Hydantoins tautomerism

Hydantoin, 5,5-diaryl-2,4-dithio-methylation, 5, 444 Hydantoin, 1,3-divinyl-polymers, 1, 280 Hydantoin, 5-methylene-polymers, 1, 280 Hydantoin, 5-phenyl-2-thio-tautomerism, 5, 370 Hydantoin, thio-... 

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

Figure 12.1 Reaction mechanism of the 5-monosubstituted hydantoins keto-enol tautomerism under alkaline conditions.

Chemical racemization of the 5-monosubstituted hydantoins proceeds via keto-enol tautomerism under alkaline conditions, as shown in Figure 12.1 [6]. The racemization velocity is highly dependent on the buUdness and electronic factors of the substituent in 5-position (see Table 12.1) and is usually a very slow process... 

Cases in which more than one tautomerizable substituent are present in the imidazole ring are common, but tautomerism studies of such compounds are less so. Work on the hydantoins (imidazolidine-2,4-diones) is not at all definitive UV and IR studies suggest the dioxo form (78), but evidence for the existence of other tautomeric structures (79,80) is not convincing (Scheme 26). ImidazoIidine-4,5-diones also exist as the dioxo forms. NMR studies show that the 2-benzyl derivatives must exist in solution as the NH form (81 R = H, Me), and the methoxy derivatives, similarly, are NH (82) rather than OH (83) derivatives (Scheme 26). 

H-NMR, IR, and UV techniques have been used to study ionization and tautomerism in hydantoins.140... 

From reports in the early literature resting cell bioconversions of hydantoin derivatives, which do not racemize with high velocities, indicated an enzymatic racemization and the presence of a hydantoin racemase. In addition, the chemical and the enzymatic racemization proceed via the keto-enol tautomerism, which is shown in Fig. 12.4-20. Stabilizing effects on the enolate structure such as electronegative substituents are responsible for the velocity of the racemization12, 71. Increased racemization rates can be also seen at more alkaline pH-values and with increased temperatures[71. 

Figure 12.4-20. Keto-enol-tautomerism of 5-monosubstituted hydantoin derivatives.

Dihydroxyimidazoles are involved in keto-enol tautomerism, and the diketo structure, which is known as hydantoin (9.42), is favored. This is the framework for many valuable compounds with anticonvulsant activity. These are nonh5 notic and are used in the treatment of epilepsy. Dilantin (9.55) is the best known of these. Its synthesis is shown in Scheme 9.29. The Strecker reaction of benzophenone with HCN gives an alpha-aminonitrile, which adds ammonia to form compound 9.54 (an amidine). The ring is then closed with phosgene or diethyl carbonate. Hydrolysis gives phenytoin (dilantin, Pfizer, Inc.). 

For example, n-p-hydroxyphenylglydne, a key intermediate in the synthesis of semisynthetic cephalosporins and penicillins, is currently manufactured on a multi-thousand ton scale. The hydantoinase-catalysed reaction is also suitable for the production of unnatural D-amino acids, although the in situ racemization of the remaining substrate via keto-enol tautomerization is generally slow. To facilitate the stereoinversion, base or hydantoin racemase of Pseudomonas and Arthrobacter strains is often used. 

There is evidence that the first product of uricase oxidation is not HDC, but an unknown compound which contains the same number of carbon atoms as uric acid 266-269). HDC and the unknown compound had different ultraviolet absorption spectra the unknown compound was quite unstable and yielded allantoin more readily than HDC. It is possible that this unknown compound (X) had the structure shown in Fig. 14, and that it is converted to allantoin without preliminary hydration to HDC 261). Although X per se is not symmetrical, a tautomeric shift of the double bond to the other ring would result in equivalent structures and explain the equal labeling found in both the urea and hydantoin moieties of allantoin derived from uric-1,3-Ni acid 202). However, hydration of X at the 3 4 double bond could have also produced HDC, which would be converted subsequently to allantoin. Thus, both X and HDC might have been intermediates in the oxidation of uric acid. 

Hydantoinase process, outlined in Fig. 1, includes two hydrolases—hydantoin-hydrolyzing enzyme (hydantoinase) and AT-carbamoyl amino acid-hydrolyzing enzyme (carbamoylase)—and is one of the most efficient and versatile methods for the production of optically active a-amino acids. DL-5-Monosubstituted hydantoins, which are used as common precursors for the chemical synthesis of DL-a-amino acids [1], are the starting material of this enzymatic process. Keto-enol tautomerism is a typical feature of the hydantoin structure. Under neutral conditions, the keto form is dominant in alkaline solution, enolization between the 4 and 5 positions can occur, as has been concluded from the fact that optically pure hydantoins readily racemize. This feature is of practical relevance for the complete conversion of racemic hydantoin derivatives to optically pure L- or D-a-amino acids without any chemical racemization step. A variety of hydantoinase and carbamoylase with different stereospecificity were found. They are D-specific hydantoinase (D-hydantoinase), L-specific hydantoinase (L-hydantoinase), none-specific hydantoinase (DL-hydantoinase), D-specific carbamoylase (D-carbamoylase), and L-specific carbamoylase (L-carbamoylase). With the combination of these enzymes, optically pure amino acids are obtained from DL-5-monosubstituted hydantoins (Fig. 2). The wide substrate range of hydantoinases and carbamoylases also gives generality to the hydantoinase process. 

---