Design and Synthesis of Novel 6,7‐Dihydrobenzo[d]isoxazol‐4(5H)‐one Derivatives Bearing 1,2,3‐Triazole Moiety as Potential Hsp90 Inhibitors and their Evaluation as Antiproliferative Agents

An effective approach to 5‐triazolyl‐substituted 6,7‐dihydrobenzo[ d ]isoxazol‐4(5 H )‐ones and 4,5,6,7‐tetrahydrobenzo[ d ]isoxazoles was developed. The approach included α‐keto bromination of 6,7‐dihydrobenzo[ d ]isoxazol‐4(5 H )‐one followed by nucleophilic substitution of bromine with the azide group. For the preparation of 5‐azido‐6,7‐dihydrobenzo[ d ]isoxazol‐4(5 H )‐one, the carbonyl group at position 4 was reduced to a methylene group under ionic hydrogenation conditions using triethylsilane as a reducing agent. Cu(I)‐catalyzed [2+3] cycloaddition of terminal alkynes to both obtained azides was used for the synthesis of two series of triazolyl derivatives. Compounds, which contain in their structures common for some Hsp90 inhibitors 2,4‐dihydroxy‐5‐isopropylphenyl fragment, were evaluated as antiproliferative agents against two breast cancer cell lines: hormone‐dependent MCF7 and HER2‐positive HCC1954. The lead compound showed half‐maximal inhibitory concentration (IC ₅₀ ) values below 5 μM and induced significant changes in the Hsp90 signaling pathways in HER2‐positive HCC1954 cells. It increased the expression of Hsp70 (used as a pharmacodynamic marker of Hsp90 inhibition) and inhibited the expression of HER2, p‐c‐Met, c‐Met, and p‐AKT. The combination of the selected isoxazole‐triazole hybrid molecule and the earlier described apoptosis inducer LCTA‐3344 demonstrated a significant antiproliferative effect against HCC1954 cells. The lead compound was revealed to be a promising candidate for future anticancer drug design, particularly against aggressive breast cancer positive for HER2.