The invention provides compounds of formula (I) the prodrugs thereof, and the pharmaceutically acceptable salts of the compounds and prodrugs; wherein R', R', R'', X, and Z are as defined herein; pharmaceutical compositions thereof; and uses thereof in treating diabetes, insulin resistance, diabetic neuropathy, diabetic nephropathy, diabetic retinopathy, cataracts, hyperglycemia, hypercholesterolemia, hypertension, hyperinsulinemia, hyperlipidemia, atherosclerosis, and tissue ischemia.