The marine alkaloids, longamide B (1), longamide B methyl ester (2), hanishin (3), and a series of non-naturally occurring analogues were synthesized in an efficient manner from inexpensive commercially available dl-aspartic acid dimethyl ester. The cytotoxicities of these natural products (1-3) and their analogues (9-15) were evaluated against human lung adenocarcinoma (A549) and human prostate cancer (PC3) cells. This is the first evaluation of the cytotoxicities of these alkaloids in these cancer cell lines and it revealed that analogue 15 had comparable cytotoxic activity to its natural parent compound, (±)-hanishin (3). This study provides useful information for further structural modification of these alkaloids in order to develop novel antitumor agents.