The present invention relates to a novel Betulinic acid derivative and a composition comprising the same. Field of the Invention The invention relates to the use of betulinic acid and its derivatives for the inhibition/prevention of cancer growth, novel betulinic acid derivatives useful for the inhibition of tumor/cancer cells and a process for the preparation of the said derivatives. The invention also relates to the antileukaemic, and anti-lymphoma activity of said novel betulinic acid derivatives, useful for treatment of prostate, ovarian and lung cancer. Background and prior art references to the Invention Under the auspices of a National Cooperative Natural Product Drug Discovery Group supported by the National Cancer Institute, the potential antitumor activity of approximately 2500 extracts derived from globally collected plants was evaluated in a panel of enzyme based assays and in a battery of cultured human tumor cell lines. One such extract, prepared from the stem bark of Ziziphus mauritiana Lam. (Rhamnaceae), displayed selective cytotoxicity against cultured human melanoma cells (Nature Medicine, Vol. 1 (10), 1995, WO 96/29068). As a result of bioactivity guided fractionation, betulinic acid, a pentacyclic triterpene, was identified as a melanoma-specific cytotoxic agent. In follow-up studies conducted with athymic mice carrying human melanomas, tumor growth was completely inhibited without toxicity. As judged by a variety of cellular responses, antitumor activity was mediated by the induction of apoptosis. A number of triterpenoids, including betulinic acid, have several known medical applications, including use as an anticancer drug. Anderson et al., in WO 95/04526, have discussed the derivatives of triterpenoids which have been used in cancer therapy, including their activity against polyamines which are required by cells to grow at an optimal rate. Some of these triterpenoids have been found to interfere with enzymatic synthesis of polyamines required for optimal cell growth, and thus inhibit the growth of cancer cells, particularly by inhibiting ornithine decarboxylase (Yasukawa, K. et al. Oncology 48 : 72-76,1991). The anti-cancer activity of betulinic acid and some derivatives has been demonstrated using mouse sarcoma 180 cells implanted subcutaneously hi nude mice ( JP 87,301,580). Choi et al have shown that betulinic acid 3-monoacetate, and betulinic acid methyl ester exhibit EDso values of 10.5 and 6.8 ug/ml, respectively, against p388 lymphocytic leukemia cells (Choi, Y-H et al., Planta Medica vol XLVDL, pages 511 - 513, 1988). The ED*) values of these derivatives is greater than 4.0 ug/ml and hence may not be considered to be having any significant anticancer activity. Table 1 indicates anticancer activity (EDso values) of an derivatives and the applicants claim only derivatives which show ED^ < 4.0ug/ml. In addition, none of the prior art derivatives cited in literature have been tested on humans. Summary of the invention The present invention provides a pharmaceutical composition comprising betulinic acid derivatives useful for killing or inhibiting multiplication of cancer cells, novel betulinic acid derivatives, a process for the synthesis of the said novel derivatives of betulinic acid and testing the bio-activity using cultured human leukemia (MOLT-4, Jurkat E6.1, HL60, CEM.CM3), lymphoma cells (BR1STOL-8, U937), prostate cancer cells (DU 145), lung cancer cells (A 549) and ovarian cancer cells (PA-1) as the monitor. In a preferred embodiment, a pharmaceutically acceptable carrier, diluent, or solvent is used. The invention provides a method of treatment for humans, mammals, or other animals suffering from cancer or other tumors. The method may suitably comprise essentially of administering a therapeutically effective dose of the pharmaceutical composition so as to kill or inhibit the multiplication of cancer or tumor cells. The method of treatment of the present invention is particularly useful in the treatment of leukemias and lymphomas and in general in the treatment of prostate cancer, ovarian and lung cancer. Objects of the invention An object of the present invention is to provide a method and composition for inhibiting tumor growth and, particularly, for inhibiting the growth of leukemias and lymphomas and in general for inhibiting the growth of prostate, ovarian and lung cancer using a natural product-derived compound and its derivatives. Another object of the invention is to provide a treatment method using betulinic acid derivatives to prevent the growth of cancerous cells, wherein betulinic acid derivative is administered systemically. Still another object of the invention is to overcome the problem of high toxicity associated with standard chemotherapeutic agents by using a natural product-derived compound, e.g., betulinic acid derivatives. Yet another object of the invention is to overcome the problem of insufficient availability associated with synthetic anticancer agents by using synthetic derivatives of betulinic acid. It is also an object of the invention to overcome the problem of high cost of synthetic anticancer agents by utilizing the readily available natural product derived compound, e.g., betulinic acid derivatives that would be of substantially cheaper cost. One more object of the invention is to provide novel betulinic acid derivatives and a process for producing such derivatives. Yet another object of the invention is to provide a pharmaceutical formulation containing betulinic acid derivatives alone or in combination. Statement of the Invention Accordingly, the present invention relates to a novel betulinic acid derivative of formula 2: (Formula Removed) wherein R, R1, R2, R3, R4, R5 and R6 independently or in combination represent: R is H; R1 is H, Br, Cl, F or I; R2 is H and R3 is OH, OCOCH3, OCO(CH2)nCH3 where (n=l to 5), OCOC(CH3)3, OCO(CH2)n Cl (where n=l to 10), OCOC6H5, OSO2CH3, NH2, NHCH2 CH2OH, NHCH2CH2OCOCH3, N=CH C6H3 (NO2)2, N=CHC6H4Cl N-CHC6 H4Br, N=CHC6 H4N02, N=CHC6 H3 Br2, N=CHC6 H3C12, N=CHC6H3(CH3)CF3 N=CHC6 H4CF3, N=CHC6 H4F, OCOC6 H2C13, OCOCH(OCOCH3) CH3, OCOCH(OCOCH3)C6H5, OCOCH2 C6H5, OCO(CHOH)CH3, OCOC6 H4Br, OCO C6H4C1, OCOC6H4F, OCOC6 H4I, OCOC6 H3C12, OCOC6 H3F2, OCOC6 H4CF3, NHNHC6H5, NHNHC6H4OMe, or N=CHC6H3F2, or R2 and R3 together are O, NNHC6H5 or derivatives thereof such as herein described, NNHC6H2C13, NNHC6H4OCH3, NNHC6H4OH, NNHC6H3(Br)(OCH3), NNHC6H4F, NNHCH(OH)C6H5, NNHCOC6H5, NCH2C6H5, NNHCH2C6H5F, NNHC6H4F, or N-OX (X being H, COCH3, SO2C6H6CH3 or CO(CH2)nCH3 (where n=l to 5)); R4 is OH, -OCH3, 0(CH2)nCOOCH3, O(CH2)nCOOC2H5, 0(CH2)n, COOH, 0(CH2)nCOCl (where n=l to 5), OCH2CH2OC2H5, OCH2CH2OH, OCH2CH2OCOCH3, Cl, N3, NHNH2, C6H4OMe, HNNHC6H2C13, NH2, or NH(CH2)nCH3 (where n=0 to 9); R5 is H, or Br; R6 is CH3, CH2Br, CH2OH, CHO, CH2OCOCH3, COOH, COO(CH2)n COOCH3, COO(CH2)nCOOC2H5, COO(CH2)n COOH (where n=l to 5); or R5 and Re together are =CH2 or -CH3. Brief Description of the accompanying Drawings Fig. 1 represents the formula for betulinic ackL Fig. 2 represents a general formula of derivatives of betulinic acid. Fig. 3 represents a general formula of derivatives of betulinic acid. Fig. 4 represents a general formula of derivatives of betulinic acid. Fig. 5 represents a general formula of derivatives of betulinic acid. Fig. 6 represents a general formula of derivatives of betulinic acid. Detailed Description of the Invention The invention mainly relates to a method of treating a patient with leukemia or prostate, lung or ovarian cancer, said method comprising administering a pharmaceutically effective dosage of betulinic acid, or a betulinic acid derivative alone or in combination thereof to the patient. The patient includes a human, mammal or other animal and the ED50 value of active betulinic acid derivatives against leukemia or lymphoma is in the range of 0.4 to 3.5 µg/ml. The preferred ED50 values of active betulinic acid derivatives are in the ranges of 0.4 to 40 µg/ml, 0.7 to 4.0 µg/ml, 1.2 to 4.0 µg/ml against prostate cancer, lung cancer and ovarian cancer respectively. The invention also relates to novel derivatives of betulinic acid, which are used for treating patient with leukemia or prostate, lung or ovarian cancer, the general formula of betulinic acid is shown as structure 1 here below. Structure 1 (Formula Removed) The novel derivatives of betulmic acid have been shown in structures 2 to 6 herebelow Structure 2 (Formula Removed) wherein R, R1, R2, R3, R4, R5 and R6 independently or in combination represent: R is H; R1 is H,Br,Cl,ForI; R2 is H and R3 is OH, OCOCH3, OCO(CH2)nCH3 where (n-1 to 5), OCOC(CH3)3, OCOCCH2)n Cl (where n=l to 10), OCOC6H5, OSO2CH3, NH2, NHCH2 CH2OH, NHCH2CH2OCOCH3, N=CH C6H3 (NO2)2, N=CHC6H4Cl N=CHC6 H4Br, N=CHC6 H4NO2, N=CHC6 H3 Br2, N=CHC6 H3C12, N=CHC6H3(CH3)CF3 N=CHC6 H4CF3, N=CHC6 H4F, OCOC6 H2C13, OCOCH(OCOCH3) CH3, OCOCH(OCOCH3)C6H5, OCOCH2 C6H5, OCO(CHOH)CH3, OCOC6 H4Br, OCO C6H4Cl, OCOC6H4F, OCOC6 H4l, OCOC6 H3C12, OCOC6 H3F2, OCOC6 H4CF3, NHNHC6H5, NHNHC6H4OMe, or N=CHC6H3F2nor R2 and R3 together are O, NNHC6H5 or derivatives thereof such as herein described, NNHC6H2C13, NNHC6H4CH3, NNHC6H4OH, NNHC6H3(Br)OCH3), NNHC6H4F, NNHCH(OH)C6H5, NNHCOCgHs, NCH2C6H5, NNHCH2C6H5F, NNHC6H4F, or N-OX (X being H, COCH3, SO2C6H6CH3 or CO(CH2)nCH3 (where n=l to 5)); R4 is OH, -OCH3, O(CH2)nCOOCH3, O(CH2)nCOOC2H5, 0(CH2)n, COOH, 0(CH2)nCOCl (where n=l to 5), OCH2CH2OC2H5, OCH2CH2OH, OCH2CH2OCOCH3, Cl, N3, NHNH2, C6H4OMe, HNNHC6H2C13, NH2, or NH(CH2)nCH3 (where n=0 to 9); R5 is H, or Br; R6 is CH3, CH2Br, CH2OH, CHO, CH2OCOCH3, COOH, COO(CH2)n COOCH3, COO(CH2)nCOOC2H5, COO(CH2)n COOH (where n=l to 5); or R5 and R6 together are =CH2 or -CH3, Structure 3 (Formula Removed) wherein R= H; Rl = H; R2 = H; R3 = OSO2CH3 , NHCH2CH2OH, N=CHC6H4F, N=CHC6H4C1, N=CHC6H4NO2, OCOC6H4Br, OCOC6H3F2, OCOC6H4CF3 , OCOC6H4F and R4 =OH or OCH2C