Derivatives Of A CA-4 Oxazole Analogue And Natural Diphyllin Glycosides And Their Analogues: Synthesis And Biological Evaluation

In this paper, analogues based on natural compounds Combretastatin A-4 (CA-4) and diphyllin have been synthesized to develop new antitumor candidate drugs.CA-4 represents one of the key antineoplastic and cancer vascular targeting stilbenes isolated from the south African bush willow tree Combretum caffrum. However, there are two disadvantages prevent CA-4 from entering clinical trial. The first is its poor pharmacokinetics resulting from its high lipophilicity and low water solubility. Second, the susceptibility of the cis double bond is isomerized to the inactive trans configuration. It had been proved that introducing a oxazole in place of the olefin functional group has no substantial loss of potency but still had the flaw of bad bioavailability. We decided to modify the CA-4 oxazole analogue (C1) with sugar moieties and phosphate moieties to solve the two problems mentioned above.Oxazole analogue C1 has been synthesized by a revised routine from toluenesulfinic acid sodium salt in 5 steps in overall 19.0% yields. Two series of derivatives (8 glycosides and 5 phosphates) of C1 were synthesized. The sugar portions in target molecules are D-glucopyranosyl, D-galactopyranosyl, L-arabinopyranosyl, D-xylopyranosyl, L-rhamnopyranosyl, D-mannopyranosyl, L-fucopyranosyl and D-lactopyranosyl residue, respectively. The employed protocols for glycolsylation of oxazole are phase transfer catalysis reaction (PTC) with glycosyl bromides as donors and trichloroacetimidate method under acid conditions with glycosyl trichloroacetimidate donors. The phosphate derivatives synthesized in three steps by phosphorylation of C1 utilizing dibenzyl phosphite under basic conditions in acetonitrile. The resulting phosphoric acid reacted with various alkali to afford the phosphates (disodium phosphate, dipotassium phosphate, dilithium phosphate, diammonium phosphate and calcium phosphate).Derivatives were evaluated for their cytotoxic activities against two human cancer cell lines (HCT116 and A549) using a SRB assay. All the glycosides showed more or less loss of potency compare to parent oxazole. Mannoside C7 showed potencies in HCT116 and A-549 cell lines (IC50, 0.06-0.7μM). All five phosphate derivatives showed strong cytotoxicity against HCT116 (0.001-0.02μM) and A-549 cell lines (0.07-0.3μM).Diphyllin, an arylnaphthalene lignan isolated from many traditional medicinal plants, has been reported to possess a wide range of pharmacological activities, including anti-tumor, anti-viral activities. Moreover, some natural diphyllin glycosides showed more potent bioactivities than diphyllin itself. We therefore set up a project to synthesize some natural diphyllin glycosides and their analogues to fully discuss their structure-activity relationships (SAR).Diphyllin has been synthesized by a revised routine from 6-bromo vertraldehyde in five steps in overall 42.5% yield. Three naturally occurring diphyllin glycosides (D1, D2, D3) isolated from Justicia patentiflor Hemsl were synthesized by phase transfer catalysis reation and orthoesterification-orthoester rearrangement approach. Four series of diphyllin derivatives were synthesized.Derivatives were evaluated for their cytotoxic activities against three human cancer cell lines (HCT116, KB, MCF-7) using a SRB assay. D1, D-xyloside D1, and 6-methoxy-glucoside D9 showed better antitumor activity (IC50, 0.02-0.06μM) than diphyllin; deoxyhexosides D1 and D12 (having three hydroxyl groups) are more active than hexosides D5 and D6 (having four hydroxyl groups); D11, the peracetylated product of D1 showed comparable bioactivity to D1 to HCT-116 and KB cell lines but better bioactivity to MCF-7 cell line.A novel synthesis routine for preparing per-O-acetyl-D-fucosyl bromide has been archieved from D-galactose by acetonation, iodination at O-6 hydroxyl group, reduction, deacetonation, acetylation and bromination. The total yield is 35.0% for 6 steps.