[4 + 2] Cycloadditions of nitroalkenes in water. Highly asymmetric synthesis of functionalized nitronates

Article abstract of DOI:10.1021/jo010182v

The [4 + 2] cycloadditions of (E)-2-aryl-1-cyano-1-nitroalkenes 1 with achiral and enantiopure vinyl ethers 2 and 3 carried out in sole water are reported. These reactions occur in a heterogeneous phase under mild conditions and are fast and highly stereoselective. By using (-)-N,N-dicyclohexyl-(1S)-isoborneol-10-sulfonamide as a chiral auxiliary, the cycloadditions are totally asymmetric. The face selectivity is discussed in terms of the shape of the chiral auxiliary and the reactive conformation of vinyl ether.

Full text of DOI:10.1021/jo010182v

J . Org. Chem. 2001, 66, 4661-4666
4661
[4 + 2] Cycloa d d ition s of Nitr oa lk en es in Wa ter . High ly
Asym m etr ic Syn th esis of F u n ction a lized Nitr on a tes
Francesco Fringuelli,*,† Mizio Matteucci,†,§ Oriana Piermatti,*,† Ferdinando Pizzo,† and
Maria Cristina Burla‡
Dipartimento di Chimica and Dipartimento di Scienze della Terra, Universita` di Perugia, 06100 Italy
frifra@unipg.it
Received February 16, 2001
The [4 + 2] cycloadditions of (E)-2-aryl-1-cyano-1-nitroalkenes 1 with achiral and enantiopure vinyl
ethers 2 and 3 carried out in sole water are reported. These reactions occur in a heterogeneous
phase under mild conditions and are fast and highly stereoselective. By using (-)-N,N-dicyclohexyl-
(1S)-isoborneol-10-sulfonamide as a chiral auxiliary, the cycloadditions are totally asymmetric. The
face selectivity is discussed in terms of the shape of the chiral auxiliary and the reactive conformation
of vinyl ether.
Cyclic six-membered nitronates are interesting syn-
thetic intermediates easily obtained by [4 + 2] cycload-
dition of R,â-unsaturated nitroalkenes (acting as 4π
components)1 with enamines,2 silyl enol ethers,3 enolate
anions,4 silyl ketene acetals,5 allylsilanes,6 and simple
alkenes7 and can be converted into a variety of com-
pounds such as nitroalkylated enamines,2 1,4-diones,3
pyrrolidines,8 aminocyclopentanes,9 N-oxy-â-lactams,10
and highly functionalized polyheterocyclic systems.11
The cycloaddition chemistry of nitroalkenes has been
widely investigated by Denmark.8,11a-c,12,13a,b These reac-
tions are conveniently carried out in organic solvent (CH2-
Cl2, PhMe, PhH) at low temperature (-78 °C) and in the
presence of Lewis acids (TiCl4, TiBr3(Oi-Pr), SnCl4, MAD,
MAPh, ATPh).11a-c,13 Without catalyst the reaction re-
quires a large excess of dienophile,13c,14 a long reaction
time, or activated nitroalkenes.15 The use of high pressure
allows some of these difficulties to be overcome, at least
for the cycloadditions of nitrostyrenes to enol ethers.10,11d
Thermal and catalyzed asymmetric nitroalkene [4 +
2] cycloadditions in organic solvent have also been
investigated by using either optically active nitroalk-
enes16 or enantiopure vinyl ethers.8b,9
For some years we have been interested in organic
reactions performed in pure water as reaction medium,
and we have shown that the aqueous medium can
strongly favor the reactivity and selectivity of the reaction
even when it is carried out under heterogeneous condi-
tions.17
† Dipartimento di Chimica, Via Elce di Sotto, 8 I-06123 Perugia,
Italy.
‡ Dipartimento di Scienze della Terra, P.zza Universita`, I-06100
Perugia, Italy.
§ Present address: Department of Chemistry, Southampton Uni-
versity, Highfield, Southhampton S017 1BJ , U.K.
(1) The reactions of nitroalkenes as 4π components with electron-
rich olefins are mechanistically distinct involving Michael additions
to give betaines followed by a collapse to form the nitronate, a formal
[4 + 2] adduct; Denmark, S. E.; Dappen, M. S.; Cramer, C. J . J . Am.
Chem. Soc. 1986, 108, 1306 and references therein.
(2) (a) Daneo, S.; Pitacco G.; Risaliti, A.; Valentin, E. Tetrahedron
1982, 38, 1499. (b) Nielsen, A. T.; Archibald, T. G. Tetrahedron 1970,
26, 3475. (c) Varma, R. S.; Kabalka, G. W. Heterocycles 1986, 24, 2645.
(3) (a) Miyashita, M.; Yanami, T.; Yoshikoshi, A. J . Am. Chem. Soc.
1976, 98, 4679. (b) Miyashita, M.; Yanami, T.; Kumazawa, T.; Yoshi-
koshi, A. J . Am. Chem. Soc. 1984, 106, 2149. (c) Yoshikoshi, A.;
Miyashita, M. Acc. Chem. Res. 1985, 18, 284.
[4 + 2] Cycloadditions between an electron-rich diene
with an electron-deficient dienophile have been widely
investigated in aqueous medium,18 while, to our knowl-
edge, only two examples are reported in the literature
concerning [4 + 2] cycloadditions in water between an
electron-deficient diene and an electron-rich dienophile:
(13) (a) Denmark, S. E.; Stolle, A.; Dixon, J . A.; Guagnano, V. J .
Am. Chem. Soc. 1995, 117, 2100. (b) Denmark, S. E.; Schnute, M. E.;
Thorareusen, A.; Middleton, D. S.; Stolle, A. Pure Appl. Chem. 1994,
66, 2041. (c) Barco, A.; Benetti, S.; Pollini, G. P.; Spalluto, G.; Zanirato,
V. Tetrahedron Lett. 1991, 32, 2517.
(4) Nielsen, A. T.; Archibald, T. G. J . Org. Chem. 1969, 34, 1470.
(5) Miyashita, M.; Kumazawa, T.; Yoshikoshi, A. Chem. Lett. 1980,
22, 1043.
(6) Ochiai, M.; Arimoto, M.; Fujita, E. Tetrahedron Lett. 1981, 1115.
(7) Denmark, S. E.; Cramer C. J .; Sternberg, J . A. Tetrahedron Lett.
1986, 27, 3693.
(8) (a) Denmark, S. E.; Marcin, L. R. J . Org. Chem. 1993, 58, 3857.
(b) Denmark, S. E.; Marcin, L. R. J . Org. Chem. 1995, 60, 3221
(9) (a) Denmark, S. E.; Dixon, J . A. J . Org. Chem. 1998, 63, 6178.
(b) Denmark, S. E.; Hurd, A. R. J . Org. Chem. 1998, 63, 3045.
(10) Kuster, G. J .; Kalmoua, F.; de Gelder, R.; Scheeren, H. W.
Chem. Commun. 1999, 855.
(11) For a review and recent papers, see: (a) Denmark, S. E.;
Thorarensen, A. Chem. Rev. 1996, 96, 137. (b) Denmark, S. E.; Hurd,
A. R. J . Org. Chem. 1998, 63, 3045. (c) Denmark, S. E.; Seierstad, M.
J . Org. Chem. 1999, 64, 1610. (d) Uittenbogaard, R. M.; Seerden, J .-P.
G.; Sheeren, H. W. Tetrahedron 1997, 53, 11929. (e) Avalos, M.;
Babiano, R.; Cintas, P.; Higes, F. J .; J ime´nez, J . L.; Palacios, J . C.;
Silva, M. A. J . Org. Chem. 1999, 64, 1494.
(12) (a) Denmark, S. E.; Dixon, J . A. J . Org. Chem. 1997, 62, 7086.
(b) Denmark, S. E.; Guagnano, V.; Dixon, J . A.; Stolle, A. J . Org. Chem.
1997, 62, 4610. (c) Denmark, S. E.; Hurd, A. R.; Sacha, H. J . J . Org.
Chem. 1997, 62, 1668. (d) Denmark, S. E.; Middleton, D. S. J . Org.
Chem. 1998, 63, 1604.
(14) (a) Avalos, M., Babiano, R.; Cintas, P.; Higes, F. J .; J ime´nez,
J . L.; Palacios, J . C.; Silva, M. A. J . Org. Chem. 1996, 61, 1880. (b)
Papchikhin, A.; Agback, P.; Plavec, J .; Chattopadhyaya, J . J . Org.
Chem. 1993, 58, 2874.
(15) (a) Tohda, Y.; Tani, K.; Yamawaki, N.; Ariga, M.; Nishiwaki,
N.; Kotani, K.; Matsumura, E. Bull. Chem. Soc. J pn. 1993, 66, 1222.
(b) Tohda, Y.; Yamawaki, N.; Matsui, H.; Kawashima, T.; Ariga, M.;
Mori, Y. Bull. Chem. Soc. J pn. 1988, 61, 461.
(16) Serrano, J . A.; Ca´ceres, L. E.; Roma´n, E. J . Chem. Soc., Perkin
Trans. 1 1992, 941.
(17) (a) Fringuelli, F.; Germani, R.; Pizzo, F., Santinelli, F.; Savelli,
G. J . Org. Chem. 1992, 57, 1198. (b) Fringuelli, F.; Pani, G.; Piermatti,
O.; Pizzo, F. Tetrahedron 1994, 50, 11499. (c) Brufola, G.; Fringuelli,
F.; Piermatti, O.; Pizzo, F. Heterocycles 1996, 43, 1257; 1997, 45, 1715.
(d) Ye, D.; Fringuelli, F.; Piermatti, O.; Pizzo, F. J . Org. Chem. 1997,
62, 3248. (e) Fringuelli, F.; Piermatti, O.; Pizzo, F. In Organic Synthesis
in Water; Grieco, P. A., Ed.; Blackie Acad. and Professional: London,
1998; pp 223-249 and 250-261. (f) Fringuelli, F.; Piermatti, O.; Pizzo,
F.; Vaccaro, L. J . Org. Chem. 1999, 64, 6094. (g) Fringuelli, F.; Pizzo,
F.; Vaccaro, L. Synlett 2000, 311. (h) Fringuelli, F.; Piermatti, O.; Pizzo,
F.; Vaccaro, L. Eur. J . Org. Chem. 2001, 439.
10.1021/jo010182v CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/31/2001

4662 J . Org. Chem., Vol. 66, No. 13, 2001
Fringuelli et al.
Ta ble 1. [4 + 2] Cycloa d d ition s of Nitr oa lk en es 1 w ith
Sch em e 1
Eth yl Vin yl Eth er 2
product (%)
nitroalkene medium T (°C) t (m)
4
5
yield (%)a
1a
1a
1b
1b
1c
1c
H2O
CH2Cl2
H2O
CH2Cl2
H2O
CH2Cl2
0
0
0
0
rt
rt
3
2
20
5
10
5
80
90
96
95
98
97
20
10
4
5
2
75
82
90
90
85
85
3
a
Yield of isolated main reaction product.
diastereoselective because the amount of trans adducts
5, which, in principle, originated from exo-type additions,
is probably the result of the epimerization at C-6 of
kinetically favored cis adducts 4. This nitronate epimer-
ization was mentioned previously,8b and we have ob-
served that by maintaining the pure adduct 4b in acetone
at 40 °C for 15 min, adduct 5b is produced. The rate of
epimerization of the acetal center depends on the solvent
and is accelerated by heating. The cis-trans epimeriza-
tion causes problems in the purification of the reaction
products by recrystallization, but it is not important in
the elaboration of nitronate for synthetic purposes be-
cause the stereogenic acetal center is removed.
the cycloadditions of di(R-pyridyl)-1,2,4,5-tetrazine with
para-substituted styrenes,19 and the cycloadditions of (E)-
1,2-diaza-1,3-butadienes with enol ethers, phenyl vinyl
sulfide, and cyclopentadiene.20
The endo diastereoselectivity of the reaction is justified
on the basis of the strong secondary orbital interactions
between the oxygen of electron-rich vinyl ether 2 and the
positively charged nitrogen atom of nitroalkene 1 which
is present in the endo transition state and absent in the
exo-mode orientation.
As part of our program of organic synthesis in water,
we are interested in demonstrating the potentiality of
aqueous medium in the [4 + 2] cycloaddition of nitroalk-
enes working either as 4π or 2π components. Here we
report the results of [4 + 2] cycloaddition of (E)-2-aryl-
1-cyano-1-nitroalkenes 1 with ethyl vinyl ether 2 and
enantiopure vinyl ethers 3, performed in pure water and
in organic solvent for comparison.
The cis-trans configuration of nitronates 4 and 5 has
been assigned on the basis of 1H NMR coupling constant
values and NOE experiments. This procedure was also
followed for the other nitronates 6-9 (see below). The
data recorded for cis nitronate 4b are illustrated as an
example.
Resu lts a n d Discu ssion
2-Aryl-1-cyano-1-nitroalkenes 1 (Scheme 1) are reactive
compounds that are easily accessible in pure form.21a,b
Their chemistry has been little-studied,21 and they have
never been used in [4 + 2] cycloadditions. The results of
cycloaddition reactions of 1 with 2 in water and CH2Cl2
are reported in Table 1. All the reactions are totally
regioselective and highly diastereoselective. The reactions
carried out in aqueous medium occur in heterogeneous
phase and are slower than those performed in homoge-
neous solution in organic solvent, but high selectivity and
high reaction yields were achieved just the same. The [4
+ 2] cycloadditions can be regarded as totally endo
The saturation of H6 proton frequency gives a NOE
effect on protons H4 (2%), H5R (6.1%), and H5â (0.9%) and
the saturation of the H4 proton frequency gives a NOE
effect on H6 (2.3%) and H5R (6.1%), but no NOE effect
was observed on H5â. The protons H4, H6, and H5R are
therefore cis to each other. The vicinal coupling constant
values of proton H6 and H5 [J (H6-H5â) ) 4.6 Hz; J (H6-
H
5R) ) 4.0 Hz] indicate that the proton at C-6 is
pseudoequatorial, and the high value of the vicinal
coupling constant of protons H4 and H5â [J (H4-H5â) )
7.1 Hz] and the absence of NOE effect between the two
protons suggest a pseudoaxial position for H4.
(18) (a) Garner, P. P. In Organic Synthesis in Water; Grieco, P. A.,
Ed.; Blackie Acad. and Professional: London, 1998; p 1. (b) Parker, D.
T. Ibid 1998, 47. (c) Fringuelli, F.; Piermatti, O.; Pizzo, F. In Targets
in Heterocyclic Synthesis; Attanasi, O. A., Spinelli, D., Eds.; Italian
Society of Chemistry: Rome, 1997; Vol. 1, p 57.
(19) Vijnen, J . W.; Zavarise, S.; Engberts, J . B. F. N. J . Org. Chem.
1996, 61, 2001.
(20) Attanasi, O. A.; De Crescentini, L.; Filippone, P.; Fringuelli,
F.; Mantellini, F.; Matteucci, M.; Piermatti, O.; Pizzo, F. Helv. Chim.
Acta 2001, 84, 513.
(21) (a) Ried, W.; Koler, E. Ann. Chem. 1956, 145. (b) Dore´, J .-C.;
Rumpf, P.; Viel, C. Chim. The´r., J anvier-Fe´vrier 1973, 1, 80. (c)
Demireva, Z. I.; Polyanskaya, A. S.; Mladenov, I.; Perekalin, V. V. Zh.
Org. Khim. 1976, 12, 1192. (d) Demireva, Z. I.; Binev, I. G.; J uchnovski,
I. N. Tetrahedron Lett. 1976, 1523. (e) Mechkow, T.; Demireva, Z. Z.
Chem. 1985, 25, 169. (f) Polyanskaya, A. S.; Bodina, R. I.; Shchadrin,
V. Y.; Aboskalova, M. I. Zh. Org. Khim. 1984, 20, 2481. (g) Mechkov,
T.; Demireva, Z.; Panaiotova, T. Dokl. Bulg. Akad. Nauk. 1986, 39,
51. (h) Kislyi, V. P.; Nesterov, V. N., Shestopalov, A. M.; Semenov, V.
V. Russian. Chem. Bull. 1999, 48, 1131. (i) Kislyi, V. P.; Nesterov, V.
N., Shestopalov, A. M.; Semenov, V. V. Russian Chem. Bull. 1999, 48,
1135.
The configurations of 4 and 5 are further supported
by epimerization of 4 to 5 as mentioned above and from
X-ray analyses of optically active nitronates (see below).
1
The H NMR data, for many cis and trans nitronates,
collected in the course of this study (Table S1, see
Supporting Information), are diagnostic and allow simple
rules to be defined for determining the cis or trans
configurations of nitronate-like 4 and 5: (a) the signals
for H5R and H6 of cis isomer are at lower field than those
for the trans one; (b) the ∆(δ5R - δ5â) is higher for the cis
than for the trans isomer: =0.7 and =0.15 ppm, respec-
tively; (c) the J (H6-H5R) and J (H6-H5â) are higher for
the cis than for the trans nitronates (3.6-4.9 vs 2.0-2.3
Hz and 4.5-5.1 vs 2.2-2.7 Hz, respectively); and (d) the
J (H4-H5â) is higher for the trans than for the cis adducts
(11.6-11.9 vs 6.3-8.1 Hz).

[4 + 2] Cycloadditions of Nitroalkenes in Water
J . Org. Chem., Vol. 66, No. 13, 2001 4663
Ta ble 2. Asym m etr ic [4 + 2] Cycloa d d ition s of
Nitr oa lk en es 1 w ith En a n tiop u r e Vin yl Eth er s 3 in
P u r e Wa ter
asymmetric induction obtained using (-)-3z is a good
omen for the subsequent objectives aimed at converting
nitronate into highly enantiomerically enriched com-
pounds.
products (%)d
The following absolute configurations of (+)-6bx, 7bx,
8bx, and 9bx were assigned. The proton coupling con-
stants (Table S1, see Supporting Information) and NOE
experiments (see the discussion above for 4b as an
example) allowed the cis nitronates (+)-6bx and 7bx to
be recognized with respect to the trans ones 8bx and 9bx.
The cis adduct (+)-6bx epimerizes to the trans 8bx and
the cis 7bx to 9bx. The X-ray analysis of (+)-6bx (Figure
S1, see Supporting Information) allowed the configura-
tion of stereogenic centers 4R and 6S to be assigned. The
absolute configurations of the other three diastereoiso-
mers were consequently assigned.
reactants
6
7
8
9
endo/exo
de (4R) %
1b + 3xa
1b + 3ya
1a + 3zb
1b + 3zc
1c + 3zb
54
60
85
85
83
35
30
5
6
5
89:11
90:10
85:15
85:15
83:17
18
30
100
100
100
5
15
15
17
a
b
At 0 °C for 30 min. At rt for 60 min. c At 0 °C for 60 min.
d
For the yield of isolated products see Experimental Section.
Sch em e 2
Similarly, the NMR data and X-ray analyses of (+)-
7by, (+)-8by,23 and (-)-6bz (Figures S2-S4, see Sup-
porting Information) allowed the absolute configurations
of four diastereoisomers [6by, (+)-7by, (+)-8by, and 9by]
to be assigned, as well as two epimers [(-)-6bz and (+)-
8bz] produced by cycloadditions of 1b with (+)-3y and
(-)-3z, respectively. The cis and trans configurations of
6a z, (-)-6cz and (+)-8a z, 8cz were assigned by proton
coupling constants (Table S1) and NOE experiments. The
absolute configurations were assigned on the basis of
structures of strictly analogous compounds (-)-6bz and
(+)-8bz.
The facial selectivity of [4 + 2] cycloadditions of
nitroalkenes 1 with optically active vinyl ethers 3 are
dictated by the shape of the chiral auxiliary, as well as
by the s-cis or s-trans reactive conformation of vinyl
ether.8,9
The asymmetric cycloaddition of nitroalkenes 1 in pure
water was explored by using enantiopure vinyl ethers
(+)-3x, (+)-3y, and (-)-3z. The cycloaddition of 1b with
(+)-2-isocaranyl vinyl ether (3x) in water was completed
in 30 min at 0 °C and produced a 89:11 mixture of endo
and exo adducts (Table 2). The endo diastereoisomers (+)-
6bx and 7bx which constituted 54% and 35% of the
mixture and adducts 8bx and 9bx, that accounted for
5% and 6%, respectively, are probably the epimerization
products of 6bx and 7bx, respectively (Scheme 2). The
54:35:5:6 ratio, therefore, reflects a 59:41 (4R/4S) ratio
of diastereoisomers or a 18% de (4R). Only the adduct
(+)-6bx was isolated in pure form; the other three
stereoisomers were characterized by using an enriched
mixture.
The cycloaddition of 1b with (+)-isopinocampheyl vinyl
ether (3y) gave similar results: the 60:30:5:5 ratio of
nitronates 6by, (+)-7by, (+)-8by, and 9by reflects a 9:1
endo/ exo ratio and a 65:35 (4R/4S) ratio of diastereoiso-
mers or a 30% de (4R). Also in this case, the nitronates
(+)-8by and 9by came from the epimerization of 6by and
(+)-7by, respectively, rather than from an exo addition.22
The adducts (+)-7by and (+)-8by were isolated in pure
form; while 6by and 9by were characterized by using
enriched mixtures.
By reacting 1b with the vinyl ether (-)-3z, derived
from (-)-N,N-dicyclohexyl-(1S)-isoborneol-10-sulfona-
mide, only (-)-6bz and (+)-8bz were obtained (endo/ exo
) 85/15), reflecting a 100% de (4R) (Table 2). The totally
asymmetric production of 4R diastereoisomers was also
found in the cycloadditions of (-)-3z with 1a and 1c
(Table 2) allowing (+)-8a z and (-)-6cz, respectively, to
be isolated in pure form. The adduct 6a z was the main
reaction product of the cycloaddition of (-)-3z with 1a ,
but it isomerized easily to (+)-8a z during purification by
recrystallization (see Experimental Section). The total
Molecular calculations using the Hyperchem program
on (+)-3x and (+)-3y suggest that the ground state
conformation of both vinyl ethers is a pseudo-s-trans
geometry with the CdCO-C dihedral angles Φ )
-174.25° and -137.12°, respectively, in which the re-face
24
of the vinyl group is only moderately shielded with
respect to the si-face (Figure 1, A and B). This is in
agreement with the low facial selectivity (18 and 30% de
(4R), Table 2) in favor of an endo-unlike combination of
reagents: approach of the si-face of the vinyl ether to
the re-face of the nitroalkene.
The facial selectivity of vinyl ether (-)-3z, in contrast,
is total. Molecular calculations on (-)-3z suggest that the
ground state conformation is a pseudo-s-trans geometry,
Φ ) -178.52°, in which one of the cyclohexyl ring
systems effectively shields the re-face of the vinyl π-sys-
tem (Figure 1, C), strongly favoring the unlike topicity
(combination of si-face of vinyl ether with re-face of
nitroalkene) in the endo-approach of the reagents. The
endo-approach is favored by secondary orbital interac-
tions Nx‚‚‚OR* as already mentioned above.
In conclusion, a [4 + 2] cycloaddition of (E)-2-aryl-1-
cyano-1-nitroalkenes 1 with achiral and enantiopure
vinyl ethers 2 and 3 in pure water are highly endo
selective, and by using (-)-N,N-dicyclohexyl-(1S)-iso-
(23) The X-ray structure plot of (+)-8by (Figure S3, see Supporting
Information) refers to crystal data of not refined structure (R ) 19%);
crystal system: orthorombic; space group: P2(1)2(1)2(1); unit cell
dimension a ) 9.92 Å, b ) 26.18 Å, c ) 7.91 Å, R ) â ) γ ) 90°; V )
2056.9 Å3; Z ) 4; density (calculated) 1.24 mg/m3. The low R value
does not impugn the constitution and stereochemistry of compound.
(24) The si and re faces of vinyl ether are defined with respect to
the C-alkoxy-bearing carbon atom.8b The si and re face of nitroalchene
are defined with respect to the â-carbon.9
(22) When the reaction mixture 60:30:5:5 [endo/ exo ) 90: 10, 30%
de (4R)] of nitronates 6by, (+)-7by, (+)-8by, and 9by (Table 2) is
maintained in DMSO for 30 min at room temperature, the ratio of
components changes to 11:11:54:24 [endo/ exo ) 22:78, 30% de (4R)]
showing that the epimerization rate of 6by to (+)-8by is faster than
that of (+)-7by to 9by. The constancy of the de (4R) value, before and
after the treatment of nitronate, supports the idea that an epimeriza-
tion reaction occurs.

4664 J . Org. Chem., Vol. 66, No. 13, 2001
Fringuelli et al.
(+)-6bx, (+)-7by, and (+)-8by23 were measured with a single-
crystal diffractomer equipped with graphite-mono-chromatized
Mo-KR radiation (λ ) 0.71069 Å); the X-ray data for com-
pounds (-)-6bz were measured on an Xcalibur system, devel-
oped by Kuma diffraction Ltd equipped with a CCD detector
and a 4-circle kappa geometry goniometer. The structures were
solved by direct methods using the SIR97 program package25
and refined by the full-matrix least-squares methods with
SHELX93.26 Column chromatography was carried out on silica
gel (0.04-0.063 mm, 230-240 mesh ASTM) pretreated with
2.5% v/v of Et3N and n-hexane as eluent. To avoid or reduce
the cis-trans epimerization at C-6 of nitronates, the recrys-
tallizations were carried out dissolving the product in the suit-
able solvent at room temperature and cooling at 0°/-20 °C.
(+)-2-Isoca r a n yl Vin yl Eth er (3x). A mixture of (-)-2-
isocaranol27 (1.54 g, 0.01 mol), Hg(OAc)2 (0.80 g, 0.025 mol),
and n-butyl vinyl ether (85 mL) was stirred at 60 °C for 4.5 h.
Hg(OAc)2 (0.80 g, 0.025 mol) was again added and the mixture
heated at 60 °C for 4.5 h. The mixture, cooled to 40 °C, was
quenched with an aqueous saturated solution of K2CO3 and
extracted with tert-butyl methyl ether. The organic phase was
dried (Na2SO4), and after filtration and evaporation of the
solvent under reduced pressure, the residue was purified by
flash column chromatography to give 0.81 g of pure 3x as oil
(yield 45%).
1
[R]20 + 52.0 (c 1.24 CHCl3); H NMR 200 MHz (CDCl3) δ:
D
0.66 (m, 2H); 0.80 (m, 1H) 0.93 (d, 3H, J ) 6.2 Hz); 1.01 (s,
3H); 1.02 (s, 3H); 1.35-1.95 (m, 4H); 3.15 (db, 1H, J ) 10.6
Hz); 4.01 (dd, 1H, J ) 6.8, 1.3 Hz); 4.20 (dd, 1H, J ) 14.4, 1.2
Hz); 6.4 (dd, 1H, J ) 14.4, 6.8 Hz). 13C NMR 50 MHz (CDCl3)
δ: 15.1, 16.5, 18.2, 19.0, 20.1, 27.4, 28.6, 30.1, 34.0, 78.7, 86.9,
150.6. Anal. Calcd for C12H20O: C, 79.94; H, 11.18. Found: C,
79.89; H, 11.21.
(+)-Isop in oca m p h eyl Vin yl Eth er (3y). A mixture of (+)-
isopinocampheol (1.54 g, 0.01 mol), Hg(OAc)2 (0.80 g, 0.025
mol), and n-butyl vinyl ether (85 mL) was stirred at 60 °C for
2.5 h. Hg(OAc)2 (0.80 g, 0.025 mol) was again added and the
mixture heated at 60 °C for 2.5 h. The mixture workup, as
described for 3x, affords after purification by flash column
chromatography pure 3y (0.86 g, 48%) as an oil.
1
[R]20 +71.3 (c 1.08 CHCl3); H NMR 200 MHz (CDCl3) δ:
D
0.93 (s, 3H); 1.08 (d, 1H, J ) 8.5 Hz), 1.12 (d, 3H, J ) 7.4 Hz);
1.23 (s, 3H); 1.73-1.86 (m, 2H); 1.94 (m, 1H), 2.14 (m, 1H),
2.25-2.55 (m, 2H); 4.00 (dd, 1H, J ) 6.7, 1.5 Hz); 4.10 (ddd,
1H, J ) 8.9, 4.5, 4.5 Hz), 4.24 (dd, 1H, J ) 14.2, 1.5 Hz); 6.36
(dd, 1H, J ) 14.2, 6.7 Hz). 13C NMR 50 MHz (CDCl3) δ: 20.9,
23.7, 27.4, 33.1, 35.6, 38.3, 41.3, 44.2, 47.4, 78.7, 88.1, 151.2.
Anal. Calcd for C12H20O: C, 79.94; H, 11.18. Found: C, 79.91;
H, 11.17.
(1S)-(-)-10-(N,N-Dicycloh exylsu lfon a m id e) Isobor n eyl
Vin yl Eth er (3z). A mixture of (-)-N,N-dicyclohexyl-(1S)-
isoborneol-10-sulfonamide (1.59 g, 0.004 mol), Hg(OAc)2 (0.86
g, 0.0027 mol), and n-butyl vinyl ether (35 mL) was stirred at
120 °C for 15 h in a closed reactor. The mixture workup, as
above-described for 3x and 3y, affords after purification by
flash column chromatography pure 3z (1.35 g, 80%) as a white
F igu r e 1. Lowest energy conformer of A: (+)-3x (Φ )
-174.25°); B: (+)-3y (Φ ) -137.12°); C: (-)-3z (Φ ) -178.52°)
with endo-mode, si-face (-)-3z - re-face 1, approach.
solid.
1
mp 115-117 °C (n-hexane); [R]20 -17.11 (c 1.2 CHCl3). H
D
borneol-10-sulfonamide as chiral auxiliary, totally asym-
metric cycloadditions are observed.
This is the highest asymmetric induction value of [4
+ 2] cycloaddition performed in sole water reported in
the literature.
Although the reactions were carried out in heteroge-
neous conditions, they were fast and occurred quantita-
tively under mild conditions. Further studies on the
conversion of optically pure nitronates into enantiomeri-
cally enriched compounds are in progress.
NMR 200 MHz (CDCl3) δ: 0.87 (s, 3H); 0.97 (s, 3H); 0.8-2.1
(m, 27 H), 2.65 (d, 1H, J ) 13.3 Hz); 3.26 (m, 2H); 3.38 (d, 1H,
J ) 13.3 Hz); 3.95 (dd, 1H, J ) 6.6, 1.4 Hz), 4.20 (dd, 1H, J )
14.1, 1.4 Hz); 4.20 (m, 1H); 6.33 (dd, 1H, J ) 14.1, 6.6 Hz). 13
C
NMR 50 MHz δ: 20.1, 20.4, 25.2 (2 C), 26.4 (4 C), 27.2, 29.2,
32.5 (2 C), 33.0 (2 C), 38.7, 44.5, 48.7, 50.2, 53.4, 57.2 (2 C),
83.2, 87.6, 151.1. Anal. Calcd for C24H41NO3S: C, 68.04; H,
9.75; N, 3.31. Found: C, 68.22; H, 9.67; N, 3.35.
[4 + 2] Cycloa d d ition s of Nitr oa lk en es 1 w ith Eth yl
Vin yl Eth er 2. A mixture of nitroalkene 1 (1 mmol) and ethyl
(25) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J . Appl. Crystallogr. 1999, 32, 115.
Exp er im en ta l Section
(26) Sheldrick, G. M., SHELXL 93, Program for the Refinement of
Crystal Structures, University of Go¨ttingen, 1993.
(27) Fringuelli, F.; Piermatti, O.; Pizzo, F.; Scappini, A. M. Gazz.
Chim. Ital. 1995, 125, 195.
Melting points are uncorrected. 1H NMR and 13C NMR
spectra were recorded in CDCl3 or acetone-d6 solutions on 200
and 400 MHz spectrometers. The X-ray data for compounds

[4 + 2] Cycloadditions of Nitroalkenes in Water
J . Org. Chem., Vol. 66, No. 13, 2001 4665
vinyl ether 2 (4 mmol for 1a and 1b; 8 mmol for 1c) in pure
water or in CH2Cl2 (5 mL) was stirred at the temperature and
for the time reported in Table 1. Usual workup gave a mixture
of 4 and 5 (see Table 1). Recrystallization allowed the pure
cycloadducts 4 to be isolated. The yields are reported in Table
1.
cis-6-E t h oxy-4-p h en yl-5,6-d ih yd r o-4H -1,2-oxa zin e-3-
ca r bon itr ile 2-Oxid e (4a ). mp 101-103 °C (diethyl ether);
1H NMR 200 MHz (acetone-d6) δ: 1.23 (t, 3H, J ) 7.1 Hz);
2.18 (ddd, 1H, J ) 14.0, 7.9, 5.1 Hz), 2.80 (ddd, 1H, J ) 14.0,
8.1, 4.1 Hz); 3.77 (dq, 1H, J ) 9.7, 7.1 Hz), 3.97 (dq, 1H, J )
9.7, 7.1 Hz); 4.25 (t, 1H, J ) 8.0 Hz); 5.83 (dd, 1H, J ) 5.1, 4.1
Hz); 7.30-7.50 (m, 5H). 13C NMR 50 MHz (acetone-d6) δ: 15.1,
34.9, 40.3, 66.6, 105.6, 106.2, 113.0, 128.8, 129.1 (2 C), 129.6
(2 C), 139.1. Anal. Calcd for C13H14N2O3: C, 63.40; H, 5.73; N,
11.38. Found: C, 63.42; H, 5.70; N, 11.42.
1H, J ) 13.9, 7.2, 2.1 Hz); 2.62 (m, 1H); 4.29 (ddd, 1H, J )
9.3, 4.9, 4.9 Hz); 4.40 (dd, 1H, J ) 11.8, 7.2 Hz); 5.90 (t, 1H, J
) 2.4 Hz); 7.50 (m, 4H). 13C NMR 50 MHz (CDCl3) δ: 20.3,
23.7, 27.4, 33.2, 33.8, 35.0, 38.2, 38.4, 41.3, 44.1, 47.3, 78.0,
101.4, 103.0, 112.0, 129.1 (2 C), 129.7 (2 C), 134.6, 135.8. Anal.
Calcd for C21H25N2O3Cl: C, 64.86; H, 6.48; N, 7.20. Found: C,
64.89; H, 6.18; N, 7.20.
(4R,6S)-cis-6-[(1S)-10-(N,N-Dicycloh exylsu lfon a m id e)-
isob or n eyl]-4-p h en yl-5,6-d ih yd r o-4H -1,2-oxa zin e-3-ca r -
bon itr ile 2-Oxid e (6a z).28 1H NMR 200 MHz (acetone-d6) δ:
0.86 (s, 3H); 1.00 (s, 3H); 0.8-2.0 (m, 27H); 2.30 (ddd, 1H, J )
12.9, 7.8, 4.8 Hz); 2.71 (d, 1H, J ) 13.4 Hz); 2.92 (ddd, 1H, J
) 12.9, 7.8, 4.8 Hz); 3.23 (m, 2H); 3.26 (d, 1H, J ) 13.4 Hz);
4.05 (dd, 1H, J ) 7.8, 3.7 Hz); 4.29 (t, 1H, J ) 8.1 Hz); 5.85 (t,
1H, J ) 4.7 Hz); 7.30-7.55 (m, 5H). 13C NMR 50 MHz (CDCl3)
δ: 20.1, 20.4, 25.0 (2 C), 26.3 (4 C), 26.9, 30.6, 32.4 (2 C), 33.0
(2 C), 35.4, 39.6, 40.1, 44.3, 48.7, 50.7, 53.9, 57.5 (2 C), 87.9,
104.9, 108.0, 111.9, 127.8 (2 C), 128.5, 129.2 (2 C), 137.3.
cis-4-(4-Ch lor op h en yl)-6-eth oxy-5,6-d ih yd r o-4H-1,2-ox-
a zin e-3-ca r bon itr ile 2-Oxid e (4b). mp 111-112 °C (diethyl
1
ether); H NMR 200 MHz (acetone-d6) δ: 1.23 (t, 3H, J ) 7.1
(4R,6R)-tr a n s-6-[(1S)-10-(N,N-Dicycloh exylsu lfon am ide)-
isob or n eyl]-4-p h en yl-5,6-d ih yd r o-4H -1,2-oxa zin e-3-ca r -
bon itr ile 2-Oxid e (8a z). Yield 85%; mp 170-171 °C (n-
Hz); 2.19 (ddd, 1H, J ) 14.0, 7.1, 4.6 Hz), 2.80 (ddd, 1H, J )
14.0, 8.2, 4.0 Hz); 3.77 (dq, 1H, J ) 9.7, 7.1 Hz), 3.96 (dq, 1H,
J ) 9.7, 7.1 Hz); 4.30 (dd, 1H, J ) 8.2, 7.1 Hz); 5.82 (dd, 1H,
J ) 4.6, 4.0 Hz); 7.40-7.50 (m, 4H).13C NMR 50 MHz (Acetone-
d6) δ: 15.1, 34.5, 39.7, 66.7, 105.1, 106.2, 113.0, 129.7 (2 C),
131.1 (2 C), 134.3, 138.3. Anal. Calcd for C13H13N2O3Cl: C,
55.62; H, 4.67; N, 9.98. Found: C, 55.65; H, 4.64; N, 9.95.
cis-6-Eth oxy-4-(4-m eth oxyp h en yl)-5,6-d ih yd r o-4H-1,2-
oxa zin e-3-ca r bon it r ile 2-Oxid e (4c). mp 126-129 °C (n-
hexane/ethyl acetate); 1H NMR 200 MHz (acetone-d6) δ: 1.24
(t, 3H, J ) 7.1 Hz); 2.16 (ddd, 1H, J ) 14.0, 7.9, 5.1 Hz), 2.78
(ddd, 1H, J ) 14.0, 8.1, 4.2 Hz); 3.79 (dq, 1H, J ) 9.8, 7.1 Hz),
3.81 (s, 3H); 3.98 (dq, 1H, J ) 9.8, 7.1 Hz); 4.19 (t, 1H, J ) 8.0
Hz); 5.82 (dd, 1H, J ) 5.1, 4.2 Hz); 6.96 (m, 2H); 7.37 (m, 2H).
Anal. Calcd for C14H16N2O4: C, 60.86; H, 5.84; N, 10.14.
Found: C, 60.89; H, 5.82; N, 10.11.
hexane/ethyl acetate); [R]20 +18.3 (c 0.75 CHCl3). 1H NMR
D
200 MHz (acetone-d6) δ: 0.88 (s, 3H); 1.05 (s, 3H); 0.9-2.0
(m, 27H); 2.36 (ddd, 1H, J ) 13.8, 11.6, 2.5 Hz); 2.50 (ddd,
1H, J ) 13.8, 7.3, 2.3 Hz); 2.82 (d, 1H, J ) 13.4 Hz); 3.30 (m,
2H); 3.42 (d, 1H, J ) 13.4 Hz); 4.13 (dd, 1H, J ) 6.6, 4.0 Hz);
4.29 (dd, 1H, J ) 11.6, 7.3 Hz); 5.80 (t, 1H, J ) 2.4 Hz); 7.3-
7.5 (m, 5H). 13C NMR 50 MHz (CDCl3) δ: 20.1 (2 C), 25.0 (2
C), 26.4 (2 C), 26.5 (2 C), 26.9, 30.6, 32.6 (2 C), 32.9 (2 C),
32.8, 39.0, 40.1, 44.3, 48.9, 50.7, 54.0, 57.6 (2 C), 86.4, 103.3,
106.0, 112.0, 127.6 (2 C), 128.7, 129.4 (2 C), 137.1. Anal. Calcd
for C33H47N3O5S: C, 66.30; H, 7.92; N, 7.03. Found: C, 66.35;
H, 7.88; N, 7.00.
(4R,6S)-cis-4-(4-Ch lor op h en yl)-6-[(1S)-10-(N,N-d icyclo-
h exylsu lfon am ide)isobor n eyl]-5,6-dih ydr o-4H-1,2-oxazin e-
3-ca r bon itr ile 2-Oxid e (6bz). Yield 78%; mp 174-176 °C
(diethyl ether); [R]20D -30.46 (c 0.86 CHCl3). 1H NMR 400 MHz
(acetone-d6) δ: 0.87 (s, 3H); 1.00 (s, 3H); 0.9-2.1 (m, 27H);
2.33 (ddd, 1H, J ) 14.0, 7.4, 4.3 Hz); 2.73 (d, 1H, J ) 13.5
Hz); 2.95 (ddd, 1H, J ) 14.0, 8.3, 4.8 Hz); 3.24 (m, 2H); 3.26
(d, 1H, J ) 13.5 Hz); 4.05 (dd, 1H, J ) 8.0, 3.5 Hz); 4.35 (t,
1H, J ) 7.8 Hz); 5.84 (t, 1H, J ) 4.6 Hz); 7.51 (m, 4H). 13C
NMR 50 MHz (CDCl3) δ: 20.0, 20.3, 24.9 (2 C) 26.2 (4 C), 26.8,
30.6, 32.3 (2 C), 32.9 (2 C), 34.7, 38.7, 40.0, 44.2, 48.6, 50.6,
53.8, 57.4 (2 C), 87.9, 104.0, 107.7, 111.8, 129.2 (4 C), 134.2,
136.1. Anal. Calcd for C33H46N3O5SCl: C, 62.69; H, 7.33; N,
6.65. Found: C, 62.75; H, 7.38; N, 6.57.
[4 + 2] Cycloa d d ition s of Nitr oa lk en es 1 w ith En a n -
tiop u r e Vin yl Eth er s 3. A mixture of nitroalkene 1 (1 mmol)
and vinyl ether 3 (1.2 mmol) in pure water (5 mL) was stirred
at the temperature and for the time reported in Table 2. Usual
workup gave a mixture of nitronates 6-9 (see Table 2).
Recrystallization allowed the pure cycloadducts to be isolated.
(4R,6S)-cis-4-(4-Ch lor op h en yl)-6-(2-isoca r a n oxy)-5,6-
dih ydr o-4H-1,2-oxazin e-3-car bon itr ile 2-Oxide (6bx). Yield
82%; mp 145-146 °C (n-hexane/ethyl acetate); [R]20 +94.1 (c
D
0.55 CHCl3). 1H NMR 400 MHz (acetone-d6) δ: 0.65-0.75 (m,
2H); 0.87 (m, 1H); 0.96 (d, 3H, J ) 6.4 Hz); 0.99 (s, 3H); 1.01
(s, 3H); 1.35-1.85 (m, 4H); 2.25 (ddd, 1H, J ) 14.1, 6.6, 4.4
Hz); 2.90 (ddd, 1H, J ) 14.1, 8.7, 4.0 Hz); 3.33 (dd, 1H, J )
10.5, 2.7 Hz); 4.35 (dd, 1H, J ) 8.7, 6.6 Hz), 6.01 (t, 1H, J )
4.2 Hz), 7.50 (m, 4H). 13C NMR 50 MHz (CDCl3) δ: 15.4, 17.2,
18.1, 18.3, 21.0, 26.7, 28.7, 30.3, 33.7, 34.4, 39.0, 79.6, 101.7,
104.0, 111.9, 129.3 (2 C), 129.7 (2 C), 134.4, 136.4. Anal. Calcd
for C21H25N2O3Cl: C, 64.86; H, 6.48; N, 7.20. Found: C, 64.91;
H, 6.42; N, 7.21.
(4R,6R)-tr a n s-4-(4-Ch lor op h en yl)-6-[(1S)-10-(N,N-d icy-
cloh exylsu lfon a m id e)isobor n eyl]-5,6-d ih yd r o-4H-1,2-ox-
a zin e-3-ca r bon itr ile 2-Oxid e (8bz). Yield 82%; mp 175-176
°C (n-hexane/ethyl acetate); [R]20 +30.36 (c 0.5 CHCl3). 1H
D
NMR 400 MHz (acetone-d6) δ: 0.88 (s, 3H); 1.04 (s, 3H); 0.9-
2.0 (m, 27H); 2.41 (ddd, 1H, J ) 13.9, 11.9, 2.6 Hz); 2.53 (ddd,
1H, J ) 13.9, 7.1, 2.2 Hz); 2.77 (m, 1H, J ) 13.4 Hz); 3.31 (m,
2H); 3.42 (m, 1H, J ) 13.4 Hz); 4.13 (dd, 1H, J ) 7.5, 3.8 Hz);
4.36 (dd, 1H, J ) 11.9, 7.1 Hz); 5.81 (t, 1H, J ) 2.3 Hz); 7.50
(m, 4H). 13C NMR 50 MHz (CDCl3) δ: 20.2 (2 C), 25.1 (2 C)
26.4 (2 C), 26.5 (2 C), 26.9, 30.6, 32.7 (2 C), 32.9 (2 C), 32.7,
38.6, 40.1, 44.3, 48.9, 50.8, 54.2, 57.7 (2 C), 86.5, 102.7, 105.9,
111.9, 129.0 (2 C), 129.7 (2 C), 134.7, 135.7. Anal. Calcd for
(4S,6R)-cis-4-(4-Ch lor op h en yl)-6-(2-isop in oca m p h oxy)-
5,6-d ih yd r o-4H-1,2-oxa zin e-3-ca r bon itr ile 2-Oxid e (7by).
Yield 80%; mp 126-128 °C (diethyl ether); [R]20 + 109.0 (c
D
1
0.22 CHCl3). H NMR 400 MHz (acetone-d6) δ: 0.92 (s, 3H);
0.94 (d, 1H, J ) 9.9 Hz); 1.16 (d, 3H, J ) 7.4 Hz); 1.21 (s, 3H);
1.75 (m, 1H); 1.82 (m, 1H); 1.93 (m, 1H); 2.10 (m, 1H); 2.22
(ddd, 1H, J ) 14.1, 6.4, 4.5 Hz); 2.37 (m, 1H); 2.57 (m, 1H);
2.83 (ddd, 1H, J ) 14.1, 8.4, 3.9 Hz); 4.26 (ddd, 1H, J ) 9.4,
4.6, 4.6 Hz); 4.34 (dd, 1H, J ) 8.4, 6.4 Hz); 5.93 (t, 1H, J ) 4.2
Hz); 7.48 (m, 4H). 13C NMR 50 MHz (CDCl3) δ: 20.1, 23.5,
27.1, 33.5, 34.7, 36.5, 38.0, 38.6, 41.0, 43.8, 47.0, 78.4, 102.4,
103.8, 112.0, 128.8 (2 C), 129.5 (2 C), 133.8, 136.5. Anal. Calcd
for C21H25N2O3Cl: C, 64.86; H, 6.48; N, 7.20. Found: C, 64.92;
H, 6.45; N, 7.22.
C
33H46N3O5SCl: C, 62.69; H, 7.33; N, 6.65. Found: C, 62.66;
H, 7.31; N, 6.62.
(4R,6S)-cis-6-[(1S)-10-(N,N-Dicycloh exylsu lfon a m id e)-
isobor n eyl]-4-(4-m eth oxyp h en yl)-5,6-d ih yd r o-4H-1,2-ox-
a zin e-3-ca r bon itr ile 2-Oxid e (6cz). Yield 80%; mp 174-176
°C (diethyl ether); [R]20D -34.1 (c 0.7 CHCl3). 1H NMR 200 MHz
(acetone-d6) δ: 0.88 (s, 3H); 1.03 (s, 3H); 0.8-2.0 (m, 27H);
2.29 (ddd, 1H, J ) 13.9, 8.1, 4.6 Hz); 2.72 (d, 1H, J ) 13.5
Hz); 2.92 (ddd, 1H, J ) 13.9, 8.1, 4.9 Hz); 3.25 (m, 2H); 3.28
(d, 1H, J ) 13.5 Hz); 3.79 (s, 3H) 4.04 (dd, 1H, J ) 7.5, 3.5
(4R ,6R )-t r a n s-4-(4-Ch lor op h e n yl)-6-(2-isop in oca m -
p h oxy)-5,6-d ih yd r o-4H-1,2-oxa zin e-3-ca r bon itr ile 2-Ox-
id e (8by). Yield 85%; mp 171-172 °C (n-hexane/ethyl acetate);
1
[R]20 + 244.6 (c 0.56 CHCl3). H NMR 400 MHz (acetone-d6)
D
δ: 0.94 (s, 3H); 1.04 (d, 1H, J ) 9.8 Hz); 1.11 (d, 3H, J ) 7.4
Hz); 1.23 (s, 3H); 1.8-1.9 (m, 2H); 1.96 (m, 1H); 2.11 (m, 1H);
2.33 (ddd, 1H, J ) 13.9, 11.8, 2.7 Hz); 2.40 (m, 1H); 2.49 (ddd,
(28) The epimerization of 6a z to 8a z which occurs during recrys-
tallization prevents 6a z from being obtained in pure form, and the 1H
and 13C NMR data refers to a 6a z:8a z ) 85:15 mixture.

4666 J . Org. Chem., Vol. 66, No. 13, 2001
Fringuelli et al.
Hz); 4.22 (t, 1H, J ) 8.1 Hz); 5.83 (t, 1H, J ) 4.8 Hz); 6.97
(dd, 2H, J ) 6.7, 2.1 Hz); 7.41 (dd, 2H, J ) 6.7, 2.1 Hz). Anal.
Calcd for C34H49N3O6S: C, 65.04; H, 7.87; N, 6.69. Found: C,
65.13; H, 7.85; N, 6.65.
Su p p or tin g In for m a tion Ava ila ble: ORTEP figures of
6bx, 7by, 8by, and 6bz (Figures S1-S4); tables of crystal data,
atomic coordinates, bond lengths and angles, anisotropic
thermal parameters, hydrogen coordinates, and torsion angles
for compounds (+)-6bx, (+)-7by, and (-)-6bz; Table S1 of 1H
NMR data of nitronates 4-9 and 1H and 13C NMR peak
assignment of compounds 3x, 3y, 3z, 4a , 4b, 4c, 6bx, 7by,
8by, 6a z, 8a z, 6bz, 8bz, and 6cz. This material is available
free of charge via Internet at http://pub.acs.org.
Ack n ow led gm en t. The Ministero dell′Universita`
della Ricerca Scientifica (MURST) and the Consiglio
Nazionale delle Ricerche (CNR-Roma) are thanked for
financial support.
J O010182V