Isosteric Phosphonate Analogues 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.