Derivatives of Pseudoacids

Like carboxylic acids, cyclic oxo-carboxylic acids (or pseudoacids) can have a range of structural derivatives in the same oxidation states.  For example, pseudoacids and carboxylic acids form halides: the pseudoacid halide and the acyl halide, respectively.  In fact, the transformation can be effected with thionyl chloride in both cases.

Example of the pseudoacid chloride of o-carboxybenzaldehyde ( picture or .pdb )

Pseudoacids can form pseudoesters as carboxylic acids form normal esters: by chemical replacement of the OH with OR functions.   Pseudoesters are conveniently prepared through the pseudoacyl halide, while normal esters can be made by reaction of the acyl halide or by Fischer esterification.

Example of the methyl pseudoester of o-benzoylbenzoic acid ( picture or .pdb )

After these, the similarities between the two systems begin to diverge.  While carboxylic acids form three kinds of amides (primary, secondary, tertiary), pseudoacids form amides with either endo- or exocyclic nitrogen.  Therefore there are two kinds of primary and secondary pseudoamides and a single kind of tertiary amide (exocyclic N), for a total of five pseudoamides.

An endocyclic N secondary pseudoamide example: N-ethyl-3-hydroxyisoindolone ( picture or .pdb )
An exocyclic N tertiary pseudoamide example: 3-morpholinoisobenzofuranone ( picture or .pdb )

Things get even more interesting for the pseudoanhydrides.  Since this functional group is normally acyl-O-acyl, involvement with pseudoacids leads to the possibilities for pseudoacyl-O-acyl and pseudoacyl-O-pseudoacyl.    Additionally, since the pseudoacyl carbon (old aldehyde or ketone carbonyl carbon) is chiral, the symmetrical di-pseudoacid anhydride can have three forms, (R;R), (S;S) and the meso isomer.   This expands necessarily to four kinds for unsymmetrical di-pseudoacid anhydrides (two pairs of diastereomers).

A 'symmetrical' normal-pseudo anhydride example of o-benzoylbenzoic acid ( picture or .pdb )
A symmetrical dipseudo anhydride example of o-formylbenzoic acid (RR&SS) ( picture or .pdb )
A symmetrical dipseudo anhydride example of o-formylbenzoic acid (RS&SR) ( picture or .pdb )
A symmetrical dipseudo anhydride example with a six-ring analog; o-carboxyphenylethanal ( picture or .pdb )

Then there are the pseudoimides, with even more variations.

Protein data base (pdb) files can be viewed with Rasmol/Raswin and other viewers, available online.