Synthesis of C3- and C2-symmetric tris- and bis-sulfoxide ligands by asymmetric oxidation

Article abstract of DOI:10.1016/j.tet.2011.02.023

A series of tris- and bis-sulfoxides were synthesized by multiple asymmetric oxidation of the corresponding sulfides. High enantioselectivities were obtained based on Horeau-type amplification of selectivity. Naphthyl-substituted sulfoxides allowed for higher selectivity and greater ease of purification. These sulfoxides were tested as ligands in rhodium-catalyzed olefin hydroacylation and 1,4-addition of phenyl boronic acid to 2-cyclohexen-1-one. While no enantioinduction was observed in hydroacylation, up to 80% ee was obtained for a tris-sulfoxide and a bis-sulfoxide ligand in the 1,4-addition. Bis-sulfoxides with flexible backbones gave lower and inversed enantioselectivity, suggesting backbone rigidity plays a key role in enantioinduction.

Full text of DOI:10.1016/j.tet.2011.02.023

Tetrahedron 67 (2011) 4378e4384
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of C3- and C2-symmetric tris- and bis-sulfoxide ligands by asymmetric
oxidation
*
Peter K. Dornan, Priscilla L. Leung, Vy M. Dong
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, Canada M5S 3H6
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of tris- and bis-sulfoxides were synthesized by multiple asymmetric oxidation of the corre-
sponding sulfides. High enantioselectivities were obtained based on Horeau-type amplification of se-
lectivity. Naphthyl-substituted sulfoxides allowed for higher selectivity and greater ease of purification.
These sulfoxides were tested as ligands in rhodium-catalyzed olefin hydroacylation and 1,4-addition of
phenyl boronic acid to 2-cyclohexen-1-one. While no enantioinduction was observed in hydroacylation,
up to 80% ee was obtained for a tris-sulfoxide and a bis-sulfoxide ligand in the 1,4-addition. Bis-sulf-
oxides with flexible backbones gave lower and inversed enantioselectivity, suggesting backbone rigidity
plays a key role in enantioinduction.
Received 13 December 2010
Received in revised form 2 February 2011
Accepted 9 February 2011
Available online 3 March 2011
Keywords:
Sulfoxide
Tridentate ligand
Asymmetric oxidation
1,4-Addition
Ó 2011 Elsevier Ltd. All rights reserved.
Hydroacylation
1. Introduction
a favorable interaction between the substrate and the oxygen of the
sulfoxide leads to high levels of enantioselectivity.11 White has used
Incorporating symmetry in the design of chiral ligands has
resulted in a large number of privileged ligand architectures for
asymmetric catalysis.1 While C2 symmetry has dominated the field,
there is growing evidence that C3-symmetric ligands can favorably
decrease the number of possible stereoisomeric intermediates/
transition states in octahedral environments.2e4 The synthesis of
C3-symmetric chiral ligands is challenging due to the scarcity of
chiral C3-symmetric scaffolds on which to build ligands. Herein, we
present the design and synthesis of C3-symmetric tripodal sulfox-
ide ligands and a series of related C2-symmetric sulfoxide ligands.
In initial studies, we tested these ligands in rhodium-catalyzed
hydroacylation and conjugate addition reactions. Although
enantioinduction was not observed for hydroacylation, these novel
ligands promote conjugate addition with good levels of enantio-
control (up to 80% ee).
bis-sulfoxide ligands in palladium-catalyzed allylic CeH activa-
tion.14 Moreover, our group has shown that sulfoxides are excellent
directing groups in diastereoselective rhodium-catalyzed intra-
molecular olefin hydroacylation (Fig. 1).15
Fig. 1. Representative bis-sulfoxide ligands; (i) James’ dios ligand for asymmetric olefin
hydrogenation6 (ii) Dorta’s p-tol-BINASO ligand for asymmetric 1,4-additions of aryl
boronic acids10 (iii) White’s palladium bis-sulfoxide catalyst for allylic CeH
activation.14
Sulfoxide ligands5 are increasingly attractive targets in asym-
metric transition metal catalysis. Early work in this field demon-
strated modest levels of enantioselectivity in hydrogenation,6
DielseAlder,7 and allylic substitution8,9 reactions. Recently,
Dorta,10,11 Liao,12 and Li13 have shown that bis-sulfoxide ligands can
induce excellent levels of enantioselectivity in 1,4-additions of aryl
A common strategy for the synthesis of chiral bis-sulfoxides is
the nucleophilic substitution of chiral sulfinate esters with organ-
ometallic reagents.16 Licini17 and Siedlecka18 have reported, how-
ever, promising results in the synthesis of chiral bis-sulfoxides via
double catalytic asymmetric oxidation of the corresponding bis-
sulfide. In this approach, only a catalytic amount of chiral material
is needed and Horeau-type19 amplification of enantioselectivity can
in principle give highly enantioenriched material.
boronic acids to a,b-unsaturated ketones. It has been proposed that
* Corresponding author. Tel.: þ1 416 978 6484; e-mail address: vdong@chem.-
utoronto.ca (V.M. Dong).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.02.023

P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
4379
In considering possible designs for a C3-symmetric chiral ligand,
we imagined using sulfoxides as a simple way to incorporate chi-
rality into a tripodal scaffold (Fig. 2). An alkylation of commercially
available trichloride 3 followed by a triple asymmetric oxidation
would provide the chiral C3-symmetric tris-sulfoxide. Together
with the growing utility of sulfoxide ligands in transition metal
catalysis, this ligand could have multiple applications. This syn-
thetic strategy can also be applied to the synthesis of analogous bis-
sulfoxides in order to probe the effect of the third arm of the ligand.
3,5-diiodosalicylaldehyde.20,21 We were pleased to find that this
asymmetric oxidation system reversed the dr to 1.8:1 with the
major homo diastereomer being formed in 94% ee. Under the as-
sumption of an independent oxidation mechanism, 94% ee would
be obtained with a dr of 0.8:1 homo/hetero. The deviation from this
dr indicates a small amount of substrate control in subsequent
oxidations, however the oxidation is still largely catalyst controlled
(see Supplementary data for details). Unfortunately, the separation
of the diastereomers was not possible on large scale. Furthermore,
the product was a viscous liquid and recrystallization was not an
option to further upgrade the optical purity.
Fig. 2. Retrosynthesis of tris-sulfoxide 1.
2. Results and discussion
2.1. Design and synthesis of a C3-symmetric ligand
The oxidation of cyclohexyl tris-sulfide 2b (Table 1, entry 2) was
poorly enantioselective (32% ee), despite non-statistical diaster-
eoselectivity (1.8:1 homo/hetero). The Davis oxaziridine 5 is known
to give high levels of enantioselectivity for certain dialkyl sulfides,
which are typically problematic substrates for asymmetric sulfide
oxidation.22,23 Only a modest improvement in enantioselectivity
was observed in this case (35% ee).
Next we examined naphthyl substituents in the synthesis of tris-
sulfoxides. Both 1-naphthyl and 2-naphthyl derivatives (2c and 2d)
gave high levels of enantioselectivity (97% and 99% ee, respectively)
with good dr (3.1:1 homo/hetero in both cases). Moreover, both
products were isolable by column chromatography and could be
obtained in 40e60% yield. The synthesis requires only two steps,
with the first being a high yielding alkylation (>90%), and thus
represents a practical synthesis of a C3-symmetric chiral ligand. A
single crystal of 1c suitable for X-ray diffraction confirmed the (S)
absolute stereochemistry of the sulfoxides in accordance with lit-
erature precedence for asymmetric oxidation of alkyl aryl sulfides
(Fig. 3).20 The naphthyl based ligands 1c and 1d thus resulted in the
most scalable ligand synthesis.
Enhancing enantiomeric excess by preparing and then sepa-
rating diastereomeric mixtures was first demonstrated by Horeau
in 1973.19 Inspired by this concept, we postulated that upon
asymmetric oxidation of tris-sulfide 2, the formation of three chiral
centers would lead to significant enhancements of enantiose-
lectivity with respect to the enantioselectivity of a single oxidation.
The resulting homochiral diastereomer (RRR and SSS) could then be
separated from the undesired heterochiral diastereomer (RRS and
SSR), and material resulting from undesired catalyst selectivity
could be removed by chromatography.
We prepared a series of tridentate sulfoxides with various
substituents (Table 1). These ligands varied in their ease of isolation
and scalability. We began our studies by examining the oxidation of
phenyl tris-sulfide 2a (readily available by alkylation of the corre-
sponding trichloride with thiophenol, see Supplementary data).
With an achiral oxidant, and assuming each oxidation is in-
dependent, one would expect the statistical diastereomeric ratio to
be 1:3 homo/hetero. Using mCPBA as the oxidant, a dr of 1:2.5
homo/hetero was observed (Table 1, entry 1). This diaster-
eoselectivity validates the independent nature of the oxidations
because the stereochemistry of a sulfoxide in an intermediate does
not significantly influence the stereochemical outcome of the next
sulfide oxidation. Next, we applied Jackson’s sulfide oxidation
catalyst using ligand
4
derived from tert-leucinol and
Table 1
Synthesis of tris-sulfoxides by triple oxidation of tris-sulfides
Fig. 3. (i) Molecular structure of 1c. (ii). Molecular structure of 7d. The thermal el-
lipsoids are drawn at the 50% probability level.
Entry
R
1
Yield homo-1 (%)
dra
ee (%)
2.2. C2-Symmetric bis-sulfoxide ligands
1
Ph
Cy
Cy
1-Nap
2-Nap
1a
1b
1b
1c
1d
21
44b
76d
40
1.8:1 (1:2.5)
1.8:1 (1:2.6)
3.7:1 (1:2.6)
3.7:1 (1:1.8)
3.1:1 (1:2.5)
94
32b
35
97
99
2
In addition to the C3-symmetric tris-sulfoxides, we decided to
investigate C2-symmetric bis-sulfoxides to compare their efficien-
cies and selectivities as ligands. Because the third sulfoxide arm and
the methyl group in ligands 1a and 1b reduce the flexibility of the
three-carbon backbone linker between the sulfoxides, a ligand was
devised to have a similar rigidity, in order to make a direct com-
parison between the tris-sulfoxide and bis-sulfoxide series. Ligand
3c
4
5
60
a
Homo-3:hetero-3, dr with mCPBA in parentheses.
See Section 4.3.2.2.
Using 5 as oxidant.
b
c
d
Mixture of diastereomers.

4380
P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
7a, which contains a quaternary carbon center, was synthesized in
an analogous fashion to 1a. The enantiomeric excess of the reaction
mixture in this double asymmetric oxidation was 95% (Table 2, 7a),
which is slightly less than for the single asymmetric oxidation of 2-
naphthylmethylsulfide (98% ee, under identical oxidation condi-
tions). This decrease in stereoselectivity is likely due to the pres-
ence of the quaternary carbon center, which increases the steric
bulk of the alkyl side. Column chromatography was successful in
separating the diastereomers, and one recrystallization resulted in
an upgrade of the enantiomeric excess from 95% to >99% ee, but
a modest overall yield (38%).
a bulkier aryl group on the sulfoxide. Perhaps steric clash between
arms of the ligand in 1c contributed to an unfavorable coordination
geometry. Ligand 7a, which was designed to have a similar back-
bone rigidity to 1d, gave identical enantioselectivity to 1d. This
result supports the idea that the third arm of the tris-sulfoxide li-
gand is a spectator in this particular reaction. Ligands 7bee pro-
vided lower levels of enantioselectivity (0e43% ee), however, there
was a reversal of the observed sense of enantioinduction despite
having the same sulfoxide configuration. Moreover, the enantio-
selectivity increased as the bite angle increased. The reason for this
inversion of selectivity is not known at present, however, we
speculate that flexibility in the backbone might lead to alternative
coordination geometries (O- vs S-binding).
Table 2
Synthesis of bis-sulfoxides by double oxidation of bis-ssulfides
Table 3
Rhodium-catalyzed 1,4-addition reactiona
Entry
R
X
7
Yield homo-7 (%) dra,b
2-Nap CH2C(Me)2CH2 7a 38 9.8:1 (1:1.8) 95
Ph CH2CH2 7b
eeb (%)
1
2
3
4
5
9
5.2:1 (1:4.0) 99
4.0:1 (1:1.2) 98
4.6:1 (1:1.0) 98
4.3:1 (1:1.0) 98
2-Nap (CH2)3
2-Nap (CH2)4
2-Nap (CH2)5
7c 12
7d 49
7e 22
Entry
Ligand
Time
Yield (%)
ee (%)
1
2
3
4
5
6
7
1c
1d
7a
7b
7c
7d
7e
24
24
18
18
18
18
18
15
72
89
82
66
35
15
50
80
80
a
Homo-7:hetero-7, dr with mCPBA in parentheses.
Stereoselectivities are reported for the crude materialdpurification by column
b
chromatography and recrystallization yielded enantio- and diastereo-pure material.
0
ꢀ20
ꢀ31
ꢀ43
To probe the effect of backbone flexibility, ligands with linear al-
kane backbones were synthesized (Table 2, 7bee). While the ligands
were all obtained in high enantioselectivities (98e99% ee), they var-
ied significantly in their ease of isolation. Diastereomers were not
separable by column chromatography: two recrystallizations were
generally necessary to obtain diastereomerically pure ligand. Because
the meso diastereomers tended to crystallize more easily than the
desired homo diastereomer, low mass recovery of the desired di-
astereomer was obtained. A single crystal of 7d suitable for X-ray
diffraction confirmed the (S) absolute stereochemistry of the sulfox-
ide, which matches the stereochemistry of 1c (Fig. 3). Therefore, the
asymmetric oxidation occurs with the same sense of enantioinduc-
tion for the tris-sulfoxides and bis-sulfoxides.
a
Conditions: 2-cyclohexen-1-one (1.0 equiv), phenyl boronic acid (4 equiv),
[Rh(C2H4)2Cl]2 (1.5 mol %), ligand (3.6 mol %), KOH (2.5 M, 50 mol %), EtOAc, 40 ꢁC.
3. Conclusion
In summary, a series of chiral tridentate and bidentate sulfoxide
ligands have been synthesized by multiple asymmetric oxidation.
The synthesis of naphthyl tris-sulfoxides was particularly efficient
due to the high intrinsic enantioselectivity of the oxidation and the
ease of separating the resulting diastereomers by column chro-
matography. The ligands were tested in a rhodium-catalyzed
intramolecular olefin hydroacylation and a rhodium-catalyzed 1,4-
addition of phenyl boronic acid to 2-cyclohexen-1-one. No enan-
tioselectivity was observed in hydroacylation. On the other hand,
ligands 1d and 7a provided good levels of enantioinduction (80%
ee) in the 1,4-addition. An interesting reversal of enantioinduction
in the 1,4-addition was observed in the case of flexible bis-sulfox-
ides. Future work in this area will include exploring the use of these
readily available sulfoxide ligands for other catalytic applications.
2.3. Sulfoxide ligands in catalysis
Tris-sulfoxide 1d and bis-sulfoxide 7d were tested in the rhodium-
catalyzed intramolecular hydroacylation of 2-(1-phenylvinyl)
benzaldehyde (Scheme 1). Morehead has previously reported that
using 2 mol % [Rh((R)-BINAP)]ClO4 in dichloromethane at room
temperature, a 98% yield of the indanone product could be obtained
in 98% ee.24 Both ligands resulted in full conversion to the desired
hydroacylation product after 20 h, with no decarbonylation. How-
ever, no enantioenrichment of the products could be detected.
4. Experimental section
4.1. General considerations
Commercial reagents were purchased from SigmaeAldrich,
Strem or Alfa Aesar and used without further purification. All re-
actions were carried out under an argon atmosphere unless oth-
erwise indicated. Reactions were monitored using thin-layer
chromatography (TLC) on EMD Silica Gel 60 F254 plates and a Wa-
ters 2795 LCeMS. Visualization of the developed plates was per-
formed under UV light (254 nm) or KMnO4 stain. Organic solutions
Scheme 1. Intramolecular olefin hydroacylation with tris-sulfoxide 1d.
€
were concentrated under reduced pressure on a Buchi rotary
evaporator. 1H and 13C NMR spectra were recorded on a Varian
Mercury 300, Varian Mercury 400, VRX-S (Unity) 400, or Bruker
AV-III 400 spectrometer. NMR spectra were internally referenced to
the residual solvent signal or TMS. Data for 1H NMR are reported as
We next examined the rhodium-catalyzed 1,4-addition of phe-
nyl boronic acid to 2-cyclohexen-1-one. While ligand 1d was able
to achieve product in 80% ee and 72% yield, ligand 1c gave only 50%
ee and low yield (15%) (Table 3, entries 1 and 2), despite possessing
follows: chemical shift (
d
ppm), multiplicity (s¼singlet, d¼doublet,

P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
4381
t¼triplet, q¼quartet, m¼multiplet, br¼broad), coupling constant
4.2.2. General procedure for synthesis of bis-sulfides. To a stirred
solution of diol (19.7 mmol) in CH2Cl2 in a water bath at room
temperature were added triethylamine (59.1 mmol, 5.98 g,
8.24 mL) and methanesulfonyl chloride (49.3 mmol, 5.64 g,
3.84 mL). The cloudy red solution was stirred for 12 h, then
quenched with aqueous 1 M HCl (20 mL). The aqueous phase was
extracted twice with CH2Cl2 (50 mL). The combined organic layers
were dried over Na2SO4 and concentrated in vacuo to yield the
bismesylate, which was used without further purification. To a so-
lution of the bismesylate (4.07 mmol) in acetone (20 mL) were
added 2-thionaphthol (9.36 mmol, 1.5 g) and potassium carbonate
(12.2 mmol, 1.7 g). The resulting thick slurry was stirred for 12 h at
room temperature. The crude mixture was then diluted with ethyl
acetate (50 mL) and the organic extracts were washed with aque-
ous 1 M NaOH (20 mL) and brine (2ꢂ20 mL). The organic phase was
dried over Na2SO4 and concentrated to dryness. The resulting solid
was triturated with ether (2ꢂ5 mL) to yield the title compound as
a white solid.
(Hz), integration. Data for 13C NMR are reported in terms of
chemical shift (d ppm).
High resolution mass spectra (HRMS) were obtained on
a micromass 70S-250 spectrometer (EI) or an ABI/Sciex QStar Mass
Spectrometer (ESI). Infrared (IR) spectra were obtained on a Per-
kineElmer Spectrum 1000 FT-IR Systems and are reported in
wavenumber (cmꢀ1). Melting point ranges were determined on
a Fisher-Johns Melting Point Apparatus. Enantiomeric excesses
(ee’s) were ascertained on an Agilent 1100 Series HPLC. Di-
astereomeric ratios were determined by integration of crude 1H
NMR spectra. Optical rotations were measured on a Rudolph Re-
search Analytical Autopol IV Automatic Polarimeter. Column
chromatography was performed with Silicycle Silia-P Flash Silica
Gel, using either glass columns or a Biotage SP-1 system. All salts
were purchased from Aldrich and used without purification. Sol-
vents were purchased from Caledon and were purified according to
standard procedures.
4.2.2.1. 1,3-Bis(naphthalen-2-ylthio)propane 6c. Prepared accord-
ing to the general procedure for bis-sulfides. Propane-1,3-diol bis-
methanesulfonate was synthesized from propane-1,3-diol to yield
a red oil (4.05 g, 88%). Alkylation then yielded the title compound as
a white solid (1.06 g, 72%); mp 85e88 ꢁC. 1H NMR (400 MHz, CDCl3)
4.2. Synthesis of sulfides
4.2.1. General procedure for synthesis of tris-sulfides. Sodium hy-
dride (6 equiv, 60% dispersion in oil) and sodium iodide (1 equiv)
were dissolved in N,N-dimethylformamide (0.1 M) in a flame dried
round bottom flask. The appropriate thiol (6 equiv) was added
dropwise, followed by 2,2,2-tris(chloromethyl)-ethane (1 equiv).
The mixture was stirred at 100 ꢁC for 24 h. The crude mixture was
diluted with ethyl acetate, washed twice with 1:1 1 M NaOH/brine
and the combined organic extracts were dried over Na2SO4. The
solvent was removed in vacuo. The tris-sulfide product was purified
as described below for each entry.
d
2.06 (quint, J¼7.0 Hz, 2H), 3.18 (t, J¼7.0 Hz, 4H), 7.37e7.47 (m, 6H),
7.64e7.79 (m, 8H); 13C NMR (100 MHz, CDCl3)
d
28.3, 32.3, 125.7,
126.5, 127.0, 127.2, 127.4, 127.7, 128.4, 131.8, 133.5, 133.7; IR (neat):
1499,1133,1071, 852, 816, 737 cmꢀ1; HRMS (EI) calcd for [C23H20S2]þ
360.1006, found 360.1010.
4.2.2.2. 1,4-Bis(naphthalen-2-ylthio)butane 6d. Prepared accord-
ing to the general procedure for bis-sulfides. Butane-1,4-diol bis-
methanesulfonate was prepared from butanediol to yield a brown
solid (2.57 g, 55%). Alkylation then provided the title compound as
a white solid (945 mg, 70%); mp 168e170 ꢁC. 1H NMR (400 MHz,
4.2.1.1. 2,2,2-Tris((phenylthio)methyl)-ethane
2a. Prepared
according to the general procedure for tris-sulfides. The product was
isolated as a clear oil (480 mg, 95%) by column chromatography
eluting with 2e5% ethyl acetate in hexanes (Rf¼0.5 in 5% ethyl ac-
CDCl3)
d
1.85e1.95 (m, 4H), 3.01e3.07 (m, 4H), 7.38e7.49 (m, 6H),
28.1, 33.1, 125.6,
7.70e7.80 (m, 8H); 13C NMR (100 MHz, CDCl3)
d
etate in hexanes). 1H NMR (400 MHz, CDCl3)
d
1.17 (s, 3H), 3.17 (s,
126.5, 126.9, 127.0, 127.4, 127.7, 128.4, 131.7, 133.8, 144.6; IR (neat):
2931, 1584, 1498, 1453, 1129, 1068, 849, 812, 737 cmꢀ1; HRMS (EI)
calcd for [C24H22S2]þ 374.1163, found 374.1160.
6H), 7.15 (t, J¼7.2 Hz, 3H), 7.24 (t, J¼7.4 Hz, 6H), 7.33 (d, J¼8.1 Hz, 6H);
13C NMR (100 MHz, CDCl3)
d 23.9, 41.5, 43.6, 126.1, 128.9, 129.7, 137.0;
IR (neat): 2961, 1582, 1479, 1438, 1088, 1024, 733, 688 cmꢀ1; HRMS
(EI) calcd for [C23H24S3]þ 396.1040, found 396.1039.
4.2.2.3. (2,2-Dimethylpropane-1,3-diyl)bis(naphthalen-2-ylsul-
fane) 6a. To a solution of 2,2-dimethyl-1,3-propanediol (29 mmol,
3.0 g) in CH2Cl2 (25 mL) were added pyridine (4 mL) and toluene-
sulfonyl chloride (72 mmol,13.7 g). The mixture was stirred at room
temperature for 24 h. Another 5.5 g (29 mmol) of tosyl chloride was
added and the reaction was stirred for another 24 h. The crude
mixture was diluted with ethyl acetate (75 mL) and washed with
aqueous 1 M HCl (2ꢂ20 mL), and brine (20 mL). The organic phase
was dried over Na2SO4 and concentrated to dryness. 2,2-dimethyl-
1,3-propanediol bistosylate was isolated by column chromatogra-
phy eluting with 30% ethyl acetate in hexanes as a white solid
(3.15 g, 26%). To a flame dried round bottom flask were added NaH
(60% dispersion in mineral oil) (8 mmol, 321 mg) and NaI
(1.6 mmol, 242 mg). A solution of 2-thionaphthol (7.3 mmol, 1.17 g)
in DMF (20 mL) was then added, followed by 2,2-dimethyl-1,3-
propanediol bistosylate (2.4 mmol, 1.0 g). The mixture was stirred
at 100 ꢁC for 24 h. The crude mixture was diluted with ethyl acetate
(100 mL) and washed with aqueous 1 M NaOH (20 mL). The organic
phase was then washed with brine (20 mL), and the aqueous phase
was extracted with ethyl acetate (40 mL). The combined organic
extracts were dried over Na2SO4 and concentrated to dryness. The
product was purified by column chromatography eluting with
0e10% ethyl acetate in hexanes to yield a white solid (733 mg, 79%);
4.2.1.2. 2,2,2-Tris((cyclohexylthio)methyl)-ethane 2b. Prepared
according to the general procedure for tris-sulfides. The product
was isolated as a clear oil (1.1 g, 93%) by column chromatography
eluting with 5% ethyl acetate in hexanes (Rf¼0.9 in 10% ethyl acetate
in hexanes). 1H NMR (400 MHz, CDCl3)
d 1.07 (s, 3H), 1.20e1.37 (m,
15H), 1.56e1.64 (m, 3H), 1.71e1.79 (m, 6H), 1.93e2.02 (m, 6H),
2.58e2.66 (m, 3H), 2.66 (s, 6H); 13C NMR (100 MHz, C6D6)
d
24.3,
26.2, 26.3, 34.3, 40.1, 40.1, 45.3; IR (neat): 2924, 2850, 1447, 1262,
1200, 999 cmꢀ1; HRMS (EI) calcd for [C23H42S3]þ 414.2449, found
414.2443.
4.2.1.3. 2,2,2-Tris((2-naphthylthio)methyl)-ethane 2d. Prepared
according to the general procedure for tris-sulfides. Upon addition
of 1:1 diethyl ether/acetone to the crude concentrated mixture
after aqueous workup, a white solid precipitated. This solid was
isolated by filtration and washed with diethyl ether to yield the
product as a white solid (2.83 g, 91%); mp 89e92 ꢁC. 1H NMR
(400 MHz, CDCl3)
7.53e7.58 (m, 3H), 7.62e7.67 (m, 6H), 7.70e7.74 (m, 3H); 13C NMR
(100 MHz, CDCl3) 24.1, 41.8, 43.4, 125.6, 126.4, 127.0, 127.3, 127.6,
d 1.29 (s, 3H), 3.33 (s, 6H), 7.35e7.44 (m, 9H),
d
127.6, 128.3, 131.7, 133.6, 134.3; IR (neat): 3054, 1584, 1499, 1131,
1071, 808, 734 cmꢀ1; HRMS (EI) calcd for [C35H30S3]þ 546.1510,
found 546.1489.
mp 79e81 ꢁC. 1H NMR (400 MHz, CDCl3)
7.36e7.46 (m, 6H), 7.60e7.76 (m, 8H); 13C NMR (100 MHz, CDCl3)
d 1.19 (s, 6H), 3.18 (s, 4H),

4382
P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
d
26.9, 37.2, 45.7, 125.5, 126.4, 126.8, 127.0, 127.5, 127.6, 128.3, 131.6,
4.3.2.3. 2,2,2-Tris((1-naphthylsulfinyl)methyl)-ethane 1c. 2,2,2-
133.7, 135.0; IR (neat): 1587, 1132, 1072, 846, 820, 812, 737 cmꢀ1
HRMS (EI) calcd for [C25H24S2]þ 388.1319, found 388.1329.
;
Tris((1-naphthylthio)methyl)-ethane was synthesized according to
general procedure for the synthesis of tris-sulfides (reaction time:
2.5 days, 0.78 mmol 2,2,2-tris(chloromethyl)-ethane). Purification
was difficult, therefore the crude material was subjected to asym-
metric oxidation directly according to the general procedure for
4.3. Synthesis of sulfoxides
4.3.1. General procedure for racemic oxidation for the development of
chiral assays. Sulfide (1 equiv) was dissolved in CH2Cl2 (1 M) and the
solution was cooled to 0 ꢁC. mCPBA (3.05 equiv for tris-sulfides,
2.05 equiv for bis-sulfides; 74% by weight) was then added, and the
reaction mixture was stirred at 0 ꢁC for 30 min. The crude mixture
was poured into brine and aqueous 1 M NaOH was added to basify.
This mixture was extracted twice with CH2Cl2 and the organic ex-
tracts were dried over Na2SO4. The solvent was removed in vacuo.
The homochiral diastereomer was isolated by preparative TLC. For
ligands 7bee, the meso diastereomer could not be removed by
chromatography, but separationwas achieved during HPLC analysis.
asymmetric oxidation (reaction time: 6 h, 287 mg crude,
0.535 mmol (assuming pure tris-sulfide)). The product was isolated
by column chromatography (60% ethyl acetate in hexanes) to give
a white solid (110 mg, 35% over two steps). A single crystal suitable
for X-ray diffraction was grown by slow evaporation from a solution
of 60% ethyl acetate in hexanes. Mp 190 ꢁC (decomposed). 1H NMR
(400 MHz, CDCl3)
d
2.37 (s, 3H), 3.36 (d, J¼14.0 Hz, 3H), 3.75 (d,
J¼14.0 Hz, 3H), 7.66e7.56 (m, 9H), 7.95 (d, J¼8.3Hz, 3H), 7.97 (d,
J¼8.2 Hz, 3H), 8.09 (d, J¼8.4 Hz, 3H), 8.14 (d, J¼7.3 Hz, 3H); 13C NMR
(100 MHz, CDCl3)
d 24.6, 40.4, 65.2, 121.9, 123.7, 126.0, 127.1, 127.9,
128.8, 129.3, 131.8, 133.8, 140.1; IR (neat): 3049, 2955, 1503, 1379,
1045, 808, 772, 743 cmꢀ1; HRMS (ESIþ) calcd for [C35H30S3O3H]þ
D25
4.3.2. General procedure for asymmetric oxidation. Ligand
4
595.1429, found 595.1406. [
sis: 97% ee (AD-H, 4:6 isopropanol/hexanes, 1 mL/min, 254 nm,
tR1¼17.5 min (minor, R), tR2¼41.9 min (major, S)).
a
]
ꢀ783 (c 0.24, CHCl3). HPLC analy-
(8 mol % for tris-sulfides, 5 mol % for bis-sulfides; synthesized
according to Pelotier20), was dissolved in one third of total volume
CH2Cl2 to give a yellow solution. VO(acac)2 (4 mol % for tris-sulfides,
2.5 mol % for bis-sulfides) was dissolved in another equal portion of
CH2Cl2, and this solution was added to the first solution. The mix-
ture was initially greenish-blue, and after stirring at room tem-
perature for 30 min, the solution turned green-brown. The tris-
sulfide (1 equiv, 0.17 M final concentration) was added in the final
portion of CH2Cl2, and this was stirred at room temperature for
30 min. The reaction mixture was then cooled to 0 ꢁC and 30% H2O2
was added dropwise (3.2 equiv for tris-sulfides, 2.4 equiv for bis-
sulfides). The reaction mixture was stirred at 0 ꢁC until the first
appearance of over-oxidation as judged by LCeMS analysis. The
reaction mixture was quenched by addition of sodium thiosulfate,
and extracted twice with CH2Cl2. The organic layer was dried over
Na2SO4, and the solvent was removed in vacuo.
4.3.2.4. 2,2,2-Tris((2-naphthylsulfinyl)methyl)-ethane
1d.
Synthesized according to the general procedure for asymmetric
oxidation (reaction time: 3 h, 1.41 mmol tris-sulfide). The product
was isolated by column chromatography (65% ethyl acetate in hex-
anes) to give a white solid (500 mg, 60%); mp 87e90 ꢁC. 1H NMR
(400 MHz, CDCl3)
d
2.11 (s, 3H), 3.21 (d, J¼14.0 Hz, 3H), 3.71 (d,
J¼13.9 Hz, 3H), 7.55e7.63 (m, 6H), 7.74 (dd, J¼8.6, 1.8 Hz, 3H),
7.89e7.95 (m, 6H), 8.00 (d, J¼8.6 Hz, 3H), 8.26 (s, 3H); 13C NMR
(100 MHz, CDCl3)
d 25.3, 39.6, 67.5, 120.0, 124.6, 127.3, 127.9, 128.0,
128.6, 129.8, 132.9, 134.5, 140.9; IR (neat): 2957, 1714, 1504, 1345,
1066, 1032, 812, 744 cmꢀ1; HRMS (ESIþ) calcd for [C35H30S3O3H]þ
595.1429, found 595.1434. [
a
]
ꢀ383 (c 1.1, CHCl3). HPLC analysis:
D25
99% ee (AD-H, 4:6 isopropanol/hexanes, 1 mL/min, 254 nm,
tR1¼49.8 min (minor), tR2¼54.1 min (major)).
4.3.2.1. 2,2,2-Tris((phenylsulfinyl)methyl)-ethane 1a. Synthesized
according to the general procedure for asymmetric oxidation (re-
action time: 3 h, 0.151 mmol tris-sulfide). The product was isolated
by column chromatography (90% ethyl acetate in hexanes) to give
4.3.2.5. 2,20-(2,2-Dimethylpropane-1,3-diyldisulfinyl)dinaph-
thalene 7a. The title compound was prepared according to the
general procedure for asymmetric oxidation (reaction time: 6 h,
2.78 mmol bis-sulfide) to give a white solid. This solid was pu-
rified by column chromatography (65% ethyl acetate in hexanes)
and then recrystallized from boiling ethyl acetate to give a white
solid (441 mg, 38%); mp 155e157 ꢁC. 1H NMR (400 MHz, CDCl3)
a viscous semi-solid (14 mg, 21%). 1H NMR (400 MHz, CDCl3)
d 2.04
(s, 3H), 3.14 (d, J¼14.0 Hz, 3H), 3.53 (d, J¼14.0 Hz, 3H), 7.47e7.56 (m,
9H), 7.72 (dd, J¼8.0, 1.6 Hz, 6H); 13C NMR (100 MHz, CDCl3)
d 25.0,
39.5, 67.5, 124.1, 129.5, 131.2, 143.9; IR (neat): 3056, 2912, 1477, 1443,
1084, 1032, 998, 748, 688 cmꢀ1; MS (ESIþ) calcd for [C23H24O3S3H]þ
d
1.60 (s, 6H), 3.06 (d, J¼13.9 Hz, 2H), 3.12 (d, J¼13.9 Hz, 2H),
445.10, found 445.32. [
a
]
ꢀ237 (c 0.42, CHCl3). HPLC analysis: 94%
7.56e7.62 (m, 4H), 7.64 (dd, J¼8.6, 1.8 Hz, 2H), 7.89e7.96 (m, 4H),
D25
ee (OD-H,1:9 isopropanol/hexanes,1 mL/min, 254 nm, tR1¼34.3 min,
7.99 (d, J¼8.7 Hz, 2H), 8.24 (d, J¼1.5 Hz, 2H); 13C NMR (100 MHz,
tR2¼41.9 min).
CDCl3)
d 28.3, 36.0, 70.3, 119.9, 124.4, 127.3, 127.8, 128.0, 128.5,
129.6, 133.0, 134.4, 141.7; IR (neat): 1503, 1061, 1033, 812,
4.3.2.2. 2,2,2-Tris((cyclohexylsulfinyl)methyl)-ethane
1b.
744 cmꢀ1; HRMS (ESIþ) calcd for [C25H24O2S2þH]þ 421.1290,
D25
Synthesized according to the general procedure for asymmetric oxi-
dation (reaction time: 2.3 h, 0.723 mmol tris-sulfide). A small aliquot
of the crude material was purified by preparative TLC (8% MeOH in
CH2Cl2) to give material with 32% ee. The remaining crude material
was purified by recrystallization from ethyl acetate/methanol. The
resulting crystals were washed three times with cold ethyl acetate
and dried under vacuum to give light pink crystals (148 mg, 44%) of
low optical purity (5% ee). Mp 189e191 ꢁC. 1H NMR (400 MHz, CDCl3)
found 421.1293. [
a
]
ꢀ270 (c 1.1, CHCl3). HPLC analysis: >99% ee
(AD-H, 4:6 isopropanol/hexanes, 1 mL/min, 254 nm, tR1¼13.2 min
(minor), tR2¼24.1 min (major)).
4.3.2.6. 1,3-Bis(phenyl-2-sulfinyl)ethane 7b. The title compound
was prepared from 1,2-bis(thiophenyl)ethane according to the
general procedure for asymmetric oxidation (reaction time: 3 h,
1.62 mmol bis-sulfide; one extra equivalent of H2O2 (0.98 mmol)
was added after 2 h) to give a white solid. This solid was purified by
column chromatography (90% ethyl acetate in hexanes) and then
twice recrystallized from boiling acetone to give a crystalline white
solid (41 mg, 9%). This compound has been synthesized pre-
viously.18,25 Spectral data are in agreement with the literature.
HPLC analysis: >99% ee (AD-H, 4:6 isopropanol/hexanes, 1 mL/min,
233 nm, tR1¼6.4 min (major), tR2¼7.2 min (meso), tR3¼8.1 min
(minor)).
d
1.20e1.50 (m,15H), 1.69 (d, J¼11.1 Hz, 3H),1.81 (s, 3H),1.82e1.95 (m,
9H), 2.11 (t, J¼13.5 Hz, 3H), 2.60 (tt, J¼11.6 Hz, 3.4 Hz, 3H), 2.81 (d,
J¼13.9 Hz, 3H), 3.54 (d, J¼13.9 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d
23.4, 24.7, 25.1, 25.4, 25.4, 26.5, 38.2, 59.4, 60.2; IR (neat): 2935,
2854, 1441, 1398, 1070, 1023, 996 cmꢀ1; HRMS (ESIþ) calcd for
[C23H42O3S3H]þ 463.2368, found 463.2351. [
a]
D25
ꢀ2.9 (c 1.3, CHCl3,
32% ee). HPLC analysis: 32% ee (OD-H, 6% isopropanol in hexanes,
1 mL/min, 210 nm, tR1¼19.0 min, tR2¼24.0 min).

P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
4383
4.3.2.7. 1,3-Bis(naphthalen-2-ylsulfinyl)propane 7c. The title
compound was prepared according to the general procedure for
asymmetric oxidation from 1,3-bis(naphthalen-2-ylthio)propane
(2.32 mmol bis-sulfide, reaction time¼4.5 h, one extra equivalent
(2.32 mmol) H2O2 was added after 3 h). The product was isolated by
column chromatography in 70% ethyl acetate in hexanes, followed
by two recrystallizations from boiling acetone to yield a white solid
J¼8.6, 1.7 Hz, 2H), 7.55e7.60 (m, 4H), 7.87e7.92 (m, 4H), 7.94 (d,
J¼8.6 Hz, 2H), 8.14 (s, 2H); 13C NMR (100 MHz, CDCl3)
d 21.6, 27.7,
56.2, 119.7, 124.6, 127.3, 127.8, 128.0, 128.5, 129.5, 132.8, 134.4, 140.7.
IR(neat): 3050, 2935, 1070, 1029, 867, 813, 755, 742 cmꢀ1; HRMS
(ESI) calcd for [C25H24O2S2þH]þ 421.1290, found 421.1286. [
a]
D25
ꢀ193 (c 0.27, CHCl3). HPLC analysis: >99% ee (AD-H, 4:6 iso-
propanol/hexanes, 1 mL/min, 210 nm, tR1¼17.4 min (minor),
tR2¼20.4 min (major), tR3¼21.6 min (meso)).
(105 mg, 12%); mp 168e170 ꢁC. 1H NMR (400 MHz, CDCl3)
d 2.13
(quint, J¼7.4 Hz, 2H), 2.93 (dt, J¼14.0, 7.1 Hz, 2H), 3.11 (dt, J¼13.6,
7.7 Hz, 2H), 7.45 (dd, J¼8.6,1.8 Hz, 2H), 7.55e7.63 (m, 4H), 7.84e7.89
4.3.3. Procedure for the oxidation of 2b with the Davis oxazir-
idine. The Davis oxaziridine, N-(phenylsulfonyl)(3,3-dichlor-
ocamphoryl)oxaziridine, was synthesized according to a literature
procedure.22,23 Since the final step of the synthesis was low yielding,
a 0.97:1 mixture of the oxaziridine/sulfonylimine was used as the
oxidant. The oxidant mixture (200 mg, 100.4 mg oxaziridine,
0.276 mmol) wasdissolved in 2.5 mLCH2Cl2 and cooled to 0 ꢁC. 2,2,2-
Tris((cyclohexylthio)methyl)-ethane (41 mg, 0.099 mmol) was dis-
solved in 2.5 mL CH2Cl2, and this was added to the first flask. The
solution was stirred at 0 ꢁC for 40 h. The solvent was removed under
reduced pressure and 2,2,2-tris((cyclohexylsulfinyl)methyl)-ethane
was isolated by preparative TLC eluting with 8% MeOH in CH2Cl2 as
a 3.7:1 (homo/hetero) mixture of diastereomers (35 mg, 76%).
(m, 6H), 8.09 (d, J¼1.6 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d 15.5,
54.7, 119.8, 124.9, 127.6, 128.0, 128.3, 128.7, 129.7, 133.0, 134.6, 140.2;
HRMS (ESIþ) calcd for [C23H20O2S2þH]þ 393.0983, found 393.0972.
D25
[
a
]
ꢀ240 (c 0.9, CHCl3). HPLC analysis: >99% ee (AD-H, 4:6 iso-
propanol/hexanes, 1 mL/min, 254 nm, tR1¼16.3 min (major),
tR2¼18.1 min (minor), tR3¼20.5 min (meso)).
4.3.2.8. 1,4-Bis(naphthalen-2-ylsulfinyl)butane 7d. The title
compound was prepared according to the general procedure for
asymmetric oxidation from 1,4-bis(naphthalen-2-ylthio)butane
(2.32 mmol bis-sulfide, reaction time¼4.5 h). The product was
isolated by column chromatography in 70% ethyl acetate in hex-
anes, followed recrystallization from boiling acetone to yield
a white solid (460 mg, 49%). A single crystal suitable for X-ray
diffraction was grown by layering hexanes onto a dichloromethane
4.4. Rhodium-catalyzed olefin hydroacylation
solution. Mp 155e157 ꢁC. 1H NMR (400 MHz, CDCl3)
d
1.65e1.78 (m,
4.4.1. Procedure for olefin hydroacylation. 2-(1-Phenylvinyl)benz-
aldehyde was synthesized according to the procedure of More-
head.24 Inside a glovebox, 1d (0.005 mmol, 3.0 mg) was dissolved in
CH2Cl2 (1 mL) and added to [Rh(NBD)2]BF4 (0.005 mmol, 1.9 mg).
This solution was added to a Schlenk tube (AF-0096-01 from
Chemglass) containing a magnetic stir bar and the tube was sealed.
The tube was removed from the glovebox and placed on a Schlenk
line. The headspace in the tube was removed by one freeze-pump-
thaw cycle, and then H2 was flowed into the tube for 30 min. The
solution was then degassed by three freeze-pump-thaw cycles and
taken back into the glovebox. A solution of 2-(1-phenylvinyl)
benzaldehyde (0.1 mmol, 20.8 mg) was then added to the catalyst
solution, and the reaction was stirred at 100 ꢁC for 20 h. The crude
mixture was concentrated in vacuo and purified by column chro-
matography. Spectral data of the purified material are in agreement
with the literature.27
2H), 1.92e2.04 (m, 2H), 2.75e2.84 (m, 2H), 2.86e2.95 (m, 2H), 7.51
(dd, J¼8.6,1.8 Hz, 2H), 7.55e7.62 (m, 4H), 7.87e7.93 (m, 4H), 7.95 (d,
J¼8.7 Hz, 2H), 8.14 (d, J¼1.4 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d
21.3, 56.0, 119.7, 124.6, 127.4, 127.8, 128.0, 128.5, 129.5, 132.8, 134.4,
140.5; IR(neat): 1588,1345,1066,1040, 818, 750 cmꢀ1; HRMS (ESIþ)
calcd for [C24H22O2S2þH]þ 407.1134, found 407.1142. [
a]
ꢀ173 (c
D25
1.1, CHCl3). HPLC analysis: >99% ee (AD-H, 2:8 isopropanol/hex-
anes, 1.5 mL/min, 233 nm, tR1¼35.7 min (major), tR2¼37.3 min
(minor), tR3¼44.1 min (meso)).
4.3.2.9. 1,5-Bis(naphthalen-2-ylsulfinyl)pentane 7e. To 1,5-pen-
tanediol (2.0 g, 0.019 mol) was added 4 mL of pyridine and the
solution was stirred and cooled to 0 ꢁC. p-Toluenesulfonyl chloride
(7.2 g, 0.038 mol, 2 equiv) in 12 mL pyridine was added by addition
funnel over 30 min. The reaction mixture was stirred at 0 ꢁC for 4 h.
Addition of 20 mL of cold water precipitated a white solid. The
desired product was separated from the mono-tosylate (3:1 ratio)
by recrystallization from hot methanol, affording 2.54 g (32%) of
pentane-1,5-diyl bis(p-toluenesulfonate), as clear long crystals
whose characterization data are in agreement with literature.26
Pentane-1,5-diyl bis(p-toluenesulfonate) (2.54 g, 0.0061 mol), 2-
thionaphthol (2.44 g, 0.015 mol, 2.5 equiv), and potassium car-
bonate (2.5 g, 0.018 mol, 3 equiv) were dissolved in 50 mL of ace-
tone and stirred at 45 ꢁC for 24 h. The mixture was then filtered,
concentrated, taken up in dichloromethane, and washed with 1 M
NaOH and brine. The aqueous layer was washed twice with
dichloromethane. Combined organic layers were dried over
Na2SO4, and evaporated to afford a beige solid. 1H NMR analysis
indicated a 3:1 ratio of the desired bis-sulfide to the disulfide
resulting from oxidation of 2-thionaphthol, which was carried
through to the oxidation step without further purification.
4.5. Rhodium-catalyzed 1,4-addition
4.5.1. Procedure for asymmetric 1,4-addition of phenyl boronic acid
to cyclohex-2-ene-1-one. In
a glovebox, the sulfoxide ligand
(0.0136 mmol) was dissolved in ethyl acetate and added to
[Rh(C2H4)2Cl]2 (2.2 mg, 0.006 mmol) in ethyl acetate (3 mL total).
The resulting mixture was allowed to stir at room temperature for
20 min, upon which degassed 2-cyclohexen-1-one (36 mg,
0.375 mmol) and phenyl boronic acid (183 mg, 1.5 mmol, 4 equiv)
were added. The reaction flask was sealed with a rubber stopper
and brought out of the glovebox, upon which 2.5 M KOH(aq)
(0.075 mL, degassed) was added via syringe. The reaction was
performed under argon at 40 ꢁC and monitored by LCeMS. Upon
completion, the mixture was filtered through Celite, concentrated,
and purified by preparative TLC (eluent 30% ethyl acetate in hex-
anes). HPLC analysis: OD-H, 1:99 isopropanol:hexanes, 0.5 mL/min,
254 nm, tR1¼34.5 min (R), tR2¼38.9 min (S).
The title compound was prepared according to the general
procedure for asymmetric oxidation from the crude mixture of 1,3-
bis(naphthalen-2-ylthio)pentane (1.0 g crude material, reaction
time: 3.75 h). The off-white product was isolated by column
chromatography eluting with ethyl acetate. Two recrystallizations
from boiling acetone yielded a white solid (181 mg, 22%, calculated
based on the NMR determined ratio of bis-sulfide added in the
oxidation reaction); mp 104e105 ꢁC. 1H NMR (400 MHz, CDCl3)
Acknowledgements
We thank the University of Toronto, Canadian Foundation for
Innovation, Ontario Ministry of Research and Innovation, Boeh-
ringer Ingelheim Ltd. Canada, and Natural Sciences and Engineering
Research Council of Canada (NSERC) for funding. V.M.D is grateful
d
1.52e1.67 (m, 4H), 1.77e1.86 (m, 2H), 2.76e2.90 (m, 4H), 7.52 (dd,

4384
P.K. Dornan et al. / Tetrahedron 67 (2011) 4378e4384
4142; (d) Rendina, V. L.; Moebius, D. C.; Kingsbury, J. S. Org. Lett. DOI:10.1021/ol/
200402m.
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Asymmetry 2009, 20, 1953.
for an Alfred P. Sloan Fellowship and P.K.D for an NSERC Vanier
Canada Graduate Scholarship. We thank Dr. Alan Lough for X-ray
structure analysis.
Supplementary data
10. Mariz, R.; Luan, X. J.; Gatti, M.; Linden, A.; Dorta, R. J. Am. Chem. Soc. 2008,130, 2172.
11. Burgi, J. J.; Mariz, R.; Gatti, M.; Drinkel, E.; Luan, X. J.; Blumentritt, S.; Linden, A.;
Dorta, R. Angew. Chem., Int. Ed. 2009, 48, 2768.
NMR spectra and HPLC traces for all new compounds can be
found in the supplementary data available in the online version.
Crystallographic data for compounds 1c and 7d have been de-
posited with the Cambridge Crystallographic Data Centre as sup-
plementary publication nos. CCDC 797193 and CCDC 802122.
Copies of the data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44 (0)1223
336033 or e-mail: deposit@ccdc.cam.ac.Uk). Supplementary data
related to this article can be found online at doi:10.1016/
j.tet.2011.02.023.
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