Variable Markovnikov orientation and "cis effect" in ene reactions of nitrosocarbonyl intermediates

Article abstract of DOI:10.1021/jo0622025

Nitrosocarbonyl intermediates, generated at room temperature by the mild oxidation of nitrile oxides, undergo clean ene reactions with trisubstituted olefins. Allylic hydrogens on the more congested side of the alkene are exclusively abstracted (the "cis

Full text of DOI:10.1021/jo0622025

SCHEME 1
Variable Markovnikov Orientation and
“cis Effect” in Ene Reactions of Nitrosocarbonyl
Intermediates
Paolo Quadrelli,*,† Mariella Mella,† Andrea Piccanello,†
Silvano Romano,‡ and Pierluigi Caramella*,†
Dipartimento di Chimica Organica, UniVersita` degli Studi di
PaVia, Viale Taramelli 10, 27100 PaVia, Italy, and Unita´ di
Ricerca CNISM e Dipartimento di Fisica “A. Volta”,
UniVersita` degli Studi di PaVia, Via Bassi 6,
27100 PaVia, Italy
SCHEME 2
paolo.quadrelli@unipV.it; pierluigi.caramella@unipV.it
ReceiVed October 24, 2006
Nitrosocarbonyl intermediates, generated at room temperature
by the mild oxidation of nitrile oxides, undergo clean ene
reactions with trisubstituted olefins. Allylic hydrogens on
the more congested side of the alkene are exclusively
abstracted (the “cis effect”), thus resembling singlet oxygen
behavior. Nitrosocarbonyl benzene follows a Markovnikov
orientation and abstracts preferentially the twix hydrogens
over the lone ones. With the more sterically demanding
nitrosocarbonyl mesitylene, the Markovnikov directing effect
is relieved, and comparable twix and lone abstraction are
observed.
generated by clean photochemical4 fragmentation of 1,2,4-
oxadiazole-4-oxides 5, which represents the softest way for the
nitrosocarbonyl generation.
On pursuing our investigations on the ene reactions of
nitrosocarbonyls generated through the mild oxidation of nitrile
oxides, we wish to report here the intriguing changes in the
regioselectivity of the ene reactions of trisubstituted olefins upon
variation of the nitrosocarbonyl substituent, as well as the
remarkable “cis” selectivity of these ene reactions.
The ene reaction of nitrosocarbonyl benzene 8A with 2-meth-
yl-2-butene 9a (trimethylethylene) afforded a single ene adduct
in an almost quantitative yield.3 A few other isopentenyl
derivatives have been tested in the ene reaction with the
nitrosocarbonyl benzene 8A, and the results are reported in
Scheme 2. Addition of benzhydroximoyl chloride 6 to a stirred
CH2Cl2 solution of N-methylmorpholine N-oxide (NMO, 1.2
equiv), triethylamine (1.1 equiv), and an excess of the olefins
9a-c (10 equiv) at room temperature afforded, after 2 h of
stirring, the ene adducts 10Aa-c in excellent yields (Table 1).
The structures of all of the isolated ene adducts rely upon
the corresponding analytical and spectroscopic data that show
the presence of an olefinic methylene. Compounds 10Aa-c
derive from the addition of nitrosocarbonyl 8A to the less
substituted carbon atom of the double bond, in full accordance
with a prevailing HOMO(olefin)-LUMO(nitrosocarbonyl) interaction
that orientates the addition of the nitrosocarbonyl electrophilic
nitrogen to the alkene in a Markovnikov (M) fashion.3
We have extended our investigations to nitrosocarbonyl
mesitylene, which is generated under simple and clean experi-
mental conditions. Mesitonitrile oxide 7 was added to a stirred
CH2Cl2 solution of NMO (1.2 equiv) and an excess of olefins
9a-c (10 equiv) at room temperature. After stirring overnight
Nitrosocarbonyl intermediates 1 (RCONO) are highly reactive
species discovered by Kirby and usually generated by periodate
oxidation of hydroxamic acids.1 Recently we have proposed a
mild alternative for their generation through the oxidation of
nitrile oxides 2 with tertiary amine N-oxides,2 which easily lead
to a variety of aromatic and aliphatic nitrosocarbonyls. These
fleeting intermediates can be trapped with dienes to afford the
corresponding hetero Diels-Alder (HDA) cycloadducts 3 in
high yields (Scheme 1). Nitrosocarbonyls 1 also behave as
“super enophiles” in ene processes3 with a variety of olefins,
affording the ene adducts 4. The ene adducts are unstable under
the periodate oxidative conditions but stable under the milder
oxidation of nitrile oxides and were obtained in excellent yields
with tri- and tetra-substituted olefins. With less substituted
ethylenes the ene pathway is still active, but the oxidation step
of the nitrile oxides competes with the cycloadditions to the
olefins. To avoid this competition, nitrosocarbonyls 1 can be
† Dipartimento di Chimica Organica.
‡ Unita´ di Ricerca CNISM e Dipartimento di Fisica “A. Volta”.
(1) (a) Kirby, G. W. Chem. Soc. ReV. 1977, 6, 1-24. (b) Vogt, P. F.;
Miller, M. J. Tetrahedron 1998, 54, 1317-1348.
(2) (a) Quadrelli, P.; Gamba Invernizzi, A.; Caramella, P. Tetrahedron
Lett. 1996, 37, 1909-1912. (b) Quadrelli, P.; Mella, M.; Gamba Invernizzi,
A.; Caramella, P. Tetrahedron 1999, 55, 10497-10510.
(3) Quadrelli, P.; Mella, M.; Caramella, P. Tetrahedron Lett. 1998, 39,
3233-3236.
(4) (a) Quadrelli, P.; Mella, M.; Caramella, P. Tetrahedron Lett. 1999,
40, 797-800. (b) Quadrelli, P.; Scrocchi, R.; Piccanello, A.; Caramella, P.
J. Comb. Chem. 2005, 7, 887-892.
10.1021/jo0622025 CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/02/2007
J. Org. Chem. 2007, 72, 1807-1810
1807

TABLE 1. Yields of Adducts in the Reactions of Nitrosocarbonyls
8A,B with Alkenes 9a-c and E,Z-12
Ar-CONO
alkene
10
11
8A
9a
9b
9c
9a
9b
9c
95
94
96
43
44
45
8B
54
53
52
13
14
15
8A
8B
12 E
12 Z
12 E
12 Z
73
3
10
29
54
FIGURE 1. Sites of hydrogen abstraction in ene reactions of trimethyl
ethylene. Arrows indicate the preferred sites of H abstraction of the
different enes.
67
41
63
SCHEME 3. Full Arrows Indicate the M and AM “cis”
Paths; Dashed Lines Specify the Not Observed “trans” M
Paths
and column chromatographic separation, comparable amounts
of the M adducts 10Ba-c and the anti-Markovnikov (AM)
adducts 11Ba-c were isolated. As shown in Table 1, the
Markovnikov selectivity is fully relieved and even slightly
reversed in the ene additions of mesityl nitrosocarbonyl 8B. The
structures of the AM adducts rely upon the spectroscopic data
that show the presence of a vinyl substituent in 11Ba and two
vicinal olefinic protons in a trans arrangement in 11Bb,c. In
the AM additions to 9b,c only the (E) stereoisomers 11Bb,c
were formed owing to the severe A(1,3) strain5 involved in the
formation of the (Z) stereoisomers.
To gain further insights on the origin of this variable Markov-
nikov selectivity, we have investigated the ene reactions with the
stereoisomeric (E)- and (Z)-3-methyl-2-pentenes 12E and 12Z.
These stereoisomeric enes have been extensively used in the
investigations of ene reactions because they allow for fully
defining the stereochemical paths and assessing the intriguing
phenomenon known as the “cis effect”.6 The cis effect typically
occurs in singlet oxygen ene reactions and refers to the neat
preference of singlet oxygen for the abstraction of the twix and
lone7 allylic hydrogens from the more congested side of an olefin
(Figure 1). Other enes display different selectivities. Aromatic
nitroso compounds (Ar-NOs)8 and triazolinediones (TADs)9
favor the M “end” selectivity but differ in the “cis” preferences.
The Ar-NOs maintain some “cis” selectivity favoring the twix
over the twin abstraction, whereas TADs do not discriminate
between the two sites.
Addition of benzhydroximoyl chloride 6 to a stirred CH2Cl2
solution of NMO (1.2 equiv), Et3N (1.1 equiv), and excess of
olefins 12E,Z (10 equiv) at room temperature afforded exclu-
sively the “cis” ene adducts with a less strict M control with
respect to the analogous addition to olefins 9a-c (Scheme 3).
The M adducts 13A (from (E)-12) and 15A (from (Z)-12) were
predominantly formed besides minor amounts of the (identical)
AM adducts 14A (Table 1). The structures of the adducts 13-
5A rely upon the spectroscopic data that show the presence of
an olefinic methylene, a vinyl moiety, and a single olefinic
proton, respectively. The major adducts derive from the M
coupling of the nitrogen atom of the nitrosocarbonyl 8A to the
less substituted carbon of the olefin with abstraction of the twix
hydrogens (dots). In the case of 12Z, the twix abstraction from
the ethyl group takes place selectively affording the (E) adduct
15A by avoidance of the A(1,3) strain as observed in the related
cases of aromatic nitroso compounds.7,8 The minor adducts
derive instead from an AM coupling of the nitrogen with
abstraction of the lone hydrogens (squares). Thus, subtle
differences between nitrosocarbonyls and Ar-NO compounds
begin to emerge. The dominating path involves twix abstraction
in both cases while the minor products derive from different
paths, nitrosocarbonyls being inclined to the AM lone route and
Ar-NOs to the M twin route.
The experiments performed with nitrosocarbonyl mesitylene
8B and olefins 12E,Z confirm the neat cis effect, as well as the
relief of M control observed in the addition of 8B to enes 9a-c
(Table 1). In the case of the ene reaction with olefin 12Z the
major AM ene adduct 14B is slightly prevalent over the (E) M
adduct 15B, whereas the ene reaction of olefin 12E affords
instead the prevailing M adduct 13B along with the AM adduct
14B. We trace this slight preference for the M path to the
geminal group effect6c attributable to some steric hindrance in
the AM path because of the approach of the bulkier mesitoyl
and ethyl groups.
(5) Hoffmann, R. W. Chem. ReV. 1989, 89, 1841-1860.
(6) (a) Stephenson, L. M.; Grdina, M. J.; Orfanopoulos, M. Acc. Chem.
Res. 1980, 13, 419-425. (b) Clennan, E. L. Tetrahedron 2000, 56, 9151-
9179. (c) Stratakis, M.; Orfanopoulos, M. Tetrahedron 2000, 56, 1595-
1615. (d) Leach, A. G.; Houk, K. N. Chem. Commun. 2002, 1243-1255.
(e) Singleton, D. A.; Hang, C.; Szymanski, M. J.; Meyer, M. P.; Leach, A.
G.; Kuwata, K. T.; Chen, J. S.; Greer, A.; Foote, C. S.; Houk, K. N. J. Am.
Chem. Soc. 2003, 125, 1319-1328.
(7) For definition, see: Adam, W.; Bottke, N.; Krebs, O. J. Am. Chem.
Soc. 2000, 122, 6791-6792.
(8) (a) Adam, W.; Krebs, O. Chem. ReV. 2003, 103, 4131-4146. (b)
Leach, A. G.; Houk, K. N. Org. Biomol. Chem. 2003, 1, 1389-1403. (c)
Leach, A. G.; Houk, K. N. J. Am. Chem. Soc. 2002, 124, 14821-14822.
(d) Adam, W.; Krebs, O.; Orfanopoulos, M.; Stratakis, M.; Vougioukalakis,
G. C. J. Org. Chem. 2003, 68, 2420-2425. (e) See also: Adam, W.; Bottke,
N.; Engels, B.; Krebs, O. J. Am. Chem. Soc. 2001, 123, 5542-5548.
(9) (a) Singleton, D. A.; Hang, C. J. Am. Chem. Soc. 1999, 121, 11885-
11893. (b) Vougioukalakis, G. C.; Orfanopoulos, M. Synlett 2005, 713-
731.
1808 J. Org. Chem., Vol. 72, No. 5, 2007

FIGURE 2. Comparison of regioselective outcome in ene reactions
of enophiles in the presence of olefins 12E,Z. Numerical values
represent percentage of H abstraction.
FIGURE 3. Side view of the B3LYP/6-31G* TSs 16A,B′ and 17 for
the ene reactions of TME with 8A, 8B′ (where the mesityl was replaced
by 2,6-dimethylphenyl in the calculations) and p-NO2Ph-NO. Activation
enthalpies ∆Hq are given in kcal/mol. Bond distances are in Å; the
CH‚‚‚O contacts (dotted) and steric clashes (wavy lines) are shown in
parentheses. The attacking NCC angles are shown in red.
SCHEME 4
1
The selectivity of O2 arises instead because of a different
approach of the addends. In the ene reaction with tetra- and
trimethylethylene, 1O2 adopts a perepoxide-like TS6e involving
the linear attack of oxygen at the center of the double bond
and almost symmetrical contacts of the outer oxygen with the
twix and lone allylic hydrogens. The H transfer takes place in
the subsequent fly through a valley-ridge inflection point with
no intermediates. In frontier orbital terms the Stephenson/Fukui
model6a accounts well for the features of the perepoxide-like
TS. Nitrosocarbonyl benzene 8A closely approaches the main
pNO2Ph-NO selectivity, but 8B diverges remarkably. How to
account for the convergence of the selectivities of nitrosocar-
The results show the remarkable influence of the nitrosocar-
bonyl substituent on the selectivities in the ene reactions with
trisubstituted olefins. The reactions are “cis” specific and differ
for the “end” selectivity. Nitrosocarbonyl benzene 8A gives
mainly the M twix adducts, whereas the bulkier nitrosocarbonyl
mesitylene 8B affords mixtures of M twix and AM lone ad-
ducts.
1
bonyl 8B and O2?
Model calculations on the reaction of 8A,B′ with tetrameth-
ylethylene (TME) shed light on the factors involved in the
varying selectivities (Figure 3). The lowest B3LYP/6-31G*12
transition structures (TSs) 16A,B′ of the initial addition step
show in both cases the typical side-on attack at a double bond
carbon and the two favorable CH‚‚‚O contacts. TS 16B′ displays
however the usual twisting of the 2,6-dimethylphenyl ring out
of the nitrosocarbonyl plane causing visible steric crowding13
with the methyl group attached to the distal olefinic carbon and
lying on the opposite side of the nitroso oxygen. This hindrance
should affect and raise energetically the M twix path in the
reaction of trimethylethylene, too.
Comparing these results with those of other enophiles,7 such
as singlet oxygen, Ar-NOs, and TADs, some similarities and
differences can be noted. As far as Markovnikov selectivity is
concerned, nitrosocarbonyls locate between the unselective 1O2
and the regiospecific Ar-NO and TAD ene reactions (Figure
2). Detailed mechanistic studies and kinetic isotope effects have
provided evidence for the formation of intermediates in the ene
reactions of Ar-NOs.8 The stepwise mechanism involves a side-
on attack at the least substituted double bond carbon similar to
the nonlinear10 approach of carbene cycloadditions. The addition
leads to unsymmetrical bonded intermediates (the so-called
polarized diradicals),8b,c which can revert to the addends,
rearrange to the ene adducts by H transfer, or cyclize to the
aziridine N-oxide (Scheme 4). The transition structures (TSs)
of the various steps are energetically quite close, and the addition
step is mostly reversible in keeping with experiments8d and
calculations.8b,c The regiochemistry depends upon the addition
rate and the partitioning of the intermediate among reversion,
H-transfer, and cyclization as well.11 The preference for the twix
over the twin abstraction is attributed to the cis effect because
of the two favorable allylic CH‚‚‚O interactions assisting the
initial approach of the addends and comparable partitioning of
the intermediates.8b,c The same mechanism applies to the TAD
reactions, which display a rather unselective abstraction of twix
and twin hydrogens with no notable cis effect owing to the less
favorable CH‚‚‚N interactions.9
For comparison the TS 17 of the ene reaction of p-NO2Ph-
NO with TME is also given in Figure 3. TS 17 is later and its
(11) (a) In the simpler case of consecutive first-step reversible reactions,
the rate depends upon the partitioning of the intermediate according to the
familiar steady-state expressions:11b,c
In other words the reversibility of the addition step decreases the reaction
rate with respect to the addition rate k1, and only a fraction of diradicals
enter the H-abstraction channel. (b) Gellene, G. I. J. Chem. Ed. 1995, 72,
196-9. (c) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry,
4th ed.; Kluwer Academic/Plenum Publishers: New York, 2000, Part A,
pp 192-204.
(12) Frisch, M. J. et al. Gaussian 98, Revision A.9; Gaussian, Inc.:
Pittsburgh PA, 1998; Gaussian 03, revision B. 02; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(10) (a) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital
Symmetry; Academic Press: New York, 1970; pp 152-163. (b) Hoffmann,
R. J. Am. Chem. Soc. 1968, 90, 1475-1485. (c) Keating, A. E.; Merrigan,
S. R.; Singleton, D. A.; Houk, K. N. J. Am. Chem. Soc. 1999, 121, 3933-
3938 and references therein.
(13) Steric crowding raises the energy of the TS since the two methyls
involved adopt higher energy conformations to minimize steric clashes
thereby increasing the distortion energy of the addends. An energetic
evalution of the steric hindrance in the case at hand (1.6 kcal/mol) is detailed
in Supporting Information.
J. Org. Chem, Vol. 72, No. 5, 2007 1809

Nitrosocarbonyls are versatile synthetic tools in allylic
amination15 and the knowledge of the factors determining their
selectivities should increase the potentialities and synthetic use
of these fleeting intermediates.
Experimental Section
General Procedure for Ene Reactions of Nitrosocarbonyl
Benzene 8A. A solution of 1.56 g (10 mmol) of benzhydroximoyl
chloride 6 in 30 mL of CH2Cl2 was added dropwise to a stirred
solution of N-methylmorpholine N-oxide (NMO, 1.2 equiv), tri-
ethylamine (1.1 equiv) and an excess of the olefins 9a-c and 12E,Z
(10 equiv) in 100 mL of CH2Cl2 and allowed to react at room
temperature for 2 h. The reaction mixtures were washed twice with
water (2 × 50 mL), and the organic phases were dried on Na2SO4.
The residues, collected upon evaporation of the solvent, were
crystallized to afford the ene adducts 10Aa-c (see Supporting
Information) or submitted to chromatographic separation to yield
the ene adducts 13-15A, which were isolated and crystallized.
13A: colorless crystals, mp 95-96 °C from i-Pr2O. IR: νmax 3123,
FIGURE 4. Enthalpic profile of the reaction of nitrosocarbonyl
benzene 8A and TME. Numbers are enthalpic differences in kcal/mol.
The key distances in TS 19 and 21 are given in Å.
activation enthalpy higher than those of TSs 16A,B in keeping
with the increased stability of the Ar-NOs. A remarkable steric
hindrance shows up between an aromatic proton and the methyl
group attached to the olefinic carbon undergoing attack and lying
on the opposite side of the nitroso oxygen. This hindrance should
raise energetically the AM approaches in the cases of trimeth-
ylethylene thus favoring M selectivity as observed.
The enthalpic profile of the reaction of 8A with TME is given
in Figure 4. At variance with the case of the Ar-NOs,14 the TSs
of the various steps are well spaced. The profile points out that
the addition is not reversible and the addition step is either rate-
and product-determining. The addition step leads (IRC)12 to the
polarized diradical 18, which readily undergoes H abstraction
through TS 19 to yield the ene adduct 20.
1
1590 cm-1. H NMR (DMSO): δ 1.00 (t, 3H, J 7 Hz); 1.31 (d,
3H, J 7 Hz); 2.06 (m, 2H); 4.94 and 4.97 (bs, 1H + 1H); 5.00 (b,
1H); 7.3-7.7 (m, 5H); 9.41 (s, 1H). Anal. Calcd for C13H17NO2
(MW 219.27): C, 71.20; H, 7.82; N, 6.39. Found: C, 71.28; H,
7.81; N, 6.30. 14A: colorless crystals, mp 91-93 °C from i-Pr2O.
1
IR: νmax 3135, 1597 cm-1. H NMR (DMSO): δ 0.89 (t, 3H, J 7
Hz); 1.42 (s, 3H); 1.94 (m, 2H); 5.02 (bd, 1H, J 11 Hz); 5.08 (bd,
1H, J 17 Hz); 6.16 (dd, 1H, J 17, 11 Hz); 7.3-7.7 (m, 5H); 9.52
(s, 1H). Anal. Calcd for C13H17NO2 (MW 219.27): C, 71.20; H,
7.82; N, 6.39. Found: C, 71.14; H, 7.74; N, 6.46. 15A: colorless
crystals, mp 62-64 °C from i-Pr2O. IR: νmax 3180, 1596 cm-1
.
1H NMR (DMSO): δ 1.25 (d, 3H, J 7 Hz); 1.59 (d, 3H, J 6 Hz);
1.61 (s, 3H); 4.88 (q, 1H, J 7 Hz); 5.41 (q, 1H, J 7 Hz); 7.3-7.7
(m, 5H); 9.32 (s, 1H). The E configuration was assigned through
a NOESY experiment, which showed cross-peaks between the vinyl
and the CH-N protons. Anal. Calcd for C13H17NO2 (MW
219.27): C, 71.20; H, 7.82; N, 6.39. Found: C, 71.29; H, 7.81; N,
6.30.
The polarized diradical 18 is also connected through a
cyclization TS 21 to the aziridine N-oxide 22, but cyclization
is more difficult than H-transfer presumably because the N-acyl
substituent substantially destabilizes the aziridine N-oxide, which
carries a positive charge on nitrogen. As already noted in the
case of Ar-NOs,8b,c restricted and unrestricted calculations yield
identical structures for all species except for the diradical itself.
General Procedure for Ene Reactions of Nitrosocarbonyl
Mesitylene 8B. A solution of 1.61 g (10 mmol) of mesitonitrile
oxide 7 in 30 mL of CH2Cl2 was added dropwise to a stirred
solution of NMO (1.2 equiv) and an excess of olefins 9a-c and
12E,Z (10 equiv) in 100 mL of CH2Cl2 at room temperature. After
stirring overnight the reaction mixtures were washed twice with
water (2 × 50 mL), and the organic phases were dried on Na2SO4.
Column chromatographic separations afforded the ene adducts 10
and 11Ba-c and 13-15B, which were isolated and crystallized
(see Supporting Informations).
Spin unrestricted calculations give a diradical with <S2
)
0.49>, whereas the restricted ones afford a slightly different
structure that lies 0.2 kcal/mol above the unrestricted diradical.
In summary, the results point out that the ene reactions of
nitrosocarbonyls follow a nonlinear “cis” path because of the
two favorable CH‚‚‚O interactions assisting the twix and lone
approach. In the reactions of trimethylethylenes the Markovni-
kov twix path is favored in the case of nitrosocarbonyl benzene
8A, whereas steric hindrance in the twix approach of 8B
compensates somewhat its electronic preference and mixtures
of twix and lone adducts are formed. Conversely in the ene
reaction of p-NO2Ph-NO steric hindrance disfavors AM ap-
proaches enhancing M selectivity. The formal closeness of the
selectivities of 1O2 and 8B rests therefore on different grounds.
Acknowledgment. Financial support by University of Pavia
(FAR) and MIUR (PRIN 2004 and 2005) is gratefully acknowl-
edged.
Supporting Information Available: Physical and spectral
characterization data for all new compounds, Cartesian coordinates
and thermodynamic data of reactants and TSs, full quotation of ref
12, steric hindrance in TS 16B′, and profile of the ene reaction of
p-NO2Ph-NO with TME. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
At variance with the O2 case, nitrosocarbonyls adopt side-on
approaches like Ar-NOs and TADs and ordinary steric effects
modulate the Markovnikov bias.
JO0622025
(14) In the ene reaction of the Ar-NOs the TSs of the various steps are
mostly energetically quite close.8b,c The profile of the ene reaction of p-NO2-
Ph-NO with TME conforms to the trend and is given in Supporting
Information.
(15) (a) Johannsen, M.; Jørgensen, K. A. Chem. ReV. 1998, 98, 1689-
1708. (b) Matunas, R.; Lai, A.; Lee, C. Tetrahedron 2005, 61, 6298-6308.
(c) Zhang, H.; Cai, Q.; Ma, D. J. Org. Chem. 2005, 70, 5164-5173. (d)
Srivastava, R. S.; Nicholas, K. M. Organometallics 2005, 24, 1563-1568.
1810 J. Org. Chem., Vol. 72, No. 5, 2007