Stereocontrolled Synthesis of Sulfonyl 2,5-Diaryltetrahydrofurans

Article abstract of DOI:10.1055/s-0035-1560423

BF₃·OEt₂-mediated stereocontrolled annulation of 4-alkenols affords sulfonyl 2,5-diaryltetrahydrofurans in good yields. The key synthetic route combines the facile stereoselective reduction of α-styryl-β-ketosulfones and an intramole

Full text of DOI:10.1055/s-0035-1560423

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
M.-Y. Chang, Y.-C. Cheng
Letter
Synlett
Stereocontrolled Synthesis of Sulfonyl 2,5-Diaryltetrahydrofurans
M.-Y. Chang*
R
R
Y.-C. Cheng
O
O
S
S
O
O
1. NaBH4
16 examples
Me
Ar1
Department of Medicinal and Applied Chemistry,
Kaohsiung Medical University, Kaohsiung 807,
Taiwan
Ar1
Ar
2. BF3⋅OEt2
70–82% yields
Ar
O
O
mychang@kmu.edu.tw
Ar = Ph, 4-FC6H4, Tol, 4-O2NC6H4, 4-F3CC6H4, 4-PhC6H4, 2-naphthyl
Ar1 = Ph, 4-FC6H4, 4-PhC6H4
R = Tol, Ph
Received: 24.11.2015
Accepted after revision: 23.01.2016
Published online: 04.03.2016
molecular cycloetherification of 1,4-butandiols and reduc-
tive cyclization of γ-hydroxyketones are common path-
ways. Synthetic routes to organometal (Mg, Cu or Zn)-
mediated nucleophilic addition of cyclic hemiacetals or lac-
tones have been documented (see Scheme 1).
However, the reported preparation of these derivatives
often presents drawbacks (e.g. multistep operations and
harsh conditions, poorer stereoselectivity), and this has en-
couraged organic researchers to explore more efficient syn-
thetic protocols. To the best of our knowledge, for the syn-
thesis of sulfonyl 2,5-diaryltetrahydrofurans, no examples
of the intramolecular electrophilic annulation of 4-alkenols
have been reported.
DOI: 10.1055/s-0035-1560423; Art ID: st-2015-w0911-l
Abstract BF3·OEt2-mediated stereocontrolled annulation of 4-alkenols
affords sulfonyl 2,5-diaryltetrahydrofurans in good yields. The key syn-
thetic route combines the facile stereoselective reduction of α-styryl-β-
ketosulfones and an intramolecular Friedel–Crafts electrophilic cycliza-
tion of the resulting 4-alkenols. A plausible mechanism has been stud-
ied and proposed.
Key words 2,5-diaryltetrahydrofurans, reduction, 4-alkenols, β-keto-
sulfones, cyclization
The tetrahydrofuran moiety is an important component
in numerous diversified molecules with biological activities
of synthetic intermediates and natural products.1 Among
these building blocks, substituted 2,5-diaryltetrahydrofu-
ran can be widely found from natural sources and bioactive
molecules including manassantins A and B,2a,b virgatusin,2c–f
talaumidin2g and MK-287.2h,i As a result, numerous synthet-
ic approaches have been developed for the construction of
diversified 2,5-diaryltetrahydrofurans.2 For various proto-
cols on the synthesis of 2,5-diaryltetrahydrofurans 1, intra-
In continuation of our investigation into the synthetic
applications of β-ketosulfones,3 a three-step stereoselective
synthetic route to sulfonyl 2,5-diaryltetrahydrofurans 6 has
been developed, including (i) a K2CO3-mediated α-allylation
of β-ketosulfones 2 with styryl bromide 3 (prepared from
allylic bromination of α-methylstyrene with NBS in reflux-
ing CHCl3) in boiling acetone, (ii) a NaBH4-mediated stereo-
selective reduction of α-styryl-β-ketosulfones 4 in cooling
co-solvent of THF and MeOH (1:1) and (iii) an intramolecu-
lar BF3·OEt2-mediated stereocontrolled Friedel–Crafts an-
FG
FG
O
FG
cycloetherification
reductive cyclization
HO
Ar1
O
Ar1
γ
Ar
Ar1
Ar
Ar
O
H
O
H
nucleophilic
addition
reduction then
nucleophilic addition
FG
FG
O
H
then
Ar
Ar
Ar1
OH
Ar1
O
O
Scheme 1 Synthetic routes of 2,5-diaryltetrahydrofurans
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
M.-Y. Chang, Y.-C. Cheng
Letter
Synlett
ture and relative stereochemistry of 6a were determined
R
O
R
O
Br
Ar1
1
1
S
S
from H NMR and J coupling analysis. In the H NMR spec-
trum, the equatorial proton (blue symbol) of C-2 shows a
doublet with a coupling constant J = 8.0 Hz at δ = 5.12 ppm,
which indicates that the proton (black symbol) of C-3 is
coupling with an axial position [δ = 3.99 (q, J = 8.0 Hz)]. The
structural frameworks of 6a (Figure 1) and 6b (Figure 2)
with trans-diphenyl substituents were determined by sin-
gle-crystal X-ray crystallography.6
O
O
3
NaBH4
Ar
O
Ar1
Ar
K2CO3, acetone
THF/MeOH
O
2
4
R
O
R
O
S
S
O
O
BF3·OEt2
CH2Cl2
Me
Ar1
Ar1
Ar
Ar
O
HO
5
6
Scheme 2 Our synthetic route to 6
nulation of the resulting sulfonyl 4-alkenols 5 in CH2Cl2 at
25 °C (see Scheme 2).
After further comparison of literature and our previous
studies on the Lewis acid triggered annulation,3,4 substrate
4a was first examined. α-Styryl-β-ketosulfone 4a (Ar = Ar1 =
Ph; R = Tol) was chosen as the model’s starting material3f to
examine NaBH4-mediated stereoselective reduction. Under
the above conditions, 5a was isolated in a 90% yield. Ac-
cording to the Felkin–Anh model,5 the steric hindrance of
sulfonyl substituent should inhibit the carbonyl addition of
hydride such that the hydride should attack the carbonyl
face with the face with less repulsion to form sulfonyl 4-
alkenols 5a via a possible intermediate A. Next, intramolec-
ular annulation of 5a with BF3·OEt2 provided 6a in a 90%
yield. The possible mechanism should be initiated to form
B1 or B2 by complexation of a hydroxyl motif of 5a with
BF3·OEt2 via a chair conformation (Scheme 3). B1 should
form the preferred orientation with a syn-protonation due
to B2 providing more repulsion between the phenyl group
and the OBF3 complex. Proton exchange of B1 affords a ter-
tiary carbocation C, which, following an intramolecular ad-
dition and loss of BF3, is able to provide 6a (72%). The struc-
Figure 1 X-ray crystal structure of 6a
Figure 2 X-ray crystal structure of 6b
less repulsion face
Ph
H
Tol
O
Tol
O
H
O
H
S
S
O
O
NaBH4
Ph
BH3
A
S
Ph
Ph
Ph
Ph
O
O
H
Tol
O
HO
4a
5a (90%)
Felkin–Anh model
BF3·OEt2
more repulsion
H
syn-protonation
(preferred)
δ 3.99
J = 8.0 Hz
Ph
Ph
H
O
H
H
O
O
O
S
Ph
Me
H
3
Tol
or
Tol
Tol
Ph
S
S
H
O
S
Tol
2
H
H
O
O
H
F3B
O
H
H
O
O
O
O
F3B
F3B
Ph
Ph
Ph
δ 5.12
Ph
J = 8.0 Hz
BF3
B2
B1
C
6a (72%)
Scheme 3 Possible mechanism
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
M.-Y. Chang, Y.-C. Cheng
Letter
Synlett
However, 5a was converted slowly into 6a at 25 °C in the
presence of a CDCl3 solution. It is also very likely that the
trace acid in CDCl3 catalyzed the intramolecular annulation.
To prevent this unexpected result, 5a was immediately re-
acted with BF3·OEt2. With these results in hand, the one-pot
conversion from 4a into 6a was examined next (Table 1).
hours. As shown in entries 6 and 7 (Table 1), Brønsted acids,
such as p-TsOH and polyphosphoric acid (PPA), showed dif-
ferent catalytic activities. p-TsOH provided a similar result
with entry 4 (Table 1), but PPA provided unknown products.
On the basis of a higher yield and activity, we believe that
1.0 equivalent of BF3·OEt2 should be the optimal reagent
(Table 1, entry 1) after examining the formation of skeleton
6. With the one-pot reaction conditions in hand (Table 1,
entry 1, 4a → 6a), we further explored the scope for the
conversion of other substrates, and the results are shown in
Table 2. For the Ar, Ar1 and R groups of 4a–p, the aryl rings
(Ar = Ph, 4-FC6H4, Tol, 4-NO2C6H4, 4-CF3C6H4, 4-PhC6H4,
Naph; Ar1 = Ph, 4-FC6H4, 4-PhC6H4; R = Tol, Ph), with elec-
tron-neutral or electron-donating groups were well tolerat-
ed, providing the desired 6a–p in moderate to good yields
(70–82%).7
Table 1 One-Pot Conditionsa
Tol
O
Tol
O
Tol
O
O
O
O
S
S
S
1) NaBH4
2) Lewis acid
Me
Ph
Ph
Ph
Ph
Ph
Ph
O
O
HO
5a
4a
6a
Entry
Lewis acid (equiv), solvent, temp (°C)
BF3·OEt2 (1.0), CH2Cl2, 25
BF3·OEt2 (2.0), CH2Cl2, 25
BF3·OEt2 (0.5), CH2Cl2, 25
BF3·OEt2 (1.0), (CH2Cl)2, 25
BF3·OEt2 (1.0), (CH2Cl)2, 84
p-TsOH (1.0), CH2Cl2, 25
PPA (1.0), CH2Cl2, 25
6a (%)b
75
1
2
3
4
5
6
7
Table 2 Synthesis of 6a
70
48c (58)d
69
R
R
O
O
S
S
O
O
1) NaBH4
Me
Ar1
40e (26)f
63g
Ar1
Ar
2) BF3·OEt2
Ar
O
O
6
4
h
–
a Reaction conditions: (1) 4a (1.0 mmol), NaBH4 (3 equiv), MeOH–THF (1:1;
10 mL), 0 °C, 1 h. (2) Resulting crude 5a, solvent (5 mL), 25 °C, 20 h.
b Isolated yields.
Entry
4, Ar, Ar1, R
6, Yield (%)b
1
2
4a, Ph, Ph, Tol
6a, 75
6b, 70
6c, 70
6d, 80
6e, 82
6f, 73
6g, 76
6h, 75
6i, 70
6j, 73
6k, 70
6l, 76
c Compound 5a was recovered (13%).
d Reaction time was 40 h and trace amounts of 5a were recovered.
e A complex mixture was isolated in 32% yield.
f Reaction time was 40 h and 43% of complex mixture was isolated.
g Compound 5a was recovered (10%).
4b, 4-FC6H4, Ph, Tol
4c, 4-MeC6H4, Ph, Tol
4d, 4-O2NC6H4, Ph, Tol
4e, 4-F3CC6H4, Ph, Tol
4f, 4-PhC6H4, Ph, Tol
4g, Naph, Ph, Tol
3
4
h Unknown products were observed.
5
6
7
Following the sequential reduction–hydroalkoxylation,
6a was isolated in a 75% yield over two steps via a one-pot
NaBH4 (3 equiv)-mediated reduction followed by treatment
of the resulting 5a with BF3·OEt2 (1.0 equiv). Compared
with the yield (65%) of the two divided steps, the one-pot
reaction condition could increase the efficiency of the over-
all yields of 6a (Table 1, entry 1). Furthermore, the amounts
(2.0 or 0.5 equiv) of BF3·OEt2, reaction solvents [CH2Cl2 or
(CH2Cl)2] and temperatures (25 °C or 84 °C) were also ex-
amined. However, attempts to provide a higher yield of 6a
were unsuccessful. In entry 2 (Table 1), no obvious yield
changes were observed when 2.0 equivalents of BF3·OEt2
were used. To decrease the amount of BF3·OEt2 (0.5 equiv),
6a was isolated in a 48% yield along with the recovery of 5a
(13%), as shown in entry 3 (Table 1). With a long reaction
time (40 h), a similar yield distribution was observed.
Changing the reaction solvent from CH2Cl2 to (CH2Cl)2, 6a
was produced in a 69% yield (see entry 4 in Table 1). After
elevating the temperature (r.t. → reflux), the desired 6a was
only isolated in a 40% yield (see entry 5 in Table 1). When
the reaction time was prolonged, a complex mixture was
isolated in a higher (43%) yield in boiling (CH2Cl)2 after 40
8
4h, Naph, 4-FC6H4, Tol
4i, Naph, 4-PhC6H4, Tol
4j, Ph, 4-PhC6H4, Tol
4k, Tol, 4-PhC6H4, Tol
4l, 4-PhC6H4, 4-PhC6H4, Tol
4m, Ph, Ph, Ph
9
10
11
12
13
14
15
16
6m, 70
6n, 78
6o, 75
6p, 74
4n, 4-FC6H4, Ph, Ph
4o, 4-PhC6H4, Ph, Ph
4p, 4-PhC6H4, 4-PhC6H4, Ph
a Reaction conditions: (1) 4 (1.0 mmol), NaBH4 (3.0 equiv), MeOH (5 mL),
THF (5 mL), 0 °C, 1 h (2) BF3·OEt2 (1.0 equiv), CH2Cl2 (5 mL), 25 °C, 20 h.
b Isolated yield.
In summary, we have developed a mild, facile and one-
pot synthesis of 2,5-diaryltetrahydrofurans 6 in good yields
via a NaBH4-mediated stereocontrolled reduction of α-sty-
ryl-β-ketosulfones 4 and an intramolecular BF3·OEt2-medi-
ated Friedel–Crafts electrophilic cyclization of the resulting
4-alkenols 5 under a reduction–hydroalkoxylation process.
A plausible mechanism has been discussed and proposed.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
M.-Y. Chang, Y.-C. Cheng
Letter
Synlett
Further investigation regarding the synthetic applications
of β-ketosulfones will be conducted and published in due
course.
939. (g) Tang, X.; Huang, L.; Xu, Y.; Yang, J.; Wu, W.; Jiang, H.
Angew. Chem. Int. Ed. 2014, 53, 4205. (h) Chawla, R.; Singh, A. K.;
Yadav, L. D. S. Eur. J. Org. Chem. 2014, 10, 2032. (i) Singh, A. K.;
Chawla, R.; Yadav, L. D. S. Tetrahedron Lett. 2014, 55, 4742.
(j) Shi, X.; Ren, X.; Ren, Z.; Li, J.; Wang, Y.; Yang, S.; Gu, J.; Gao,
Q.; Huang, G. Eur. J. Org. Chem. 2014, 10, 5083. (k) Xuan, J.; Feng,
Z.-J.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Chem. Eur. J. 2014, 20,
3045.
Acknowledgment
The authors would like to thank the Ministry of Science and Technol-
ogy of the Republic of China for financial support (MOST 104-2113-
M-037-012).
(5) (a) Mengel, A.; Reiser, O. Chem. Rev. 1999, 99, 1191. (b) Gung, B.
W. Tetrahedron 1996, 52, 5263. (c) Ager, D. J.; East, M. B. Tetra-
hedron 1992, 48, 2803.
(6) CCDC 1429280 (6a) and 1434699 (6b) contain the supplemen-
tary crystallographic data for this paper. This data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the CCDC, 12 Union Road, Cambridge CB2
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560423.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
1EZ,
UK;
fax:
+44(1223)336033;
e-mail:
deposit@ccdc.cam.ac.uk].
(7) Representative Synthetic Procedure of Skeleton 6: NaBH4
(100 mg, 3.0 mmol) was added to a solution of skeleton 4 (1.0
mmol) in a co-solvent of THF (5 mL) and MeOH (5 mL) at 0 °C.
The reaction mixture was stirred for 1 h at 0 °C and the solvent
was concentrated. The residue was diluted with H2O (10 mL)
and the mixture was extracted with CH2Cl2 (3 × 20 mL). The
combined organic layers were washed with brine, dried, filtered
and evaporated to afford the crude product. Without further
purification, BF3·OEt2 (142 mg, 1.0 mmol) was added to a solu-
tion of the resulting skeleton 5 in CH2Cl2 (5 mL) at 25 °C. The
reaction mixture was stirred at 25 °C for 20 h. The reaction
mixture was concentrated and the residue was diluted with H2O
(10 mL) and the mixture was extracted with CH2Cl2 (3 × 20 mL).
The combined organic layers were washed with brine, dried, fil-
tered and evaporated to afford the crude product under reduced
pressure. Purification on silica gel (hexanes–EtOAc, 8:1 to 3:1)
yielded the skeleton 6. Compound 6a: yield: 75% (294 mg); col-
orless solid; mp 173–174 °C (recrystallized from hexanes and
EtOAc). 1H NMR (400 MHz, CDCl3): δ = 7.39–7.42 (m, 4 H), 7.16–
7.30 (m, 8 H), 7.07 (d, J = 8.0 Hz, 2 H), 5.12 (d, J = 8.0 Hz, 1 H),
3.99 (q, J = 8.0 Hz, 1 H), 2.88 (dd, J = 1.2, 8.4 Hz, 2 H), 2.37 (s, 3
H), 1.78 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 145.27, 143.82,
136.26, 135.70, 129.36 (2 ×), 128.62 (2 ×), 128.50 (2 ×), 128.14,
127.82 (2 ×), 127.58 (2 ×), 127.18, 124.50 (2 ×), 83.80, 79.46,
67.64, 40.14, 30.31, 21.48. HRMS (ESI): m/z [M+ + 1] calcd for
References and Notes
(1) For reviews on the synthesis of tetrahydrofurans, see:
(a) Rjabovs, V.; Turks, M. Tetrahedron 2013, 69, 10693.
(b) Bellur, E.; Feist, H.; Langer, P. Tetrahedron 2007, 63, 10865.
(c) Lorente, A.; Lamariano-Merketegi, J.; Albericio, F.; Alvarez,
M. Chem. Rev. 2013, 113, 4567. (d) Jalce, G.; Franck, X.; Figadere,
B. Tetrahedron: Asymmetry 2009, 20, 2537. (e) Wolfe, J. P.; Hay,
M. B. Tetrahedron 2007, 63, 261.
(2) For representative examples of 2,5-diaryltetrahydrofuran skele-
ton in natural products, e.g. manassantin
A and B, see:
(a) Hanessian, S.; Reddy, G. J.; Chahal, N. Org. Lett. 2006, 8, 5477.
(b) Kim, H.; Kasper, A. C.; Moon, E. J.; Park, Y.; Wooten, C. M.;
Dewhirst, M. W.; Hong, J. Org. Lett. 2009, 11, 89. For virgatusin,
see: (c) Akindele, T.; Marsden, S. P.; Cumming, J. G. Org. Lett.
2005, 17, 3685. (d) Kim, H.; Wooten, C. M.; Park, Y.; Hong, J. Org.
Lett. 2007, 9, 3965. (e) Jahn, U.; Rudakov, D. Org. Lett. 2006, 8,
4481. (f) Martinet, S.; Meou, A.; Brun, P. Eur. J. Org. Chem. 2009,
2306. For talaumidin, see: (g) Harada, K.; Kubo, M.; Horiuchi, H.;
Ishii, A.; Esumi, T.; Hioki, H.; Fukuyama, Y. J. Org. Chem. 2015,
80, 7076. For MK-287, see: (h) Girotra, N. N.; Ponpipom, M. M.;
Acton, J. J.; Alberts, A. W.; Bach, T. N.; Ball, R. G.; Bugianesi, R. L.;
Parsons, W. H. J. Med. Chem. 1992, 35, 3474. (i) Thompson, A. S.;
Tschaen, D. M.; Simpson, P.; McSwine, D. J.; Reamer, R. A.;
Verhoeven, T. R.; Shinkai, I. J. Org. Chem. 1992, 57, 7044. During
the preparation of this manuscript, Saikia and co-workers
demonstrated the feasibility of this BF3·OEt2-mediated annula-
tions, see: (j) Sultana, S.; Devi, N. R.; Saikia, A. K. Asian J. Org.
Chem. 2015, 4, 1281. (k) Ghosh, P.; Deka, M. J.; Saikia, A. K. Tet-
rahedron 2016, 72, 690.
C
C
24H25O3S: 393.1524; found: 393.1530. Anal. Calcd for
24H24O3S: C, 73.44; H, 6.16. Found: C, 73.70; H, 6.28. Single-
crystal X-ray diagram: crystal of compound 6a was grown by
slow diffusion of EtOAc into a solution of compound 6a in
CH2Cl2 to yield colorless prisms. The compound crystallizes in
the monoclinic crystal system, space group
P 21/c, a =
(3) (a) Chang, M.-Y.; Chen, Y.-C.; Chan, C.-K. Tetrahedron 2014, 70,
8908. (b) Chang, M.-Y.; Cheng, Y.-J.; Lu, Y.-J. Org. Lett. 2014, 16,
6252. (c) Chang, M.-Y.; Lu, Y.-J.; Cheng, Y.-C. Tetrahedron 2015,
71, 1192. (d) Chang, M.-Y.; Cheng, Y.-J.; Lu, Y.-J. Org. Lett. 2015,
17, 1264. (e) Chang, M.-Y.; Cheng, Y.-J.; Lu, Y.-J. Org. Lett. 2015,
17, 3142. (f) Chang, M.-Y.; Cheng, Y.-J. Org. Lett. 2015, 17, 5702.
(4) For recent syntheses of β-ketosulfones, see: (a) Pospisil, J.; Sato,
H. J. Org. Chem. 2011, 76, 2269. (b) Sreedhar, B.; Rawat, V. S.
Synlett 2012, 23, 413. (c) Kumar, A.; Muthyala, M. K. Tetrahedron
Lett. 2011, 52, 5368. (d) Lu, Q.; Zhang, J.; Zhao, G.; Qi, Y.; Wang,
H.; Lei, W. J. Am. Chem. Soc. 2013, 135, 11481. (e) Tsui, G. C.;
Glenadel, Q.; Lau, C.; Lautens, M. Org. Lett. 2011, 13, 208.
(f) Zhou, G.; Ting, P. T.; Aslanian, R. G. Tetrahedron Lett. 2010, 51,
13.9520(8) Å, b = 14.5100(7) Å, c = 11.1921(6) Å, V = 2096.6(2)
Å3, Z = 4, dcalcd = 1.243 mg/cm3, F(000) = 832, 2θ range 1.577–
26.410°, R indices (all data) R1 = 0.0729, wR2 = 0.1568. Com-
pound 6b: yield: 70% (287 mg); colorless solid; mp 154–155 °C
(recrystallized from hexanes and EtOAc). 1H NMR (400 MHz,
CDCl3): δ = 7.50–7.53 (m, 2 H), 7.37–7.42 (m, 2 H), 7.32 (d, J =
8.0 Hz, 2 H), 7.27–7.29 (m, 3 H), 7.15 (d, J = 8.0 Hz, 2 H), 6.89–
6.94 (m, 2 H), 5.49 (d, J = 8.0 Hz, 1 H), 4.39 (q, J = 8.0 Hz, 1 H),
2.85 (dd, J = 10.0, 12.8 Hz, 1 H), 2.56 (dd, J = 8.4, 12.8 Hz, 1 H),
2.39 (s, 3 H), 1.57 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 162.55
(d, J = 244.8 Hz), 146.02, 144.22, 136.30, 134.10 (d, J = 3.0 Hz),
130.26 (d, J = 8.4 Hz, 2 ×), 129.45 (2 ×), 128.24 (2 ×), 127.77 (2 ×),
127.00, 124.48 (2 ×), 114.32 (d, J = 22.0. Hz, 2 ×), 83.24, 78.72,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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M.-Y. Chang, Y.-C. Cheng
Letter
Synlett
67.46, 40.53, 27.80, 21.41. HRMS (ESI): m/z [M+ + 1] calcd for
24H24FO3S: 411.1430; found: 411.1435. Single-crystal X-ray
diagram: crystal of compound 6b was grown by slow diffusion
of EtOAc into a solution of compound 6b in CH2Cl2 to yield col-
orless prisms. The compound crystallizes in the monoclinic
crystal system, space group P 21/c, a = 5.8096(3) Å, b =
15.7969(8) Å, c = 22.8181(12) Å, V = 2093.57(19) Å3, Z = 4, dcalcd
= 1.302 mg/cm3, F(000) = 864, 2θ range 1.568–26.542°, R indices
(all data) R1 = 0.0645, wR2 = 0.1086.
C
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E