Isosteric phosphonate analogs of ET-16-OMe. Synthesis and biological evaluation of the enantiomers of 2'-(Trimethylammonio)ethyl 4-(hexadecyloxy)-3-methoxybutanephosphonate and 2'-(trimethylammonio)ethyl 4-(hexadecylthio)-3-methoxybutanephosphonate
(8)] by LiCH2P(O)(OMe)2 using BF3.Et2O in tetrahydrofuran at low temperature. The cytotoxicactivities of the hexadecyloxy and hexadecylthio phosphonate analogs of ET-18-OMe (1 and 2) against the murine leukemias WEHI-3B,L1210, and P388 were similar, indicating that substitution of a sulfur atom for oxygen in the long-chain ether does not result in a significant difference in cytotoxicity. The IC50 values