Hydrolysis of 4-imino-imidazolidin-2-ones in acid and the mechanism of cyclization of hydantoic acid amides.

The hydrolysis of iminohydantoins generates the same tetrahedral intermediate as that obtained in the cyclization of hydantoic acid amides to hydantoins. The ratio of the products of imine hydrolysis under kinetic control is determined by the relative height of the barriers of the breakdown of to amide or to hydantoin. Thus the partitioning of products unequivocally proves which is the rate determining step in the cyclization reaction-formation or breakdown of .

Thorpe-Ingold effects in cyclizations to five-membered and six-membered rings containing planar segments. The rearrangement of N(1)-alkyl-substituted dihydroorotic acids to hydantoinacetic acids in base.

While the gem-dimethyl effect (GDME) is quantitatively similar for cyclizations to cyclopentane and cyclohexane rings and their homomorphs, in systems containing planar segments the GDME is stronger for the formation of five-membered rings. Planar pentagons have smaller angles than planar hexagons and their formation is helped by the decrease in the potential internal bond angle caused by substituents, as suggested by Thorpe and Ingold for small rings. The phenomenon is illustrated with crystal structure data on five-membered hydantoins and six-membered dihydrouracils containing four-atom planar segments.

Kinetics and mechanism of the cyclization of omega-(p-nitrophenyl)-hydantoic acid amides: steric hindrance to proton transfer causes a 10(4)-fold change in rate.

The pH-rate profiles for the cyclization of primary 2,3-dimethyl and 2,2,3-trimethyl-hydantoinamides (2-UAm and 3-UAm respectively) differ strikingly from those for the cyclizations of the corresponding N-methylated amides 2-MUAm and 3-MUAm; which are dominated by the water reaction, spanning some 6 pH units. For the cyclization of UAm the plateau extends over no more than two pH units. The difference is due to the slower base-catalyzed cyclization of the N-methylamides.

Schiff base addition to cyclic dicarboxylic anhydrides: an unusual concerted reaction. An MO and DFT theoretical study.

The existing controversy as to whether dicarboxylic anhydride iminolysis by Schiff bases is a concerted [4 + 2] addition or a stepwise reaction following either a Perkin-like route or occurs as iminolysis via zwitterionic intermediates is solved computationally by DFT and MO theory. Both types of theory favor a concerted mechanism starting as bimolecular addition of imine to the carbonyl carbon of anhydride monoenol, accompanied by simultaneous elimination of carboxylate. The reaction proceeds further without forming any intermediate and completes as intramolecular charge transfer from enol HOMO to imine LUMO with ring closure.