Application of the Ugi reaction for the one-pot synthesis of uracil polyoxin C analogues

Article abstract of DOI:10.1021/jo900244m

(Chemical Equation Presented) A simple, two-step synthesis of amide derivatives of uracil polyoxin C (UPOC) methyl ester using the Ugi reaction is described. The four components employed in the Ugi reaction are 2′,3′-isopropylidine-protected uridine-5′-al

Full text of DOI:10.1021/jo900244m

CHART 1. Structures of Selected Polyoxins and
Nikkomycins
Application of the Ugi Reaction for the One-Pot
Synthesis of Uracil Polyoxin C Analogues
Andrew Plant,*,† Peter Thompson,‡ and David M. Williams*
Centre for Chemical Biology, Richard Roberts Building,
Department of Chemistry, UniVersity of Sheffield,
Sheffield S3 7HF, U.K., and Research Chemistry, Syngenta,
Jealott’s Hill International Research Centre,
Bracknell, Berkshire RG42 6EY, U.K.
d.m.williams@sheffield.ac.uk; andrew.plant@syngenta.com
ReceiVed February 3, 2009
requirements for biological activity. These include the pyrimi-
dine base,2,3 the C5′ carboxyl group and associated C5′-(S)-config-
uration4-7 and the presence of a free NH on the amide backbone
of the side chain.8,9
One of the simplest reported routes to uracil polyoxin C
(UPOC) analogues involves a modified Strecker reaction
between 2′,3′-isopropylidene-protected uridine-5′-aldehyde (1),
an amino acid, and TMSCN in the presence a Lewis acid.10
This affords UPOC precursors, 5′-R-amino nucleosides, in
reasonable yield, but as a mixture of two diastereoisomers with
ratios of between 6.5:1 and 4:1 in favor of the natural (C5′-S)
configuration of UPOC. Stereoselective synthetic routes to
polyoxin C and analogues thereof have also been described by
a number of groups, including ourselves.10-18
A simple, two-step synthesis of amide derivatives of uracil
polyoxin C (UPOC) methyl ester using the Ugi reaction is
described. The four components employed in the Ugi reaction
are 2′,3′-isopropylidine-protected uridine-5′-aldehyde, 2,4-
dimethoxybenzylamine, an isoxazolecarboxylic acid, and the
convertible isonitrile N-(2-{[(tert-butyldimethylsilyl)oxy]meth-
yl}phenyl)carbonitrile. Following the Ugi reaction, treatment
with HCl in MeOH achieves deprotection of the isopropyl-
idene group and the N-benzyl group and conversion of the
isonitrile-derived amide (the Ugi product) into the corre-
sponding methyl ester. The procedure is amenable to auto-
mated multiparallel synthesis of novel compounds related to
the polyoxin and nikkomycin nucleoside-peptide antibiotics.
In our own recent studies,18 we reported a highly stereose-
lective, three-step synthesis of both natural (C5′-S) and unnatural
(C5′-R) diastereoisomers of uracil polyoxin C methyl ester. A
(2) Azuma, T.; Isono, K. J. Chem. Soc., Chem. Commun. 1977, 159–160.
(3) Azuma, T.; Isono, K. Chem. Pharm. Bull. 1977, 25, 3347–3353.
(4) Azuma, T.; Saita, T.; Isono, K. Chem. Pharm. Bull. 1977, 25, 1740–
1748.
(5) Cooper, A. B.; Desai, J.; Lovey, R. G.; Saksena, A. K.; Girijavallabhan,
V. M.; Ganguly, A. K.; Loebenberg, D.; Parmegiani, R.; Cacciapuoti, A. Bioorg.
Med. Chem. Lett. 1993, 3, 1079–1084.
(6) Isono, K.; Azuma, T.; Suzuki, S. Chem. Pharm. Bull. 1971, 19, 505–
513.
(7) Isono, K.; Suzuki, S.; Azuma, T. Agric. Biol. Chem. 1971, 35, 1986–
1989.
(8) Boehm, J. C.; Kingsbury, W. D. J. Org. Chem. 1986, 51, 2307–2314.
(9) Emmer, G.; Ryder, N. S.; Grassberger, M. A. J. Med. Chem. 1985, 28,
278–281.
(10) Fiandor, J.; Garcialopez, M. T.; Delasheras, F. G.; Mendezcastrillon,
P. P. Synthesis 1987, 978–981.
(11) Akita, H.; Uchida, K.; Chen, C. Y.; Kato, K. Chem. Pharm. Bull. 1998,
46, 1034–1038.
(12) Barrett, A. G. M.; Lebold, S. A. J. Org. Chem. 1990, 55, 3853–3857.
(13) Evina, C. M.; Guillerm, G. Tetrahedron Lett. 1996, 37, 163–166.
(14) Harding, K. E.; Southard, J. M. Tetrahedron: Asymmetry 2005, 16, 1845–
1854.
Polyoxins and nikkomycins (examples shown in Chart 1) are
nucleoside-peptide antibiotics that display biological activity
against fungal chitin synthase (CS) derived from Candida
albicans and/or Saccharomyces cereVisiae.1 Since chitin is
absent from mammals and plants, CSs are attractive targets for
inhibition in both fungi and insects. Literature-derived struc-
ture-activity relationships for the polyoxins, nikkomycins and
their synthetic analogues suggest several important structural
(15) Kato, K.; Chen, C. Y. Synthesis 1998, 10, 1527–1533.
(16) Moffatt, J. G.; Damodaran, N. P.; Jones, G. H. J. Am. Chem. Soc. 1971,
93, 3812–13.
(17) More, J. D.; Finney, N. S. Synlett 2003, 1307–1310.
(18) Plant, A.; Thompson, P.; Williams, D. M. J. Org. Chem. 2008, 73, 3714–
3724.
† Present address: Syngenta Crop Protection, Mu¨nchwilen AG, WST-810.3.38,
CH-4332 Stein, Switzerland.
‡ Present address: Piramal Healthcare, P.O. Box 521, Leeds Road, Hudder-
sfield HD1 9GA, UK.
(1) Cohen, E. Ann. ReV. Entomol. 1987, 32, 71–93.
4870 J. Org. Chem. 2009, 74, 4870–4873
10.1021/jo900244m CCC: $40.75  2009 American Chemical Society
Published on Web 05/20/2009

FIGURE 3. Components required for synthesis of UPOC analogues 2
via the Ugi reaction.
FIGURE 4. Polyoxin C analogues synthesized by the Ugi reaction.8
component Ugi reaction might be applicable (Figure 3),
especially if a suitable convertible isonitrile could be used which
could be easily transformed into the C5′-methyl ester (or other
functionality likely to give rise to the corresponding carboxylic
acid in vivo).
In a brief communication, Tsuchida et al. reported the
synthesis of a protected C5′-carboxamide precursor to UPOC
using the Ugi reaction20 but provided no details of the
conversion of this compound into UPOC. The Ugi reaction has
also been employed in the solid-phase, combinatorial synthesis
of nikkomycin analogues using a Rink amide resin.21 However,
since no convertible isonitriles were used, analogues with the
important carboxyl group at the C5′-position were not synthe-
sized. Boehm and Kingsbury have employed the Ugi reaction
to synthesize polyoxin analogues containing N-methylated
peptide bonds.8 Thus, 2′,3′-protected uridine-5′-aldehyde,
L-(carbobenzyloxy)phenylalanine (L-ZPhe), methylamine, and
two different isonitriles were reacted in the presence of methanol
to give Ugi products as a mixture of diastereoisomers (i.e., no
stereocontrol at C5′). Treatment of the Ugi product 4 (derived
using cyclohexenyl isonitrile) with acetic acid afforded the
corresponding C5′-carboxamide 5 in 25% overall yield (Figure
4). Armstrong has shown that conversion of isonitriles with
similar structures to 4 to the corresponding carboxylic acids
requires a strong electron-donating N-acyl group22 and often
harsh acid conditions. Indeed, conversion of the carboxamide
5 to the C5′-carboxylic acid required treatment with nitrosyl
sulfuric acid.8 In the context of our own goals, the use of
cyclohexenyl isonitrile was not considered to be ideal. First,
the post-Ugi conversion to the carboxylic acid was expected to
be difficult since our target compounds were not N-methylated,
and in addition, we anticipated that this would be accompanied
by hydrolysis of the secondary amide function of the polyoxin
side chain. Futhermore, based on our own experience and that
of others,23 cyclohexenyl isonitrile is often difficult to prepare,
is generally obtained in only moderate yield, and must be stored
at -30 °C due to its low thermal stability.
FIGURE 1. Structures of protected uridine aldehyde precursor (1) and
polyoxin C derivatives designed as plant pro-pesticides.18
FIGURE 2. Putative transition state structure during chitin synthesis.
key step of this synthesis is the addition of TMSCN to an
appropriate chiral sulfinimine derived from uridine aldehyde 1.
Further elaboration of UPOC methyl ester by conjugation to
various isoxazole carboxylic acids gave compounds of general
structure 2 (Figure 1), designed as potential pro-pesticidal agents
directed against CS. In designing these compounds we sought
to include features of both polyoxins/nikkomycins and of the
putative transition state structure derived from the natural
substrate UDP-GlcNAc during its enzymatic incorporation into
chitin (Figure 2). The isoxazole unit was incorporated as a
lipophilic, masked, metal-binding site, which we postulated
would facilitate transmembrane permeability in plants and
subsequently, upon cleavage of the heterocycle, form a 1,3-
dicarbonyl moiety (structure 3, Figure 1). This strategy has been
shown to improve transmembrane permeability and root uptake
of the herbicide isoxaflutole.19 The analogues we have prepared
to date do not display significant pesticidal activity, the reasons
for which are currently under further investigation.
Initially, to confirm that our target compounds could indeed
be prepared by the Ugi reaction, we employed a simple
(nonconvertible) isonitrile, together with the other three com-
ponents shown in Scheme 1. Thus, the Ugi reaction with
One goal of our initial studies was to produce compound
libraries of structures based on 2 for high-throughput screening
to identify new lead structures for novel crop protection agents.
With this task in mind, we considered a short and effective
means of generating compounds of general structure 2. Ret-
rosynthetic analysis of structure 2 suggested that the four-
(20) Tsuchida, K.; Mizuno, Y.; Ikeda, K. Nucleic Acids Res. Symp. Ser. 1980,
s49-52.
(21) Suda, A.; Ohta, A.; Sudoh, M.; Tsukuda, T.; Shimma, N. Heterocycles
2001, 55, 1023–1028.
(22) Keating, T. A.; Armstrong, R. W. J. Am. Chem. Soc. 1996, 118, 2574–
2583.
(19) Pallett, K. E.; Cramp, S. M.; Little, J. P.; Veerasekaran, P.; Crudace,
A. J.; Slater, A. E. Pest Manag. Sci. 2001, 57, 133–142.
(23) Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336–
337.
J. Org. Chem. Vol. 74, No. 13, 2009 4871

SCHEME 1. Ugi Reactions Using Cyclohexane Isonitrile
SCHEME 2. Ugi Reactions Using Linderman’s Convertible
Isonitrile23
aldehyde 124 in methanol gave compound 6 in 70% yield. To
evaluate the viability of the Ugi reaction for synthesizing
analogues using parallel synthesis and to generate a library of
novel compounds based on structure 2, we used a Zymark robot
to perform the reaction. Thus, nucleoside 1,24 a variety of 20
different amines, 5-(chloromethyl)isoxazole-4-carboxylic acid,18
and six different isonitriles were reacted for 24 h in methanol
at room temperature to generate a 120-component library of
Ugi products.
In order to obtain the desired C5′-NH derivatives of com-
pound 6, we investigated a variety of different conditions to
remove the allyl protecting group. Unfortunately, all of these
methods including Pd(0) and NDMBA,25 sulfinic acid26 or
2-mercaptobenzoic acid,27 zirconium cyclopentadiene,28 rhod-
ium(I),29 or Grubbs’ first-generation carbene30 proved unsuc-
cessful. We attributed this to the possibility that the allyl group
might be sterically shielded, thereby preventing the effective
co-ordination of the respective metal complexes. Consequently,
as an alternative, we investigated using 2,4-dimethoxybenzyl-
amine as an ammonia equivalent in the analogous Ugi reaction,
based on its utility during peptide synthesis and subsequent ease
of debenzylation under acid conditions.31 The Ugi product 7
was obtained in good yield (68%) as a 1:1 mixture of
diastereoisomers. Treatment of compound 7 with HCl in
methanol resulted in the successful cleavage of the 2,4-
dimethoxybenzyl group and simultaneous removal of the sugar
isopropylidene group.
When Ugi reactions were performed with aldehyde 1,24
isonitrile 8, dimethoxybenzylamine, and isoxazoles18 9, 10, or
11, the desired Ugi products 12-14 were obtained with yields
of 50%, 35%, and 45%, respectively (Scheme 2). In each case,
the Ugi products are obtained as mixtures of diastereoisomers.
Although for compounds 12 and 13 the two diastereoisomers
were only partially resolved by silica TLC, for compound 14 it
proved possible after repeated chromatography to obtain a faster
and slower running component which we have characterized
by NMR. This confirms that the individual diatereoismers are
formed in the Ugi reaction in an approximately 1:1 ratio.
Interestingly, the faster running diastereoisomer of 14 appears
to consist of two conformational isomers as evidenced by NMR.
Treatment of compounds 12-14 with HCl in methanol facili-
tated ketal cleavage, N-debenzylation, and conversion of the
isonitrile-derived amides to the corresponding C5′-methyl ester
derivatives of UPOC. The products 15-17 were obtained after
flash chromatography in yields of 35%, 35%, and 31%,
respectively, as mixtures of diastereoisomers.
With this result in hand, we next focused on the Ugi reaction
using the convertible isonitrile 8 (Scheme 2) described by
Linderman.23 The synthesis of 8 is considerably easier than the
synthesis of cyclohexenyl isonitrile. Furthermore, the formamide
precursor to 8 is stable to long-term storage while in our hands
the isonitrile itself also proved to be stable when stored at
-18 °C.
(24) More, J. D.; Finney, N. S. Org. Lett. 2002, 4, 3001–3003.
(25) Garrohelion, F.; Merzouk, A.; Guibe, F. J. Org. Chem. 1993, 58, 6109–
6113.
(26) Honda, M.; Morita, H.; Nagakura, I. J. Org. Chem. 1997, 62, 8932–
8936.
(27) Lemaire-Audoire, S.; Savignac, M.; Genet, J. P.; Bernard, J. M.
Tetrahedron Lett. 1995, 36, 1267–1270.
(28) Ito, H.; Taguchi, T.; Hanzawa, Y. J. Org. Chem. 1993, 58, 774–775.
(29) Davies, S. G.; Fenwick, D. R. J. Chem Soc., Chem. Commun. 1997,
565–566.
(30) Alcaide, B.; Almendros, P.; Alonso, J. M.; Luna, A. Synthesis 2005,
668–672.
(31) Sasaki, Y.; Abe, J. Chem. Pharm. Bull. 1997, 45, 13–17.
Previously, we have shown that compounds 15 and 17
undergo isoxazole ring-opening under basic conditions and in
the presence of Fe(II), respectively, to afford diketonitriles
analogous to compound 2.18 The novel compound 16 could be
converted under basic conditions (Et3N in MeOH) to produce
the diketonitrile 18 (Figure 5). Despite this, compound 16 did
4872 J. Org. Chem. Vol. 74, No. 13, 2009

(bs, 3H), 3.55-3.61 (m, 3H), 3.68-3.77 (m, 3H), 4.52-5.18 (m,
9H), 5.47-5.73 (m, 2H), 6.22-6.41 (m, 2H), 6.93-7.54 (m, 9H),
8.20-8.34 (m, 1H), 8.35-8.40 (m, 1H), 8.87-8.92 (m, 1H), 9.62
(bs, 1H), 9.72 (bs, 1H). 13C NMR (126 MHz, CDCl3): δ -5.3,
-5.2, 14.0, 18.2, 23.5, 23.6, 24.6, 24.9, 25.1, 25.8, 26.1, 26.9, 27.0,
55.0, 55.2, 60.2, 62.3, 62.7, 63.0, 80.7, 81.9, 82.0, 83.1, 83.7, 83.8,
85.7, 93.7, 95.3, 95.6, 98.5, 98.6, 102.1, 102.6, 102.8, 104.3, 104.4,
112.4, 112.5, 113.4, 114.9, 120.0, 120.1, 121.9, 122.0, 122.3, 122.6,
122.9, 124.4, 124.7, 124.8, 126.8, 127.3, 127.6, 127.7, 129.9, 130.1,
135.3, 136.1, 142.0, 142.3, 142.8, 148.4, 148.5, 148.8, 149.3, 149.4,
149.8, 150.5, 156.0, 156.1, 158.1, 160.9, 163.3, 163.8, 164.5, 165.1,
166.2, 166.7, 170.2.
FIGURE 5. Structure of diketonitrile 18 derived from 16 under mildly
basic conditions.
not display any significant biological activity using the assays
that we have employed previously.18
In summary, we have shown that the four-component Ugi
reaction may be employed for the synthesis of polyoxin/
nikkomycin analogues from a readily available uridine 5′-
aldehyde, dimethoxybenzylamine, Linderman’s convertible isoni-
trile,23 and novel isoxazolecarboxylic acids. Treatment of the
Ugi products with HCl in methanol provides the polyoxin/
nikkomycin analogues as their C5′-methyl esters in two steps
overall and provides a simple means of producing compound
libraries for biological evaluation.
Methyl 1,5-Dideoxy-1-(3,4-dihydro-2,4-dioxo-1-(2H)-pyrim-
idinyl)-5-{5-([pyridin-2-ylthio)methyl]isoxazole-4-carbonylamino}-
ꢀ-D-allofuranuronate (r-L-Talofuranuronate) (15). Nucleoside
12 (200 mg, 0.22 mmol) was dissolved in MeOH (10 mL) and
cooled to 0 °C. HCl was then bubbled through the solution for 1 h
at 0 °C. The solution was then allowed to warm to room temperature
and stirred for 5 h. Water (6 mL) was then added and the mixture
stirred for a further 12 h and then evaporated. EtOAc (20 mL) and
satd aq NaHCO3 (20 mL) were added to the residue, the two layers
were separated, and the aqueous layer was extracted with EtOAc
(3 × 20 mL). The combined organic layers were dried (MgSO4)
and evaporated, and the crude product was purified by flash
chromatography (silica, 0-20% MeOH in EtOAc) to give a beige-
colored solid as a 1:1 mixture of diastereoisomers (40 mg, 0.08
Experimental Section
N-[(tert-Butyldimethylsilyloxymethyl)phenyl] 1,5-Dideoxy-
1-(3,4-dihydro-2,4-dioxo-1-(2H)-pyrimidinyl)-5-{5-[(pyridin-
2-ylthio)methyl]isoxazole-4-carbonyl(2,4-dimethoxybenzyl)-
amino}-ꢀ-D-allofuranuronamide (r-L-Talofuranuronamide)
(12). Nucleoside 124 (250 mg, 0.887 mmol), 2,4-dimethoxyben-
zylamine (148 mg, 0.887 mmol), and isoxazole 918 (209 mg, 0.887
mmol) were dissolved in dry MeOH (10 mL). Isonitrile 723 (219
mg, 0.887 mmol) was then added and the reaction stirred at room
temperature under argon for 24 h. The mixture was then evaporated
and the residue dissolved in EtOAc (50 mL) which was then washed
sequentially with satd aq NaHCO3 solution, water, and brine. The
organic layer was dried (MgSO4) and evaporated to a yellow solid.
Purification by flash chromatography (silica, 1:1 hexane/EtOAc,
then EtOAc) gave a beige-colored solid (405 mg, 0.44 mmol, 50%).
HRMS (ESI): m/z calcd for C45H54N6O11SSi ·Na [M + Na]+
937.3214, found 937.3238. 1H NMR (500 MHz, CDCl3): δ
-0.01-0.07 (m, 6H), 0.86-0.89 (m, 9H), 1.20-1.29 (m, 3H), 1.53
1
mmol, 35%). HRMS (ESI), H NMR, and 13C NMR data were in
agreement with those described previously.18
Acknowledgment. We are grateful to Gavin Bluck (auto-
mated parallel synthesis, Syngenta) and thank the BBSRC and
Syngenta for funding.
Supporting Information Available: Experimental details for
1
the syntheses of compounds 13, 14, and 16-18. H and 13C
1
NMR spectra for compounds 12-14, 16, and 18. H and 13C
NMR spectra and 1H-13C-coupled and 1H-1H-coupled spectra
for fast and slow diastereoisomers of 14. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO900244M
J. Org. Chem. Vol. 74, No. 13, 2009 4873