The invention relates to novel betulinic acid derivatives of formula (I), wherein R is C(═CH2)CH3 or CH(CH3)2; R2 together with the adjacent carbonyl group forms a carboxylic acid, carboxylic acid ester or amide or substituted amide; R3 or R4 are hydrogen or aryl with the proviso that both are not independently hydrogen or alkyl or R3 and R4 are combined together to form an aryl ring optionally substituted with a group X, wherein X is selected from halogen, alkyl, cyano, nitro, alkoxy, amino or substituted amine; Y is N or O; and R1 is zero when Y is O, and R1 is hydrogen, alkyl or aryl alkyl when Y is N, useful for inhibition of tumor cancer cells.