Compounds formally derived from an oxoacid $\ce{R_{k}E(=O)_{l}(OH)_{m}}$, ($l \neq 0$) and an alcohol, phenol, heteroarenol, or enol by linking with formal loss of water from an acidic hydroxy group of the former and a hydroxy group of the latter. By extension acyl derivatives of alcohols, etc. Acyl derivatives of chalcogen analogues of alcohols (thiols, selenols, tellurols) etc. are included. E.g. $\ce{R'C(=O)(OR)}$, $\ce{R'C(=S)(OR)}$, $\ce{R'C(=O)(SR)}$, $\ce{R'S(=O)2(OR)}$, $\ce{(HO)2P(=O)(OR)}$, $\ce{(R'S)2C(=O)}$, $\ce{ROCN}$ (but not $\ce{R–NCO}$) ($\ce{R} \neq \ce{H}$).