Bioisosterism represents one approach used by the medicinal chemist for the rational modification of lead compounds into safer and more clinically effective agents. Bioisosteres are substituents or groups that have chemical or physical similarities and that produce broadly similar biological effects. The sulfone moiety is recognized as a nonclassical bioisostere for replacement of the carbonyl group. When sulfonyl derivatives 5a-e were compared with carbonyl compounds 4a-e, the sulfone substitution dramatically decreased the antiproliferative activity of the series.