Exploiting the co-reliance of tumours upon transport of amino acids and lactate: Gln and Tyr conjugates of MCT1 inhibitors

Article abstract of DOI:10.1039/c5md00579e

Glutamine and tyrosine-based amino acid conjugates of monocarboxylate transporter types 1 and 2 inhibitors (MCT1/2) were designed, synthesized and evaluated for their potency in blocking the proliferation of a human B lymphoma cell line that expresses the transporters Asct2, LAT1 and MCT1. Appropriate placement of an amino acid transporter recognition element was shown to augment anti-tumour efficacy vs. Raji cells. Amino acid conjugation also improves the pharmacodynamic properties of experimental MCT1/2 inhibitors.

Full text of DOI:10.1039/c5md00579e

MedChemComm
View Article Online
View Journal
RESEARCH ARTICLE
Exploiting the co-reliance of tumours upon
transport of amino acids and lactate: Gln and Tyr
Cite this: DOI: 10.1039/c5md00579e
conjugates of MCT1 inhibitors†‡
Reji N. Nair,§a Jitendra K. Mishra,§a Fangzheng Li,a Mariola Tortosa,a
Chunying Yang,b Joanne R. Doherty,c Michael Cameron,d John L. Cleveland,b
a
a
*
William R. Roush and Thomas D. Bannister
Received 14th December 2015,
Accepted 19th February 2016
Glutamine and tyrosine-based amino acid conjugates of monocarboxylate transporter types 1 and 2 inhibi-
tors (MCT1/2) were designed, synthesized and evaluated for their potency in blocking the proliferation of a
human B lymphoma cell line that expresses the transporters Asct2, LAT1 and MCT1. Appropriate placement
of an amino acid transporter recognition element was shown to augment anti-tumour efficacy vs. Raji cells.
Amino acid conjugation also improves the pharmacodynamic properties of experimental MCT1/2 inhibitors.
DOI: 10.1039/c5md00579e
www.rsc.org/medchemcomm
cells that are reliant upon amino acid transporters. Examples
include the imaging agents 1 (ref. 9) and 2 (ref. 10) (Fig. 1),
Introduction
Proteomics and functional analyses of many tumour types
have shown a shared reliance upon active transport systems
to drive rapid cancer cell growth and to sustain survival. For
example, a greatly enhanced rate of uptake of amino acids,
particularly glutamine and large neutral amino acids, is ubiq-
uitous in malignant cells.1–5 These cells largely rely upon the
Asct2 and LAT1 transporters for import of glutamine and
large neutral amino acids, respectively, thus these trans-
porters have been dubbed “partners in crime” for their role in
driving cancer progression.5 Monocarboxylate transporters
(MCTs) are also often up-regulated to transport lactic acid, a
by-product of energy production via glycolysis (the Warburg
effect).6–8 The co-reliance on MCT1, Asct2, and LAT1 is partic-
ularly manifest in tumours with MYC oncogene involvement.4
Tethered amino acids have been used to direct molecular
imaging agents to tumour cells, which leads to the selective
accumulation of the amino acid/imaging agent conjugates in
which are substrates for the Asct2 and LAT1 transporters,
respectively.
The tethered glutamine residue of 1 and the tyrosine core
of 2 deliver, respectively, the gadolinium-containing MRI con-
trast warhead and the radioactive fluorine-containing PET
warhead to affected cells, permitting high sensitivity in
detecting tumours. The transport of the Gd-containing com-
plex 1, in particular, indicates that even large complexes can
be accommodated as substrates by Gln transporters.
A logical extension of this concept is to use amino acid
transport to deliver not an imaging agent but rather an anti-
tumour agent to cancer cells. This principle also has prece-
dent in the mustard agent melphalan (3), which has en-
hanced brain exposure and efficacy relative to other aniline
mustards because it is a LAT1 substrate (LAT1 is abundant in
the blood–brain barrier, as well as in many tumour types).11
Indeed, the melphalan analogue
greater brain exposure vs. compound 3 due to improved
LAT1 transport (Fig. 2).12
4
achieves ∼1000-fold
a Department of Chemistry, The Scripps Research Institute, 110 Scripps Way,
Jupiter, FL 33458, USA. E-mail: tbannist@scripps.edu
b Department of Tumor Biology, Moffitt Cancer Center & Research Institute, 12902
A conceptually similar delivery strategy, using folates teth-
ered to imaging agents or anti-tumour agents, has been
Magnolia Drive, Tampa, FL 33612, USA
c Department of Cancer Biology, The Scripps Research Institute, 110 Scripps Way,
Jupiter, FL 33458, USA
d Department of Molecular Therapeutics, The Scripps Research Institute, 110
Scripps Way, Jupiter, FL 33458, USA
† The authors declare no competing interests.
‡ Electronic supplementary information (ESI) available: All experimental
methods and supporting data for new compounds. See DOI: 10.1039/
c5md00579e
§ RNN (current location – Pacific Northwest National Laboratory, Richland, WA)
and JM (current location – St. Jude's Children's Research Hospital, Memphis,
TN) were equal primary contributors to this work.
Fig. 1 Molecular imaging agents using amino acid transport.
This journal is © The Royal Society of Chemistry 2016
Med. Chem. Commun.

View Article Online
Research Article
MedChemComm
Fig. 2 Mustard agents transported by LAT1.
Fig. 4 Tyr and Gln conjugates of MCT1 inhibitor 5.
recently reviewed.13,14 Here drug targeting to tumours relies
upon enhanced folate receptor expression, which is manifest
in ∼40% of human cancers. Phase II clinical results of folate-
conjugated small molecules have shown significant promise,
though no agents have yet met stringent phase III clinical
trial endpoints.13
5 nM, respectively, in a standard MTT assay using a Raji hu-
man B-cell lymphoma line).20
The bromine substituent21 provided access to alkyne-
substituted analogues via Sonagashira coupling reactions.22
For example, a Pd-catalyzed coupling reaction of 7 with
N-phthaloyl-protected 1-amino-4-pentyne gave alkyne 8. Fur-
ther, hydrazinolysis of 8 followed by coupling with N-Boc-Gln
and Boc deprotection gave the Gln conjugate 9. Coupling of
bromide 7 with a terminal alkyne O-linked to a protected ty-
rosine residue gave, after deprotection, the Tyr conjugate 10.
Details of the synthetic route are discussed below; experimen-
tal conditions are provided in the ESI‡ that accompanies this
article.
We have begun to explore the potential of LAT1 and/or
Asct2 transporters to facilitate the selective delivery of experi-
mental therapeutic agents to tumour cells abundantly ex-
pressing these transporters. As a case study, we chose to in-
vestigate amino acid conjugation strategies to augment
effectiveness of the MCT1/2 inhibitor 5 (Fig. 3) that has been
reported by AstraZeneca15–19 and that has also been studied
in our labs for its anti-tumour potential.20 A linker group
(whether permanently attached or designed to be biodegrad-
able) would be used to connect the MCT1/2 inhibitor to an
amino acid transporter recognition element, as shown sche-
matically in compound 6 in Fig. 3. The complex should direct
MCT inhibitors to cells that over-express the relevant amino
acid transporter. We reasoned that since most cancers are re-
liant upon glycolysis and many overexpress MCT1 (and are
thus sensitive to this class of MCT inhibitors), their co-
reliance upon lactate and the amino acid transporters LAT1
and Asct2 should result in additive or synergistic therapeutic
benefits.
The Raji B-cell lymphoma cell line was selected for these
studies based upon expression profiling analyses, wherein we
found that, relative to levels expressed in wild type (WT) B
cells, Asct2, Lat1 and Mct1 mRNA levels were highly elevated
in premalignant and neoplastic B cells from the Eμ-Myc
transgenic mouse, a validated model of human Burkitt B cell
lymphoma such as Raji cells that have MYC/Immunoglobulin
gene translocations (Fig. 5).20 Co-elevation of all three trans-
porters is undoubtedly not unique to Burkitt lymphoma. By
qRT-PCR this cell line expresses the MCT1 and MCT2 iso-
forms but not the MCT4 isoform (Fig. 6). The absence of
MCT4 blocks a possible pathway for the development of treat-
ment resistance upon exposure to MCT1/2 inhibitors. To-
gether, these findings indicate that this human lymphoma cell
line is appropriate for biological assessment of our new conju-
gates. As a control we used MDA-MB-231 cells, which highly
express MCT4, moderately express MCT2, and do not express
MCT1 (Fig. 6). Cytoxicity (MTT assay) of compounds in Raji
cells with no toxicity in MDA-MB-231 cells is thus consistent
with an MCT1/2 inhibition-driven effect. Unless indicated, all
Results and discussion
Inhibitor design
Preparing amino acid-conjugated analogues of inhibitor 5
required the identification of regions that are tolerant to
substitution by large appended groups. In the published
AstraZeneca synthesis, the naphthylmethyl group was at-
tached to the pyrrole core by alkylation.19 Thus, we explored
the anti-cancer cell-based activity of substituted analogues.
We were encouraged to find that the bromide 7 (Fig. 4) was
similar in activity to MCT1/2 inhibitor 5 (EC50 values 11 and
Fig. 5 Expression of the Gln transporter Asct2 (left), the essential
neutral amino acid transporter Lat1 (middle) and Mct1 (right) in WT vs.
premalignant Eμ-Myc splenic B cells and in Eμ-Myc lymphomas. Each
symbol represents one mouse.
Fig. 3 MCT1 inhibitor and its amino acid conjugates.
Med. Chem. Commun.
This journal is © The Royal Society of Chemistry 2016

View Article Online
MedChemComm
Research Article
Fig. 6 Expression profiling for MCT isoforms by qRT-PCR.
compounds reported here with activity vs. Raji cells were in-
active vs. MDA-MB-231 cells.
The Tyr conjugate 10 (Fig. 4) was significantly more active
in blocking lymphoma cell proliferation (EC50 = 0.8 nM) than
was the parent MCT1/2 inhibitor 5 (EC50 = 5 nM), which was
similar in potency to the Gln conjugate 9 (EC50 = 4 nM). En-
hanced cell-based efficacy in Gln conjugates was, however,
observed when the naphthyl group was replaced with an iso-
quinoline group and a diamine linker was also used instead
of an alkyne-containing linker (see compound 11, Fig. 7).
This compound was highly efficacious vs. Raji cells (EC50
=
0.16 nM). An attempt to reduce the lipophilicity of the parent
MCT1/2 inhibitor, using a pyridine ring in place of the iso-
quinoline group, gave a compound (12) with reduced cell-
based efficacy (EC50 ∼ 100 nM), supporting the known prefer-
ence of the MCT1 transporter for large aromatic groups in
this region.19 A quinoline group was only slightly less pre-
ferred (compound 13, EC50 = 0.8 nM) and was superior to the
pyridine analogue 14 (EC50 ∼ 500 nM). Glutamine conjugates
9 and 11–15 were in every case more highly efficacious vs.
Raji cells than were related compounds lacking the Gln resi-
due (compounds 5 and 15–18), see Fig. 7. The enhanced effi-
cacy of the LAT1 conjugate 10 and the Gln conjugates 11–14
in MTT assays in Raji cells is consistent with the hypothesis
that upregulated amino acid transporters in this tumour cell
line recognize the conjugates as substrates and thus augment
tumour exposure to these compounds. Based upon modelling
studies that use a published MCT1 homology-based struc-
ture,23 the linker and conjugated amino acid of 9–11 should
occupy a solvent-exposed region and should contribute very
little to their intrinsic affinity for MCT1. In support of this,
an assay using 14C-labeled lactate showed no appreciable dif-
ference in effects upon lactate flux among compounds 9–11
(data not shown).
Fig. 7 Gln conjugates and comparator compounds.
pound residence time in all tissues. While this experiment
was not done in tumour-bearing mice, a similar increase in
compound residence time within tumour cells is consistent
with the observation of increased efficacy for compound 10
vs. compound 5 in the four day MTT assays used to assess ef-
fects upon tumour cell growth. Also in support of this pro-
posed mechanism of efficacy enhancement for the conju-
gates, a single point study of six tumour-bearing NOD/SCID
mice showed that, at 24 h, the concentration of conjugate 10
in tumours ranged from 210–460 nM, with a mean of 300
nM. Thus, tumour drug levels significantly exceed the drug
levels in plasma and blood. Increased accumulation in the
liver, spleen and kidney may be lipophilicity-driven. Accord-
ingly, more hydrophilic MCT1 inhibitors should localize to
tumours with higher specificity.
Amino acid conjugation also affects both the rate of com-
pound uptake and the tissue distribution of compounds over
time. In a head-to-head study, the tyrosine conjugate 10 and
the parent compound 5 were each dosed in mice at 20 mg
kg−1 IP using a 1 mg ml−1 formulation in 10 : 10 : 80 DMSO :
Tween80 : water. Compound 10 was found to be present at
higher levels in nearly all tissues at the 6 h and 24 h time
points (Table 1). Further, unlike parent compound 5, com-
pound 10 was detectable in all tissues a full seven days after
IP administration. Amino acid conjugation thus apparently
facilitates uptake following IP dosing and also increases com-
Table 1 Tissue distribution studies for compounds 5 and 10, each dosed
at 20 mg kg−1 IP, using solutions of 1 mg mL−1 in a vehicle of 10 : 10 : 80
DMSO :Tween80 :water
[5] at 6 [5] at 24 [5] at 7 [10] at 6 [10] at 24 [10] at 7
Tissue h (nM) h (nM)
d (nM) h (nM)
h (nM)
d (nM)
Plasma
Blood
Brain
Liver
Spleen 740
Kidney 120
27
140
190
63
4
63
270
60
420
160
74
0
0
0
0
43
0
3270
4330
110
2500
2260
1620
1540
12
76
62
940
1060
960
520
4
41
43
98
190
210
110
Lung
61
0
This journal is © The Royal Society of Chemistry 2016
Med. Chem. Commun.

View Article Online
Research Article
MedChemComm
Synthesis procedures
The preparation of test MCT1/2 inhibitors in this series relied
upon a scalable synthesis of the core scaffold with all of the
required substituents except for the aryl-CH2– or heteroaryl-
CH2 chain. Compound 27 (Scheme 1) was prepared in a
7-step route that featured an olefin metathesis reaction24 and
one-pot directed lithiation–alkylation strategy, as we previ-
ously reported.25,26 Alkylation with 1-(bromomethyl) naphtha-
lene and desilylation of 27 is used to prepare the MCT1/2 in-
hibitor 5.19
Using other electrophiles to alkylate the pyrrole nucleus
gave other parent MCT1/2 inhibitors and amino acid conju-
gates. For example (Scheme 2), alkylation with bromide 29
gave access to compound 7 by deprotection, compound 8 by
Scheme 2 a. NBS, CCl4, 62%; b. NaH, KI, DMF, 80%; c. TBAF, THF,
88%; d. N-phthaloyl 1-amino-4-pentyne,29 PdIJPPh3)4, pyrrolidine, 42%;
e. H2NNH2–H2O, EtOH; then N-Boc-Gln, HATU, DIPEA, DMF, 21%; then
4 M HCl in dioxane, 97%; f) PdIJPPh3)4, pyrrolidine, 80 °C, 42%; then 4
N HCl in dioxane, 51%; g) 5-chloro-1-pentyne, K2CO3, DMF, 97%; then
LiOH–H2O, MeOH–THF, 99%.
Sonogashira coupling, compound
9
by phthalimide
deprotection, Gln residue coupling, and Boc deprotection,
and compound 10 by Sonagashira coupling using alkyne 31,
prepared from Boc-Tyr methyl ester in two steps.27 For the
full structures of compounds 7, 9, and 10, please see Fig. 4.
The alkylation using electrophiles containing quinoline,
isoquinoline, 4-pyridyl, and 3-pyridyl-containing heterocycles
is shown in Scheme 3. The chloro groups of alkylation prod-
ucts 32–35 were present in test MCT1 inhibitors 36–39.
Buchwald–Hartwig reactions28 of the chloroaryl compounds
with 1,5-di-aminopentane gave the diamine-containing ana-
logues 40–43, and after desilylation gave the test MCT1 inhib-
itors 15–18. Diamines 40–43 were then used to access the glu-
tamine conjugates 11–14 by amide coupling reactions
followed, in each case, by TBDMS deprotection.
were slightly preferred over quinolines (11 vs. 13). Amino
acid-containing compounds were among the most potent
compounds studied in this cell-based assay (compounds 9
and 11–14). As discussed earlier, in six separate instances the
amino acid-containing MCT1 inhibitor was more cytotoxic to
Raji cells than were related compounds lacking the tethered
amino acid: compare the tyrosine (LAT1-transported) conju-
gate 10 to the parent MCT1 inhibitor 5, and also compare the
glutamine (Asct2-transported) conjugates 9 and 11–14 to com-
pounds 5 and 15–19.
Structure–activity relationships
Results from a 96 h MTT assay using Raji Burkitt B-cell lym-
phoma20 are shown in Table 2. Larger aromatic groups were
preferred (36 vs. 38, 37 vs. 39, 17 vs. 18, 15 vs. 16, 13 vs. 14,
11 vs. 12). Isoquinolines, in the most potent compounds,
Scheme 1 a. Vinyl magnesium chloride, Et2O, 70%; b. methyl acrylate
(5 eq.), Grubbs 2nd gen. Metathesis catalyst24 (3%), CuI (6%), Et2O, 35
°C, 81%; c. activated MnO2 (10 eq.), 99%; d. NaH (2 eq.), TOSMIC acid
(1.5 eq.), DMF, 0–25 °C, 53%; e. methyl hydrazine (3 eq.), EtOH, reflux,
16 h, 90%; f. Boc anhydride (2 eq.), DMAP (0.1 eq.), CH3CN, RT, 95%; g.
LDA (2.0 eq.), THF, dithiane 26 (2 eq.), 68%.
Scheme 3 a. NaH, DMF, HET-CH2–Br, yields 68–81% (see ESI‡); b. HCl,
THF; yields 56–68%; c. 1,5-diaminopentane (5 eq.), Pd(OAc) (0.02 eq.),
BiNAP (0.04 eq.), NaOtBu (1.5 eq.), toluene, MW 100 °C, 4 h, yields 42–
77%; d) HATU, N-Boc-Gln-CO2H, DIPEA, CH2Cl2, 12 h, then HCl, yields
29–53%.
Med. Chem. Commun.
This journal is © The Royal Society of Chemistry 2016

View Article Online
MedChemComm
Research Article
Table 2 Results from 96 h MTT assay using Raji Burkitt B-cell lymphoma.
All compounds were nontoxic over 96 h at 10 μM to the control MCT1-
deficient breast cancer cell line MDA-MB-231 using an MTT assay, indi-
cating that selective cytotoxicity is driven by MCT1 inhibition
Acknowledgements
Financial support was provided by the Bankhead-Coley New
Investigator Grant 1BN01 from the Florida Biomedical Re-
search Program, by grants U54MH084512, CA154739 and P30
CA076292-14 from the National Cancer Institute/National In-
stitutes of Health, and by financial support from the State of
Florida given to Scripps Florida and to the Moffitt Cancer
Center. We thank Dr. Xiangming Kong, NMR core facility
manager at Scripps Florida, for assistance with NMR tech-
niques. We also thank Dr. Kristen E. N. Scott for review of the
biological data and for assistance in the construction of
figures.
X =
X =
EC50 (nM)
5
EC50 (nM)
11
5
7
60
55
1000
690
Notes and references
36
38
1 V. Ganapathy, M. Thangaraju and P. D. Prasad, Pharmacol.
Ther., 2009, 121(1), 29.
2 J. D. McGivan and C. I. Bungard, Front. Biosci., Landmark
Ed., 2007, 12, 874.
3 B. C. Fuchs, J. C. Pérez, J. E. Suetterlin, S. B. Chaudhry and
B. P. Bode, Am. J. Physiol., 2004, 286, G467.
4 A. N. McCracken and A. L. Edinger, Trends Endocrinol.
Metab., 2013, 24(4), 200.
5 B. C. Fuchs and B. P. Bode, Semin. Cancer Biol., 2005, 15,
254.
37
17
39
18
24
520
420
0.8
6 O. Warburg, Science, 1956, 123, 309–314.
15
13
7 M. G. Vander Heiden, L. C. Cantley and C. B. Thompson,
Science, 2009, 324, 1029.
8 J. R. Doherty and J. L. Cleveland, J. Clin. Invest., 2013, 123(9),
3685.
16
14
0.8
500
100
9 R. Stefania, L. Tei, A. Barge, S. G. Crich, I. Szabo, C. Cabella,
G. Cravotto and S. Aime, Chem. – Eur. J., 2009, 15, 76.
10 V. Makrides, R. Bauer, W. Weber, H.-J. Wester, S. Fischer, R.
Hinz, K. Huggel, T. Opfermann, M. Herzau, V. Ganapathy, F.
Verrey and P. Brust, Brain Res., 2007, 1147, 25.
11 N. H. Greig, S. Momma, D. J. Sweeney, Q. R. Smith and S.
Rapoport, Cancer Res., 1987, 47, 1571.
12 Y. Takada, D. T. Vistica, N. H. Greig, D. Purdon, S. I.
Rapoport and Q. R. Smith, Cancer Res., 1992, 52, 2191.
13 J. A. Ledermann, S. Canevari and T. Thigpen, Ann. Oncol.,
2015, 26, 2034.
0.16
11
12
10
4
0.8
9
14 P. S. Low and S. A. Kularatne, Curr. Opin. Chem. Biol.,
2009, 13(3), 256.
15 W. F. Michne, J. D. Schroeder, J. W. Guiles, A. M.
Treasurywala, C. A. Weigelt, M. F. Stansberry, E. McAvoy,
C. R. Shah, E. Bump and D. Schlegel, J. Med. Chem.,
1995, 38, 2557.
16 C. M. Murray, R. Hutchinson, J. R. Bantick, G. P. Belfield,
A. D. Benjamin, D. Brazma, R. V. Bundick, I. D. Cook, R. I.
Craggs, S. Edwards, L. R. Evans, R. Harrison, E. Holness,
A. P. Jackson, C. G. Jackson, L. P. Kingston, M. W. D. Perry,
A. R. J. Ross, P. A. Rugman, S. S. Sidhu, M. Sullivan, D. A.
Taylor-Fishwick, P. C. Walker, Y. M. Whitehead, D. J.
Wilkinson, A. Wright and D. K. Donald, Nat. Chem. Biol.,
2005, 1, 371.
Conclusions
Appending Gln and Tyr residues to an MCT1 inhibitor, simi-
lar to their use in contrast imaging agents, consistently en-
hanced toxicity vs. Raji cells, which express MCT1 and the
amino acid transporters LAT1 and Asct2. The enhanced cyto-
toxicity likely arises from facilitated transport into Raji cells.
Future work includes linker and transporter recognition ele-
ment optimization, MCT inhibitor optimization and the
application of such conjugation strategies to other classes of
anti-tumour agents.
This journal is © The Royal Society of Chemistry 2016
Med. Chem. Commun.

View Article Online
Research Article
MedChemComm
17 S. D. Guile, J. R. Bantick, D. R. Cheshire, M. E. Cooper,
A. M. Davis, D. K. Donald, R. Evans, C. Eyssade, D. D.
Ferguson, S. Hill, R. Hutchinson, A. H. Ingall, L. P.
Kingston, I. Martin, B. P. Martin, R. T. Mohammed, C.
Murray, M. W. D. Perry, R. H. Reynolds, P. V. Thorne, D. J.
Wilkinson and J. Withnall, Bioorg. Med. Chem. Lett.,
2006, 16, 2260.
18 S. D. Guile, J. R. Bantick, M. E. Cooper, D. K. Donald, C.
Eyssade, A. H. Ingall, R. J. Lewis, B. P. Martin, R. T.
Mohammed, T. J. Potter, R. H. Reynolds, S. A. St-Gallay and
A. D. Wright, J. Med. Chem., 2007, 50, 254.
19 J. Bantick, M. Cooper, P. Thorne and M. Perry, US Pat., No.
6342601B1, issued Jan 29, 2002.
20 J. R. Doherty, C. Yang, K. E. N. Scott, M. D. Cameron, M.
Fallahi, W. Li, M. A. Hall, A. L. Amelio, J. K. Mishra, F. Li,
M. Tortosa, H. M. Genau, R. J. Rounbehler, Y. Lu, C. V.
Dang, K. G. Kumar, A. A. Butler, T. D. Bannister, A. T.
Hooper, K. Unsal-Kacmaz, W. R. Roush and J. L. Cleveland,
Cancer Res., 2014, 74, 908.
21 Bromide 23 was prepared following the method of K. Fujita,
S. Hatano, D. Kato and J. Abe, Org. Lett., 2008, 10, 3105.
22 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron
Lett., 1975, 16, 4467.
23 B. Nancolas, R. B. Sessions and A. P. Halestrap, Biochem. J.,
2015, 466, 177.
24 R. H. Grubbs, Tetrahedron, 2004, 60, 7117.
25 R. Nair and T. D. Bannister, J. Org. Chem., 2014, 79, 1467.
26 R. Nair and T. D. Bannister, Eur. J. Org. Chem., 2015, 8, 1764.
27 J. Maza, J. R. McKenna, B. K. Raliski, M. T. Feedman and
D. D. Young, Bioconjugate Chem., 2015, 26(9), 1884.
28 J. F. Hartwig, Acc. Chem. Res., 2008, 41(11), 1534 and
references cited therein.
29 J. Maza, J. R. McKenna, B. K. Raliski, M. T. Feedman and
D. D. Young, Bioconjugate Chem., 2015, 26(9), 1884.
Med. Chem. Commun.
This journal is © The Royal Society of Chemistry 2016