The present invention provides a novel class of compounds that have the activity of inhibiting HSP90 enzyme and are useful as anti-cancer agents or such, and compounds that are useful as synthetic intermediates thereof. Specifically, the present invention provides compounds represented by the following formula (1), and pharmaceutically acceptable salts thereof: wherein X, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , L 1 , L 2 , and L 3 are as defined in the specification.