Table 2.

Structures and inhibitory activities of compounds 19–24.^a

Compd.	R2	Heterocycle	IC50 (μM)
19			3.7
20			10.7
21			2.1
22			9.3
23			19.3
24			3.5

^aStandard errors of all IC₅₀ values are less than 30%.