Synthesis and cytotoxicity of 28-carboxymethoxy lupane triterpenoids. Preference of 28-O-acylation over 28-O-alkylation of betulin by ethyl bromoacetate

28-Carboxymethoxy lupane tritepenoids 3 and 4 were synthesized by alkylation of betulin with the THP protected 2-hydroxyethyl iodide followed by oxidation and reduction. Direct reaction of betulin (5) or betulone (10) with ethyl bromoacetate led to 28-O-acylation, instead of 28-O-alkylation. The targeted compounds 3 and 4 were not cytotoxic at the highest concentration tested (75 μmol/L), suggesting that elongation of the chain length at the 28-position in both betulinic acid (1) and betulonic acid (2) was detrimental to the cytotoxicity. The acylation products 28-O-bromoacetates (8a, 8b and 11) and 28-O-methoxyacetate 13 exhibited cytotoxicity against several cancer cell lines tested.