Communications
DOI: 10.1002/anie.200905095
Enzymatic Synthesis
Biocatalytic Friedel–Crafts Alkylation Using Non-natural Cofactors**
Harald Stecher, Martin Tengg, Bernhard J. Ueberbacher, Peter Remler, Helmut Schwab,
Herfried Griengl, and Mandana Gruber-Khadjawi*
À
The formation of C C bonds is a central aspect of
synthetic organic chemistry. However, in biocatal-
ysis only few enzymes capable to perform this
reaction are known, among which aldolases, trans-
ketolases, and hydroxynitril lyases have been
investigated thoroughly.[1] Some have even found
their way into industrial applications.[2]
Friedel–Crafts alkylation is a classic organic
reaction of great importance. However, in partic-
ular for large scale application, this transformation
is ecologically very critical and regiospecific mono-
alkylation is difficult to achieve. Therefore, an
environmentally friendly and selective alternative
would be highly desirable.
In nature methyl groups are selectively intro-
duced into reactive aromatic rings by methyltrans-
ferases (Mtases), in particular with S-adenosyl-l-
methionine (SAM) as the cofactor. Furthermore,
enzyme-catalyzed reactions are important for
access to isoprenoids. Also, prenylation of aro-
matic rings has been performed.[3] For phenyl-
alanine ammonia lyases a Friedel–Crafts-type
mechanism has been proposed.[4]
Recently, it has been shown that besides the
methyl group other alkyl, alkenyl, and alkinyl
groups can be introduced into S-adenosyl-l-homo-
Scheme 1. C-Mtases involved in the biosynthesis of the antibiotics coumermycin A1
in Streptomyces rishiriensis and Novobiocin in Streptomyces spheroides.
cystein. These modified cofactors of transferases
were used for a sequence-specific alkylation of
DNA.[5]
Having cofactor and modified cofactors in hand, we
investigated the possibility of alkylation of aromatic sub-
strates, thus transferring the biosynthesis into the laboratory
(Scheme 1).
[*] Dipl.-Ing. H. Stecher, M. Sc. M. Tengg, Dr. B. J. Ueberbacher,
Dr. P. Remler, Prof. Dr. H. Griengl, Dr. M. Gruber-Khadjawi
Kompetenzzentrum Angewandte Biokatalyse
Petersgasse 14, 8010 Graz (Austria)
Aminocoumarins are antibiotics produced by some Strep-
tomyces species and are targets for the methyl transfer from
the natural cofactor SAM. The Mtase A and B are involved in
Fax: (+43)316-873-8740
E-mail: mandana.gruber@a-b.at
[7]
the biosynthesis[6] of the antibiotics coumermycin A1 (pro-
duced by Streptomyces rishiriensis) and novobiocin[8] (pro-
duced by Streptomyces spheroides; Scheme 1).
Dipl.-Ing. H. Stecher, Dr. M. Gruber-Khadjawi
Institut fꢀr Organische Chemie, Technische Universitꢁt Graz
Stremayrgasse 16, 8010 Graz (Austria)
SAM analogues were synthesized by modifying the
ˇ
strategy published by Klimasauskas, Weinhold, and co-work-
M. Sc. M. Tengg, Prof. Dr. H. Schwab
Institut fꢀr Molekulare Biotechnologie, Technische Universitꢁt Graz
(Austria)
ers.[9] S-Adenosyl-l-homocysteine (SAH) was alkylated by
seven different alkyl bromides using formic acid as the solvent
and AgOTf as a Lewis acid activator and catalyst. We
observed quantitative conversion in less than 2 days (average
reaction time 24 h; Table 1). The chemical synthesis of SAM
analogues results in approximately 1:1 diastereomeric mix-
tures at the sulfonium center. In the first screenings the
diastereomers were separated by preparative HPLC and used
as cofactors for the alkylation of coumarin compound 3a.
Both epimers were accepted by the enzymes NovO and CouO
[**] The ꢂsterreichische Forschungsfꢃrderungsgesellschaft (FFG), the
Province of Styria, and the Styrian Business Promotion Agency
(SFG)—within the framework of the Kplus programme as well as
GASS programme within NAWI Graz—are acknowledged for
financial support, as well as Birgit Krenn for technical support.
M.G.-K. thanks Prof. Dr. Rolf Breinbauer for his fruitful contribution
to this manuscript.
Supporting information for this article is available on the WWW
9546
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 9546 –9548