Stereoselectivity control in Diels-Alder reactions of 4-thiosubstituted 5-alkoxyfuranones: Synthesis and reactivity of enantiopure 4-sulfinyl and sulfonyl 5-(l-menthyloxy)furan-2(5H)-ones

Article abstract of DOI:10.1016/S0957-4166(00)00471-7

The synthesis of enantiomerically pure (S)-4-phenylsulfinyl- and 4-phenylsulfonyl-(5S)-5-(l-menthyloxy)furan-2(5H)-ones 5a and 6 and (5R)-5-(l-menthyloxy)-4-phenylsulfonylfuran-2(5H)-one 8 and the study of their reactions with cyclopentadiene are described. The sulfur substituents at C-4 modify (sulfone 8, anti/syn = 78/22) and invert (sulfoxide 5a and sulfone 6, anti/syn≈27/73) the trend imposed by C-5 on the π-facial selectivity and the syn-adducts become the favoured ones. The endo or exo approach mode is favoured for anti (endo/exo ratio ranging between 26 and 5) or syn (exo/endo ratio ranging between 4.5 and 2.3) attack, respectively.

Full text of DOI:10.1016/S0957-4166(00)00471-7

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4737–4752
Stereoselectivity control in Diels–Alder reactions of
4-thiosubstituted 5-alkoxyfuranones: synthesis and reactivity
of enantiopure 4-sulfinyl and sulfonyl
5-(l-menthyloxy)furan-2(5H)-ones
Jose´ L. Garc´ıa Ruano,* Fernando Bercial, Alberto Fraile, Ana M. Mart´ın Castro and
M. Rosario Mart´ın*
Departamento de Qu´ımica Orga´nica, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain
Received 23 October 2000; accepted 21 November 2000
Abstract
The synthesis of enantiomerically pure (S)-4-phenylsulfinyl- and 4-phenylsulfonyl-(5S)-5-(l-menthyl-
oxy)furan-2(5H)-ones 5a and 6 and (5R)-5-(l-menthyloxy)-4-phenylsulfonylfuran-2(5H)-one 8 and the
study of their reactions with cyclopentadiene are described. The sulfur substituents at C-4 modify (sulfone
8, anti/syn=78/22) and invert (sulfoxide 5a and sulfone 6, anti/syn:27/73) the trend imposed by C-5 on
the p-facial selectivity and the syn-adducts become the favoured ones. The endo or exo approach mode is
favoured for anti (endo/exo ratio ranging between 26 and 5) or syn (exo/endo ratio ranging between 4.5
and 2.3) attack, respectively. © 2001 Published by Elsevier Science Ltd.
1. Introduction
Homochiral 5-alkoxyfuran-2(5H)-ones have played an important role as building blocks in
the asymmetric synthesis of a number of natural products.1 In particular, (5R)-5-(l-menthyloxy)-
2(5H)-furanone 12 (Fig. 1) is a versatile dienophile in asymmetric Diels–Alder reactions,1c,2–4
since it has been reported that it controls efficiently both the p-facial and the endo/exo selectivity
(only one endo adduct is formed).† The spatial arrangement of the g-alkoxy substituent, which
shields one of the p-faces of the molecule, has usually been presumed as the origin of the high
stereocontrol observed in Diels–Alder reactions, as well as in other cycloadditions.5 As trans-
addition of the diene relative to the menthyloxy substituent of the furanone was exclusively
* Corresponding authors. E-mail: joseluis.garcia.ruano@uam.es; rosario.martin@uam.es
†
Nevertheless, in Ref. 1c there was described one example of low stereoselectivity in the Diels–Alder reaction of the
furanone 1 with an arylisobenzofuran.
0957-4166/00/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0957-4166(00)00471-7

4738
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
observed, the configuration at C-5 was considered as entirely responsible for the p-facial
selectivity. The introduction of a sulfonyl group at C-4 strongly enhances the dienophilic
reactivity of the furanone but it does not affect its stereoselectivity. Thus, reaction of (5S)-5-(d-
menthyloxy)-4-(phenylsulfonyl)-2(5H)-furanone 2 with cyclopentadiene afforded only one endo-
adduct,6 which demonstrated once again the important role played by C-5 configuration in
determining the stereochemical course of the g-alkoxybutenolides cycloadditions.
The sulfinyl group has proved to be very efficient in the control of the p-facial selectivity of
Diels–Alder reactions on many different dienophiles.7 In particular, reactions of cyclopentadiene
with (S)-3-(p-tolylsulfinyl)-2(5H)-furanone 3, in the presence of chelating Lewis acids, afforded
mixtures of endo- and exo-adducts,8 resulting from an almost exclusive approach of the diene to
only one of the faces of the dienophile. In the absence of catalysts the observed p-facial
selectivity is just the opposite. In order to evaluate the relative influence of the configurations at
sulfur and C-5 in the control of the p-facial selectivity of Diels–Alder reactions of 2(5H)-furan-
ones, the study of the reactions of (5R,SS)- and (5S,SS)-5-ethoxy-3-(p-tolylsulfinyl)furan-2(5H)-
ones 4a and 4b with cyclopentadiene9 was undertaken. The two main stereochemical conclusions
of this study were the high predominance of the endo-adducts, which indicates that the endo/exo
control is mainly exerted by the lactone moiety, and the fact that, depending on the reaction
conditions, the control of the p-facial selectivity is mainly exerted by the configuration at C-5
(under thermal conditions) or by the sulfur configuration (under ZnBr2 catalysis). As a
continuation of these studies, we decided to incorporate the sulfinyl moiety at the C-4 position
of 5-alkoxyfuran-2(5H)-ones and to study the reactions of this dienophile with cyclopentadiene.
As the results obtained were unexpectedly contradictory to the previously described ones, it was
necessary to carry out the study of other 5-alkoxyfuranones to clarify their behaviour and the
actual influence of the configuration at C-5 on the stereoselectivity of the cycloadditions. The
synthesis and reactions with cyclopentadiene of 4-thioderivatives of 5-(l-menthyloxy)-2(5H)-
furanones 5a and 6 (Fig. 1), as well as those of (5S)-5-(l-menthyloxy)furan-2(5H)-one 7, and the
sulfone 8 (enantiomer of compound 2) are reported herein.
Figure 1.

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4739
Scheme 1.
2. Results and discussion
The synthesis of enantiomerically pure (5S,SS)-5-(l-menthyloxy)-4-phenylsulfinylfuran-2(H)-
one 5a, as well as that of the sulfone 6 were carried out from the sulfide 9 (Scheme 1). The preparation
of 9 was performed following the method previously described by us,10 consisting in the reaction
of the 4-bromo derivative 10 with PhSH and Et3N (Scheme 1). The oxidation of 9 with m-CPBA‡
(1 equiv., −78°C) afforded a 5:1 mixture of the two sulfoxides, 5a and 5b, in 65% yield (after
chromatography), easily separated from the sulfone 6 and the starting sulfide 9, also present in
the reaction mixture. The major epimer 5a could be isolated diastereoisomerically pure (46% yield
from 9) by precipitation with hexane from the epimeric mixture previously dissolved in xylene.
When the oxidation of 9 was accomplished at room temperature using 2 equiv. of m-CPBA, the
sulfone 6 was obtained as a sole product in a 95% yield.
The configuration at C-5 for sulfoxides 5a and 5b and sulfone 6 was assigned as S on the basis
of the known (5S)-configuration of starting sulfide 9.§ The configuration at sulfur for 5a and 5b
was initially established from the relative Dl values (lsulfoxide−lsulfide) for H-3 and H-5 in both
epimers. The shielding effect of the phenyl group and the deshielding effect of the sulfinyl one,
exerted on the protons adopting a 1,3-parallel arrangement with respect to them, would explain
the chemical shifts observed for compounds 5a and 5b (see Scheme 1). In this sense, we assume
that they exist mainly in the conformations depicted in Scheme 1, which will be favoured by steric
and electrostatic grounds. Unequivocal assignment of the absolute configuration at sulfur and C-5
for 5a was established by X-ray analysis of the adduct exo-11syn (see later), resulting from its
reaction with cyclopentadiene.
The results obtained in the reactions of sulfoxide 5a with cyclopentadiene (Scheme 2) are collected
in Table 1. In the absence of Lewis acids (entries 1 and 2), the reaction required 24 h at room
temperature in CH2Cl2 or 0.5 h in refluxing CHCl3 to be completed, affording a mixture of three
adducts 11, easily separated by flash chromatography. The use of ZnBr2 as a catalyst (entry 3)
slightly increased the reactivity (lower reaction times at rt are needed) but the proportion of the
1
obtained adducts (determined in all cases by H NMR of the crude mixtures) remained almost
unaltered.
‡ We have unsuccessfully tested other oxidising reagents, such as NaIO4, chlorobenzotriazol, and several hydroper-
oxides (TBHP, CHP, H2O2/TiCl3) and m-CPBA under conditions different to those indicated in Section 3, to
achieve the oxidation of sulfide into sulfoxides.
§ Under the conditions used, no epimerisation at C-5 for any of the three thio derivatives was observed. This
contrasts with the results reported by Feringa.6

4740
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
Scheme 2.
Table 1
Reactions of 5a with cyclopentadiene
Entry
Reaction conditionsa
Yield (%)b
Adduct ratioc
Exo-11syn
Endo-11syn
Endo-11anti
1
2
3
A
B
C
88
86
76
53
56
60
21
21
20
26
23
20
a A: CH2Cl2, rt, 24 h; B: CH3Cl, reflux, 0.5 h; C: ZnBr2 (2.3 equiv.), CH2Cl2, rt, 24 h.
b Combined yield of pure products after chromatography.
1
c (%) Determined by H NMR.
1
The endo or exo geometry for bicyclic adducts 11 was deduced from their H NMR spectra.
Compounds endo-11syn and endo-11anti exhibit coupling constants of 5.7 and 5.8 Hz between
H-7 and H-7a, whereas the absence of J7,7a and the existence of a long range coupling constant
4J7a,8x (which was also observed by a COSY experiment) are characteristic features of the major
adduct exo-11syn. The differentiation between endo-11syn and endo-11anti was based on the relative
l value of their acetallic proton (it is higher for endo-11syn due to the deshielding effect of the
sulfinyl group in a cis arrangement) and mainly on the NOE enhancement observed for H-3 with
H-5 (16%) in endo-11anti, which is absent in endo-11syn. The unequivocal assignment of exo-11syn
was established by X-ray analysis (Fig. 2), which, in its turn, allowed us to confirm the absolute
configuration at sulfur, as well as that at C-5 of the starting product 5a (see above).
The results observed in Table 1 clearly contrast with those reported by Feringa for compound
12 (only the endo-adduct from the opposite anti face to that displayed by the menthyloxy group
was formed). Initially, this suggested to us that, in the absence of any catalyst, the role of the
sulfinyl group in the control of both endo/exo and p-facial selectivities was much more important
than that of C-5 of the furanone. Thus, the major adduct from 5a was exo-11syn, resulting from
the exo approach of diene to the same syn face bearing the menthyloxy moiety. Moreover, the
p-facial selectivity, measured as the syn/anti adducts ratio (]3:1), indicated that the approach
of the diene from the same face of the alkoxy group was the favoured one, and the exo/endo ratio
(>1) revealed the exo addition mode as the slightly preferred one. The comparison between these
results and those obtained for 4a (Fig. 1) in the absence of catalysts, which evidenced the
predominant role of the configuration at C-5 of the furanone ring (see above), suggested to us
that the position of the sulfinyl group on the furanone skeleton was of utmost importance.

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4741
Figure 2.
On the other hand, the relative size of the sulfonyl and sulfinyl groups suggested no important
differences in their relative endo orientating character, thus determining that the endo/exo
adducts ratio should be similar for sulfonyl- and sulfinyl-furanones. Nevertheless, the compari-
son between the results obtained in reactions of cyclopentadiene with 5a (exo-11syn was
obtained as the major adduct) and 2 (only the endo–anti adduct was detected)6 indicated that
this was not the case. All these considerations prompted us to study the reaction of the sulfone
6 with cyclopentadiene. It required 1 hour at room temperature to afford a mixture of
stereoisomers exo-12syn, exo-12anti, endo-12syn and endo-12anti, in a 50:4:22:24 relative ratio
(Scheme 3). In our hands their separation could not be achieved successfully.
Characterisation of the compounds was performed by 1H NMR of the crude reaction mixture.
The signals corresponding to compounds exo-12syn, endo-12anti and endo-12syn were identified
by comparison with those of actual samples of such compounds, independently obtained by
m-CPBA oxidation of the sulfoxides¶ exo-11syn, endo-11anti and endo-11syn (the configuration
of which had been unequivocally established (see above)), respectively. The remaining signals in
the spectra of the crude reaction mixture were assigned to the minor diastereoisomer exo-12anti.
The coupling constant (J7,7a and J7,8) and the chemical shift of the acetallic proton for the
sulfonyl adducts 12 (see Scheme 3), as well as those for the sulfinyl adducts 11, allowed us to
assign the endo–exo and syn–anti stereochemistry and validate the criteria used for the
assignment of the stereochemistry of this type of adduct.
¶ The formation of sulfonyl epoxides (6 and 8% yield) was observed in the oxidation of the sulfoxides exo-11syn
and endo-11syn.

4742
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
Scheme 3.
As expected, the proportion of adducts resulting from sulfone 6 (Scheme 3) was quite similar
to that obtained from sulfoxide 5a. The almost identical endo/exo ratio can be rationalised by
assuming a similar endo orientating character for the sulfinyl and sulfonyl groups. On the
contrary, the fact that the change of sulfur function resulted in no significant variation in the
observed p-facial selectivity was unexpected, because it suggests a hardly relevant role of the
sulfur configuration on the stereochemical course of these cycloadditions. Additionally, the
exclusive formation of the endo–anti adduct reported from sulfone 26 strongly contrasted with
the behaviour of compound 6, which yielded a mixture, the exo-12syn adduct being the major
one. Taking into account that sulfones 2 and 6 only differ in the stereochemistry of their
menthyloxy residues (d- and l-, respectively), the results reported herein suggested that this
difference, and not the configuration at C-5 (identical in both substrates), must be responsible
for their different stereochemical course. This being the case, the predominant role so far
assigned to the configuration at C-5 in cycloadditions of 5-menthyloxyfuranones ought to be
reconsidered.
In order to investigate this point we decided to study the reaction of compound 7 (Fig. 1) with
cyclopentadiene. Compound 7, which differs from 1 only in the stereochemistry at the C-5, was
studied in order to verify the influence of the relative configuration of the involved stereogenic
centres on the p-facial selectivity. The synthesis of 7 was accomplished by DIBALH reduction
of 5a (Scheme 4), which yielded a 1:2 mixture of sulfide 9 and the desired butenolide 7 (54%
isolated yield). The formation of the latter compound can be explained as the result of a
reduction of the double bond (by 1,4-hydride addition from the associated species A, see Scheme
4) followed by desulfinylation of the resulting sulfoxide.
Scheme 4. (a) DIBALH (1.5 equiv.)/THF, −78°C, 30 min, R=menthyl(l).

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4743
The reaction of 7 with cyclopentadiene (4 equiv.) at room temperature required 12 days to be
completed and yielded a 92:8 mixture of two adducts in 82% combined yield (Scheme 5), which
could be separated and characterised as endo-13anti (77% isolated yield) and exo-13anti (5%
1
isolated yield) from their H NMR spectra (J3,3a=1.7 and 1.8 Hz) and COSY experiment
(endo-13anti shows a correlation between H-3a and H-4 but not between H-8 and H-3a or H-7a;
however, exo-13anti exhibits a correlation signal between H-8 and H-3a and H-7a but not
between H-3a and H-4).
Scheme 5.
As we can see, both compounds are the result of the endo and exo approaches of the diene
from the less hindered face of the dienophile (anti with respect to the alkoxy group), thus
suggesting that configuration at C-5 is totally efficient in the control of the p-facial selectivity.
This conclusion agrees with that previously established by Feringa2,3 from the results obtained
from compound 1. On the other hand, the endo/exo selectivity is not complete for 7 (84% de),
whereas 1 evolved with exclusive (>96%) formation of the endo adduct, which suggests that the
relative configuration of the stereogenic centres could have some influence on the control of this
kind of stereoselectivity. In order to verify this possibility, the study of the reaction of
cyclopentadiene with racemic methoxyfuran-2(5H)-one, with no stereogenic centre additional to
C-5, was undertaken. In our hands the results obtained from the above reaction were quite
similar to those from 7 (only anti adducts as a 93:7 endo/exo mixture were formed), which
revealed that the endo/exo selectivity for 5-alkoxyfuran-2(5H)-one was high but not complete.
The comparison of the results obtained from butenolides 7 (Scheme 5) and 5a (Table 1)
clearly shows the important role of the sulfinyl group in the control of the stereoselectivity of
these reactions. The four possible transition states for evolution of 5a in its presumably most
stable conformation are shown in Fig. 3.
Figure 3.

4744
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
As we can see, the interaction CH2/Ph must destabilise the endo TSs, thus favouring the exo
ones. Additionally, the H/O interactions present in each endo TS must be similar, thus
determining a low facial selectivity for the endo approach. By contrast, the two exo TSs have
different stabilities because of the O/H interaction only exhibited by the exo–anti TS (see Fig. 3),
thus justifying the complete facial selectivity observed for the exo approach (exo-11anti was not
detected). The comparison of the results observed for compounds 5a and 7 suggests that the
CH2/Ph interaction of the endo adducts (Fig. 3) must be responsible for the significant increase
in the proportion in the exo ones, observed for sulfoxide 5a. Moreover, the influence of the
sulfinyl group at C-4 in the p-facial selectivity control is clearly higher than that of the
configuration at C-5 (syn adducts are predominant) and also higher than that exerted by the
sulfinyl group when it is at C-3 in the furanone ring (compounds 4a and 4b, see above). This
behaviour can be explained by assuming that the conformational mobility around the CꢀS bond
is completely restricted for compound 5a (only the rotamer depicted in Fig. 3 must be present
due to the strong electrostatic and steric interactions of the other two possible rotamers).** In
this sense, the influence of the SOPh group in 5a was similar to that observed for SOTol in 4a
in reactions conducted under ZnBr2 catalysis, conditions favouring the chelation of the sulfinyl
and lactone oxygens, thus restricting the conformational mobility around the CꢀS bond.
At this point, the most striking results are the behaviour of sulfones. We decided to study the
behaviour of the sulfone 8. The synthesis of this compound was performed from the correspond-
ing sulfide, as shown in Scheme 6.
Scheme 6.
The reaction of 8 with cyclopentadiene took place at rt in 5 h, both in CH2Cl2 and C6H6, and
yielded a mixture of four adducts in a 13:18:65:4 ratio (Scheme 7). Chromatographic separation
allowed the isolation of the three major ones, the configurational assignment of which was based
on their spectroscopic parameters. The stereochemistry of the minor endo-14syn was assigned by
exclusion.
1
The stereochemistry of adducts 14 was deduced from their H NMR spectra following the
same criteria as in adducts 11 and 12. Thus, compounds endo-14anti and endo-14syn exhibited
coupling constants of 4.4 and 4.7 Hz, respectively, between H-7 and H-7a, whereas the low
4
values of J7,7a and the existence of a long range coupling constant J7,8 were characteristic of the
adducts exo-14anti and exo-14syn. For adducts 14, with a configuration at C-3 opposite to that
of 12, the acetallic proton of the syn-adducts is deshielded with respect to the corresponding
anti-adducts, maybe due to the deshielding effect of the sulfone group adopting a cis arrange-
ment in the former.
** This explanation suggests that the relative influence of the sulfinyl group and the C-5 configuration could be
completely different for compound 5b, which must exhibit smaller conformational restrictions.

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4745
Scheme 7.
The results obtained from sulfone 8 (Scheme 7) revealed that the endo addition mode was
favoured with respect to the exo one (endo/exo ratio=69:31) and the p-facial selectivity for the
endo approach was quite high (88% de), but it substantially decreased for the exo approach
mode (16% de). These results indicated that those previously reported by Feringa for compound
2 (enantiomer of 8) should be reconsidered. The endo–anti-adduct is the major one but neither
the endo/exo selectivity nor the p-facial selectivity were complete starting from these sulfones.
The comparison between sulfones 6 and 8, both epimers at C-5, is also interesting in order to
establish the origin of the stereoselectivity of the 5-alkoxy furanones. In these compounds the
endo/exo ratio depends on the relative configuration of both epimers. Thus, compound 8
exhibited a predominance of the endo-adducts (endo/exo=69:31), whereas it disappeared for
compound 6 (endo/exo=46:54). A similar but more striking fact was related with the p-facial
selectivity. Compound 8 exhibited the expected behaviour, the anti-adducts being the major ones
(anti/syn=78:22); the change in the relative configuration of C-5 or menthyl group determined
the inversion of such a selectivity, as observed for compound 6 (anti/syn=28:72). These results
must be a consequence of the different spatial arrangement of the menthyl moiety in both
epimers, which will be able to modify the conformational preferences around the CꢀS bond
(Scheme 8).
Scheme 8.

4746
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
The Ph/Omenthyl interaction will be more destabilising for compound 6, thus shifting the
conformational equilibrium towards 6A. The lower magnitude of such an interaction in compound
8 (the menthyl group is far away from the sulfonyl group) would explain the predominance of
8A with no electrostatic (O/Omenthyl)1,3p interaction. As the Ph group is effectively bulkier than
the O-menthyl one, compound 6 must exhibit a syn selectivity, whereas 8 will show a net
predominance of anti-adducts.
In conclusion, we can state that the configuration at C-5 is obviously important in the control
of the p-facial selectivity of the cycloadditions to 5-menthyloxyfuran-2(5H)-ones (anti-adducts were
clearly favoured), but its influence in the endo/exo selectivity is modulated by the relative
configuration of C-5 and the menthyl moiety. Substrates bearing a substituent at C-4 can modify
(compound 8) and even invert (compounds 5a and 6) the trend imposed by C-5 on the p-facial
selectivity, the syn-adducts becoming the favoured ones.
3. Experimental
Melting points are uncorrected. Microanalyses were performed with a Perkin–Elmer 2400 CHN
analyser. IR spectra were recorded on Perkin–Elmer model 681 and FT 1600 grating spectropho-
1
tometers, w values are in cm−1. H and 13C NMR spectra were determined with Bruker AC-200
and AC-300 spectrometers, in CDCl3 solution. Chemical shifts were reported in ppm (l) downfield
from Me4Si. Silica gel Merck 60 (230–400 mesh) was used for flash column chromatography. Optical
rotation was measured with a Perkin–Elmer model 241 polarimeter at room temperature in the
solvent and concentration indicated in each case [g/100 mL]. X-Ray structure was refined by
full-matrix least-squares (Sheldrich, G. M., SHELXL96, Program for Crystal Structure Refinement,
University of Go¨ttingen, Go¨ttingen, 1996) on F2. All non-hydrogen atoms were refined
anisotropically.
3.1. Oxidation of (5S)-5-(l-menthyloxy)-4-phenylthiofuran-2(5H)-one 9
Method A: To a stirred solution of (5S)-5-(l-menthyloxy)-4-(phenylthio)furan-2(5H)-one (9, 3
g, 8.65 mmol) in dry dichloromethane (150 mL) was added dropwise (1 h) a solution of m-CPBA
(1.7 g, 90% purity, 8.7 mmol) in dry dichloromethane (250 mL) at −78°C. The mixture was warmed
to −10°C and allowed to stand for 3 h, and then washed with saturated aqueous sodium bicarbonate
topH7. Theaqueouslayerwasextractedseveraltimeswithdichloromethane. Thecombinedorganic
layers were dried and the solvent evaporated to dryness. The signals corresponding to three new
products 5a (58%), 5b (11%) and 6 (8%) and those of the starting material (23%) were observed
by 1H NMR of the crude reaction. The residue was purified by flash chromatography
(hexane/dichloromethane, 1:1) to afford, in decreasing order of Rf, compounds 5a+5b, 6 and 9
in 65, 8 and 19% yields, respectively. The pure sulfoxide 5a was isolated in 46% yield from a solution
of a 5a+5b mixture in xylene by precipitation with hexane. An analytical sample of 5b was obtained
by cooling a solution of epimers (5b/5a>1) in xylene at −33°C.
Method B: To a stirred solution of (5S)- or (5R)-5-(l-menthyloxy)-4-(phenylthio)furan-2(5H)-
one (150 mg, 0.43 mmol) in dry dichloromethane (6 mL) was added a solution of m-CPBA (275
mg, 90% purity, 1.4 mmol) in dry dichloromethane (20 mL) at room temperature. The mixture
was allowed to stand for 3 h, and then washed with saturated aqueous sodium bicarbonate to
pH 7. The aqueous layer was extracted several times with dichloromethane. The combined organic
layers were dried and the solvent evaporated to dryness to afford the sulfone 6 or 8 in quantitative
yield.

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4747
3.1.1. (5S,SS)-5-(l-Menthyloxy)-4-(phenylsulfinyl)furan-2(5H)-one 5a
46% Yield. Crystallised from ethyl acetate–hexane (white solid), mp 147–149°C. [h]2D0=+176
(c=1, CHCl3). Anal. calcd for C20H26O4S: C, 66.27; H, 7.23; S, 8.85. Found: C, 66.28; H, 7.01;
1
S, 9.02. IR (Nujol): 1790, 1760, 1615, 1050. H NMR: 7.75–7.54 (m, 5H), 6.22 (d, 1H, J 1.1),
5.96 (d, 1H, J 1.1), 3.60 (dt, 1H, J 10.7 and 4.3), 2.30–0.78 (18H). 13C NMR: 169.7, 166.8, 140.4,
132.8, 129.7, 126.0, 123.5, 102.2, 83.8, 47.9, 42.0, 33.8, 31.5, 25.1, 22.6, 22.0, 21.0, 15.7.
3.1.2. (5S,SS)-5-(l-Menthyloxy)-4-(phenylsulfinyl)furan-2(5H)-one 5b
6% Yield. Crystallised from ethyl acetate–hexane (white solid), mp 165–167°C. [h]2D0=−142
(c=1, CHCl3). Anal. calcd for C20H26O4S: C, 66.27; H, 7.23; S, 8.85. Found: C, 66.12; H, 7.12;
1
S, 8.66. IR (KBr): 1799, 1764, 1616, 1050. H NMR: 7.75–7.50 (m, 5H), 6.81 (d, 1H, J 1.1), 5.38
(d, 1H, J 1.1), 3.36 (dt, 1H, J 10.2 and 4.7), 2.20–0.65 (18H). 13C NMR: 172.1, 166.6, 140.5,
133.1, 130.2, 126.0, 123.5, 101.4, 83.2, 47.8, 42.1, 33.7, 31.5, 25.0, 22.6, 22.0, 21.0, 16.0.
3.1.3. (5S)-5-(l-Menthyloxy)-4-(phenylsulfonyl)furan-2(5H)-one 6
95% Yield. Crystallised from ethyl acetate–hexane (white solid), mp 79–80°C. [h]2D0=+39.4
(c=1, CHCl3). Anal. calcd for C20H26O5S: C, 63.47; H, 6.92; S, 8.47. Found: C, 63.09; H, 6.60;
1
S, 8.17. IR (CHCl3): 1795, 1765, 1585, 1330, 1165, 1130. H NMR: 7.92 (m, 2H), 7.72 (m, 1H),
7.64 (m, 2H), 6.61 (d, 1H, J 0.8), 6.19 (d, 1H, J 0.8), 3.57 (dt, 1H, J 10.5 and 4.4), 2.30–0.75
(18H). 13C NMR: 166.2, 161.8, 138.0, 134.9, 129.4, 128.4, 102.0, 83.4, 48.1, 42.0, 33.8, 31.6, 24.5,
22.6, 22.1, 21.1, 15.7.
3.1.4. (5R)-5-(l-Menthyloxy)-4-(phenylsulfonyl)furan-2(5H)-one 8
1
95% Yield. [h]2D0=−124.2 (c=1.05, CH2Cl2). H NMR: 7.93 (m, 2H), 7.71 (m, 1H), 7.58 (m,
2H), 6.76 (d, 1H, J 0.9), 6.28 (d, 1H, J 0.9), 3.55 (dt, 1H, J 10.6 and 4.1), 2.25–0.57 (18H). 13C
NMR: 166.3, 162.2, 138.0, 134.8, 129.2, 128.9, 97.8, 79.4, 47.7, 39.1, 34.1, 31.3, 25.1, 23.1, 22.2,
20.5, 15.6.
3.2. Diels–Alder reactions of sulfoxide 5a with cyclopentadiene
Method A: To a solution of furanone 5a (350 mg, 0.96 mmol) in dichloromethane (6 mL),
cyclopentadiene (383 mg, 5.79 mmol) was added. The mixture was kept at room temperature for
1
24 h. Then the solvent was removed in vacuo and the resulting residue analysed by H NMR
and purified by chromatography on silica gel. The first flash chromatography (ether/hexane, 2:3)
afforded pure endo-11syn (88 mg, 0.21 mmol, 21% yield) and a mixture of exo-11syn and
endo-11anti. The latter compounds were isolated diastereoisomerically pure by a second column
chromatography (ether/hexane/dichlorometane, 1:9:3): exo-11syn (41%), endo-12anti (26%). 88%
combined yield.
Method B: To a stirred solution of ZnBr2 (140 mg, 0.62 mmol) in 0.5 mL of THF was added
at room temperature to a solution of 5a (100 mg, 0.27 mmol) in dichloromethane (2 mL). The
mixture was stirred for 1 h and then cyclopentadiene (91 mg, 1.38 mmol) was added and stirring
was continued for 5 h. Water was added to the reaction mixture and the aqueous layer was
extracted several times with dichloromethane. The combined organic layers were dried over dry
1
magnesium sulfate and the solvent removed in vacuo. The resulting residue was analysed by H
NMR (Table 1) and purified by chromatography on silica gel following the above procedure.

4748
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
3.2.1. Adduct-exo-11syn
42% (Method B) and 41% (method A) yield. Crystallised from ethyl acetate–hexane (white
solid), mp 154–156°C. [h]2D0=+213 (c=1, CHCl3). Anal. calcd for C25H32O4S: C, 70.06; H, 7.53;
1
S, 7.48. Found: C, 70.03; H, 7.42; S, 7.50. IR (Nujol): 1765, 1650, 1080, 1050. H NMR: 7.54
(m, 5H), 6.70 (dd, 1H, J 5.6 and 2.9), 6.54 (dd, 1H, J 5.6 and 3.1), 5.80 (s, 1H), 3.69 (m, 1H),
3.55 (dt, 1H, J 10.6 and 4.4), 3.36 (m, 1H), 2.42 (m, 1H), 2.40–2.00 (m, 2H), 1.95 (m, 1H), 1.80
(m, 1H), 1.75–0.85 (16H). 13C NMR: 172.6, 139.9, 139.4, 138.0, 131.9, 129.4, 125.0, 102.0, 82.6,
78.7, 52.8, 48.0, 47.8, 47.5, 46.9, 42.1, 34.0, 31.4, 25.2, 22.4, 22.0, 21.1, 15.7.
3.2.2. Adduct-endo-11syn
21% (Method A) and 17% (method B) yield. Crystallised from ethyl acetate–hexane (white
solid), mp 169–171°C. [h]2D0=+97.4 (c=0.50, CHCl3). Anal. calcd for C25H32O4S: C, 70.06; H,
1
7.53; S, 7.48. Found: C, 69.77; H, 7.45; S, 7.62. IR (Nujol): 1775, 1760, 1085, 1055. H NMR:
7.70–7.53 (m, 5H), 6.43 (dd, 1H, J 5.5 and 3.1), 6.17 (dd, 1H, J 5.5 and 3.1), 5.66 (s, 1H), 3.60
(m, 1H), 3.50 (dt, 1H, J 10.6 and 4.3), 3.31 (m, 1H), 3.07 (d, 1H, J 4.8), 2.40–0.84 (m, 20H). 13C
NMR: 172.6, 139.4, 138.4, 134.1, 132.3, 129.6, 125.4, 102.1, 82.3, 77.5, 52.5, 50.2, 48.7, 48.1,
45.7, 42.0, 34.1, 31.4, 25.3, 22.6, 22.1, 21.2, 15.8.
3.2.3. Adduct-endo-11anti
26% (Method A) and 17% (method B) yield. Crystallised from ethyl acetate–hexane (white
solid), mp 175–176°C. [h]2D0=−12 (c=0.45, CHCl3). Anal. calcd for C25H32O4S: C, 70.06; H,
1
7.53; S, 7.48. Found: C, 69.67; H, 7.34; S, 7.45. IR (CHCl3): 1760, 1070, 1040. H NMR: 7.82
(m, 2H), 7.50 (m, 3H), 6.42 (m, 2H), 5.27 (s, 1H), 3.59 (m, 1H), 3.50 (dt, 1H, J 10.7 and 4.3),
3.11 (m, 1H), 2.82 (d, 1H, J 4.7), 2.50–2.10 (m, 3H), 1.75–0.82 (m, 17H).13C NMR: 174.3, 143.0,
140.8, 137.2, 131.1, 129.3, 125.2, 107.4, 85.2, 77.5, 53.6, 51.5, 50.2, 48.4, 44.0, 42.5, 33.9, 31.7,
25.4, 22.6, 22.0, 21.1, 15.7.
3.3. Diels–Alder reaction of sulfone 6 with cyclopentadiene
To a solution of furanone 6 (250 mg, 0.66 mmol) in dichloromethane (4 mL), cyclopentadiene
(174 mg, 2.64 mmol) was added. The mixture was kept at room temperature for 1 h. The solvent
1
was removed in vacuo. H NMR analysis of the crude reaction showed the presence of adducts
exo-12syn, exo-12anti, endo-12syn and endo-12anti in a 50:4:22:24 ratio. By flash chromatogra-
phy (ethyl acetate/hexane, 5:1) a mixture of adducts was obtained in 94% yield. All attempts to
separate the adducts by column chromatography on silica gel or crystallisation were
unsuccessful.
3.3.1. Oxidation of sulfinyl adducts
To a stirred solution of the adducts: exo-11syn, endo-11syn, endo-11anti (0.31 mmol) in dry
dichloromethane (3 mL) was added dropwise a solution of m-CPBA (62 mg, of 90% purity) in
dry dichloromethane (3 mL) at −78°C. The mixture was warmed to −33°C and allowed to stand
for the time indicated in Table 2, and then washed with saturated aqueous sodium bicarbonate
to pH 7. The aqueous layer was extracted several times with dichloromethane. The combined
organic layers were dried and the solvent evaporated to dryness. The compounds obtained from
exo-11syn and endo-11syn were isolated by column chromatography (ether/hexane/
dichlorometane, 1:4:1).

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4749
Table 2
Oxidation of the sulfinyl adducts
Adduct
Time (h)
Products (%)a
Yield (%)b
Exo-11syn
Endo-11syn
Endo-11anti
72
96
24
Exo-12syn (94): epoxide (6)
Endo-12syn (92): epoxide (8)
Endo-12anti (100)
12 (78): epoxide (6)
12 (76): epoxide (8)
12 (92)
1
a Determined by H NMR from the crude reaction mixture.
b Of pure products.
3.3.2. Adduct-exo-12syn
78% Yield, from oxidation of sulfinyl adduct exo-11syn (ether/hexane/dichloromethane,
1:4:1). White solid, mp 146–147°C. [h]2D0=+106 (c=1, CHCl3). Anal. calcd for C25H32O5S: C,
67.54; H, 7.25; S, 7.21. Found: C, 67.66; H, 7.16; S, 7.16. IR (Nujol): 1780, 1325, 1315, 1150,
1
1140. H NMR: 7.86 (m, 2H), 7.75–7.55 (m, 3H), 6.36 (m, 2H), 6.07 (s, 1H), 3.39 (m, 1H),
3.30–3.23 (m, 2H), 3.18 (d, 1H, J 1.5), 2.20–0.45 (m, 20H). 13C NMR: 172.8, 137.7, 137.0, 136.6,
134.5, 129.6, 129.1, 103.3, 82.5, 81.5, 52.5, 48.8, 48.2, 47.9, 47.6, 42.0, 33.8, 31.4, 25.0, 22.6, 22.0,
21.0, 15.7.
3.3.3. Adduct-endo-12syn
76% Yield, from oxidation of sulfinyl adduct endo-11syn (ether/hexane/dichloromethane,
1:4:1). White solid, mp 136–138°C. [h]2D0=+3.8 (c=0.50, CHCl3). Anal. calcd for C25H32O5S: C,
1
67.54; H, 7.25; S, 7.21. Found: C, 67.63; H, 7.28; S, 7.03. IR (KBr): 1772, 1320, 1175, 1148. H
NMR: 7.95 (m, 2H), 7.75–7.60 (m, 3H), 6.36 (dd, 1H, J 5.4 and 2.9), 6.23 (dd, 1H, J 5.4 and
3.0), 5.97 (s, 1H), 3.78 (d, 1H, J 4.7), 3.37 (m, 2H), 3.23 (dt, 1H, J 10.6 and 4.3), 2.40 (m, 1H),
2.10–0.55 (m, 19H). 13C NMR: 172.5, 139.8, 136.8, 135.3, 134.7, 129.7, 129.6, 103.1, 82.8, 81.8,
52.6, 50.5, 49.5, 48.1, 45.6, 41.7, 33.9, 31.4, 25.1, 22.7, 22.1, 21.2, 15.9.
3.3.4. Adduct-endo-12anti
92% Yield, from oxidation of endo-11anti. Crystallised from ethyl acetate–hexane (white
solid), mp 147–148°C. [h]2D0=+58.4 (c=0.50, CHCl3). Anal. calcd for: C25H32O5S: C, 67.54; H,
1
7.25; S, 7.21. Found: C, 67.70; H, 7.19; S, 7.12. IR (Nujol): 1780, 1320, 1305, 1190, 1150. H
NMR: 7.93 (m, 2H), 7.70–7.45 (m, 3H), 6.51 (dd, 1H, J 5.6 and 3.0), 6.35 (dd, 1H, J 5.6 and
3.3), 5.09 (s, 1H), 4.06 (d, 1H, J 4.4), 3.81 (m, 1H), 3.47 (m, 1H), 3.20 (dt, 1H, J 10.8 and 4.4),
2.87 (m, 1H), 1.90–0.60 (m, 19H). 13C NMR: 173.1, 141.4, 140.4, 137.7, 133.4, 129.9, 128.4,
106.0, 82.3, 81.3, 54.0, 52.0, 50.2, 48.3, 46.3, 42.0, 33.9, 31.6, 24.6, 22.5, 21.9, 21.2, 15.7.
3.3.5. Epoxide of adduct-exo-12syn
Isolated by chromatography (ether/hexane/dichlorometane, 1:4:1). Yield 6%. Crystallised
from ethyl acetate–hexane (white solid), mp 198–199°C. [h]2D0=+58.6 (c=1, CHCl3). Anal. calcd
C25H32O6S: C, 65.19; H, 7.00; S, 6.96. Found: C, 65.03; H, 6.85; S, 6.48. IR (KBr): 1782, 1374,
1
1324, 1191, 1143. H NMR: 7.91 (m, 2H), 7.75–7.62 (m, 3H), 5.79 (s, 1H), 3.93 (d, 1H, J 3.5),
3.53 (d, 1H, J 3.5), 3.36 (d, 1H, J 2), 3.11 (dt, 1H, J 10.6 and J 4.4), 3.04 (m, 2H), 2.06–0.36
(m, 20H). 13C NMR: 172.8, 136.4, 134.9, 129.9, 128.9, 103.8, 82.9, 80.7, 50.5, 49.8, 48.7, 47.8,
43.7, 42.3, 41.9, 33.7, 31.4, 25.1, 24.9, 22.6, 22.0, 21.0, 15.6.

4750
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
3.3.6. Epoxide of adduct-endo-12syn
Isolated by chromatography (ether/hexane/dichlorometane, 1:4:1). Yield 8%. Crystallised
from ethyl acetate–hexane (white solid), mp 117–119°C. [h]2D0=+24.7 (c=0.70, CHCl3). Anal.
calcd for C25H32O6S: C, 65.19; H, 7.00; S, 6.96. Found: C, 65.04; H, 6.70; S, 6.53. IR (KBr):
1
1776, 1368, 1312, 1224, 1152. H NMR: 7.91 (m, 2H), 7.80–7.62 (m, 3H), 6.12 (s, 1H), 3.92 (d,
1H, J 3.6), 3.65 (d, 1H, J 5.4), 3.36 (d, 1H, J 3.6), 3.31 (dt, 1H, J 10.6 and 4.3), 3.10 (m, 1H),
3.01 (m, 1H), 2.12–0.55 (m, 20H). 13C NMR: 170.8, 136.2, 135.0, 129.9, 129.8, 103.4, 83.5, 81.1,
50.9, 48.7, 48.4, 48.2, 44.8, 41.8, 40.1, 33.9, 31.5, 26.7, 25.3, 22.8, 22.1, 21.3, 16.0.
3.4. (5S)-5-(-l-Menthyloxy)furan-2(5H)-one 7: desulfinylation of (5S)-5-(-l-menthyloxy)-
4-(phenylsulfinyl)furan-2(5H)-one 5a
To a solution of 5a (300 mg, 0.83 mmol) in THF (15 mL) at −78°C, under an argon
atmosphere, DIBALH (1.25 mL of 1.0 M in hexane, 1.25 mmol) was added dropwise and the
mixture was further stirred for 30 min at the same temperature. Then ethanol (2 mL), ethyl
acetate (10 mL) and a sodium/potassium tartrate solution (5 mL) were added and the mixture
was stirred for 15 min. The aqueous layer was extracted with ethyl acetate and the combined
organic extracts were dried (MgSO4), filtered and concentrated under reduced pressure. The
1
crude mixture was analysed by H NMR (7/9=67:33) and the products were separated by
column chromatography (hexane/ethyl acetate, 10:1).
3.4.1. (5S)-5-(-l-Menthyloxy)furan-2(5H)-one 7
1
54% Yield. H NMR: 7.19 (dd, 1H, J 5.7 and 1.3), 6.21 (dd, 1H, J 5.7 and 1.3), 5.95 (t, 1H,
J 1.3), 3.52 (dt, 1H, J 10.6 and 4.4), 2.25 (m, 1H), 2.12 (m, 1H), 1.66 (m, 2H), 1.56–1.25 (m, 2H),
1.12–0.79 (m, 12H).
3.5. Diels–Alder reaction of (5S)-5-(-l-menthyloxy)furan-2(5H)-one 7 with cyclopentadiene
To a solution of furanone 7 (94 mg, 0.39 mmol) in dichloromethane (2.5 mL), cyclopentadiene
(103 mg, 1.56 mmol) was added. The mixture was kept at room temperature under inert
1
atmosphere for 12 days. The solvent was removed in vacuo. H NMR analysis showed the
presence of endo-13anti, exo-13anti and 7 in a 85:7:8 ratio. The residue was purified by flash
chromatography (hexane/ethyl acetate, 14:1) to afford, in decreasing order of Rf, compounds
exo-13anti and endo-13anti in 5 and 77% yields, respectively.
3.5.1. Adduct-endo-13anti
77% Yield. White solid, mp 62–63°C. [h]2D0=+11.9 (c=1, CHCl3). Anal. calcd for C19H28O3:
1
C, 74.96; H, 9.27. Found: C, 74.99; H, 9.33. IR (KBr): 1757, 1358, 1180, 1118. H NMR: 6.24
(dd, 1H, J 5.7 and 2.8), 6.18 (dd, 1H, J 5.7 and 2.9), 4.92 (d, 1H, J 1.7), 3.31 (m, 3H), 3.16 (m,
1H), 2.95 (ddd, 1H, J 8.5, 4.3 and 1.7), 2.16–2.03 (m, 2H), 1.65–1.57 (m, 3H), 1.45–1.17 (m, 3H),
1.02–0.78 (m, 12H). 13C NMR: 177.4, 136.5, 134.2, 107.6, 82.3, 51.9, 48.6, 48.2, 47.9, 45.5, 44.8,
42.7, 34.1, 31.6, 25.9, 23.3, 22.0, 20.9 and 16.3.
3.5.2. Adduct-exo-13anti
1
5% Yield. Colourless oil. H NMR: 6.21 (m, 2H), 5.17 (d, 1H, J 1.8), 3.40 (dt, 1H, J 10.7 and
4.4), 3.24 (m, 1H), 2.96 (m, 1H), 2.74 (dt, 1H, J 8.0 and 1.4), 2.36 (m, 1H), 2.23–2.05 (m, 2H),
1.63 (m, 2H), 1.50 (m, 1H), 1.37–1.21 (m, 3H), 1.08–0.80 (m, 12H).

J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
4751
3.6. Diels–Alder reaction of (5R)-5-(-l-menthyloxy)-4-(phenylsulfonyl)furan-2(5H)-one 8 with
cyclopentadiene
To a solution of furanone 8 (850 mg, 2.25 mmol) in dichloromethane (15 mL), cyclopenta-
diene (594 mg, 9 mmol) was added. The mixture was kept at room temperature under an
1
inert atmosphere for 5 h. The solvent was removed in vacuo. H NMR analysis showed the
presence of endo-14anti, exo-14anti, exo-14syn and endo-14syn in a 65:13:18:4 ratio. The
residue was purified by flash chromatography (hexane/ethyl acetate, 8:1) to afford, in
decreasing order of Rf, compounds exo-14syn+endo-14syn, endo-14anti and exo-14anti in 22,
61 and 8%, respectively. An analytical sample of exo-14syn was obtained by precipitation
with ethyl acetate–hexane.
3.6.1. Adduct-endo-14anti
61% Yield. White solid, mp 57–58°C. [h]2D0=−137.4 (c=1, CHCl3). Anal. calcd for
C25H32O5S: C, 67.54; H, 7.25; S, 7.21. Found: C, 67.70; H, 7.23; S, 6.93. IR (KBr): 1777,
1
1308, 1146. H NMR: 7.92 (m, 2H), 7.60 (m, 1H), 7.49 (m, 2H), 6.48 (dd, 1H, J 5.5 and
2.9), 6.43 (dd, 1H, J 5.5 and 3.2), 5.05 (s, 1H), 4.00 (d, 1H, J 4.4), 3.84 (m, 1H), 3.48 (m,
1H), 3.22 (dt, 1H, J 10.9 and 4.2), 2.84 (m, 1H), 1.74 (m, 2H), 1.56 (m, 3H), 1.27–0.55 (m,
14H). 13C NMR: 173.3, 141.9, 140.8, 137.0, 133.2, 129.6, 128.4, 102.4, 80.8, 80.6, 53.6, 51.6,
50.7, 47.2, 46.6, 39.4, 34.1, 31.3, 24.4, 22.5, 22.0, 21.0, 14.9.
3.6.2. Adduct-exo-14anti
8% Yield. White solid, mp 66–67°C. [h]2D0=−46.6 (c=1, CHCl3). Anal. calcd for
C25H32O5S: C, 67.54; H, 7.25; S, 7.21. Found: C, 67.27; H, 7.02; S, 7.02. IR (KBr): 1773,
1
1316, 1149. H NMR: 7.79 (m, 2H), 7.59 (m, 1H), 7.50 (m, 2H), 6.50 (dd, 1H, J 5.6 and
2.9), 6.40 (dd, 1H, J 5.6 and 3.2), 5.30 (s, 1H), 3.43 (m, 2H), 3.37 (m, 1H), 3.30 (dt, 1H, J
10.8 and 4.3), 1.77–1.56 (m, 6H), 1.24 (m, 1H), 1.05 (m, 1H), 0.93–0.63 (m, 11H), 0.50 (m,
1H). 13C NMR: 172.6, 142.2, 135.7, 133.0, 128.8, 128.5, 104.5, 81.3, 80.3, 53.1, 51.3, 48.8,
47.2, 46.5, 39.4, 34.1, 31.3, 24.4, 22.6, 22.0, 21.1, 15.0.
3.6.3. Adduct-exo-14syn
White solid, mp 168–170°C. [h]D20=−190.8 (c=0.5, CHCl3). Anal. calcd for C25H32O5S: C,
67.54; H, 7.25; S, 7.21. Found: C, 67.31; H, 7.17; S, 7.19. IR (KBr): 1774, 1321, 1152, 1122.
1H NMR: 7.89 (m, 2H), 7.73 (m, 1H), 7.62 (m, 2H), 6.40 (m, 1H), 6.12 (s, 1H), 3.44 (m,
2H), 3.26 (m, 1H), 3.23 (m, 1H), 1.94 (m, 2H), 1.60 (m, 4H), 1.26 (m, 1H), 1.10–0.67 (m,
12H), 0.28 (m, 1H). 13C NMR: 172.7, 137.6, 137.2, 136.7, 134.4, 129.5, 129.2, 100.1, 80.6,
79.0, 52.9, 48.7, 48.3, 47.8, 47.7, 39.1, 33.9, 31.1, 25.3, 22.4, 22.1, 21.0, 15.1.
3.6.4. Adduct-endo-14syn
1H NMR: 7.95 (m, 2H), 6.30 (dd, 1H, J 5.5 and 3.2), 6.23 (dd, 1H, J 5.5 and 3.0), 5.96 (s,
1H), 3.92 (d, 1H, J 4.7), 2.53 (m, 1H), 0.66 (d, 3H, J 6.9), 0.34 (m, 1H) (the remaining
.
signals were not included because they overlap with the signals of the adduct-exo-14syn).

4752
J. L. Garc´ıa Ruano et al. / Tetrahedron: Asymmetry 11 (2000) 4737–4752
Acknowledgements
We thank Direccio´n General de Investigacio´n Cient´ıfica y Te´cnica (grant PB98-0078) for
financial support.
References
1. (a) Bertrand, S.; Hoffmann, N.; Pete, J.-P. Eur. J. Org. Chem. 2000, 2227; (b) Van Oeveren, A.; Jansen, J. F. G.
A.; Feringa, B. L. J. Org. Chem. 1994, 59, 5999; (c) Pelter, P.; Ward, R. S.; Qianrong, L.; Pis, J. Tetrahedron:
Asymmetry 1994, 5, 909.
2. Feringa, B. L.; de Jong, J. C. J. Org. Chem. 1988, 53, 1125.
3. (a) de Jong, J. C.; Jansen, J. F. G. A; Feringa, B. L. Tetrahedron Lett. 1990, 31, 3047; (b) de Jong, J. C.; Feringa,
B. L. Tetrahedron Lett. 1989, 30, 7239.
4. de Jong, J. C.; van Bolhuis, F.; Feringa, B. L. Tetrahedron: Asymmetry 1991, 2, 1247.
5. (a) de Lange, B.; Feringa, B. L. Tetrahedron Lett. 1988, 29, 5317; (b) Rispens, T. M.; Keller, E.; de Lange, B.;
Zijlstra, W. J.; Feringa, B. L. Tetrahedron: Asymmetry 1994, 5, 607.
6. de Jong, J. C.; van den Berg, K. J.; van Leusen, M.; Feringa, B. L. Tetrahedron Lett. 1991, 32, 7751.
7. (a) Carren˜o, M. C. Chem. Rev. 1995, 95, 1717; (b) Cid, M. B.; Garc´ıa Ruano, J. L. In Topics in Current
Chemistry. Organosulfur Chemistry; Page, P. C. B., Ed.; Springer: Berlin, 1999; p. 1.
8. Carretero, J. C.; Garc´ıa Ruano, J. L.; Mart´ın, M. L. Tetrahedron: Asymmetry 1997, 53, 14115.
9. Carretero, J. C.; Garc´ıa Ruano, J. L.; Lorente, A.; Yuste, F. Tetrahedron: Asymmetry 1993, 4, 177.
10. Mart´ın, M. R.; Mateo, A. I. Tetrahedron: Asymmetry 1994, 5, 1385.
.