Recently, we reported substrate-based pentapeptidic BACE1 inhibitors possessing a hydroxymethylcarbonyl isostere as a substrate transition-state mimic. These inhibitors showed potent inhibitory activities in enzymatic and cell assays. We also designed and synthesized non-peptidic and small-sized inhibitors possessing a heterocyclic scaffold at the P-2 position. By studying the structure-activity relationship of these inhibitors, we found that the sigma-pi interaction of an inhibitor with the BACE1-Arg235 side chain played a key role in the inhibition mechanism. Hence, we optimized the inhibitors with a focus on their P-2 regions. In this Letter, a series of novel BACE1 inhibitors possessing a 5-nitroisophthalic scaffold at the P-2 position are described along with the results of the related structure-activity relationship study. These small-sized inhibitors are expected improved membrane permeability and bioavailability. (C) 2012 Elsevier Ltd. All rights reserved.