The diastereoselective direct one-pot three-components Mannich reaction of cyclododecanone (CDD), which has a unique conformation with aromatic aldehydes and anilines in the presence of catalytic amount of ammonium acetate was studied. The catalyzed reaction offered high yield, high diastereoselectivity, and simple workup. The addition of aniline to the aldol product took place, and this addition was made to the less hindered α-side. The Mannich derivatives were found to exhibit anti-cancer activities against six human cancer cell lines. This was observed using the CCK-8 nonradioactive colorimetric assay and the results were compared with MCFA10 normal breast epithelium cell line. A significant difference was observed in anti-cancer activity of anti (7f–7h) and syn (7ff–7hh) isomers in human cancer cell lines. The diastereoselective direct one-pot three-components, Mannich reaction of CDD, having unique conformation, with aromatic aldehydes and anilines in the presence of catalytic amount of NH4OAc giving anti/syn products was studied. The β-amino-carbonyl compounds are potent human anti-cancer cell lines and analogues of 7h–g anti-isomers are active in anti-cancer cell line whereas 7hh–gg syn-isomer is inactive.