FIELD OF INVENTION
The present invention relates to an improved process for the preparation N-(4-fluorophenylmethyO-N-Cl-rnethylpiperidin^-yO-N'^-Cl-methylpropyloxy) phenylmethyl) carbamide (2:1) represented by following structural formula-1
Further, the present invention also provides a process for the preparation of crystalline form-C of N-(4-fluorophenylmethyl)-N-(l-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy) phenylmethyl) carbamide (2:1) compound of formula-!
BACKGROUND OF THE INVENTION
N-(4-fluorophenylmethyl)-N-(l-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy) phenylmethyl) carbamide is commonly known as Pimavanserin. Pimavanserin is marketed as a Pimavanserin hemitartrate under the brand name NUPLAZID® in the United States. Pimavanserin is related to serotonin 2A/AC receptor inverse agonists to treat a variety of human neurodegenerative diseases such as Parkinson's disease, Huntington's disease, Lewy Body Dementia and Alzheimer's disease.
Pimavanserin or its pharmaceutically acceptable salts was first generically disclosed in US patent 6,756,393 & specifically disclosed in US patent No. 7,601,740. The process for the preparation of Pimavanserin and its salts as per US patent No. 7,601,740 is depicted in Scheme-1.

In US patent No. 7732615 (herein after US'615), was first reported the crystalline Form-A, Form-B, Form-C of Pimavanserin liemitartrate, amorphous and Form-Y of Pimavanserin free base and Form-D, Form-E and Form-F of Pimavanserin solvates.
In US patent No. 7868176 (herein after US'176), discloses the solid forms of pimavanserin such as edisylate (2:1) & citrate (1:1)

In an international publication WO 2018007842A1, discloses the salts of Pimavanserin such as besylate (1:1) , cyclamate (1:1), tosylate (1:1) , benzoate (1:1) and D-(-) mandelate (1:1), L-(+)- mandelate (1:1), in crystalline, amorphous and hydrate & solvate forms.
Crystalline forms such as amorphous hemitartrate , crystalline hemhydrate form-3, crystalline hemihydrate form-1, 2 are reported in literature. But the available crystalline form of Pimavanserin hemitartrate is in Form-C, which is thermodynamically more stable.
Thus there is a need to develop an improved method to prepare crystalline form-C, by avoiding contamination with other polymorphs.
OBJECT OF THE INVENTION
The main object of the present invention is to prepare an improved process for the preparation of Pimavanserin or its pharmaceutical acceptable salts.
Yet another object of the present invention is to prepare a crystalline form-C of Pimavanserin in an industrial feasible method.
DETAILED DESCRIPTION OF THE INVENTION
The process for the preparation of lntermediate-1, intermediate-2 are well reported in the prior art. Intermediate process of the present invention is same as the teachings of the patent publications US7, 601,740 and US7, 790,899.

EXAMPLES
In the following examples, the preferred embodiment of the present invention is described only by way of illustrating the processes of the invention. However, these are not intended to limit the scope of the invention in any way.
Example-1: Preparation of Pimavanserin free base
Charge Triphosgene (62g) into a round bottomed flask containing MDC (1.0 L) under nitrogen atmosphere at 0-5 °C. Slowly add 4-(2-Methyl propoxy) Benzene Methanamine freebase solution in MDC over a period of 30-40 min by controlling the temperature. Stir the mass for 30 min and add Triethylamine (235 mL) slowly to the above mass. Maintain the stirring for 2hrs at same temperature and add solution of N-(4-fluorobenzyl)- 1-methyl piperidin-4-amine in MDC to the above mass. Stir the mass for 1-2 hrs and after completion of reaction the reaction mass was quenched with water (300 ml) at same temperature. Raise the temperature to 28±2°C and stir for 15 mins and settle for 10 mins. Separate the layers and extract aqueous layer with MDC (300 ml) and combined organic layer was washed with 5% aq. Sodium hydroxide solution (300 ml) and followed by water (300 ml), 10% aq. Sodium chloride solution. Collect MDC and distill completely under vacuum below 45 °C and co-distil with n-heptane. Charge heptane (1 Lt) and heat to 90±2 °C and stir for 30 min. Filter the mass to eliminate the undissolved sticky material and collect the filtrate. Allow to cool the mass to 28±2°C and stir for 4-6 hrs. Filter the mass and wash with n-heptane (200 ml) and suck dry the material. Dry the material in hot air oven to get the required compound in 70-80% yield.

Charge Pimavanserin base in to a R.B. flask containing acetonitrile (700 ml) under nitrogen atmosphere and heat to 65-70 °C. Maintain for 30±5 min and filter the mass over micron paper under hot condition and collect the filtrate. Add THF solution of L-Tartaric acid over a period of 10-15 min and stir for 1-2'hrs at 65-70 °C. Distill THF under vacuum at same temperature and co-distill again with acetonitrile at 65°-70°C to remove traces of THF. Collect part of the sample, filter and dry and check the DSC of the sample. It should be above 165°C. After DSC complies; cool the mass to 55-60 °C and filter total mass under nitrogen atmosphere. Suck dry the material and dry under vacuum oven at 45-50 °C to get Pimavanserin hemi tartrate form-C about 75-85% yield.