Thiohydantoin anions

In one example, reaction of 4-hydroxybenzaldehyde (109) with the enolate anion of 108, generated by reaction with the base pyridine, leads to a new thiohydantoin, 110. When 110 is heated with thiourea (111) at 100°C for several hours, S=C=NH is lost and the other product is the amino acid tyrosine (60). 

Another amino acid synthesis is called the azlactone synthesis. Remember from before that an azlactone is an oxazolone (see 95). When glycine (52) is converted to its AT-benzoyl derivative (112 known as hippuric acid) by reaction with benzoyl chloride, treatment with acetic anhydride (AC2O) gives the azlactone 113. This is the reaction presented in the preceding section (see compormd 95). Compound 110 has the common name of hippuric acid azlactone. As with the thiohydantoin, the -CH2- unit in 113 is susceptible to an enolate anion condensation reaction with aldehydes (Chapter 22, Section 22.7.2), and reaction with 2-methylpropanal in the presence of pyridine gives azlactone 114. Catalytic hydrogenation of the alkene unit (Chapter 19, Section 19.3.2) and acid hydrolysis lead to the amino acid leucine (55). 

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