Phosphonopyruvates: Syntheses, NMR Investigations, and ReactionsThe new 3‐(diethoxyphosphoryl)‐2‐oxopropanoates5–24and ‐propanamides25–38with various substituents at C(3) were prepared in moderate‐to‐good yields (Schemes 2and3, Tables 1and2). It was shown that they adopt a preferred conformation in which the diethoxyphosphoryl group and the substituent at C(4) are antiperiplanar to each other (seeB). The keto‐enol tautomerization of phosphonopyruvates with PhC(3) (see20) and MeSC(3) (see24and33) was examined. In CHCl3, two tautomeric species exist, whereas in dimetylsulfoxide (DMSO), three tautomeric forms are observed. Oxime ethers, an oxime, and a phenylhydrazone of unsubstituted phosphonopyruvates were prepared (see40–44), and quinoxalin‐2(1H)‐ones could be obtained from the reaction of pyruvates with 4,5‐dimethylb