The present invention is directed to compounds which are beta-lacatamase inhibitors. The compounds and their pharmaceutically acceptable salts, are useful in combination with beta-lactam antibiotics, or alone, for the treatment of bacterial infections, including infections caused by drug resistant organisms, including multi-drug resistant organisms. The present invention includes compounds according to formula (Ia): or a pharmaceutically acceptable salt thereof, wherein the values of R1, R2, R3 and R4 are described herein.本發明係關於ß-內醯胺酶抑制劑化合物。該等化合物及其醫藥學上可接受之鹽適合與ß-內醯胺抗生素組合或單獨用於治療細菌感染,包括由耐藥生物體(包括多重耐藥生物體)所引起之感染。本發明包括式(Ia)之化合物:或其醫藥學上可接受之鹽,其中R1、R2、R3及R4之值描述於本文中。