Site-selective methylation of N β-Nosyl hydrazides of N -nosyl protected α-amino acids

Article abstract of DOI:10.1021/jo1003168

The methylation reaction of N^β-nosyl hydrazides of N-nosyl protected α-amino acids by using diazomethane shows a controlled regiochemical trend and makes it possible to obtain the corresponding products methylated at specific positions depending on the amount of diazomethane used. The observed selectivity is closely connected with the different acidity of sulfonyl hydrazide, sulfonamide, and acyl hydrazine protons present in the analyzed substrates. The reactivity order of these three diverse reactive sites is supported by theoretical calculations. The hydrazine derivatives considered in this work belong to a class of compounds with interesting biological activity and of great interest in organic synthesis.

Full text of DOI:10.1021/jo1003168

pubs.acs.org/joc
Site-Selective Methylation of Nβ-Nosyl Hydrazides of N-Nosyl Protected
r-Amino Acids
Rosaria De Marco,† Antonella Leggio,† Angelo Liguori,*,† Tiziana Marino,‡
Francesca Perri,† and Nino Russo‡
†
ꢀ
Dipartimento di Scienze Farmaceutiche, Universita della Calabria, Ed. Polifunzionale, 87036,
Arcavacata di Rende, Italy, and ‡Dipartimento di Chimica, Universitaꢀ della Calabria, Via P. Bucci, 87036,
Arcavacata di Rende, Italy
a.liguori@unical.it
Received March 5, 2010
The methylation reaction of Nβ-nosyl hydrazides of N-nosyl protected R-amino acids by using
diazomethane shows a controlled regiochemical trend and makes it possible to obtain the corre-
sponding products methylated at specific positions depending on the amount of diazomethane used.
The observed selectivity is closely connected with the different acidity of sulfonyl hydrazide,
sulfonamide, and acyl hydrazine protons present in the analyzed substrates. The reactivity order
of these three diverse reactive sites is supported by theoretical calculations. The hydrazine derivatives
considered in this work belong to a class of compounds with interesting biological activity and of
great interest in organic synthesis.
Introduction
N-Alkyl-N,N0-bis(arylsulfonyl)hydrazines display signi-
ficant antineoplastic activity against a variety of tumor cells.3
Their tumor-inhibitory properties can be attributed to their
capacity to generate, under physiological conditions, alkyl-
ating species that act by modifying directly the DNA.4
Acyl hydrazine substrates substituted on the β nitrogen
atom with an arenesulfonyl leaving group are of great
interest also in organic chemistry.5 Oxidizing agents convert
N-acyl-N0-(arylsulfonyl)hydrazines to the corresponding
acyl radicals,6 which are useful reaction intermediates in
carbon-carbon bond formation and in particular in the
synthesis of carbocycles.7
Hydrazine derivatives are useful reaction intermediates in
organic synthesis.1 Furthermore, some biologically active
molecules or intermediates involved in their synthesis are
characterized by the presence of the hydrazine functional
group.2
(1) (a) Hydrazine and its Derivatives. Kirk-Othmer Encyclopedia of Chemi-
cal Technology, 5th ed.; Wiley & Sons: New York, 2005; Vol. 13, pp 562-607.
(b) Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A., Ed.; Wiley:
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DOI: 10.1021/jo1003168
r
Published on Web 04/20/2010
J. Org. Chem. 2010, 75, 3381–3386 3381
2010 American Chemical Society

De Marco et al.
JOCArticle
Also, many types of sulfonamide compounds show biologi-
cal activity13 and are widely used in therapy as antibacterial,14
hypoglycemic,15 diuretic,16,17 anticarbonic anhydrase,16,18 and
antithyroid19 drugs.
Results and Discussion
With the aim to obtain new and potentially active sub-
strates we have undertaken the synthesis of molecules having
both sulfonyl and sulfonamide functionalities.
FIGURE 1. Formation of diimide intermediates from N-alkyl-N,
N0-bis(arylsulfonyl)hydrazines.
For this purpose, the Nβ-4-nitrobenzenesulfonyl hydra-
zides (Nβ-nosyl hydrazides) of N-nosyl protected R-amino
acids were chosen as starting model systems. The selective
alkylation of these substrates was performed using diazo-
methane as methylating agent,20 exploiting the different
acidity of sulfonamide and amide protons.21
Diazomethane acts as a base and removes an acidic proton
of the reacting molecule. The methyl diazonium ion obtained
is capable of alkylating the nucleophile species generated
from the initial acid-base reaction.22
As reported in the literature, the acidity in DMSO of both
sulfonamide and sulfonyl hydrazide protons is higher than
that of the acyl hydrazine protons (pKa PhSO2NH2 = 16.1;
pKa PhSO2NH2NH2 = 17.1; pKa PhCONH2NH2 = 18.9);21
instead, the pKa values, measured in DMSO, of sulfonamides
and sulfonyl hydrazides are rather similar.21
These data suggest that the protons of the sulfonamide
and sulfonyl hydrazide functions present in the same mole-
cule of the N0-(N-nosyl-R-aminoacyl)-N00-nosyl hydrazines
could have comparable acidity, with both more acidic than
the acyl hydrazine proton.
With the aim to estimate the acidic properties of this kind
of compounds, density functional theory (DFT) computa-
tions have been performed on the synthesized species. The
gas-phase acidities were calculated following the procedure
described in the Experimental Section.
The three considered anionic forms, named An(Na),
An(Nb), and An(Nc) with regard to the amino groups of
different nature present in the 3a molecule, are shown in
Figure 2.
From these calculations it is evident that the An(Na)
anionic species (ΔΔH = 0.0 kcal/mol) is the most stable
one followed by An(Nb) (ΔΔH = 12.4 kcal/mol) and An(Nc)
(ΔΔH = 14.1 kcal/mol), respectively, hence with regard to
acidic properties of the three sites in the 3a molecule, we
propose the acidity order of Na > Nb > Nc.
Generally, the activity of these hydrazine derivatives takes
place through the generation of a reactive diimide inter-
mediate.3-6 The formation and the nature of this species
determine the biological activity of the N-alkyl-N,N0-bis-
(arylsulfonyl)hydrazine derivatives and the chemical reacti-
vity of N-acyl-N0-(arylsulfonyl)hydrazines. In the first case,
the diimide intermediate formation depends on the acidity of
the hydrazide proton and the leaving group ability of the
arenesulfinate ion (ArSO2-) (Figure 1).3 After nucleophilic
attack on the alkyl group, the diimide intermediate decom-
poses with loss of a molecule of nitrogen and expulsion of the
second arenesulfinate ion and gives rise to the alkylated
nucleophile (Figure 1). The presence of the alkyl substituent
is essential for the generation of reactive species involved in
the antineoplastic activity. In fact N,N0-bis(arylsulfonyl)-
hydrazines not alkylated on the hydrazine moiety do not
show antitumor properties.3a
The evolution of the N-acyl-N0-(arylsulfonyl)hydrazine
systems is also associated with the formation of diimide
species. The anion resulting from the removal of the sulfon-
amide proton, undergoes a 1,2-hydride shift, expulsion of
sulfinate ion, and migration of an electron pair to form the
NdN double bond.5b The acyl diimide thus obtained decom-
poses spontaneously into the corresponding aldehyde.5 This
reaction represents an alternative to the use of hydride
reducing agents in the conversion of carboxylic acid deriva-
tives to aldehydes; differently, the acyl alkyl diimides tauto-
merize readily to the corresponding hydrazones.8 Further-
more, an acyl diimide derivative is also involved in the
formation of the acyl radical that is the key intermediate
responsible for the antitubercolar activity of isoniazid.9
Several unalkylated N-acyl-N0-(arylsulfonyl)hydrazines
show inhibitory activity against a series of serine proteases
involved in the pathogenesis or in the control of some human
diseases. In particular, a series of N-acyl-N0-(arylsulfonyl)
substituted cyclic hydrazide derivatives are selective inhibi-
tors of dipeptidyl peptidase IV.10 Furthermore, tripeptides
containing a C-terminal sulfonyl hydrazide functionality11
have proven to be potent and selective inhibitors of a serine
protease that is essential for the replication of the hepatitis
C virus12 and involved in viral persistence in organisms.
(13) (a) Ghorab, M. M.; Ragab, F. A.; Hamed, M. M. Eur. J. Med. Chem.
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(17) Maren, T. H. Annu. Rev. Pharmacol. Toxicol. 1976, 16, 309.
(18) Supuran, C. T.; Scozzafava, A. Curr. Med. Chem. Immunol. Endocr.
Metab. Agents 2001, 1, 61.
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Biochem. J. 2003, 375, 613. (b) Amos, R. I. J.; Schiesser, C. H.; Smith,
J. A.; Yates, B. F. J. Org. Chem. 2009, 74, 5707.
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Leggio, A.; Liguori, A. J. Org. Chem. 2005, 70, 3892. (c) Belsito, E.; Di Gioia,
M. L.; Greco, A.; Leggio, A.; Liguori, A.; Perri, F.; Siciliano, C.; Viscomi,
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metallic Compounds; VCH Publisher: New York, 1995.
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2000, 74, 2046.
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De Marco et al.
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SCHEME 2. Synthesis of Hydrazines 4a-c
TABLE 2. Synthesis of Hydrazines 4a-c
compound
R
4a
4b
4c
CH3
CH2CH(CH3)2
CH2Ph
Treatment of the hydrazines 3b,c (Scheme 2) under the
same conditions as 3a led to the formation of the correspond-
ing derivatives 4b,c, selectively methylated on the sulfonyl
hydrazide function. In all cases, the products were recovered
in quantitative yields and high purity without need of
chromatographic separation. The methylation reaction is
chemospecific for the alkylation of the sulfonyl hydrazide
function. Both small (1 mmol, 0.55 g of 3c) and larger scale
(14 mmol, 8.0 g of 3c) reactions were performed successfully
and with safety.
FIGURE 2. B3LYP optimized structures of the 3a neutral molecule
and its related anionic forms An(Na), An(Nb), and An(Nc).
These theoretical findings also quantify the different acidic
nature of the sulfonamide and sulfonyl hydrazide protons.
On the basis of these predictions, we were interested in
using the diazomethane as probe molecule to test the acidity
of the Na, Nb and Nc protons of the hydrazine substrates. The
subsequent formation of the alkylation products, resulting
from the reaction of the generated nucleophilic sites with the
methyl diazonium ion, represents a measure of the base
activity of diazomethane, depending on the acidity of the
Na, Nb, and Nc protons.
To investigate the behavior of its acidic protons, B3LYP
calculations were carried out also on 4a species and its
anionic forms. The optimized structures are depicted in
Figure 3.
The nosyl-substituted acyl hydrazines 3a-c were
easily prepared by treatment of nosylhydrazide (2)23 with
the appropriate N-nosyl-R-aminoacyl chloride 1a-c20b,c
(Scheme 1, Table 1).
SCHEME 1. Synthesis of Hydrazines 3a-c
TABLE 1. Synthesis of Hydrazines 3a-c
compound
R
yield (%)
3a
3b
3c
CH3
CH2CH(CH3)2
CH2Ph
80
73
70
FIGURE 3. B3LYP-optimized structures of the 4a neutral mole-
cule and its two related anionic forms An(Nb) and An(Nc).
The N0-(N-nosyl-L-alanyl)-N00-nosylhydrazine (3a), cho-
sen as model system, was dissolved in dry THF and treated
with 1.5 equiv of a 0.66 M methylene chloride solution of
diazomethane24 at room temperature. After 50 min, eva-
poration of the solvent under vacuum afforded N0-(N-nosyl-
L-alanyl)-N00-methyl-N00-nosylhydrazine (4a) in quantitative
yield (Scheme 2, Table 2).
Also in this case, the acidities of Nb and Nc were calculated
and showed the same trend as the 3a species (see Figures 2
and 3). In particular the anion having the negative charge on
the Nb site results more stable than that having the negative
charge on the Nc one, by about 5 kcal/mol.
The theoretical acidity order proposed for the mono-
methylated substrate 4a was confirmed by experimental
data. In fact, the treatment of hydrazines 4a-c with 2 equiv
of a 0.66 M methylene chloride solution of diazomethane at
room temperature led to the formation of the corresponding
(23) Myers, A. G.; Zheng, B.; Movassaghi, M. J. Org. Chem. 1997, 62,
7507.
(24) Arndt, F. Organic Synthesis; Wiley: New York, 1943; Collect. Vol. II,
p 165.
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De Marco et al.
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SCHEME 3. Synthesis of Hydrazines 5a-c
TABLE 5. NBO Charges in |e| for 3a and 5a Species and Their
Anionic Forms
Na
Nb
Nc
O
3a
neutral species
An(Na)
An(Nb)
-0.684
-0.806
-0.716
-0.663
-0.904
-0.914
-0.920
-1.007
-0.475
-0.440
-0.542
-0.489
-0.619
-0.708
-0.736
-0.636
TABLE 3. Synthesis of Hydrazines 5a-c
compound
An(Nc)
R
5a
5a
5b
5c
CH3
CH2CH(CH3)2
CH2Ph
neutral species
An(Nc)
-0.476
-0.541
-0.627
-0.740
dimethylated products 5a-c (Scheme 3), which are recov-
ered in quantitative yields and high purity without need of
chromatographic separation (Table 3). The methodology
was expanded to the large-scale synthesis (gram quantity).
Furthermore, in an additional experiment, the hydrazines
5a-c were completely methylated by treating them with
5 equiv of a 0.66 M methylene chloride solution of diazo-
were also obtained by treatment of the hydrazines 3a-c with
a large excess of diazomethane (10 equiv) for 5 h at room
temperature. Therefore, the theoretical acidity order pro-
posed above agrees well with the observed reactivity of the 3a
species with different stoichiometric ratios of diazomethane.
Similar reasons can be advanced to account for the forma-
tion of the 5a molecule starting from the 3a one when an
increased amount of diazomethane was considered.
1
methane at room temperature (Scheme 4). The H NMR
spectrum of each crude reaction product, obtained after
evaporation of the solvent under vacuum, indicated the
presence of resonance signals corresponding to a mixture
of the two isomers 6a-c and 7a-c in an approximately 1:3
ratio, respectively (Scheme 4).
A further increase of the quantity of diazomethane added
to the 3a species yielded the 6a and 7a trimethylated pro-
ducts. These species show the peculiarity to have the third
methyl group introduced on the carbonyl oxygen atom
rather than on the Nc site. In this case the acidic properties
do not represent the only factor to explain the results, but the
charge distribution can contribute to rationalize the experi-
mental evidence. For this purpose an NBO analysis on the
anionic forms of the 3a species was performed, and the
obtained net charge values of the acidic sites and the carbo-
nyl oxygen present in the 3a species are collected in Table 5.
At first glance on this table, it is possible to deduce that the Nc
site is still the less favored one in the methylation process
because its charge value is the least negative with respect to
the other considered sites. These results support the presence of
the O-methylated products in the course of the synthetic
process. In fact the An(Nc) species shows an NBO charge value
on the carbonyl oxygen (-0.636 e) of about 0.147 e more
negative than that found in the Nc deprotonated site (-0.489 e).
The NBO charges calculations were also extended to the
5a neutral species and its relative anion and the obtained
results are collected in Table 5. In the anionic species derived
from 5a, the charge value on the carbonyl oxygen (-0.740 e)
is more negative than that present on the Nc site (-0.541 e,
Table 5). These reasons can be advanced to explain the lack
of the trimethylated form, having the third methyl group on
the Nc position, as a possible product during the synthesis
described in the present work.
SCHEME 4. Synthesis of Trimethylated Compounds 6a-c and
7a-c
TABLE 4. Synthesis of Trimethylated Products 6a-c and 7a-c
R
6 yield (%)
7 yield (%)
a
b
c
CH3
CH2CH(CH3)2
CH2Ph
21
24
27
60
72
77
Chromatographic separation gave the trimethylated pro-
ducts 6a-c and 7a-c in 21-27% and 60-77% yield,
respectively (Table 4). The methylation reaction was also
performed successfully on a larger scale than 1 mmol. The
formation of these compounds definitely confirms a chemo-
specific control of the methylation reaction of the sulfonyl
hydrazide substrates by using diazomethane.
The hydrazines 3a-c were then subjected to the reaction with
different ratios of diazomethane: the treatment with 3.5 equiv of
a 0.66 M methylene chloride solution of diazomethane afforded
the corresponding dimethylated derivatives 5a-c. Also in this
case, the methylation reaction shows chemospecificity, provid-
ing the corresponding products methylated on the nitrogen
atoms directly bound to the nosyl groups.
Conclusions
The described methylation procedure of the Nβ-nosyl
hydrazides of the N-nosyl protected R-amino acids with
diazomethane provides in high yields and purity the corre-
sponding products selectively methylated at specific posi-
tions. The theoretical calculation of the acidity in the gas
phase of some representative derivatives of the synthesized
compounds is useful in the rationalization of the peculiar
behavior shown by the hydrazine derivatives in the methyl-
ation reaction using different stoichiometric ratios of diazo-
methane. The different acidity of the three different reactive
The same trimethylated products 6a-c and 7a-c, obtai-
ned from 5a-c under the conditions previously described,
3384 J. Org. Chem. Vol. 75, No. 10, 2010

De Marco et al.
JOCArticle
sites present in the analyzed molecules determines the selec-
tivity of the methylation reaction; this reaction is controlled
by the formation of the more stable conjugated base. In any
case, the formation of the methylated products in the reac-
tion of used substrates with diazomethane follows the acidity
trend of the NH groups. The observed selective O-methyla-
tion, during the treatment of the substrates 3a-c and 5a-c
with diazomethane, could be justified by a hard-hard inter-
action driven by the charges present on the methyldiazonium
ion and the oxygen atom of the anion derived from the
deprotonation of the dimethylated compounds.
mixture showed complete conversion of the precursor. Evapora-
tion of the solvent under vacuum provided the respective N0-(N-
methyl-N-nosyl-R-aminoacyl)-N00-methyl-N00-nosyl hydrazine
5a-c in quantitative yield. The methylation reaction was also
performed successfully starting from 13 mmol (7.5 g) of the
hydrazine 4c.
N0-(N-Methyl-N-nosyl-L-alanyl)-N00-methyl-N00-nosylhydrazine
(5a). Obtained as a pale yellow oil (quantitative yield); 1H NMR
(300 MHz, DMSO-d6) δ 10.35 (s, 1H), 8.48-8.38 (m, 4H), 8.01 (d,
J = 9.0 Hz, 4H), 4.49 (q, J = 7.1 Hz, 1H),2.98 (s, 3H), 2.80 (s, 3H),
1.12 (d, J = 7.1 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 169.8,
155.0, 150.7, 149.9, 144.0, 130.2, 128.9, 125.2, 124.9, 53.3, 38.47,
30.7, 15.5. Anal. Calcd for C17H19N5O9S2: C, 40.71; H, 3.82; N,
13.97. Found: C, 40.69; H, 3.80; N, 14.04.
Experimental Section
Treatment of N0-(N-Methyl-N-nosyl-r-aminoacyl)-N00-meth-
yl-N00-nosyl Hydrazines 5a-c with Diazomethane. A 0.66 M
methylene chloride solution of diazomethane (5 mmol) was
added dropwise to a stirred solution of the appropriate N0-(N-
methyl-N-nosyl-R-aminoacyl)-N00-methyl-N00-nosyl hydrazine
5a-c (1 mmol) in dry tetrahydrofuran at room temperature.
The mixture was maintained under stirring for about 80-90
min, until TLC analysis (diethyl ether/petroleum ether 70:30
v/v) of the reaction mixture showed complete conversion of the
precursor. The organic solvent was removed under vacuum, and
the oily residue was subjected to chromatography to provide
compounds 6a-c in 21-27% yields and compounds 7a-c in
60-77% yields.
Chromatographic purification of the crude reaction product
obtained by the reaction performed starting from 5 mmol (3.0 g)
of 5c afforded the corresponding trimethylated products 6c
(0.64 g) and 7c (2.2 g) in 22% and 75% yields, respectively.
(E)-Methyl N-Methyl-N-nosyl-2-(N-methyl-4-nitrophenylsul-
fonamido)propanhydrazinoate (6a). Obtained as a pale yellow oil
(21%); 1H NMR (300 MHz, DMSO-d6) δ 8.30 (d, J = 8.7 Hz, 4
H), 7.83 (d, J = 8.7 Hz, 4 H), 5.45 (q, J = 7.4 Hz, 1H), 3.50 (s,
3H), 2.92 (s, 3H), 2.74 (s, 3H), 1.32 (d, J = 7.2 Hz, 3H); 13C
NMR (75 MHz, DMSO-d6) δ 175.1, 166.0, 151.5, 150.0, 143.4,
132.7, 128.9, 125.3, 124.2, 56.4, 44.8, 38.5, 31.1, 15.7. Anal.
Calcd for C18H21N5O9S2: C, 41.94; H, 4.11; N, 13.59. Found: C,
42.09; H, 4.09; N, 13.53.
Synthesis of N0-(N-Nosyl-r-aminoacyl)-N00-nosyl Hydrazines
3a-c. General Procedure. Sodium bicarbonate (10 mmol) and N,
N-dimethylformamide (0.53 mmol) were added to a stirred solu-
tion of 4-nitrobenzenesulfonylhydrazide (1, 1 mmol) in dry tetra-
hydrofuran (10 mL); a solution of the appropriate N-nosyl-R-
aminoacyl chloride20b,c (1 mmol) in dry tetrahydrofuran (10 mL)
was then added, and the resulting mixture was stirred at room
temperature for 60-90 min, until TLC analysis (chloroform/ethyl
acetate 70:30 v/v) of the reaction mixture showed complete
conversion of the precursor. Distilled water (15 mL) was added
to the reaction mixture, and the solution was extracted with
chloroform (3 ꢀ 10 mL). The combined organic extracts were
washed once with 10% aqueous hydrochloric acid (10 mL) and
once with brine (10 mL) and then dried over Na2SO4. The solvent
was evaporated under vacuum to provide the corresponding
nosyl-substituted acyl hydrazine 3a-c in 70-80% yields.
N0-(N-Nosyl-L-alanyl)-N00-nosylhydrazine (3a). Obtained as a
pale yellow oil (80%); 1H NMR (300 MHz, DMSO-d6) δ 10.40
(d, J = 2.4 Hz, 1H), 10.30 (d, J = 2.4 Hz, 1H), 8.50 (d, J = 8.4,
1H), 8.47-8.39 (m, 4H), 8.12-7.98 (m, 4H), 3.97-3.81 (m,
1H),1.02 (d, J = 7.0 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ
169.8, 150.7, 150.3, 144.6, 144.3, 130.1, 129.0, 125.2, 125.0, 52.6,
15.5. Anal. Calcd for C15H15N5O9S2: C, 38.05; H, 3.19; N, 14.79.
Found: C, 38.19; H, 3.18; N, 14.74.
Synthesis of N0-(N-Nosyl-r-aminoacyl)-N00-methyl-N00-nosyl
Hydrazines 4a-c. General procedure. A 0.66 M methylene chloride
solution of diazomethane (1.5 mmol) was added dropwise to a
stirred solution of the appropriate N0-(N-nosyl-R-aminoacyl)-N00-
nosyl hydrazine 3a-c (1 mmol) in dry tetrahydrofuran at room
temperature. The mixture was maintained under stirring for about
40-50 min, until TLC analysis (chloroform/ethyl acetate 60:40
v/v) of the reaction mixture showed complete conversion of
the precursor. Evaporation of the solvent under vacuum provided
the respective N0-(N-nosyl-R-aminoacyl)-N00-methyl-N00-nosyl
hydrazine 4a-c in quantitative yield. The methylation reaction
was also performed successfully starting from 14 mmol (8.0 g) of
the hydrazine 3c.
N0-(N-Nosyl-L-alanyl)-N00-methyl-N00-nosylhydrazine (4a). Ob-
tained as a pale yellow oil (quantitative yield); 1H NMR (300 MHz,
DMSO-d6) δ10.33 (s, 1H), 8.57 (d, J = 8.4 Hz, 1H), 8.41-8.32 (m,
4H), 8.04-7.92 (m, 4H), 3.85-3.73 (m, 1H), 2.89 (s, 3H), 1.05 (d,
J = 6.9 Hz, 3H); 13C NMR (75 MHz, DMSO-d6) δ 170.3, 150.3,
149.7, 146.8, 142.0, 129.8, 128.1, 124.6, 124.5, 50.3, 38.0, 18.9.
Anal. Calcd for C16H17N5O9S2: C, 39.42; H, 3.52; N, 14.37.
Found: C, 39.55; H, 3.50; N, 14.32.
Synthesis of N0-(N-Methyl-N-nosyl-r-aminoacyl)-N00-methyl-
N00-nosyl Hydrazines 5a-c. General procedure. A 0.66 M methy-
lene chloride solution of diazomethane (2 mmol) was added
dropwise to a stirred solution of the appropriate N0-(N-nosyl-R-
aminoacyl)-N00-methyl-N00-nosyl hydrazine 4a-c (1 mmol) in
dry tetrahydrofuran at room temperature. The mixture was
maintained under stirring for about 60-70 min, until TLC
analysis (chloroform/ethyl acetate 60:40 v/v) of the reaction
(Z)-Methyl N-Methyl-N-nosyl-2-(N-methyl-4-nitrophenylsul-
fonamido)propanhydrazinoate (7a). Obtained as a pale yellow oil
1
(60%); H NMR (300 MHz, DMSO-d6) δ 8.40-8.52 (m, 4H),
8.17-8.00 (m, 4H), 5.12 (q, J = 6.7 Hz, 1H), 4.04 (s, 3H), 2.74 (s,
3H), 2.71 (s, 3H), 1.03 (d, J = 6.7 Hz, 3H); 13C NMR (75 MHz,
DMSO-d6) δ 161.2, 152.4, 148.7, 148.1, 143.2, 129.9, 128.1,
125.1, 123.3, 56.3, 51.5, 38.3, 30.9, 16.1 Anal. Calcd for
C18H21N5O9S2: C, 41.94; H, 4.11; N, 13.59. Found: C, 41.89;
H, 4.12; N, 13.56.
Computational Details. Density Functional Theory calcula-
tions were performed on the 3a and 4a compounds and their
relative neutral and anionic forms. The hybrid Becke three para-
meter exchange and Lee Yang and Parr correlation (B3LYP)25,26
functional was used in both geometry optimization and frequency
calculations as implemented in the Gaussian03 code.27 All the
calculations were carried out using the extended 6-311þG(2df,2p)
basis set on all atoms.
Vibrational frequency calculations performed to determine
the nature of stationary point of all the investigated species and
to take into account the zero-point frequencies and the enthalpy
terms. This approach proved to be adequate to describe with
(25) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: Oxford, 1989.
(26) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(27) Gaussian 03, Frisch, M. J. et al. Gaussian, Inc.: Wallingford, CT,
2004.
J. Org. Chem. Vol. 75, No. 10, 2010 3385

De Marco et al.
JOCArticle
reasonable accuracy both the gas-phase acidities and basicities
where ΔE°elec, ΔZPE, and ΔE298
refer to the differences
vib
of a wide variety of molecular systems.28-33
between the electronic energies at 0 K, the ZPE, and the thermal
vibrational corrections of AH and A-, respectively. The term
5/2RT, necessary to convert the energy in enthalpy, includes the
PV work term and the differences between the translational and
rotational energy contributions of the species involved in the
deprotonation process. The relative acidity values (ΔΔacH) were
computed as differences between the value of the more acidic
compound and the given ones.
In this work the enthalpy variation for the deprotonation
process (ΔacH) at 298 K of the neutral compound AH
AH a A- þ Hþ
ð1Þ
is used as acidity of the AH neutral species in the gas phase and
can be calculated as follows:
5
Natural bond orbital (NBO) analysis34 was performed on all
the neutral and charged species.
ΔacH ¼ ΔE°elec þ ΔZPE þ ΔE298vib þ RT
ð2Þ
2
(28) Marino, T.; Russo, N.; Sicilia, E.; Toscano, M.; Mineva, T. Adv.
Quantum Chem. 2000, 36, 93.
Acknowledgment. This work was supported by grants
ꢀ
from Ministero Italiano dell’Istruzione dell’Universita e
della Ricerca (MIUR).
(29) Marino, T.; Russo, N.; Tocci, E.; Toscano, M. Int. J. Quantum Chem.
2001, 84, 264.
(30) Leopoldini, M.; Marino, T.; Russo, N.; Toscano, M. J. Phys. Chem.
A 2004, 108, 4916.
(31) Leopoldini, M.; Marino, T.; Russo, N.; Toscano, M. Theor. Chem.
Acc. 2004, 111, 210.
Supporting Information Available: Complete list of authors
for ref 27. Characterization of compounds 3b,c, 4b,c, 5b,c, 6b,c,
and 7b,c. 1H NMR and 13C NMR spectra of compounds 3a-c,
4a-c, 5a-c, 6a-c, and 7a-c. Tables of atom coordinates and
absolute energies. This material is available free of charge via the
Internet at http://pubs.acs.org.
(32) Martins, H. F. C.; Leal, J. P.; Fernandez, M. T.; Lopes, V. H. C.;
Cordeiro, M. N. D. S. J. Am. Soc. Mass Spectrom. 2004, 15, 848.
(33) Ragnarsson, U.; Grehn, L.; Koppel, J.; Loog, O.; Tsubrik, O.;
Bredikinin, A.; Maeorg, U.; Koppel, I. J. Org. Chem. 2005, 70, 5916.
(34) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO,
Version 3.1. (part of the Gaussian suite of programs).
3386 J. Org. Chem. Vol. 75, No. 10, 2010