Alkyne Ligation Handles: Propargylation of Hydroxyl, Sulfhydryl, Amino, and Carboxyl Groups via the Nicholas Reaction

Article abstract of DOI:10.1021/acs.orglett.6b02088

The Nicholas reaction has been applied to the installation of alkyne ligation handles. Acid-promoted propargylation of hydroxyl, sulfhydryl, amino, and carboxyl groups using dicobalt hexacarbonyl-stabilized propargylium ions is reported. This method is us

Full text of DOI:10.1021/acs.orglett.6b02088

Letter
pubs.acs.org/OrgLett
Alkyne Ligation Handles: Propargylation of Hydroxyl, Sulfhydryl,
Amino, and Carboxyl Groups via the Nicholas Reaction
Sarah M. Wells,† John C. Widen,‡ Daniel A. Harki,*,‡ and Kay M. Brummond*,†
†Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15206, United States
‡Department of Medicinal Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
S
* Supporting Information
ABSTRACT: The Nicholas reaction has been applied to the
installation of alkyne ligation handles. Acid-promoted
propargylation of hydroxyl, sulfhydryl, amino, and carboxyl
groups using dicobalt hexacarbonyl-stabilized propargylium
ions is reported. This method is useful for introduction of
propargyl groups into base-sensitive molecules, thereby
expanding the toolbox of methods for the incorporation of
alkynes for bio-orthogonal reactions. High-value molecules are used as the limiting reagent, and various propargylium ion
precursors are compared.
idespread use of the Huisgen 1,3-dipolar cycloaddition
Another commonly used protocol for installing an alkynyl
Wbetween azides and alkynes to form 1,2,3-triazoles, a click group is a carbodiimide-mediated coupling reaction between 5-
reaction,1 has led to increased interest in transformations used to
synthesize and/or install alkynyl groups.2 Typically, when
readying substrates for a click reaction, late-stage propargylation
or 5-hexynoylation reactions of hydroxyl or amino groups are
used to attach the desired alkynes.2 Propargylation of a hydroxyl
group is usually achieved by a Williamson ether synthesis under
basic conditions where the corresponding alkoxide is reacted
with propargyl bromide (Figure 1, eq 1). Src-directed probe 1
was prepared using this approach but required protection of the
3′- and 5′-hydroxyl groups and 6-amino group to avoid over-
propargylation.3 Propargylation has also been accomplished by
converting a hydroxyl group into a leaving group (i.e., a mesylate)
and replacing it with propargylamine.4
hexynoic acid and a hydroxyl or amino group (Figure 1, eq 2).2b,5
The Duocarmycin probe 2 exemplifies a product obtained from
an EDC6-mediated coupling reaction between a cyclic, secondary
amino group and 5-hexynoic acid.7 While carbodiimide
couplings offer nonbasic, neutral conditions, they require
expensive reagents and/or the tedious removal of urea-related
byproducts.
Many other methods are available for the functionalization of a
compound with an alkynyl group;2,8 however, despite these
options, challenges still arise when alkynylating functionally
dense natural products and chemical probes for applications such
as activity-based protein profiling9 for target identification.10 For
example, during investigations to label two different sesquiter-
pene analogues with alkynyl groups (vide infra), these analogues
were unstable to the basic conditions required for propargylation.
Although the hexynoylation reaction could serve as an alternative
for appendage of an alkyne ligation handle via an allylic ester
linkage, concerns about the metabolic stability of ester-
containing probes in cell culture lowered enthusiasm for this
approach.11 Consequently, a method for propargylation of these
sesquiterpene analogues and other biomechanistic probes under
nonbasic conditions was needed.
We report our studies to establish the Nicholas reaction as an
alternative protocol for the propargylation of high-value small
molecules. The Nicholas reaction involves the addition of a
nucleophile to the Co-stabilized propargylic carbocation 3,
generated by treating the corresponding dicobalt hexacarbonyl
complexed (Co2(CO)6)-propargyl alcohol with acid. Alkyne 4 is
formed after oxidative decomplexation (Figure 1, eq 3).12 While
it is well-known that the Nicholas reaction can be used to effect
Received: July 27, 2016
Figure 1. Synthetic methods for alkyne incorporation.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02088
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Letter
propargylation reactions of heteronucleophiles, classical con-
ditions require excess nucleophile relative to the cobalt−carbonyl
complex, even as the solvent in some cases, limiting its utility in
the preparation of alkyne ligation handles.13 Conditions where
the nucleophile is the limiting reagent would expand the utility of
this approach.
purification (Scheme 1). Use of N-methylmorpholine-N-oxide as
an oxidant in this transformation resulted in decomposition of
7.15
Scheme 1. Decomplexation of Co2(CO)6-Alkyne 7
To increase the efficiency of the Nicholas reaction, we began
our investigations using molecularly complex alcohol 5 as the
limiting reagent. Initially, a reaction was carried out with a
1:1.1:1.4 ratio of nucleophile 5/6a/BF3OEt2. However, these
conditions led to a moderate yield of 40%, so we focused on using
higher equivalents of 6a and BF3OEt2. We varied the molar
equivalencies of 6a and the Lewis acid (BF3OEt2) while keeping
the order of addition constant. To reduce the likelihood of the
alcohol and ester groups of 5 tying up the BF3OEt2, Co2(CO)6-
propargyl alcohol 6a was added to BF3OEt2 to form the
propargyl cation, followed by the addition of alcohol 5. Using this
addition order and a 1:2:2.5 molar ratio of alcohol 5/6a/
BF3OEt2, Co2(CO)6-propargyl ether 7 was obtained in 47% yield
(Table 1, entry 1) after stirring 4.5 h at 0 °C. Increasing the
Next, the generality of these optimized reaction conditions was
tested on hydroxyl-, sulfhydryl-, amino-, and carboxyl-containing
amino acids: a class of compounds selected for their richness of
functionality and the utility of propargylated peptides for
biochemical applications.1d,2a,16 Unfortunately, when subjecting
N-Boc-L-serine methyl ester (9a) to the optimized reaction
conditions, 10a was obtained in 20% yield while 76% of the
starting material 9a was recovered (Table 2, entry 1). Similarly,
when N-Fmoc-L-serine methyl ester (9b) was subjected to the
same conditions, 10b was isolated in 29% yield with 63%
recovered 9b (entry 3). In both of these examples, 6a was fully
consumed and the dimerized product of 6a was obtained,
resulting from the propargylium cation reacting more readily
with the hydroxyl group of 6a. To overcome this competing
homodimerization reaction, Co2(CO)6-methyl propargyl ether
6b was examined.12c Reaction of 9a with 6b afforded 10a in 97%
yield (entry 2). The yield of 10b also increased significantly to
54% when using 6b (entry 4). Use of propargyl acetate for the
synthesis of 10a and 10b gave yields comparable to those of 6a
(see Supporting Information (SI)).
Next, we tested this method for the propargylation of cysteine
thiols, a transformation typically accomplished using basic
alkylation conditions.2a,17 Thiols react efficiently in the Nicholas
reaction; however, application has been limited to the synthesis
of sulfur-containing macrocycles.12c,18 N-Acetyl- and N-Fmoc-L-
cysteine ethyl ester (9c and 9d) were reacted with 6a, giving the
corresponding Co2(CO)6-alkynes 10c and 10d in high yields of
86 and 71% (entries 5 and 6). N-Fmoc cysteine 9d was also
reacted with 6b, which gave a comparable yield of 67% for 10d
(entry 7).
To evaluate the phenolic side chain of tyrosine in the Nicholas
reaction, N-Boc-L-tyrosine methyl ester (9e) was reacted with
6a.13a Two major products were observed; the desired product,
10e, was isolated in 45% yield (57% based on recovered 9e)
(entry 8), and an unstable byproduct was obtained in trace
amounts. 1H NMR analysis of this byproduct revealed aromatic
signals integrating for three protons, resulting from electrophilic
aromatic substitution (see SI, S5).12c Because 9e was recovered
along with complete consumption of 6a, 6b was tested. This
reaction required a longer reaction time and did not improve the
yield of 10e (23% yield, entry 9) due to Boc instability.19 When
the N-Fmoc tyrosine ester 9f was reacted with complex 6a, 10f
was formed in 6% yield (56% based on recovered starting
material) (entry 10). Employing 6b resulted in a significantly
improved yield to 73% (entry 11). A byproduct, presumably
formed by electrophilic aromatic substitution, was also observed
by TLC for these reactions.
Table 1. Optimization of Nicholas Reaction with Alcohol 5
equiv
(5/6a/LA)
order of
addition
temp
(°C)
time
(h)
yield
(%)
entry
a
1
1:2:2.5
1:3:5
LA, 6a, 5
LA, 6a, 5
0
0
0
0
0
0
0
4.5
4
47
36
28
44
55
22
60
2
3
c
1:2:2.5
1:2:2.5
1:2:2.5
1:3:5
LA, 6a, 5
LA, 5, 6a
6a, 5, LA
6a, 5, LA
6a, 5, LA
4
a
4
4.5
4
b
5
6
5
b
d
7
1:2:2.5
3.5
a
b
Dimerized product of 6a was isolated in 33−34% yield. Dimerized
c
product of 6a was isolated in 25−28% yield. Alcohol 5 was added
dropwise over 5 min. Complex 6a was generated in situ from
d
propargyl alcohol and Co2(CO)8.
equivalents of BF3OEt2 and 6a afforded 7 in 36% yield (entry 2).
Adding alcohol 5 more slowly lowered the yield of 7 to 28%
(entry 3).
Due to these results, the order of addition was examined.
Adding 5 to BF3OEt2 prior to addition of 6a did not effect the
yield of 7, obtained in 44% yield (entry 4). Next, 5 (1 equiv) and
BF3OEt2 (2.5 equiv) were added sequentially to 6a (2 equiv),
which increased the yield of 7 to 55% (entry 5). With this same
order of addition, increasing the amount of 6a and BF3OEt2
lowered the yield of 7 to 22% (entry 6). For all of these examples,
6a was prepared, isolated, and purified by column chromatog-
raphy before the reaction. Although this complex is stable to air
and moisture, it was reasoned that forming 6a in situ may be
advantageous.13c To this end, 6a was formed in situ from
propargyl alcohol and dicobalt octacarbonyl, followed by the
sequential addition of 5 and BF3OEt2 to afford the highest yield
of 7 (60%, entry 7). A final attempt to improve the reaction
conditions by lowering the reaction temperature only resulted in
decreased yields of 7.14 Decomplexation of cobalt complex 7 was
achieved using ceric ammonium nitrate (CAN) in acetone to
readily afford alkyne 8 in 97% yield without the need for
Amino groups were tested by subjecting L-proline methyl ester
(9g) to the Nicholas reaction with 6a. Consumption of 9g was
observed by TLC within 15 min with no evidence of 10g (entry
B
DOI: 10.1021/acs.orglett.6b02088
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with tetrafluoroboric acid in diethyl ether at 0 °C.12b,20 Reaction
of 6c with 9g in DCM at 0 °C afforded Co2(CO)6-alkyne 10g in
46% yield (entry 13). The primary amine of L-phenylalanine
methyl ester (9h) also proved to be an effective nucleophile;
when reacted with 6c, dialkylation afforded amine 10h in 59%
yield (entry 14).
Table 2. Synthesis of Alkyne-Modified Amino Acids (AA)
(Nucleophilic Group (−XH) of AA Highlighted in Red)
Carboxyl groups were also subjected to the Nicholas reaction
conditions. Only a few examples of carboxyl groups serving as a
nucleophile in the Nicholas reaction have been reported.21
Reaction of N-Bz-D-phenylalanine (9i) with 6a and BF3OEt2
afforded Co2(CO)6-propargyl ester 10i in 60% yield (entry 15).
Reaction of 9i with 6c afforded a lower yield for 10i (18%, entry
16); thus, the utility of preformed propargylium salt is not
necessarily general.
Co2(CO)6-alkyne-modified amino acids 10a−i underwent
oxidative decomplexation with CAN. Propargyl derivatives of
serine, cysteine, tyrosine, and phenylalanine 11a−f,i were
afforded in high yields (75−94%). A moderate yield of 56%
was observed for the formation of dipropargylamine 11h (entry
14). Proline alkyne derivative 11g appeared to be unstable,
permitting isolation and NMR characterization only once prior
to decomposition (entry 12).
To effect monoalkynylation of primary amines, an alternative
tetrafluoroborate salt 12 was prepared from Co2(CO)6-2-
methyl-3-butyn-2-ol (Scheme 2). Reaction of 12 with 9h
afforded the monoalkynylated propargylamine 13 after oxidative
decomplexation.
Scheme 2. Reaction of 9h with BF4− 12
Finally, to show the synthetic utility of these conditions for
base-sensitive, functionally dense molecules, we applied the
Nicholas reaction conditions to two sesquiterpene analogues.
Base-sensitive guaianolide analogue 14, previously synthesized in
our group, was reacted with 6a, formed in situ, and BF3OEt2 to
give the Co2(CO)6-alkyne derivative in 46% yield.22 Reaction
with CAN generated alkyne probe 15 in quantitative yield.
Melampomagnolide B (MelB) (16) was used as a parthenolide
mimic for conjugation to biotin via an ester linkage.23,24
However, these biotinylated compounds may have metabolic
stability issues for in vivo biochemical experiments. Formation of
the alternative ether linkage using the allylic alcohol handle has
proven to be difficult; MelB is base-sensitive, and the allylic
hydroxyl group was unreactive in our hands toward oxidation or
bromination.25 Reaction of 16 with 6a and BF3OEt2 afforded the
corresponding Co2(CO)6-alkyne product after 1 h in 19% yield.
A shortened reaction time of 10 min gave a 41% yield (45% yield
based on recovered 16), suggesting the Co2(CO)6-alkyne
product was unstable to the reaction conditions. Reacting 16
with 6b gave a 39% yield of the coupled product. Cobalt
decomplexation afforded the MelB alkyne probe 17 in 94% yield
(Scheme 3).
a
b
Complexes 6a and 6b were formed in situ. BF3OEt2 is not used
c
d
when using 6c. Use of isolated 6b gave the highest yield. 11g is
unstable. For additional examples, see SI.
In conclusion, the Nicholas reaction conditions described
provide an acid-mediated alternative for propargylation of
molecularly complex compounds. Reaction conditions were
optimized for use of high-value nucleophiles as limiting reagents,
12). We presume the BF3OEt2 coordinates with the nitrogen of
proline. To circumvent this issue, the cationic propargylium ion
was prepared as tetrafluoroborate salt 6c by reacting complex 6a
C
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ASSOCIATED CONTENT
* Supporting Information
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Chem. 2010, 75, 6660−6672.
(14) Reactions at −40 and −10 °C afforded 7 in 23 and 38% yield,
respectively.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02088.
1
Full experimental details, characterization data, and H
and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: daharki@umn.edu.
*E-mail: kbrummon@pitt.edu.
Notes
(15) Hayashi, Y.; Yamaguchi, H.; Toyoshima, M.; Okado, K.; Toyo, T.;
Shoji, M. Chem. - Eur. J. 2010, 16, 10150−10159.
(16) Ahmad Fuaad, A.; Azmi, F.; Skwarczynski, M.; Toth, I. Molecules
2013, 18, 13148−13174.
The authors declare no competing financial interest.
(17) Struthers, H.; Spingler, B.; Mindt, T. L.; Schibli, R. Chem. - Eur. J.
2008, 14, 6173−6183.
ACKNOWLEDGMENTS
■
(18) (a) Hope-Weeks, L. J.; Mays, M. J.; Woods, A. D. J. Chem. Soc.,
We acknowledge the NIH (R21-CA194661 to D.A.H. and R01-
GM054161 to K.M.B.) and the Department of Defense
(PC141033 to D.A.H.) for funding. Mass spectrometry
performed at the University of Minnesota was conducted at
the Masonic Cancer Center Analytical Biochemistry Core
Facility, which is supported by the National Institute of Health
(P30-CA77598).
Dalton Trans. 2002, 1812−1819. (b) Hagendorn, T.; Brase, S. RSC Adv.
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2014, 4, 15493−15495.
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Synth. Commun. 1997, 27, 1819−1825.
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