ORGANIC
LETTERS
2012
Vol. 14, No. 6
1378–1381
An Asymmetric Synthesis of L-Pyrrolysine
Margaret L. Wong,† Ilia A. Guzei,† and Laura L. Kiessling*,†,‡
Departments of Chemistry and Biochemistry, University of Wisconsin;Madison,
Madison, Wisconsin 53706, United States
Received January 10, 2012
ABSTRACT
An efficient asymmetric synthesis of the 22nd amino acid L-pyrrolysine has been accomplished. The key stereogenic centers were installed by an
asymmetric conjugate addition reaction. A Staudinger/aza-Wittig cyclization was used to form the acid-sensitive pyrroline ring. Pyrrolysine was
synthesized in 13 steps in 20% overall yield.
Pyrrolysine is the 22nd genetically encoded amino acid.1
It consists of a (4R,5R)-4-methyl-5-carboxypyrroline ring
linked to the ε-nitrogen of L-lysine.2,3 Pyrrolysine was
identified by X-ray crystallography, when it was first
observed in the structure of Methanosarcina barkeri mono-
methylamine methyltransferase.1 It has been hypothesized
that this unique amino acid plays an important role in
methane production in some archaeal species.4
Pyrrolysine is encoded by an in-frame UAG codon,
which is nonterminating, and its incorporation is mediated
by a dedicated tRNA and cognate tRNA synthetase.5 The
mechanism of pyrrolysine incorporation into proteins
offers a platform for developing new protein labeling
methods.6 Small molecules, such as fluorophores,6a biotin,5e
ubiquitin,6b and oligosaccharides,5e have been used for
labeling pyrrolysine and pyrrolysine-surrogate residues.7
Although pyrrolysine has been exploited for protein label-
ing, an understanding of its biosynthesis,5e,8 evolutionary
purpose, and distribution in the proteome is incomplete.
New synthetic methods that provide sufficient quantities of
this novel amino acid would facilitate an understanding of
pyrrolysine biochemistry.
Pyrrolysine is a synthetic target.2,9 One reported chemi-
cal synthesis2 involves the coupling of (4R,5R)-4-methyl-
pyrroline-5-carboxylic acid to lysine. While this innovative
route affords the desired product, the yield is modest (9%
overall). Moreover, we and others5a,9 have found that the
reactions were irreproducible. We therefore sought to
devise an alternative synthetic route.
† Department of Chemistry.
‡ Department of Biochemistry.
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J. A.; Chan, M. K. Science 2002, 296, 1462. (c) Srinivasan, G.; James,
C. M.; Krzycki, J. A. Science 2002, 296, 1459.
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(9) Berube, A. Progress Toward the Total Syntheses of the Polycyclic
Terpenes Providencin and Bacchopetiolone, and Study of Pyrrolysine;
Yale University, 2006.
r
10.1021/ol300045c
Published on Web 03/06/2012
2012 American Chemical Society