A flow chemistry approach to norcantharidin analogues

Article abstract of DOI:10.2174/157018011795906730

Acid-ester and acid-amide norcantharidin derivatives are prepared using a 'one-pot' synthetic procedure utilizing the ThalesNano H-cube flow hydrogenator. Traditionally, rapid library generation and reaction scale up of these analogues was limited by the batch wise hydrogenation of 5,6-dehydronorcantharidin. This was resolved with the use of flow chemistry. With no associated scale up issues, a method was devised to produce norcantharidin, along with acid-ester and acid-amide analogues on any scale necessary for biological screening.

Full text of DOI:10.2174/157018011795906730

568
Letters in Drug Design & Discovery, 2011, 8, 568-574
A Flow Chemistry Approach to Norcantharidin Analogues
Mark Tarleton1, Kelly A. Young1, Elli Unicomb2, Siobhann N. McCluskey2, Mark J. Robertson1,
Christopher P. Gordon1 and Adam McCluskey*,1
1Discipline of Chemistry, Chemistry Building, School of Environmental and Life Sciences, The University of Newcastle,
Callaghan, NSW 2308, Australia
2Lambton High School, Young and Womboin Roads Lambton NSW 2299, Australia
Received January 14, 2011: Revised March 18, 2011: Accepted March 29, 2011
Abstract: Acid-ester and acid-amide norcantharidin derivatives are prepared using a ‘one-pot’ synthetic procedure utiliz-
ing the ThalesNano H-cubeTM flow hydrogenator. Traditionally, rapid library generation and reaction scale up of these
analogues was limited by the batch wise hydrogenation of 5,6-dehydronorcantharidin. This was resolved with the use of
flow chemistry. With no associated scale up issues, a method was devised to produce norcantharidin, along with acid-ester
and acid-amide analogues on any scale necessary for biological screening.
Keywords: Cantharidin, Norcantharidin, Flow hydrogenation, Flow chemistry, Protein phosphatase inhibition.
INTRODUCTION
While simple approaches to multiple analogues of 2 are
available commencing with the Diels–Alder addition of
furan and maleic anhydride followed by hydrogenation to 2
(Scheme 1). The direct synthesis of 1 is more complex due to
the presence of two methyl groups preventing simple Diels-
Alder approaches [13], but its total synthesis has been
greatly accelerated by the use of high pressure chemistry
approaches [14]. Additionally the presence of these methyl
groups has been deemed responsible for the known nephro-
toxicity of 1. Hence 2 is the more promising lead compound
in a drug development program as it retains most of the PP1 /
PP2A inhibition and anti-cancer properties of 1 including the
lack of myelosuppresion, with greatly reduced nephrotoxic-
ity [3-5].
The process of drug development is a long and arduous
one. From lead discovery and compound library develop-
ment the synthetic chemistry requirements change from the
production of milligram to grams and at time kilograms
quantities of active compound(s) [1]. This places consider-
able pressure on the robustness of the chemistry utilised [2].
One of our teams primary areas of research has been in the
development of small molecule inhibitors, based on cantha-
ridin (1) and norcantharidin (2) (Fig. 1), of the serine /
threonine protein phosphatases 1 and 2A (PP1 and PP2A) [3-
5].
O
O
Notwithstanding the ease of synthesis of 2, the need for
hydrogenation in the second synthetic step stalls rapid library
development in a standard laboratory environment. Typically
this step is limited to a batch wise approach, with the quan-
tity of each batch limited by the volume of the hydrogenating
system. This also introduces batch-to-batch variations in the
amount and quality of 2 produced. This is a disadvantage not
only in regards to library development but to reaction scale
up.
O
O
O
O
O
O
1
2
Fig. (1). Chemical structures of cantharidin (1) and norcantharidin
(2).
Our initial interest was sparked by the structural simplic-
ity of 1 and 2 relative to the archetypal protein phosphatase
inhibitors such as okadaic acid (3) and microcystin-LR (4)
(Fig. 2) [6,7]. We, and others, have developed a number of
highly focused norcantharidin derived compound libraries
[4,8-11]. Our efforts in this area have been rewarded by the
development of new classes of inhibitor with a broad range
of protein phosphatase inhibition levels and PP1/PP2A selec-
tivity [4]. We have also explored the effect of these inhibi-
tors potential as anti-malarial, anti-parasitic and anticancer
drugs [4,12].
In efforts to accelerate both analogue development and
reaction scale up, our team has invested heavily in flow
chemistry technology via the Australian Cancer Research
Foundation Centre for Kinomics. Having access to the
ThalesNano H-cubeTM (H-cube) flow hydrogenator we
turned our attention to the synthesis of, initially, norcantha-
ridin (2) and later the ‘one-pot’ synthesis of norcantharidin
analogue libraries.
Typically our batch hydrogenation of 5 to 2 is conducted
using a Parr hydrogenator: 10% Pd-C, 4 bar H2, 24 hours on
a 5 g scale. Our initial evaluation of this reaction under flow
hydrogenation conditions was conducted with a 0.05M solu-
tion of 5 in acetone at 1 mL/min at 50 ºC and 50 bar H2 pres-
sure [15]. These conditions were chosen as being at the mid-
range of the capability of the H-cube (upper limit is 100 ºC,
100 bar H2 and a flow rate of 3 mL/min) [16]. In this in-
*Address correspondence to this author at the Discipline of Chemistry,
Chemistry Building, School of Environmental and Life Sciences, The Uni-
versity of Newcastle, Callaghan, NSW 2308, Australia Tel: +61 29 216486;
Fax: +61 29 215472; E-mail: Adam.McCluskey@newcastle.edu.au
1570-1808/11 $58.00+.00
© 2011 Bentham Science Publishers Ltd.

A Continuous Production of Norcantharidin Analogues
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6 569
OH
O
H
H
H
O
O
HO
O
O
O
OH
H
H
O
O
H
OH
OH
3
CO2H
O
N
HN
NH
O
O
O
O
NH
HN
H
N
H
N
O
O
O
CO2H
HN
NH2
4
Fig. (2). Okadaic acid (3) and microcystin LR (4), two archetypal members of the okadaic acid class of compounds.
O
O
O
i
ii
O
+
O
O
O
O
O
O
O
O
5
2
Scheme 1. Reagents and Conditions: i) Et2O, rt, 24-48 hrs; (ii) Dry Acetone, 10% Pd-C; H2, 4 Bar, 24 hrs, rt.
stance we observed 100 % conversion of 5 to 2 (Table 1,
entry 1). Subsequent optimisation studies commenced with
changes in the H2 pressure, and as can be seen from the data
presented in Table 1, all conditions excepting entry 2, ef-
fected complete conversion of 5 to 2. Controlled H2 mode at
0 bar (atmospheric pressure), at room temperature, only af-
forded a 60% conversion at 1 mL/min.
In previous studies we have shown that simple stirring of
2 in the presence of amines or alcohols afforded excellent
yields of ring opened acid-amides or acid-ester analogues
[8,18]. Having successfully converted our batch hydrogena-
tion synthesis of 2 to a flow approach, we examined the pos-
sibility of coupling this process in a ‘one-pot’ manner with
the aforementioned reactions with simple aliphatic alcohols.
We rationalised that such an approach would permit access
to the required quantities of analogues for biological screen-
ing in a rapid manner, and facilitate compound scale up
should the biological data suggest a favourable outcome
[19].
The continuous production of 2 from 5 using the condi-
tions outlined in Table 1, entry 1, affords the desired com-
pound at a rate of 12g/day which compares very favourably
with 5g/day via our batch hydrogenation approaches. Addi-
tionally, hydrogen is produced “on demand” from deionized
water and the catalyst cartridges are supplied pre-packaged,
which eliminates the safety concerns associated with flam-
mable gases and active catalyst disposal are negated [16,17].
Thus we modified the synthetic procedure used for the
flow synthesis of 2 from 5 with the simple incorporation of
Table 1. Optimisation of the Flow Hydrogenation of 5,6-dehydronorcantharidin (5) to Norcantharidin (2)
Entry
Catalyst
H2 Pressure (bar)
Temperature (°C)
Yield (%)
1
2
3
4
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
50
50
rt
100
60
0 (atmospheric pressure)
30
40
30
40
100
100

570 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6
Tarleton et al.
O
R
R
O
O
O
CO2H
6 - 10
O
O
ii
i
O
O
O
O
iii
O
O
O
2
5
N
H
CO2H
11 - 20
Scheme 2. Reagents and Conditions: i) 0.05M solution of 5 in acetone, 50 bar, 50°C, 1 mL/min, 10% Pd/C; (ii) 0.05M solution of 3 in ROH,
50 bar, 50°C, 1 mL/min, 10% Pd/C; (iii) 0.055M (or 0.03M) solution of RNH2 in acetone added to 0.05M solution of 3 in acetone, 50 bar,
50°C, 1 mL/min, 10% Pd/C.
Table 2. A ‘One-Pot’ Coupled Flow Hydrogenation and Ring Opening Synthesis of Norcantharidin Ring Opened Acid-Esters and
Acid-Amides (Protein Phosphatase Inhibition Data Taken from Refs. [8, 15, 18])
Product
Entry
Alcohol
Yield (%)
PP1 IC50 (ꢀM)
PP2A IC50 (ꢀM)
O
O
O
1
CH3OH
100
4.71
0.41
CO2H
6
O
O
O
2
3
4
5
CH3CH2OH
CH3(CH2)2OH
CH3(CH2)3OH
CH3(CH2)4OH
100
100
0%
0%
2.96
4.82
0.45
0.47
CO2H
7
O
O
O
CO2H
8
O
O
O
>100
>100
>100
>100
CO2H
9
O
O
O
CO2H
10
either an alcohol or an amine into the reagent stream that
contained 5. A series of solutions of 5 (0.05M) in methanol
(Table 2, entry 1), ethanol (Table 2, entry 2) and propanol
(Table 2, entry 3) were subjected to flow hydrogenation con-
ditions of 1 mL/min reagent flow, 50 bar H2 at 50ºC. The
outcomes of these reactions are tabulated in Table 2.

A Continuous Production of Norcantharidin Analogues
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6 571
Table 3. A ‘One-Pot’ Coupled Flow Hydrogenation and Ring Opening Synthesis of Norcantharidin Ring Opened Acid-Amides
(Protein Phosphatase Inhibition Data Taken from Ref. [8, 15, 18])
Product
Yield
(%)
PP1 IC50
(ꢀM)
PP2A IC50
(ꢀM)
Entry
Amine
O
N
H
O
1
CH3(CH2)7NH2
63
58
56
25
9
7
25
5
CO2H
11
O
N
H
O
2
3
CH3(CH2)9NH2
8.6 1.5
CO2H
12
O
NH2
N
H
0%
0%
n.d.a
n.d.
O
CO2H
13
NO2
O
NH2
N
H
4
74 13
23 1.5
O
O2N
CO2H
14
O
NH2
O
N
H
O
5
6
0%
n.d.
n.d.
O
CO2H
15
O
NH2
N
H
55
(68 3)b
(87 2)b
O
CO2H
16
O
NH2
N
H
O
7
8
100
35
5
13
1
O
CO2H
O
17
O
N
NH2
100c
n.d.
n.d.
H
O
CO2H
18

572 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6
Tarleton et al.
(Table 3). Contd…..
Product
Yield
(%)
PP1 IC50
(ꢀM)
PP2A IC50
(ꢀM)
Entry
Amine
O
Cl
Cl
Cl
Cl
N
H
NH2
O
9
52
91
>100
>100
CO2H
19
O
N
H
NH2
O
10
35
0
12
0
CO2H
20
an.d. = not determined; bpercentage inhibition at 100 ꢀM drug concentration; creagents were recycled through the catalyst bed until MS showed complete consumption of the starting
anhydride 5.
The data presented in Table 2 shows that for short chain
alcohols the ring opening of the anhydride by the alcohol and
reduction of the 5,6-double bond of 5 occurs in an excellent
yield. The longer chain alcohols failed to give any observ-
able product under these conditions (Table 2, entries 4 and
5), nor as 0.05M solutions in dry acetone (data not shown).
We have previously noted that the reaction of 2 with longer
chain alcohols is sluggish and difficult to drive to completion
this is reflected when conducting a two-step process via flow
hydrogenation [18].
prevent the product from precipitating and thus, with 4-
methylbenzylamine afforded quantitative conversion to the
desired product (Table 3, entry 8). This process was repeated
for the other amines that precipitated and also effected quan-
titative conversion to the desired products (data not shown).
The other amines evaluated (Table 3, entries 6, 7, 9 and 10)
all proceed to afford the desired products in good to excel-
lent yields (52-100%).
An additional advantage flow chemistry here was the
ability to loop the reagents stream, i.e. in the event of incom-
plete conversion the post-catalyst eluant was delivered back
to the reagent vessel and the reaction continued until com-
plete conversion observed (by MS). The effect of such rea-
gent looping is shown with 17 (Table 3, entry 7).
In the development of our protein phosphatase inhibitor
libraries we have previously reported that the acid amide
analogue is more potent than the equivalent acid ester ana-
logue [18,20]. Thus, an acetone solution of 5 (0.05M) and an
amine (0.055M, see Table 3 for details) were subjected to
flow hydrogenation conditions of 1 mL/min reagent flow, 50
bar H2 at 50ºC. The outcomes of these reactions are tabulated
in Table 3.
In conclusion herein we have reported the combined re-
duction and nucleophilic anhydride ring opening of a fo-
cused library of norcantharidin analogues. This reaction se-
quence works best with amine nucleophiles but due care and
consideration must be given to the product solubility as flow
chemistry has a low tolerance for materials that precipitate in
the reaction lines.
As we had observed no conversion to the acid ester with
issues with long chain alcohols above, we first evaluated the
addition of octyl and decylamine to 5 (Table 3, entries 1 and
2). As can be seen the addition proceeded smoothly with
yields of 63% and 58% respectively. While quantitative con-
version was not obtained, the outcome was more favourable
than that noted with alcohols. This is most likely a function
of the amine versus alcohol nucleophilicity. In a more thor-
ough investigation of this flow hydrogenation/nucleophilic
anhydride ring opening reaction sequence we specifically
targeted products that were of interest to our protein phos-
phatase inhibition program [3-5, 8, 12, 18, 20].
This two-step procedure was conducted in excellent
yields, typically quantitative, allowing for the rapid devel-
opment of novel potentially biologically active analogues.
The flow chemistry approaches developed herein are inde-
pendent of scale and facilitate both the initial screening
stages of a drug design and discovery program, and the in-
crease in compound quantity required as such a program
progresses through to in cell and animal studies.
The attempted addition of three anilines (Table 3, entries
7-9) failed. For example, under the conditions used, addition
of the 0.055 M aniline solution to the 0.05 M solution of 5
resulted in the immediate precipitation of the (1S,4R)-3-
(anilino)-7-oxabicyclo[2.2.1]hept-5-ene-2-carboxylic acid.
The precipitation of products during a flow chemistry trans-
formation is a limitation of this technology, but one that is
simply overcome by a ‘solvent-switch’ approach [21]. Sol-
vent switching was not required in this instance with dilution
of the feeder solution to 0.03 M (in acetone) sufficient to
EXPERIMENTAL
General Experimental
All starting materials were purchased from Aldrich
Chemical Co. and Lancaster Synthesis. Solvents were bulk,
and distilled from glass prior to use. Reaction progress was
monitored by TLC, on aluminium plates coated with silica
gel with fluorescent indicator (Merck 60 F254) and flash
chromatography was conducted utilising SNAP Biotage KP-
1
SIL columns. H and 13C spectra were recorded on a Bruker

A Continuous Production of Norcantharidin Analogues
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6 573
Advance AMX 300 MHz spectrometer at 300.13 and 75.48
MHz, respectively. Chemical shifts are relative to TMS as
internal standard. All compounds returned satisfactory Mass
spectra were obtained using a micromass liquid chromatog-
raphy Z-path (LCZ) platform spectrometer. Mass to charge
ratios (m/z) are stated with their peak intensity as apercent-
age in parentheses. All mass spectra were obtained via the
ES method thus fragmentation patterns were not observed.
The University of Wollongong, Australia, Biomolecular
Mass Spectrometry Laboratory, analysed samples for
HRMS. The spectra were run on a micromass QTof2 spec-
trometer using polyethylene glycol or polypropylene glycol
as the internal standard.
(1S,4R)-3-(Benzylcarbamoyl)-7-oxabicyclo[2.2.1]heptane-
2-carboxylic acid (20)
1H NMR (DMSO-d6) (300 MHz): ꢀ 7.97 (t, J = 5.7 Hz,
1H), 7.62-7.59 (m, 1H), 7.43-7.37 (m, 2H), 7.31-7.23 (m,
2H), 4.73 (s, 1H), 4.49 (d, J = 4.1 Hz, 1H), 4.28-4.14 (m,
2H), 2.95 (d, J = 9.6 Hz, 1H), 2.85 (d, J = 9.6 Hz, 1H), 1.63-
1.41 (m, 4H); 13C NMR (DMSO-d6) (75 MHz): ꢀ 173.0,
172.2, 130.9, 128.1, 127.6, 126.2, 78.9, 77.1, 53.1, 51.9,
42.4, 28.0, 27.7; MP 170°C [8].
ACKNOWLEDGEMENTS
The authors acknowledge the financial support of the
Australian Research Council and the Australian Cancer Re-
search and Ramaciotti Foundations. MT acknowledges the
UNI-PRS postgraduate funding from the University of New-
castle. KAY acknowledges John Morris Scientific (Austra-
lia) for generous scholarship provision. SNM and EU ac-
knowledge the Lambton High School Work Experience re-
lease program.
In a Typical Synthesis of Acid-Ester Analogues [18]
A mixture of 5,6-dehydronorcantharidin (5) (0.05 M in
methanol) was stirred until dissolved. The corresponding
solution was then passed through the H-cube flow hydro-
genation apparatus under the conditions of 50 bar, 50°C, 1
mL/min, controlled H2, 10% Pd/C. The reaction eluant was
collected, and the solvent removed in vacuo to leave a white
solid which did not require further purification.
(1S,4R)-3-(Methoxycarbonyl)-7-oxabicyclo[2.2.1]heptane-
2-carboxylic acid (6)
1H NMR (acetone-d6) (300 MHz): ꢀ 4.78-4.77 (m, 2H),
3.55 (s, 3H), 3.10-3.00 (m, 2H), 1.70-1.55 (m, 4H); 13C
NMR (acetone-d6) (75 MHz): ꢀ 170.9, 170.7, 77.8, 77.4,
51.5, 50.9, 50.1, 28.2, 27.2; MP 140-141°C [18].
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
DiMasi, J. A. The value of improving the productivity of the drug
development process: faster times and better decisions. Pharmaco-
econ. 2002, 20, 1-10.
Colombo, M.; Peretto, I. Chemistry strategies in early drug discov-
ery: an overview of recent trends. Drug Discov. Today 2008, 13,
677-684.
McCluskey, A.; Sim, A. T. R.; Sakoff, J. A. Serine-threonine pro-
tein phosphatase inhibitors: development of potential therapeutic
strategies. J. Med. Chem. 2002, 45, 1151-1175
(1S,4R)-3-(Ethoxycarbonyl)-7-oxabicyclo[2.2.1]heptane-2-
carboxylic acid (7)
McCluskey, A.; Sakoff, J. A. Small Molecule inhibitors of serine /
threonine protein phosphatases. Mini Rev. Med. Chem. 2001, 1, 43-
55
1H NMR (CDCl3) (300 MHz): ꢀ 8.88 (bs, 1H), 4.93 (d, J
= 2.9 Hz, 1H), 4.87 (d, J = 2.9 Hz, 1H), 4.11 (q, J = 7.1 Hz,
2H), 3.03-2.95 (m, 2H), 1.83-1.79 (m, 2H), 1.55-1.48 (m,
2H), 1.22 (t, J = 7.1 Hz, 3H); 13C NMR (CDCl3) (75 MHz):
ꢀ 175.7, 170.3, 78.0, 77.8, 60.5, 51.8, 51.7, 28.5, 28.4, 13.4;
MP 112-113°C [18].
Sakoff, J. A.; McCluskey, A. Protein phosphatase inhibition: struc-
ture based design. Towards new therapeutic agents. Cur. Pharm.
Des. 2004, 10, 1139-1159.
Bagu, J. R.; Sykes, B. D.; Craig, M. M.; Holmes, C. F. B. A mo-
lecular basis for different interactions of marine toxins with protein
phosphatase-1. Molecular models for bound motuporin, microcys-
tins, okadaic acid, and calyculin A. J. Biol. Chem. 1997, 272, 5087-
5097.
In a Typical Synthesis of Acid-Amide Analogues [8,20]
A mixture of 5,6-dehydronorcantharidin (5) (0.05M in
acetone) was stirred until dissolved. To this, a solution of
amine (0.055M in acetone) was added and the resulting solu-
tion was passed through the H-cube flow hydrogenation ap-
paratus under the conditions of 50 bar, 50°C, 1 mL/min, con-
trolled H2, 10% Pd/C. The reaction was collected, and the
solvent removed in vacuo to leave an off white precipitate.
This was purified by either adding ether (10 mL) and collect-
ing the observed precipitate under suction or subjected to
flash silica chromatography (~5% MeOH/DCM) to afford
the desired norcantharidin acid amide.
[7]
[8]
Sheppeck, J. E.; Gauss, C. M.; Chamberlin, A. R. Inhibition of the
Ser-Thr phosphatases PP1 and PP2A by naturally occurring toxins.
Bioorg. Med. Chem. 1997, 5, 1739-1750.
Hill, T. A.; Stewart, S. G.; Gordon, C. P.; Ackland, S. P.; Gilbert,
J.; Sauer, B.; Sakoff, J. A.; McCluskey, A. Norcantharidin Ana-
logues: Synthesis, Anticancer Activity and Protein Phosphatase 1
and 2A Inhibition. ChemMedChem. 2008, 3, 1878-1892
Tatlock, J. H.; Linton, A.; Hou, X. J.; Kissinger, C. R.; Pelletier, L.
A.; Showalter, R. E.; Tempczyk, A.; Villafranaca, J. E. Structure-
based design of novel calcineurin (PP2B) inhibitors Bioorg. Med.
Chem. Lett. 1997, 7, 1007-1012
[9]
[10]
[11]
Sodeoka, M.; Baba, Y.; Kobayashi, S.; Hirukawa, N. Structure-
activity relationship of cantharidin derivatives to protein phospha-
tases 1, 2A1, and 2B Bioorg. Med. Chem. Lett. 1997, 7, 1833-1836
Laidley, C. W.; Dauben, W. G.; Guo, Z. R.; Lam, J. Y. L.; Casida,
J. E. 2-Carboxymethylendothal analogues as affinity probes for
stabilized protein phosphatase 2A. Bioorg. Med. Chem. 1999, 7,
2937-2944.
(1S,4R)-3-(4-Methoxybenzylcarbamoyl)-7-
oxabicyclo[2.2.1]heptane-2-carboxylic acid (17)
1H NMR (DMSO-d6) (300 MHz): ꢀ 7.68 (t, J = 5.5 Hz,
1H), 7.17 (d, J = 8.7 Hz, 2H), 6.83 (d, J = 8.7 Hz, 2H), 4.69
(s, 1H), 4.46-4.44 (m, 1H), 4.19 (dd, J = 14.9, 5.5 Hz, 1H),
4.01 (dd, J = 14.9, 5.5 Hz, 1H), 3.71 (s, 3H), 2.82 (d, J = 9.8
Hz, 1H), 2.72 (d, J = 9.8 Hz, 1H), 1.71-1.38 (m, 4H); 13C
NMR (DMSO-d6) (75 MHz): ꢀ 173.1, 171.2, 158.0, 131.3,
128.4, 113.5, 78.6, 77.5, 54.9, 53.9, 44.9, 28.7, 27.9, 21.1;
MP 143-144°C [8].
[12]
[13]
Bajsa, J.; McCluskey, A.; Gordon, C. P.; Stewart, S. G.; Hill, T. A.;
Sahu, R.; Duke, S. O.; Tekwani, B. L. The antiplasmodial activity
of norcantharidin analogues that may act by protein phosphatase
inhibition. Bioorg. Med. Chem. Lett. 2010, 20, 6688-6695.
Stork, G.; van Tamelen, E. E.; Friedman, L. J.; Burgstahler, A. W.
A Stereospecific Synthesis of Cantharidin J. Am. Chem. Soc. 1953,
75, 384–392.

574 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 6
Tarleton et al.
[14]
[15]
Dauben, W. G.; Kessel, C. R.; Kazuo, H.; Takemura, K. H. Simple,
efficient total synthesis of cantharidin via a high-pressure Diels-
Alder reaction. J. Am. Chem. Soc. 1980, 102, 6893–6894.
Sakoff, J. A.; Ackland, S. P.; Baldwin, M. L.; Keane, M. A.;
McCluskey, A. Anticancer activity and protein phosphatase 1 and
2A inhibition of a new generation of cantharidin analogues. Inves-
tig. New Drugs 2002, 20, 1-11.
[19]
In a typical library development study 25 mg of compound is re-
quired for complete chemical characterisation and biological
evaluation. At 1 mL/min a 0.05M solution reaction produces ca 10
mg/min (assumes average MW of product ~ 350 and a 60% puri-
fied yield). Thus a 5 minute flow chemistry run affords ca 50 mg of
pure analogue. The equivalent batch process requires 24 hrs.
Hart, M. E.; Chamberlin, A. R.; Walkom, C.; Sakoff, J. A.;
McCluskey, A. Modified norcantharidins: synthesis, protein phos-
phatases 1 and 2A inhibition, and anticancer activity. Bioorg. Med.
Chem. Lett. 2004, 14, 1969-1973.
Baxendale, I. R.; Deeley, J.; Griffiths-Jones, C. M.; Ley, S. V.;
Saaby, S.; Geoffrey K. Tranmer, G. K. A flow process for the
multi-step synthesis of the alkaloid natural product oxomaritidine: a
new paradigm for molecular assembly. Chem. Commun. 2006,
2566-2568.
[20]
[21]
[16]
[17]
www.thalesnano.com accessed 16th December 2010.
Clapham, B.; Wilson, N. S.; Michmerhuizen, M. J.; Blanchard, D.
P.; Dingle, D. M.; Nemcek, T. A.; Pan, J. Y.; Sauer, D. R. Con-
struction and validation of an automated flow hydrogenation in-
strument for application in high-throughput organic chemistry. J.
Comb. Chem. 2008, 10, 88-93.
McCluskey, A.; Bowyer, M. C.; Collins, E.; Sim, A. T. R.; Sakoff,
J. A.; Baldwin, M. L. Anhydride modified cantharidin analogues:
synthesis, inhibition of protein phosphatases 1 and 2A and antican-
cer activity. Bioorg. Med. Chem. Lett. 2000, 10, 1687-1690.
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