Optically active cyclic diaryl(alkoxy)sulfonium salts with intramolecular S···O interaction: Synthesis, absolute configuration and stereoselective hydrolysis

Article abstract of DOI:10.1016/S0957-4166(00)00064-1

Optically active cyclic sulfonium salts {(S)-(-)-1-[2'- (methoxycarbonyl)phenyl]-3H-2,1-benzoxathiol-1-ium tetrafluoroborate (S)-(-)- 2, (R)-(-)-1-[8'-(methoxycarbonyl)-1'-naphthyl]-3H-2,1-benzoxathiol-1-ium tetrafluoroborate (R)-(-)-4 and (R)-(-)-1-[2'-(

Full text of DOI:10.1016/S0957-4166(00)00064-1

Tetrahedron: Asymmetry 11 (2000) 1303±1312
Optically active cyclic diaryl(alkoxy)sulfonium salts with
.
..
intramolecular S O interaction: synthesis, absolute
con®guration and stereoselective hydrolysis
0
0
Denes Szabo, * Jeno Varga, Antal Csampai and Istva
   Â
n Kapovitsa
a,
a
b
aDepartment of Organic Chemistry, EoÈtvoÈs LoraÂnd University, H-1518 Budapest 112, PO Box 32, Hungary
bDepartment of General and Inorganic Chemistry, EoÈtvoÈs Lor a nd University, H-1518 Budapest 112, PO Box 32, Hungary
Received 18 January 2000; accepted 11 February 2000
Abstract
0
Optically active cyclic sulfonium salts {(S)-(^)-1-[2 -(methoxycarbonyl)phenyl]-3H-2,1-benzoxathiol-1-
0
0
ium tetra¯uoroborate (S)-(^)-2, (R)-(^)-1-[8 -(methoxycarbonyl)-1 -naphthyl]-3H-2,1-benzoxathiol-1-ium
tetra¯uoroborate (R)-(^)-4 and (R)-(^)-1-[2 -(methoxycarbonyl)phenyl]-3H-2,1-naphtho[1,8-d,e]-oxathiin-
-ium tetra¯uoroborate (R)-(^)-6} were prepared from optically active diaryl(acyloxy)(alkoxy)spiro-l -
sulfanes (R)-(+)-1, (S)-(^)-3 and (S)-(+)-5, respectively. The molecular structures determined by H and
NMR measurements can be described with spiro-l -sulfane-like trigonal bipyramidal geometry about the
.
central sulfur owing to the S O(carbonyl) intramolecular interaction in the axial position. The stereo-
chemistry of the hydrolysis reactions of sulfonium salts 2, 4, 6 and spiro-l -sulfanes 1, 3, 5 depending on
0
4
1
1
13
C
4
..
4
pH is discussed in detail. # 2000 Elsevier Science Ltd. All rights reserved.
1
. Introduction
Recently we described the stereospeci®c synthesis of optically active diaryl(acyloxy)(alkoxy)-
4
spiro-l -sulfanes 1, 3 and 5 starting from optically active precursor sulfoxides (see e.g. 7!1 in
1
Scheme 2). The absolute con®gurations were determined by X-ray di€raction method and a
2
comparative analysis of CD spectra. It has also been published that cyclic sulfonium salts with
carbonyl oxygen atom in 1,5 or 1,6 position to sulfur (prepared in racemic form) show molecular
structures of slightly distorted trigonal bipyramidal (TBP) geometry about the sulfonium centre
4
3±6
similarly to those ones found in analogous spiro-l -sulfanes.
In these cases the molecular
...
arrangement is stabilized by intramolecular S O(carbonyl) interaction with the oxygen atom in
the axial position. To get more information about the stereochemistry and reactivity of compounds
of such type, we decided to prepare optically active diaryl(alkoxy)sulfonium salts stabilized by
*
Corresponding author. Tel: 36-1-209-0555; fax: 36-1-372-2620; e-mail: szabod@szerves.chem.elte.hu
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00064-1

1
304
D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
...
intramolecular O(alkoxy)±S O(carbonyl) interactions and to investigate their structure and
reactivity including the determination of absolute con®gurations.
As a ®rst part of this work we report now the stereospeci®c synthesis of diaryl(alkoxy)-
sulfonium tetra¯uoroborates (S)-(^)-2, (R)-(^)-4 and (R)-(^)-6. In addition, the hydrolysis reac-
4
tions of the above compounds and their optically active spiro-l -sulfane precursors {(R)-(+)-1,
(S)-(^)-3 and (S)-(+)-5} leading to optically active sulfoxides are also described. From the results
obtained one can reach some conclusions about the stereochemistry of nucleophilic reactions
4
occurring at the four-coordinated sulfur atom of spiro-l -sulfanes and sulfonium salts having an
+
...
axial O(alkoxy)±S±O(acyloxy) and O(alkoxy)±S O(carbonyl) part, respectively.
2
. Results and discussion
2
.1. Synthesis (see Scheme 1)
0
0
(
R)-(+)-1,1 -Spirobi[3H-2,1-benzoxathiol]-3-one, (S)-(^)-spiro[3H-2,1-benzoxathiol-1,1 -naph-
0
tho[1,8-d,e]-3H-2,1-oxathiin-3-one] and (S)-(+)-spiro[3H-2,1-benzoxathiol-3-one-1,1 -naphtho[1,8-
4
1
d,e]-3H-2,1-oxathiin] {spiro-l -sulfanes (R)-(+)-1, (S)-(^)-3 and (S)-(+)-5 } were treated with
trimethyloxonium tetra¯uoroborate at room temperature in dichloromethane to give (S)-(^)-1-
0 0
2 -(methoxycarbonyl)phenyl]-3H-2,1-benzoxathiol-1-ium tetra¯uoroborate, (R)-(^)-1-[8 -(methoxy-
[
0
0
carbonyl)-1 -naphthyl]-3H-2,1-benzoxathiol-1-ium tetra¯uoroborate and (R)-(^)-1-[2 -(methoxy-
carbonyl)-phenyl]-3H-2,1-naphtho[1,8-d,e]-oxathiin-1-ium tetra¯uoroborate {sulfonium salts (S)-
(
^)-2, (R)-(^)-4 and (R)-(^)-6}.
The formation of sulfonium salts can be explained by the structure of the precursor spiro-l -
sulfanes. X-Ray determinations revealed that in asymmetrically substituted (acyloxy)(alkoxy)-
4
4
O BF^
+
Scheme 1. Conversion of spiro-l -sulfanes into sulfonium tetra¯uoroborates. Reagents and conditions: (i) Me
3
4
,
ꢀ
CH Cl , 20 C, 4 h
2
2

D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
1305
4
Scheme 2. Stereospeci®c conversion of spiro-l -sulfanes to sulfoxide-carboxylic acids by hydrolysis. Reagents and
ꢀ
ꢀ
ꢀ
conditions: (i) KHCO
then 1,2-dichloroethane, 83 C, 20 min
3
aq., 20 C, 30 min; (ii) H
ꢀ
2
SO
4
aq., 20 C, 30 min±48 h; (iii) AcCl, 1 equiv. Et
3
N, DMF, ^60 C,
4
spiro-l -sulfanes the axial S±O bonds di€er markedly. The S±O(alkoxy) bond is a usual covalent
Ê
1
5
bonding (1.71, 1.66 and 1.69 A in (R)-(+)-1, (R)-(+)-3 and racemic 5, respectively), whereas the
Ê
1
5
S±O(acyloxy) bond lengths (2.05, 2.13 and 2.11 A in (R)-(+)-1, (R)-(+)-3 and racemic 5,
respectively) point to a weak hypervalent bond, which results in a sulfonium-carboxylate
zwitterion structure. Trimethyloxonium tetra¯uoroborate can methylate the carboxylate-like
oxygen, thus leading to the formation of diaryl(alkoxy)sulfonium salts carrying an o-methoxy-
carbonyl substituent on one of the aromatic rings.
2
.2. Enantiomeric excess
Sulfonium salts 2, 4 and 6 can be hydrolysed in KHCO aq. into diaryl sulfoxides 8, 10 and 12,
3
respectively (cf. Scheme 3). Because this reaction is stereospeci®c, the hydrolysis of the enantio-
mers of 2, 4 and 6 leads to optically active sulfoxide-methyl esters 8, 10 and 12, respectively.
Measuring the optical activity of the latter compounds the minimal ee of the starting sulfonium
salts can be calculated (for determining the ee of 8, 10 and 12, see Ref. 7). The following minimal
enantiomeric excess data were obtained: >86% for (^)-2, >89% for (^)-4 and >99% for (^)-6.

1
306
D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
Scheme 3. Stereospeci®c conversion of sulfonium salts to sulfoxide-methyl esters by hydrolysis. Reagents and conditions:
ꢀ ꢀ
(
3 2 4
i) KHCO aq., 0 C, 30 min; (ii) H SO aq., 20 C, 48 h. No reaction with (R)-(^)-6
2
.3. Molecular structures
NMR measurements showed that optically active compounds 2, 4 and 6 exhibit sulfonium
4
structure with a spiro-l -sulfane-like trigonal bipyramidal arrangement about sulfur. This means
+
.
..
that the nearly linear O(alkoxy)±S O(carbonyl) moieties occupy the axial position as was expected
from the structures of analogous diaryl(alkoxy) and diaryl(acylamino) sulfonium salts.3 Owing
to the close contact between sulfonium centre and the methoxycarbonyl oxygen, the endo meth-
ylene hydrogen and the aromatic hydrogen attached in ortho position to sulfur on the aromatic
ring carrying the methoxycarbonyl group come into steric proximity in the rigid ring systems of
the investigated sulfonium salts 2, 4 and 6. This conformation is unambiguously evidenced in
±6
DMSO-d by mutual NOE interaction between these hydrogens (6.5% for 2, 5.8% for 4 and
6
4
.1% for 6). On the other hand the proximity of the carbonyl oxygen ®xed in close contact and
the hydrogen atom bonded to the other aromatic ring in ortho position relative to the sulfonium
centre is clearly revealed by their characteristic down®eld shifts (8.07 ppm for 2, 8.27 ppm for 4
y
and 8.42 ppm for 6).
y
Each 1H and 13C NMR assignment based on 2D-COSY, 2D-HSQC, 2D-HMBC and DNOE measurements is
provided in supplementary material.

D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
1307
2
.4. Absolute con®gurations
Knowing the absolute con®gurations of precursor spiro-l -sulfanes (R)-(+)-1, (S)-(^)-3 and
4
1,2
(S)-(+)-5, we may attribute the con®gurations to the sulfonium salts (S)-(^)-2, (R)-(^)-4 and
4
(R)-(^)-6, because the sulfonyl moiety of spiro-l -sulfanes does not take part in the methylating
reaction, which converts only the oxycarbonyl part of the molecule (see e.g. 1!2 in Scheme 1).
4
{
1,2
The similar CD spectra of spiro-l -sulfanes (R)-(+)-1, (S)-(^)-3 and (S)-(+)-5, and the corre-
sponding sulfonium salts (S)-(^)-2, (R)-(^)-4 and (R)-(^)-6 (Fig. 1), gave convincing evidence for
both the same absolute con®gurations and the similar TBP geometry of the compounds men-
tioned. A comparative analysis of CD spectra for the enantiomers of 2, 4, 6 and other sulfonium
salts as well as their precursor compounds will be published in detail elsewhere.8
Figure 1. CD spectra of sulfonium salts (S)-(^)-2, (R)-(^)-4 and (R)-(^)-6
4
It should be pointed out that the spatial arrangement of the above mentioned spiro-l -sulfanes
and corresponding sulfonium salts {(R)-(+)-1 and (S)-(^)-2; (S)-(^)-3 and (R)-(^)-4; (S)-(+)-5
and (R)-(^)-6} are analogous although the designations of con®guration are di€erent. The con-
4
®
guration of sulfur in spiro-l -sulfanes was designated by the convention proposed by Martin
9
and Balthazor for compounds with pentacoordinated stereogenic atom, while the Cahn±Ingold±
Prelog convention was used for sulfonium salts.
2
.5. Hydrolysis
By using kinetic methods the mechanism of hydrolysis of racemic spiro-l -sulfanes and related
4
10±13
sulfonium salts has been investigated extensively in our laboratory by Ru€ et al. In contrast,
4
only a few l -sulfanes were prepared in optically active form, thus the stereochemistry of their
nucleophilic reactions has been slightly studied. Martin et al. investigated the basic hydrolysis of
4
9
an optically active chloro-l -sulfane. Zhang et al. found that the basic and acidic hydrolysis of
4
optically active spiro-l -sulfanes leads to sulfoxide diastereomers with opposite con®gurations of
the sulfur atom, therefore they suggested di€erent stereomechanisms depending on pH.14
{
In Refs. 1 and 2 the CD spectrum of (R)-(+)-3 is given. The spectrum of (S)-(^)-3 was also recorded and shows the
mirror image of (R)-(+)-3.

1
308
D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
Having a set of optically active sulfonium salts, (S)-(^)-2, (R)-(^)-4, (R)-(^)-6, and the corre-
4
sponding optically active precursor spiro-l -sulfanes, (R)-(+)-1, (S)-(^)-3, (S)-(+)-5, of known
absolute con®gurations, we submitted them to hydrolysis under di€erent conditions (in 1N
KHCO and 1N H SO solutions). Experimental results are summarized in Schemes 2 and 3.
3
2
4
1
In a previous paper we showed that the stereospeci®c dehydration of sulfoxide (R)-(+)-7
4
yielding spiro-l -sulfane (R)-(+)-1 (see Scheme 2) proceeds with `retention of con®guration'
because the C (acyl±O)!C (alkyl±O)!lone pair sequence of substituents attached to the
ar
ar
stereogenic sulfur traces the same direction both in the case of sulfoxide (R)-(+)-7 (Cahn±
4
9
Ingold±Prelog convention) and in spiro-l -sulfane (R)-(+)-1 (Martin±Balthazor convention ).
Thus, the reverse reaction, e.g. the hydrolysis of (R)-(+)-1 into (R)-(+)-7, may be assumed to
proceed with retention. However, this is true for reaction 1!7 only if it is conducted in aqueous
KHCO solution, whereas in acidic media inversion does occur (e.e. 84%). To exclude racemi-
3
zation as a contributing factor in the above experiments, we treated optically active sulfoxide 7
4
under the conditions as employed for the acidic hydrolysis of spiro-l -sulfane (R)-(+)-1 (1N
ꢀ
H SO , 30 min, 25 C, cf. Exp.), which resulted in ca. 16% of racemization, while total race-
2
4
mization took ca. 60 h (for racemization of sulfoxides including the e€ect of acid catalysis and
neighbouring participation of carboxylic group, see Ref. 15). Under basic conditions 10% of
racemization was observed for sulfoxide (R)-(+)-7, while stirring the solution of the methyl ester
(
R)-(+)-8 with aqueous KHCO or H SO a€orded only the starting enantiomer.
3
2
4
Thus we may assume that the hydrolysis of (R)-(+)-1 is stereospeci®c and can follow two
di€erent pathways depending on pH. This assumption is in accordance with the results of Zhang
et al.14 obtained for the hydrolysis of other ®ve-membered (acyloxy)(alkoxy)spiro-l -sulfanes.
The proposed mechanisms are shown in Scheme 4.
4
Owing to the polar and weak S±O(acyloxy) hypervalent bond, the acyloxy-ring of diaryl(acyloxy)-
4
(
alkoxy)spiro-l -sulfanes splits easily, and hydroxide ion can attack the sulfonium centre from
the `acyloxy-side' of the monocyclic alkoxy-sulfonium salt 13. A monocyclic (alkoxy)(hydroxy)-
4
l -sulfane intermediate 14 is formed, which transforms into sulfoxide-carboxylic acid (R)-(+)-7 by
splitting of the `alkoxy-ring' and by proton-transfer. The reaction proceeds with retention, e.g.
the con®guration of sulfur is preserved.
In acidic media the alkoxy ring may open after protonation and water molecule can hydrolyse the
acyloxy-sulfonium intermediate 16 through the formation of the monocyclic (acyloxy)(hydroxy)-
4
l -sulfane 17 intermediate. In compounds 14 and 17 sulfur atoms have opposite con®gurations,
thus the ring-splitting and proton-transfers convert 17 to the enantiomeric sulfoxide±carboxylic
acid (S)-(^)-7. This mechanism is supported by the following data: (i) isotopically labelled H O
2
experiments proved that the water molecule attacks the sulfur during hydrolysis;1
4,16
therefore,
regarding the inversion of sulfur the attack of water takes place from the `alkoxy-side'; (ii) the
basicity of ethers is stronger than that of esters17 (pKBH+=^3.5 and ^6.5, respectively); (iii)
kinetic investigations revealed that water as a poor nucleophile can hydrolyse asymmetrically
4
11
substituted spiro-l -sulfanes in a slow reaction, but even water can hydrolyse the very unstable
acyloxy-sulfonium intermediate 16 formed by the splitting of the S±O H(alkoxy) bond.
+
4
In alkaline media the hydrolysis of spiro-l -sulfanes (S)-(^)-3 and (S)-(+)-5 containing
naphthalene rings (see path i in Scheme 2) proceeds in a similar way as that of compound (R)-
(
+)-1, yielding the expected stereoisomer of sulfoxide-carboxylic acids (S)-(+)-9 and (S)-(+)-11.
2
5
46
25
546
The formation of a nearly racemic product (9) {‰ꢀŠ =^23; ‰ꢀŠ =673 (c=0.5, DMF) for the
5
7
pure enantiomer } from (S)-(^)-3 in acidic media indicates, however, that two parallel mechan-
isms are operative in this case. A water molecule can attack the sulfonium centre after the

D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
1309
4
Scheme 4. The stereospeci®c hydrolysis of spiro-l -sulfane (R)-(+)-1 in basic (1N KHCO
3
) and acidic (1N H
2
4
SO )
media
+
cleavage of the S±OH (alkoxy) bond (inversion), and from the side of the acyloxy group (retention),
due to the very weak S±O(acyloxy) bond in the six-membered ring (cf. Ref. 11). In the acidic
hydrolysis of (S)-(+)-5 the ee of (R)-(^)-11 was 66%. In control experiments sulfoxide-carboxylic
acid enantiomers 9 and 11 were treated with H SO and KHCO and only in the case of 9 was ca.
2
4
3
1
0% of racemization observed.
As shown in Scheme 3, the hydrolyses of diaryl(alkoxy)sulfonium salts (S)-(^)-2 and (R)-(^)-4
gave the corresponding diaryl sulfoxide-methyl esters (R)-(+)-8 and (S)-(+)-10 with opposite
sulfur con®gurations both in 1N KHCO and 1N H SO solutions. It should be mentioned that
3
2
4
(R)-(^)-6 gives (S)-(+)-12 in basic media but did not hydrolyse under acidic conditions, which may

1
310
D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
be attributed to the slight reactivity of water toward the six-membered alkoxy-ring, which can be
cleaved by hydroxide.11 In reference experiments sulfoxide-methyl ester enantiomers 8, 10 and 12
were treated with H SO and KHCO and all of them were isolated in enantiomerically pure form.
2
4
3
The detailed mechanism proposed for the hydrolysis of the sulfonium salt (S)-(^)-2 is illustrated
in Scheme 5. We may assume that the sulfonium centre is attacked by hydroxide ion (in
KHCO aq.) or by a water molecule (in H SO aq.), which gives the monocyclic (alkoxy)-
3
2
4
4
(
hydroxy)-l -sulfane 18 intermediate, from which (R)-(+)-8 sulfoxide-methyl ester is formed by
cleavage of the alkoxy-ring and by proton-transfer, involving inversion of the con®guration of the
sulfonium centre. (R)-(+)-8 can also be obtained from (R)-(+)-7 by methylation with diazo-
methane.
Scheme 5. The stereospeci®c hydrolysis of sulfonium salt (S)-(^)-2 in basic (1N KHCO ) or in acidic (1N H SO )
3
2
4
media
3
. Experimental
Melting points were determined on a Boetius micro melting point apparatus. IR spectra were
1
13
taken on a Specord IR 75 (Zeiss, Jena) spectrophotometer. H and CNMR spectra were
recorded in DMSO-d solution at room temperature on a Bruker DRX 500 spectrometer at
5
6
1
13
00.13 ( H) and 125.76 ( C) MHz with the deuterium signal of the solvent as the lock and TMS
as internal reference. The standard Bruker microprogram NOEMULT.AU to generate NOE was
used with a selective preirradiation time. The 2D-COSY, 2D-HSQC and 2D-HMBC spectra were
obtained by using the standard Bruker pulse programs COSYGS, INV4GS and INV4GSLPLRND.
3
.1. General procedure for the preparation of sulfonium tetra¯uoroborates (S)-(^)-2, (R)-(^)-4
and (R)-(^)-6
4
1
To a stirred solution of spiro-l -sulfanes (R)-(+)-1, (S)-(^)-3 and (S)-(+)-5 (3 mmol) in
dichloromethane (40 mL) was added trimethyloxonium tetra¯uoroborate (0.46 g, 3.03 mmol).

D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
1311
The stirring continued for 4 h at room temperature then ether (100 mL) was poured into the
mixture. The precipitate was ®ltered o€, washed with ether and dried over P O .
2
5
3
3
(
.1.1. Selected data for sulfonium tetra¯uoroborates
0
.1.1.1. (S)-(^)-1-[2 -(Methoxycarbonyl)phenyl]-3H-2,1-benzoxathiol-1-ium tetrafluoroborate
25
ꢀ
(KBr)/cm^1 1662vs
max
S)-(^)-2. Yield 84%; ‰ꢀŠ =^204, c=0.5, DMF; mp 155±158 C; IR ꢁ
546
1
(
CˆO), 1050vs (BF ); H NMR ꢂ 6.18 (d, 1H, J=13.2 Hz), 6.54 (d, 1H, J=13.2 Hz), 4.20 (s, 3H),
4
7
matic ring).
.67±8.23 (m, ArH); 13C NMR ꢂ 169.1 (s, CˆO), 84.2 (t, CH ), 55.9 (q, CH ), 124.5±140.4 (aro-
2
3
0 0
.1.1.2. (R)-(^)-1-[8 -(Methoxycarbonyl)-1 -naphthyl]-3H-2,1-benzoxathiol-1-ium tetrafluoro-
3
25
ꢀ
^1
borate (R)-(^)-4. Yield 80%; ‰ꢀŠ =^279, c=0.5, DMF; mp 183±193 C; IR ꢁ
(KBr)/cm
546
max
1
1
(
1
674vs (CˆO), 1050vs (BF ); H NMR ꢂ 6.06 (d, 1H, J=12.8 Hz), 6.47 (d, 1H, J=12.8 Hz), 4.16
4
13
s, 3H), 7.79±8.53 (m, ArH); C NMR ꢂ 172.1 (s, CˆO), 83.3 (t, CH ), 55.1 (q, CH ), 124.6±
2
3
39.3 (aromatic ring).
0
.1.1.3. (R)-(^)-1-[2 -(Methoxycarbonyl)-phenyl]-3H-2,1-naphtho[1,8-d,e]-oxathiin-1-ium tetra-
3
25
ꢀ
fluoroborate (R)-(^)-6. Yield 86%; ‰ꢀŠ =^87, c=0.5, DMF; mp 198±204 C; IR ꢁ
(KBr)/
546
max
^
1
1
cm 1710vs (CˆO), 1070vs (BF ); H NMR ꢂ 5.13 (d, 1H, J=12.8 Hz), 5.87 (d, 1H, J=12.8 Hz),
4
13
4
1
.08 (s, 3H), 6.91±8.59 (m, ArH); C NMR ꢂ 164.9 (s, CˆO), 68.2 (t, CH ), 53.8 (q, CH ), 124.0±
2
3
36.1 (aromatic ring).
4
3
(
.2. General procedure for the hydrolysis of spiro-l -sulfanes (R)-(+)-1, (S)-(^)-3 and (S)-
+)-5
4
(
A) To the solution of 1N KHCO (2 mL) was added a spiro-l -sulfane (0.1 mmol) and the
3
reaction mixture was stirred for 30 min at room temperature. The solution was acidi®ed with 1N
H SO (pH 2) then the crystals were ®ltered, washed with water and dried. Yields: 88, 87 and
2
4
9
0% for sulfoxide-carboxylic acids (R)-(+)-7, (S)-(+)-9 and (S)-(+)-11, respectively.
4
(B) To the solution of a spiro-l -sulfane (0.1 mmol) in dichloromethane (2 mL) was added 1N
H SO (2 mL) and the mixture was stirred for 30 min {for (R)-(+)-1)} or 48 h {for (S)-(^)-3 and
2
4
(
S)-(+)-5} at room temperature. Dichloromethane was removed in vacuo then the crystals were
ltered, washed with water and dried. Yields: 93, 91 and 91% for sulfoxides (S)-(^)-7, racemic 9
and (R)-(^)-11, respectively.
®
3
.3. General procedure for the hydrolysis of sulfonium tetra¯uoroborates (S)-(^)-2, (R)-(^)-4
and (R)-(^)-6
(A) To the solution of a sulfonium salt (0.1 mmol) in dichloromethane (2 mL) was added 1N
KHCO (2 mL) then the mixture was stirred for 30 min at 0 C. The organic phase was separated,
ꢀ
3
dried and then the solvent was removed in vacuo. Yields: 83, 87 and 88% for sulfoxide-methyl
esters (R)-(+)-8, (S)-(+)-10 and (S)-(+)-12, respectively.
(B) To the solution of a sulfonium salt (0.1 mmol) in dichloromethane (2 mL) was added 1N
H SO (2 mL) and the mixture was stirred for 48 h at room temperature. The organic phase was
2
4
separated, dried and then the solvent was removed in vacuo. Yields: 94 and 89% for sulfoxide-
methyl esters (R)-(+)-8 and (S)-(+)-10; the unchanged (R)-(^)-6 was isolated in the yield of 95%.

1
312
D. Szab o et al. / Tetrahedron: Asymmetry 11 (2000) 1303±1312
Physical data of sulfoxide-carboxylic acids and sulfoxide-methyl esters isolated by the hydrolysis
reactions are identical with those given in Ref. 7.
Acknowledgements
This work was supported by the Hungarian Scienti®c Research Foundation (OTKA, No.
Â
T 029799). The authors thank Professor Arpad Kucsman and Professor Ferenc Ru€ for the
Â
discussions to produce the ®nal form of the manuscript.
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1
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Â
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Â
n, A.; Nagy, P. Tetrahedron: Asymmetry
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2
3
4
5
Â
, D.; Kapovits, I.; Hollo
Â
si, M. Enantiomer 1998, 3, 323.
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. Szabo
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