allowing introduction of these sterically demanding substituents very late in the synthesis (Schemes 5 and 6). Benzhydryl derivative (±)-2 was found to be the most selective member (Ki (trypsin)/Ki (thrombin)=1200) of this class of nonpeptidic thrombin inhibitors, while the ‘dipiperonyl' analog (±)-3 (Ki=9 nM, 7.60-fold selectivity) displays the highest potency of all compounds prepared so far (Table 1). A