Page 242 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

222 acylations of a chiral secondary alcohol. Entry 3 is a desymmetrization of a 3-
phenylpentane-1,5-diol. Entry 4 is a resolution of an ester group attached to a
CHAPTER 2 methylene-
-lactone. Entries 5 and 6 are desymmetrizations of diesters.
Stereochemistry,
Conformation,
and Stereoselectivity
T.2.2.2. Proteases and Acylases
Proteases and acylases have the capacity to catalyze hydrolysis and formation of
amide bonds. Proteases find extensive application in analytical biochemistry. 223 The
proteases are used to break down large proteins into polypeptides. Various proteases
exhibit selectivity for particular sequences in peptides and proteins. Many of the
proteases are digestive enzymes and in general they have much more stringent structural
requirements for substrates than esterases and lipases. For example, chymotrypsin is
selective for hydrolysis at the carboxy group of aromatic amino acids, while trypsin
cleaves at the carboxy group of the basic (protonated) amino acids lysine and arginine.
The proteases, like the esterases and lipases, function on the basis of a catalytic triad
involving a serine, histidine, and aspartic acid. 224 They can catalyze formation and
hydrolysis of esters as well as amides. 225
Considerable attention has also been given to enantioselective enzymatic
hydrolysis of esters of -amino acids. This is of particular importance as a means of
preparing enantiopure samples of unusual (non-proteinaceous) -amino acids. 226 The
readily available proteases -chymotrypsin (from bovine pancreas) and subtilisin (from
Bacillus lichenformis) selectively hydrolyze the L-esters, leaving D-esters unreacted.
These enzymatic hydrolysis reactions can be applied to N-protected amino acid esters,
such as those containing t-Boc 227 and Cbz 228 protecting groups.

CO 2 C 2 H 5 α-chymotrypsin CO 2 H
NHBoc NHBoc
(R,S) S-enantiomer, 95% yield, 93% e.e.
subtilisin
PhCH 2 CH 2 CHCO 2 CH 3 PhCH 2 CH 2 CHCO 2 H + PhCH 2 CH 2 CHCO 2 CH 3
NHCbz
(R,S) NHCbz NHCbz
S-enantiomer 87% yield, 99% e.e. R-enantiomer 97% yield, 98% e.e.

Much of the interest in acylases originated from work with the penicillins. Struc-
turally modified penicillins can by obtained by acylation of 6-aminopenicillamic acid.
For example, the semisynthetic penicillins such as amoxicillin and ampicillin are
obtained using enzymatic acylation. 229 Acylases are used both to remove the phenyl-
acetyl group from the major natural penicillin, penicillin G, and to introduce the
modified acyl substituent.
223 A. J. Barrett, N. D. Rawlings, and J. F. Woessner, eds., Handbook of Proteolytic Enzymes, 2nd Edition,
Elsevier, 2004.
224
L. Polgar in Mechanisms of Protease Action, CRC Press, Boca Raton, FL, Chap. 3, 1989; J. J. Perona
and C. S. Craik, Protein Sci., 4 337 (1995).
225 K. Watanabe and S. Ueji, J. Chem. Soc., Perkin Trans. 1, 1386 (2001); T. Ke, C. R. Westcott, and
A. M. Klibanov, J. Am. Chem. Soc., 118, 3366 (1996).
226
T. Miyazawa, Amino Acids, 16, 191 (1999).
227
B. Schricker, K.Thirring, and H. Berner, Biorg. Med. Chem. Lett., 2, 387 (1992).
228 S.-T. Chen, S.-Y. Chen, S.-C. Hsiao, and K.-T. Wang, Biotechnol. Lett., 13, 773 (1991).
229
A. Bruggink, E. C. Roos, and E. de Vroom, Org. Process Res. Dev., 2, 128 (1998).