PROCESS FOR THE PREPARATION OF PALBOCICLIB

1 . A process for the preparation of palbociclib, a compound of formula I, (the “compound I”),

comprising the steps of:

a) reacting 6-bromo-2-chloro-8-cyclopentyl-5-methylpyrido[2,3-d]pyrimidin-7(8H)-one, a compound of formula II, (the “compound II”) with tert-butyl 4-(6-amino-3-pyridyl) piperazine-1-carboxylate, a compound of formula III (the “compound III”) in the presence of a Grignard reagent,

to obtain 4-{6-[6-bromo-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydro-pyrido[2,3-d]pyrimidin-2-ylamino]-pyridin-3-yl}-piperazine-1-carboxylic acid-butyl ester, a compound of formula IV, (the “compound IV”), wherein the Grignard reagent is added in one lot, and the compound II is added in at least two portions;

b) reacting the compound IV obtained in the step a) with n-butyl vinyl ether, a compound of formula V, (the “compound V”)

to obtain tert-butyl 4-(6-{[6-(1-butoxyethenyl)-8-cyclopentyl-5-methyl-7-oxo-7,8-dihydropyrido[2,3-d]pyrimidin-2-yl]amino}pyridin-3-yl)piperazine-1-carboxylate, a compound of formula VI (the “compound VI”); and

c) reacting the compound VI obtained in the step b) with an acid followed by treating with a base to obtain palbociclib, the compound I.

2 . The process according to claim 1 , wherein the step a) comprises the steps of:

(a-i) reacting the compound II with the compound III in a mixture comprising a solvent and a Grignard reagent, wherein less than or equal to one molar equivalent of the compound II is used with respect to compound III, and wherein the Grignard reagent is added in one lot;

(a-ii) stirring the reaction mixture of the step (a-i) for a period of about 15 minutes to about 1 hour;

(a-iii) adding second portion of the compound II to the mixture of step (a-ii) to obtain the compound IV;

(a-iv) optionally, repeating the step (a-ii) and the step (a-iii) to have total molar equivalent of compound II with respect to compound III stoichiometrically between 0.8 to 1.0 molar equivalents; and

(a-v) isolating the compound IV obtained in the step (a-iii) or the step (a-iv).

3 . The process according to claim 2 , wherein the step (a-i) comprises addition of the Grignard reagent at a temperature below 40° C.

4 . A The process according to claim 1 , wherein the compound VI is crystalline Form G, which is characterised by an X-ray powder diffraction (XRPD) pattern having peaks at about 5.87, 9.29, 10.52, 11.77, 12.97, 13.72, 16.31, and 22.44±0.2 degrees 2 theta.

5 . A The process according to claim 1 , wherein the compound VI is crystalline Form G, which is as substantially illustrated in FIG. 1 .

6 . A process for the preparation of crystalline Form G of the compound VI, wherein the process comprises the steps of:

(i) providing a solution of, the compound VI in an ester solvent;

(ii) heating the solution of the step (i);

(iii) optionally, filtering the hot solution of the step (ii);

(iv) cooling the solution of the step (ii) or the step (iii); and

(v) isolating the crystalline Form G of the compound VI, characterised by an XRPD pattern having peaks at about 5.87, 9.29, 10.52, 11.77, 12.97, 13.72, 16.31, and 22.44±0.2 degrees 2 theta.

7 . A compound selected from:

a) a compound of formula VII (the “compound VII”),

wherein R 1 is hydroxy or isopropyl; or

b) a compound of formula VIII (the “compound VIII”),

8 . (canceled)

9 . The process according to claim 1 , wherein the palbociclib represented by the compound of formula I,

contains a compound VII, a compound VIII and a compound IX represented by the following structures:

wherein R is hydroxy or isopropyl,

in an amount ranging from about 0.15% w/w to about 0.03% w/w as determined by High-performance liquid chromatography (HPLC).

10 . (canceled)