C2-Symmetric sulfur derivatives of 2,2′,3,3′'-tetramethoxybiphenyl

Article abstract of DOI:10.1016/S0957-4166(01)00247-6

A practical route to prepare dithioether, thiophene and thiophene S-dioxide derivatives of 2,2′,3,3′-tetramethoxy-1,1′-biphenyl 1 is described. Resolution of 6,6′-bis(methylthio)-3,3′-dimethoxy-[1,1′-biphenyl]-2,2′ -diol 15 was achieved and its absolute configuration was assigned by X-ray analysis of the corresponding phosphorothioamidate diastereomer 18.

Full text of DOI:10.1016/S0957-4166(01)00247-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 1451–1458
C2-Symmetric sulfur derivatives of 2,2%,3,3%-tetramethoxybiphenyl
Giovanna Delogu,a,* Davide Fabbri,a Maria Antonietta Dettori,a Alessandra Fornib,† and
Gianluigi Casaloneb
aIstituto CNR Applicazione delle Tecniche Chimiche Avanzate ai Problemi Agrobiologici, Via Vienna 2, I-07100 Sassari, Italy
bCentro del CNR per lo Studio delle Relazioni tra Struttura e Reattivita` Chimica, Via Golgi 19, I-20133 Milan, Italy
Received 27 April 2001; accepted 24 May 2001
Abstract—A practical route to prepare dithioether, thiophene and thiophene S-dioxide derivatives of 2,2%,3,3%-tetramethoxy-1,1%-
biphenyl 1 is described. Resolution of 6,6%-bis(methylthio)-3,3%-dimethoxy-[1,1%-biphenyl]-2,2%-diol 15 was achieved and its absolute
configuration was assigned by X-ray analysis of the corresponding phosphorothioamidate diastereomer 18. © 2001 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Furthermore, transannular interactions between sulfur
atoms are of interest in determining the geometry,
reactivity and biological proprieties of S-disubstituted
molecules.10 Although the design and preparation of
chiral hydroxylated biphenyls is of central interest in
the chemistry of biologically active substances11 very
little has been developed around homochiral sulfur-con-
taining hydroxylated biphenyls.12
The rapid development of biphenol chemistry in materi-
als science1 as well as in catalysis,2,3 and in particular
the increasing interest in sulfur-containing biphenyls,4
has encouraged us to investigate the preparation of
these compounds. Substituted dibenzothiophenes5
attract the attention of research groups involved in
desulfurization reactions since dibenzothiophenes are
present in diesel fuel and represent a stringent environ-
mental problem. Synthesis of new substituted diben-
zothiophenes and their respective S-dioxides should
lead to a greater knowledge of hydrodesulfurization
mechanisms and provide new ideas for the design of
more effective and selective catalysts.6
We have recently reported the use of 2,2%,3,3%-tetra-
methoxybiphenyl 113 as a key intermediate for the
preparation of enantiopure dibromobiphenol 214 (Fig.
1). In our continuous studies on the chemistry of this
class of compounds, we have investigated the synthesis
of their sulfur derivatives.
Biphenyl thioethers are used as ligands in Cu(II) and
Cu(I) complex models.7 In these compounds the steric
constraints of the biphenyl moiety allow control of the
geometry at the metal ion site and the presence of bulky
substituents in the biphenyl moiety forces the metal
complexes into specific geometries. In fact, the struc-
tural features of these synthetic models have been
shown to recreate many of the properties of the natu-
rally occurring systems.8 Biphenyl thioethers are also
attracting attention as linkers9 by virtue of the flexible
geometry of biphenyl as well as the propensity of the
thioether linkage to be inert to a broad spectrum of
reaction conditions.
Figure 1.
2. Results and discussion
We applied a synthetic strategy that would allow us to
* Corresponding author. E-mail: g.delogu@iatcapa.ss.cnr.it; aforni@
csrsrc.mi.cnr.it
substitute both bromine atoms of
dithiomethyl groups by metal–halide exchange reaction.
2 with two
† Author to whom inquires concerning the X-ray structure analysis
should be directed.
We protected the hydroxyl groups of 2 and treated
0957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00247-6

1452
G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
Scheme 1. When R=Br, R%=H: (a) K2CO3, CH2I2 DMF, 70°C, 70% yield; (b) n-BuLi in dry THF at −50°C, then CH3I, 25%
yield. When R=Br, R%=CH3: (c) [BTEA·Br3], CH2Cl2, MeOH, rt, 5 (45% yield), 6 (50% yield), 7 (5% yield).
acetal 3 with n-BuLi at −50°C in dry THF (Scheme 1a).
We chose this synthetic strategy because it provided an
efficient means of functionalizing the biphenyl at the 6-
and 6%-positions. Since the two acetal enantiomers (+)-3
and (−)-3 are available starting from biphenyl 2, the
method would have allowed us to form enantiopure
6,6%-disubstituted biphenyls. Initially we quenched the
reaction mixture with iodomethane, a reactive elec-
trophile, in order to optimize the experimental condi-
tions (Scheme 1b).
6 was observed as a side reaction. When iodomethane
and methyldisulfide were used as electrophiles,
biphenyls 8 and 9 were obtained in 65 and 63% yield,
respectively. Low yields were seen when bulky elec-
trophiles
such
as
chlorotrimethylsilane
and
chlorodiphenylphosphine were used. Biphenyls 10 and
113 were obtained in 25 and 31% yield, respectively.
Use of sulfur monochloride as the electrophile gave
dibenzothiophene 12 in 80% yield, whereas from reac-
tion with elemental sulfur, biphenyl 12 was recovered in
lower yield from a mixture of products. 1,2,8,9-Tetra-
methoxy-dibenzo[c,e][1,2]-dithiin 13 was not detected.16
No resolution of thiophene 12 or of thiophene S-diox-
ide 14 (the latter obtained by oxidation of 12 in the
presence of 2 equiv. of mCPBA) was observed after
injection on chiral HPLC.17a This is in accord with the
literature reports for similar compounds,18 although we
expected that the presence of methoxy groups at the 1,
2, 8 and 9 positions of the dibenzothiophene backbone
would lead to a considerable deviation from planarity
and aid separation of the two enantiomers of diben-
zothiophenes 12 and 14. In contrast, biphenyl 6 showed
a clear separation (h=1.4) in the two enantiomers after
analysis by chiral HPLC.17b
Although repeated attempts at changing the reaction
conditions (temperature, reaction time, different alkyl-
lithium reagents) were made in order to increase the
yield, biphenyl 4 was obtained in at best only 25%
yield. Difficulties encountered in the preparation of 4
prompted us to increase the reactivity of the starting
material towards the metal–halide exchange reaction.
3,3%,2,2%-Tetramethoxybiphenyl 1 was brominated at rt
by treatment with BTEA·Br3 using a dichloromethane–
methanol mixture as the solvent (Scheme 1c).15 Altering
the reaction conditions allowed us to reduce bromina-
tion at the aromatic ring, which is strongly activated to
electrophilic substitution. These reactions were not
regioselective and biphenyls 5, 6 and 7 were recovered
in 45, 50 and 5% yield, respectively (Scheme 1c).
All of the compounds prepared which possess a C2
symmetry axis were easily purified by flash chromatog-
raphy. Our strategy was to transform biphenyl 9 in the
corresponding 2,2%-diol 15 in order to achieve its resolu-
tion and wide application as a ligand both in catalysis
and bioinorganic chemistry. Previously19 we have
observed that complete desulfurization was achieved
when biphenyl 2,2%,6,6%-tetramethoxy-3,3%-dithiomethyl-
1,1%-biphenyl 16 was treated with 2 equiv. of (CH3)3SiI
at rt in CH2Cl2 giving 2,2%,6,6%-tetramethoxy-1,1%-
biphenyl 17 in 80% yield. Surprisingly, on switching the
positions of the thiomethyl and methoxyl groups, e.g.
Biphenyl 6, which possesses a C2 symmetry axis, was
treated with 2.2 equiv. of n-BuLi at −50°C in dry THF.
Metal–halide exchange proceeded rapidly giving, after
quenching at −50°C with the appropriate electrophile,
derivatives 8–12 (Scheme 2a).
We used several electrophiles so as to introduce differ-
ent functionalities at the 6,6% positions. It was antici-
pated that the size and nature of the electrophile would
influence the yield of the product since sterically con-
gested positions at the biphenyl are involved in the
reaction. In all experiments, debromination of biphenyl
Scheme 2. (a) n-BuLi, dry THF, −50°C, then electrophile, −50°C“rt (8, CH3I, 65% yield; 9, CH3SSCH3, 63% yield; 10, (CH3)3SiI,
25% yield; 11, PPh2Cl, 31% yield; 12, S2Cl2, 80% yield); (b) mCPBA in CH2Cl2 at rt, 80% yield.

G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
1453
compound 9, regioselective O-demethylation occurred
at the ortho–ortho% positions in 90% yield in the pres-
ence of 2.2 equiv. of (CH3)3SiI at rt (Scheme 3a).
vent which is lost too quickly to allow X-ray analysis.
Then we decided to transform diol (+)-15 into the
phosphorothioamidate (+)-18 and to submit crystals of
the latter compound to diffraction analysis. Both the
structure and absolute configuration of (P,S)-(+)-18
were determined unequivocally by X-ray analysis. A
perspective view of the molecule, showing the atom
numbering scheme, is reported in Fig. 2.
In order to apply a rapid resolution procedure of 15, we
prepared the phosphorothioamidate 18, treating 15
with (S)-(−)-Cl2P(S)NHCH(CH3)Ph 19, in the presence
of pyridine (Scheme 3b).20 In this case we chose a cheap
chiral source, (S)-(−)-a-methylbenzylamine, that was
used in equimolar ratio and that we expected to
recover, under reduction conditions, without loss of
enantiomeric purity. Diastereomers 18 were obtained in
75% yield but, unfortunately, all attempts to separate
the two diastereomers failed. Among the several chiral
resolving agents applied to the resolution of biaryl
diols21 we preferred to use, according to our experience
in this field,22 (1R,2S,5R)-(−)-menthyl chloroformate as
a result of its efficiency and the generally practical
separation of the corresponding diastereomers. Treat-
ment of ( )-15 with 2.2 equiv. of this resolving agent in
benzene or toluene and in the presence of Et3N at rt
gave diastereomers (M,1R,1%R,2S,2%S,5R,5%R)-20 and
(P,1R,1%R,2S,2%S,5R,5%R)-20 in 88% yield (Scheme 3c).
Diastereomers 20 were readily separated by flash chro-
matography and were isolated with ]99% d.e. Reduc-
tion of each dicarbonate diastereomer 20 with LiAlH4
provided enantiopure diols (M)-15 and (P)-15 in virtu-
ally quantitative yield (Scheme 3d, e). X-ray diffraction
analysis of (−)-20, aimed at correlating the absolute
configuration with the specific rotation, in order to
achieve information on distortion of the biphenyl unit,
was first attempted. Unfortunately all attempts failed
because the crystals were unstable and broke down
within a few minutes. They contain co-crystallized sol-
Figure 2. ORTEP plot of diastereomer (P,S)-(+)-18 with
atom numbering scheme. Displacement ellipsoids are drawn
at the 30% probability.
Scheme 3. (a) (CH3)3SiI (2.2 equiv.) at rt in CH2Cl2, 80% yield; (b) (S)-(−)-Cl2P(S)NHCHCH3Ph 19, pyridine, 85°C, 75% yield;
(c) (1R,2S,5R)-(−)-menthyl chloroformate, Et3N, benzene or toluene, rt, 88% yield; (d) separation by flash chromatography; (e)
LiAlH4, THF, rt, 90% yield.

1454
G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
Dibenzothiophene 12 and its S-dioxide 14 represent the
first example of dibenzothiophene with four substitu-
tions at 1,2,8,9 positions. Studies in order to achieve
more information on the conformation distortion of
structures 12 and 13 are under investigation.
The presence of the two bulky thiomethyl groups at the
ortho- and ortho%-positions in (P,S)-(+)-18 causes sig-
nificant distortion of the biphenyl skeleton. The dihe-
dral angle (~) between the least-squares planes of the
two phenyl rings measures 54.49(9)° and the resulting
,
separation between S(2) and S(3) is 3.310(1) A, which
indicates a strong repulsive interatomic interaction (cf.
23
3. Experimental
3.1. General procedures
,
the sum of the sulfur van der Waals radii, 3.6 A ). The
repulsive interaction between the sulfur atoms is further
reduced through other geometrical factors. Both sulfur
atoms are significantly driven away from the phenyl
ring to which they are bonded. The distance of S(2)
from the plane of the ring, C(1)ꢀC(6), is equal to
Melting points were determined on a Bu¨chi 530 appara-
1
tus and are uncorrected. All H NMR and 13C NMR
spectra were recorded in CDCl3 solution with a Varian
VXR 5000 spectrometer at 299.94, 75.42 and 121.42
MHz, respectively. 31P NMR chemical shifts are rela-
tive to H3PO4 (external standard) in CDCl3. Chemical
shifts are given in ppm (l); multiplicities are indicated
by s (singlet), d (doublet), t (triplet), m (multiplet) or dd
(doublet of doublets). Elemental analyses were per-
formed using an elemental analyzer Perkin–Elmer
model 240 C. Optical rotations were measured with a
Perkin–Elmer 343 spectropolarimeter. Tetrahydrofuran
(THF), benzene and toluene were freshly distilled from
sodium–benzophenone ketyl. Triethylamine (Et3N) was
dried over KOH, N,N-dimethylformamide (DMF) was
,
0.226(1) A and the corresponding distance of S(3) from
C(7)ꢀC(12) is 0.243(1) A. Moreover, the bond angles
,
indicate that the C(1) and C(11) carbon atoms are not
symmetric.
The
angles
S(2)ꢀC(1)ꢀC(2)
and
S(2)ꢀC(1)ꢀC(6) measure 123.0(2) and 118.2(2)°, respec-
tively, indicating that the S(2)ꢀC(1) bond is driven
towards the biphenyl bond. The corresponding angles
S(3)ꢀC(11)ꢀC(10) and S(3)ꢀC(11)ꢀC(12) measure
118.5(2) and 121.7(2)°, respectively, showing that
S(3)ꢀC(11) is driven away from the biphenyl bond. This
asymmetry of the thiomethyl groups can also be seen in
the slightly different interatomic distances where
,
,
S(2)···C(12) is 3.070(2) A and S(3)···C(6) is 3.174(3) A.
,
dried over 4 A molecular sieves and both solvents were
distilled before use. All reagents were of commercial
quality and used as purchased. 3-Chloroperoxybenzoic
acid (mCPBA) was used with 50–55% purity. Flash
chromatography was carried out with silica gel 60
(230–400 mesh, Kiesgel, EM Reagents) eluting with
appropriate solution in the stated v:v proportions. Ana-
lytical thin-layer chromatography (TLC) was per-
formed with 0.25 mm thick silica gel plates (Polygram®
Sil G/UV254, Macherey–Nagel).
It is interesting to compare this structure with that of
the related phosphorothioamidate derivative with
methoxyl groups at the ortho-ortho% positions (com-
pound 17 of Ref. 19). In that structure ~=49.93(9)°,
showing clearly the intramolecular effect of the reduced
overcrowding of the oxygen atoms with respect to the
sulfur atoms. It is also worth noting the completely
different orientation which the C(CH3)Ph group
assumes in the two structures (compare Fig. 2 with
figure 1 of Ref. 19). For example, the S(1)ꢀPꢀNꢀC(17)
torsions measure −171.7(2) and −18.6(2)°, in the
dithiomethyl and the dimethoxy19 derivatives, respec-
tively. In this case the different, still near isoenergetic,
geometries are exclusively imputable to the different
intermolecular packing forces.
3.2. 1,1-Dibromo-4,8-bis(methoxy)dibenzo[2,1-d:1%,2%-f ]-
[1,3]dioxepine 3
To a solution of 2 (0.94 g, 2.32 mmol) and K2CO3 (0.96
g, 6.97 mmol) in dry DMF (30 mL), CH2I2 (0.94 g, 3.49
mmol) was added, under N2. The reaction mixture was
stirred at 70°C under N2 for 12 h. Water (200 mL) was
added to the mixture and the organic phase was
extracted with ether (2×50 mL). The organic layer was
dried over Na2SO4 to obtain 3 as an orange solid that
was purified by flash chromatography using a 1:1 mix-
ture of CH21Cl2:petroleum as eluent (0.67 g, 70%): mp
230–232°C; H NMR l 3.90 (s, 6H), 5.43 (s, 2H), 6.91
The enantiomeric purity of each diol 15 was related to
the diastereomeric excess of the corresponding phos-
1
phorothioamidate 18, which was verified by H NMR.
Interconversion of the biphenyl structure was moni-
tored
by
NMR
spectroscopy.
Diastereomer
(M,1R,1%R,2S,2%S,5R,5%R)-20 does not racemize at the
stereogenic axis in organic solvents even when heated to
160°C for 12 h in DMSO-d6.
(d, J=9.1 Hz, Ar, 2H), 7.48 (d, J=9.1 Hz, Ar, 2H); 13
C
NMR l 56.42, 101.38, 113.73, 113.78, 130.09, 134.07,
142.14, 151.28. Anal. calcd for C15H12Br2O4: C, 43.30;
H, 2.91; found: C, 43.39; H, 3.33%.
In summary, the article gives a contribution to the
chemistry of sulfur-containing hydroxylated biphenyls
and 2,2%,3,3%-tetramethoxy-1,1%-biphenyl 1 has proved to
be a versatile starting material for the efficient prepara-
tion of such compounds. Both enantiomers of the new
biphenyl thioether 15 were obtained by a practical
route. Crystallographic data for diastereomer (P,S)-(+)-
18 gave important information on the torsional angle,
which has been influenced by the bulky dithiomethyl
groups at the ortho-ortho% positions.
The same procedure was employed starting from enan-
tiopure diols (M)-(−)-2 (95% e.e.) and (P)-(+)-2 (99%
e.e.), to obtain (M)-(+)-3, [h]2D0=377.3 (c 1, CHCl3),
and (P)-(−)-3, [h]2D0=−393.2 (c 1, CHCl3), respectively.
3.3. 1,11-Bis(methyl)-4,8-bis(methoxy)dibenzo-
[2,1-d:1%,2%-f ][1,3]dioxepine 4
To a stirred solution of 3 (1 g, 2.4 mmol) in dry THF

G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
1455
(20 mL), n-BuLi (1.6 M in hexanes, 3.3 mL, 5.28 mmol)
3.8. General procedure of metal–halide exchange and
quenching with electrophiles
was added dropwise at −50°C under N2. After 4 h,
CH3I (1.42 g, 10 mmol) was added dropwise. The
mixture was allowed to warm to rt over 12 h. Water
and 10% aqueous HCl were cautiously added. The
organic phase was extracted with ether, dried over
Na2SO4 and the solvent evaporated to afford a solid.
After purification by flash chromatography using a 1:1
mixture of CH2Cl2:petroleum as eluent, 4 was obtained
as a white solid (0.17 g, 25%): mp 128–129°C; 1H NMR
l 2.14 (s, 6H), 3.87 (s, 6H), 5.42 (s, 2H), 6.93 (d, J=8.7
Hz, Ar, 2H), 7.07 (d, J=8.7 Hz, Ar, 2H); 13C NMR l
19.22, 56.30, 101.00, 112.04, 120.60, 125.23, 127.20,
129.27, 145.22. Anal. calcd for C17H18O4: C, 71.31; H,
6.34; found: C, 71.10; H, 6.20%.
To a stirred solution of 6 (1 equiv.) in dry THF (20
mL), n-BuLi (1.6 M in hexanes, 2.2 equiv.) was added
dropwise at −50°C under N2. After 4 h the appropriate
electrophile (2.6 equiv.) was added dropwise. The mix-
ture was allowed to warm to rt over 12 h. Water (100
mL) and 10% aqueous HCl were cautiously added. The
organic phase was extracted with ether, dried over
Na2SO4 and the solvent evaporated to afford a solid.
After purification by flash chromatography using a
mixture (ꢀ1:1) of CH2Cl2:petroleum as the eluent,
biphenyls 8–12 were obtained in pure form.
3.9. 6,6%-Dimethyl-2,2,3,3%-tetramethoxy-1,1%-biphenyl 8
3.4. Bromination of 3,3%,2,2%-tetramethoxy-1,1%-
biphenyl 1
1
65%; mp 145–146°C; H NMR l 1.88 (s, 6H), 3.61 (s,
6H), 3.85 (s, 6H), 6.83 (d, J=9 Hz, Ar, 2H), 6.95 (d,
J=9 Hz, Ar, 2H); 13C NMR l 16.80, 55.51, 60.02,
111.12, 124.48, 129.08, 131.57, 150.40, 159.09. Anal.
calcd for C18H22O4: C, 71.49; H, 7.34; found: C, 71.59;
H, 7.21%.
To a solution of 1 (1 g, 3.64 mmol) in CH2Cl2 (20 mL)
and CH3OH (5 mL), BTEA·Br3 (6.93 g, 16.05 mmol)
was added. The reaction mixture was stirred at rt under
N2 for 5 h until the initial orange color faded. Aqueous
Na2S2O5 was added to the mixture, then the organic
phase was extracted with CH2Cl2. The organic layer
was dried over Na2SO4 and the solvent evaporated to
obtain an orange solid that was purified by flash chro-
matography using a 1:1 mixture of CH2Cl2:petroleum
as eluent. Compounds 5 (0.70 g, 45%), 6 (0.78 g, 50%)
and 7 (0.06 g, 5%) were obtained and properly
separated.
3.10. 6,6%-Dithiomethyl-2,2,3,3%-tetramethoxy-1,1%-
biphenyl 9
1
63%; mp 201–202°C; H NMR l 2.34 (s, 6H), 3.70 (s,
6H), 3.88 (s, 6H), 6.98 (d, J=8.7 Hz, Ar, 2H), 7.06 (d,
J=8.7 Hz, Ar, 2H); 13C NMR l 16.69, 55.60, 60.29,
111.52, 121.78, 129.84, 130.40, 146.74, 150.56. Anal.
calcd for C18H22O4S2: C, 58.99; H, 6.05; found: C,
59.37; H, 6.12%.
3.5. 5,6%-Dibromo-2,2%,3,3%-tetramethoxy-1,1%-biphenyl 5
1
Mp 212–213°C; H NMR l 3.64 (s, 6H), 3.86 (s, 3H),
3.88 (s, 3H), 6.82 (d, J=9 Hz, Ar, 1H), 6.84 (d, J=2.4
Hz, Ar, 1H), 7.05 (d, J=2.4 Hz, Ar, 1H), 7.32 (d, J=9
Hz, Ar, 1H); 13C NMR l 56.97, 55.98, 60.50, 60.84,
113.24, 114.42, 115.53, 116.63, 125.28, 127.35, 132.89,
133.46, 145.90, 147.87, 152.20, 153.33. Anal. calcd for
C16H16Br2O4: C, 44.66; H, 3.75; found: C, 44.70; H,
3.87%.
3.11. 6,6%-Bis(trimethylsilyl)-2,2,3,3%-tetramethoxy-1,1%-
biphenyl 10
1
25%; oil; H NMR l −0.91 (s, 18H), 3.69 (s, 6H), 3.89
(s, 6H), 6.94 (d, J=8.1 Hz, Ar, 2H), 7.25 (d, J=8.1 Hz,
Ar, 2H); 13C NMR l 0.13, 55.39, 60.00, 110.95, 129.70,
132.56, 138.44, 152.49, 175.95. Anal. calcd for
C22H34O4Si2: C, 63.11; H, 8.19; found: C, 63.50; H,
8.12%.
3.6. 6,6%-Dibromo-2,2%,3,3%-tetramethoxy-1,1%-biphenyl 6
1
Mp 110–122°C; H NMR l 3.70 (s, 6H), 3.89 (s, 6H),
3.12. 6,6%-Bis(diphenylphosphyl)-2,2,3,3%-tetramethoxy-
1,1%-biphenyl 11
6.88 (d, J=8.7 Hz, Ar, 2H), 7.37 (d, J=8.7 Hz, Ar,
2H); 13C NMR l 56.84, 60.53, 113.23, 114.62, 127.27,
133.48, 147.53, 152.15. Anal. calcd for C16H16Br2O4: C,
44.47; H, 3.73; found: C, 44.53; H, 3.80%.
1
31%; mp 170–172°C [lit.3 mp 171–172°C]; H NMR l
3.32 (s, 6H), 3.89 (s, 6H), 6.85–7.35 (series of m, Ar,
24H); 13C NMR (aliphatic only) l 55.40, 60.18; 31P
NMR l −16.30.
3.7. 6-Bromo-2,2%,3,3%-tetramethoxy-1,1%-biphenyl 7
1
Mp 104–105°C; H NMR l 3.63 (s, 3H), 3.68 (s, 3H),
3.87 (s, 3H), 3.90 (s, 3H), 6.68 (dd, J=1.8, 7.8 Hz, Ar,
1H), 6.82 (d, J=8.1 Hz, Ar, 1H), 6.96 (dd, J=1.8, 7.8
Hz, Ar, 1H), 7.10 (t, J=8.1 Hz, Ar, 1H), 7.33 (d,
J=8.1 Hz, Ar); 13C NMR l 55.71, 55.95, 60.45, 60.75,
112.11, 112.87, 114.88, 122.74, 123.49, 127.26, 132.14,
134.28, 146.56, 147.97, 152.22, 152.68. Anal. calcd for
C16H17BrO4: C, 54.41; H, 4.85; found: C, 53.91; H,
5.01%.
3.13. 1,2,8,9-Tetra(methoxy)dibenzo[2,1-b:1%,2%d]-
thiophene 12
1
80%; oil; H NMR l 3.90 (s, 6H), 3.94 (s, 6H), 7.13 (d,
J=8.4 Hz, Ar, 2H), 7.45 (d, J=8.4 Hz, Ar, 2H); 13C
NMR l 56.63, 62.27, 112.73, 117.84, 129.34, 133.26,
146.48, 151.08. Anal. calcd for C16H16O4S: C, 63.14; H,
5.30; found: C, 63.00; H, 5.22%.

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G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
3.14. 1,2,8,9-Tetra(methoxy)dibenzo[2,1-b:1%,2%-d]thio-
phene-S-dioxide 14
The same procedure was employed starting from enan-
tiopure diol (P)-(+)-15 to obtain (+)-(P,S)-18, which
was recrystallized from CH2Cl2:n-hexane to obtain
crystals suitable for X-ray analysis. (+)-(P,S)-18: mp
180°C; [h]2D0=+75.5 (c 1, CHCl3). Anal. calcd for
C24H26NO4PS3: C, 55.48; H, 5.05; N, 2.70; found: C,
55.54; H, 5.10; N, 2.44%.
To a solution of 12 (1 g, 4.16 mmol) in CH2Cl2 (20 mL)
was added dropwise a solution of m-chloroperbenzoic
acid (55% purity, 5.18 g, 15 mmol) in CH2Cl2 with
stirring at 0°C under N2. The mixture was allowed to
warm to rt over 12 h. Water (100 mL) was added, and
the organic layer was separated and washed with a
saturated solution of Na2S2O5 and NaHCO3. The
organic solution was dried over Na2SO4 and the solvent
evaporated to afford a solid, which was purified by
flash chromatography using CH2Cl2 as eluent. Biphenyl
14 was obtained as a colorless solid (1.12 g, 80%); mp
3.17. 6,6%-Bis(methylthio)-3,3%-dimethoxy-[1,1%-biphenyl]-
2,2%-diyl-O,O%-bis[5-methyl-2-(1-methylethyl)-cyclo-
hexyl]-carbonic ester 20
A solution of ( )-15 (0.32 g, 0.95 mmol) and Et3N (2
mL) in benzene or toluene (15 mL) was added, drop-
wise, to a solution of (−)-(1R,2S,5R)-menthyl chloro-
formate (0.45 g, 2.05 mmol) in benzene or toluene (15
mL) at rt under N2. The solution was stirred at rt for 1
h, washed with 10% aqueous HCl and water and the
organic phase extracted with CH2Cl2. The crude, dried
over Na2SO4, gave 20 (0.56 g, 88%) as a 1:1 mixture of
two diastereomers that were completely separated by
flash chromatography using CH2Cl2 as eluent.
1
185–187°C; H NMR l 3.82 (s, 6H), 3.93 (s, 6H), 7.03
(d, J=8.7 Hz, Ar, 2H), 7.55 (d, J=8.7 Hz, Ar, 2H); 13
C
NMR l 56.35, 62.39, 112.67, 118.55, 122.99, 123.16,
124.00, 130.88. Anal. calcd for C16H16O6S: C, 57.13; H,
4.79; found: C, 57.19; H, 4.62%.
3.15. 6,6%-Bis(methylthio)-3,3%-dimethoxy-[1,1%-biphenyl]-
2,2%-diol 15
To a solution of 9 (1.4 g, 3.8 mmol) in CH2Cl2 (30 mL)
at rt and under N2, (CH3)3SiI (1.7 g, 8.4 mmol) was
added. The mixture was stirred at rt for 12 h. MeOH
(10 mL) was added and the mixture was poured into ice
water, stirred for 0.5 h, saturated with salt and
extracted with ether. The organic extract was dried
(Na2SO4) and evaporated to afford a brown solid. The
crude material was purified by flash chromatography
using CH2Cl2 as eluent, to give 15 as an oil (1.15 g,
1
(M,1R,1%R,2S,2%S,5R,5%R)-20: mp 170–171°C; H NMR
l 0.71 (d, J=6.9 Hz, 6H), 0.82 (d, J=6.9 Hz, 6H), 0.89
(d, J=6.9 Hz, 6H), 0.80–1.95 (series of m, 18H), 2.32
(s, 6H), 3.82 (s, 6H), 4.45 (m, 2H), 6.98 (d, J=8.7 Hz,
Ar, 2H), 7.24 (d, J=8.7 Hz, Ar, 2H); 13C NMR l
16.38, 17.45, 20.68, 22.04, 23.42, 25.81, 31.56, 34.11,
40.30, 46.99, 55.92, 78.98, 112.85, 125.50, 128.87,
130.58, 138.70, 149.6, 152.11; [h]2D0=−98.8 (c 1, CHCl3).
Anal. calcd for C38H54O8S2: C, 64.93; H, 7.74; found:
C, 64.32; H, 7.71%.
1
90%); H NMR l 2.35 (s, 6H), 3.90 (s, 6H), 5.67 (bs,
2H), 6.91 (d, AB, J=8.7 Hz, Ar, 4H); 13C NMR l
16.73. 55.97, 111.15, 118.00, 130.40, 130.71, 143.49,
144.87. Anal. calcd for C16H18O4S2: C, 56.78; H, 5.36;
found: C, 56.29; H, 5.40%.
1
(P,1R,1%R,2S,2%S,5R,5%R)-20: mp 69–70°C; H NMR l
0.73 (d, J=6.9 Hz, 6H), 0.84 (d, J=6.9 Hz, 6H), 0.86
(d, J=6.9 Hz, 6H), 0.80–1.95 (series of m, 18H), 2.34
(s, 6H), 3.83 (s, 6H), 4.45 (m, 2H), 6.99 (d, J=8.4 Hz,
Ar, 2H), 7.25 (d, J=8.4 Hz, Ar, 2H); 13C NMR l
16.17, 17.96, 20.63, 21.96, 23.27, 25.82, 31.24, 34.06,
40.19, 46.78, 55.86, 78.95, 112.70, 126.18, 130.56,
138.56, 149.80, 151.56, 153.57; [h]2D0=54.9 (c 1, CHCl3).
Anal. calcd for C38H54O8S2: C, 64.93; H, 7.74; found:
C, 64.88; H, 7.34%.
3.16. Dibenzo-[d,f ](1,3,2)-dioxaphosphepin-6-amine-
1,11-bis(methylthio)-4,8-dimethoxy-N-(1-phenylethyl)-6-
sulfide 18
N-((S)-a-Methylbenzyl)-dichlorothiophosphoroamidate
19 (0.17 g, 0.7 mmol) was added dropwise to a solution
of 15 (0.18 g, 0.53 mmol) in pyridine (20 mL) at rt
under N2. After stirring the mixture under reflux for 12
h, the reaction mixture was cooled and made acidic
with 10% H2SO4. Water was added and the organic
phase was extracted with CH2Cl2, dried over Na2SO4
and evaporated to dryness to obtain a colorless solid.
The crude was purified by flash chromatography using
CH2Cl2 as eluent, to give 18 as a 1:1 mixture of the two
diastereomers (P,S)-18 and (M,S)-18 (0.21 g, 75%).
3.18. (M)-(−)-6,6%-Bis(methylthio)-3,3%-dimethoxy-[1,1%-
biphenyl]-2,2%-diol 15
A solution of (M,1R,1%R,2S,2%S,5R,5%R)-(−)-20 (1.8 g,
2.55 mmol) in dry THF (30 mL) was cooled at 0°C
under N2. To this solution, LiAlH4 (0.46 g, 12 mmol)
was added in portions with vigorous magnetic stirring.
After stirring the mixture for 12 h at rt, water and then
10% aqueous HCl were cautiously added. The organic
phase was extracted with ether, dried over Na2SO4 and
evaporated to afford a colorless solid. After purification
by flash chromatography using a 1:4 mixture of ethyl
acetate:petroleum, as eluent, enantiomerically pure
(M)-(−)-15 (0.77 g, 90%) and enantiomerically pure
(−)-menthol (0.72 g, 90%) were obtained. (M)-(−)-15:
[h]2D0=−16.0 (c 1, CHCl3).
1
(P,S)-18: H NMR l 1.52 (d, J=5.1 Hz, 3H), 2.21 (s,
3H), 2.28 (s, 3H), 3.51 (m, 1H), 3.67 (s, 3H), 3.93 (s,
3H), 4.80 (m, 1H), 6.80–7.40 (series of m, Ar, 9H); 13C
NMR (aliphatic only) l 19.35, 19.60, 25.38, 53.00,
1
55.21, 56.60; 31P NMR l 77.71. (M,S)-18: H NMR l
1.48 (d, J=5.1 Hz, 3H), 2.06 (s, 3H), 2.33 (s, 3H), 3.42
(s, 3H), 3.51 (m, 1H), 3.80 (s, 3H), 4.80 (m, 1H),
6.80–7.40 (series of m, Ar, 9H); 13C NMR (aliphatic
only) l 19.28, 19.55, 25.23, 53.04, 55.43, 56.70; 31P
NMR l 76.80.

G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
1457
3.19. (P)-(+)-6,6%-Bis(methylthio)-3,3%-dimethoxy-[1,1%-
biphenyl]-2,2%-diol 15
3. Schmid, R.; Foricher, J.; Cereghetti, M.; Schonholzer, P.
Helv. Chim. Acta 1991, 74, 370.
4. (a) Robert, F.; Winum, J.-Y.; Sakai, N.; Gerard, D.;
Matile, S. Org. Lett. 2000, 2, 37; (b) Liupel, B.; Niederalt,
C.; Nieger, M.; Grimme, S.; Vo¨gtle, F. Angew. Chem.,
Int. Ed. 1998, 37, 3031; (c) Morita, Y.; Kashiwagi, A.;
Nakasuji, K. J. Org. Chem. 1997, 62, 7464; (d) Cossu, S.;
De Lucchi, O.; Fabbri, D.; Valle, G.; Painter, G. F.;
Smith, R. A. J. Tetrahedron 1997, 53, 6073; (e) Ueda;
Adachi, T.; Sumiya, K.; Yoshida, T. J. Chem. Soc.,
Chem. Commun. 1995, 935; (f) Parson, A. S.; Garcia, J.
M.; Snieckus, V. A. Tetrahedron Lett. 1994, 35, 7537; (g)
Haenel, M. W.; Fiesel, H.; Jakubik, D.; Gabor, B.;
Goddar, R.; Kru¨ger, C. Tetrahedron Lett. 1993, 34, 2107;
(h) Dakova, B.; Carbonelle, P.; Walcarius, A.; Lamberts,
L.; Evers, M. Electrochim. Acta 1992, 37, 725; (i)
Nabeshima, T.; Sakiyama, A.; Yagyu, A.; Furukawa, N.
Tetrahedron Lett. 1989, 30, 5287; (j) Kai, Y.; Morii, H.;
Moki, K.; Kasai, N. J. Am. Chem. Soc. 1985, 107, 4802.
5. (a) Meille, V.; Schulz, E.; Lemarie, M.; Faure, R.; Vrinat,
M. Tetrahedron 1996, 52, 3953; (b) Yasui, M.; Murata,
S.; Iwasaki, F.; Furukawa, N. Bull. Chem. Soc. Jpn. 1995,
68, 744; (c) Kimura, T.; Ishikawa, Y.; Ueki, K.; Horie,
Y.; Furukawa, N. J. Org. Chem. 1994, 59, 7117; (d)
Nink, G.; Boberg, F. Phosphorus, Sulfur, Silicon 1991, 60,
281; (e) Stender, K.-W.; Wolki, N.; Klar, G. Phosphorus,
Sulfur, Silicon 1989, 42, 111; (f) Dunkerton, L. V.; Barot,
B. C.; Nigam, A. J. Heterocyclic Chem. 1987, 24, 749; (g)
Novi, M.; Garbarino, G.; Petrillo; Dell’Erba, C. J. Chem.
Soc., Perkin Trans. 2 1987, 623; (h) Kundo, H.; Castle, R.
N.; Lee, M. L. J. Heterocyclic Chem. 1985, 22, 215.
6. (a) Kabe, T.; Aoyama, Y.; Wang, D.; Ishihara, A.; Qian,
W.; Hosoya, M.; Zhang, Q. Appl. Catal. A 2001, 209,
237; (b) Chan, M.-C.; Cheng, K.-M.; Ho, K. M.; Ng, C.
T.; Yam, T. M.; Wang, B. S. L.; Luh, T.-Y. J. Org.
Chem. 1988, 53, 4466; (c) Eisch, J. J.; Hallenbeck, L. E.;
Han, K. I. J. Am. Chem. Soc. 1986, 108, 7763.
Using the above procedure, diastereomer (P,1R,1%R,
2S,2%S,5R,5%R)-(+)-20 (99% d.e.) gave (P)-(+)-15
(90%); [h]2D0=+16.3 (c 0.5, CHCl3).
3.20. X-Ray structure determination of (P,S)-(+)-18
Diffracted intensities were collected with a Bruker P4
diffractometer, using graphite-monochromated Mo-Ka
,
radiation=0.71073 A. Crystal description: colorless
prism 0.40×0.39×0.32 mm. Mr=519.61, monoclinic,
space group P21, a=9.880(1), b=8.308(1), c=15.667(2)
3
,
,
A, i=104.40(1)°, V=1245.6(3) A , Z=2, T=293(2) K,
v=0.393 mm−1. ꢀ/2q scans, 7527 measured reflections,
6949 independent reflections, 5670 reflections with I>
2|(I), Rint=0.020, 4.3°<2q<60.0°. The structure was
solved by SIR-92 and refined on F2 by full-matrix
least-squares using SHELXL-97. 6949 reflections used
in refinement, 367 parameters. Heavy atoms were
anisotropic, H atoms isotropic. Flack parameter24 for
determination of the absolute configuration=0.00(7).
Final R=0.0453 and wR=0.1010 for data with I>
−3
,
2|(I), (D/|)max=0.004, Dzmax=0.422 e A , Dzmin
=
−3
,
−0.167 e A .
Acknowledgements
This work was supported by the C.N.R., Rome, and
partially by Sardinia Autonomous Region.
References
1. (a) Hohnholz, D.; Schweikart, K.-H.; Subramanian, L.
R.; Wedel, A.; Wischert, W.; Hanack, M. Synth. Met.
2000, 110, 141; (b) Sakai, N.; Matile, S. Chem. Eur. J.
2000, 6, 1731; (c) Takanishi, Y.; Takezoe, H.; Suzuki, Y.;
Kobayashi, I.; Yajima, T.; Terada, M.; Mikami, K.
Angew. Chem., Int. Ed. 1999, 38, 2354; (d) Ko¨lbel, M.;
Tschierske, C.; Diele, S. J. Chem. Soc., Chem. Commun.
1998, 1511; (e) Solladie´, G.; Hugele´, P.; Bartsch, R. J.
Org. Chem. 1998, 63, 3895; (f) Itoh, T.; Shirakami, S.;
Nakao, Y.; Yoshida, T. Chem. Lett. 1998, 979; (g) Vizi-
tiu, D.; Halden, B. J.; Lemieux, R. P. J. Chem. Soc.,
Chem. Commun. 1997, 1123; (h) Costero, A. M.; Pitarch,
M.; Andrei, C.; Ochando, L. E.; Amigo`, J. M.; Debaerde-
maeker, T. Tetrahedron 1996, 52, 669.
2. (a) Bringmann, G.; Hinrichs, J.; Peters, K.; Peters, E.-M.
J. Org. Chem. 2001, 66, 629; (b) Zhang, Z.; Qian, Z.;
Longmire, J.; Zhang, X. J. Org. Chem. 2000, 65, 6223; (c)
Trabesinger, G.; Albinati, A.; Feiken, N.; Kunz, R. W.;
Pregosin, P. S.; Tschoerner, M. J. Am. Chem. Soc. 1997,
119, 6315; (d) Harada, T.; Takeuchi, M.; Hatsuda, M.;
Ueda, S.; Oku, A. Tetrahedron: Asymmetry 1996, 7, 2479;
(e) Jendralla, H.; Li, C. H.; Paulus, E. Tetrahedron:
Asymmetry 1994, 5, 1297; (f) Heiser, B.; Broger, E. A.;
Crameri, Y. Tetrahedron: Asymmetry 1991, 2, 51; (g)
Consiglio, G.; Indolese, A. Organometallics 1991, 10,
3425.
7. (a) Malachowski, M. R.; Adams, M.; Elia, N.; Rhein-
gold, A. L.; Kelly, R. S. J. Chem. Soc., Dalton Trans.
1999, 2177; (b) Chautemps, P.; Gellon, G.; Morin, B.;
Pierre, J.-L.; Provent, C.; Refaif, S. M.; Beguin, C. G.; El
Marzouki, A.; Serratrice, G.; Saint-Aman, E. Bull. Soc.
Chim. Fr. 1994, 131, 434.
8. (a) Baret, P.; Beaujolais, V.; Be´guin, C.; Gaude, D.;
Pierre, J.-L.; Serratrice, G. Eur. J. Inorg. Chem. 1998,
613; (b) Knapp, S.; Keenan, T. P.; Liu, J.; Potenza, J. A.;
Schugar, H. J. Inorg. Chem. 1990, 29, 2191; (c) Anderson,
O. P.; Becher, J.; Frydendahl, H.; Taylor, L. F.;
Toftlund, H. J. Chem. Soc., Chem. Commun. 1986, 699;
(d) Hanson, G.; Kemp, D. S. J. Org. Chem. 1981, 46,
5441; (e) Averill, B. A.; Bale, J. R.; Orme-Johnson, W. H.
J. Am. Chem. Soc. 1978, 100, 3034.
9. (a) Breidenbach, S.; Harren, J.; Neumann, S.; Nieger, M.;
Rissanen, K.; Vo¨gtle, F. J. Chem. Soc., Perkin Trans. 1
1996, 2061; (b) Breslow, R.; Halfon, S.; Zhang, B. Tetra-
hedron 1995, 51, 377.
10. (a) Fujii, T.; Kusanagi, H.; Takahashi, O.; Horn, E.;
Furukawa, N. Tetrahedron 1999, 55, 5027; (b) Kimura,
T.; Nakayama, H.; Obinata, T.; Furukawa, N. J. Org.
Chem. 1994, 59, 7117; (c) Cossu, S.; De Lucchi, O.; Piga,
E.; Valle, G. Phosphorus, Sulfur, Silicon 1991, 63, 51.
11. (a) Tohma, H.; Morioka, H.; Takizawa, S.; Arisawa, M.;
Kita, Y. Tetrahedron 2001, 57, 345; (b) Bringmann, G.;

1458
G. Delogu et al. / Tetrahedron: Asymmetry 12 (2001) 1451–1458
Tasler, S. Tetrahedron 2001, 57, 331; (c) Staroske, T.;
50% yield: Delogu, G.; Cossu, S.; Fabbri, D.; Maglioli, P.
Org. Prep. Proc. Int. 1991, 23, 455.
O’Brien, D. P.; Jørgensen, T. J. D.; Roepstorff, P.;
Williams, D. H.; Heck, A. J. R. Chem. Eur. J. 2000, 6,
504; (d) Baumeinster, B.; Sakai, N.; Gerard, D.; Matile,
S. Angew. Chem., Int. Ed. 2000, 39, 1955; (e) Itoh, T.;
Shirakami, S.; Ishida, N.; Yamashita, Y.; Yoshida, T.;
Kim, H.-S.; Wataya, Y. Biorg. Med. Chem. Lett. 2000,
10, 1657; (f) Nicolaou, K. C.; Boddy, C. N. C.; Bra¨se, S.;
Winssinger, N. Angew. Chem., Int. Ed. 1999, 38, 2096; (g)
Feldman, K. S.; Sahasrabudhe, K. J. Org. Chem. 1999,
64, 209; (h) Kyasnoor, R. V.; Sargent, M. V. J. Chem.
Soc., Chem. Commun. 1998, 2713; (i) Sakai, N.; Brennan,
C. K.; Weiss, L. A.; Matile, S. J. Am. Chem. Soc. 1997,
119, 8726; (j) Dusemund, C.; Mikami, M.; Shinkai, S.
Chem. Lett. 1995, 157; (k) Fujihashi, T.; Hara, H.;
Sakata, T.; Mori, K.; Higuchi, H.; Tanaka, A.; Kaji, H.;
Kaji, A. Antimicrob. Agents Chemother. 1995, 39, 2000;
(l) Costero, A. M.; Pitarch, M. J. Org. Chem. 1994, 59,
2939; (m) Nelson, T. D.; Meyers, A. I. J. Org. Chem.
1994, 59, 2577; (n) Kondo, K.; Takahashi, M.; Ohmizu,
H.; Matsumoto, M.; Taguchi, I.; Iwasaki, T. Chem.
Pharm. Bull. 1994, 42, 62.
17. (a) HPLC analysis was performed at rt using a Chiracel
OD column (10 mm, 25 cm×0.46 I.D.) at flow rate of 0.6
mL/min, 254 UV detection, using different mixtures
(w:w) of n-hexane:propan-2-ol as mobile phase; (b) Sepa-
ration of biphenyl 6 in two enantiomers was performed
by chiral HPLC using a Chiracel OD column with 98:2
hexane:propan-2-ol as mobile phase, at a flow rate of 0.6
mL/min and with UV detector at 254 nm.
18. (a) Kimura, T.; Nakayama, H.; Furukawa, N. Heterocy-
cles 1996, 42, 71; (b) Kimura, T.; Nakayama, H.;
Furukawa, N. Chem. Lett. 1994, 27.
19. Delogu, G.; Fabbri, D.; Dettori, M. A.; Forni, G.; Casa-
lone, A. Tetrahedron: Asymmetry 2000, 11, 4417.
20. We applied conditions that were developed successfully
by us for resolution of BINOL and biphenols: Fabbri,
D.; Delogu, G.; De Lucchi, O. J. Org. Chem. 1993, 58,
1748.
21. (a) Periasamy, M.; Venkatraman, L.; Sivakumar, S.;
Sampathkumar, N.; Ramanathan, C. R. J. Org. Chem.
1999, 64, 7643; (b) Shan, Z.; Xiong, Y.; Wezhong, L.;
Zhao, D. Tetrahedron: Asymmetry 1998, 9, 3985; (c)
Toda, F.; Tanaka, K. J. Chem. Soc., Chem. Commun.
1997, 1087; (d) Chow, H.-F.; Wan, C.-W.; Ng, M.-K. J.
Org. Chem. 1996, 61, 8712; (e) Venkatraman, L.; Peri-
asamy, M. Tetrahedron: Asymmetry 1996, 7, 2471; (f)
Mazaleyrat, J.-P.; Wakselman, M. J. Org. Chem. 1996,
61, 2695; (g) Wang, M.; Liu, S. Z.; Liu, J.; Hu, B. F. J.
Org. Chem. 1995, 60, 7364; (h) Hu, Q.-S.; Vitharana, D.;
Pu, L. Tetrahedron: Asymmetry 1995, 6, 2123; (i) Pakul-
ski, Z.; Zamojski, A. Tetrahedron: Asymmetry 1995, 6,
111; (j) Brunel, J.-M.; Buono, G. J. Org. Chem. 1993, 58,
7313.
12. (a) Delogu, G.; Fabbri, D.; Dettori, M. A. Tetrahedron:
Asymmetry 1998, 9, 2819; (b) Capozzi, G.; Delogu, G.;
Dettori, M. A.; Fabbri, D.; Menichetti, S.; Nativi, C.;
Nuti, R. Tetrahedron Lett. 1999, 40, 4421.
13. Prepared in two steps in 80% overall yield, by known
procedure: Horner, L.; Weber, K. H. Chem. Ber. 1963,
96, 1568.
14. Delogu, G.; Fabbri, D.; Dettori, M. A.; Casalone, G.;
Forni, A. Tetrahedron: Asymmetry 2000, 11, 1827.
15. Benzyltriethylammonium tribromide [BTEA·Br3] was
used instead of benzyltrimethylammonium tribromide
[BTMA·Br3]: Kajigaeshi, S.; Moriwarki, M.; Tanaka, T.;
Fujisaki, S.; Kakinami, T.; Okamoto, T. J. Chem. Soc.,
Perkin Trans. 1 1990, 897.
22. Ref. 14. Fabbri, D.; Delogu, G.; De Lucchi, O. J. Org.
Chem. 1995, 60, 6599.
16. When 2,2%-dilithium-1,1%-biphenyl was quenched with ele-
23. Bondi, A. J. Phys. Chem. 1964, 68, 441.
mental sulfur, dibenzo[c,e][1,2]-dithiin was recovered in
24. Flack, H. D. Acta Crystallogr., Sect. A 1983, 39, 876.
.