Field of Invention :
The present invention relates to a high yielding, safe, efficient and industrially viable process for the synthesis of 3-[[(2S)-2-amino-3-methyl-l-oxobutyl] amino] 1-propanesulfonic acid (Formula /), also known as Valitramiprosate (ALZ-801).
Background of the Invention :
Valitramiprosate (ALZ-801; Formula-1), namely 3-[[(2S)-2-amino-3-methyl-l-oxobutyl] amino]-1-propanesulfonic acid, was developed by BHI Limited Partnership. Valitramiprosate is an oral active small-molecule inhibitor of beta amyloid (A(3) oligomer formation and is currently under Phase-Ill clinical trials for tlje treatment of Alzheimer's disease (AD). Valitramiprosate is a prodrug of tramiprosate with improved pharmacokinetic properties and gastrointestinal tolerability. During clinical studies, it was discovered that the primary metabolite of tramiprosate and its prodrug 3-Amino propane sulfonic acid (3-APS), is an endogenous molecule in the human brain and present in the cerebrospinal fluid (CSF) of patients with AD and other neurodegenerative brain diseases (CNS Drugs (2018), 32(9), 849-861).
Valitramiprosate and its synthesis was disclosed in US 8,748,656, (hereinafter '656 patent). The synthesis of Valitramiprosate disclosed in '656 patent involves making of BOC-L-valine N-hydroxysuccinimide ester (active ester) by reacting BOC-L-valine with N-hydroxysuccinimide (NHS) in presence of /VFA',A^A''-tetramethyl-0-(lA/-benzotriazol-]-yl-uroniurn-hexafiuorophosphate (HBTU, coupling agent). Amidation was earned out with BOC-L-valine N-hydroxysuccinimide ester and 3-amino propanesulfonic acid (3-APS) under basic conditions

followed by resin treatment, BOC deprotection and purification with EtOH and water mixture to give Valitramiprosate (Scheme-I).
Amidation is the key step in this prior art process. The process suffers from disadvantages of number of steps, use of activating agent N-hydroxy succinimide (NHS) and the coupling reagent HBTU, followed by column purification of the active ester by flash chromatography to remove the liberated urea derivatives and unreacted coupling reagent. These steps are not suitable for large scale production. The high cost of activating and coupling reagents increases the process cost in addition to slow column purification. Use of HBTU and NHS also leads to formation of unwanted by-products like urea derivatives causing purification issues and
loading of effluents. Thus this process suffers from drawbacks of number of stages and safety and environmental concerns.
Summary of the Invention :
By considering the drawbacks in the prior art processes as discussed above, we were able to plan and develop an industrially viable process for the preparation of Valitramiprosate avoiding hazardous reagents and column purification, besides suppressing formation of undesirable impurities. The aimed objective could be achieved in a single step process.

N-Protected-L-valine-N-carboxy anhydride was reacted with 3-aminopropane sulfonic acid (3-APS) to obtain Vaiitramiprosate sodium (Formula-2), which is then converted to Valitramiprosate (Formula-1) by H+ resin treatment. The amidation reaction gives the desired product overall in good yield and purity. The protecting groups are selected to enable easy cleavage after the reaction.
The present process makes use of L-valine-N-carboxy anhydride (L-VALNCA; Formula-3A) or N-BOC-L-valine-N-carboxy anhydride (N-BOC-L-VALNCA, Formu!a-3B) and 3-APS; Formula-4) at the amidation step to prepare highly pure Valitramiprosate.
The present invention has the advantage of less number of stages and is free from formation of impurities compared to the prior art process.
Detailed Description of the Invention :
The present invention relates to the synthesis of Valitramiprosate as depicted in Scheme 11 below.

When trying to repeat the prior art processes for preparing Valitramiprosate, low yields and formation of unwanted impurities were observed when using N-hydroxy succinimide (NHS) and W,MiV',/^'-tetramethyl-0-(tH-benzotriazol-l-yl)uronium hexafluorophosphate (HBTU).
On the other hand it was found that the reaction of L-valine-N-carboxy anhydride (L-VALNCA, Formula-3A) or N-BOC- L-valine-N-carboxy anhydride (N-BOC-L-VALNCA, Formula-3B) with 3-aminopropane sulfonic acid (3-APS, Formula-4) at low temperatures under basic conditions in a medium of a mixture of 1,4-dioxane and water provides sodium salt of 3-[[(2S)-2-amino-3-methyl-1-oxobutyl] amino]-1-propanesulfonic acid (Formula 2). The sodium salt thus obtained was dissolved in water and passed through a strong H+ ion-exchange resin column. The strong acidic fraction was collected and was concentrated to dryness followed by purification to afford Valitramiprosate with high chemical and chiral purities of pharmaceutical grade in a facile manner.
This process avoids the formation of water-soluble byproducts like urea derivatives and unreacted N-hydroxy succinimide as described in the prior art process.
The protecting groups may be obtained by using reagents selected from DIBOC, methyl chloroformate, ethyl chloroformate, isobutyl chloroformate, benzyl chloroformate etc. The preferred protecting group is BOC. Amidation reaction also works equally well without any protecting group.

As per the present invention, amidation reaction needs low temperatures and basic conditions in a polar solvent or water or mixtures thereof. To our surprise and satisfaction, we found that use of 1,4-dioxane and water as medium among other solvents and at a temperature of about 0-5°C resulted not only uverall yield of
63-65% but also of purity of both chemical and chiral purities over 99% of product. Other solvents, particularly THF, acetone, acetonitrile, dimethylformamide, dichloromethane, chloroform, ethylacetate, isopropyl acetate, toluene-isopropylacetate mixtures, were also studied at same conditions but 1,4-dioxane was found to perform optimally.
Thus, one embodiment of the present invention consists of amidation of L-valine-N-carboxy anhydride (L-VALNCA) or N-BOC- L-valine-N-carboxy anhydride (N-BOC-L-VALNCA) and 3-aminopropane sulfonic acid (3-APS) either in a polar solvent such as 1,4-dioxane, Tetrahydrofuran, Dimethyl formamide, Acetone, Acetonitrile, Dimethyl acetamide etc. or water medium or in a mixture of 1,4-dioxane and water.
The reaction is generally over in about a couple of hours.
Efforts were made to study the effects of pH and the base needed for the reaction. No specific advantage could be established with the bases studied, namely NaOH,

KOH, LiOH, hydroxides of barium, magnesium or calcium as also some organic bases like pyridine, di-isopropylamine and triethylamine. Sodium hydroxide (NaOH) was found to yield satisfactory results. A pH of 10.5 to 11.0 was found to be quite satisfactory. Higher pH in the reaction medium was causing reduction in yield and quality of product.
Thus, the present invention consists of amidation of L-valine-N-carboxy anhydride (L-VALNCA) or N-BOC- L-valine-N-carboxy anhydride (N-BOC-L-VALNCA) and 3-aminopropane sulfonic acid (3-APS) in 1,4-dioxane and water-mixture (35:65) at a pH of about 10.5-11.0, using a base such as NaOH and conducting the reaction for a period of about one hour at about 0°C.
, The following detailed examples describe the preparation of valitramiprosate and are to be construed as illustrative and not limitations of the preceding disclosure in any way. The starting materials, L-valine-N-carboxy anhydride was prepared by known prior art method US 8,394,81362 , N-BOC-L-valine-N-carboxy anhydride was prepared as per FR2858976A1 and 3-aminopropane sulfonic acid (3-APS) was prepared by known prior art methods, Industrial and Engineering Chemistry Vol. 56 (3) 41-45 (1964) and US 7253306B2.

Examples:
Chemical purity was determined using HPLC under the following conditions:
Column: YMC ODS AQ, 250 x 4.6mm, 5um. Mobile phase: Water and Acetonitrile; Flow rate: 1.0 mL/min; Column temperature: 30°C and detection: 205nm. Peak eluted linear gradient.
Chiral purity was determined using HPLC under the following conditions:
Column: INTERSIL ODS-2, 250 x 4.6 mm, 5 urn.
Mobile phase: Buffer (72%) and Acetonitrile (28%), Flow rate: 2.0 mL/min.
Column temperature: 27°C; detection: 340nm.
Example-1: Preparation of 3-|(2S)-2-amino-3-methyl-l-oxobutyl|arnino|- 1-propanesulfonic acid \ Valitramiprosate (ALZ-801; Formula-1)] :
To a solution of 3-aminopropane sulfonic acid (3-APS) (24.3 gm, 0.17 mol) in water (150.0 mL) whose pH was adjusted to 10.5-11.0 with 2M NaOH solution, was added a solution of L-valine-N-carboxy anhydride (L-VALNCA) (25.0 gm, 0.17 mol) in 200.0 mL of 1,4-dioxane dropwise during 1 hrat0-5°C while maintaining the pH of the reaction mass at 10.5-11.0 with simultaneous addition of 2M NaOH solution. The reaction mixture was stirred for 2 to 3 hr at 0-5°C. The reaction was monitored by TLC. After reaction is completed, reaction mass was washed with methyl tert butyl ether (1 x 150 mL). The aqueous layer was distilled off completely. Off white solid material obtained as sodium salt containing 93-95% of Valitramiprosate (HPLC) was not isolated but immediately converted to Valitramiprosate by an ion-exchange resin.
The solid material was dissolved in water (50 mL). The solution was passed through an ion-exchange column of length 2 ft and 2 inch dia (Indion-252H resin 500 gm, strongly acidic) during 30 min. The strong acidic fractions were combined and then were concentrated to dryness. The residual material was co-evaporated with ethanol to remove water completely. The solid material was dissolved in water (100 mL) at 80-85°C and EtOH (100 mL) was added drop wise. After allowing to ambient temperature the product slowly crystallized. The suspension was stirred at room temperature for 48hrs. The solid material was collected by filtration and dried in a vacuum

oven at 60° C to provide 27.0 gm (65%) of a white solid substance containing 99.85% of chemical purity and 99.98% chiral purity of Vali tram i pros ate (HPLC).
'HNMR(D20)5: 1.03-1.06 (m, 6H), 194-2.04 (m, 2H), 2 .16-2025-2 .09 (m, 1H), 2 .93-2.98 (m, 2H), 3 .38-3.43 (m, 2H), 3.75-3.77 (d, 2H).
13CNMR (D20) 6: 17.09, 17.65, 23.98, 29.83, 38.15, 48.51, 58.77, 169.15. m/z: 237 (M-l)
Example-2: Preparation of 3-[(2S)-2-amino-3-methyl-l-oxobutyl|amino|- 1-propanesulfonic acid {Valitramiprosate):
To a solution of 3-aminopropane sulfonic acid (3-APS) (10.0 gm, 0.069 mol) in water (60.0 mL) whose pH was adjusted to 10.5-11.0 with2M NaOH solution was added a solution of L-valine-N-carboxy anhydride (L-VALNCA) (11.5 gm, 0.082 mol) in 75 ml of THF dropwise during 1.0 hr at 0-5°C while maintaining the pH of the reaction mass at 10.5-11.0 with 2M NaOH solution. The mixture was stirred for 1.0 h at 0-5°C and the mixture was washed with methyl tert butyl ether (1 x50mL).
The aqueous layer was distilled off completely. Off white solid material obtained (75%) of a white substance containing 85-90% of Valitramiprosate sodium salt according to HPLC.
Resin treatment and purification, performed as described in example-1, provided 7.5 gm (45%) of a white solid substance containing 99.5% of chemical purity and 99.8% of chiral purity of Valitramiprosate according to HPLC.
Example-3: Preparation of 3-((2S)-2-amino-3-mcthyl-l-oxobutyl|aniinol- 1-propanesulfonic acid [Valitramiprosate] :
To a solution of 3-aminopropane sulfonic acid (3-APS) (10.0 gm, 0.069 mol) in water (60.0 mL) whose pH was adjusted to 10.5-11.0 with 2M NaOH solution, was added a solution of L-valine-N-carboxy anhydride (L-VALNCA) (11.5 gm, 0.082 mol) in 100.0 mL of ethyl acetate dropwise during 1.0 hr at 0-5°C while maintaining the pH of the reaction mass at 10.5-11.0 with 2M NaOH

solution. The mixture was stirred for 1.0 h at 0-5°C. Separated aqueous layer was distilled off completely. Off white solid material obtained (75%) of a white substance containing 85-90% of Valitramiprosate sodium salt according to HPLC.
Resin treatment and purification, performed as described in the example-1, provided 8.3 gm (50%) of a white solid substance containing 99.5% of chemical purity and 99.8% of chiral purity of valitramiprosate according to HPLC.
Example-4: Preparation of 3-[(2S)-2-amino-3-methyl-l-oxobutyl|amino|-1-Propanesulfonic acid [Valitramiprosate] from N-BOC-L-VAL NCA :
To a solution of 3-aminopropane sulfonic acid (3-APS) (5.0 gm, 0.036 mol) in water (30.0 mL) and whose pH was adjusted to 10.5-11.0 with 2M NaOH solution, was added a solution of N-BOC-L-valine-N-carboxy anhydride (N-BOC-L-VALNCA) (13.1 gm, 0.053 mol) in 75.0 mL of 1,4-dioxane dropwise during 1.0 hr at 0-5°C while maintaining the pH of the reaction mass at 10.5-11.0 with 2M NaOH solution. The mixture was stirred for 10-12.0 h at 0-5°C. After reaction completes, reaction mass was washed with methyl tert butyl ether (1 x 50 mL). The aqueous layer was distilled off completely. Off white solid material was obtained.
The solid material was dissolved in water (30.0 mL). The solution was passed through an Indion-252H ion-exchange column (strongly acidic). The strong acidic fractions were combined and treated with concentrated HC1 (10.0 mL). The mixture was stirred at 50° C for 30 minutes and then concentrated to dryness. The residual materia! was co-evaporated with EtOH (ethanol) to completely remove water. EtOH (50 mL) was added to the residue. The mixture was stirred at reflux for 1 h, and then cooled to room temperature. The solid material was collected by filtration and was dried in a vacuum oven at 60° C providing 5.10 gm (60%) of a white solid substance containing 99.54% of chemical purity and 99.8% of chiral purity of Valitramiprosate according to HPLC.