NOVEL INTERMEDIATES FOR THE PREPARATION OF CAMPTOTHECIN ANALOGUES Field of the invention : The present invention relates to a process for the preparation of camptothecin analogues useful as anti-cancer drugs, in particular camptothecin analogue irinotecan and its salts thereof, More particularly, the invention relates to novel intermediate compound of formula (la) and its use in the process for the preparation of camptothecin analogues and its salts thereof. Background of the invention : US Patent No. 6,121,451 reveals a process for preparing antineoplastic drug 7-ethyl-10-hydroxy camptothecin 10-[t,4'-carboxylate] or CPT-11 (V) free base. In the described process a compound therein identified as 14CPT (I) is first reacted with compound of formula (II) to form an intermediate of formula (III), which is further reacted with 4-pipcridino piperidine earbomyl chloride of formula (IV) to produce CPT-ll free base (V) as shown below. WO 03/089413 reveals the novel compound of formula (VI). and its use in the preparation of camptothecin analogues and particularly irinotecan free base and its pharmaceutically acceptable salts. Alternative materials and methods for preparing irinotecan and camptothecin analogues are described in the prior art. The prior art processes reported for the preparation of Irinotecan and its salts involves chromatographic purification step at one or the other stage of the process. However, in the process of present invention for the preparation of Irinotecan and its salt the tedious step of chromatographic purification has been totally eliminated. Thus the process of present invention is economical and easily viable for the commercial scale preparation of the anticancer drug. This provides an added advantage to the process of present invention over the prior art processes. Objects of the invention: An object of the invention is to provide novel intermediate compounds of formula (la) Another object of the invention is to provide process for the preparation of compounds of formula (la) Yet another object of the invention is to use novel intermediate compounds of formula (la) for the preparation of Camptothecin analogues and its salts thereof. Still another object of the invention is to provide a process using novel intermediate compound of formula (la, Ri= C2Hs) for the preparation of anti cancer drug Irinotecan and its pharmaceutically acceptable salts thereof. Still yet another object of the invention is to provide a process for the preparation of Camptothccin analogues with out involving chromatography purification at any stage of the process, more particularly for the preparation of Irinotecan and its salts thereof. An objective of the invention is to provide an easy and economically viable process for the commercial scale preparation of Irinotecan and its pharmaceutic ally acceptable salts. Summary of the invention : The present invention provides novel intermediate compounds of formula (la) and its salts thereof. Wherein R is amino protecting group defined as -CO (CH2) n CH3 (n= 0 to4); - CO (CH2) n CH2 X (X= Halogen; n= 0 to 4); -COCXj nionhenone(Ia;R= COCH2 CH3, R,=C2Hs) Charge chloroacetic anhydride (27ml) into a reactor, add 2- Amino-5-hydroxy - propiophenone[AHP](9g) to it. The mixture is heated to 130DC and maintained at this temperature for 2-3 hr. Monitor the reaction mixture by I IPLC for the disappearance of AHP. Work up the reaction mixture by pouring on to crushed ice water (270ml) with stiring and continued stiring between 0°-5°C for at least an hour. Filter the precipitated solid, wash with water (100ml), dried and crystallised to yield step - I product (13.5g); HPLC purity not less than 98.00%. Step II: Preparation of 4- propionimido -3-propionylphenyl-l,4'- bipiperidine-1 '-carboxylate (la; R= COCH2 CH3and Ri=C2 Hs Dissolve Step- I product (12g) in dry pyridine (192ml). Add 4-Dimethylamino pyridine (24), followed by 4-piperidino piperidine carbomylchloride (26.4g) to it. Stirred the mixture at room temperature for 48h-60h. Monitor the reaction mixture by HPLC. Work up the reaction mixture when step- I product is NMT 3.0% by adding water (72m[).Extract with ethylacetate (240m!) .Separate layers, evaporate ethyl acetate layer to dryness.Dissolve the residue in chloroform 260ml), washed with 10% aqueous sodium bicarbonate solution (6x 260ml), followed by water (6x 260ml).Dry the washed chloroform over anhydrous sodium sulphate, filterand evaporate to dryness to yield step- II product (11.5g), HPLC purity not less than 94.00%. Step III: Preparation of Irinotecan base ( VII, R/ = C2II5) Dissolve Step- II productfl 1.5) in ethyl alcohol (115ml) ,added con. HCl (25.4ml) , glacial acetic acid (115ml), 14CPT (10.62g) and heat the mixture to 110°- I I5°C for a period of 24 hrs. Evaporate the reaction mixture to dryness, add water (345m!) and stir for 10-15 min and filter the aqueous solution through celite bed. The filtered aqueous solution is extracted with chloroform (4x85ml). Separated organic and aqueous layer. Aqueous layer is evaporated to dryness under vacuum. Add ethyl alcohol (80ml) to dissolve the residue, add activated charcoal (1.15g) and heat to 70°C with string for an hour. Filter the hot solution and cool to room temperature with stirring and then to 0°C for an overnight. The precipitated pale yellow solid is filtered to obtain is step- III producl(10.0 g) having HPLC purity of not less than 98.00%. Step IV: Preparation of Irinotecan Hydrochloride Trihydrate; Take Step- IN (lO.Og) product in DM H20 (90.0ml) and add cone. HCl (19.00ml). Heat the misture to 60°when a pale yellow solution is obtained. Add activated charcoal (l.OOg) and continue to maintain the mixture at this temperature for further 30 min and filter.Cool the filtered solution to room temperature and then to 0°C with stirring for an overnight. Filter the separated solid, dry the solid under high vacuum to yield irinotecan hydrochloride trihydrate (5.9g); HPLC purity 99.60% Example IH Step I: Preparation of2-chioroacetinildo-5-hvdroxv-propiophenone(Ia;R= COCinCl. R1=C2IF5 Charge chloro acetic anhydride (30ml) into a reactor, add 2- Amino-5-hydroxy - propiophenone[AHP](10g) to it. The mixture is heated to 130°C and maintained al this temperature for 2-3 hr. Monitor the reaction mixture by HPLC for the disappearance of AHP. Work up the reaction mixture by pouring on to crushed ice water (300ml) with sliring and continued stiring between 0°-5°C for at least an hour. Filter the precipitated solid, wash with water (100ml), dried and crystallised to yield step -1 product (11 g); HPLC purity not less than 98.50%, Step II: Preparation of 4-chloroacetimido-3-propionylphenyi-l,4'- bipiperidine-J '-carboxylate (la; R=-COCH2Cl and Ri=C2 H5) Step- I product (lOg) in dry pyridine (160ml). Add 4-Dimethylamino pyridine (18.3g), followed by 4-piperidino piperidine carbomylchloride (20.Og) to it. Stirred the mixture at room temperature for 48h-60h. Monitor the reaction mixture by HPLC. Work up the reaction mixture when step- 1 product is NMT 3.0% by adding water (60ml).Extract with cthylacetate (200ml) .Separate layers, evaporate ethyl acetate layer to dryness.Dissolve the residue in chloroform 215ml), washed with 10% aqueous sodium bicarbonate solution (6x 215ml), followed by water (6x 215ml).Dry the washed chloroform over anhydrous sodium sulphate, filterand evaporate to dryness to yield step- II product (1 LOg), HPLC purity not less than 95.00%. Step III: Preparation of Irinotecan base ( VII, Rf = C2Hs) Dissolve Step- II product(l 1.0) in ethyl alcohol (110ml) .added con. HC1 (24.2ml) , glacial acetic acid (110ml), 14CPT (lO.Og) and heat the mixture to 110°- 115°C for a period of 24 hrs. Evaporate the reaction mixture to dryness, add water (345ml) and stir for 10-15 min and filter the aqueous solution through eclite bed. The filtered aqueous solution is extracted with chloroform (4x85ml). Separated organic and aqueous layer. Aqueous layer is evaporated to dryness under vacuum. Add ethyl alcohol (80ml) to dissolve the residue, add activated charcoal (1.15g) and heat to 70QC with string for an hour. Filter the hot solution and cool to room temperature with stirring and then to 0°C for an overnight. The precipitated pale yellow solid is filtered to obtain is step- III product(lO.Og) having I1PI.C purity of not less than 98.00%. Step IV: Preparation afhinatecan Hydrochloride Trihydrate Take Step- 111 (10.0 g) product in DM H20 (90.0ml) and add cone. HC1 (19. 0ml), Heat the mixture to 60°when a pale yellow solution is obtained. Add activated charcoal (1.04g) and continue to maintain the mixture at this temperature for further 30 min and filter.Cool the filtered solution to room temperature and then to 0°C with stirring for an overnight. Filter the separated solid, dry the solid under high vacuum to yield irinotecan hydrochloride trihydrate (6.03g); HPLC purity 99.62% We Claim: 1. A compound of formula(Ia) and its sails thereof Wherein R is amino protecting group defined as -CO (CH2) n CH3 (n= 0 to4); - CO (CH2) n CH2 X (X= Halogen; n= 0 to 4); -COCX3 (X- Halogen) and R, is hydrogen, alkyl, araalkyl, hydroxyRg ethyl, carboxymethyl, acyloxy methyl silyl or N-substituted alkyl amine -CH2-N-R2 where N is a linking nitrogen atom, wherein R2 and R3 substituents on the N-atom may be defined as follows. i)R2 and Rj are each independently selected from hydrogen, alkyl, alkemyl, hydroxyalkyl, acyloxyalky! or acycloxy alkyl substituted group. ii)R2 and R3 together with the nitrogen linking atom can form heterocyclic unit 2. A compound of claim 1, wherein Rs is an ethyl group. 3. A compound of claim 1 that is an acid addition salt of compound of formula(Ia) with an acid selected from hydrochloric, hydrobromic,benzoic, tartaric, citric, fumaric. maleic ,alkyl monocarboxylic acid and alkyldicarboxylic acid having Q-C4 alkyl chain. 4. A process for the preparation of camptothecin analogues or its salts thereof, the said process comprising first step of contacting compound of formula (la) and a second step of preparing a sail of camptothecin analogues thus obtained. 5. A process of claim 4 , wherein irinotecan or its salt thereof is prepared by contacting compound of formula (la ,R1=C2H5) or its sail thereof with compound of formula (I). 6. A process for preparing compounds of formula(Ia) comprising step of contacting compound of of formula (Ha) with 4-piperidinopiperidine carbamyl chloride (IV), 7. A drug substance comprising therapeutically effective amount of irinotecan and/ or its salt thereof and compound of formula(Ia ) and /or its atleast one salt thereof in a detectable amount. 8. The drug substance of claim 7, wherein compound of formula(Ia ) and /or its atleast one salt thereof present in an amount not more than about 1%. 9. The drug substance of claim 7, wherein compound of formula(Ia ) and /or its atleast one salt thereof present in an amount not more than about 0.5%. 10. The drug substance of claim 7, wherein compound of formula(Ia ) and /or its atleast one salt thereof present in an amount not more than about 0.1%. 11. A drug substance comprising therapeutically effective amount of irinotecan and/ or its salt thereof and 7- ethyM O-hydroxy-camptothecin and /or its atleast one salt thereof in a detectable amount. 12. The drug substance of claim 11, wherein 7-ethyl-i O-hydroxy-camptothecin and /or its atleast one salt thereof present hi an amount, not more than about 1%. 13. The drug substance of claim 11, wherein 7-ethyl-10-hydroxy-camptothecin and /or its atleast one salt thereof present in an amount, not more than about 0.5%. 14. The drug substance of claim 11, wherein 7-ethyl-10-hydroxy-camptothecin and /or its atieast one salt thereof present in an amount, not more than about 0.1%. 15. A drug substance comprising therapeutically effective amount of irinotecan and/or its salt thereof and compound of formula(la) and /or its atieast one salt thereof and 7-ethyl-10-hydroxy-camptothecin and /or its atieast one salt thereof each present in a detectable amount. 16. The drug substance of claim 15, wherein compound of formula(la) and/or its atieast one salt thereof and 7-ethyi-10-hydroxy-camptothecin and /or its atieast one salt thereof each present in an amount not more thanl%. 17 , The drug substance of claim ]5, wherein compound of formulafia) and /or its atieast one salt thereof and 7-ethyl-10-hydroxy-camptothecin and /or its atieast one salt thereof each present in an amount not more than 0.5%. 18. The drug substance of claim 15 wherein compound of formulafla )and /or its atieast one salt thereof and 7-ethyl-10-hydroxy-camptothecin and /or its atieast one salt thereof each present in an amount not more than 0.1%.