflux, 18h, 90%.

Meng-Yang Chang (2003)51 Meng-Yang et al reported a facile [3+2] annulation reaction as key step between sulfonyl-acetamide derivatives and α-bromo substituted unsaturated alkyl esters that lead to corresponding pyroglutamic acid in good yields. Benzyl amine was treated with chloroacetyl chloride and triethylamine to produce α-chloro acetamide, which was then treated with p-toluenesulﬁnic acid sodium salt to give α-sulfonyl acetamide in 85% yield from the two-steps reaction. The reaction of α-sulfonyl-acetamide 86 with ethyl 2-bromo-3-(4-chlorophenyl) acrylate 85 resulted in the formation of single pyroglutamate isomer 87. The one-pot desulfonation and hydrolysis of pyroglutamate was accomplished by treatment of 6% sodium amalgam (Na/Hg) and sodium phosphate. After addition of water to the resulting mixture and then acidiﬁcation, yielded 1-benzyl-3-(4-chlorophenyl)-5-oxopyrrolidine-2-carboxylic acid 88 in 60% yields. Decarboxylation under Barton conditions and debenzylation gave γ-lactam, which on acid hydrolysis (±)-Baclofen was obtained as final product (Scheme-4).

+ Scheme-4: Reagent and conditions: (a) NaH, THF, 50%; (b) Na/Hg, H2O, H , 60%; (c) N-methylmorpholine, isobutyl chloroformate, N-hydroxy-2-thiopyridone, TEA, THF, hν, -15 oC; (d) 6N HCl, reflux, 18h, 90%.

Mohammad Hassan Houshdar Tehrani’s approach (2003)52 Mohammad et al. have synthesised (±)-Baclofen from 4- chlorobenzaldehyde. The key steps involved the condensation of β-nitro styrene 77 thus prepared from nitro methane and p-chloro benzaldehyde with diethyl malonate to

28 produce diethyl 2-(1-(4-chlorophenyl)-2-nitroethyl)malonate 90. Further reduction and decarboxylation of 90 gave access to baclofen with a good yield (Scheme-5).

Scheme-5: Reagent and conditions: a) CH3NO2, NaOH, CH3OH, 0 °C; b) ‒ diethylmalonate, ethanol, NaOEt; c) H2, Raney Ni, CH3OH, rt; d) -OH , reflux then + ‒ H3O , reflux then -OH .

Zhenliang Chen (2005)53 Zhenliang et al. used N-tert-butyl γ-lactam hydrolysis obtained from Rh(II) catalyzed intramolecular C–H insertion of diazoacetamides for synthesis of Baclofen. N-tert-butyl-N-p-chlorophenylethyl amine 92 was treated with diketene and 4- acetamido- benzene sulfonyl azide to produce N-tert-Butyl-N-p-chlorophenylethyl α- diazoacetamide 93. Intramolecular rearrangement catalyzed by Rh2(cap)4 in the solution of α-diazoacetamide gave N-tert-Butyl-β-p-chlorophenyl-γ-lactam 94, which on acid hydrolysis yields (±)-Baclofen (Scheme-6).

Scheme-6: Reagent and conditions: (a) Diketene, THF; p-ABSA, DBU, THF, LiOH,

H2O, THF, 86% yield. (b) Rh2(cap)4 (1 mol%), reflux, CH2Cl2, 94% yield. (c) 28% HCl, reflux, 95%.

Ravi Varala and Srinivas R. Adapa (2006)54 Ravi et al. synthesized Baclofen using Pd(II)-bipyridine catalyzed conjugative addition of 4-chloroboronic acid as key step. N-Allyl phthalimide 97 was subjected to ozonolysis at -78 oC to produce N-Phthalimido acetaldehyde 98, which was treated with ethyl 2-(triphenylphosphoranylidene)acetate to gave ethyl-4-(1,3-dioxo-1,3-dihydro-2H- isoindol-2-yl)but-2-enote 99. Michael addition reaction was carried out between compound 99 and (4-chlorophenyl)boronic acid 100 using Pd(II)-bipyridine catalyst to gave Ethyl 4-(1,3-Dioxo-1,3-dihydro-2H-isoindol-2-yl)-3-(4chlorophenyl)butanoate 101. Finally phthalimide deprotection and acid catalyzed ester hydrolysis gives (±)- Baclofen (Scheme-7).

Scheme-7: Reagent and conditions: (a) Et3N, toluene, reflux, 3.5h, 95%; (b) O3, DCM/MeOH, DMS, 62%; (c) Dry DCM, rt, 78%; (d) Pd(OAc)2, bipyridine, H2O, THF, AcOH, 82%; (e) NH2NH2.H2O, EtOH, reflux; (f) 6N HCl, 58%.

Meng-Yang Chang (2006)55 Chang et al. used MCPBA epoxidation and Baeyer Villiger oxidation reaction for the synthesize (±)-Baclofen. 4-Hydroxypiperidine 102 was protected as N- carboxylate followed by Jones oxidation to produce compound 103 which was subjected to Grignard addition of 4-Chlorophenylmagnesium bromide to gave benzyl 4- (4-chlorophenyl)-5,6-dihydropyridine-1(2H)-carboxylate 104 as a starting material for (±)-Baclofen synthesis (Figure-13).

Figure-13

Compound 104 under the repeated combination of MCPBA and BF3-OEt2 condition provides benzyl 3-(4-chlorophenyl)-2,5-dihydro-1H-pyrrole-1-carboxylate 106. Double bond was hydrogenated with H2, Pd/C followed by N-Boc protection. Selective oxidation of tert-butyl 3-(4-chlorophenyl)- pyrrolidine-1-carboxylate 107 with ruthenium oxide and subsequent hydrolysis with aqueous hydrochloric acid yielded (±)- baclofen (Scheme-8).

Scheme-8: Reagent and conditions: (a) i) MCPBA, ii) BF3(OEt)2; (b) i) MCPBA, ii) BF3(OEt)2; (c) i) H2, Pd/C, ii) Boc2O; (d) i) RuCl3, NaIO4, ii) 2N HCl.

Koichi Kato (2009)56 Koichi et al. synthesized [4-11C]-Baclofen via Michael addition of nitro methane labelled with short lived 11C which can be used in PET image analysis and promote the development of new PET tracers. Positron emission tomography (PET) is a non invasive in vivo imaging technology using molecules labelled with short lived positron 11 18 emitting radionuclide’s such as C and F (T1/2 = 20.3 and 109.7 min).

Figure 14

A tetrabutylammonium fluoride promoted Michael addition of nitro[11C]methane (Figure 14) to methyl p-chlorocinnamate 108, followed by the nitro group reduction and alkaline hydrolysis yielded (R,S)-[4-11C]Baclofen 110 in 36.4 ± 1.8% radiochemical conversion in three steps within 20 min (Scheme-9).

Scheme-9: Reagent and conditions: (a) TBAF, THF, rt, 3 min.; (b) NiCl2, NaBH4, o MeOH/H2O, rt, 3 min.; (c) NaOH aq., 80 C, 3 min.

Kallolmay Biswas (2014)57 Kallolmay et al. synthesised Baclofen by suing Suzuki coupling reaction as a key step between mucochloric acid 111 and 4-chlorophenylboronic acid 100 to produce 3-chloro-4-(4-chlorophenyl)-5-hydroxyfuran-2(5H)-one 112 which was then treated with 2,4-dimethoxybenzylamine (DMB-NH2) to produce an α, β-unsaturated γ- butyrolactam 113. The DMB group was removed by treatment with TFA. Unsaturated double bond was reduced to get the corresponding γ-butyrolactam 76 using nickel boride in high yield. Finally hydrolysis of butyrolactam gives γ-aminobutaric acid i.e. (±)-Baclofen (Scheme-10).

Scheme-10: Reagent and conditions: (a) PdCl2(PPh3)2, CsF, Bu4NCl, PhMe/H2O (1:1), rt, 18 h, 68%; (b) DMB-NH2, NaBH(OAc)3, AcOH, CHCl3, rt, 12h, 50%; (c) TFA, o DCE, 80 C , 2h, 75%; (d) Ni2B, MeOH/THF (1:1), rt, 1h, 57%; (e) 6M HCl, reflux, 16h (81%).

Fabricio F. Naciuk (2015)58 Fabricio et al. used Michael addition reaction of 1,3-dicarbonyl compound (Meldrum’s acid) 115 to nitrostyrene 77 effectively promoted by hydrotalcite [Mg–Al] to afford the respective γ-nitroester 79 through a one pot domino process.

NaBH4/NiCl2.6H2O system used to reduce γ-nitroester to give γ-lactam 76. Further hydrolysis of γ- lactam with HCl produced (±)-Baclofen (Scheme-11).

Scheme-11: Reagent and conditions: (a) Hydrotalcite, EtOH, 90 ºC, 24h, 85%; (b)

NaBH4, NiCl2.6H2O, EtOH, 0 ºC, 2h, 89%; (c) 6M HCl, 12h, reflux, 89%.

Claudio Mazzini (1997)59 Mazzini et al. developed the strategy for the synthesis of (R)-Baclofen involved a microbiologically mediated Baeyer Villiger oxidation of the prochiral 3-(4'- chlorobenzyl)-cyclobutanone 116 obtained from 4-chlorostyrene 73 and trichloro acetyl chloride as a key step which led to the optically pure (R)-(‒)-lactone 117 (Scheme-12).

Scheme-12: Reagent and conditions: (a) (i) Cl3CCOCl, Et2O, POCl3, reflux; (ii) Zn, AcOR reflux, 65% ; (b) Culture of C. echinulata, 31%; (c) Me3SiI, EtOH, CH2C12, 0 °C o to rt, 95%; (d) NaN3, DMF, 75 °C, 95%; (e) (i) 2M NaOH, rt; (ii) HCI, 0 C ‒ rt, 95%; (f) (i) H2, 1 atm, Pd/C, Et2O/EtOH, rt; (ii) HCl, 80%.

Milind D. Nikalje (2003)60 M. Nikalje et al. synthesized (R)-(‒)-Baclofen using Ru(II)-(S)-BINAP catalyzed asymmetric hydrogenations of C=C and C=O groups introducing stereogenic centre into the molecule constitute the key steps with 90% ee and 26% yield. P-Chloroacetophenone 121 and ethyl bromoacetate was refluxed in dry benzene containing zinc powder at 80 oC to produce Ethyl 3-(4-chlorophenyl)-2-butenoate 122. Allylic bromination with NBS gives bromoester 123 which was then treated with sodium azide to produce ethyl 4-azido-3-(4-chlorophenyl)-2-butenoat 124. Reaction mixture of unsaturated azidoester, Ru(II)–(S)-BINAP, dry MeOH was degassed with N2 and then pressurized with H2 to 200 psi pressure at 50 °C for 20 h gave (R)-ethyl 4- azido-3-(4-chlorophenyl)-1-butanoate 119. Reduction of azide group and γ-lactam formation was achieved by CoCl2·6H2O, NaBH4 which on acid hydrolysis (R)-Baclofen was produced (Scheme-13).

Scheme-13: Reagent and conditions: (a) BrCH2CO2Et, Zn, benzene, reflux, p-TSA, toluene, reflux, 78%; (b) NBS, AIBN, CCl4, reflux, 10h, 92%; (c) NaN3, EtOH:H2O (80:20), 80 °C, 8h, 78%. (d) Ru(II)–(S)-BINAP, H2 (200 psi), MeOH, 50 °C, 20h; 68%; (e) CoCl2, NaBH4, H2O, 25 °C, 30 min; 80%; (f) 6N HCl, 100 °C, 10h, 76%.

Yongcan Wang (2008)61 Yongcan et al. used the enantioselective Michael addition reaction of nitromethane to 3-(4-chlorophenyl)-acrylaldehyde 126 (α,β-unsaturated aldehydes) under the catalysis of (R)-2-(diphenyl(trimethylsilyloxy)methyl)-pyrrolidine and lithium acetate as additive to synthesize (R)-3-(4-chlorophenyl)-4-nitrobutanal 127 with 62% yield and 91% ee. Oxidation of aldehyde 127 to corresponding γ-nitroacid 128 and further reduction of the nitro group led to formation of amino acid (R)-Baclofen (Figure-15).

Figure-15

Xiao-Fei Yang (2012)62 Yang et al. used the Pd-catalyzed asymmetric allylic alkylation (AAA) reaction of nitromethane with (E)-3-(4-chlorophenyl)allyl methyl carbonate 129 (monosubstituted allyl substrates) as a key step for the synthesis of (R)-1-chloro-4-(1- nitrobut-3-en-2-yl)benzene 130. The alkylated product 130 was treated with 9-BBN, followed by hydroboration oxidation reaction aﬀord the alcohol 132. Dess Martin oxidation of alcohol 132 gives the corresponding aldehyde 133 which was subjected to a Pinnick oxidation to provide the acid 134 in 84% yield, followed by esterification with DCC/MeOH. The nitro group of γ-nitroester 135 was reduced to amine and acid hydrolysis offered (R)-Baclofen (Scheme-14).

S cheme-14: Reagent and conditions: (a) CH3NO2, Pd2(dba)3.CHCl3 (5 mol%), L6 (10 mol%), DABCO (100 mol%), THF, rt; (b) 9-BBN, H2O2, NaOH; (c) DMP, DCM; (d) NaH2PO4.2H2O, NaClO2, t-BuOH; (e) DCC, DMAP, MeOH; (f) i) Raney-Ni, H2; ii) 2M HCl.

M. N. Lokhande, M. D. Nikalje (2015)63 Mahendra used asymmetric Michael addition of diethyl malonate to 1-chloro-4- (2-nitrovinyl) benzene 77 in the presence of scandium triflate and spartiene as organo catalyst to produce adduct 136. The Michael adduct 136 containing nitro group is reduced with NaBH4/NiCl2 and hydrolysis of ester 137 under basic condition followed by decarboxylation under reflux condition gives γ-lactam 125, which on acid hydrolysis produces R-(‒)-Baclofen (Scheme-15).

Scheme-15: Reagent and conditions: (a) Diethyl malonate, Sc(OTf)3, (‒)-Separtiene, Et3N, THF, rt, 4h; (b) NaBH4, NiCl2, MeOH, rt, 7h; (c) i) NaOH, EtOH, rt, 12h; ii) toluene, reflux, 6h; (d) 6N HCl, reflux, 24h.

Kozo Shishido (1998)64 Kozo et al. used Lipase mediated asymmetric acetylation of δ-symmetrical 2- aryl-1,3 propanediols as a key step for the synthesis of (S)-Baclofen. 2-(4- chlorophenyl)propane-1,3-diol 138 was treated with Lipase (PPL) to form R-3-Acetoxy- 2-4-chlorophenyl-1-propanol 139 with a tertiary stereogenic center at the benzylic position. The free hydroxy group was protected as its misyl ether 140, and then converted to corresponding nitrile 141. Acetate group from the nitrile compound 141 was deprotected to alcohol 142 by LiOH and further converted to azide 143 under Mitsunobu reaction conditions. Finally oxidation of nitrile group and reduction of azide gave (S)-Baclofen (Scheme-16).

Scheme-16: Reagent and conditions: (a) Lipase (PPL), Et2O, rt, 92% yield, 99% ee; (b) MsCl, iPr2NEt, 4-DMAP; (c) KCN, 18-Crown-6; (d) LiOH; (e) DEAD, PPH3, (PhO)2PON3, 75%; (f) aq. HCl; (g) H2, PtO2.

Jan Vesely (2008)65 Vasely et al. used a novel organocatalytic highly enantioselective nitrocyclopropanation reaction of α,β-unsaturated aldehydes as a key step for the synthesis of Baclofen. (E)-3-(4-chlorophenyl)acrylaldehyde 126 was treated with bromonitromethane using (S)-2-(diphenyl((trimethylsilyl)oxy)methyl)pyrrolidine as an organo catalyst to form corresponding nitrocyclopropane derivatives 145 and 146. Ring opening was made to give 3-nitromethyl acid ester 147 in 85% yield and 92% ee which was further used to form (S)-Baclofen (Scheme-17).

Scheme-17: Reagent and conditions: (a) BrCH2NO2, Et3N, CHCl3, rt; (b) DIPAE (40 mol %), MeOH, DCM, rt, 15h.

Anna Fryszkowska (2010)66 Anna et al. developed a enantioselective biocatalytic reduction of β-aryl-β- cyano-α,β-unsaturated carboxylic acids from anaerobic bacteria. 2-(4- chlorophenyl)acetonitrile 148 was treated with glyoxalic acid to produce potassium (Z)- 3-(4-chlorophenyl)-3-cyanoacrylate 149. The double bond was reduced enantioselective manner with the help of different anaerobic bacteria obtained from crude extracts to produce potassium (S)-3-(4-chlorophenyl)-3-cyanopropanoate 150. The potassium salt was converted to corresponding acid 151 and methyl ester i.e. (S)-methyl 3-(4- chlorophenyl)-3-cyanopropanoate 152. Nitrile group from the ester 152 was reduced with the help of NaBH4/NiCl2 to form γ–lactam 153 which on acid hydrolysis gives (S)- 4-amino-3-(4-chlorophenyl)butanoic acid hydrochloride i.e. (S)-Balclofen (Scheme-18).

Scheme-18: Reagent and conditions: (a) Glyoxalic acid, K2CO3, MeOH; (b) C. + sporogenes, A. woodii and R. product; (c) H ; (d) Me3SiCH2N2; (e) NaBH4, NiCl2, MeOH; (f) 6N HCl.

Han Yong Bae (2011)67 Bae et al. used highly enantioselective bio mimetic Michael addition reactions of malonic acid half thioesters (MAHTs) to a variety of nitro olefins, affording the optically active γ-amino acid precursors. The Michael addition of MAHT 154 to the (E)-1-chloro-4-(2-nitrovinyl)benzene 77 afford the corresponding Michael adduct 155 in high yield and excellent enantioselecitivity (82% yield and 91% ee). The reduction of the nitro group, followed by intramolecular cyclization affords enantiomerically pure γ- butyro lactam 153 which on acid hydrolysis gives (S)(+)-Baclofen-HCl salt (Scheme- 19).

Scheme-19: Reagent and conditions: (a) catalyst 1a (5mol%), MTBE (0.1 M), 45 oC,

30h; (b) Raney Ni, H2, H3PO4, THF; (c) HCl reflux.

Fuzhong Han (2011)68 Han et al. used efficient Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to ethyl-γ-phthalimidocrotonate by using bis-sulfoxide ligand affords γ- aminobutyric acid (GABA) derivatives with high enantioselectivities (90–96% ee) under mild conditions. γ-phthalimidocrotonate 156 was treated with (4-chlorophenyl)boronic acid 100 using Rh-catalyst to produce the intermediate 157 which on hydrolysis gave (S)-Baclofen (Scheme-20).

Scheme-20: Reagent and conditions: (a) [(R,R)-L1-RhCl]2 (2.5 mol%), CH2Cl2/H2O (10:1), KOH (50 mol%), 40 oC, 1.5h; (b) 6N HCl, reflux, 12h.

1.1.2 Present work 1.1.2.1 Objective In the view of high biological activity associated with the Baclofen (GABA), a number of methods have been reported in the literature. However these methods suffer from many drawbacks such as low overall yield, use of expensive reagents or catalysts. In this context, a more practical approach for the synthesis of Baclofen and analogs is highly desirable. This chapter describes, for the first time, the synthesis of (±)-Baclofen and analogs using Wittig-Claisen rearrangement protocol developed by our group.

1.1.2.2 Preparation of Wittig Salt We have developed Wittig-Claisen rearrangement protocol for the synthesis of different molecules. HCl gas was passed in the mixture of allyl alcohol and paraformaldehyde at 5‒10 oC to obtain corresponding chloroether in high yield. The chloroether was treated with triphenylphosphine in dry toluene to produce corresponding phosphonium chloride salt in high yield (Figure 16). This salt can be synthesized in kilogram scale and cab be preserved for years at room temperature under anhydrous condition without any significant change in chemical reactivity.

Figure-16

The salt was dried at 100 oC under vacuum and used as a source of corresponding allyloxy-methylene-triphenylphosphorane by in situ treatment with tert- butoxide (base). Such an in situ generated Wittig reagent was treated with variety of aromatic and aliphatic aldehydes/ketones to access corresponding allyl-vinyl ethers which were latter rearranged using Claisen rearrangement to get corresponding aldehyde.

1.1.3 Result and discussion The present strategy for the synthesis of (±)-Baclofen starting from commercially available 4-chlorobenzaldehyde is depicted in Scheme-21.

Scheme-21: Reagent and conditions: (a) ClPh3PCH2-O-CH2CH=CH2, t-BuOK, toluene, o o o 0 C, 1h, 90%;(b) toluene, 120 C, 10h, 98 %; (c) NaBH4, MeOH, 0 C, 1h, 96% (d) o o TsCl, pyridine, DMAP, DCM, 0 C, 7h, 93%; (e) NaN3, DMF, 60 C, 10h, 80% ; (f) o O3, H2O2, DCM, 0 C, 10 min, 65%; (g) H2, Pd/C, MeOH, HCl, 84%.

(±)-Baclofen 165 and analogs can be synthesized from the precursor 2-(4- chlorohenyl)pent-4-en-1-ol 161 with functional group transformations and which can be obtained by Wittig-Claisen rearrangement done on starting material 4-chlorobenzaldehyde followed by reduction (Scheme-21). Alcohol 161 can be used for the synthesis of different natural products and biologically active useful compounds (Figure-17).

1-(2-(allyloxy)vinyl)-4-Chlorobenzene(159)

1H and 13C-NMR Spectrum of Compound 159

Powdered and dried under vacuum at 100 oC, allyloxy-methylene-triphenyl phosphonium chloride salt (1.5 eq.) was treated with 4-chlorobenzaldehyde (1 eq.) and t-BuOK (2 eq.) in dry toluene at 0 oC for 1h. After complete consumption of the starting aldehyde, water was added till two distinct layers are formed. Organic layer was separated and aqueous portion was extracted with ethyl acetate (2x20 ml). Combined organic layer was dried over anhydrous sodium sulphate, concentrated under reduced pressure and purified by column chromatography. The product allyl vinyl ether was pale yellow viscous liquid in 90-96 % yield.69

IR spectrum of the ether shows sharp peaks at 1592 and 2921 cm-1 due to alkene function. 1H NMR show multiplet resonating between δ 7.60–7.55 corresponded for two meta substituted aromatic protons and multiplet between 7.31–7.27 attributed to two ortho-substituted aromatic protons with respective to side chain. Doublet resonated at δ 6.25 with coupling constant 7 Hz attributed to one methine proton from internal double bond carbon attached to oxygen atom. A multiplet resonated at δ 6.01 (ddt, J = 17.2, 10.5, 5.3 Hz) attributed to one methyne proton of terminal double bond. Two multiplets resonating at δ 5.41 (ddd, J = 17.2, 3.2, 1.6 Hz) and δ 5.31 (ddd, J = 10.5, 2.8, 1.4 Hz) were attributed to two methylene protons from the terminal double bond. Doublet at δ 5.25 with coupling constant 7.0 Hz attributed to benzylic methylene proton for internal double bond. Doublet of triplet observed at δ 4.47 with coupling constants (J) 5.3 Hz and 1.5 Hz attributed to two protons from allylic bridged methylene. 13C NMR spectrum shows peaks at δ 134.47, 131.05, 129.50 and δ 128.29 were attributed to four aromatic carbons. A peak at δ 146.64 attributed to methine carbon adjacent to the oxygen atom from an internal double bond. Methine carbon from a terminal double bond was observed to be resonated at δ 133.42. Peak at δ 117.76 corresponded to methylene carbon from a terminal double bond. Benzylic methine carbon resonated at δ 104.74. A peak at δ 73.97 attributed to bridged methylene carbon neighbouring to terminal double bond. From these methine, methylene and bridged methylene protons we confirmed the structure of allyl-vinyl ether 159.

2-(4-chlorophenyl)pent-4-enal (160) The isomeric mixture of allyl vinyl ether 159 as such was refluxed in toluene to affect the Claisen rearrangement69 to produce 4-pentenal 160 in enantiomeric form. IR spectrum show strong absorption bands at 2930 and 1722 cm-1 indicating the presence of aldehyde functional group. The peaks appearing at 1642 and 917 cm-1 shows the terminal olefin. In 1H NMR a doublet at δ 9.69 integrating for one proton with coupling constant 1.6 Hz assigned to the proton of aldehyde group. Multiplet resonating between δ 7.40-7.35 was attributed to two aromatic protons meta to side chain, while multiplet at δ 7.17-713 attributed to two aromatic protons ortho to side chian. Broad multiplet at δ 5.71 (ddt, J = 17.1, 10.2, 6.9 Hz) attributed to one methine proton from a terminal double bond and a multiplet resonating between δ 5.01-5.02 integrated for two protons attributed to methylene protons of terminal double bond. Multiplet resonating between δ 3.65-3.60 was attributed to one benzylic methanetriyl proton adjacent to aldehyde functionality.

Two multiplets resonated between δ 2.88 – 2.81 and δ 2.49 were attributed to two protons from the allylic bridged methylene.

1H and 13C-NMR Spectrum of Compound 160

The 13C NMR spectrum showed single at δ 199.6 confirmed the presence of aldehyde group in the molecule. Peaks at δ 134.18, 133.70, 130.19 and 129.26 corresponded to aromatic carbon atoms. A peak at δ 134.42 attributed to methine carbon from a double bond and peak δ 117.60 was corresponded to methylene carbon for terminal olefin. Peak observed at δ 58.01 show benzylic methanetriyl carbon while peak resonating at δ 33.97 was attributed to bridge methylene carbon allylic to terminal olefin.

2-(4-chlorohenyl)pent-4-en-1-ol (161)

1H and 13C-NMR Spectrum of Compound 161

Enantiomeric mixture of aldehyde 160 was reduced to get alcohol 161 using sodiumborohydride69 at 0 oC. In the IR spectrum of alcohol 161, strong signal at 3332 cm-1 and at 1014 indicate the presence of hydroxyl group. Also, two sharp signals at 1646 and 914 cm-1 corresponded to terminal olefin. In 1H NMR spectrum, multiplet resonating between δ 5.76–5.66 integrated for one proton was assigned to the internal methine of olefinic double bond. Two terminal methylene olefinic protons show a multiplet resonated between δ 5.08–4.97. A multiplet resonated between 3.85–3.72 integrating for two protons was assigned to methylene carrying hydroxy group.

One homoallylic methine proton was observed to be resonated as multiplet at δ 2.90. Two allylic protons show two multiplets resonated between δ 2.54–2.45 and between δ 2.42–2.32 corresponding to one proton each. In the 13C NMR spectrum signals appearing at δ 140.49, 132.47, 129.28 and 128.81 were due to aromatic carbons. Peaks resonated at δ 135.84 and 116.76 were corresponded to internal and terminal olefin carbons respectively.

2-(4-chlorophenyl)pent-4-en-1-yl4-methylbenzenesulfonate(162)

1H and 13C-NMR Spectrum of Compound 162

Alcohol 161 was protected as its tosyl ether 162 by using excess of pyridine and tosyl chloride in DCM solvent.70 In the IR spectrum of the tosylate 162, peak at 1448 and 919 cm-1 indicates the presence of double bond. In 1H NMR spectrum, two multiplets resonating between δ 7.65–7.60 attributed to two aromatic protons and multiplet between δ 7.31–7.26 attributed to two aromatic protons from tosyl ring. Again a multiplet resonated between δ 7.23–7.19 was attributed to two aromatic protons meta to side chain and multiplet resonated between δ7.02–6.98 was attributed to two ortho protons from the benzene ring carrying chlorine atom. Two methylene protons of carbon carrying tosyl group were resonated at δ 4.16 and δ 4.09 corresponded to one proton each and appears as doublet of doublet. One benzylic methine proton was resonated at δ 3.00 as a multiplet. Two multiplets resonated at δ 2.47 and δ 2.34 shows two protons from bridged methylene present at allylic position to the terminal double bond. Three methyl protons from toluene ring show singlet resonating at δ 2.46 merged with a multiplet of one proton from bridged methylene carbon bearing tosyl group. In the 13C NMR spectrum, peaks resonated at δ 144.77, 132.84, 129.15 and 127.79 were corresponded to aromatic carbons from the benzene ring carrying chlorine atom. The signals appeared at δ 138.45, 132.67, 129.75 and 128.65 show aromatic carbons from benzene ring of tosyl group. Peaks resonated at δ 134.52 and δ 117.67 corresponded to internal methine and terminal methylene carbons of the terminal double bond respectively. Methylene carbon bearing tosyl group resonated at δ 72.81. Benzylic methine carbon showed peak at δ 44.13. Bridged methylene carbon allylic to olefinic double bond was resonated at δ 35.93. A peak resonated at δ 21.65 show methyl carbon from tosyl group.

1-(1-azidobut-3-en-2-yl)-4-chlorobenzene (163) Tosyl ether was heated with sodium azide at 60 oC to produce corresponding azide 163.70 In the IR spectrum, peak at 2130 cm-1 indicated the presence of azide group. Peaks at 2917, 1491 and 917 cm-1 show presence of double bond. In the 1H NMR spectrum, a multiplet resonated between δ 7.37 – 7.31 was attributed to two aromatic protons at meta position and multiplet between δ 7.20–7.11 was attributed to two aromatic protons ortho to the side chain. Multiplet observed between δ 5.72 – 5.58 show one internal methine proton while multiplet resonated between δ 5.11–4.98 show two terminal methylene protons from an olefinic group.

1H and 13C-NMR Spectrum of Compound 163

A multiplet resonating at δ 3.70 corresponded to two methylene protons attached to azide group. Methine proton from benzylic position show multiplet resonated between δ 3.10–2.96. Internal bridged methylene attached to olefin show two multiplets between δ 2.67–2.56 and between δ 2.50–2.37 corresponded to one proton each. In the 13C NMR spectrum, peaks resonated at δ 139.90, 132.79, 129.19 and 128.69 were due to aromatic carbons. Signals observed at δ 134.97 and 117.53 were contributed to methine and methylene carbons of terminal double bond. Methylene carbon bearing azide group show peak at δ 48.56 and methylene carbon allylic to double bond was resonated at δ 37.42. Peak at δ 47.06 attributed to methanetriyl carbon at benzylic position.

4-azido-3-(4-chlorophenyl)butanoic acid (164)

1H and 13C-NMR Spectrum of Compound 164

Double bond from the azide 163 was transformed to corresponding acid functionality using ozone and hydrogen peroxide to get corresponding azido acid 164.71 In the IR spectrum, broad peak observed at 3373 cm-1 indicated the presence of acid group and peak at 2125 cm-1 show presence of azide group. In the 1H NMR spectrums, there is no any peak in the olefinic region between δ 4.0‒7.0 indicates the absence of terminal double bond. Two multiplets resonated between δ 7.38–7.30 and δ 7.23–7.17 attributed to two aromatic protons each from the benzene ring. Multiplet resonated

50 between δ 3.83–3.73 attributed to two methylene protons attached to acid group. Multiplet resonated at δ 3.69 show methanetriyl proton at benzylic position. Methylene protons on the carbon bearing azide group show two multiplets at δ 3.06 and δ 2.75. The 13C NMR spectrum show peak at δ 175.15 confirmed the presence of acid group in the molecule. Peaks resonated at δ 137.82, 135.53, 129.50 and 128.91 were due to aromatic carbon atoms. Methylene carbon bearing azide group was resonated at δ 48.56 while methylene carbon bearing acid functionality was resonated at δ 37.42. A peak at 47.06 was attributed to benzylic methylene carbon.

4-amino-3-(4-chlorophenyl)butanoic acid hydrochloride (165) Azido acid 164 was hydrogenated70 with 10% Pd/C under 200 psi pressure in presence of hydrogen gas to reduce azide functionality to corresponding amine to gave amino acid, (±)-Baclofen 32 as product. (±)-Baclofen 32 was treated with HCl to get corresponding hydrochloride salt i.e. (±)-Baclofen hydrochloride 165 as final product.

IR spectrum show peak at 3406 cm-1 corresponded to acid group while peak at 3078 cm-1 corresponded to amine group from the molecule. In the 1H NMR spectrums, multiplet at δ 7.31 showing doublet was due to meta substituted proton with coupling constant 8.5 Hz while multiplet at 7.18 corresponded to ortho substituted proton with coupling constant 8.5 Hz showing vicinal protons i.e. ortho to each others from benzene ring. Multiplet resonating between δ 3.80-3.58 corresponded to two methylene protons neighbouring to amine group. Multiplet at 3.50 corresponded to methanetriyl proton.

Two methylene protons adjacent to acid group resonated between δ 3.12-2.93 and δ 2.79-2.62 corresponding to one proton each.

1H and 13C-NMR Spectrum of Compound 165

In the 13C NMR spectrum, single resonating at δ 178.97 was attributed to carboxylic acid carbon confirms the presence of acid functionality. Peak at δ 136.23, 134.43, 129.48 and 128.89 were corresponded to aromatic carbons from benzene ring. Peak at δ 50.68 show methylene carbon carrying amine group, and peak at δ 37.01 corresponded to methanetriyl carbon at benzylic position. A peak at δ 29.72 show bridged methylene carbon bearing acid functionality. The IR, 1H NMR and 13C NMR data of the product confirmed the structure and was in agreement with the reported data.

1.1.4 Experimental section All solvents were distilled before use. All liquid reagents were distilled and stored under anhydrous conditions. Dry Toluene was prepared by distilling it over sodium and stored over sodium wire. Dry DMSO was obtained by refluxing over o anhydrous K2CO3 and was stored over molecular sieves (4 A ). Dry DMF was obtained using molecular sieves (4 Ao). Dry THF was obtained by distilling from dark-blue solution of sodium benzophenone radical anion under nitrogen atmosphere. Dry CH2Cl2 was prepared by distilling over calcium hydride and stored over molecular sieves (4 Ao). All anhydrous reactions were carried out under nitrogen atmosphere. Silica gel (100 - 200 mesh) was used for column chromatography. IR spectra were recorded on Shimadzu FTIR-8400 series instrument. 1H NMR spectra [ppm, TMS as an internal 13 standard in CDCl3] were recorded on VARIAN Mercury 400 MHz instrument. C NMR spectra were recorded on VARIAN Mercury 100 MHz instrument. Low- resolution mass spectra were recorded on Shimadzu GCMS - QP 5050A series instrument in Chemical Ionization mode and the fragmentation pattern is given after the corresponding M+ value. The elemental analyses were obtained on a HOSLI semiautomatic C, H analyzer.

1-(2-(allyloxy)vinyl)-4-chlorobenzene (159) To a suspension of 4-chlorobenzaldehyde (5 g, 35.56 mmol) and allyloxy methylene triphenylphosphoniumchloride (19.68 g, 53.35 mmol) in dry toluene (60 mL), potassium tert-butoxide (4.2 g, 53.35 mmol) was added portion wise over the period of 10 min. at 0 oC. The reaction mixture was stirred for 1h at the room temperature. After the completion of reaction (TLC check, 10% ethyl acetate:pet ether), toluene was removed under reduced pressure and the crude product was extracted with ethyl acetate (3x 25 mL). The combine organic layer was washed with water, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography using ethyl acetate:pet ether (2:98) as a mobile hase, gave pure allyl vinyl ether 159 as coloueless thick liquid. The product in hand was the mixture of E and Z isomers (12.5 g, 89.2%).

IR (Neat, cm-1): 2921, 1700, 1592, 1162, 1084, 820. 1 H NMR (400 MHz, CDCl3): δ 7.60 – 7.55 (m, 2H, Ar-H), 7.31 – 7.27 (m, 2H, Ar-H), 6.25 (d, J =7 Hz,

1H), 6.01 (ddt, J = 17.2, 10.5, 5.3 Hz, 1H), 5.41 (ddd, J = 17.2, 3.2, 1.6 Hz, 1H), 5.31 (ddd, J = 10.5, 2.8, 1.4 Hz, 1H), 5.25 (d, J= 7.0 Hz, 1H), 4.47 (dt, J = 5.3, 1.5 Hz, 2H); 13 C NMR (100 MHz, CDCl3): δ 146.64, 134.47, 133.42, 131.05, 129.50, 128.29, 117.83, 104.84, 73.97.

2-(4-chlorophenyl)pent-4-enal (160) The isomeric mixture of allyl vinyl ether 7 (12.5 g) was dissolved in toluene (30ml) and solution was refluxed at 120 oC for 10h. After the completion of reaction (TLC check, 20% ethyl acetate:pet ether), toluene was removed under reduced pressure. The crude product aldehyde 160 obtained as viscous liquid was used as such for the further reaction. The product in hand was the mixture of two inseparable mixtures of enantiomers (12.4g, 99% yield).

IR (Neat, cm-1):2930, 1722, 1642, 1091, 917, 822. 1 H NMR (400 MHz, CDCl3) δ 9.69 (d, J = 1.6 Hz, 1H), 7.40 – 7.35 (m, 2H), 7.17 – 7.13 (m, 2H), 5.71 (ddt, J = 17.1, 10.2, 6.9 Hz, 1H), 5.09 – 5.02 (m, 2H), 3.65 – 3.60 (m, 1H), 2.88 – 2.81 13 (m, 1H), 2.49 (dddt, J = 14.9, 8.1, 6.9, 1.3 Hz, 1H); C NMR (100 MHz, CDCl3): δ 199.60, 134.42, 134.18, 133.70, 130.20, 129.26, 117.60, 58.01, 33.97.

2-(4-chlorohenyl)pent-4-en-1-ol (161) To the solution of aldehyde 160 (5 g, 25.64 mmol) in 5% aqueous methanol (50 ml), sodium borohydride (0.96 g, 25.64 mmol) was added portion wise over a period of 10 min at 0 oC. The reaction mixture was stirred at 0 oC for 30 min. After completion of reaction (TLC check, 30% ethyl acetate:pet ether) the methanol was removed under reduced pressure and the crude residue was partitioned between ethyl acetate and water. Layers were separated and aqueous layer was extracted with ethyl acetate (3x10mL). Combined organic layer was washed with water, dried over anhydrous sodium bisulphate and concentrated under reduced pressure. The crude product was purified by column chromatography using ethyl acetate/pet ether (5:95) as an eluent to give the alcohol 161 (4.8 g, 96% yield).

IR (Neat, cm-1): 3332, 2924, 1641, 1490, 1014, 914, 823 cm-1 1 H NMR (400 MHz, CDCl3) δ 7.34 – 7.31 (m, 2H), 7.20 – 7.16

(m, 2H), 5.76 – 5.66 (m, 1H), 5.08 – 4.97 (m, 2H), 3.85 – 3.72 (m, 2H), 2.90 (dt, J = 13.9, 7.1 Hz, 1H), 2.54 – 2.45 (m, 1H), 2.42 – 2.32 (m, 1H); 13C NMR (100 MHz,

CDCl3): δ 140.49, 135.84, 132.47, 129.28, 128.81, 116.76, 66.68, 47.63, 36.60.

2-(4-chlorophenyl)pent-4-en-1-yl 4-methylbenzenesulfonate(162) To a solution of alcohol 161(9 g, 45.91 mmol) in dry DCM at 0 oC under a nitrogen atmosphere was added excess of pyridine (18 g) and catalytic amount of DMAP. After stirring for 15 min, p-toluenesulfonyl chloride (13.15 g, 68.87 mmol) was added portion wise, and the reaction mixture was stirred for 8h. After completion of reaction (TLC check, 20% ethyl acetate:pet ether), solvent was removed under reduced pressure. Water was added in the crude product and extracted with ethyl acetate (3x10 ml). Combined organic layer was dried over sodium sulphate and concentrated under reduced pressure. Further purification was done using column chromatography (hexane/EtOAc, 96:4) to give tosylate compound 162 as a white solid (15 g, 93%).

IR (Neat, cm-1): 2978, 1448, 1175, 919, 814. 1 H NMR (400 MHz, CDCl3): δ 7.65 – 7.60 (m, 2H), 7.31 – 7.26 (m, 2H), 7.23 – 7.19 (m, 2H), 7.02 – 6.98 (m, 2H), 5.59 (ddt, J = 17.0, 10.3, 7.0 Hz, 1H), 5.03 – 4.95 (m, 1H), 4.16 (dd, J = 9.8, 6.0 Hz, 1H), 4.09 (dd, J = 9.8, 7.0 Hz, 1H), 3.00 (dt, J = 13.2, 6.5 13 Hz, 1H), 2.47 (m, 1H), 2.46 (s, 3H), 2.34 (m, 1H); C NMR (100 MHz, CDCl3): δ 144.77, 138.45, 134.52, 132.84, 132.67, 129.75, 129.15, 128.65, 127.79, 117.67, 72.81, 44.13, 35.95, 21.65.

1-(1-azidobut-3-en-2-yl)-4-chlorobenzene (163)

To a stirred solution of tosylate 162 (8 g) in dry DMF, was added NaN3 (4 g) and reaction mixture was stirred at 60 oC for 10h. After completion of reaction (TLC check), reaction mixture was cooled to room temperature, water was added and extraction was done with ethyl acetate. The combined organic layer was washed with brine solution and dried over anhydrous Na2SO4. Removal of the solvent under reduced pressure, crude product was obtained which was purified by column chromatography (hexane/ethyl acetate, 97:3) to give azide 163 as a yellow viscous liquid (4g, 80%).

IR(Neat, cm-1): 2917, 2130, 1491, 1092, 917, 823. 1 H NMR (400 MHz, CDCl3): δ 7.37 – 7.31 (m, 2H), 7.20 – 7.11 (m, 2H), 5.72 – 5.58 (m, 1H), 5.11 – 4.98 (m, 2H), 3.70 (qd, J = 10.9, 6.6 Hz, 2H), 3.10 – 2.96 (m, 1H), 2.67 13 – 2.56 (m, 1H), 2.50 – 2.37 (m, 1H); C NMR (100 MHz, CDCl3): δ 139.90, 134.97, 132.79, 129.19, 128.69, 117.53, 48.56, 47.06, 37.42.

4-azido-3-(4-chlorophenyl)butanoic acid (164) To a stirred solution of azide 163 (1g) in dry DCM, ozone gas was passed at -78 oC for 15 minutes till complete conversion of the reactant into corresponding ozonide. After complete conversion of reactant into ozonide (TLC check), hydrogen peroxide (2 eq.) was added in the reaction mixture to convert ozonide in to corresponding acid. DCM was removed under reduced pressure and reaction mixture was extracted with ethyl acetate (2x10 ml). The crude product obtained was purified with base/acid treatment followed by column chromatography (hexane/ethyl acetate, 80:20) to give azido acid 164 as a yellow viscous liquid (0.70g, 65%). IR (Neat, cm-1): 3373, 2125, 1445, 967, 910. 1 H NMR (400 MHz, CDCl3): δ 7.38 – 7.30 (m, 2H), 7.23 – 7.17 (m, 2H), 3.83 – 3.73 13 (m, 2H), 3.69 (m, 1H), 3.06 (m, 1H), 2.75 (m, 1H); C NMR (100 MHz, CDCl3): δ 175.15, 137.82, 135.53, 129.50, 128.91, 49.45, 40.27, and 5.96.

4-amino-3-(4-chlorophenyl)butanoic acid hydrochloride (165) A mixture of azido acid 164 (200 mg) in MeOH (15 ml) and 10 % Pd/C (10 mg) was hydrogenated at 200 psi at room temperature for 10 h. The reaction mixture filtered off in vacuum through cilite. The filtrate was evaporated under reduced pressure to give (±)-Baclofen 32 as colourless solid (0.149g, 84%). The HCl gas was passed through the solution of residue dissolved in DCM/MeOH for 15-30 minutes. The precipitate thus formed was collected by filtration to obtain salt of (±)-Baclofen hydrochloride 165. IR (Neat, cm-1): 3406, 3078, 1705, 1090, 913 1 H NMR (400 MHz, D2O): δ 7.31 (d, J = 8.5 Hz, 2 H), 7.18 (d, J = 8.5 Hz, 2 H), 3.80-3.58 (m, 2H), 3.50 (dt, J = 12.9, 6.3 Hz, 1H), 3.12-2.93 (m, 1H), 2.79-2.62 (m, 1H); 13 C NMR (100 MHz, D2O): δ 178.97, 136.23, 134.43, 129.48, 128.89, 50.68, 37.01, 29.72.

Section II: Synthesis of (±)-Homobaclofen 1.2 Introduction

Baclofen 32 is a potent and selective agonist for bicuculline insensitive GABAB receptors and is used clinically as an antispastic and muscle relaxant agent. In the search for new bioactive chemical entities that bind specifically to GABAB receptors, number of Baclofen homologues have been synthesized and tested for pharmacological activity. Among the analogs, (R,S)-5-amino-3-(4-chlorophenyl)pentanoic 166 and (R,S)-5- amino-4-(4-chlorophenyl) pentanoic acid 167, show activity as like Baclofen and well known as Homobaclofen. Compound 166 is an agonist to GABAB receptors with an EC50 value of 46 μM on tsA201 cells transfected with GABAB1b/ GABAB2/Gqz5 being the most active congener among all the synthesized compounds.72

Figure-18: Homobaclofen

(R)-Homobaclofen 47, the homologue of baclofen (32) has been shown to exhibit a quite remarkable functional pharmacological profile in guinea pig ileum as compared to that of baclofen. Compound 48 (EC50 = 310 ± 16 µM) was shown to be one half as potent as 47. In GABA receptor selective [3H]GABA binding to rat brain synaptic membranes, R-homobaclofen 47 was 50-fold less potent than R -baclofen, whereas S-enantiomer 48 was inactive. Homobaclofen 167 did not show detectable affinity for GABAA or GABAB receptor sites and was inactive as an agonist or an 73 antagonist at GABAB receptors in the guinea pig ileum. When tested in an assay of electrically induced contractions of guinea pig ileum, R-homobaclofen was still less

57 potent than R-Baclofen (13-fold) but interestingly, S-homobaclofen was shown to be almost equipotent with the R-enantiomer. R-homobaclofen could be antagonized by CGP35348 and S-homobaclofen displayed no effect on the GABA 1b receptor. Recent study using [3H]GABA binding to rat brain synaptic membranes and electrically induced contractions of guinea pig ileum R-homobaclofen was approximately 20-fold less potent than R-Baclofen. When the Ser130 residue in the GABA 1b receptor was mutated to an alanine (mutant S130A), the abilities of GABA, R-Baclofen and R-homobaclofen to activate the receptor were dramatically impaired. The concentration–response curve for GABA on the mutant started at concentrations above 1 mM, whereas no rsponse was detected for either R-baclofen or R-homobaclofen in concentrations up to 3 mM. Mimicking the insignificant effect of the S153A mutation on the pharmacology of R-baclofen, R-homobaclofen retained its potencies at the wild type receptor on the mutant.74 Homobaclofen can provide important information for the structure of the substance having a stimulating and blocking activity which may be used to design new, more potent compounds with the same properties.

1.2.1 Review of literature Literature search revealed that there are few reports available on the synthesis of (±)-Homobaclofen along with R and S form are described below. Rolf Karla (1999)75 Rolf Karla et al. synthesized Homobaclofen 175 starting from (±)-Baclofen as shown in Scheme 22. Acid functionality was transformed into ester 170, amine group was protected as its Boc-derivative 171. Ester functionality was transformed to corresponding alcohol 172 and nitrile 173. Further cyclization was done using con. HCl to produce δ-lactam 174. Hydrolysis of lactam 174 gave salt of Homobaclofen, 175 (scheme 22). Rolf also synthesized (R)-Homobaclofenhydrochloried 180 and (S)- Homobaclofenhydrochloried 181 from the starting material 176 and 177 respectively (Scheme-23).

Scheme-22: Reagent and conditions: (a) MeOH, HCl; (b) (Boc)2O, Et3N; (c) LiBEt3H, THF; (d) Ph3P, CCl4; (e) NaCN, DMF; (f) MeOH, HCl, NaOH; (g) 6M HCl.

Scheme-23: Reagent and conditions: (a) phenyl chlorothionoformate, DCM, DMAP;

(b) SnBu3H, acetone, di-tert-butyl peroxyoxalate; (c) 1 M LiOH, THF, 0.5 M HCl; (d) 6 M HCl.

Marcos Jose S. (2002)76 Marcos et al. synthesized Homobaclofen 175 by Heck arylation of N-acyl- tetrahydro pyridine with aryldiazonium tetrafluoroborates under phosphine free conditions as key step. Heck arylation of tetrahydropyridine 182 with p-Cl-benzenediazonium tetrafluoroborate 183 was performed to produce lactamol 184 and 185, which are oxidized after workup with tetrapropylammonium perruthenate (TPAP) to provide a mixture of lactams 187 and 188 together with a minor amount of the olfefinic compound 186 in an overall yield of 66% over the two steps. Compounds 187, 188 and 186 were obtained in a ratio of 50:36:13 after flash chromatography. Lactams 187 and 188 are known compounds and both have been used as precursors to the synthesis of isomeric homobaclofens.

Scheme-24: Reagent and conditions: (a) 2 mol% Pd(OAc)2, CH 3CN/ H2O (2:1 ratio, o v/v); (b) TPAP, NMO, CH2Cl2,4A MS (66% over two steps); (c) 6 M HCl, reflux, 18h.

Meng-Yang Chang (2003)77 Chang et al synthesized Homobaclofen by using Michel reaction between methyl 2-(4-chlorophenyl)acrylate 190 and N-benzyl-2-tosylacetamide 191 to form the pyroglutamate adduct 192 which was then transformed into compound 193 and then δ- lactam 174. Lactam 174 on acid hydrolysis produced Homobaclofen 175.

Scheme-25: Reagent and conditions: (a) NaH, THF; (b) i) NaH, ii) BF3(OEt)2; (c) i) Na(Hg), ii) H2, Pd(OH)2

Fabio Simonelli (2004)78 Fabio used the oxazoline cyanocuprates addition to conjugated cyanoalkenes at - 78 oC in THF for the synthesis of a varietyof δ-cyanooxazolines which can be used for the synthesis of different target compounds. (E)-3-(4-chlorophenyl)acrylonitrile 194 was added into the reaction mixture containing in situ generated oxazoline cyanocuprate complex 195 to form the γ-cyno Oxazoline 196 which on treatment with Sn/HCl, produces Homobaclofen 189 (Figure 19).

Figure-19

Zhi-Tao He (2012)79 Zhi-Tao et al. used Rhodium-catalyzed asymmetric arylation (RCAA) reactions as a key step for the synthesis of optically pure 4-aryl-2-piperidinones which can be used for the synthesis of pharmacologically active molecules. 1-N-Boc-2-oxo-5,6- dihydropyridine 197 was treated with (4-chlorophenyl)boronic acid to form the Boc- protected piperidinone 198.

(R)-tert-butyl 4-(4-chlorophenyl)-2-oxopiperidine-1-carboxylate 198 was base hydrolysized to N-Boc protected amino acid 199, through which HCl gas was passed to produce (R)-Homobaclofen hydrochloride 180.

Scheme-26: Reagent and conditions: (a) (4-chlorophenyl)boronic acid, [Rh(L1)OH]2 (5.0 mol %), KHF2 (20 mol %), toluene/isopropanol (40:1), rt; (b) i) NaOH, MeOH/H2O, ii) 2M HCl, (c) HCl g, dioxane.

Mohamed I. Attia (2013)80 Mohamed et al. used Horner-Wadsworth-Emmons (HWE) reaction for the synthesis of Homobaclofen as a key step. 2-chloro-1-(4-chlorophenyl)ethanone 200 was treated with ethyl 2-(diethoxyphosphoryl)acetate to form mixture of (E) and (Z) isomers 201A and 201B. The major isomer (Z)-ethyl 3-(4-chlorophenyl)-4-cyanobut-2- enoate 201A was subjected to a nucleophilic displacement of the halogen with potassium cyanide to obtain 3-aryl-4-cyano-2-butenoic acid ethyl ester 202, which on catalytic hydrogenation using a catalytic PtO2 or 10% Pd/C and concentrated hydrochloric acid in 95% gave (R,S)-5-amino-3-aryl-pentanoic acid ethyl ester hydrochloride 203. Acid hydrolysis of compound 203 leads to Homobaclofen 189.

Scheme-27: Reagent and conditions: (a) (EtO)2POCH2COOEt, NaH, dry 1,2- dimethoxyethane, 50 °C, 18 h; (b) (C2H5)4N CN/CH3CN, 50 °C, 18 h; (c) H2, Pd/C or PtO2, 4 bar, 95%, C2H5OH, conc. HCl , 25 °C, 18 h; (d) 5N HCl, reflux, 4h.

1.2.2 Present work 1.2.2.1 Objective Homobaclofen acts as agonists of GABA and Baclofen. Homobaclofen can also provide important information for the structure of the substance having a stimulating and blocking activity which may be used to design new, more potent compounds with the same properties. In the view of biological activity associated with the Homobaclofen (GABA), very few methods have been reported in the literature. However these methods suffer from many drawbacks such as low overall yield, use of expensive reagents or catalysts. In this content, a more practical approach for the synthesis of Homobaclofen is desirable. This part of chapter describes, for the first time, the synthesis of (±)-Homobaclofen using Wittig-Claisen rearrangement protocol developed by our group.

1.2.3 Result and discussion The present strategy for the synthesis of (±)-Homobaclofen hydrochloride 189 and 175 starting from precursor 161, which was synthesized from commercially available 4-chlorobenzaldehyde by Wittig-Claisen rearrangement followed by reduction is depicted in Scheme 28.

Scheme 28. Reagent and conditions: (a) NaCN, NaI, Me3SiCl, CH3CN:DMF (1:1), 60 o o o C, 3h, 80%; (b) LiAlH4, THF, 0 C, 2h, 85%; (c) O3, H2O2, DCM, -10 C, 1h, 74%; o (d) TPP, Phthalimide, DEAD, 0 C, 3h, 85%; (e) Borane complex, H2O2, NaOH, THF, o 0 C, 2h, 78%; (f) KMnO4, NaOH, acetone, 3h, 69%; (g) NH2NH2, EtOH, rt, 30 min, 90%.

3-(4-chlorophenyl)hex-5-enenitrile (204) 4-Pentene-1-ol 161 was heated with sodium cynide in presence of sodium iodide and trimethyl silyl chloride to form 5-enenitrile 204. This is a single step reaction in which an alcohol can be converted to corresponding nitrile in presence of catalytic sodium iodide.81 In the IR spectrum of 204, absorption band at 2235 cm-1 corresponded to nitrile functionality. Peaks at 1652, 1466, 970 and 815 cm-1 indicate the presence of benzene ring and olefinic double bond. In the 1H NMR spectrum, multiplet resonated between δ 7.33-7.29 and a multiplet at δ7.16 integrating together for four aromatic protons. A multiplet at δ 5.67 integrating for one proton was attributed to the internal proton of the terminal double bond. The terminal methylene protons of olefin showed a multiplet resonated between δ 5.12-4.95. A multiplet observed between δ 4.39-4.26 integrating for two protons were attributed to the methylene protons attached to the carbon bearing nitrile group. Methanetriyl proton shows multiplet between δ 3.12-2.99. A multiplet between δ 2.56-2.45 and δ 2.45-2.33 integrating for two protons were attributed to the bridged methylene protons allylic to terminal double bond.

1H NMR spectrum of the compound 204

In the 13C NMR spectrum, a signal at δ 139.51 was assigned to carbon from nitrile group. Peaks at δ 134.97 and δ 117.39 were assigned to two carbons, methine and methylene carbon from a terminal double bond. Aromatic carbons from the benzene ring resonated at δ 132.71, 129.75, 129.21 and 128.72. A benzylic methine carbon showed peak at δ 43.90 methylene carbon attached to nitrile group resonated at δ 35.74

66 while allylic methylene carbon attached to olefinic terminal double bond showed peak at δ 66.75. This data confirmed the structure of nitrile 204.

13C NMR spectrum of the compound 204

3-(4-chlorophenyl)hex-5-en-1-amine (205)

1H NMR spectrum of the compound 205

A nitrile group from the compound 204 was reduced to corresponding amine 205 using 82 LiAlH4 keeping terminal double bond intact. In the IR spectrum of amine 205, a peak between 3270-3335 cm-1 showed the presence of amine functionality.

13C NMR spectrum of the compound 205

In the 1H NMR spectrum multiplet between δ 3.82-3.64 integrating for two protons attributed to the methylene bearing amine functionality. A methine proton was resonated between δ 2.79-2.72. Two methylene protons allylic to terminal double bond showed two multiplets close to each other between δ 2.55-2.42 and δ 2.41-2.29. A multiplet between δ 2.15-1.93 integrating for two protons attributed to the bridged methylene protons neighbouring to benzylic methine carbon and methylene carbon carrying amine group. In the 13C NMR spectrum, there is no peak corresponding to nitrile carbon but presence of one extra peak in up field region shows the difference from starting material. Signals at δ 140.54, 132.40, 129.41 and 128.72 corresponds to aromatic carbons from the benzene ring. Peaks at δ 135.93 and 116.73 showed internal and terminal carbons of the olefinic terminal double bond. Allylic carbon resonated at δ 66.50 methine carbon showed signal at δ 47.51. Signals at δ 37.47 and 28.65 corresponded to methylene carbons α and β to amine group.

5-amino-3-(4-chlorophenyl)pentanoic acid hydrochloride (189) Amine 205 was converted in to Homobaclofen hydrochloride 189 using oxidative 71 ozonolysis using O3 and H2O2. In the IR spectrum a broad peak between 3200-2725 and peak at 1727 cm-1 showed presence of acid functionality.

1H NMR spectrum of the compound 189

In the 1H NMR spectrum, aromatic protons showed two multiplets between δ 7.21-7.15 and 7.14-7.06 attributed to two protons each. Signal at 3.67-3.52 as multiplet corresponded to methylene protons attached to acid functionality. Methine proton was resonated between δ 3.15-3.10. Two methylene protons adjacent to amine functionality showed two close multiplets between δ 2.80-2.71 and 2.65-2.51. A multiplet between δ 3.32-2.20 attributed to two methylene protons adjacent to methine carbon.

13C NMR spectrum of the compound 189

In the 13C NMR spectrum, peak resonated at δ 175.89 was attributed to carbon from the acid functionality. This showed the presence of acid group in the molecule. Peaks at δ 140.21, 133.90, 129.58 and 128.22 attributed to aromatic carbons from a benzene ring. Methylene carbon attached to acid functionality resonated at δ 69.10. Methine carbon was resonated at δ 52.09. Peaks resonated at δ 39.20 and δ 33.91corresponds to methylene α-carbon and β-carbon to amine functionality.

2-(2-(4-chlorophenyl)pent-4-en-1-yl)isoindoline-1,3-dione (206) For the synthesis of 5-amino-4-(4-chlorophenyl)pentanoic acid 167, alcohol 161 was subjected to phthalimide protection under mitsunobu reaction83 conditions to get the corresponding phthalimide 206. IR spectrum shows peaks at 2962, 1630, 1465, 936 and 823 cm-1 corresponding to aromatic and olefinic double bond. The 1H NMR spectrum show two multiplets between δ 7.82-7.65 and δ 7.25-7.12 attributed to total eight aromatic protons, four from phthalimide ring and four from chloro benzene ring respectively. Two multiplets resonated at δ 5.65 and 5.06-4.89 attributed to three protons from the terminal double bond. A multiplet at δ 3.88 attributed to two methylene protons attached to phthalimide group. Methine proton was resonated between δ 3.40-3.27 as multiplet. Bridged allylic methylene protons showed multiplet resonated between δ 2.51-2.37.

1H NMR spectrum of the compound 206

13C NMR spectrum of the compound 206

In the 13C NMR spectrum, a signal resonating at δ 168.19 was attributed to carbonyl carbons of phthalimide group. Peaks at δ 139.65, 133.91, 131.82, 129.10, 128.58 and 123.23 correspond to aromatic carbons from phthalimide and chloro benzene ring. Carbons from terminal double bond showed peaks at δ 135.25 and δ 116.97 corresponded to internal and terminal carbons. Methylene carbons attached to phthalimide and terminal olefin showed peaks at δ 43.32 and δ 43.12 respectively. A peak at δ 38.06 corresponded to benzylic methine carbon.

2-(2-(4-chlorophenyl)-5-hydroxypentyl)isoindoline-1,3-dione (207) Phthalimide 206 was subjected to hydroboration oxidation reaction84 to offer the alcohol 207. In the IR spectrum peak at 3345 cm-1 and 1022 cm-1 showed the presence of hydroxy functionality. Peaks at 2958, 1635, 1483, 925 cm-1 and 819 cm-1 attributed to aromatic region. In the 1H NMR spectrum, multiplet resonated between δ 7.92-7.62 attributed to four protons from phthalimide ring and multiplet observed between δ 7.40- 7.05 corresponded to four protons from chloro benzene ring. Four methylene protons from two methylene carbons attached to phthalimide and hydroxy functionality showed multiplet between δ 4.12-3.93. Methine proton showed multiplet between δ 3.55-3.35. Peaks between δ 1.90-1.63 and δ 1.62-1.39 attributed to four methylene protons from two bridged methylene carbons attached to methine in the side chain carrying alcohol functionality respectively.

1H NMR spectrum of the compound 207

13C NMR spectrum of the compound 207

In the 13C NMR spectrum, a signal resonating at δ 168.19 was attributed to carbonyl carbons of phthalimide group. Peaks resonating at δ 139.93, 133.95, 132.10, 129.08, 128.77 and 122.96 were attributed to aromatic carbons from phthalimide ring and chloro benzene ring. Carbon atom carrying hydroxy group was resonates at δ 61.22 while that of carrying phthalimide group was resonated at δ 51.65. Methine carbon allylic to chloro benzene ring showed peak at δ 41.94. Two methylene carbons from a side chain carrying hydroxy group attached to allylic methine carbon were resonated at δ 29.49 and 28.87 respectively.

4-(4-chlorophenyl)-5-(1,3-dioxoisoindolin-2-yl)pentanoic acid (208)

1H NMR spectrum of the compound 208

13C NMR spectrum of the compound 208

85 Alcohol 207 was oxidized to corresponding acid 208 using KMnO4 and NaOH. IR spectrum shows broad peak at 3385 for carboxylic acid. In the 1H NMR spectrum, multiplet between δ 7.98-7.68 showed four protons from phthalimide ring and multiplet between δ 7.33-7.13 attributed to four protons from chloro benzene ring. A multiplet between δ 4.01-3.55 attributed to four protons from the methylene carbons attached to carboxylic acid and phthalimide ring. Methine proton was resonated between δ 3.42-

3.25. Two methylene protons from bridged methylene carbon attached to methine carbon in the side chain of acid showed two multiplets resonated between δ 2.69-2.50 and 2.44-2.25 corresponded to one proton each. In the 13C NMR spectrum, peak at δ 180.95 showed carboxylic acid carbon and peak at δ172.13 showed carbonyl carbons of phthalimide group. Aromatic carbons from phthalimide and chloro benzene ring were resonated at δ 148.98, 134.92, 132.51, 129.86, 128.89 and 123.03. Signal at δ 52.02 attributed to methylene carbon bearing phthalimide ring. A peak at δ 40.95 showed methine carbon. Methylene carbon carrying acid functionality resonated at δ 29.03 and peak at δ 25.92 showed β-carbon to acid functionality.

5-amino-4-(4-chlorophenyl)pentanoic acid (175) To synthesise homobaclofen 175, phthalimide 208 was deprotected with hydrazinhydrate and treated with acid to form hydrochloride salt of homobaclofen.86,87 In the IR spectrum, peaks at 3415 and 3129 cm-1 showed presence of acid and amine functionality.

1H NMR spectrum of the compound 175

In the 1H NMR spectrum, a peak at δ 8.87 showed proton from acid functionality. Multiplet resonated between δ 7.40-7.25 attributed to two aromatic protons meta to side chain and multiplet between δ 7.25-7.710 attributed to two aromatic protons ortho to side chain. Multiplet between δ 4.05-3.90 attributed to four methylene protons present on methylene carbons carrying acid and amine functionality. Methine proton was

74 resonated as multiplet between δ 3.20-3.05. Two bridged methylene protons resonated as multiplet between δ 2.62-2.51 and δ 2.50-2.41.

13C NMR spectrum of the compound 175

In the 13C NMR spectrum, peak at δ 174.62 attributed to carbon atom from acid functionality. Peaks resonating at δ 136.01, 132.23, 129.22 and 128.53 corresponded to aromatic carbons. Signal at δ 47.55 was attributed to methylene carbon bearing amine group, and peak at δ 42.19 was attributed to methylene carbon carrying acid functionality. Benzylic methine carbon was resonated at δ33.32 and bridged methylene carbon was resonated at δ 28.94.

1.2.4. Experimental Section

3-(4-chlorophenyl)hex-5-enenitrile (204) Alcohol 161 (0.5 g, 2.55 mmol), NaCN (0.250 g, 5.10 mmol), NaI (5-7 mg),

CH3CN (10 ml), and DMF (10 ml) is deaerated, and under an argon atmosphere,

Me3SiCl (0.554 g, 5.10 mmol) is added at room temperature. The reaction mixture was then heated in the oil bath at 60 oC for 10h. After completion of reaction (TLC check), water (30 ml) was added in the reaction mixture and extracted with ethyl acetate (3x 10 ml). The combined organic layer was dried over sodium sulphate and concentrated under reduced pressure. The crude product obtained was then purified by column chromatography using ethyl acetate:pet ether (3:97) as solvent system (0.4 g, 80%).

IR (Neat, cm-1): 2235, 1652, 1466, 970, 815. 1 H NMR (400 MHz, CDCl3): δ 7.33 – 7.29 (m, 2H), 7.16 (dq, J = 8.3. 2.3Hz, 2H), 5.77 (ddt, J = 17.1, 10.1, 7.0 Hz, 1H), 5.12 – 4.95 (m, 2H), 4.39 –4.26 (m, 2H), 3.12-2.99 (m, 13 1H), 2.56 – 2.45 (m, 1H), 2.45 – 2.33 (m, 1H); C NMR (100 MHz, CDCl3): δ 139.51, 134.97, 132.71, 129.75, 129.21, 128.72, 117.39, 66.75, 43.90, 35.74

3-(4-chlorophenyl)hex-5-en-1-amine (205) Nitrile 204 (0.5 g, 2.439 mmol) dissolved in THF was added drop wise to a o stirred suspension of LiAlH4 (92 mg, 1 equiv.) in THF at 0 C over a period of 30 minutes under the atmosphere of nitrogen. The reaction mixture was stirred further for 2 hours. After the completion of reaction (TLC check), ethyl acetate (10 ml) was added drop wise followed by water (5ml). The reaction mixture was filtered through cilite bed and washed with ethyl acetate (20 ml). The combined reaction mixture was dried over sodium sulphate and concentrated under reduced pressure to give a sticky product 205 in good yield. Further purification was done by column chromatography (ethyl acetate:pet ether, 10:90)(0.45g, 85%).

IR (Neat, cm-1): 3270_3335, 1448, 945, 845. 1 H NMR (400 MHz, CDCl3): δ 7.35-7.28 (m, 2H), 7.19- 7.15 (m, 2H), 5.78-5.59 (m, 1H), 5.10-4.93 (m, 2H), 3,82- 3.64 (m, 2H), 2.92-2.79 (m, 1H), 2.55-2.42 (m, 1H), 2.41-

13 2.29 (m, 1H), 2.15-1.93 (m, 2H); C NMR (100 MHz, CDCl3): δ 140.54, 135.93, 132.40, 129.41, 128.72, 116.73, 66.50, 47.51, 37.47, 28.65.

5-amino-3-(4-chlorophenyl)pentanoic acid hydrochloride (189) Ozone gas was passed through a stirred solution of amine 205 (0.25g) in dry DCM at -75 oC still the completion of reaction (TLC check). After the completion of reaction, 30% solution of H2O2 was added and reaction mixture was stirred for 20 minutes. Then organic layer was collected separately and water layer was extracted with DCM (2x10 ml). The combined organic layer was dried under reduced pressure to gate amino acid which was further refluxed with aqueous solution of hydrochloric acid for 3h. The solvent from the reaction mixture was removed on rotary evaporator under reduced pressure to got corresponding hydrochloride salt of homobaclofen 189 (0.2g, 74%).

IR (Neat, cm-1): 3200-2725, 1727, 925, 870 1 H NMR (400 MHz, D2O): δ 7.21-7.15 (m, 2H), 7.14-7.06 (m, 2H), 3.65-3.52 (m, 2H), 3.15-3.10 (m, 1H), 2.80-2.71 (m, 1H), 2.65-2.51 (m, 1H), 2.32-2.20 (m, 2H); 13C NMR

(100 MHz, D2O): δ 175.89, 140.21, 133.90, 129.58, 128.22, 69.10, 52.09, 39.20, 33.91.

2-(2-(4-chlorophenyl)pent-4-en-1-yl)isoindoline-1,3-dione (206) To a stirred solution of alcohol 161 ( 1 g, 5.08 mmol), triphenyl phosphine ( 2.66 g, 10.16 mmol) and phthalimide (1.5 g, 10.16 mmol) in dry THF (25 mL) at 0 °C was added diethyl azo-dicarboxylate (DEAD, 1.6 mL, 10.16 mmol) and reaction mixture was allowed to stir at room temperature for 2h. After completion of reaction water was added to the reaction mixture. Organic layer was separated, collected in a conical flask and aqueous layer was extracted with ethyl acetate (3 x 15 ml). Combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography using ethyl acetate/pet ether (5:95) mobile phase to give phthalimide 206 as solid compound (1.42g, 85%).

IR (Neat, cm-1): 2962, 1630, 1465, 936, 823. 1 H NMR (400 MHz, CDCl3): δ 7.82 – 7.65 (m, 1H), 7.25 – 7.12 (m, 1H), 5.65 (ddt, J = 17.1, 10.2, 6.9 Hz, 1H), 5.06 – 4.89 (m, 1H), 3.88 (qd, J = 13.7, 8.0 Hz, 1H), 3.40 – 13 3.27 (m, 1H), 2.51 – 2.37 (m, 1H); C NMR (100 MHz, CDCl3): δ 168.19, 139.65, 135.25, 133.91, 131.82, 129.10, 128.58, 123.23, 116.97, 43.32, 43.12, 38.06.

2-(2-(4-chlorophenyl)-5-hydroxypentyl)isoindoline-1,3-dione (207) To a stirred solution of phthalimide 206 (0.5 g, 1.53 mmol) in dry THF was added borane dimethyl sulfinde (0.072 ml, 0.76 mmol) at 0 oC. The reaction mixture was stirred for 6h at the same temperature. After completion of reaction (TLC check) aqueous solution of NaOH and H2O2 solution was added in the reaction mixture at 0 oC and reaction mixture was further stirred for 1h. The organic layer was separated and aqueous layer was extracted with ethyl acetate (3x10 ml). The combined organic layer was washed with brine, dried over sodium sulphate and concentrated under reduced pressure to get alcohol 207 as viscous compound in good yield. Further purification was done by column chromatography using ethyl acetate:pet ether (5:95) as solvent system (0.4g, 78%).

IR (Neat, cm-1): 3345, 2958, 1635, 1483, 1022, 925, 819. 1 H NMR (400 MHz, CDCl3): δ 7.92-7.62 (m, 4H), 7.40-7.05 (m, 4H), 4.12-3.93(m, 4H), 3.55-3.35 (m, 1H), 1.90-1.63 (m, 2H), 1.62-1.39 (m, 2H); 13C NMR (100 MHz,

CDCl3): δ 168.19, 139.93, 133.95, 132.10, 129.08, 128.77, 122.96, 61.22, 51.65, 41.94, 29.49, 28.87. 4-(4-chlorophenyl)-5-(1,3-dioxoisoindolin-2-yl)pentanoic acid (208) In a stirred solution of phthalimide 207 (0.3g, 344mmol) dissolved in aqueous acetone (10 ml, 30%) was added solution of KMnO4 (0.275 g, 1.744 mmol) and NaOH (0.070 g, 1.744mmol) dissolved in water, drop wise over a period of 30 minutes at room temperature. The reaction mixture was stirred for further 2h. After the completion of reaction (TLC check) solvent was removed under reduced pressure and extraction was done with ethyl acetate (3x15 ml). The combined organic layer was dried over sodium sulphate and concentrated under reduced pressure to get crude acid. Further purification was done by base-acid treatment and column chromatography to get the acid as colourless viscous liquid (0.21g, 69%)..

IR (Neat, cm-1): 3385, 2936, 1457, 971, 897. 1 H NMR (400 MHz, CDCl3): δ 7.98-7.68 (m, 4H), 7.33-7.13 (m, 4H), 4.01-3.55 (m, 4H), 3.42-3.25 (m, 1H), 2.69-2.50 (m, 1H), 2.44-2.25 (m, 1H); 13C NMR (100 MHz,

CDCl3): δ 180.95, 172.13, 140.98, 134.92, 132.51, 129.86, 128.89, 123.03, 52.02, 40.95, 29.03, 25.92.

5-amino-4-(4-chlorophenyl)pentanoic acid hydrochloride (175) To a stirred solution of phthalimide 208 (600 mg, 1.69 mmol) in ethanol (5 ml) was added 80% aqueous hydrazine hydrate (0.20 mL, 13.38 mmol) and stirred at room temperature for 30 min. After completion of reaction the mixture was diluted with

CH2Cl2 and washed with water. Aqueous layer was extracted with CH2Cl2 (3x5 mL).

Combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. Crude residue was filtered through short bed of silica gel using methanol/chloroform (5:95) as mobile phase to get corresponding aminoacid (345 mg, 90% yield). The crude amino acid was treated with concentrated hydrochloric acid to get corresponding hydrochloride salt 175.

IR (Neat, cm-1): 3415, 3129, 1724, 1083, 909. 1 H NMR (400 MHz, CDCl3): δ 8.87 (S, 1H), 7.40-7.25 (m, 2H), 7.25-7.10 (m, 2H), 4.05-3.90 (m, 4H), 3.20-3.05 (m, 1H), 2.62-2.51 (m, 1H), 2.50-2.41(m, 1H); 13C NMR

(100 MHz, CDCl3): δ 174.62, 136.01, 032.23, 129.22, 128.53, 47.55, 42.19, 33.32, 28.94.

Section III: Synthesis of (±)-PCPGABA

1.3 Introduction

PCPGABA [4-amino-2-(4-chlorophenyl)butyric acid] is typical GABAB receptor agonists. Recent pharmacological studies88 using selective ligands for GABA- A and GABA-B receptors suggest that the GABA-B receptor subtype is a potentially important receptor controlling lung and airway functions, and therefore GABA-B receptor agonists may have wider applicability as general antitussive drugs. Baclofen [4amino-3-(4-chlorophenyl)butyric acid] and PCPGABA [4-amino-2-(4- chlorophenyl)butyric acid] are typical GABA-B receptor agonists. PCPGABA was used always in the racemic form for pharmacological tests on vagal efferent activity.89 It has been also reported that peripheral administration of PCPGABA stimulated gastric acid secretion in many species of animals such as rats, guinea pigs, dogs and human.90 Neural discharges produced in the multifibers of vagus preparation suggests involvement of vagus excitation in the mechanism of acid stimulation caused by PCPGABA.91 It is also observed the acid stimulatory effect of PCPGABA in anesthetized rats at a BT of 37 oC.92 Recently it has been reported that PCPGABA at considerably lower doses significantly decreased the body temperature (BT) of rats suggesting an involvement of GABAergic system in the mechanism of thermoregulation.93 Koji Takeuchi showed that, acid secretion, motility and mucosa investigated in the anesthetized rat stomach under various body temperatures affected by PCPGABA (BT: 28‒38 oC) and they were compared with those of 2-deoxy-D-glucose (2DG), an acid stimulant through cytoglycopenia. PCPGABA induces lesions dose-dependently (>1 mg/kg) in both the stomach and duodenum. This activity of PCPGABA was dependent on BT, lowering of BT enhanced the ulcerogenicity. At higher BT (36‒38 oC) there was no effect of PCPGABA on acid secretion but produced a marked increase of acid output at lower BT (30‒32 oC). Gastric motility was enhanced significantly by 2DG and PCPGABA to similar degrees at either high or low BT but they don’t affected alkaline secretion in the duodenum. These results suggest that acid stimulatory and ulcerogenic action of PCPGABA may involve a temperature-dependent process but does not relate to a cytoglycopenia and the vagus nerve mediating acid secretion and motility may be different in the temperature dependency.94

1.3.1 Review of literature Literature search revealed that there are only three reports available on the synthesis of PCPGABA which are described below.

G. Michael Wall95 Michael et al. synthesized α-Baclofen (PCPGABA) 209, started from esterification of p-chlorophenylacetic acid 210. The resulting ethyl p-chloro- phenylacetate 211, on oxalate addition was converted to the atropic ester derivative 212 which on treatment with nitromethane in the presence of the base Triton B produced the nitroester 213. Reduction of the nitroester 213 under high pressure (1800 psi) and high temperature (70 oC) gave α-baclofen ester (7%) 214 and α-baclofen lactam 215 (18%). The amino ester 214 was easily converted to α-baclofen lactam 215 by heating. Hydrolysis of α-baclofen lactam 215, to the amino acid, α-Baclofen 209, was accomplished by refluxing the lactam in 3 N HCl for several hours (Scheme-29).

+ Scheme-29: Reagent and conditions: (a) EtOH, H (b) NaOEt, EtOH, K2CO3, HCHO; o (c) Triton B/CH3NO2; (d) Ra-Ni, H2 (1800 psi), 70 C; (e) on standing/heat; (f) 3N HCl, heat.

Shigeyuki Yoshifuji96 Shigeyuki et al. used trans-4-hydroxy-L-proline as a starting material for the synthesis of (R)-PCPGABA and (S)-PCPGABA. L-Proline 216 was converted to the 4- ketoproline derivative 217a. Grignard reaction of the ketone 217a with 4- chlorophenylmagnesium bromide in presence of CeCl3 proceeded stereoselectively to afford almost a single adduct, 218a which was treated to provide isomeric mixture of olefins 219a. Catalytic hydrogenation of the mixture 219a was accomplished using Pt

81 catalyst at 1 atm to give only one (220a) of the possible diastereoisomers in 84 % yield. N-Deprotection and ester cleavage of 220a provide (2S,4R)-4-(4-chlorophenyl)proline 221a. Decarboxylation of the α-amino acid 221a was done by Hashimoto's method and immediately protected as Boc-derivative 222a. Oxidation of 222a using RuO4 gave lactam 223a which was deprotected and hydrolysized to (R)-PCPGABA 224a (Scheme- 30).

Scheme-30: Reagent and conditions: (a) 4-Chlorophenyl magnesium bromide, CeCl3, o ether, rt, 78%; (b) SOCl2, Pyridine, 79%; (c) H2/Pt, 84%; (d) 6N HCl, AcOH, 110 C; o (e) cyclohexanol, 2-cyclohexen-1-one,155 C , then Boc protection, 98%; (f) RuO4, NaIO4, EtoAc, H2O, rt, 3h, 32%; (g) 6N HCl, reflux, 99%.

For the synthesis of (S)-PCPGABA, Grignard adduct 218a was refluxed with DBU in methanol for three days to get epimarized adduct 218b in 54% yield with recovery of 218a. Adduct 218b was used for the synthesis of (S)-PCPGABA repeating the same steps (Figure-20).

Figure-20

Rolf H. Prager97 Diethyl 2-(4-chlorophenyl)malonate 225 was treated with 2-bromoacetonitrile to form the diethyl 2-(4-chlorophenyl)-2-(cyanomethyl)malonate 226. Reduction of nitrile group using Pt/H2 gave diethyl amine 227, which on hydrolysis and decarboxylation gave PCPGABA (Figure-21).

Figure-21

1.3.2 Present work 1.3.2.1 Objective In view of recent interest in the pharmacology of these compounds, it is necessary to examine the relative activities of baclofen and PCPGABA and for this purpose it must be synthesized on a practical scale. We have prepared PCPGABA using intermediate 4-pentenenol obtained using Wittig-Claisen reaggangement protocol.

1.3.2.2 Result and discussion Our strategy for the synthesis of PCPGABA is outlined in Scheme 31. We started our synthesis from alcohol 161 obtained from 4-chlorobenzaldehyde employing Wittig olefination-Claisen rearrangement protocol followed by reduction. The alcohol was protected as benzyl ether using benzyl bromide98 and double bond was cleaved to get alcohol 229 using ozone and sodium borohydride which was then subjected to phthalimide substitution under mitsunobu reaction conditions83 to give compound 230. Benzyl ether was deprotected99 and alcohol 231 was oxidised to carboxylic acid 232 by 85 KMNO4 oxidation. Phthalimide 232 was deprotected to free amine using hydrazinhydrate86 to give final compound PCPGABA 209.

o Scheme-31: Reagent and conditions: (a) BnBr, NaH, PTC, THF, 0 C, 90%; (b) O3, o o DMS, NaBH4, DCM, 0 C 60%; (c) TPP, DIAD, HNPhth, THF, 0 C, 80%, (d) H2, Pd/C, MeOH, 90%, (e) KMnO4, NaOH, Acetone:Water, 3h, 70% (f) NH2NH2.H2O, EtOH, 1h, 60%.

1-(1-(benzyloxy)pent-4-en-2-yl)-4-chlorobenzene (228)

1H NMR (400 MHz) Spectrum of the Compound 228

13C NMR (100 MHz) Spectrum of the Compound 228

Alcohol 161 was protected as benzyl ether using benzyl bromide and sodium hydride to give benzyloxy 4-pentene 228. In the IR spectrum, peak at 1020 cm-1 show ether linkage. In the 1H NMR spectrum, multiplet resonated between δ 7.40-7.18 was attributed to seven aromatic protons. A doublet resonated at δ 7.13 with coupling constant 8.4 Hz attributed to two aromatic protons from the chloro benzene ring ortho substituted to side chain. A multiplet at 5.64 showing doublet of doublet of tiplet with

85 coupling constant (J) 17.1, 10.1 and 7.0 Hz was attributed to one internal methine olefinic proton from the terminal double bond. A multiplet between δ 5.03-4.86 attributed to two methylene protons from the terminal double bond. Two protons from benzylic methylene carbon attached to ether linkage resonated between δ 4.63-4.35 shows multiplet. A multiplet between δ 3.64-3.47 attributed to two protons for a bridged methylene group carrying benzyl group. Methine proton shows doublet of quartet at δ 2.96 (J = 12.7, 6.4 Hz). From the allylic methylene, two protons were resonated separately. One proton shows a multiplet at δ 2.55 while another shows a multiplet between δ 2.40-2.21. In the 13C NMR spectrum, peaks at δ 135.06, and 115.36 attributed to internal methine and terminal methylene carbon atoms of terminal olefinic double bond. Peaks resonating at δ 139.90, 132.90, 129.50, 129.23, 129.23, 128.80, 128.57 and 128.39 attributed to aromatic carbons from benzyl and chloro benzene rings. A peak at δ 71.87 attributed to benzyl methylene carbon, while a peak resonating at δ 70.66 attributed to methylene carbon carrying benzyl group. Benzylic methine carbon was resonated at δ 39.97, and bridged allylic methylene carbon was resonated at δ 34.00.

4-(benzyloxy)-3-(4-chlorophenyl)butan-1-ol (229) Double bond from the benzyl ether 228 was transformed into aldehyde by ozonolysis and aldehyde was further reduced to alcohol 229. In the IR spectrum peak at 3462 cm-1 show presence of alcohol functionality. Peaks at 2963, 1674, 1425, 969 and 874 cm-1 show the presence of aromatic ring.

1H NMR (400 MHz) Spectrum of the Compound229

13C NMR (100 MHz) Spectrum of the Compound 229

In the 1H NMR spectrum, a multiplet between δ 7.60-6.86 was attributed to nine protons from the two benzene rings. A multiplet between δ 4.61-4.27 was attributed to four protons from the two methylene carbons attached to oxygen atom. A multiplet between δ 4.02-3.78 attributed to two protons from the methylene carbon bearing hydroxy group. Methine proton was resonated between δ 3.14-2.93 and a multiplet for two protons between δ 2.21-1.92 attributed to methylene carbon attached to benzylic carbon in hydroxy side chain. In the 13C NMR spectrum, peaks at δ 139.82, 133.06, 129.53, 129.26, 129.17, 128.85, 128.63, and δ 128.41 attributed to carbons from aromatic rings. A peak at δ 68.45 corresponded to benzyl carbon and a peak δ 60.31 attributed to methylene carbon carrying benzyl group. A peak at δ 41.17 shows methylene carbon bearing hydroxy group. Methine carbon was resonated at δ 35.07 and bridged methylene carbon from the free hydroxy side chain was resonated at δ 29.71.

2-(4-(benzyloxy)-3-(4-chlorophenyl)butyl)isoindoline-1,3-dione (230) Alcohol 229 was subjected to nucleophilic substitution under mitsunobu reaction condition to form corresponding phthalimide derivative 230.

1H NMR (400 MHz) Spectrum of the Compound 230

13C NMR (100 MHz) Spectrum of the Compound 230

In the 1H NMR spectrum, a multiplet between δ 7.78-7.62 attributed to four protons from the phthalimide ring. A multiplet between δ 7.34-7.17 attributed for seven protons and a multiplet at δ 7.14 attributed for two protons corresponded to protons from benzyl and chloro benzene rings. A singlet at δ 4.44 attributed to two protons from benzyl group. A multiplet at δ 3.63 attributed to two protons from methylene carbon attached to phthalimide. Two methylene protons from a carbon carrying benzyl group

88 were resonated between δ 3.55-3.43 as multiplet. Methine proton shows triplet of doublet (J = 11.4 and 6.6 Hz) at δ 2.94, while two bridged methylene protons neighbouring to methine carbon from a phthalimide side chain resonated between δ 2.26-1.99 as multiplet. 13C NMR spectrum show a peak at δ 168.31attributed to carbonyl carbon from phthalimide ring. Aromatic carbons were resonated at δ140.12, 137.96, 133.85, 132.00, 129.25, 128.51, 128.37, 127.53 and δ 123.02. A peak at δ 74.68 attributed to benzyl carbon from benzyl group and peak at δ 72.95 attributed to methylene carbon carrying benzyl group. A peak at δ 43.62 was attributed to methine carbon. Peak at δ 36.11show methylene carbon bearing phthalimide group and peak at δ 29.72 corresponded to bridged methylene carbon from phthalimide side chain.

2-(3-(4-chlorophenyl)-4-hydroxybutyl)isoindoline-1,3-dione (231) Benzyl ether 230 was deprotected using Pd/C under hydrogen gas balloon pressure to get corresponding phthalimide alcohol 231. IR spectrum shows peak at 3362 cm-1 and 1025 cm-1 indicates the presence of hydroxy group. 1H NMR spectrum shows a multiplet resonated between δ 7.76-7.51 attributed to four aromatic protons from the phthalimide ring and a multiplet between δ 7.18-6.97 attributed to four protons from the chloro benzene ring. A doublet of doublet at δ 3.58 with coupling constant (J) 16.2 and 6.9 Hz attributed to four protons from the two methylene carbons carrying phthalimide and hydroxy groups.

1H NMR (400 MHz) Spectrum of the Compound 231

13C NMR (100 MHz) Spectrum of the Compound 231

A multiplet between δ 2.84-2.65 corresponded to one methine proton. A multiplet between δ 2.11-1.93 corresponded to two protons from a bridged methylene in phthalimide side chain. A broad singlet δ 1.89 shows the hydroxy proton. In the 13C NMR spectrum, peak resonated at δ 168.30 show carbonyl carbon from phthalimide ring. Peaks at δ 139.74, 133.90, 131.84, 129.30, 128.73 and δ 123.06 corresponded to aromatic carbons from phthalimide and chloro benzene rings. A signal at δ 67.32 attributed to methylene carbon bearing hydroxy group. Peak at δ 45.97 corresponded to methine carbon. Peak at δ36.34 shows methylene carbon bearing phthalimide ring and peak at δ 30.08 attributed to bridged methylene carbon.

2-(4-chlorophenyl)-4-(1,3-dioxoisoindolin-2-yl)butanoic acid (232) A free primary alcohol 231 was oxidized to corresponding carboxylic acid 232 using -1 -1 KMnO4/NaOH solution. In the IR spectrum broad peak at 3390 cm and 1720 cm shows the presence of carboxylic group. In the 1H NMR spectrum, a multiplet resonatedbetween δ 7.67-7.90 attributed to four protons from phthalimide ring. Multiplet observed between δ 7.32-7.14 attributed to four aromatic protons from chloro benzene ring. Two protons from methylene carrying phthalimide ring were resonated between δ 3.95-3.77 as multiplet. Methine proton was resonated between δ 3.29-3.42 as multiplet. A multiplet resonating between δ 2.53-2.39 attributed to two protons from the bridged methylene carbon.

1H NMR (400 MHz) Spectrum of the Compound 232

13C NMR (100 MHz) Spectrum of the Compound 232

In the 13C NMR spectrum, we can saw carboxylic acid carbon resonated at δ 180.00. A peak at δ 168.36 corresponded to carbonyl carbon from phthalimide ring. Peaks resonating at δ 138.96, 134.89, 131.86, 129.25, 128.47 and 123.92 show aromatic carbons from phthalimide ring and chloro benzene ring. Methylene carbon bearing phthalimide was resonated at δ 48.87. Methine carbon show peak at δ 39.03. A bridged methylene carbon was resonated at δ 29.09.

4-amino-2-(4-chlorophenyl)butanoic acid (209)

1H & 13C NMR (100 MHz) Spectrum of the Compound 209

Phthalimide acid 232 was deprotected to give corresponding amino acid, PCPGABA 209. IR spectrum show peaks at 3440 cm-1 and 1730 cm-1 corresponded to carboxylic acid. In the 1H NMR spectrum, a doublet of doublet resonated at δ 7.31 with coupling constant (J) 25.0 Hz and 8.4 Hz attributed to four aromatic protons from the benzene ring. Two protons from the primary amine show singlet at δ 4.95. A multiplet observed between δ 3.73-3.57 attributed to one methine proton. A multiplet resonated between δ 2.96-2.65 attributed to two protons from a methylene carrying amine functionality. Two methylene protons adjacent to benzylic methine carbon resonated separately as multiplets between δ 2.26-2.04 and at δ 2.04-1.85. 13C NMR spectrum shows a peak at δ 176.62 was attributed to carboxylic carbon, and peaks resonated at δ140.01, 133.32, 129.92, 128.84 corresponded to aromatic carbons. A peak at δ 48.03 shows methine carbon. A peak resonated at δ 37.89 was attributed to methylene carbon bearing amine functionality and a bridged methylene carbon was resonated at δ 31.81.

1.3.3 Experimental section

1-(1-(benzyloxy)pent-4-en-2-yl)-4-chlorobenzene (228) To a suspension of sodium hydride (1.2 g, 51 mmol) in dry THF (40 ml) at 0 oC, the solution of alcohol 161 (5 g, 25.5 mmol) in dry THF (20 ml) was added slowly over a period of 15 min. The solution was stirred at same temperature for 10 min. and benzyl bromide (3.7 ml, 30 mmol) was added drop wise. Reaction mixture was allowed to come at room temperature and stirred for 7‒8 hours. After the completion of reaction (TLC check), the mixture was cooled and quenched by slowly addition of cold water. The THF was removed under reduced pressure and the crude product was extracted with ethyl acetate (3x10 ml). The combined organic layer was washed with water, dried over anhydrous sodium sulphate and concentrated under reduced pressure. The crude product obtained was further purified by column chromatography using hexane:ethyl acetate (95:5) as mobile phase to give the benzyl ether 228 as colourless viscous liquid (1.58 g, 90%).

IR (Neat, cm-1): 2948, 1655, 1458, 1020, 965, 884. 1 H NMR (400 M Hz, CDCl3): δ 7.40-7.18 (m, 7H), 7.13 (d, J = 8.4 Hz, 2H), 5.64 (ddt, J = 17.1, 10.1, 7.0 Hz, 1H), 5.03-4.86 (m, 2H), 4.63-4.35 (m, 2H), 3.64-3.47 (m, 2H), 2.96 (dq, J = 12.7, 6.4 Hz, 1H), 2.55 (dt, J = 13.2, 6.6 Hz, 1H), and δ 2.40-2.21 (m, 1H); 13 C NMR (100 M Hz, CDCl3): δ 139.90, 135.06, 132.90, 129.50, 129.23, 129.23, 128.80, 128.57, 128.39, 115.36, 71.87, 70.66, 39.97, 34.00.

4-(benzyloxy)-3-(4-chlorophenyl)butan-1-ol (229) To a stirred solution of benzyl ether 229 (5.96 g, 37.7 mmol) dissolved in

CH2Cl2 (50 mL) a stream of ozonized oxygen was bubbled through it at -78 °C until the blue color persisted (25‒40 min). After the solution was flushed with nitrogen for 15 min, dimethyl sulfide (25 ml) was added and the reaction mixture was allowed to warm to room temperature. After evaporation of solvent, the crude aldehyde was obtained as colorless viscous liquid. The crude aldehyde was dissolved in 5% aqueous methanol (50 ml) and cooled to 0 °C. NaBH4 (1.58 g, 41.5 mmol) was added portion wise over the period of 10 min. and the mixture was stirred for additional 15 minutes. After

94 completion of reaction (TLC check), the reaction was quenched with saturated ammonium chloride solution. The mixture was concentrated under reduced pressure and the residue was partitioned between ethyl acetate and brine solution. The layers were separated and the aqueous layer was extracted with ethyl acetate (2 x 10 ml). The combined organic layers was dried over anhydrous Na2SO4 and concentrated under reduced pressure to give 4.2 g (68%) of crude alcohol 229 as colourless viscous liquid.

IR (Neat, cm-1): 3462, 2963, 1674, 1425, 969, 874. 1 H NMR (400 M Hz, CDCl3): δ 7.60-6.86 (m, 9H), 4.61-4.27 (m, 4H), 4.02-3.78 (m, 13 2H), 3.14-2.93 (m, 1H), and δ 2.21-1.92 (m, 2H); C NMR (100 M Hz, CDCl3): δ 139.82, 133.06, 129.53, 129.26, 129.17, 128.85, 128.63, 128.41, 68.45, 60.31, 41.17, 35.07, 29.71.

2-(4-(benzyloxy)-3-(4-chlorophenyl)butyl)isoindoline-1,3-dione (230) To a stirred solution of alcohol 229 (957 mg, 4.23 mmol), triphenyl phosphine (1.66 g, 6.35 mmol) and phthalimide (934 mg, 6.35 mmol) in dry THF (25 ml) at 0 °C was added diethylazodicarboxylate (DEAD, 1.25 ml, 6.35 mmol) drop wise and reaction mixture was allowed to stir at room temperature for 3 hours. After completion of reaction, water was added to the mixture. Layers were separated and aqueous layer was extracted with ethyl acetate (3x15 ml). Combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude residue was purified by silica gel column chromatography using ethyl acetate/pet ether (5:95) mobile phase to give phthalimide 230 as colourless viscous liquid (1.18 g, 86% yield).

IR (Neat, cm-1): 2941, 1684, 1456, 965, 843. 1 H NMR (400 M Hz, CDCl3): δ 7.78-7.62 (m, 4H), 7.34-7.17 (m, 7H), 7.14(m, 2H), 4.44 (s, 1H), 3.63 (t, J = 7.0 Hz, 2H), 3.55-3.43 (m, 2H), 2.94 (td, J = 11.4, 6.6 Hz, 1H), and δ 2.26-1.99 (m, 2H);

13 C NMR (100 M Hz, CDCl3): δ 168.31, 140.12, 137.96, 133.85, 132.00, 129.25, 128.51, 128.37, 127.53, 123.02, 74.68, 72.95, 43.62, 36.11, 29.72.

2-(3-(4-chlorophenyl)-4-hydroxybutyl)isoindoline-1,3-dione (231) A mixture of benzyl ether 230 (1.23 g, 2.14 mmol) and 10% palladium/carbon (62 mg, 5 wt%) in methanol (15 ml) was hydrogenated at room temperature under balloon pressure for 14h. After completion of reaction, the mixture was filtered through short bed of cilite and concentrated under reduced pressure. The crude residue was purified by passing through short bed of silica gel using ethyl acetate/pet ether (10:90) as mobile phase to get alcohol 231 as colourless viscous liquid (0.845 g, 88% yield).

IR (Neat, cm-1): 3362, 2950, 1630, 1480, 1025, 930, 842. 1 H NMR (400 M Hz, CDCl3): δ 7.76-7.51 (m, 4H), 7.18-6.97 (m, 4H), 3.58 (dd, J = 16.2, 6.9 Hz, 4H), 2.84-2.65 (m, 1H), 2.11-1.93 (m, 2H), and δ 1.89 (bs, 1H); 13C NMR

(100 M Hz, CDCl3): δ 168.30, 139.74, 133.90, 131.84, 129.30, 128.73, 123.06, 67.32, 45.97, 36.34, 30.08.

2-(4-chlorophenyl)-4-(1,3-dioxoisoindolin-2-yl)butanoic acid (232) In a stirred solution of alcohol 231(0.4 g) in aqueous acetone (30%) was added aqueous solution of KMnO4 and NaOH dissolved in water drop wise over a period of 30 minutes at room temperature. The reaction mixture was stirred for further 2h. After the completion of reaction (TLC check), solvent was removed under reduced pressure and extraction was done with ethyl acetate (3x15 ml). The combined organic layer was dried over sodium sulphate and concentrated under reduced pressure to get crude acid. Further purification was done by base-acid treatment and column chromatography to get the acid as colourless viscous liquid (0.321g, 77%).

IR (Neat, cm-1): 3390, 2940, 1720, 1446, 935, 890. 1 H NMR (400 M Hz, CDCl3): δ 7.67-7.90 (m, 4H), 7.32-7.14(m, 4H), 3.95-3.77 (m, 13 2H), 3.29-3.42 (m, 1H), and δ 2.53-2.39 (m, 2H); C NMR (100 M Hz, CDCl3): δ 180.00, 168.36, 138.96, 134.89, 131.86, 129.25, 128.47, 123.92, 48.87, 39.03, 29.09.

4-amino-2-(4-chlorophenyl)butanoic acid (209) To the stirred solution of phthalimide 232 (897 mg, 1.40 mmol) in ethanol (5 ml) was added solution of 80% aqueous hydrazine hydrate (0.17 ml, 2.80 mmol) and stirred at room temperature for 30 min. After completion of reaction the mixture was diluted with CH2Cl2 and washed with water. Aqueous layer was extracted with CH2Cl2

(3x7 ml). Combined organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. Crude residue was filtered through short bed of silica gel using methanol-chloroform (3:97) as mobile phase to get corresponding amino acid 209 as white solid (0.485g, 87%).

IR (Neat, cm-1): 3440, 2972, 1730, 963, 841. 1 H NMR (400 M Hz, CDCl3): δ 7.31 (dd, J = 25.0, 8.4 Hz, 4H), 4.95 (s, 2H), 3.73-3.57 (m, 1H), 2.96-2.65 (m, 2H), 2.26-2.04 (m, 1H), and δ 2.04-1.85 (m, 1H); 13C NMR

(100 M Hz, CDCl3): δ 176.62, 140.01, 133.32, 129.92, 128.84, 48.03, 37.89, 31.81.

Section IV: Synthesis of (±)-Phaclofen and (±)-Saclofen

1.4 Introduction

Baclofen acts as a potent GABAB agonist but inactive at GABAA receptors.

However there is a lack of potent and specific GABAB receptor antagonists. Aminovaleric acid (δ-AVA) has been shown to antagonize the action of (‒)-baclofen in the periphery and CNS, which is equally potent at GABAA and GABAB receptors. Kerr 100 et al. in 1987 described the first GABAB antagonist phaclofen. Phaclofen (51) and

Saclofen (52) act as GABAB antagonists specific for GABA and Baclofen. Phaclofen, a phosphono acid analog of Baclofen antagonize the actions of baclofen in the periphery and this compound has been recently reported to antagonize central GABAB receptors. Phaclofen (1 mM) produced a potentiation of the forskolin effect. (‒)-Baclofen (10µM) potentiated isoprenaline stimulated cyclic AMP accumulation, an effect antagonized by phaclofen (1 mM).101 3-amino-2-(4-chlorophenyl)propanesulfonic acid (Saclofen) is about three times more effective than its hydroxy analogue (240) as a GABAB antagonist. 102

Figure-22: δ-AVA, Saclofen and phosphono derivatives of GABA.

The phosphono-analogues of GABA, 4-amino-butylphosphonic acid (49), 3- amino-2(4-chlorophenyl)-propylphosphonic acid (51) and 3-amino-2-cyclohexylpropyl- phosphonic acid (238) antagonized the GABA and baclofen induced GABAB receptor

98 mediated depression of twitch responses to transmural stimulation in the guinea pig isolated ileum, in reversible, surmountable and concentration dependent manner. No such activity was found in a variety of related analogues. 3-amino-propylphosphonic acid (3-APPA, 235) behaved as a partial agonist.103 The antagonist activity of the

Phaclofen and DAVA and their low affinity for the GABAB receptor compared to their carboxylic acid analogues might be due to the unique ability of phosphonic acids to be diionized at physiological pH. On a GABA functional assay, the rat anococcygeus, α- difluro Phaclofen (239) showed very weak agonist activity and no antagonist of the effect of GABAB agonist. These results suggested that diionization of the acidic moiety 104 may be detrimental for GABAB antagonist activity. (‒)-(R)-Phaclofen was shown to inhibit the binding of [3H]-(R)-baclofen to GABA, receptor sites on rat cerebellar membranes (IC50 = 76 ± 13 µM), whereas (+)-(S)-phaclofen was inactive in this binding assay (IC50 > 1000 µM). (+)-(S)-phaclofen (200 µM) was inactive with (±)-phaclofen (400 µM) in antagonizing the action of baclofen in rat cerebral cortical slices, while (‒)- (R)-Phaclofen (200 µM ) was equipotent. The structural similarity of the antagonist (‒)- (R)-phaclofen and the agonist (R)-baclofen suggests that these ligands interact with the

GABAB receptor sites in a similar fashion. Thus, it may be concluded that the different pharmacological effects of phosphono derivatives of GABA essentially result from the different spatial and proteolytic properties of their acid groups. 105 Giovanni et al synthesized 3-amino-3-(4-chlorophenyl)propanoic acid 242, and the corresponding phosphonic and sulfonic acids, lower homologues of baclofen, phaclofen and saclofen respectively (Figure 54). The chlorinated acids were all weak specific antagonists of

GABA at the GABAB receptor, with the sulfonic acid (pA2 4.0) being stronger than the 106 phosphonic acid (pA2 3. 8) and carboxylic acid (pA2 3. 5). The enantiomer (R)- Saclofen reversibly antagonised the (R,S)-baclofen induced depression of cholinergic twitch contractions in the guinea pig ileum with an apparent pA2 of 5.3, but not (S)- saclofen. Also, 2-hydroxy-saclofen was resolved by the same method, its (S)- 107 enantiomer yielding an apparent pA2 of 5.0. Saclofen and its analogs are currently used as biological probes to understand the functions of the receptor.

Figure-23: Phosphono and sulphono derivatives of GABA.

100

1.4.1 Review of literature There are very few reports are available for the synthesis of Phaclofen and Saclofen, which are described below.

John Chiefari (1987)108 Dehydration of compound 259, which is obtained from the Reformatsky reaction of 4-chloroacetophenone and ethyl bromoacetate with phosphorus oxychloride, gives unsaturated ester 260. Bromination of 260 with NBS on irradiation in the presence of dibenzoyl peroxide (DBP) gave allylic bromide 261. The Arbusov reaction with phosphite gave phosphonate ester 262, which on catalytic hydrogenation gives compound 263. Ester 263 was treated with sodium azide in sulfuric acid to give Phaclofen in 35% yield after purification by ion exchange chromatography.

Scheme-32: Reagent and conditions: (a) POCl3, 94%; (b) NBS, DBP, hν, CCl4, 4h, o 84%; (c) P(OMe)3, 150 C, 45 min. 55%; (d) H2/Pt, MeOH; (e) i) 10 M HCl, reflux, 10 o h; ii) H2SO4 NaN3, CHCl3, 45 C, 20 min then 10h, rt.

Roger G. Hall (1989)109 Roger used Michael addition of a Phosphonate to a β-nitrostyrene as a key step to form an adduct 262. On hydrogenation with Ra-Ni, amine 263 was obtained which on acid treatment, Phaclofen produced.

101

o Scheme-33: Reagent and conditions: (a) LDA, P(O)(OMe)2CH3, THF, ‒78 C, 71%; (b) H2, Ra-Ni, EtOH, 85%; (c) i) HCl, reflux, 15h; ii) Propylene oxide, EtOH.

T. N. Robinson101 Robinson et al. treated 2-(4-chlorophenyl)acetonitrile 148 with diethyl oxalate to produce the adduct, ethyl 3-(4-chlorophenyl)-3-cyano-2-oxopropanoate 264 which was then reduced to hydroxypropanenitrile 265. Alcohol functionality was substituted by bromide (266) and then treated with triethyl phosphite to give diethyl (2-(4- chlorophenyl)-2-yanoethyl)phosphonate 267. The nitrile functionality was reduced to corresponding amine derivative (268) and then phosphonate was hydrolysed to give Phaclofen 51.

Scheme-34: Reagent and conditions: (a) (COOEt)2, NaOEt, 78%; (b) Na2CO3, HCHO, 57%; (c) PBr3; (d) P(OEt)3, reflux; (e) PtO2, H2(g); (f) 6N HCl

William Howson and Judy M. Hills (1991)104 Willam et al. synthesized α-difluro Phaclofen to study diionization effect at physiological pH. The lithium anion of diethyl difluoro methane phosphonate reacted in a 1,4 addition to 4-chloro β-nitro styrene (77) at -78 oC to give the nitro compound 269 in 78% yield. Reduction of the nitro group using Raney nickel in presence of hydrogen

102 atmosphere gave amine 270 in 72% yields. Hydrolysis with concentrated hydrochloric acid produced the final product as the hydrochloride salt. Treatment with propylene oxide gave the racemic difluorinated phaclofen 271 in 53% yields as the zwitterions.

Scheme-35: Reagent and conditions: (a) LiCF2P(O)(OEt)2; (b) Ra-Ni, H2, EtOH; (c) i) 12 M HCl, reflux, ii) Propylene oxide, MeOH/EtOH.

Wolfgang Froestl (1995)110 Wolfgang et al synthesised substituted Phaclofen derivatives by using Michel addition reaction of substituted phosphonic acids on nitro styrene as shown in Figure 24.

Figure-24

Zheng-Chao Duan (2010)111 Zheng et al. synthesized (R)-Phaclofen using the Rh-catalyzed asymmetric hydrogenation with a P-stereogenic BoPhoz-type phosphine-aminophosphine ligand. Unsaturated ester (122) was obtained by the Horner Wittig reaction on 1-(4- chlorophenyl)ethanone (121). Bromination of (E)-122 with N-bromosuccinimide in the presence of benzoyl peroxide gave the allylic bromides (123) in high yields. The Arbusov reaction with phosphite gives the target phosphonate 260. The double bond was reduced asymmetrically to give compound 276 which was then treated with

103 hydrochloric acid and sodium azide in presence of sulphuric acid to give (R)-Phaclofen (277).

Scheme-36: Reagent and conditions: (a) P(O)(OCH3)2CH2COOCH3, NaH, THF, rt; (b) NBS, BP, CCl4; (c) P(OCH3)3, reflux, 4h; (d) Rh(COD)2 1mol%, L1 1.1 mol%, CH2Cl2, 60 bar, rt, 24 h; (e) i) 10 M HCl, reflux, 10 h; ii) NaN3, H2SO4, rt, 12h.

Chun-Sing Li (1990)112 Chun-Sing Li treated (E)-2(4-Chlorophenyl)-3-dimethylamino-2-propene-nitrile (278) with sodiumcyanoborohydride in presence of acetic acid followed by in situ oxidation with sodiumperiodate and elimination of the intermediate amine oxide at rt produced 2-(4-chlorophenyl)acrylonitrile (279). On Michael addition of sodium bisulfite product obtained was sodium 2-(4-chloro phenyl)-2-cyanoethanesulfonate

(280). Hydrogenation of the nitrile by Ra-Ni, H2 in ammonium hydroxide solution gave Saclofen (52).

o Scheme-37: Reagent and conditions (a) i) NaCNBH3/ CH2Cl2, AcOH, 0 C, 20 min; ii) NaIO4/ H2O, 16h, 69% (b) NaHSO3/MeOH, H2O, reflux, 8h, 71 % (c) H2, Ra-Ni/NH3, H2O, rt, 3.4 bar 59%

104

Giovanni Abbenante (1990)113 Giovanni et al. synthsized Saclofen by bromination of 1-chloro-4-(prop-1-en-2- yl)benzene (281) with NBS. Dibromo compound (282) was treated with potassium phthalimide to give bromo phthalimide compound (283). Deprotection of phthalimide (283) gave 3-bromoamine (284) which was treated with 5M solution of ammonium bisulphite to gate unsaturated sulphonic acid. Double bond from the amino sulphonic acid was reduced by palladium carbon under atmosphere of hydrogen to give Saclofen (52).

Scheme-38: Reagent and conditions (a) NBS, BPO, CCl4, reflux; (b) KNPhth, DMF, rt, 10 h, 77 % (c) NH2NH2, EtOH, reflux; (d) 5M ABS, H2O, O2, 3 days, then HCl; (e) H2, 10% Pd/C .

Giovanni Abbenante and Rolf H. Prage (1992)114 1-chloro-4-(prop-1-en-2-yl)benzene (281) was brominated using N-bromo succinimide to give monosubstituted allyl bromide compound (285). Bromide was substituted by phthalimide to give allyl phthalimide 286. Radical addition of thioacetic acid to the phthalimidoalkene 286 gave the desired thioacetate (287). Oxidation of compound 287 with chlorine gave the sulfonyl chloride (288) (94%), which gave saclofen (52) on hydrolysis.

Scheme-39: Reagent and conditions: (a) NBS, BPO, CCl4, reflux; (b) KNPhth, DMF, rt, 10h; (c) Thioacetic acid, CCl4, mercury lamp (8W, 254 nm) 24h, 46%; (d)Thioacetate:H2O, DCM, Cl2 gas, 1h; (e) i) 0.5 M NaOH, ii) 5M HCl, reflux, 3h.

105

Rolf H. Prager (1995)115 Rolf et al. synthesized allyl alcohol using modified procedure of Barluenga. Sharpless epoxidation of (289) with diethyl D-(-)-tartrate gives the (R)-epoxyalcohol (290). Ring opening of (290) with sodium azide in the presence of ammonium chloride yields the azidodiol (291). The diol was converted slowly to the tosylate, which on reaction with KOtBu in t-butyl alcohol readily formed epoxide (292). Azidoepoxide on hydrogenation produces the aminoepoxide (293) which was then converted to (S)-(+)- hydroxysaclofen (294) by reaction with sodium hydrogen sulfite in refluxing ethanol- water and isolated as its hydrochloride.

Scheme-40: Reagent and conditions: (a) D-(-)-DET, Ti(O-iPr)d, t-BuOOH, CH2Cl2, -20 o C; 67%; (b) NaN3. NH4Cl, MeOH/H2O, 70 °C; 93%; (c) TsCl, pyridine, 84%; (d) o KOtBu, THF/ t-BuOH, 92%; (e) H2 /PtO2, EtOH; 94%; (f) NaHSO3, EtOH/H2O, 100 C; 51%.

106

1.4.2 Present work 1.4.2.1 Objective

Phaclofen (51) and Saclofen (52) act as GABAB antagonists specific for GABA and Baclofen. We are interested to synthesize these antagonists vai a common precursor 2-(2-(4-chlorophenyl)pent-4-en-1-yl)isoindoline-1,3-dione 206.

1.4.3 Result and discussion The present strategy for the synthesis (±)-Phaclofen and (±)-Saclofen starting from intermediate phthalimide (206), which was obtained from commercially available 4-chlorobenzaldehyde is depicted in Scheme-41.

o Scheme-41: Reagent and conditions (a) RuCl2(PPh3)3, EtOH, 80 C, 2h, 90%; (b) O3, o o DMS, DCM, ‒78 C, NaBH4, MeOH, 0 C 65%; (c) PBr3, CHCl3, 72%; (d) trimethyl phosphite (P(OMe)3), reflux, 61%; (e) a. NH2NH2, b. 6N HCl, 67%; (f) 5M Ammoniumbisulfite (ABS), H2O, O2, 55%; (g) NH2NH2.H2O, EtOH, 1hr, 60%.

(E)-2-(2-(4-Chlorophenyl)pent-3-en-1-yl)isoindoline-1,3-dione (295) 116 Terminal double bond was isomerised by RuCl3 to get the compound 295 having internal double bond which is characterized by 1H NMR spectral analysis. In the 1H NMR spectrum, a multiplet resonated at δ 5.91‒5.78 was attributed to two olefinic protons from internal double bond. In the parent compound two multiplets was observed for the terminal double bond were at δ 5.65 and δ 5.02 attributed to one methine and two methylene protons. This changes in NMR spectrum confirmers the isomerisation of terminal double bond into internal double bond. Also a double at δ 2.13 with coupling constant 5.3 Hz attributed to three protons from the allylic methyl. Multiplet resonated

107

1H NMR spectrum of the compound 295

13C NMR spectrum of the compound 295 between δ 7.90‒7.75 was attributed to two protons from the phthalimide ring. Multiplet observed between δ 7.75‒7.62 attributed to two protons from the phthalimide ring. Multiplets between δ 7.40‒7.24 and δ 7.24‒7.07 attributed to two protons each from the chloro-benzene ring. A multiplet resonated between 4.21‒4.05 was attributed to two protons from methylene attached to phthalimide ring. Methine proton was resonated between δ 3.91‒3.80 as multiplet. In the 13C NMR spectrum, peaks resonating at δ 167.89 were attributed to carbonyl carbon from the phthalimide ring. Peaks resonated at

108

δ 139.46, 134.84, 137.07, 132.59, 129.09, 128.43, 127.26 and δ123.41 corresponded to aromatic carbons. Peak at δ 51.24 attributed to methylene carbon bearing phthalimide ring. Methine carbon was resonated at δ 40.51 and allylic methyl carbon show peak at δ 19.08.

2-(2-(4-Chlorophenyl)-3-hydroxypropyl)isoindoline-1,3-dione (296)

1H NMR spectrum of the compound 296

13C NMR spectrum of the compound 296

109

The internal double bond was subjected to ozonolysis71 followed by reduction to get the alcohol 296. In the IR spectrum, peaks at 3345 and 1015 cm‒1 shows presence of hydroxy group. In the 1H NMR spectrum, aromatic carbons from phthalimide ring were resonated between δ 7.89‒7.75 and 7.75‒7.62 attributed to two protons each while aromatic protons from chloro benzene ring were resonated between δ 7.41‒7.24 and 7.24‒7.15. A broad multiplet resonating between δ 3.40‒3.26 was attributed to four protons, from two methylene carbons bearing phthalimide and hydroxy group. A Multiplet between δ 3.40‒3.26 corresponded to one methine proton. In the 13C NMR spectrum, peak at δ 168.92 was attributed to carbonyl carbon from phthalimide ring. Methylene carbon bearing hydroxy group was resonated at δ 49.98, and methylene carbon carrying phthalimide ring was resonated at δ41.93. Methine carbon was resonated at δ 31.82.

2-(3-Bromo-2-(4-chlorophenyl)propyl)isoindoline-1,3-dione (297)

1H NMR spectrum of the compound 297

Alcohol 296 was subjected to Appel reaction 62,117 to get bromide 297. In the 1H NMR spectrum, peak resonating between δ 4.34‒4.25 corresponded to one proton from the methylene bearing phthalimide ring. A multiplet at δ 4.25‒4.01 was attributed to total three protons, containing one methine proton, one methylene protons neighbouring to phthalimide and one methylene proton neighbouring to bromide. A multiplet between δ 3.98‒3.88 corresponded to one of the methylene proton adjacent to bromide.In the 13C

110

NMR spectrum, methylene carbon bearing phthalimide ring resonated at δ 50.00, methine carbon was resonated at δ 42.33, and methine carbon was resonated at δ 31.92.

13C NMR spectrum of the compound 297

Dimethyl(2-(4-chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propyl)phosphonate (298)

1H NMR spectrum of the compound 298

111

Bromo compound 297 was treated with trimethyl phosphite to get the phosphonate 298.101, 108, 111 In the 1H NMR spectrum, multiplet resonating between δ 3.84‒3.43 attributed to eight protons, containing six methoxy protons and two methylene protons neighbouring to phthalimide ring. Methine proton was resonated between δ 3.32‒3.16, and two methylene protons neighbouring to phosphonate resonated separately between δ 2.43‒2.25, and δ 2.12‒1.96. In the 13C NMR spectrum, peak at δ 168.06 corresponded to carbonyl carbon from phthalimide ring. Aromatic carbons resonated at δ 141.07, 134.22, 133.60, 129.01, 128.43 and 124.40. Two close signals at δ 51.49 and 51.27 attributed to two methoxy carbons and one methylene carbon bearing phthalimide ring. A peak at δ 41.39 attributed to methylene carbon carrying phosphate while peak at δ 37.07 corresponded to methine carbon.

13C NMR spectrum of the compound 298

(2-(4-Chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propyl)phosphonic acid (51) Phthalimide 298 was deprotected to corresponding amine 51 using hydrazine hydrates to get amino phosphate which then was treated with HCl to produce amino phosphonic acid, Phaclofen (51) as white solid compound.

112

1H NMR spectrum of the compound 51

IR spectrum shows peaks at 1390 cm-1 shows presence of the phosphonic group. In the 1H NMR Spectrum, a multiplet resonating between δ 7.62‒7.40 attributed to four aromatic protons from the benzene ring. A Multiplet resonating between δ 3.87‒3.56 was attributed to three protons containing one methine proton and two methylene protons neighbouring to amine functionality. A peak resonating between δ 2.71‒2.52 attributed to two methylene protons adjacent to phosphonic acid group.

2-(4-Chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propane-1-sulfonic acid (299)

13C NMR spectrum of the compound 299

113

Bromo phthalimide 297 was treated with sodium sulfite to give corresponding sulphoic acid 299 as solid compound. IR spectrum shows peaks at 1652, 1463, 1380, 1156 cm-1. In the 1H NMR spectrum multiplet resonating between δ 7.76‒7.41 was attributed to two aromatic protons meta to the side chain and multiplet between δ 7.39‒7.09 was attributed to ortho protons with respective to side chain. A multiplet resonating between δ 4.19‒3.75 was attributed to four methylene protons neighbouring to phthalimide and acid functionality. A peak resonated between δ 2.96‒2.76 corresponds to methine proton.

3-Amino-2-(4-chlorophenyl)propane-1-sulfonic acid (52) Phthalimide 299 was deprotected by hydrazine hydrates to give amino sulphoic acid 52. The 1H NMR spectrum shows only two multiplets. Multiplet resonating between δ 7.52‒7.02 was attributed to four aromatic protons and multiplet resonating between δ 3.96‒3.12 corresponded to four methylene protons and one methine proton.

1H NMR spectrum of the compound 52

In the 13C NMR spectrum aromatic protons resonated at δ 140.00, 131.66, 128.85 and 128.41. Methine carbon was resonated at δ 45.35. Methylene carbon bearing acid functionality resonated at δ 53.83 and methylene carbon neighbouring to amine resonated at δ 43.51.

114

13C NMR spectrum of the compound 52

115

1.4.4 Experimental section (E)-2-(2-(4-Chlorophenyl)pent-3-en-1-yl)isoindoline-1,3-dione (295) Phthalimide 206 (1g) in dry ethanol was added drop wise in the solution containing Ru-catalyst (10 mol%) kept at 80 oC under argon atmosphere and refluxed for 3-4 h. After the completion of time the reaction mixture was cooled to room temperature and filtered off. Reaction mixture was concentrated under reduced pressure and used for further reaction without any purification (0.9g, 90%).

IR (Neat, cm‒1): 2970, 1635, 1460, 930, 820. 1 H NMR (400 MHz, CDCl3): δ 7.90‒7.75 (m, 2H), 7.75‒7.62 (m, 2H), 7.40‒7.24 (m, 2H), 7.24‒7.07 (m, 2H), 5.91‒5.78 (m, 2H), 4.21‒4.05 (m, 2H), 3.91‒3.80 (m, 1H), 2.13 (d, J = 5.3 13 Hz, 3H); C NMR (100 MHz, CDCl3): δ 167.89, 139.46, 134.84, 137.07, 132.59, 129.09, 128.43, 127.26, 123.41, 51.24, 40.51, 19.08.

2-(2-(4-Chlorophenyl)-3-hydroxypropyl)isoindoline-1,3-dione (296) Ozone gas was passed through a solution of phthalimide 295 (0.9 g) dissolved in the dry DCM, at -78 oC for 15-20 minutes. After completion of reaction (checked by TLC), DMS was added in the reaction mixture to break the ozonoid into corresponding aldehyde and solution was stirred for further 1h. The solvent was removed under reduced pressure to get aldehyde as an intermediate. Aldehyde was dissolved in ethanol and sodium borohydride was added at 0 oC. After completion of reaction, ethanol was removed under reduced pressure whand extracted with ethyl acetate (3x10 ml). Combined organic layer was dried over sodium sulphate and concentrated under reduced pressure to get crude product which was further purified by column chromatography using ethyl acetate:pet ether (5:95) as mobile phase to give alcohol 296 as colourless viscous liquid (0.55g, 65% ).

IR (Neat, cm‒1): 3345, 1645, 1486, 1015, 912, 836. 1 H NMR (400 MHz, CDCl3): δ 7.89‒7.75 (m, 2H), 7.75‒7.62 (m, 2H), 7.41‒7.24 (m, 2H), 7.24‒7.15 (m, 2H), 3.95‒3.72 (m, 13 4H), 3.40‒3.26 (m, 1H); C NMR (100 MHz, CDCl3): δ 168.92, 140.10, 132.92, 131.10, 129.43, 138.87, 123.71, 49.98, 41.93, and 31.82.

116

2-(3-Bromo-2-(4-chlorophenyl)propyl)isoindoline-1,3-dione (297) Carbontetrabromide (0.6 g, 1.89 mmol) was added in the reaction mixture of alcohol 296 (0.5g, 1.58 mmol) and triphenyl phosphine (0.5g, 1.89 mmol) in chloroform at 0 oC. After completion of reaction (TLC chexk), water was added and extracted with chloroform (3 x 10 ml). Combined organic layer was dried over sodium sulphate and concentrated under reduced pressure. Crude product obtained was purified by column chromatography using ethyl acetate:pet ether (2:98) as mobile phase to give bromo compound (0.43g, 72%).

IR (Neat, cm‒1): 1650, 912, 836. 1 H NMR (400 MHz, CDCl3): δ 7.78‒7.61, 7.36‒7.21, 7.20‒7.06, 4.34‒4.25, 4.25‒4.01, 3.98‒3.88; 13C NMR (100

MHz, CDCl3): δ 168.92, 140.02, 133.02, 130.22, 129.36, 128.43, 123.84, 50.00, 42.33, 31.92.

Dimethyl (2-(4-chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propyl)phosphonate (298) Trimethyl phosphite (0.2g, 1.58mmol) was added drop wise to a solution of bromide 297 (0.3g, 0.793 mmol) in toluene at 120 oC and stirred at the same temperature for 3h. After completion of reaction, excess solvent containing trimethyl phosphite was removed in vacuo. Flash chromatography of the residue on silica gel (EtOAc/hexane 1:1) gives pure phosphonate (0.194 g, 60%).

IR (Neat, cm‒1): 1658, 1456, 930, 856. 1 H NMR (400 MHz, CDCl3): δ 7.89‒7.69, 7.46‒7.27, 7.26‒7.12, 3.84‒3.43, 3.32‒3.16, 2.43‒2.25, 13 2.12‒1.96; C NMR (100 MHz, CDCl3): δ 168.06, 141.07, 134.22, 133.60, 129.01, 128.43, 124.40, 51.49, 51.27, 41.39, 37.07.

(2-(4-Chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propyl)phosphonic acid (51) Phosphonate 298 (0.150 g) was dissolved in ethanol and hydrazine hydrate was added. The solution was refluxed for 2-3h still the completion of reaction. Solvent was removed under reduced pressure and 10M HCl was added and resulting solution was refluxed for 2h to get the amino phosphonic acid as white solid compound (0.61 g, 67%).

117

IR (Neat, cm‒1): 1660, 1510, 1390. 1 H NMR (400 MHz, CDCl3): δ 7.62‒7.40, 3.87‒3.56, 2.71‒2.52.

2-(4-Chlorophenyl)-3-(1,3-dioxoisoindolin-2-yl)propane-1-sulfonic acid (299) A solution of the bromide 297 (0.3g, 10 mmol) and sodium sulﬁte (20 mmol) in water (5 ml) and ethanol (5 ml) was reﬂuxed for 16h. The solution was concentrated under reduced pressure and extracted with ethyl acetate. The combined organic phase was concentrated under reduced pressure and further purification was done by recrystallization to get white solid (0.165 g, 55%)

IR (Neat, cm‒1): 1652, 1463, 1380, 1156, 963, 854. 1 H NMR (400 MHz, CDCl3): δ 7.76‒7.41, 7.39‒7.09, 4.19‒3.75, 2.96‒2.76.

3-Amino-2-(4-chlorophenyl)propane-1-sulfonic acid (52) Phthalimide 299 (0.150 g) was dissolved in ethanol and hydrazine hydrate was added. The solution was refluxed for 2-3 hours still the completion of reaction. The reaction mixture was concentrated and the residue was redissolved in water (5 ml). The aqueous phase was extracted with ethyl acetate (4x 5 ml) to remove any unreacted (299), then acidified with concentrated hydrochloric acid and washed with ethyl acetate (4x5 ml). The aqueous fraction was concentrated to a white solid, which was extracted with boiling ethanol (4x15 ml), which gives the pure hydrochloride of 3-amino-2-(4- chlorophenyl)propanesulfonic acid (52) (0.059 g, 60%).

IR (Neat, cm‒1): 1674, 1442, 1370, 1160, 987, 831. 1 H NMR (400 MHz, CDCl3): δ 7.52‒7.02, 3.96‒3.12; 13 C NMR (100 MHz, CDCl3): δ 140.00, 131.66, 128.85, 128.41, 53.83, 45.35, 43.51.

118

1.4.5 References 1. Jorgensen, E. M. GABA (August 31, 2005), WormBook, doi/10.1895/ wormbook.1.14.1 2. Gowravaram S., Padmaja P., P. N. Reddy, Surender S. Jadav, J. S. Yadav, Tetrahedron Letters, 2009, 50, 6166–6168 3. (a) Satoshi S., Nobuko S., Shizuo Y., T., Ryohei K.,and Akira M., Chem. Pharm. Bull. 1999, 47(8), 1188-1192; (b) Fernando C., Mariangela B. M. de Azevedo, Roberta B., Patrícia R., Synthetic Communications, 1997, 27(14), 2455-2465 4. W. Sieghart, Pharmacol. Rev., 1995, 47, 181. 5. Krogsgaard-Larsen, P.; Honore, T.; Hansen, J. J.; Curtis, D. R.; Lodge, D. R. Nature, 1980, 284, 64. 6. Bräuner-Osborne, H.; Nielsen, B.; Krogsgaard-Larsen, P. Eur. J. Pharmacol., 1998, 350, 311. 7. (a) Krogsgaard-Larsen, P.; Falch, E.; Hjeds, H. Prog. Med. Chem., 1985, 22, 67; (b) Krogsgaard-Larsen, P.; Hjeds, H,; Falch, E.; JØrgensen F. S.; Nielsen, L. Adv. Drug Res., 1988, 17, 381. 8. Curits, D. R.; Duggan, A. W.; Felix, D.; Johnston, G. A. Nature, 1970, 226, 1222. 9. Johnston, A. R.; Beart, P. M..; Curtis, D. R..; Game, C. J. A.; McCulloch, R. M.; Maclachlan, R. M. Nature, 1972, 240, 219. 10. Rognan, D.; Boulanger, T.; Hoffmann, R.; Vercauteren, D. P.; Andre, J. M.; Durant, F.; Wermuth, C. G. J. Med. Chem., 1992, 35, 1969. 11. (i) Chambon, J. P.; Feltz, P.; Heaulme, M.; Restle, S.; Schlichter, R.; Biziere, K.; Wermuth, C. G. Proc. Natl. Acad. Sci. USA, 1985, 82, 1832; (ii) Allan, R. D.; Apostopoulos, C.; Richardson, J. A. Aust. J. Chem., 1990, 43, 1767; (iii) Melikian, A.; Schlewer, G.; Chambon, J. P.; Wermuth, C. G. J. Med. Chem., 1992, 35, 4092. 12. Feldman, R. S.; Meyer, J. S.; Quenzer, L. F. Sinauer Associates Inc., 1997, pp. 417. 13. Krogsgaard-Larsen, P.; Johnston, G. A. R.; Lodge, D.; Curtis, D. R. Nature, 1977, 268, 53. 14. Braestrup, C.; Nielsen, M.; Krogsgaard-Larsen, P.; Falch, E. Nature, 1979, 280, 331. 15. Krogsgaard-Larsen, P.; Snowman, A.; Lummis, S. C.; Olsen, R. W. J. Neurochem., 1981, 37, 401.

119

16. Byberg, J. R.; Labouta, I. M.; Falch, E.; Hjeds, H.; Krogsgaard-Larsen, P.; Curtis, D. R.; Gynther, B. D. Drug Des. Discov., 1987, 1, 261. 17. FrǾlund, B.; Kristiansen, U.; Brehm, L.; Hansen, Z. B.; Krogsgaard-Larsen, P.; Falch, E., J. Med. Chem., 1995, 38, 3287. 18. FrØlund, B.; JØrgensen, A. T.; Tagmose, L.; StensbØl, T. B.; Vestergaard, H. T.; Engbolm, C.; Kristiansen, U.; Sanchez, C.; Krogsgaard-Larsen, P.; Liljefors, T. J. Med. Chem., 2002, 45, 2454. 19. Johnston, G. A. R. Pharmacol. Ther., 1996, 173. 20. Zhang, D.; Pan, Z. –H.; Awobuluyi, M.; Lipton, S. A. Trends Pharmacol. Sci. 2001, 22, 121. 21. Johnston, G. A. R. “GABAC Receptors: Molecular Biology, Pharmacology and Physiology”, in The GABA Receptors, Enna, S. J.; Bowery, N. G. (Eds.), Humana, Clifton, NJ, 1997, pp. 297. 22. Qian, H.; Dowling, J. E. J. Neurosci., 1994, 14, 4299. 23. Ragozzino, D.; Woodward, R. M.; Murata, Y.; Eusebi, F.; Overman, L. E.; Miledi, R. Mol. Pharmacol., 1996, 50, 1024. 24. Lodish, H.; Berk, A.; Zipursky, S. L.; Matsudaira, P; Baltimore, D.; Darnell, J. In Molecular Cell Biology, Freeman w. H.; Company, New York, 2000, pp. 848. 25. Bräuner-Osborne, H.; Krogsgaard-Larsen, P. Genomics, 2000, 65, 121. 26. Chen, K.; Li, H. Z.; Ye, N.; Zhang, J.; Wang, J. J. Brain Res Bull 2005, 67, 310. 27. Knight, A. R.; Bowery, N. G. Neuropharmacol., 1996, 35, 703. 28. Bowery, N. G.; Bettler, B.; Froestl W.; Gallagher, J. P.; Marshall, F.; Raiteri, M.; Bonner, T. I.; Enna, S. J. Pharmacol. Rev., 2002, 54, 247. 29. Berthelot, P.; Vaccher, C.; Musadad, A.; Flouquet, N.; Debaert, M.; Luyckx, M. J. Med. Chem., 1987, 30, 743. 30. (a) Mann, A.; Boulanger, T.; Brandau, B.; Durant, F.; Evrard, G.; Heaulme, M.; Desaulles, E.; Wermuth, C. –G. J. Med. Chem., 1991, 34, 1307; (b) Shuto, S..; Shibuya, N.; Yamada, S.; Ohkura, T.; Kimura, R.; Matsuda, A. Chem. Pham. Bull., 1999, 47, 1188. 31. Abbenante, D.; Hughes, R.; Prager, R. H. Aust. J. Chem., 1994, 47, 1441. 32. Kristiansen, U.; Fjalland, B. Pharmacol. Toxicol., 1991, 68, 332. 33. Biochemical and Biophysical Research Communications 452 (2014) 649–654 34. Grigory V., Maksim V., Aleksandrs C., Ivars K., Canadian Journal of Chemistry, 1994, 72, 2312-2317

120

35. Krogsgaard-Larsen, P. “GABA Receptors”, in Receptor Pharmacology and Function, vol. 13 of Clinical Pharmacology, Williams, M.; Glennon, R. A.; P. B. M. W. M. Timmermans (Eds.), Marcel Dekker Inc., New York and Basel, 1989, pp. 349. 36. (a) Wong, C. G. T.; Bottiglieri, T.; Snead III, O. C. Ann. Neurol. 2003, 54, S3; (b) Simpson, M. D. C.; Slater, P.; Deakin, J. F. W. Biol. Psychiatry 1998, 44, 423; (c) Pearl, P. L.; Hartka, T. R.; Cabalza, J. L.; Taylor, J.; Gibson, M. K. Future Neurol. 2006, 1, 631; (d) Goka, V. N.; Schlewer, G.; Linget, J. M.; Chambon, J. P.; Wermuth, C. G. J. Med. Chem. 1991, 34, 2547; e) Fulvia F., Valentina G., Giuliana P., Ennio V., Tetrahedron: Asymmetry, 2005, 16, 1341–134; f) Gajcy, K.; Lochynski, S.; Librowski, T. Curr.Med.Chem.2010,17, 2338. 37. Antonio L. P., Pilar G. G. and Avelino C., Angew. Chem. Int. Ed., 2014, 53, 8687–8690 38. Satoshi S., Nobuko S., Shizuo Y., takashi O., Ryohei K., and Akira M., Chem. Pharm.Bull., 1999, 47(8), 1188-1192 39. Robertc N. and Micheld E., Can. J. Chem., 1994, 72, 2312 40. (a) Bowery, N. G.; Hill, D . R.; Hudson, A. L.; Doble, A.; Middemiss, N. D.; Shaw, J. Turnbull, M, Nature, 1980, 283, 92; (b) Silverman, R. B.; Levy, M. A. J. Biol Chem, 1981, 256, 1565; (c) Mann, A.;, Boulanger, T.; Brandau, B.; Durant, F.; Evrard, G.; Haeulme, M.; Desaulles, E.; Wermuth, C. G. J Med. Chem, 1991, 34, 1307 41. (a) Pier, F. K.; Zimmerrman, P.; Brain Res, 1973, 54, 376; (b) Polc, P.; Haefely, W. Naunyn Schmiedbergs Arch Pharmacol, 1976, 294, 121 42. Fromm, G. H.; Terrence, C. F.; Chaftha, H. S.; Glass, J. D. Arch Neural, 1980, 37, 768. 43. Sachais, B. A.; Logue, J. N. Arc Neural, 1977, 34, 422. 44. Thakur V.V., Paraskar A. S., Sudalai A., Indian Journal of Chemistry-Section B, 2007, 46B(2), 326-330 45. Mohamed I. A., Claus H., Hans B. O., Molecules, 2013, 18, 10266-10284 46. Shuto S., Shibuya N., Yamada S., Ohkura T., Kimura R., Matsuda A., Chem. Pharm. Bull., 1999, 47(8) 1188—1192. 47. Lei Ji, Yuheng Ma, Jin Li, Liangren Zhang, Lihe Zhang, Tetrahedron Letters, 2009, 50, 6166–6168 48. Coelho, F.; Mariangela B.; De Azevedo, M.; Boschiero, R.; Resende, P.; Syn. Commun. 1997, 27, 2455.

121

49. Alcindo, A.; Santos, d.; Giuliano, C.; Simonelli, C. F.; Alfredo, R. M.; Oliveira, D.; Francisco, D.; Marques, A.; Paulo, H.; Zarbin, G. J. Braz. Chem. Soc., 2001, 12, 673. 50. Marcos, J.; Carpes, S.; Carlos, R.; Correia, D. Tetrahedron Lett. 2002, 43, 741. 51. Chang, M. Y.; Sun, P. P.; Cgena, S. T.; Changb, N. C. Tetrahedron Lett. 2003, 44, 5271. 52. Mohammad, H.; Houshdar, T.; Morteza, F.; Massoud, S.; Nazer, T. Iranian Journal of Pharmaceutical Research 2003, 1. 53. Chen Z.; Chen, Z.; Jiang, Y.; Hu, W. Tetrahedron 2005, 61, 1579. 54. Varala, R.; Adapa, S. R. Syn. Commun., 2006, 36, 3743. 55. Chang, M. –Y.; Paib, C. –L.; Kunga, Y. –H. Tetrahedron Lett. 2006, 47, 855. 56. Kato, K.; Zhang, M.; -R.; Suzuki, K. Bioorg. Med. Chem. Lett. 2009, 19, 6222. 57. Biswas, K.; Gholap, R.; Srinivas, P.; Kanyal, S.; Sarma, K. D. RSC Adv., 2014, 4, 2538. 58. Fabricio F. Naciuk, D. Z.; Vargas, C.; D’Oca, R. M.; Celso, C. M.; Dennis, R. New J. Chem., 2015, 39, 1643. 59. Claudio, M.; Jacques, L.; Alphand, V. –R.; Roland F. Tetrahedron Lett. 1997, 38, 1195. 60. Thakur, V. V.; Nikalje, M. D.; Sudalai, A. Tetrahedron Asymm. 2003, 14, 581. 61. Wang, Y.; Li, P.; Liang, X.; Zhangc, T. Y.; Ye, J. Chem. Commun., 2008, 1232. 62. Yang, X. -F.; Ding, C. H.; Li, X. H.; Huang, J. Q. Hou, X. L.; Dai, L. X.; Wang, P. J., J. Org. Chem. 2012, 77, 8980. 63. Lokhande, M. N.; Nikalge, M. D., IJCPS, 2015, 4, 487. 64. Kozo, S.; Toshikazu, B. J. Mol. Cat. B: Enzymatic 1998, 5, 183. 65. Vesely, J.; Zhao, G. –L.; Bartoszewicz, A.; Córdova, A. Tetrahedron Lett. 2008, 49, 4209. 66. Fryszkowska, A.; Fisher, K.; Gardinera, J. M.; Stephens, G. M. Org. Biomol. Chem., 2010, 8, 533. 67. Bae, H. Y.; Some, S.; Lee, J. H.; Kim, J. –Y.; Song, M. J.; Lee, S.; Zhang, Y. J.; Songa, C. E. Adv. Synth. Catal. 2011, 353, 3196. 68. Han, F.; Chen, J.; Zhang, X.; Liu, J.; Cun, L.; Zhu, J.; Deng, J.; Liao, J. Tetrahedron Lett. 2011, 52, 830. 69. (a) Kulkarni, M. G.; Davawala S. I.; Shinde, M. P.; Dhondge, A. P.; Borhade, A. S.; Chavhan, S. W.; Gaikwad, D. D. Tetrahedron Lett. 2006, 47, 3027; (b) Kulkarni, M. G.; Davawala, S. I.; Doke, A. K.; Pendharkar, D. S. Synthesis, 2004,

122

2919; (c) Kulkarni, M. G.; Davawala S. I.; Dhondge, A. P.; Borhade, A. S.; Chavhan, S. W.; Gaikwad, D. D. Tetrahedron Lett. 2006, 47, 1003; (d) Kulkarni, M. G.; Dhondge, A. P.; Borhade, A. S.; Gaikwad, D. D.; Chavhan, S. W.; Shaikh, Y. B.; Ningdale, V. B.; Desai, M. P.; Birhade, D. R.; Shinde, M. P. Tetrahedron Lett. 2009, 50, 2411; (e) Kulkarni, M. G.; Gaikwad, D. D.; Borhade, A. S.; Shaikh, Y. B.; Ningdale, V. B.; Chavhan, S. W.; Dhondge, A. P.; Desai, M. P.; Birhade, D. R. Synth. Commun. 2010, 40, 423. 70. (a) Ahmed Kamal, Saidi Reddy Vangala, N. V. Subba Reddy, V. Santhosh Reddy, Tetrahedron: Asymmtry, 2009, 20, 2589-2593; (b) Kalamkar N. B., Kasture V. M. And Dhavale D. D., Journal of Organic Chemistry, 2008, 73, 3619-3622; (c) Schelkun R. M., Yuen P., Wustrow, Kinsora J., Su Ti-Zhi, Vartanian M. G., Bioorganic & Medicinal Chemistry Letters, 2006, 16, 2329-2332; (d) Taedong O., Aram J., Joohee L., Jung H. L., Chang S. H., and Hee-S. L., Journal of Organic Chemistry, 2007, 72, 7390-7393. 71. (a) Yan Ma and George Marston, Phys. Chem. Chem. Phys., 2009, 11, 4198– 4209; (b) Peter Hannen, Harald Haeger, Martin Roos, United States Patent, Patent No.:US 8,703,993 B2, Apr. 22, 2014; (c) (e) Kulkarni, M. G.; Dhondge, A. P.; Borhade, A. S.; Gaikwad, D. D.; Chavhan, S. W.; Shaikh, Y. B.; Ningdale, V. B.; Desai, M. P.; Birhade, D. R.; Shinde, M. Eur. J. Org. Chem. 2009, 23, 3875. 72. Mohamed I. Attia, Claus Herdeis and Hans Bräuner-Osborne, Molecules 2013, 18, 10266. 73. Karla, R.; Ebert, B.; Thorkildsen, C.; Herdeis, C.; Johansen, T.N.; Nielsen, B.; Krogsgaard-Larsen, P. J. Med. Chem. 1999, 42, 2053. 74. Anders A. Jensen, Bo E. Madsen, Povl Krogsgaard-Larsen, Hans Brauner- Osborne, Journal of Pharmacology 2001, 417, 177. 75. Rolf, K.; Bjarke, E.; Christian, T.; Claus, H,; Tommy, N.; Johansen, B.; Krogsgaard-Larsen, P. J. Med. Chem. 1999, 42, 2053. 76. Carpes, M. J.; Carlos, R. D. Correia, Tetrahedron Lett. 2002, 43, 741. 77. Chang, M. –Y.; Chan, S. –T. Heterocycles 2003, 60, 99 78. Simonelli, F.; Francisco D.; Marques, A.; Wisniewski, A. Jr.; Wendler, E. P. Tetrahedron Lett. 2004, 45, 8099. 79. He, Z. –T.; Wei, Y. –B.; Yu, H. –J. Sun, C. –Y. Feng, C. –G.; Tian, P.; Lin, G. – Q.; Tetrahedron 2012, 68, 9186. 80. Attia, M. I.; Herdeis, C.; Bräuner-Osborne, H. Molecules 2013, 18, 10266. 81. Roman D., Karl G. U., J. Org. Chem., 1981, 46, 2985-2987

123

82. Herbert C. Brown and Nung Min Yoon, Journal of the American Chemical Society, 1966, 88:7, 1464-1472; (b) Robert F. N. and Weldon G. B., Contribution from the George Herbert Jones T, Aboratory, The University of Chicago, April 30, 1948. 83. Mitsunobu O., Wada, M., Sano, T., J. Am. Chem. Soc. 1972, 94, 679 84. (a) Peter James Smith, David AJ Crowe, Kenneth Charles Westaway, Canadian Journal of Chemistry, 2001, 79(7): 1145-1152. 85. Billman, J. H.; Parker, E. E.; J.Am.Chem.Soc. 1944, 66, 538. 86. (a) Miguel A. Gonzalez, David Perez-Guaita, Tetrahedron, 2012, 68, 9612-9615; (b) Peter R. Hewitt, Ed Cleator and Steven V. Ley, Org. Biomol. Chem., 2004, 2, 2415– 2417; (c) Marugan J. J., Leonard K., Raboisson P., Gushue J. M., Calvo R., Koblish H. K., Lattanze J., Zha S., Cummings M. D., Player M. R, Schubert C., Maroney A. C. and Lu T,, Bioorganic & Medicinal Chemistry Letters, 2006, 16, 3115–3120 87. Marcos Jose´ S. Carpes and Carlos Roque D. Correia, Tetrahedron Letters 43 (2002) 741–744 88. (a) Chapman, R. W.; Danko, G.; DelPrado, M..; ,Egan, R. W.; Kreutner, W.; Rizzo, C.; Hey, J. A. Phai'macotogy 1993, 46, 315.; (b) Chapman, R. W.; Hey, J. A.; Rizzo, C. A.; Bolser, D. C. Trends PitarmacoLSci. 1993, 14, 26.; (c) Belvisi, M. G.; Ichinose, M.; Barnes, P. J. J. Pharmacol., 1989, 97, 1225. 89. Yamasaki, K.; Goto, Y.; Hara, N.; Hara, Y. J. Pharmacology 1991, 55, 11. 90. (a) Goto Y, Watanabe K: In Drugs and Peptic Ulcers. CJ Pfeiffer (Ed). New York, CRC Press, 1982, pp 203.; a) Tsai LH, Taniyama K, Tanaka C: Am. J. Physiol 1987, 253, G601.; b) Thirlby, R. C.; Stevens, M. H.; Blair, A. J.; Petty, F.; Crawford, I. L.; Taylor, I. L.; Walsh, J. H.; Feldman, M. Am. J. Physiol 1988, 254, G723. 91. Goto, Y.; Tache, Y.; Debas, H. T.; Novin, D. Life Sci. 1985, 36, 2471. 92. Andrews, P. L. K.; Wood, K. L. J. Pharmacol 1986, 89, 461. 93. Zarrindast, M. R.; Oveissi, Y. Gen Pharmacol. 1988, 19, 223. 94. Koji, T.; Hideyuki, N.; Hiromichi, N.; Susumu, O. Digestive Diseases and Sciences, 1990, 35, 458. 95. Wall, G. M.; Baker, J. K. J. Med. Chem. 1989, 32, 1340. 96. Shigeyuki, Y.; Mamoru, K. Chem. Pharm. BulL., 1995, 43, 1302. 97. Prager, R. H.; Schafer, K. Aust. J. Chem., 1997, 50, 813.

124

98. Czernecki S.; Georgoulis C.; Provelenghiou C.; Fusey G., Tetrahedron Lett. 1976, 17, 3535. 99. Jing Li and Todd L. Lowary, Med. Chem. Commun., 2014, 5, 1130-1137. 100. Kerr, D. I. B.; Ong, J.; Prager, R. H.; Gynther, B. D.; Curtis, D. R. Res. 1987,405, 150. 101. Robinson, T. N.; Cross, A. J.; Green, A. R.; Toczek, J. M.; Boar, B. R. J. Pharmacol.,1989, 98, 833. 102. Kerr, D. I. B., Ong, J.; Prager, R. H., 1st Int. GABAB Symp., Cambridge, 1989. 103. David, I. B.; Kerr, J. O.; Rolf, H. P. J. PharmacoL 1990, 422. 104. Howson, W.; Hill; S, J. M. Bioorg. Med. Chem. Lett. 1991, 1, 501. 105. Frydenvang, K.; Hansen, J. J.; Povlkrogsgaard, L.; Mitrovic, A.; Tran, H.; Colleen, A. D.; Graham, A.; Johnston, R. Chirality 1994, 6, 583. 106. Abbenante, G.; Hughes, R.; Prager, R. H. Aust. J. Chem., 1997, 50, 523. 107. David, I. B.; Kerr, J. O.; Claude, V.; Pascal, B.; Nathalie, F.; Vaccher, M. –P.; Debaert, M. Eur. J. Pharmacology 1996, 308, R1. 108. Chiefari, J.; Galanopoulos, S.; Janowski,W. K.; Kerr, D. I. B.; Prager, R. H. Aust. J. Chem., 1987, 40, 1511. 109. Hall, R. G. Synthesis 1989, 6, 442. 110. Froestl,W.; Mickel, S. J.; Sprecher, G. V.; Diel, P. J.; Hall, R. G.; Maier, L.; Strub, D.; Melillo, V.; Baumann, P. A.; Bernasconi, R.; Gentsch, C.; Hauser, K.; Jaekel, J.; Karlsson, G.; Klebs, K.; Maitre, L.; Marescaux, C.; Pozza, M. F.; Schmutz, M.; Steinmann, M. W.; Riezen, H. V.; Vassout, A.; ondadori, C.; Olpe, H.; Waldmeier, P. C.; Bittiger; H. J. Med. Chem. 1995, 38, 3313. 111. Duan, Z. –C.; Hu, X. –P.; Zhang, C.; Zheng, Z. J. Org. Chem. 2010, 75, 8319. 112. Li, C. –S.; Howson, W.; Dolle, R. E. Synthesis 1990, 244. 113. Abbenante, G.; Prager,R. H. Aust. J. Chem., 1990, 43, 213. 114. Abbenante, G.; Prager, R. H. Aust. J. Chem., 1992, 45, 1801. 115. Prager, R. H.; Schafer, K.; Hamon, D. P. G.; Massy-Westropp, R. A. Tetrahedron 1995, 51, 11465. 116. (a) Sumeet K. Sharma, Vivek K. Srivastava, Raksh V. Jasra, Journal of Molecular Catalysis A: Chemical, 2006, 245, 200–209; (b) J. K. Stille and Y. Becker, J. Org. Chem. 1980,45, 2139-2145 117. (a) Rolf Appel, Angew. Chrm. internar. Edit., 1975, 14,(12), 801-811; (b) Travis W. B., John C. S. and Kenneth B. W., Tetrahedron, 2004, 60, 10943–10948; (c) Jose G. Calzada and John Hooz., Organic Syntheses, 1988, 6, 634; 1974, 54, 63.

125