Process for preparation of ((3r,11br)-1,3,4,6,7,11b-hexahydro-9,10-di(methoxy-d3)-3-(2-methylpropyl)-2h-benzo[a]quinolizin-2-one

A process for the preparation of Deutetrabenazine compound of formula I, which comprises the following steps:

• i. demethylation of Tetrabenazine of compound of Formula II,
• with an acid or mixture to yield di-hydroxy benzoquinoline compound of formula III or a salt thereof;
• ii. optionally protecting the keto group of di-hydroxy benzoquinoline compound of Formula III with protecting group to yield keto protected di-hydroxy benzoquinoline compound of Formula IV;
• wherein P1 is protecting group
• iii. reacting the di-hydroxy benzoquinoline compound of formula III or keto protected di-hydroxy benzoquinoline compound of Formula IV with deuterium source to yield Deutetrabenzine compound of formula I or keto protected deutetrabenazine compound of Formula V
• iv. optionally de-protecting the keto protected deutetrabenazine compound of Formula V using an acid to yield Deutetrabenazine compound of formula I,
• v. optionally purifying the obtained deutetrabenazine compound of formula I and
• vi. isolating the obtained deutetrabenazine compound of formula I