Sometimes chemists need the unnatural D enantiomer of an amino acid, often as part of a drug or an insecticide. Most L-amino acids are isolated from proteins, but the D-amino acids are rarely found in natural proteins. D-amino acids can be synthesized from the corresponding L-amino acids. The following synthetic scheme is one of the possible methods.

(a) Draw the structures of intermediates 1 and 2 in this scheme.
(b) How do we know that the product is entirely the unnatural D configuration?

Draw three-dimensional representations of the following amino acids.

(a) L-phenylalanine

Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn–Ingold–Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(a) What is the relationship between (R)-cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?

Esterase is an enzyme that catalyzes the hydrolysis of esters. It hydrolyzes esters of l-amino acids more rapidly than esters of D-amino acids. How can this enzyme be used to separate a racemic mixture of amino acids?

Draw the structure of the phenylthiohydantoin derivatives of

(c) lysine.

Suggest how you would separate the free L-amino acid from its acylated D enantiomer in Figure 24-5.

Provide the Fischer Projection representation of D-Aspartic Acid. Indicate if the chiral center is R or S.

Convert the following Fischer Projection of L-Proline into a bondline structure.

An opioid pentapeptide has the following structure: Tyr-Cys-Gly-Phe-Cys

a. Draw the structure of the pentapeptide including all the side chains.

b. Write its structure following mild oxidation.