The synthetic route for the aminobenzoxazole scaffold

The synthetic route for the aminobenzoxazole scaffold with a solvent accessible moiety is described in . Nitration of methoxyphenol and subsequent hydrogenation of the resulting nitro Zotarolimus afforded compound . Aminobenzoxazole was formed by the reaction of compound with BrCN in EtOH. The solvent accessible hydrophilic moiety, the morpholinylethyl group, was attached to the OH or NH group of the aminobenzoxazole scaffold to yield compounds and . We prepared two types of benzoxazole analogs. Two substituted groups of the aminobenzoxazole core were attached in an inverse fashion. In a series of type A analogs, the solvent accessible moiety, the morpholinyl alkoxy group, was attached to the benzene ring of benzoxazole, and additional H-bond donor/acceptor and hydrophobic moieties were attached to the nitrogen of aminobenzoxazole. Type B analogs were shaped in reverse (). Preparation of the hydrophobic moiety was started with aminobenzyl alcohol, which was treated with benzoyl chloride or substituted with phenyl isocyanate to form amide or urea compound , respectively. -Substituted aminobenzyl alcohol and were transformed to bromide by treatment with PBr or oxidized to respective aldehydes and by treatment with MnO (). Type A compounds were prepared as depicted in . Addition of compounds to isocyanate afforded urea , and reductive amination with aldehydes or gave -benzyl substituted compounds –. Type B compounds were also prepared by -alkylation of compounds with various alkyl halides (). Enzymatic activity of the prepared compounds against Aurora A and B kinases was determined using the ADP-Glo™ assay (Promega, USA), which quantifies the ADP generation consumed from ATP during a kinase reaction. The inhibitory activity was represented as the relative percentage inhibition of the positive control staurosporine. Following assessment of all the prepared compounds for Aurora A and B enzymatic activity, selected compounds with sufficient activity against Aurora B were examined for their antiproliferative activity against the various human cancer cell lines. The results of the enzymatic assay are summarized in , . At the beginning of the study, we introduced an amide or urea group directly to the benzoxazole scaffold as depicted in , which was rarely active. Docking studies indicated that the lengthening of the molecule may be useful for activity. Introduction of a benzyl group as a linker between benzoxazole and the H-bond donor/acceptor moiety, amide or urea group, enhanced the activity depending on the position of substituents, as shown in . The attachment position of the H-bond donor/acceptor moiety (X) to the benzyl linker affected the activity. Better activity was seen when group X was substituted at the position (compound ) as compared with the isomer and the isomer . A urea substituent group was more favorable than an amide as demonstrated when comparing versus and versus . Therefore, a further structure–activity relationship study was carried out with phenylurea analogs. As observed in , the -substituted urea analogs (, , and ) revealed a higher activity than the -substituted analogs (–). We investigated the effect of a halogen substituent (X) on the terminal benzene ring. Introduction of a halogen atom generally increased activity. In the case of chlorine substitutions, the activities were in the order of; 2,4- or 3,4-diCl>4-Cl>3-Cl. The hydrophilic morpholine substituent was attached at the C6 or C5 position of the benzoxazole scaffold. 6-Alkoxy analogs were more potent than 5-alkoxy analogs when comparing and with and , respectively. With respect to type B analogs, most of the compounds revealed promising activities. However, the structure–activity relationships were more obscure than those of type A analogs. Compound showed the highest relative inhibitory activity but a lower IC value. The majority of the tested compounds showed selectivity for Aurora B kinase over Aurora A kinase. Considering the relative inhibitory activities, five representative compounds were chosen and their IC values were evaluated (). Dichloro-substituted compounds and were the most potent, with IC values of 0.7 and 0.6μM, respectively.