Several diacetoxy acetal analogues have been synthesized from santonin and assessed for their ability of inducing or enhancing the differentiation of human HL-60 leukemia cells. The compounds themselves had little effect on HL-60 cell differentiation. However, three analogues, 2a, 3a, and 5b, synergistically enhanced 1,25-dihydroxyvitamin $D_3[1,25-(OH)_2D_3]-induced$ HL-60 cell differentiation when combined with 5 nM of dihydroxyvitamin $D_3[1,25(OH)_2O_3]$, a well-known differentiation inducer. Especially, the compound 5b profoundly enhanced the $1,25-(OH)2O_3]-induced$ HL-60 cell differentiation.