Syntheses of new carbanucleosides by pericyclic reactions

Article abstract of DOI:10.1002/ejoc.201300202

The synthesis of heterobase-functionalized cyclopentene derivatives as valuable substrate for the introduction of suitable substituents through pericyclic reactions is reported. The structures of ene and 1,3-dipolar adducts are discussed, and the primary antiviral activities of some adenine derivatives are reported. The synthesis of heterobase-functionalized cyclopentene derivatives as valuable substrates for the introduction of suitable substituents through pericyclic reactions is reported. The structures of ene and 1,3-dipolar adducts are discussed, and the primary antiviral activities of some adenine derivatives are reported. Copyright

Full text of DOI:10.1002/ejoc.201300202

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
FULL PAPER
Carbanucleosides
The synthesis of heterobase-functionalized
cyclopentene derivatives as valuable sub-
strates for the introduction of suitable sub-
stituents through pericyclic reactions is re-
ported. The structures of ene and 1,3-di-
polar adducts are discussed, and the pri-
mary antiviral activities of some adenine
derivatives are reported.
L. Scagnelli, M. G. Memeo, S. Carosso,
B. Bovio, P. Quadrelli* .................... 1–13
Syntheses of New Carbanucleosides by
Pericyclic Reactions
Keywords: Antiviral agents / Carbanucleos-
ides / Ene reaction / Cycloaddition / Ni-
trosocarbonyls / Nitrile oxides
0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
DOI: 10.1002/ejoc.201300202
Syntheses of New Carbanucleosides by Pericyclic Reactions
Luca Scagnelli,[a] Misal Giuseppe Memeo,[a] Serena Carosso,[a] Bruna Bovio,[a] and
Paolo Quadrelli*[a]
Keywords: Antiviral agents / Carbanucleosides / Ene reaction / Cycloaddition / Nitrosocarbonyls / Nitrile oxides
The synthesis of heterobase-functionalized cyclopentene de-
rivatives as valuable substrate for the introduction of suitable
substituents through pericyclic reactions is reported. The
structures of ene and 1,3-dipolar adducts are discussed, and
the primary antiviral activities of some adenine derivatives
are reported.
side analogues, as has been detailed in various structural
studies.[5] The carbocyclic moiety is normally represented
by a cyclopentane ring, although four-, six- and seven-mem-
bered rings are often proposed as alternatives.[6]
Introduction
Viral infections and malignancies are among the com-
monest causes of morbidity and mortality in our society,
and this motivates strong interest in the design and synthe-
sis of new molecules that can effectively counteract these
diseases. The discovery of several new series of nucleoside
analogues with antiviral activity has altered the classical
way of thinking about the structures of nucleoside ana-
logues as antiviral agents.[1] These derivatives play a funda-
mental role in viral chemotherapy, and modification of the
sugar fragment, as well as its replacement with a carbocy-
clic moiety, have resulted in the synthesis of interesting nu-
cleoside analogues that have shown remarkable activity
towards a variety of viruses.[2] Methods for the synthesis of
sugar and sugar-modified nucleosides, as well as carbocyclic
nucleosides, have been studied extensively.[3] Various syn-
thetic problems such as low yields and low stereoselectivity
have frequently been encountered, however. These, as well
as toxicity problems with the obtained compounds, are just
some of the drawbacks that characterize these synthetic ap-
proaches.
Carbanucleosides serve as more metabolically stable
structural analogues of both natural and unnatural antivi-
ral, antifungal and antibacterial nucleosides.[4] Structurally,
the typical feature of a nucleoside is the presence of a spacer
between the nucleobase and a hydroxy functionality, where
the spacer can be either an open-chain or a cyclic moiety
(Figure 1). In the cyclic case, ring size modifications and
the mono- or polycyclic nature of the spacer between the
nucleobase and the hydroxy group affect both the confor-
mational bias and the activities and functions of the nucleo-
Figure 1. Structural features of carbocyclic nucleoside analogues.
A side arm carrying a hydroxy group is normally present.
This feature discriminates between the two families of clas-
sical nucleoside analogues (x = 1), bearing hydroxymethyl-
ene moieties, and nor-nucleoside analogues (x = 0), in
which the hydroxy groups are directly linked to the carbocy-
clic rings.[7] The nucleobases can be of the purine or pyrim-
idine type, and a variety of functionalized structures, suit-
able for various types of functionalization, have been re-
ported.
Most effective antiviral agents act as prodrugs for their
corresponding phosphorylated metabolites. The hydroxy
group on the side arm is recognized and phosphorylated
by the kinases that produce the corresponding nucleotides.
Modifications in this part of the nucleoside structure are
often required.[8] Recently, N–O chemistry, based on nitroso
reagents, has been extensively applied for the synthesis of
nucleosides as antivirals and antibiotics.[4,9] In some cases
the N–O functionality serves as an isoster of the hydroxy-
methylene group [i.e., the hydroxylamine (HO–NH) moi-
ety]. In others, the mild cleavage conditions usable for the
N–O bond allow the stereoselective introduction of hydroxy
and amino groups for further synthetic elaborations.
[a] Department of Chemistry, University of Pavia,
Viale Taramelli 12, 27100 Pavia, Italy
Fax: +39-0382-987323
E-mail: paolo.quadrelli@unipv.it
Homepage: http://chimica.unipv.eu/site/home/persone/
scheda700003454.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300202.
2
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
Scheme 1.
We have recently proposed the synthesis of a new class of
carbocyclic nucleosides, starting from cyclopentadiene and
based on nitrosocarbonyl intermediates (RCONO, 1,
Scheme 1) chemistry.[10] These intermediates, generated by
the mild oxidation of nitrile oxides with tertiary amine N-
oxides, are efficiently trapped by cyclic dienes to afford the
hetero-Diels–Alder (HDA) cycloadducts 3.
These are highly reactive dipolarophiles and have been
employed to synthesize the conformationally restricted car-
bocyclic moiety aminols 7 through amide hydrolysis and N–
O bond cleavage of the cycloadducts 6.[11] Aminols 7 served
for the linear construction of purine and pyrimidine nucleo-
sides 8.
Scheme 2.
Applications of nitrosocarbonyl compounds to the syn-
theses of biologically active molecules are well-known, es-
sentially due firstly to the variety of generating methods
available (oxidation of hydroxamic acids[12] and nitrile ox-
ides,[13] as well as thermal[14] or photochemical[15] cyclore-
versions of 1,2,4-oxadiazole 4-oxides) and secondly to the
exceptional reactivity of these intermediates in HDA reac-
tions. Nitrocarbonyl intermediates are also powerful en-
Results
Convergent Synthesis of 6-Chloropurine-Substituted
Cyclopentene
Freshly distilled cyclopentadiene was subjected to epox-
ophiles,[16] but no applications of ene reactions to the syn- idation with peracetic acid. The epoxide 11 (Scheme 3) was
thesis of nucleosides have so far been published.[17] Recently obtained in 40% yield as straw yellow oil at a good level of
we have reported that nitrosocarbonyl intermediates, gener- purity.[19]
ated at room temp. by the mild oxidation of nitrile oxides,
undergo clean ene reactions with trisubstituted olefins 9 drous diethyl ether at 0 °C overnight afforded cyclopent-3-
(Scheme 1).[18] en-1-ol (12) in 56% yield,[19] and this was converted into
Reduction of 11 with lithium aluminiumhydride in anhy-
The ene reactions of nitrosocarbonyl intermediates de- the corresponding mesyl derivative 13 (reddish oil, yield
scribed above represent a valuable tool for the synthesis of 61%).[20] Condensation with the sodium salt of 6-chloro-
nucleoside analogues in which the role normally attributed purine (14) followed immediately, as the key step in the con-
to the hydroxymethylene group is instead played by the hy- vergent synthesis of the nucleoside structures.[21]
droxyamino functionality. In continuation of our studies of
6-Chloropurine (14, 1 equiv.) was suspended in anhy-
ene reactions and their applications for nucleoside synthe- drous DMF, and a slight excess of NaH (95%) was added
sis, we wish to report here the synthesis of carbanucleoside portionwise with vigorous stirring at 0 °C for a couple of
analogues through the application of pericyclic reactions. hours to generate the corresponding sodium salt 15. The
The nucleoside synthesis proceeds from the cheap cyclo- cyclopentene mesylate 13 was added dropwise to the solu-
pentadiene, through insertion of commercially available het- tion, and the reaction mixture was stirred at room tempera-
erobases, taking advantage of the chemistry of nitrosocarb- ture for 48 h. After this, the reaction was quenched by pour-
onyl intermediates and nitrile oxides (Scheme 2).
ing the mixture into ice, and the products were extracted
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
Figure 2. ORTEP plots of adduct 16a with atom labelling (ellip-
soids at 20% probability). Hydrogen atoms are omitted for clarity,
with the exception of CH–Npurine. Labels D (downward) and U
(upward) indicate the orientation of the methine carbon atom of
the cyclopentene moiety.
Scheme 3.
Curiously enough, the crystal structure of adduct 16a
showed the presence in the solid of both of the conformers
associated with the cyclopentene moiety. In Figure 2 on the
left (bottom), the structure 16aD shows the purine ring ly-
ing in the orthogonal plane with respect to the average
plane of the carbocyclic moiety attached to the methine car-
bon atom displaced downward (D). In the plot on the right
(top), the purine ring is attached to the methine carbon
atom displaced upward (U) with respect to the average
plane of the cyclopentene moiety. These results suggested a
flexible orientation of the heterobase ring linked to a cyclo-
pentene system.
In the following sections the behaviour of the cyclo-
pentene-purine 16a as dipolarophile and ene partner is
studied, together with the functionalization steps for the
synthesis of new nucleoside analogues, thanks to the
favourable positioning of the chlorine atom in the hetero-
base ring for further derivatization with suitable amines.
from the water phase with dichloromethane. The residue
obtained upon evaporation of the solvent was subjected to
chromatographic separation to isolate the regioisomeric ad-
ducts 16a and 16b (36% and 23% yield, respectively).
The structure assignment is based on the corresponding
analytical and spectroscopic data, as well as on X-ray
analysis of the major compound 16a. Regioisomer 16a is a
white, low-melting solid (m.p. 53–4 °C), the proton NMR
spectrum of which (DMSO as solvent) clearly shows a 1:1
ratio between the cyclopentene and the 6-chloropurine moi-
eties. The two allylic methylene units were found as double
doublets at δ = 2.78 and 2.97 ppm, whereas the olefinic
CH=CH system gave a singlet at δ = 5.90 ppm. The homo-
allylic proton adjacent to the nitrogen atom of the purine
ring gave a multiplet at δ = 5.37 ppm. The purine ring was
recognizable through the presence of the singlets corre-
sponding to the CH=N protons at δ = 8.66 and 8.79 ppm.
The 13C NMR spectrum was consistent with a structure
containing 10 carbon atoms.
Adduct 16a as Dipolarophile and Synthesis of Adenine
Derivatives
The minor compound 16b is a yellowish oil with a similar
spectroscopic pattern. The two allylic methylene groups
were found as double doublets at δ = 2.02 and 2.29 ppm,
We have recently illustrated the synthetic potential of
whereas the olefinic CH=CH system gave a singlet at δ = bromoisoxazoline moieties in nucleoside structures;[9b]
5.14 ppm. A multiplet at δ = 4.99 ppm corresponded to the moreover, these heterocyclic moieties are reported to be iso-
homoallylic proton, the singlets at δ = 7.87 and 7.99 ppm sters of sugars and to be able to establish positive binding
to the purine ring.
with amino acid residues in proteins.[22] A bromonitrile ox-
The evident symmetrical arrangement of adducts 16a ide BrCNO is easily generated in situ in AcOEt solution
and 16b resulted in some difficulties in the assignments of from the corresponding bromoxime shown in Scheme 4, the
the regiochemistry, and NOESY experiments were unsuc- preparation of which is achieved by treatment of glyoxylic
cessful from this point of view. A decisive contribution to acid with hydroxylamine hydrochloride in the presence of
solving the structural problem was provided by X-ray bromine.[23]
analyses of adduct 16a conducted on single crystals, which
With the cyclopentene-purine 16a as dipolarophile in its
allowed for the unequivocal attribution of both regioisom- racemic mixture form, the 1,3-dipolar cycloaddition reac-
eric structures. ORTEP views of the adduct 16a are shown tion smoothly occurred to afford the single cycloadduct 17
in Figure 2; the corresponding crystallographic data are (Scheme 4), in 98% yield and as a white solid compound
given in Table 4 and in the Supporting Information.
(m.p. 223–5 °C) easily purified by column chromatography.
4
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
Similarly, the 1H NMR spectrum (DMSO) of the cy-
clopropyl derivative 18b showed the cyclopropyl moiety in
the form of the presence of diagnostic signals at δ = 0.62
and 0.71 ppm, together with the CH–NH system at δ =
3.05 ppm. The NH group gave a doublet at δ = 7.88 ppm
(J = 1 Hz) along with the IR band at ν = 3271 cm–1.
˜
Scheme 4.
Adduct 16a as Ene Partner and Synthesis of
Hydroxylamine Derivatives
In previous works, as well as in the Introduction of this
paper, we have detailed various methods for the generation
of nitrosocarbonyl intermediates, along with the ability of
these fleeting intermediates to behave as “super-enophiles”
in ene reactions with a variety of olefins.[13–16,18] We have
demonstrated, however, that the oxidation of nitrile oxides
with NMO fails with less substituted ethylenes, because of
competing 1,3-dipolar cycloaddition reactions between the
1,3-dipoles and the C=C double bonds in the olefins. This
shortcoming can be avoided by using the mildest available
method for the generation of nitrosocarbonyls: the photo-
chemical fragmentation of the easily available 3,5-diaryl-
1,2,4-oxadiazole 4-oxides.[25] When exposed to sunlight or
310 nm lamps (15 W), these heterocycles undergo cyclore-
version into aromatic nitriles and nitrosocarbonyls, and in
the absence of competing processes and in the presence of
excess olefins these add to the C=C double bond to afford
ene adducts in high yields.[26]
The structure of cycloadduct 17 was assigned on the ba-
sis of the relevant analytical and spectroscopic data. The
1H NMR spectrum in DMSO showed complex multiplet
signals centred at δ = 2.66 ppm, corresponding to the meth-
ylene protons of the cyclopentane ring. The isoxazoline pro-
tons gave two signals at δ = 5.35 ppm (dd, J = 9, 5 Hz, 5-
H) and δ = 4.22 ppm (t, J = 9 Hz, 4-H), coupled between
them and the adjacent methylenes. A signal at δ = 4.89 ppm
is attributable to the methine proton; the singlets at δ = 8.79
and 8.81 ppm correspond to the CH=N system of the pur-
ine ring. The stereochemistry of the cycloadduct was deter-
mined through a NOESY experiment. The sole CH=N pro-
ton at δ = 8.79 ppm, corresponding to the imidazole ring
of the 6-chloropurine, gave an NOE crosspeak only with
the two deshielded protons located on the same plane of
the cyclopentane ring at δ = 2.74 ppm and belonging to the
methylene groups. The same protons correlate through
NOE crosspeaks with the H4 and H5 protons of the
isoxazoline ring that is located anti to them.
Ene reactions of nitrosocarbonyls constitute a valuable
method for the introduction of a hydroxylamino function-
ality onto a carbocyclic spacer as an isoster of the hydroxy-
methylene group in a nucleoside structure.[4] We investi-
gated the photochemical reaction between 3,5-diphenyl-
1,2,4-oxadiazole-4 oxide and the cyclopentene-purine 16a
(Scheme 6) for the preparation of new nucleoside analogues.
The chloro derivative 17 was functionalized with a cou-
ple of primary representative amines, linear and cyclic, as
shown in Scheme 5, by the well-established protocol.[10,24]
Upon heating of methanol solutions of 17 at moderate tem-
perature with excesses either of ethylamine or of cyclopro-
pylamine, the corresponding derivatives were isolated as so-
lids in high yields by simple concentration of the solvent
and were fully characterized. In its 1H NMR spectrum
(DMSO) the ethylamino derivative 18a showed the presence
of a triplet at δ = 1.19 ppm along with a broad singlet at δ
= 3.59 ppm corresponding to the ethyl moiety, whereas the
NH proton was found at δ = 7.54 ppm (broad singlet). The
presence of the NH group in the structure was also con-
firmed by the IR spectrum, which showed a strong band at
ν = 3270 cm–1.
˜
Scheme 6.
3,5-Diphenyl-1,2,4-oxadiazole 4-oxide (1.5 equiv.) was
added portionwise with stirring to a methanol solution of
16a over a period of a couple of hours under irradiation
with 2ϫ15 W lamps centred at 310 nm (Scheme 6). This
ensures an excess of the cyclopentene derivative during the
generation of the nitrosocarbonyl benzene through the frag-
mentation of the parent heterocycle. After evaporation of
the solvent, the residue was subjected to chromatographic
separation to purify the ene diastereoisomeric adducts syn-
19 and anti-19, obtained in moderate yields (26%) as an
inseparable mixture.
Scheme 5.
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
The structure assignment of the solid ene adducts (m.p. compound 19 as a mixture of diastereoisomers was dis-
176–9 °C) is based on the corresponding analytical and solved in methanol, and the solution was saturated with
spectroscopic data. In the IR spectrum the presence of a ethylamine. The sealed vial was heated at 50 °C for 48 h
broad band around 3200 cm–1 indicated the introduction of (Scheme 7).
a hydroxy group (in the hydroxylamine moiety), and a band
After this, and removal of the solvent, compound 20 was
found at 1634 cm–1 was in the expected range for a conju- obtained as a solid in 91% yield. The structural assignment
1
gated carbonyl group. The H NMR spectrum in DMSO was based on the relevant analytical and spectroscopic data.
showed two pairs of multiplets corresponding to the meth- The IR spectrum showed the C=C double bond band at ν
˜
ylene system of the cyclopentene ring at δ = 2.10 and = 1585 cm–1, along with another band at ν = 1622 cm–1 in
˜
3.02 ppm and at δ = 2.44 and 2.63 ppm coupled with the the range typical of a C=N group. At higher frequencies, a
two methine protons at δ = 5.67 and δ = 6.28 ppm. The broad band centred at ν = 3000 cm–1 and a sharper one at
˜
CH=CH protons gave a cumulative signal for both dia- ν = 3306 cm–1, corresponding to a hydroxy group and a
˜
stereoisomers at δ = 5.95 ppm. Other than the aromatic NH group, respectively, were found.
protons of the phenyl groups, the CH=N signals of the pur-
The presence of the ethylamino group was clearly shown
1
ine rings of syn-19 and anti-19 are nicely separated and in the H NMR spectrum (DMSO) by the triplet at δ =
found at δ = 8.69 and 8.77 ppm and at δ = 8.46 and 1.17 ppm (CH3) and the broad singlet at δ = 3.52 ppm
8.82 ppm. The presence of highly deshielded singlets at δ = (CH2), along with the singlet for NH at δ = 7.71 ppm. The
9.73 and 9.82 ppm indicated the presence of the acid pro- remaining parts of the proton spectrum are fully consistent
tons of the hydroxylamine moieties originating from the ene with the cyclopentene-purine structure, with the following
addition of the nitrosocarbonylbenzene. By comparison of features. In the aromatic range of the spectrum, the phenyl
the intensities of these two signals we were able to deter- system of the benzoyl group was missing, and a sharp and
mine the diastereoisomeric ratio in the mixture (2.4:1). To highly deshielded singlet appeared at δ = 10.90 ppm, indi-
complete the structural assignment, we investigated the pos- cating the presence of an acidic hydroxy group, typical of
sibility of attributing the relative positions of the purine oximes. This result can be easily explained in terms of sub-
ring and the hydroxylamine group through NOESY experi- stitution of the chlorine atom with the ethylamino group
ments. Unfortunately, as a result of the high symmetry of and concomitant base-catalysed benzoyl elimination, fav-
the ene adducts and the unfortunate coincidence of chemi- oured by the formation of the conjugated C=N double
cal shifts in the NMR spectra, we were unable to attribute bond. Clearly, the change in hybridization of the hydroxyl-
the stereochemical configurations of products 19 beyond amine carbon atom on transformation into an oxime car-
any reasonable doubt even with a change in the solvent.
bon atom erases the diastereoisomeric mixture, so a single
To functionalize the ene adducts 19 with amines to test compound is obtained.
their biochemical behaviour, we performed the reactions by
A diversion can be planned if the conservation of the N-
the standard protocol already described.[10,24] A sample of benzoyl-hydroxylamine functionality is desired or required.
Scheme 7.
6
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
The cyclopentene-purine 16a is easily convertible into the also tested against respiratory virus influenza A H1N1 cell
ethylamino derivative 21 (Scheme 7), isolatable as a solid line MDCK (strain California 7/2009) and neuramidase
compound (m.p. 105–7 °C) in 90% yield on purification by (NA) cell line HFF (strain NA). Table 1 and Table 2 report
column chromatography. The reported structure was con- the primary antiviral activities of the tested compounds and
firmed by the analytical and spectroscopic data. The main control experiments on specified drugs.
features, relative to the starting compound, were the pres-
ence of the ethylamino group [1H NMR (δ, DMSO): 1.30
(t, CH3), 3.72 (br. s, CH2), 5.75 ppm (br. s, NH)]. Com-
pound 21 was allowed to react with nitrosocarbonylbenz-
ene, photochemically generated through the fragmentation
of 3,5-diphenyl-1,2,4-oxadiazole 4-oxide under the same ex-
perimental conditions as applied previously. Upon evapora-
tion of the solvent a residue was obtained and subjected to
chromatographic separation to isolate an inseparable dia-
stereoisomeric mixture of ene adducts 22syn and 22anti as
an oil in 36% yield. The 1H NMR spectrum in DMSO
showed two pairs of multiplets corresponding to the meth-
Table 1. Primary antiviral activities of compound 19.
[#]
Virus
Cell line
HFF
EC50
EC90
CC50
SI50
HSV-1[a]
Acyclovir
HSV-2[a]
Acyclovir
VZV[a]
Ͼ60
1.75
Ͼ60
3.34
Ͼ60
0.69
Ͼ300
4.75
18
Ͼ60
3.09
Ͼ60
Ͼ100
Ͼ60
5.76
Ͼ300
8.22
295
Ͼ100
228
Ͼ100
130
Ͼ100
Ͼ300
Ͼ300
100
Ͼ1000
Ͼ100
2322
Ͼ20
Ͻ5
Ͼ57
Ͻ4
Ͼ30
Ͻ2
Ͼ145
n.a.
Ͼ63
5.6
Ͼ120
n.a.
48375
n.a.
Ͼ14
Ͼ3
HFF
HFF
Acyclovir
VV[a]
HFF
Cidofovir
PTV[b]
Vero 76
2.2.15
ylene system in the cyclopentene ring, at δ = 2.00 and Ribavirin
8
HBV[c]
Ͼ100
0.048
Ͼ100
0.135
Ͼ20
3.00 ppm and at δ = 2.32 and 2.51 ppm, together with the
Lamivudine
methine protons at δ = 5.59 and δ = 5.78 ppm. The
HCV[d]
HL/Neo ET Ͼ20
Ͼ2
CH=CH protons gave a cumulative signal for both dia-
IFNα-2b
Ͼ2
0.14
stereoisomers at δ = 6.18 ppm. In addition to the aromatic
protons of the phenyl groups, the CH=N signals of the pur-
ine rings of 22syn and 22anti are nicely separated and found
at δ = 7.95 and 8.09 ppm and at δ = 8.18 and 8.22 ppm.
The NH signals are combined in a broad singlet at δ =
7.74 ppm, and the two highly deshielded singlets at δ = 9.77
and 9.82 ppm indicate the presence of the acid protons of
the hydroxylamine moieties. The ethylamino group is found
in the expected range. The diastereoisomeric ratio was cal-
culated to be close to 1:1. Any attempt to attribute the
stereochemistry to the two sets of signals in the mixture
through NMR experiments unfortunately failed, again be-
cause of the highly symmetric nature of these type of ad-
ducts.
To verify the ability of organic bases to promote the
benzoyl group elimination in compound 22, we dissolved a
sample of 22 in methanol and the solution was saturated
with ethylamine gas. The sealed vial was heated at 50 °C for
48 h and the debenzoylated compound 20 was isolated in
80% yield.
Flu-A
MDCK
33.65
Ͼ100
Ͼ100
H1N1[e]
Ribavirin
NA[f]
5.85
13.28
Ͼ100
58.70
Ͼ300
Ͼ17
HFF
Cidofovir
[a] Drug conc. range 0.096–300 μm; control conc. range 0.032–
100 μm. [b] Drug conc. range 0.1–100 μm; control conc. range 1–
1000 μm. [c] Drug conc. range 0.1–100 μm; control conc. range 0.3–
100 μm. [d] Drug conc. range 0.063–20 μm; control conc. range
0.006–2 lUmL–1. [e] Drug conc. range 0.096–300 μm; control conc.
range 0.096–300 μm. [f] Drug conc. range 0.8–100 μm; control conc.
range 0.8–100 μm. [#] n.a.: not applicable.
Table 2. Primary antiviral activities of compound 21.
[g]
Virus
Cell line
HFF
EC50
EC90
CC50
SI50
HSV-1[a]
Acyclovir
HSV-2[a]
Acyclovir
VZV[a]
Ͼ300
1.75
Ͼ300
3.34
Ͼ300
0.69
Ͼ300
4.75
Ͼ81
8
Ͼ300
3.09
Ͼ300
Ͼ100
Ͼ300
5.76
Ͼ300
Ͼ100
Ͼ300
Ͼ100
Ͼ300
Ͼ100
Ͼ300
Ͼ300
81
Ͼ1000
Ͼ100
2322
Ͼ20
n.a.
Ͼ57
n.a.
Ͼ30
n.a.
Ͼ145
n.a.
Ͼ63
Ͻ1
Ͼ120
n.a.
48375
n.a.
HFF
HFF
Acyclovir
VV[a]
HFF
Ͼ300
8.22
Cidofovir
PTV[b]
Vero 76
2.2.15
Antiviral Tests
Ribavirin
HBV[c]
Ͼ100
0.048
Ͼ100
0.135
Ͼ20
We submitted samples of some representative com-
pounds (18a, 18b, 19 and 21 to the NIH/NIAID (USA) for
primary antiviral evaluation. Compounds 19 and 21 were
tested for their inhibitory activity against Herpes simplex
virus 1 (HSV-1), Herpes simplex virus 2 (HSV-2), Varicella-
Zoster virus (VZV), Vaccinia virus (VV), Punta Toro virus
(PTV), hepatitis B virus (HBV) and hepatitis C virus
(HCV). The antiviral activities of the above compounds
were tested in vitro in cell line HFF (strain E-377 for HSV-
1, strain G for HSV-2, strain Ellen for VZV and strain
Copenhagen for VV), the Vero 76 cell line (strain Adames)
for PTV, cell line 2.2.15 (strain ayw) for HBV and the Huh-
Lamivudine
HCV[d]
HL/Neo ET Ͼ20
IFNα-2b
Flu-A
Ͼ2
MDCK
Ͼ2
67.78
0.14
Ͼ100
Ͼ14
Ͼ100
Ͼ1
H1N1[e]
Ribavirin
n.a.[f]
5.85
13.28
Ͼ100
298.69
Ͼ300
Ͼ17
HFF
Cidofovir
[a] Drug conc. range 0.096–300 μm; control conc. range 0.032–
100 μm. [b] Drug conc. range 0.1–100 μm; control conc. range 1–
1000 μm. [c] Drug conc. range 0.1–100 μm; control conc. range 0.3–
100 μm. [d] Drug conc. range 0.063–20 μm; control conc. range
0.006–2 lUmL–1. [e] Drug conc. range 0.096–300 μm; control conc.
range 0.096–300 μm. [f] Drug conc. range 0.8–100 μm; control conc.
Luc/Neo ET cell line (strain CON-1) for HCV. They were range 0.8–100 μm. [g] n.a.: not applicable.
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
Both the hydroxylamine derivative 19 and the adenine same molecules to complete these in vitro preliminary re-
cyclopentene 21 were found to be mainly inactive in the sults, with the possibility also to introduce minor structural
reported Herpes-type viruses experiments. The same lack of modifications to upgrade the antiviral activities of our nu-
activity was also observed in the cases of PTV and HBV. cleoside analogues. Docking and protein binding studies to
Insufficient activity was found for compounds 19 and 21 in shine some light on the structure–activity relationships of
the case of Flu-A H1N1 virus and NA. Some differences these nucleoside analogues will follow.
between the two cyclopentenyl derivatives can be observed.
Compound 19 seems to be capable of inhibiting the Flu-A
Discussion
H1N1 virus, whereas compound 21 showed a CC50 value
very close to the limit value expressed by the reference Ci-
dofovir.
The introduction of a heterobase, in this case 6-chloro-
purine, onto a cyclopentene spacer generates a fairly sym-
metrical substrate in which the heterobase does not influ-
ence or restrict the conformational mobility of the cyclo-
pentene ring. X-ray analysis in fact showed the presence in
the crystal structure of the two conformers in which the
methylene bridge is displaced upward and downward with
respect to the average plane of the unsaturated moiety. The
conformational space of product 16a was explored through
optimization by the DFT approach at the B3LYP/6–31G(d)
The bromoisoxazoline derivatives 18a and 18b were also
tested, but only against the respiratory virus influenza A
H1N1 cell line MDCK (strain California 7/2009), in view
of the results previously obtained for compounds 19 and
21. Table 3 reports the primary antiviral activities of the
tested compounds and control experiments with specified
drugs.
[31]
level. Figure 3 shows the calculated structures for the
two conformers 16aD and 16aU.
Table 3. Primary antiviral activities of compounds 18a and 18b.
Entry
Virus[a]
EC50
EC90 CC50 SI50
1
2
3
18a
Flu-A H1N1 0.80
Ͼ100 Ͼ100 Ͼ125
Ͼ100 Ͼ100 Ͻ1
13.28 Ͼ100 Ͼ17
18b
Ͼ100
5.85
Ribavirin
[a] Drug conc. range 0.8–100 μm; control conc. range 0.8–100 μm.
These primary antiviral activity data might suggest po-
tential for the development of docking experiments with the
selected protein responsible for the replication of the influ-
enza type A virus of type H1N1. The outbreak of the hu-
man influenza A pandemic (H1N1) caused considerable
public concern. Recent studies focused attention on the un-
derstanding of this new virus by analysing the relationship
between its molecular characteristics and its pathogenic
properties.[27] Results of this analysis indicated that the hu-
man pandemic influenza A (H1N1) virus was a new, reas-
Figure 3. Calculated structures of the conformers 16aD and 16aU
and relative energies. Deviations from planarities are reported in
degrees.
The difference in energy between the two conformers is
sorted virus combining genetic materials from the avian flu small, and the deviation from the average plane defined by
(H1N1) virus, the classical swine flu (H1N1) virus, the hu- the unsaturated system is just over 10°. Nevertheless, an
man flu (H3N2) virus and the Eurasian swine flu (H1N1) interesting difference can be observed in the consequent po-
virus. Analysis of the sequences for receptor-binding and sition of the purine ring and in particular of the H–C=N
cleavage sites of hemagglutinin (HA) allowed it to be deter- system belonging to the imidazole moiety.
mined that replication could be inhibited by molecules such
This is the moment to comment on the X-ray structures
as Oseltamivir (Tamiflu), a diamine derivative of shikimic shown in Figure 2. The purine ring is in the special position
acid,[28] and Zanamivir (Relenza), a derivative of 3,4-di- x, 1/2, z, whereas the cyclopentene rings have a mirror
amino-2-(hydroxymethyl)-3,4-dihydro-2H-pyran-6-carb- plane. In the purine moiety, the sum angles at N9 [360.0(3)°]
oxylic acid.[29]
and N9Ј [360.0(3)°] are consistent with sp2 hybridization of
The Influenza A virus is an orthomyxovirus, and its re- N9 and N9Ј, through which the 2pz lone pair takes part in
ceptor binding complex consists of two primary structural π-bonding within the heterocyclic system. The cyclopentene
proteins: hemagglutinin (HA) and neuraminidase (NA). It rings are tilted by 90.0(2)° with respect to the purine ring.
has been determined that hemagglutinin is the primary pro- This excludes any conjugation between the cyclopentene
tein responsible for binding to receptor sites on the cell and purine systems; indeed, the N9–C10 [1.470(4) Å] and
membrane, allowing the virion to enter the cell.[30]
N9Ј–C10Ј [1.473(5) Å] distances are longer than the corre-
Being aware of the structural differences between our sponding single bonds [1.426(12) Å] in aromatic com-
compounds and the active compounds cited above, we are pounds. Interestingly, there is exact planarity in the purine
planning firstly to perform a secondary screening of the moieties, because of the x, 0.2500, z coordinates. In the pur-
8
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
ine five-membered ring the N7–C8 [1.301(5) Å] and N7Ј– wider space area created by the cyclopentane ring in the
C8Ј [1.316(5) Å] distances in molecules 16aD and 16aU envelope conformation as in 18aD.
correspond to double bonds, whereas the remaining four
distances correspond to values intermediate between single
and double bonds. In the purine six-membered rings, N3–
C4 [1.323(5) Å] and N5–C6 [1.311(5) Å] in molecule 16aD
and N3Ј–C4Ј [1.326(6) Å] and N5Ј–C6Ј [1.312(6) Å] in mo-
lecule 16aU correspond to double bonds, whereas the re-
maining distances are longer, thus suggesting π delocaliza-
tion over the purine system.
The cyclopentene ring in molecule 16aD is slightly puck-
ered, with the maximum deviations from the least-squares
plane of 0.075(4) Å below this plane for C11 and of
0.090(3) Å above it for C10. The cyclopentene ring in mole-
cule 16aU is more puckered, with the maximum deviations
from the least-squares plane of 0.1699(5) Å below the plane
(for C10Ј) and 0.139(4) Å above it. The puckering param-
eters of the cyclopentene moieties for molecules 16aD and
16aU calculated as described by Cremer and Pople[32] are
Q = 0.1460, φ = 0° and Q = 0.2704, φ = 0°, respectively. The
cyclopentene rings each show an envelope (E) conformation
(ideal value φ = 0°). If the pseudorotation concept[33] is ap-
plied to this ring system in the two independent molecules
16aD and 16aU, the phase angle (P) is 180° or 0°; a pseudo-
rotation over P = 180° gives the mirror image of the rings
with all signs of torsion angles θ inverted. The P value is
calculated from a formula:
tanP = (θ2 + θ4) – (θ1 + θ3)/2θ0 (sin36° + sin72°)
where θ0, θ1, θ2, θ3 and θ4 are the five torsion angles about
C12–C13*, C13*–C14*, C14*–C10, C10–C11 and C11–C12
for the two independent molecules 16aD and 16aU, respec-
tively (see values in Table S1 in the Supporting Infor-
mation).
In the conformer 16aD the purine ring occupies the more
favourable space, from the steric point of view, in a pseudo-
equatorial position around the cyclopentene ring, and the
imidazolyl hydrogen atom points out of the plane of the
cyclopentene ring. In the other case, 16aU, the purine occu-
pies a pseudoaxial position that shifts the hydrogen atom
of the imidazole ring just over the plane of the cyclopentene
ring. Presumably this conformer, although more stable, is
less populated when the C=C double bond is oriented to
receive the addition by the nitrile oxide or by the ni-
trosocarbonyl intermediate in the 1,3-dipolar cycloaddition
or ene reaction, respectively.
Figure 4. Calculated structures of the conformers 18aD, 18bD,
18aU and 18bU and relative energies. Deviations from planarity
are reported in degrees.
Analogous considerations apply to comparison of the
two cyclopropylamino derivatives 18bD and 18bU.
These conformational studies should serve for the
planned programmed docking studies on HA and NA re-
ceptors, allowing for the evaluation of the levels of binding
between the active compounds and the proteins responsible
for the viral activity. This pivotal step should suggest struc-
tural modifications for introduction to improve the activi-
ties of the selected molecules.
Conclusions
An analogous conformational analysis was conducted on
the ethylamino and cyclopropylamino cycloadducts to the
We report the synthesis of heterobase-functionalized cy-
bromonitrile oxide 18a and 18b, these being the compounds clopentene derivatives as valuable substrates for the intro-
found to be active against the flu virus. Figure 4 shows the duction of suitable substituents through pericyclic reac-
calculated structures for both series of conformers. The two tions. The use of 6-chloropurine in this specific case was
ethylamino conformers differ more significantly in energy considered more convenient because of the further func-
then the cyclopropylamino ones, with 18aD being more tionalization made possible by the presence of the chlorine
stable by 2.15 kcalmol–1. The reduced flexibility of the cy- atom in the 6-position in the heterobase and the robust
clopentane moiety is caused by the fusion with the bromo- methods of derivatization already applied in previous syn-
isoxazoline ring, and presumably this increases the difficult- theses. The problem of the high symmetry that characterizes
ies in interconversion between the two forms; in addition, the chloropurine cyclopentene makes the structural attri-
the ethylamino-purine ring can be easily located in the bution of the further derivatized compounds somewhat dif-
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
9

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
Scheme 8.
added portionwise with vigorous stirring at 0 °C over a couple of
hours to generate the corresponding sodium salt 15. Cyclopentene
mesylate 13 (1.5 g, 9 mmol) was added dropwise to the solution,
and the reaction mixture was stirred at room temperature for 48 h.
After this, the reaction was quenched by pouring the mixture into
ice, and the products were extracted from the water phase with
dichloromethane. The residue obtained upon evaporation of the
solvent was subjected to chromatographic separation to isolate the
products. Regioisomers 16a and 16b were obtained in 36% and
23% yields, respectively, and fully characterized.
ficult, although both the ene and 1,3-dipolar cycloaddition
reactions seem to offer new contributions to the search for
biologically active compounds (Scheme 8).
Future developments – in particular, the application of
the mild and high-yielding Al(Hg) N–O bond cleavage of
the isoxazoline ring for the convenient introduction of new
functionalities – are currently under study.
This scaled-up strategy for the synthesis of new nucleo-
side analogues will be applied to prepare new products to
be tested in secondary screening against the virus found to
be most promising in the previous observations.
Compound 16a: 0.73 g (36%), m.p. 53–54 °C from acetone. 1H
NMR (300 MHz, DMSO, 25 °C): δ = 2.78 (dd, J = 15, 8 Hz, 2 H,
CH2), 2.97 (dd, J = 15, 5 Hz, 2 H, CH2), 5.37 (m, 1 H, N-CH),
5.90 (s, 2 H, CH=CH), 8.66 (s, 1 H, CH=N), 8.79 (s, 1 H,
CH=N) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 39.2, 54.3,
Experimental Section
129.2, 131.5, 146.2, 149.3, 151.6, 152.0 ppm. IR: ν = 1592 (C=C),
˜
General: All melting points are uncorrected. Elemental analyses
were performed with a C. Erba 1106 elemental analyzer. IR spectra
(Nujol mulls) were recorded with a Perkin–Elmer RX-1 FTIR spec-
1557 (C=N) cm–1. C10H9ClN4 (220.66): calcd. C 54.43, H 4.11, N
25.39; found C 54.6, H 4.2, N 25.2.
1
trometer. H and 13C NMR spectra were recorded with a Bruker
1
Compound 16b: 0.47 g (23%). Yellowish oil. H NMR (300 MHz,
AVANCE 300 instrument in the specified deuterated solvents.
Chemical shifts (δ) are expressed in ppm from internal tetramethyl-
silane. Column chromatography and tlc: silica gel 60 (0.063–
0.200 mm, Merck), eluent cyclohexane/ethyl acetate from 9:1 to
5:5. MPLC chromatographic separations were performed with a
Biotage Flash Master Personal apparatus and eluent cyclohexane/
ethyl acetate from 9:1 to 5:5. The identification of samples from
different experiments was achieved through mixed m.p.s and super-
imposable IR spectra.
DMSO, 25 °C): δ = 2.02 (dd, J = 16, 2 Hz, 2 H, CH2), 2.29 (m, 2
H, CH2), 4.99 (m, 1 H, N-CH), 5.14 (s, 2 H, CH=CH), 7.87 (s, 1
H, CH=N), 7.99 (s, 1 H, CH=N) ppm. 13C NMR (75 MHz,
DMSO, 25 °C): δ = 38.7, 55.8, 80.4, 125.4, 126.8, 127.0, 138.5,
144.4, 146.3, 150.3 ppm. IR: ν = 1592 (C=C), 1538 (C=N) cm–1.
˜
C10H20ClN4 (220.66): calcd. C 54.43, H 4.11, N 25.39; found C
54.2, H 4.1, N 25.5.
Cycloaddition between Bromonitrile Oxide and Adduct 16a: Sodium
hydrogen carbonate (0.8 g, 9.5 mmol) was suspended in a solution
of adduct 16a (1.0 g, 4.5 mmol) in AcOEt (150 mL). An AcOEt
solution (80 mL) of the bromoxime shown in Scheme 4 (1.84 g,
9.1 mmol) was added dropwise with vigorous stirring at room temp.
The reaction mixture was stirred at room temperature for 72 h.
After this, the inorganic salts were filtered, and the solvent was
removed under vacuum. The residue obtained was subjected to
chromatographic purification to isolate the cycloadduct 17 in 98%
yield.
Materials: Cyclopentadiene was freshly distilled from the dimer
purchased from Sigma–Aldrich. 6-Chloropurine (14) was pur-
chased from Sigma–Aldrich. All other reagents and solvents were
purchased from Sigma–Aldrich or Alfa–Aesar and used without
any further purification.
Synthesis of Adducts of 6-Chloropurine (14) and Cyclopentene Mes-
ylate 13: 6-Chloropurine (14, 3 g, 19 mmol) was suspended in anhy-
drous DMF, and a slight excess (1.2 equiv.) of NaH (95%) was
10
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
Compound 17: 1.52 g (98%), m.p. 223–225 °C from methanol. 1H
NMR (300 MHz, DMSO, 25 °C): δ = 2.66 (m, 4 H, CH2), 4.22 (t,
with gaseous ethylamine and kept in a sealed tube at 50 °C for 48 h.
After this, the solvent was evaporated and the residues were taken
J = 9 Hz, 1 H, 4-H isox.), 4.89 (m, 1 H, N-CH), 5.35 (dd, J = 9, up with methanol to allow crystallization of compound 20, which
5 Hz, 1 H, 5-H isox.), 8.79 (s, 1 H, CH=N), 8.81 (s, 1 H,
was fully characterized.
CH=N) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 33.9, 53.4,
Compound 20: 19.8 mg (91%), m.p. 175–178 °C from methanol. 1H
NMR (300 MHz, DMSO, 25 °C): δ = 1.17 (t, J = 7 Hz, 3 H, CH3),
2.73 (dd, J = 18.5, 3 Hz, 1 H, H-CH), 3.26 (dd, J = 18.5, 8 Hz, 1
H, HC-H), 3.52 (br, 2 H, N-CH2), 5.77 (m, 1 H, N-CH), 6.63 (ddd,
J = 12.5, 5.5, 2 Hz, 2 H, CH=CH), 7.71 (br, 1 H, NH), 8.04 (s, 1
H, CH=N), 8.19 (s, 1 H, CH=N), 10.90 (s, 1 H, OH) ppm. 13C
NMR (75 MHz, DMSO, 25 °C): δ = 16.2, 34.0, 35.8, 40.8, 57.7,
55.1, 84.3, 131.4, 141.2, 146.5, 149.1, 151.3, 151.9 ppm. IR: ν =
˜
1589 (C=N), 1558 (C=N) cm–1. C11H9BrClN5 (342.58): calcd. C
38.57, H 2.65, N 20.44; found C 38.6, H 2.4, N 20.3.
Syntheses of the Amino Derivatives 18a and 18b. General Method:
Solutions of chloronucleoside 17 (30 mg, 0.09 mmol) in MeOH
(2 mL) were either saturated with gaseous ethylamine and kept in
a sealed tube at 50 °C for 48 h or, in the case of the liquid cyclo-
propylamine, an excess (50 equiv.) was added to the solution and
the reaction was allowed to proceed in a sealed tube under the
same conditions. The solvent was evaporated, and the residues were
taken up with methanol to allow crystallization of the nucleoside
derivatives 18a and 18b, which were fully characterized.
133.9, 140.0, 142.5, 153.6, 155.7, 163.6 ppm. IR: ν = 3306 (NH),
˜
3000 (OH), 1622 (C=C), 1585 (C=N) cm–1. C12H14N6O (258.28):
calcd. C 55.80, H 5.46, N 32.54; found C 55.6, H 5.4, N 32.5.
Syntheses of the Ethylamino Derivative 21: A solution of the chloro-
nucleosides 16a (30 mg, 0.14 mmol) in MeOH (2 mL) was saturated
with gaseous ethylamine and kept in a sealed tube at 50 °C for 48 h.
After this, the solvent was evaporated, and the residues were taken
up with methanol to allow crystallization of compound 21, which
was fully characterized.
Compound 18a: 27.7 mg (90%), m.p. 204–205 °C from methanol.
1H NMR (300 MHz, DMSO, 25 °C): δ = 1.19 (t, J = 9 Hz, 3 H,
CH3), 2.60 (m, 4 H, CH2), 3.59 (br, 2 H, N-CH2), 4.17 (t, J = 9 Hz,
1 H, 4-H isox.), 4.78 (m, 1 H, N-CH), 5.33 (dd, J = 9, 5.5 Hz, 1
H, 5-H isox.), 7.54 (br, 1 H, NH), 8.19 (s, 1 H, CH=N), 8.20 (s, 1
H, CH=N) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 15.0,
30.6, 34.1, 34.9, 52.6, 55.3, 84.5, 119.5, 139.3, 141.1, 152.2,
Compound 21: 28.1 mg (90%), m.p. 105–107 °C from methanol. 1H
NMR (300 MHz, DMSO, 25 °C): δ = 1.30 (t, J = 7 Hz, 3 H, CH3),
2.68 (dd, J = 16, 3 Hz, 2 H, CH2), 3.06 (dd, J = 16, 8 Hz, 2 H,
CH2), 3.72 (br, 2 H, N-CH2), 5.36 (m, 1 H, N-CH), 5.75 (br, 1 H,
NH), 5.94 (s, 2 H, CH=CH), 7.78 (s, 1 H, CH=N), 8.42 (s, 1 H,
CH=N) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 15.0, 30.8,
154.6 ppm. IR: ν = 3270 (NH), 1621 (C=N) cm–1. C H BrN O
˜
13 15
6
(351.20): calcd. C 44.46, H 4.30, N 23.93; found C 44.5, H 4.4, N
23.9.
Compound 18b: 29.3 mg (92%), m.p. 207–210 °C from methanol.
1H NMR (300 MHz, DMSO, 25 °C): δ = 0.62 (m, 2 H, CH2 cy-
cloprop.), 0.71 (m, 2 H, CH2 cycloprop.), 2.55 (m, 4 H, CH2), 3.05
(br, 2 H, N-CH cycloprop.), 4.17 (t, J = 9 Hz, 1 H, 4-H isox.), 4.76
(m, 1 H, N-CH), 5.32 (dd, J = 9, 5.5 Hz, 1 H, 5-H isox.), 7.88 (d,
J = 1 Hz, 1 H, NH), 8.23 (s, 1 H, CH=N), 8.24 (s, 1 H,
CH=N) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 6.4, 23.8,
34.0, 39.8, 52.5, 55.2, 84.4, 119.6, 139.5, 141.2, 152.1, 155.6 ppm.
35.7, 40.4, 52.4, 77.1, 119.5, 129.0, 137.4, 152.9, 154.7 ppm. IR: ν
˜
= 3276 (NH), 1618 (C=N) cm–1. C12H15N5 (229.28): calcd. C 62.86,
H 6.59, N 30.54; found C 62.8, H 6.4, N 30.3.
Photochemical Derivatization of Compound 21: 3,5-Diphenyl-1,2,4-
oxadiazole 4-oxide (0.8 g, 1.5 equiv.) was added portionwise with
stirring over a period of a couple of hours to a methanol solution
of 21 (0.5 g, 2.2 mmol), during irradiation with 2ϫ15 W lamps
centred at 310 nm. The reaction mixture was stirred for a further
3 h for completion. Upon evaporation of the solvent, the residue
was subjected to chromatographic separation to purify the ene ad-
ducts syn-22 and anti-22, obtained in 36% yield as an inseparable
mixture of diastereoisomers. The mixture was fully characterized
spectroscopically; in the NMR spectra we report the chemical shift
of the major diastereoisomer, with the signals of the minor one in
square brackets.
IR: ν = 3271 (NH), 1614 (C=N) cm–1. C H BrN O (363.21):
˜
14 15
6
calcd. C 46.30, H 4.16, N 23.14; found C 46.4, H 4.2, N 23.2.
Photochemical Derivatization of Adduct 16a: 3,5-Diphenyl-1,2,4-
oxadiazole 4-oxide (2.9 g, 1.5 equiv.) was added portionwise with
stirring to a methanol solution of 16a (1.8 g, 8.2 mmol), over a
period of a couple of hours during irradiation with 2ϫ15 W lamps
centred at 310 nm. The reaction mixture was stirred for a further
3 h for completion. After evaporation of the solvent, the residue
was subjected to chromatographic separation to purify the ene ad-
ducts 19syn and 19anti, obtained in 26% yields as an inseparable
mixture of diastereoisomers. The mixture was fully characterized
spectroscopically; in the NMR spectra we report the chemical shift
of the major diastereoisomer, with the signals of the minor one in
square brackets.
Compound syn/anti-22: Orange oil, 0.29 g (36%). 1H NMR
(300 MHz, DMSO, 25 °C): δ = 1.16 (t, J = 7 Hz, 3 H, CH3), 2.00
[2.32] (m, 1 H, H-CH), 3.00 [2.51] (m, 1 H, HC-H), 3.51 (br, 2 H,
N-CH2), 5.58–5.90 (m, 1 H, 1 H, N-CH), 6.18 (m, 2 H, CH=CH),
7.43 (m, 3 H, arom.), 7.63 (m, 2 H, arom.), 7.74 (br, 1 H, 1 H,
NH), 7.95 [8.18] (s, 1 H, CH=N), 8.09 [8.22] (s, 1 H, CH=N), 9.77
[9.82] (s, 1 H, 1 H, OH) ppm. 13C NMR (75 MHz, DMSO, 25 °C):
δ = 14.9, 22.8, 30.7, 34.3, 34.5, 57.1, 59.1, 62.2, 63.4, 127.8, 128.2,
130.1, 132.8, 133.8, 134.9, 135.0, 135.1, 138.2, 138.8, 152.3, 152.4,
Compound syn/anti-19: 0.75 g (26%), m.p. 176–179 °C from ace-
tone. 1H NMR (300 MHz, DMSO, 80 °C): δ = 2.44 [2.10] (m, 1 H,
H-CH), 3.02 [2.63] (m, 1 H, HC-H), 6.28 [5.67] (m, 1 H, N-CH),
5.95 (m, 2 H, CH=CH), 7.42 (m, 3 H, arom.), 7.66 (m, 2 H, arom.),
8.69 [8.46] (s, 1 H, CH=N), 8.77 [8.82] (s, 1 H, CH=N), 9.82 [9.73]
(s, 1 H, OH) ppm. 13C NMR (75 MHz, DMSO, 80 °C): δ = 33.8
[34.2], 39.5 [39.8], 60.4 [58.3], 127.8, 128.2, 130.2, [131.3], [131.9],
132.9, [134.8], 134.9, 135.8, [136.1], [145.4], 146.2, [149.0], 151.3,
154.4, 168.8, 168.9 ppm. IR: ν = 3304 (OH), 1667 (C=O), 1621
˜
(C=N), 1611 (C=N) cm–1. C19H20N6O2 (364.40): calcd. C 62.62, H
5.53, N 23.06; found C 62.5, H 5.5, N 23.1.
X-ray Crystallographic Analysis of 16a: Unit cell dimensions for
compound 16a were obtained by least-squares fitting of 2θ values
for 25 reflections, with an Enraf–Nonius CAD4 diffractometer and
graphite-monochromated Mo–Kα radiation at the Centro Grandi
Strumenti (CGS) of the University of Pavia, Italy. A summary of
crystal data, data collection and structure refinement is presented
in Table 4.
151.5, [151.6], 168.9 [169.0] ppm. IR: ν = 3200 (OH), 1634 (C=O),
˜
1592 (C=C), 1557 (C=N) cm–1. C17H14ClN5O2 (355.78): calcd. C
57.39, H 3.97, N 19.68; found C 57.5, H 4.0, N 19.6.
Synthesis of the Ethylamino Derivative 20: A solution of chloronu-
cleosides 19 (30 mg, 0.08 mmol) in MeOH (2 mL) was saturated
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
11

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
L. Scagnelli, M. G. Memeo, S. Carosso, B. Bovio, P. Quadrelli
FULL PAPER
Table 4. Crystal data, data collection and structure refinement.
Acknowledgments
Empirical formula
Formula weight
Crystal size [mm]
Temperature [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α
β
C10H9N4Cl
220.661
0.49ϫ0.42ϫ0.07
293
monoclinic
P 21/m
12.104(4)
6.684(2)
12.880(3)
90.0
99.72(1)
90.0
1027.1(5)
4
Financial support by the University of Pavia, the Ministero dell’U-
niversità
e
della Ricerca (MIUR) (PRIN 2008, CUP:
F11J10000010001 and PRIN 2011, CUP B11J12002450001) and
Steroid S. p. A. - V. le Spagna, 156-20093 Cologno M. se (MI), Italy
for a research grant is gratefully acknowledged. Prof. M. Prichard
(University of Alabama, Birmingham) is warmly thanked for Her-
pes viruses tests, Dr. D. Smee (Utah State University) for PTV tests,
Prof. J. Noah (Southern Research Institute) for influenza viruses
tests, Dr. M. Murray (Southern Research Institute) for hepatitis C
virus tests and Prof. B. Korba (Georgetown University) for hepati-
tis B virus tests. The authors also acknowledge the CINECA
Award (number HP10CWAWUL, 2012) for the availability of high-
performance computing resources and support.
γ
V [Å3]
Z
Dcalcd. [gcm–3]
Absorption coeff., μ
[mm–1]
1.427
0.339
Diffractometer/scan
Radiation
λ [Å]
Enraf–Nonius CAD-4, ω/2 θ
MoKα
0.71073
456
1.60 Ͻ θ Ͼ 25.0
–14 Ͻ h Ͼ 14, 0 Ͻ k Ͼ 7, 0 Ͻ l Ͼ 15
1980
1980
[1] a) G. L. Patrick, in: An Introduction to Medicinal Chemistry
Oxford U. P., Oxford, 2001; b) P. Herdewijn, Drug Discovery
Today 1997, 2, 235–242; c) P. Wang, J. H. Hong, J. S. Coo-
perwood, C. K. Chu, Antiviral Res. 1998, 40, 19–44; d) A. Pip-
erno, M. A. Chiacchio, D. Iannazzo, R. Romeo, Curr. Med.
Chem. 2006, 13, 3675–3695; e) Y. Liu, M. Li, X. Xing, Zhong-
guo Xinyao Zazhi 2012, 21, 2493–2498; E. S. Matyugina, A. L.
Khandazhinskaya, S. N. Kochetkov, Russ. Chem. Rev. 2012, 81,
729–746.
[2] a) C. McGuigan, P. Murziani, M. Slusarczyk, B. Gonczy, J.
Vande Voorde, S. Liekens, J. Balzarini, J. Med. Chem. 2011, 54,
7247–7258; b) W. Lin, J. K. Buolamwini, Bioconjugate Chem.
2011, 22, 1221–1227; c) E. De Clercq, Biochim. Biophys. Acta
2002, 1587, 258–275; d) E. De Clercq, Med. Res. Rev. 2002, 22,
531–542; e) A. D. Bricaud, P. Herdewijn, E. De Clercq, Bio-
chem. Pharmacol. 1983, 3583–3588.
F(000)
Range [°] for data coll.
Index ranges
Reflections measured
Unique reflections
Corrections applied
Refinement method
Variables
Lorentz polarization
full-matrix, least-squares on F2
211
1.057
Goodness-of-fit (1980)
R1 (I) Ͼ 2 σ (I), (1355) 0.0466
R1 (1980)
0.0754
0.35, –0.32
(Δρ)max., min., eÅ–3
[3] a) A. Adam, L. A. Agrofoglio, Modified Nucleosides 2008, 393–
424; b) X.-S. Fan, X.-Y. Zhang, X. Wang, G.-R. Qu, Youji Hu-
axue 2008, 28, 1888–1898; c) F. Casu, M.-A. Chiacchio, R.
Romeo, G. Gumina, Curr. Org. Chem. 2007, 11, 999–1016; d)
Q. Wu, C. Simons, Synthesis 2004, 1533–1553 and references
cited therein e) V. P. Rajappan, X. Yin, S. W. Schneller, Tetrahe-
dron 2002, 58, 9889–9895; f) M. Ishikura, A. Murakami, N.
Katagiri, Org. Biomol. Chem. 2003, 1, 452–453; g) L. S. Jeong,
S. J. Yoo, K. M. Lee, M. J. Koo, W. J. Choi, H. O. Kim, J. G.
Park, S. K. Lee, M. W. Chun, J. Med. Chem. 2003, 46, 201–
202; h) V. Gouverneur, S. J. McCarthy, C. Mineur, D. Belotti,
G. Dive, L. Ghosez, Tetrahedron 1998, 54, 10537–10554; i)
G. W. Kirby, M. Nazeer, J. Chem. Soc. Perkin Trans. 1 1993,
1397–1402.
The structure was solved by direct methods, and the E-map cor-
rectly revealed the non-hydrogen atoms in the molecules. The posi-
tions of the hydrogen atoms were located by difference Fourier syn-
thesis, compared with those calculated from the geometry of the
molecules and refined isotropically in the subsequent least-squares
refinement. The programs SHELXL[34] was used to solve the struc-
ture. The ORTEP[35] program was used for molecular graphics.
Antiviral Assays: The National Institute of Allergy and Infectious
Diseases (NIAID) established the AACF under a contract with
Southern Research Institute. The NIAID, through the AACF, pro-
vides free and confidential services for suppliers interested in sub-
mitting compounds for evaluation for antiviral activity. Tested
compounds were delivered in standard DMSO solutions. The
methods applied for the different assays can be found at the URL
via the internet at http://niaid-aacf.org.
[4] M. J. Mulvihill, M. J. Miller, Tetrahedron 1998, 54, 6605–6626
and references cited therein.
[5] a) L. Estevez, G. M. J. Moa, C. Teran, R. A. Mosquera, Tetra-
hedron 2007, 63, 717–726 and references cited therein b) I.
Perez-Castro, O. Caamaño, M. D. Garcia, G. Gomez, F. Fer-
nandez, J. Ferreira, C. Lopez, Synthesis 2007, 2621–2626.
[6] a) B. S. Bodnar, M. J. Miller, Angew. Chem. 2011, 123, 5746;
Angew. Chem. Int. Ed. 2011, 50, 5630–5647; b) B. S. Bodnar,
M. J. Miller, J. Org. Chem. 2007, 72, 3929–3932; c) J. Wang,
M. Froeyen, P. Herdewijn, Adv. Antiviral Drug Des. 2004, 4,
119–145; d) M. J. Miller, Proceedings of the International Sym-
posium on Advances in Synthetic, Combinatorial and Medicinal
Chemistry, Moscow (Russia) May 5–8, 2004, T14 and P140; e)
P. F. Vogt, M. J. Miller, Tetrahedron 1998, 54, 1317–1348; f) L.
Boger, S. M. Weinreb, Hetero Diels–Alder Methodology in Or-
ganic Synthesis Academic Press, San Diego, 1987.
[7] a) T. Sato, S. Aoyagi, C. Kibayashi, Org. Lett. 2003, 5, 3839–
3842; b) N. Momiyama, H. Yamamoto, Org. Lett. 2002, 4,
3579–3582; c) K. R. Flower, A. P. Lightfoot, H. Wan, A. Whit-
ing, J. Chem. Soc. Perkin Trans. 1 2002, 2058–2064; d) S. Iwasa,
A. Fakhruddin, Y. Tsukamoto, M. Kameyama, H. Nishiyama,
Tetrahedron Lett. 2002, 43, 6159–6161; e) R. W. Ware Jr., C. S.
The following control assays were applied: Herpes Simplex virus 1
(Crystal violet), Herpes Simplex virus 2 (Crystal violet), Varicella-
Zoster virus (Crystal violet), Vaccinia virus (Crystal violet), Punta
Toro virus (Visual), hepatitis B virus (DNA hybridization) and hep-
atitis C virus (Luciferase reporter/CytoTox-1). The antiviral activi-
ties of the above compounds were tested in vitro in cell line HFF
(strain E-377 for HSV-1, strain G for HSV-2, strain Ellen for VZV
and strain Copenhagen for VV), the Vero 76 cell line (strain Ad-
ames) for PTV, cell line 2.2.15 (strain ayw) for HBV and cell line
Huh-Luc/Neo ET (strain CON-1) for HCV. The following control
assays were applied: influenza A H1N1 cell line MDCK (strain
California 7/2009; cellTiter-Glo) and neuramidase (NA) cell line
HFF (strain NA; Neutral Red).
Supporting Information (see footnote on the first page of this arti-
cle): X-ray crystallographic data for 16a and copies of the 1H NMR
and 13C NMR spectra of new compounds.
12
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30202
/KAP1
Date: 30-04-13 10:22:09
Pages: 13
Syntheses of New Carbanucleosides by Pericyclic Reactions
Day, S. B. King, J. Org. Chem. 2002, 67, 6174–6180; f) C. P.
Chow, K. J. Shea, S. M. Sparks, Org. Lett. 2002, 4, 2637–2640;
g) M. D. Surman, M. J. Mulvihill, M. J. Miller, Tetrahedron
Lett. 2002, 43, 1131–1134; h) M. D. Surman, M. J. Mulvihill,
M. J. Miller, J. Org. Chem. 2002, 67, 4115–4121; i) S. Iwasa, K.
Tajima, S. Tsushima, H. Nishiyama, Tetrahedron Lett. 2001,
42, 5897–5899.
a) P. Mohan, in: Principles of Medicinal Chemistry (Ed.: D.
Balado), Williams and Wilkins, Pennsylvania, 1995, pp. 862–
868; b) E. De Clercq, Biochem. Pharmacol. 2007, 73, 911–922;
c) A. Piperno, M. A. Chiacchio, D. Iannazzo, R. Romeo, Curr.
Med. Chem. 2006, 13, 3675–3695.
a) T. A. Wencewicz, B. Yang, J. R. Rudloff, A. G. Oliver, M. J.
Miller, J. Med. Chem. 2011, 54, 6843–6858; b) M. Savion,
M. G. Memeo, B. Bovio, G. Grazioso, L. Legnani, P. Quadrelli,
Tetrahedron 2012, 68, 1845–1852.
a) P. Quadrelli, R. Scrocchi, P. Caramella, A. Rescifina, A. Pip-
erno, Tetrahedron 2004, 60, 3643–3651; b) P. Quadrelli, M.
Mella, S. Carosso, B. Bovio, P. Caramella, Eur. J. Org. Chem.
2007, 6003–6015; c) P. Quadrelli, M. Mella, G. Assanelli, A.
Piccanello, Tetrahedron 2008, 64, 7312–7317.
[20] a) P. O’Brien, T. D. Towers, M. Voith, Tetrahedron Lett. 1998,
39, 8175–8178; b) H. O. Kim, H. W. Baek, H. R. Moon, D.-K.
Kim, M. W. Chun, L. S. Jeong, Org. Biomol. Chem. 2004, 2,
1164–1168.
[21] C. K. H. Tseng, V. E. Marquez, R. W. Fuller, B. M. Goldstein,
D. R. Haines, H. McPherson, J. L. Parson, W. M. Shannon, G.
Arnett, M. Hollingshead, J. S. Driscoll, J. Med. Chem. 1989,
32, 1442–1446.
[22] P. Conti, M. De Amici, G. Grazioso, G. Roda, T. Stensbol, H.
Brasuner-Osborne, U. Madsen, L. Toma, C. De Micheli, Eur.
J. Org. Chem. 2003, 4455–4461.
[23] a) A. Pinto, P. Conti, L. Tamborini, C. De Micheli, Tetrahe-
dron: Asymmetry 2009, 20, 508–511.
[24] a) M. Ishikura, A. Murakami, N. Katagiri, Org. Biomol. Chem.
2003, 1, 452–453; b) S. Daluge, R. Vince, J. Org. Chem. 1978,
43, 2311–2320; c) N. Katagiri, Y. Yamatoya, M. Ishikura, Tet-
rahedron Lett. 1999, 40, 9069–9072; d) C. T. Evans, S. M. Rob-
erts, K. A. Shoben, A. G. Sutherland, J. Chem. Soc. Perkin
Trans. 1 1992, 589–592; e) D. Salvatori, R. Volpini, S. Vicen-
zetti, A. Vita, S. Costanzi, C. Lambertucci, G. Cristalli, S. Vit-
tori, Bioorg. Med. Chem. 2002, 10, 2973–2980.
[8]
[9]
[10]
[25] P. Quadrelli, A. Gamba Invernizzi, M. Falzoni, P. Caramella,
Tetrahedron 1997, 53, 1787–1796.
[11]
[12]
a) P. Quadrelli, M. Mella, P. Paganoni, P. Caramella, Eur. J.
Org. Chem. 2000, 2613–2620; b) P. Quadrelli, V. Fassardi, A.
Cardarelli, P. Caramella, Eur. J. Org. Chem. 2002, 2058–2065.
[26] P. Quadrelli, M. Mella, P. Caramella, Tetrahedron Lett. 1999,
40, 797–800.
a) G. W. Kirby, Chem. Soc. Rev. 1977, 6, 1–24; b) T. Sato, S.
Aoyagi, C. Kibayashi, Org. Lett. 2003, 5, 3839–3842; c) N. Mo-
miyama, H. Yamamoto, Org. Lett. 2002, 4, 3579–3582; d)
R. W. Ware Jr., C. S. Day, S. B. King, J. Org. Chem. 2002, 67,
6174–6180; e) C. P. Chow, K. J. Shea, S. M. Sparks, Org. Lett.
2002, 4, 2637–2640; f) M. D. Surman, M. J. Mulvihill, M. J.
Miller, Tetrahedron Lett. 2002, 43, 1131–1134; g) M. D. Sur-
man, M. J. Mulvihill, M. J. Miller, J. Org. Chem. 2002, 67,
4115–4121; h) S. Iwasa, K. Tajima, S. Tsushima, H. Nishiyama,
Tetrahedron Lett. 2001, 42, 5897–5899; i) K. R. Flower, A. P.
Lightfoot, H. Wan, A. Whiting, J. Chem. Soc. Perkin Trans. 1
2002, 2058–2064; j) S. Iwasa, A. Fakhruddin, Y. Tsukamoto,
M. Kameyama, H. Nishiyama, Tetrahedron Lett. 2002, 43,
6159–6161; k) S. Pulacchini, K. F. Sibbons, K. Shastri, M. Mo-
tevalli, M. Watkinson, H. Wan, A. Whiting, A. P. Lightfoot,
Dalton Trans. 2003, 2043–2052; l) W. Adam, N. Bottke, O.
Krebs, C. R. Saha-Moller, Eur. J. Org. Chem. 1999, 1963–1965.
[27] S. Payungporn, N. Panjaworayan, J. Makkock, Y. Poovorawan,
Acta Virologica 2010, 54, 155–163.
[28] M. Karpf, R. Trussardi, Angew. Chem. 2009, 121, 5871; Angew.
Chem. Int. Ed. 2009, 48, 5760–5762.
[29] a) X.-B. Zhu, M. Wang, S. Wang, Z.-J. Yao, Tetrahedron 2012,
68, 2041–2044; b) J.-J. Shie, J.-M. Fang, P.-T. Lai, W.-H. Wen,
S.-Y. Wang, Y.-S. E. Cheng, K.-C. Tsai, A.-S. Yang, C.-H.
Wong, J. Am. Chem. Soc. 2011, 133, 17959–17965; c) J. H. Be-
igel, Drugs Future 2010, 35, 385–392.
[30] a) K. Subbarao, J. Katz, Cell. Mol. Life Sci. 2000, 57, 1770–
1784; b) Q.-S. Du, H. Wei, R.-B. Huang, K.-C. Chou, Export
Opin. Drug Discov. 2011, 6, 619–631.
[31] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T.
Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pom-
elli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J.
Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, C. Gonzalez, J. A. Pople, Gaussian 03, rev. B.02,
Gaussian, Inc., Pittsburgh PA, 2003.
[13]
a) P. Quadrelli, A. Gamba Invernizzi, P. Caramella, Tetrahe-
dron Lett. 1996, 37, 1909–1996; b) P. Quadrelli, M. Mella, A.
Gamba Invernizzi, P. Caramella, Tetrahedron 1999, 55, 10497–
10510.
[14] P. Quadrelli, G. Campari, M. Mella, P. Caramella, Tetrahedron
Lett. 2000, 41, 2019–2022.
[15] P. Quadrelli, M. Mella, P. Caramella, Tetrahedron Lett. 1999,
40, 797–800.
[16] P. Quadrelli, M. Mella, P. Caramella, Tetrahedron Lett. 1998,
39, 3233–3236.
[17] a) B. S. Bodnar, M. J. Miller, Angew. Chem. 2011, 123, 5746;
Angew. Chem. Int. Ed. 2011, 50, 5630–5647; b) L. Boger, S. M.
Weinreb, Hetero Diels–Alder Methodology in Organic Synthesis
Academic Press, San Diego, 1987.
[18] a) P. Quadrelli, M. Mella, A. Piccanello, S. Romano, P. Cara-
mella, J. Org. Chem. 2007, 72, 1807–1810; b) P. Quadrelli, S.
Romano, A. Piccanello, P. Caramella, J. Org. Chem. 2009, 74,
2301–2310; c) P. Quadrelli, M. Mella, S. Carosso, B. Bovio, J.
Org. Chem. 2013, 78, 516–526.
[19] a) E. F. Healy, J. D. Lewis, A. B. Minniear, Tetrahedron Lett.
1994, 35, 6647–6648; b) J. K. Crandall, D. B. Banks, R. A. Col-
yer, R. J. Watkins, J. P. Arrington, J. Org. Chem. 1968, 33, 423–
425.
[32] P. Cremer, J. A. Pople, J. Am. Chem. Soc. 1975, 97, 1354–1358.
[33] C. Altors, M. Sundaralingam, J. Am. Chem. Soc. 1972, 94,
8205–8212.
[34] G. M. Sheldrick, SHELXL-93. Program for the Refinement of
Crystal Structure, University of Göttingen, Germany, 1993.
[35] C. K. Johnson, ORTEP, report ORNL-3793, Oak Ridge
National Laboratory, Tennessee, USA, 1966.
Received: February 6, 2013
Published Online: ᭿
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
13