Diastereomeric sulfinates derived from (L)-N-methylephedrine: Synthesis, applications and rearrangements

Article abstract of DOI:10.1016/S0957-4166(99)00336-5

The reaction of sulfinyl chlorides with (L)-N-methylephedrine alone or in the presence of tertiary amines was found to produce diastereomeric sulfinates with diastereomeric purities up to 90%. The diastereomeric ratio is strongly influenced by the nature

Full text of DOI:10.1016/S0957-4166(99)00336-5

Pergamon
Tetrahedron: Asymmetry 10 (1999) 3177–3187
Diastereomeric sulfinates derived from (L)-N-methylephedrine:
synthesis, applications and rearrangements
Józef Drabowicz,a,∗ Bogdan Bujnicki,a Paolo Biscarini b and Marian Mikołajczyk a,∗
aCentre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Łódz´, Sienkiewicza 112, Poland
bDipartimento di Chimica Fisica ed Inorganica, Universita degli Studi di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
Received 18 June 1999; accepted 9 August 1999
Abstract
The reaction of sulfinyl chlorides with (L)-N-methylephedrine alone or in the presence of tertiary amines was
found to produce diastereomeric sulfinates with diastereomeric purities up to 90%. The diastereomeric ratio is
strongly influenced by the nature of substituents on the sulfinyl chlorides and to some extent by the reaction
conditions. In a few cases, the pure diastereomers were isolated by chromatography and used for the preparation
of optically active sulfoxides. The silica gel catalyzed rearrangement of sulfinates to the corresponding sulfones is
also discussed. © 1999 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction
Chiral, diastereomeric and enantiomeric sulfinates have recently received considerable attention
mainly due to the fact that they serve as starting materials in the synthesis of sulfoxides and other optically
active sulfinic acid derivatives1 and as model compounds in the studies of nucleophilic substitution
at sulfur.2 Among diastereomeric sulfinic esters the most important role is still played by (−)-(S)-
O-menthyl p-toluenesulfinate, first prepared by Phillips as early as 1925.3 Since then, considerable
effort has been devoted to improving its synthesis.4,5 It is of interest to note that a great number of
other O-menthyl sulfinates were prepared in a similar way using the appropriate sulfinyl chlorides as
substrates.6 Recently, a few other optically active alcohols were used instead of (−)-menthol as chiral
components. For example, from (−)-cholesterol and methanesulfinyl chloride diastereomerically pure
O-cholesteryl methanesulfinates were obtained by fractional crystallization, albeit in low yields.7 Very
good results have recently been obtained with sugar derivatives, especially with diacetone-D-glucose
(DAG)8,9 and dicyclohexylidene D-glucose (DCG).9 Another procedure for the efficient preparation of
∗
Corresponding author: Tel: (0-48-42) 6844014, ext. 234; fax: (0-48-42) 6847126; e-mail: draj@bilbo.cbmm.lodz.pl
0957-4166/99/$ - see front matter © 1999 Published by Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00336-5

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diastereomerically pure sulfinate esters is based on the use of trans-2-phenylcyclohexanol as a chiral
auxiliary.10
Considering the use of diastereomeric sulfinic esters as reagents in asymmetric and stereoselective
synthesis it should be noted that their application has one, but very basic, drawback. It results from the
fact that optically active products formed from diastereomeric sulfinates are contaminated by optically
active alcohol by-products. This makes purification procedures less convenient and time consuming.
Moreover, only in a few cases can the optically active alcohols formed as by-products be recovered in
high chemical yields and enantiomeric excesses and be reused.
Looking for another group of diastereomeric sulfinates, which are free from the limitations presented
above, we turned our attention to a possible preparation of sulfinic esters from optically active aminoal-
cohols containing a tertiary nitrogen atom. Due to the presence of such a basic site the diastereomeric
sulfinate mixtures formed should be much more easily resolved into the pure diastereomers either via
classical crystallization of the corresponding ammonium salts or by chromatography of the crude reaction
products. Moreover, the presence of the basic amino group should allow efficient recovery of the optically
active aminoalcohol formed as a by-product. Thus, the aminoalcohols formed should be easily separated
from the reaction mixture by a simple wash out under appropriately low pH and recovered from the water
phase after adjustment of its pH to basic values and extraction with an organic solvent. Scheme 1 shows
this new approach to the synthesis of diastereomeric sulfinates and their application to the preparation of
other optically active sulfinyl derivatives.
Scheme 1.
2. Results and discussion
2.1. Synthesis of diastereomeric sulfinates derived form (L)-N-methylephedrine
It is obvious that in the pool of enantiomerically pure aminoalcohols one can find at least a few
compounds which contain a tertiary nitrogen atom. Among them, commerically available and relatively
cheap levorotatory (L)-N-methylephedrine, (−)-(1R,2S)-1 (HOEph), was selected as a model compound.

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3179
Preliminary experiments carried out with equimolar amounts of N-methylephedrine 1 and a variety
of sulfinyl chlorides 2 in ethyl ether as a solvent have shown that the formation of the expected
hydrochlorides of N-methylephedrine sulfinates 3 is always accompanied by the deposition of N-
methylephedrinium chloride. For this reason, the hydrochlorides of the diastereomeric sulfinates 3 could
not be isolated by crystallization of the crude reaction products. Therefore, the sulfinates 3 were isolated
as neutral species by column chromatography of the reaction mixtures obtained after conversion of
the hydrochlorides of the sulfinates 3 and N-methylephedrine 1 to neutral precursors as shown below
(Scheme 2).
Scheme 2.
The extent of asymmetric induction in the formation of the esters 3 and their yields were determined
1
by analysis of the H NMR spectra of the crude reaction products measured before chromatographic
purification. On the other hand, the direction of asymmetric induction in the formation of the esters
3 was established by their conversion into appropriate, optically active sulfoxides as configurational
standards, assuming that the reaction of organometallic reagents with 3 proceeds stereospecifically with
inversion of configuration at the sulfinyl sulfur atom (this will be discussed below). The chemical
yields, diastereomeric excess values and absolute configurations of the sulfinates 3, formed as major
condensation products, are collected together with other experimental data in Table 1.
It is evident from the results given in Table 1 that this new asymmetric synthesis of diastereomeric
sulfinates 3 is modestly efficient in terms of chemical yields and diastereomeric excess values. The de
values of 3 were found to range from 23 to 90%. It should be pointed out that there is an interesting
relationship between the de value and the steric requirement of a substituent bound to the sulfinyl
sulfur atom. The lowest degree of asymmetric induction is observed for methanesulfinate 3a and p-

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Table 1
Diastereomeric N-methylephedrine sulfinates 3, formed in the reaction of sulfinyl chlorides 2 with
(L)-N-methylephedrine 1
toluenesulfinate 3h (23.2 and 27.2%, respectively). This value increases substantially for the isopropyl
ester 3d (60%) and becomes very high (90%) for the sulfinates 3f and 3g which contain sterically
demanding substituents such as t-butyl or adamantyl. Analysis of the results in Table 1 also indicates that
a substantial amount of the starting N-methylephedrine 1 is blocked as a hydrochloride, thus decreasing
the chemical yield of the sulfinates 3. In light of these observations, it was interesting to check if the use
of an equimolar amount of an external tertiary amine could influence the outcome of the condensation
between sulfinyl chlorides 2 and N-methylephedrine 1. Namely, it could be expected that the presence
of an external tertiary amine in the reaction mixture would lead to the conversion of N-methylephedrine
hydrochloride into the free amine. Consequently, this should lead to an increase of the chemical yields
of the expected sulfinates 3. Moreover, it is reasonable to expect that their de values can also be changed.
The data collected in Table 2 show, however, that the presence of an external base has only a very limited
effect on both the chemical yields and de values of the sulfinates 3 formed.
Similarly, the data presented in Table 3 indicate that the solvent used has practically no influence on
the outcome of the condensation of t-butanesulfinyl chloride 2f with N-methylephedrine 1 in terms of the
chemical yield and diastereomeric ratio of the produced sulfinate 3f.
2.2. Preparation of optically active sulfoxides from N-methylephedrine sulfinates
Purification of the crude reaction mixtures by column chromatography on silica gel with a mixture of
diethyl ether:petroleum ether (2:10) as an eluent allows the complete separation of the sulfinates 3 from
the non-reacted N-methylephedrine 1. In some cases, it was also possible to isolate single diastereomers
of the sulfinates 3 or to obtain a mixture strongly enriched in one of them. Interestingly, we were able
to obtain the pure (−)-(S)-diastereomer of the sulfinate 3f containing the bulky t-butyl group at sulfur.

J. Drabowicz et al. / Tetrahedron: Asymmetry 10 (1999) 3177–3187
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Table 2
Influence of an external base on the outcome of the reaction of sulfinyl chlorides 2 with
(L)-N-methylephedrine 1
Table 3
Influence of solvent on the outcome of the condensation of t-butanesulfinyl chloride 2f with
(L)-N-methylephedrine 1
Sulfinates 3 purified in this way were used for the preparation of optically active sulfoxides (Eq. 1). The
results of these experiments are summarized in Table 4.

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Table 4
Optically active sulfoxides 4 from diastereomeric N-methylephedrine sulfinates 3 and Grignard
reagents R1MgX
(1)
Since the levorotatory sulfinates 3 were found to give the levorotatory aryl alkyl sulfoxides 4a–c,f,g,
and methyl t-butyl sulfoxide 4d, and dextrorotatory benzyl t-butyl sulfoxide 4e on treatment with the
appropriate organomagnesium reagents, the S configuration was assigned to the sulfinates 3 based on a
very reasonable assumption that all reactions follow the same steric course and afford sulfoxides 4 with
inverted configuration at the sulfinyl sulfur atom.
Considering preparation of optically active sulfoxides with the use of the diastereomeric sulfinates 3
as substrates it is of interest to note that the adamantyl analog 3g does not react at all even with 10 molar
excesses of methylmagnesium bromide or methyllithium (Eq. 2). The total inertness of this sulfinate
towards these two very reactive organometallic reagents results from the presence of the sterically bulky
adamantyl substituent in the ester 3g. This causes the process of nucleophilic exchange at the sulfinyl
sulfur atom in 3g to not occur at all.
(2)
2.3. Rearrangement of N-methylephedrine sulfinates into the corresponding sulfones
During purification of the crude reaction mixtures containing sulfinates 3 and unreacted N-
methylephedrine 1 by silica gel chromatography the rearrangement of the sulfinic esters 3a–e,h into the
corresponding sulfones 5a–e,h was observed (Eq. 3).

J. Drabowicz et al. / Tetrahedron: Asymmetry 10 (1999) 3177–3187
3183
(3)
It was found that the rearrangement rate strongly depends on the nature of the substituent bound to the
sulfinyl sulfur atom. Slow and repeated purification of the p-toluenesulfinate 3h gave the sulfone 5h as a
single reaction product. On the other hand, the sulfinates 3a–e, which contain alkyl substituents in their
structures, rearrange to the sulfones 5a–e much more slowly. Therefore, it was possible to isolate them
in pure form as long as chromatography was carried out rapidly. Traces of sulfones were detected in 3
when the crude reaction mixture was allowed to have contact with silica gel for a longer time. In turn, the
sulfinates 3f,g containing bulky substituents at sulfur do not undergo rearrangement during purification
on a silica gel column. It was found, however, that the t-butanesulfinate 3f could be converted to a mixture
containing equimolar amounts of sulfone 5f and t-butyl benzyl sulfone 6 (Eq. 4) when it was stirred in
ethyl ether solution over silica gel for at least 72 h.
(4)
Exclusive formation of the sulfone 5h during purification of the sulfinate 3h by silica gel chromato-
graphy and the simultaneous but much slower formation of the sulfones 5f and 6 during longer contact
of the sulfinate 3f with silica gel may be rationalized by the sequence of events shown in Scheme 3.
The rearrangement is believed to proceed in two steps. The initial slow and reversible step involves
cleavage of the oxygen–carbon bond leading to the generation of sulfinic acid anion 7 and carbenium ion
8. In the second step, the sulfinate anion 7 is alkylated by the carbenium ion 8 to give the sulfone 5f or 5h.
Both sulfones are formed as mixtures of diastereomers which provides strong support for the formation
of the carbenium ion 8. A simultaneous formation of t-butyl benzyl sulfone 6 in the rearrangement
of the sulfinate 3f may be explained if one assumes that the secondary carbenium ion 8 undergoes
rearrangement to the more stable, tertiary carbenium ion 9a which is stabilized by the presence of the
N,N-dimethylamino group. Therefore, it can exist as the immonium cation 9b. A nucleophilic attack of
the sulfinate anion 7 on the benzylic carbon of the latter should afford t-butyl benzyl sulfone 6 and the
structure 10 which should finally rearrange to the N-methylimine 11.
3. Experimental
1H and 13C NMR spectra were recorded in CDCl3 solutions using Me4Si as an internal standard. The
optical activity measurements were carried out on a photopolarimeter with a sensitivity of ꢀ0.002°. Mps
and bps are uncorrected. Diethyl ether and tetrahydrofuran were distilled from lithium aluminium hydride
before use. Petroleum ether and benzene were distilled over sodium.
Methanesulfinyl chloride was prepared by chlorination of dimethyl disulfide with sulfuryl chloride and
hexamethyldisiloxane.11 Other primary and secondary aliphatic sulfinyl chlorides and p-toluenesulfinyl
chloride were prepared by chlorination of the appropriate disulfides either with gaseous chlorine in
methylene chloride and acetic anhydride12 or with sulfuryl chloride and trimethylsilyl acetate.13 Ada-

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J. Drabowicz et al. / Tetrahedron: Asymmetry 10 (1999) 3177–3187
Scheme 3.
mantanesulfinyl chloride was obtained by the reaction of adamantane with thionyl chloride in the pre-
sence of AlCl3.14 (−)-(1R, 2S)-N-Methylephedrine used in the present work was a commercial product,
[α]589=−29.5 (methanol), mp 87–89°C.
3.1. Diastereomeric N-methylephedrine sulfinates 3: general procedure
A three-necked flask equipped with a stirring bar was charged with the appropriate sulfinyl chloride
2 (10 mmol), dissolved in a given solvent (30 mL), and cooled to −70°C by an external cooling system
(a solid CO2–acetone bath). At this temperature, a solution of N-methylephedrine (10 mmol) neat or
mixed with the equivalent amount of a tertiary amine in the appropriate solvent (20 mL) was added
dropwise. The reaction mixture was kept at −70°C for a few hours and was then allowed to reach a
temperature of around 0°C. After addition of more solvent (∼50 mL) the reaction mixture was quenched
with a 5% aqueous solution of sodium hydroxide. The organic phase was separated form the water layer.
The aqueous solution was extracted twice with ethyl ether (50 mL). The combined organic extracts
were washed twice with water and dried over anhydrous magnesium sulfate. After evaporation of the
1
solvent the crude reaction products were subjected to H NMR analysis to determine chemical yield
and diastereomeric ratio of the formed sulfinates 3. The pure sulfinates 3 were isolated by column
chromatography on silica gel using ethyl ether:petroleum ether (2:10) as an eluent.

J. Drabowicz et al. / Tetrahedron: Asymmetry 10 (1999) 3177–3187
3185
3.2. N-Methylephedrine methanesulfinate 3b
Yield 16.6%, mp 42–44°C, de=100%; [α]589=−167.0 (c 2.03, CHCl3); 1H NMR δ: 1.130 (d, J=6.62
Hz, 3H), 2.286 (s, 6H), 2.419 (s, 3H), 2.930 (q, J=6.62 Hz, 0.5H), 2.95 (q, J=6.62, 0.5H), 5.193 (d,
J=6.16, 1H), 7.323–7.458 (m, 5H). Anal. calcd for C12H19NO2S: C, 59.69; H, 7.94; N, 5.80; S, 13.29.
Found: C, 59.37; H, 7.92; N, 5.74; S, 13.65.
3.3. N-Methylephedrine n-propanesulfinate 3c
Yield 73.2%, oil, de=47.6%; [α]589=−128.0 (c 2.03, CHCl3); 1H NMR δ: 0.987 and 1.022 (2t, J=7.46
Hz, 3H), 1.026 and 1.137 (2d, J=6.71 Hz), 1.717 (m, 2H), 2.272 and 2.296 (2s, 6H), 2.750 (m, 2H), 2.820
and 2.959 (2 quintets, J=7.10 Hz, 1H), 5.195 and 5.400 (2d, J=6.00 Hz, 1H), 7.205–7.425 (m, 5H). Anal.
calcd for C14H23NO2S: C, 62.39; H, 8.61; N, 5.20; S, 11.90. Found: C, 62.19; H, 8.31; N, 5.05; S, 12.10.
3.4. N-Methylephedrine i-propanesulfinate 3d
Yield 46%, oil, de=66.4%; [α]589=−124.6 (c 1.81, CHCl3); 1H NMR δ: 1.035 and 1.150 (2d, J=6.75
Hz, 1H), 1.22 (dd, J=6.72 Hz, 6H), 2.290 and 2.380 (2s, 6H), 2.750 (sp, J=6.72 Hz, 1H), 5.158 and 5.285
(2d, J=6.72 Hz, 1H), 7.25–7.40 (m, 5H). Anal. calcd for C14H23NO2S: C, 62.39; H, 8.61; N, 5.20; S,
11.90. Found: C, 62.19; H, 8.52; N, 5.41; S, 12.13.
3.5. N-Methylephedrine t-butanesulfinate 3f
Yield 50%, mp 33–35°C, de=100%; [α]589=−160.8 (c 0.7, CHCl3); 1H NMR δ: 1.142 (d, J=6.62 Hz,
3H), 1.164 (s, 9H), 2.265 (s, 6H), 2.983 (quintet, J=6.3 Hz, 1H), 5.152 (d, J=6.3 Hz, 1H), 7.245–7.415
(m, 5H). Anal. calcd for C15H25NO2S: C, 63.54; H, 8.89; N, 4.94; S, 11.32. Found: C, 63.72; H, 8.96; N,
5.13; S, 11.57.
3.6. N-Methylephedrine adamantanesulfinate 3g
1
Yield 50%, mp 63–65°C, de=100%; [α]589=−85.1 (c 2.61, CHCl3); H NMR δ: 1.185 (d, J=6.72
Hz, 3H), 1.650–2.20 (m, 15H), 2.282 (s, 6H), 2.987 (quintet, J=6.72 Hz, 1H), 5.177 (d, J=6.72 Hz, 1H),
7.25–7.42 (m, 5H); 13C NMR δ: 8.739, 27.926, 33.504, 36.038, 40.628, 58.676, 63.759, 79.593, 126.835,
127.505, 127.805, 139.406. Anal. calcd for C20H31NO2S: C, 68.70; H, 8.99; N, 4.01; S, 9.18. Found: C,
68.91; H, 8.99; N, 4.32; S, 9.35.
3.7. Synthesis of optically active sulfoxides 4: general procedure
To a solution of the appropriate Grignard reagent (3 mmol) in ethyl ether (20 mL) a solution of the
sulfinate 3 (1 mmol) in ethyl ether (5 mL) was added at room temperature. The mixture was stirred for
a few hours and then worked up by quenching with a 5% sulfuric acid solution (10 mL). The organic
layer was separated and the aqueous solution was extracted with chloroform (3×15 mL). The combined
organic solutions were washed successively with 5% solution of sodium carbonate (5 mL) and water (5
mL) and dried over anhydrous magnesium sulfate. Evaporation of the solvent gave the crude product
which was purified by preparative thin layer chromatography. The spectroscopic data of the isolated

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sulfoxides 4 were identical with those of authentic samples of racemic mixtures. Specific rotations and
enantiomeric excesses of the sulfoxides 4 obtained in these reactions are collected in Table 4.
3.8. Rearrangement of sulfinates 3 into the corresponding sulfones
3.8.1. N-Methylephedrine p-toluenesulfinate 3h
This sulfinate underwent instantaneous rearrangement to the sulfone 5h when the crude reaction
product was purified on a silica gel column: mp 72–79°C (dec); [α]589=−138.3 (c 0.6, CHCl3); 1H NMR
δ: 1.459 (d, J=6.32 Hz, 3H), 2.024 (s, 6H), 2.261 (s, 3H), 3.720 and 3.750 (2q, J=6.20 Hz, 1H), 4.042
and 4.090 (2s, 1H), 6.90–7.30 (m, 9H); 13C NMR δ: 11.782, 21.479, 39.899, 58.235, 127.832, 128.564,
128.870, 130.114, 134.039, 143.683. Anal. calcd for C18H23NO2S: C, 68.08; H, 7.31; N, 4.41; S, 10.10.
Found: C, 68.39; H, 7.53; N, 4.39; S, 10.37.
3.8.2. N-Methylephedrine t-butanesulfinate 3f
To a solution of (−)-(S)-sulfinate (0.141 g, 0.5 mmol) in ethyl ether (20 mL) was added silica gel (2 g,
Merck 60–230 mesh) and the heterogenous mixture was stirred at room temperature for 72 h, after which
time TLC indicated disappearance of the substrate. The silica gel was filtered off and washed with ethyl
1
ether (2×20 mL). The solvent was removed and the H NMR spectrum of the residue was measured
to establish the product ratios. To separate the sulfone 5f from t-butyl benzyl sulfone 6 the residue left
after removal of the solvent was redissolved in ethyl ether (30 mL). This solution was washed with a 5%
solution of sulfuric acid (20 mL). The ether phase was washed with a 5% solution of sodium bicarbonate,
then water and dried over anhydrous magnesium sulfate. Evaporation of the solvent gave virtually pure
(TLC, 1H NMR assay) benzyl t-butyl sulfone 6 (0.035 g). The aqueous solution was made strongly basic
by the addition of a 10% solution of sodium hydroxide and extracted with ethyl ether (3×15 mL). The
organic solution was dried over anhydrous magnesium sulfate. Careful removal of the solvent gave the
virtually pure sulfone 5f (0.07 g, 50%): mp 73–76°C (dec); 1H NMR δ: 1.086 (s, 9H), 1.330 (d, J=6.74
Hz, 3H), 2.052 (g, 6H), 3.721 (quintet, J=6.74 Hz, 1H), 4.118 (d, J=7.41 Hz, 1H), 7.27–7.55 (m, 5H).
Anal. calcd for C15H25NO2S: C, 63.54; H, 8.89; N, 4.94; S, 11.92. Found: C, 63.86; H, 9.03; N, 5.16; S,
12.17.
Acknowledgements
One of us (J.D.) would like to thank the CNR Rome for a two-month visiting professor fellowship at
Bologna University, and the State Committee for Scientific Research for partial financial support (grant
no. 3T09A 077 14).
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