A potent, orally-available inhibitor of Hsp90 (Ki = 1.7 nM) that induces the degradation of oncoproteins, including HER2 (EC50 = 38 nM), while up-regulating the expression of Hsp70 and Hsp27; cytotoxic to cancer cells and, when administered orally to mice, inhibits growth of xenograft tumors.