This invention relates to a novel class of compounds, represented by the formula (I) below, wherein the meanings of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are indicated therein, which are cysteine protease inhibitors, including but not limited to, inhibitors of cathepsins K, L, S and B. These compounds are useful for treating diseases in which inhibition of bone resorption is indicated, such as osteoporosis, osteoarthritis and rheumatoid arthritis.