C O M M U N I C A T I O N
Cross-metathesis coupling of sugars and fatty acids to lysine and
cysteine
Andrea J. Vernall and Andrew D. Abell*
Department of Chemistry, University of Canterbury, Christchurch, New Zealand.
E-mail: andrew.abell@canterbury.ac.nz; Fax: +64-3-3642110; Tel: +64-3-3642818
Received 25th June 2004, Accepted 2nd August 2004
First published as an Advance Article on the web 10th August 2004
Attachment of an olefin tether to the side chain of either lysine
or cysteine allows cross metathesis (CM) conjugation with
olefin-containing sugar and fatty acid analogues.
The ability to link or conjugate a molecule to the side chain of
lysine or cysteine, be it as a separate amino acid or as part of a
peptide or protein, is fundamental to a number of important natural
and non-natural processes. Examples include the biosynthesis of
glycoproteins,1 the definition of protein structure,2 the preparation of
haptens for monoclonal antibody production,3 and the cross-linking
of proteins associated with the formation of blood clots, collagen
and food-stuffs.4 Nature has evolved a number of general methods
to achieve this goal, for lysine these include the Maillard reaction4
and cross-linking catalysed by enzymes such as transglutaminase5
and the formation of disulfide bonds in the case of cysteine. By
contrast, synthetic chemists are essentially limited to the formation
of an amide or glycosidic bond as a general method of conjugating
an organic molecule to a lysine side chain,6 and mimicking disulfide
Scheme 1 (i) DIEA, CH2Cl2, 2a (3a, 78%); EDCI, HOBt, DIEA, CH2Cl2
bond formation in the case of cysteine. These restrictions greatly
limit the choice of conjugation partners.
and 2b (3b, 99%), or 2c (3c, 58%); (ii) HCl gas, then N-Boc-L-Phe, EDCI,
HOBt, DIEA, CH2Cl2 (55%); (iii) BOP-Cl, TEA, CH2Cl2 and 2b (5b, 93%)
or 2c (5c, 82%).
We now report a simple sequence whereby an olefin tether is
attached to the side chain of either lysine or cysteine to allow
cross metathesis (CM)7 conjugation with an olefin partner. CM
conjugations of olefins to allylglycine8 and vinylglycine9 are known,
however these non-natural amino acid are expensive and difficult
to access enantiomerically pure, particularly in large quantities. By
contrast, the method reported here uses cheap, natural amino acids
(lysine and cysteine) to which is attached an olefin tether of variable
length. These amino acids are often associated with cross-linking
in their own right and as such bioconjugates resulting from CM of
the olefin tethered derivatives, with biologically important olefins,
should be of use in biosynthetic studies and as pharmaceutical
targets.
Fig. 1 Structure of the olefin coupling partners and CM catalyst.
the ethylene by-product to help drive the reaction to completion.
The reaction mixtures were worked-up by adding DMSO followed
by silica gel chromatography and the thus obtained cross coupled
1
N-Boc-L-lysine 1 and N-Boc-L-cysteine 4 were used to prepare
the N- and S-acylated derivatives that bear an olefin tether of
variable chain length for cross metathesis studies (see 3a–c and
5b,c, Scheme 1). To this end, N-Boc-L-Lys-OMe 1 was treated with
EDCI/HOBt and either 2b or 2c to give N-substituted alkyl amides
3b and 3c, respectively. The ,-unsaturated amide derivative 3a
was synthesised by reaction of 1 with acryloyl chloride 2a in the
presence of base. Reaction of the corresponding acrylic acid was
attempted using EDCI/HOBt without success. The corresponding
S-acylated cysteines 5b and 5c were prepared by BOP-Cl mediated
coupling of N-Boc-L-cysteine with the alkene acids 2b and 2c.
Finally, the lysine-based dipeptide 6 was prepared to extend the
investigation beyond a single amino acid-based substrate. The
N-Boc protecting group of 3b was removed on treatment with
HCl gas and the resultant hydrochloride salt was coupled to
N-Boc-L-Phe-OH to give 6 in 63% overall yield. The non-peptide,
organic coupling partners were chosen to allow conjugation of
lysine and cysteine amino acids to biologically important olefins
such as those derived from sugars 2,3,4,6-tetra-O-benzyl-1--C-
allylglucoside (7)10 and fatty acids (8) (Fig. 1).
products were essentially pure by H NMR spectroscopy.12 Yields
have not been optimized and some homodimer products derived
from 8 were also isolated in its couplings with 3a–c and 5c.
The fatty acid analogue 8 was successfully coupled to the
lysine-based olefins 3a, 3b and 3c to give 10a (69%), 10b (45%)
and 10c (64%), respectively, pathway A Scheme 2. The higher yield
of 10a, relative to 10b, is consistent with literature where it has been
noted that cross metathesis reactions involving an ,-unsaturated
coupling partner, as in 3a, are favored due to effective chelation of
this moiety to the catalyst.13 The extended tether of 3c would also
appear to be favored for cross metathesis couplings where 10c was
obtained in good yield. The lysine containing dipeptide 6 was also
coupled with 8 to give 11 in 39%, thus demonstrating applicability
of the methodology to systems other than simple side chain
N-acylated lysines, Scheme 2 pathway B. Future extension of this
methodology to more complex peptides will be linked to ongoing
development of metathesis catalysts with improved efficiency in
aqueous media.14
Next we coupled 3a to the sugar analogue 7 to give the conjugate
12a in 55%, Scheme 2 pathway C. The side chain N-acylated
lysines 3b and 3c, also underwent cross metathesis with 7 to give
12b (33%) and 12c (65%), respectively. Here again superior yields
were observed in reactions of 3a and 3c relative to 3b. The sugar
conjugate 12c was subsequently treated with Pd on C and hydrogen
The cross metathesis reactions as shown in Schemes 2 and 4 were
carried out using Grubb’s 2nd generation catalyst 911 with 1 equiv. of
the amino acid-based substrate and 3 equiv. of 7 or 8. The reactions
were carried out under a flow of nitrogen to facilitate removal of
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 5 5 5 – 2 5 5 7
2 5 5 5