organic compounds
with the phenyl ring. The two CÐS bonds are unequal, S1Ð
C15 being shorter than S1ÐC3, because of the different
hybridization. The endocyclic torsion angles show that the
approximate mirror symmetry of the norbornene skeleton is
only slightly distorted. The exo-ethoxycarbonyl substituent at
C2 adopts a fully extended conformation, while the endo one
is twisted, probably due to packing interactions.
The orientation of the phenyl ring in both compounds
seems to be stabilized by intramolecular hydrogen bonds
involving the ortho-H atoms of the aromatic ring, and the
Ê
carbonyl O4 and sul®nyl O5 atoms [C15ÐH15Á Á ÁO4 2.47 A
Ê
Ê
and C11ÐH11Á Á ÁO5 2.52 A in (I), and C20ÐH20Á Á ÁO4 2.38 A
Ê
and C16ÐH16Á Á ÁO5 2.61 A in (II)]. In addition, weak inter-
molecular hydrogen bonds are observed for (I) [C14Ð
Ê
H14Á Á ÁO5(x 1 , y, z) 2.54 A] and (II) [C19ÐH19Á Á ÁN1(x,
1
Ê
y, z 2) 2.62 A].
1
Figure 2
The molecular structure of the Diels±Alder adduct, (II), showing the
atom-labelling scheme. Displacement ellipsoids are shown at the 30%
probability level and H atoms are drawn as small spheres of arbitrary
radii.
The observed high ꢀ-facial selectivity (80:20 endo/exo-
sul®nyl) may be explained by invoking the possibility of these
reactions taking place on organized structures, (A) or (B)
(Fig. 3), resulting from intermolecular hydrogen bonding of
molecules. Superposition of the structure of cyclopentadiene
(Haumann et al., 1996) over the diethyl 2-[cyano(toluene-4-
sul®nyl)methylene]propanedioate showed that the organized
structure (A) favours a closer approximation [2.47 (1) versus
Fig. 2 clearly shows the endo-sul®nyl nature of the major
isomer of (II) obtained from the Diels±Alder reaction. Upon
cycloaddition, besides the change in hybridization of atoms C2
and C3, the major change in the dienophile moiety is observed
in the orientation of the ethoxycarbonyl group syn to the
cyano group, which turns from a nearly coplanar conformation
to an almost perpendicular conformation [74.77 (1)ꢀ]. The
S1ÐO5 bond is trans to C2ÐC3 and approximately coplanar
Ê
2.70 (1) A], by ®tting the space between atoms O3 and O5
Ê
[5.027 (4) A] better than does structure (B).
Experimental
For the preparation of compound (I), cyanomethyl-p-tolylsulfoxide
(5.6 mmol, 1 equivalent) in tetrahydrofuran (10 ml) was deproton-
zated using a solution of Li±HMDS [n-BuLi (6.7 mmol, 1.2 equiva-
lents) and hexamethyldisilazane (HMDS; 6.7 mmol, 1.2 equivalents)
in tetrahydrofuran (40 ml)] at 195 K for 30 min. The resulting anion
was further reacted with diethyl oxomalonate (6.2 mmol, 1.1
equivalents), added slowly and stirred for 2 h at 195 K. The reaction
was quenched with saturated ammonium chloride solution and
extracted with dichloromethane, followed by puri®cation by column
chromatography to give 3,3-diethoxycarbonyl-3-hydroxy-2-p-tolyl-
sul®nylpropionitrile in 78% yield as white crystals (m.p. 394±397 K).
Dehydration of this alcohol (4.7 mmol, 1 equivalent) in dichloro-
methane (65 ml) was performed under an argon atmosphere by
treatment with methylsulfonyl chloride (18.8 mmol, 4 equivalents)
and diisopropylethylamine (18.8 mmol, 4 equivalents) at 195 K with
constant stirring for 2 h. Water (30 ml) was then added, the organic
layer separated and the aqueous layer extracted with dichloro-
methane (2 Â 20 ml). The combined layers were dried with sodium
sulfate and concentrated. The residue was puri®ed by column chro-
matography and recrystallized from hexane±dichloromethane to
afford compound (I) in 66% yield as yellow crystals (m.p. 375±377 K).
For compound (II), cyclopentadiene (1.79 mmol, 6 equivalents) was
added to a solution of (I) (0.298 mmol, 1 equivalent) in dichloro-
methane (2 ml) at room temperature under an argon atmosphere.
The resulting solution was stirred for 2.5 h. Evaporation of the
volatiles under vacuum gave a residue that was analyzed by 1H NMR
(isomer ratio 80:20 endo-sul®nyl/exo-sul®nyl) and puri®ed by ¯ash
chromatography using hexane±ethyl acetate (85:15), yielding
compound (II) as a white solid which decomposed at 384±386 K.
Figure 3
The hypothetical transition state of the reactants during the Diels±Alder
reaction; (A) is the endo arrangement and (B) the exo arrangement.
ꢁ
Â
1314 Ruben A. Toscano et al.
C16H17NO5S and C21H23NO5S
Acta Cryst. (2001). C57, 1313±1315
Toscano, Ruben A.
