An improved synthesis of 10-isobornylsultone

Article abstract of DOI:10.1016/j.tetasy.2014.06.016

A modified and improved synthesis of 10-isobornylsultone in three steps and in 81% overall yield starting from (+)-camphor-10-sulfonic acid is described. The synthesis proceeds via methyl sulfonate ester formation, stereoselective reduction and base-catalysed intramolecular cyclisation.

Full text of DOI:10.1016/j.tetasy.2014.06.016

Tetrahedron: Asymmetry 25 (2014) 1150–1152
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An improved synthesis of 10-isobornylsultone
⇑
Frank W. Lewis , David H. Grayson
Centre for Synthesis and Chemical Biology, School of Chemistry, Trinity College, Dublin 2, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2014
Accepted 25 June 2014
A modified and improved synthesis of 10-isobornylsultone in three steps and in 81% overall yield starting
from (+)-camphor-10-sulfonic acid is described. The synthesis proceeds via methyl sulfonate ester forma-
tion, stereoselective reduction and base-catalysed intramolecular cyclisation.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Derivatives of the bicyclic monoterpene (+)-camphor have been
widely used both as chiral control elements in asymmetric synthe-
sis and as chiral templates in enantiospecific natural product syn-
theses due to its rigid bicyclic structure.1 One such derivative is
(À)-10-isobornylsultone 1 (Fig. 1). The known2 thermal rearrange-
ment of sultone 1 to sultone 2 has been exploited in the enantio-
specific total synthesis of b-santalol3 and b-santalene,4 which are
the two main constituents of East Indian sandalwood oil. In addi-
tion, sultone 1 is a key precursor to several important chiral auxil-
iaries. Cleavage of the SAO bond of 1 with various nucleophiles
gives rise to isoborneol sulfonamides 3,5 isoborneol arylsulfones
46 and 10-mercaptoisoborneol 57 which have all been frequently
employed as chiral auxiliaries in asymmetric synthesis.8–11
O
OH
O
SO2
SO2
X
2
3 X = SO2NR2
1
4
5
X = SO2Ar
X = SH
Figure 1. The structures of isobornylsultone 1, its thermal rearrangement isomer 2
and its derived chiral auxiliaries 3, 4 and 5.
synthesis,14 we required a short synthesis of the methyl sulfonate
ether intermediate 9. We proposed to synthesise 9 by the route
outlined in Scheme 1.
Accordingly, methyl sulfonate ester
7 was conveniently
Sultone 1 has previously been synthesised by the sulfonation of
camphene using acetic anhydride/fuming sulfuric acid,2a or by the
reduction of (+)-camphor-10-sulfonic acid with sodium borohy-
dride followed by exposure of the intermediate sodium isobornyl-
sulfonate salt to p-toluenesulfonyl chloride in pyridine.2c,3,5,6a In a
more recent report, Kaye obtained sultone 1 as a minor by-product
(4% yield) during the reduction of phenyl (+)-camphor-
10-sulfonate with sodium borohydride in H2O/EtOH at À8 °C, with
sultone 1 becoming the sole product (94% yield) when the reduc-
tion was performed at 25 °C in absolute ethanol.12 Herein we
report a modified and improved synthesis of sultone 1.13
obtained in 90% yield by treating (+)-camphor-10-sulfonic acid 6
CH3C(OCH3)3
DCM
O
O
SO3CH3
SO3H
7
6
NaBH4
MeOH
2. Results and discussion
During the course of our ongoing studies on the utility of cam-
phor-derived sulfones as novel chiral auxiliaries in asymmetric
NaH
OCH3
OH
CH3I
THF
SO3CH3
SO3CH3
⇑
Corresponding author at Present address: Department of Chemical and Forensic
Sciences, Faculty of Applied Sciences, Northumbria University, Newcastle upon
Tyne NE1 8ST, UK. Tel.: +44 191 227 4637; fax: +44 191 227 3519.
E-mail address: frank.lewis@northumbria.ac.uk (F.W. Lewis).
9
8
Scheme 1.
http://dx.doi.org/10.1016/j.tetasy.2014.06.016
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

F. W. Lewis, D. H. Grayson / Tetrahedron: Asymmetry 25 (2014) 1150–1152
1151
with trimethyl orthoacetate according to the procedure of Gopa-
lan.15 Reduction of 7 with excess sodium borohydride in methanol
proceeded with high stereoselectivity to afford the novel exo-
configured isoborneol sulfonate ester 8 along with ca. 10% of its
endo-configured borneol isomer 10 in 95% combined yield as deter-
mined by 1H NMR spectroscopy (Scheme 2). This mixture was used
in the next step without further purification. The exo-alcohol 8 was
characterised by a double doublet at ca. d 4.1 ppm for the C2
endo-methine proton as well as a pair of doublets at ca. d 3.0 and
3.6 ppm corresponding to the methylene protons adjacent to the
sulfonate ester group.
tone 1 (with the 8:1 ratio ranging from 17:1 to 1:1.3) in overall
yields of not higher than 44%. The reduced tendency of sulfonate
ester 8 to cyclise to 1 under these conditions compared to Kaye’s
phenyl sulfonate ester (which cyclises smoothly to 1 in the absence
of additional base)12 can be attributed to the greater leaving group
ability of the phenoxy-group in the latter compound.
However, the new synthesis of 10-isobornylsultone 1 reported
herein has several advantages compared to previous syntheses
reported in the literature. Firstly, the one-step esterification of
(+)-camphor-10-sulfonic acid 6 to form 7 using trimethyl orthoac-
etate is superior to the literature synthesis of the analogous phenyl
sulfonate ester,17 which has to be carried out over two steps from 6
[i.e., conversion of 6 into (+)-camphor-10-sulfonyl chloride using
thionyl chloride, followed by treatment of the product with phenol
in pyridine], and which employs toxic (phenol, pyridine) and corro-
sive (thionyl chloride) reagents.18 Secondly, the optimised synthe-
sis of 1 reported herein proceeds in higher overall yield (81%) than
previous syntheses of 1 from either (+)-camphor-10-sulfonyl chlo-
ride (76%)12 or from (+)-camphor-10-sulfonic acid 6 (44% overall
by Kaye’s method,12 75% overall when thionyl chloride is used to
convert 6 to (+)-camphor-10-sulfonyl chloride19). Finally, our
reduction of the sulfonate ester 7 to yield 8 requires a smaller
excess of sodium borohydride (3 equiv) than Kaye’s reduction of
the analogous phenyl sulfonate ester (which requires 10 equiv of
sodium borohydride).12
CH3C(OCH3)3
DCM
O
O
SO3CH3
SO3H
7
6
NaBH4
MeOH, rt
OH
+
OH
SO3CH3
3. Conclusion
SO3CH3
10
8
In conclusion, we have reported on an unexpected and
improved synthesis of (À)-10-isobornylsultone 1 from (+)-cam-
phor-10-sulfonic acid 6. This modified and convenient synthesis
gives 1 in three steps from 6 in a high overall yield of 81% without
the need for column chromatography or the use of toxic and corro-
sive reagents.
Scheme 2.
Treatment of 8 with sodium hydride in the presence of methyl
iodide in dry THF at ambient temperature for 1 h failed to afford
the desired methyl ether 9 (Scheme 1). Instead, isobornyl sultone
1 was obtained as the sole product. When the reaction was
repeated under identical conditions in the absence of methyl
iodide, sultone 1 was obtained in 60% yield. Performing the cyclisa-
tion reaction at 0 °C for 1 h resulted in a greatly improved yield of 1
of 95% (Scheme 3). The NMR spectroscopic data of 1 were identical
to those published by Pinho e Melo.6b Interestingly, carrying out
the reaction at ambient temperature for 24 h afforded 1 in a much
lower yield (15%). Evaporation of the aqueous phase from this reac-
tion furnished an off-white solid whose 1H NMR spectrum in D2O
suggested the presence of the corresponding sodium isobornylsulf-
onate salt,16 which was most likely obtained by simple methanol-
ysis or hydrolysis of the sulfonate ester group of 8.
4. Experimental
4.1. General
NMR spectra were recorded using a Bruker AVANCE DPX
400 MHz spectrometer (400.1 MHz for 1H and 100.6 MHz for
13C). Chemical shifts are reported in parts per million. Coupling
constants (J) are quoted in Hertz. Optical rotations were measured
using a Perkin–Elmer 141 polarimeter. IR spectra were recorded for
Nujol mulls (N) or liquid films (L) on a Mattson Genesis II FTIR
spectrometer. Mass spectra were obtained under electrospray con-
ditions using a Micromass LCT instrument. Uncorrected melting
points (Mp) were measured in unsealed capillary tubes using a
Griffin melting point apparatus. Tetrahydrofuran (THF) was dried
and distilled over sodium-benzophenone ketyl prior to use. All
other solvents and reagents were purified by standard techniques.
Organic extracts of reaction products were dried over anhydrous
magnesium sulfate.
NaH
OH
O
THF
0 °C, 1 h
S
O2
SO3CH3
8
1
4.2. (1S,4R)-Methyl (7,7-dimethyl-2-oxobicyclo[2.2.1]hept-1-
yl)methanesulfonate 7
Scheme 3.
Following the procedure of Gopalan,15 (+)-camphor-10-sulfonic
acid 6 (3.0 g, 12.9 mmol) was suspended in DCM (50 mL) and
trimethyl orthoacetate (8.21 mL, 64.5 mmol) was added. The
resulting solution was stirred at ambient temperature for 90 min.
The solution was then evaporated to yield an oil. Removal of excess
trimethyl orthoacetate in vacuo (0.1 mm Hg) afforded the title
compound 7 as a pale pink solid (2.88 g, 90%). Mp 61–63 °C
Inspired by Kaye’s synthesis of 1 via reduction of the analogous
phenyl sulfonate ester followed by cyclisation,12 we next sought to
synthesise sultone 1 from ester 7 in a one-pot procedure via reduc-
tion of
7 and subsequent base-catalysed cyclisation in situ.
However, these attempts were met with limited success. Various
reductions of 7 with excess (3–4 equiv) sodium borohydride in
methanol or in isopropanol, and subsequent additions of potas-
sium tert-butoxide (1 equiv) led at best to mixtures of 8 and sul-
(DCM); Lit.20 61 °C; [
a]
22 = +42.7 (c 1.61, CHCl3); Lit.20 +43.6 (c
D
5%, CHCl3, 23 °C); IR mmax (N) 2923, 2723, 2669, 1741 (C@O),

1152
F. W. Lewis, D. H. Grayson / Tetrahedron: Asymmetry 25 (2014) 1150–1152
1456, 1376, 1301, 1269, 1170 (S@O), 1054, 989 (S@O), 808,
747 cmÀ1 1H NMR (CDCl3) 0.88 (s, 3H, 7-CH3), 1.10 (s, 3H,
References
;
7-CH3), 1.41–1.48 (m, 1H), 1.63–1.70 (m, 1H), 1.95 (d, 2J = 18.5,
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2.39 (dt, 2J = 18.5, 3J = 4.5, 1H, 3-CH2 exo), 2.43–2.50 (m, 1H), 2.98
(d, 2J = 15.0, 1H, CH2SO3CH3), 3.59 (d, 2J = 15.0, 1H, CH2SO3CH3),
3.95 (s, 3H, SO3CH3); 13C NMR (CDCl3) 19.2 (7-CH3), 19.2 (7-CH3),
24.3 (C-5), 26.4 (C-6), 42.0 (C-3), 42.2 (C-4), 45.5 (CH2SO3CH3),
47.5 (C-7), 55.7 (SO3CH3), 57.3 (C-1), 214.1 (C-2); HRMS (CI, MeOH)
m/z calcd for C11H18O4S [M+Na]+: 269.0824; found: 269.0825.
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Methyl sulfonate ester 7 (0.25 g, 1.01 mmol) was dissolved in
methanol (10 mL) and solid sodium borohydride (0.115 g,
3.04 mmol) was added. The solution was stirred at ambient temper-
ature for 1 h. Acetic acid (2 mL) was then added and sodium carbon-
ate was added until the pH was neutral. The mixture was diluted
with ethyl acetate (100 mL) and washed with water (50 mL) and
saturated aqueous sodium hydrogen carbonate (2 Â 50 mL), dried
and evaporated to afford the title compound 8 as a colourless oil
(0.24 g, 95%) contaminated with ca. 10% of the corresponding
endo-isomer 10. IR mmax (L) 3554 (OAH), 2957, 2883, 1456, 1352
(S@O), 1257, 1166 (S@O), 1076, 992, 879, 807, 746 cmÀ1 1H NMR
;
(CDCl3) 0.86 (s, 3H, 7-CH3), 1.09 (s, 3H, 7-CH3), 1.13–1.17 (m, 1H),
1.45–1.51 (m, 1H), 1.69–1.88 (m, 5H), 2.83 (br s, 1H, OAH), 2.99
(d, 2J = 14.0, 1H, CH2SO3CH3), 3.56 (d, 2J = 14.0, 1H, CH2SO3CH3),
3.94 (s, 3H, SO3CH3), 4.09 (dd, 3J1 = 7.5, 3J2 = 4.5, 1H, 2-CH); 13C
NMR (CDCl3) 19.3 (7-CH3), 20.0 (7-CH3), 26.8 (C-5), 29.7 (C-6),
38.7 (C-3), 43.9 (C-4), 48.3 (C-7), 48.5 (CH2SO3CH3), 49.3 (C-1),
54.9 (SO3CH3), 75.7 (C-2); HRMS (CI, MeOH) m/z calcd for
C
11H20O4S [M+H]+: 249.1161; found: 249.1153.
4.4. (1S,5R,7R)-10,10-Dimethyl-4-oxa-3-thiatricyclo[5.2.1.01,5]-
decane 3,3-dioxide 1
A solution of the sulfonate ester 8 (0.58 g, 2.33 mmol) in dry
THF (10 mL) was added to a suspension of sodium hydride
(0.14 g, 60%, 3.50 mmol, 1.5 equiv) in dry THF (5 mL) at 0 °C under
an atmosphere of nitrogen. The solution was stirred at 0 °C for 1 h.
The solution was then diluted with H2O (50 mL) and extracted with
DCM (3 Â 50 mL). The combined organic extracts were dried and
evaporated to yield a semi-solid material which crystallised in
vacuo (0.1 mm Hg) to afford the title compound 1 as a white solid
(0.48 g, 95%). Mp 111–113 °C (ether); Lit.3 114–116 °C;
11. 10-Mercaptoisoborneol 5 has also been used to generate chiral ligands for
asymmetric catalysis, see: (a) Nakano, H.; Okuyama, Y.; Hongo, H. Tetrahedron
Lett. 2000, 41, 4615–4618; (b) Nakano, O.; Yanagida, H. J. Org. Chem. 2001, 66,
620–625; (c) Okuyama, Y.; Nakano, H.; Kabuto, C.; Nozawa, E.; Takahashi, K.;
Hongo, H. Heterocycles 2002, 58, 457–469; (d) Nakano, H.; Suzuki, Y.; Kabuto,
C.; Fujita, R.; Hongo, H. J. Org. Chem. 2002, 67, 5011–5014; (e) Okuyama, Y.;
Nakano, H.; Takahashi, K.; Hongo, H.; Kabuto, C. Chem. Commun. 2003, 524–
525; (f) Nakano, H.; Takahashi, K.; Fujita, R. Heterocycles 2004, 64, 407–416; (g)
Nakano, H.; Takahashi, K.; Suzuki, Y.; Fujita, R. Tetrahedron: Asymmetry 2005,
16, 609–614; (h) Okuyama, Y.; Nakano, H.; Saito, Y.; Takahashi, K.; Hongo, H.
Tetrahedron: Asymmetry 2005, 16, 2551–2557.
12. Duggan, A. R.; Kaye, P. T.; Caira, M. R. J. Chem. Res. 2006, 744–747.
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605; (b) Kawe˛cki, R. Tetrahedron: Asymmetry 1999, 10, 4183–4190; (c) Kawe˛cki,
R. J. Org. Chem. 1999, 64, 8724–8727.
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1531–1535; (b) Lewis, F. W.; McCabe, T. C.; Grayson, D. H. Tetrahedron 2011,
67, 7517–7528; (c) Lewis, F. W.; Grayson, D. H. Tetrahedron: Asymmetry 2012,
23, 643–649.
15. Trujillo, J. I.; Gopalan, A. S. Tetrahedron Lett. 1993, 34, 7355–7358.
16. This compound had 1H NMR (D2O) 0.74 (s, 3H, 7-CH3), 0.90 (s, 3H, 7-CH3),
1.00–1.03 (m, 1H), 1.28–1.32 (m, 1H), 1.58–1.67 (m, 5H), 2.76 (d, 2J = 14.6, 1H,
CH2SO3Na), 3.21 (d, 2J = 14.6, 1H, CH2SO3Na), 3.95–3.98 (m, 1H, 2-CH) ppm.
17. (a) Kaye, P. T.; Molema, W. E. Chem. Commun. 1998, 2479–2480; (b) Procter, D.
J.; Archer, N. J.; Needham, R. A.; Bell, D.; Marchington, A. P.; Rayner, C. M.
Tetrahedron 1999, 55, 9611–9622.
[a
]
D
27 = À3.7 (c 0.33, CCl4); Lit.2c À4.05 (c 10, CCl4, 25 °C); IR mmax
(N) 2920, 1459, 1412, 1375, 1345 (S@O), 1258, 1227, 1177
(S@O), 1149, 1111, 1079, 1026, 990, 940, 893, 864, 815, 737,
683 cmÀ1 1H NMR (CDCl3) 0.95 (s, 3H, 10-CH3), 1.12 (s, 3H,
;
10-CH3), 1.24–1.29 (m, 1H), 1.38–1.43 (m, 1H), 1.86–1.97 (m,
4H), 2.28 (dd, 2J = 14.0, 3J = 3.5, 1H), 3.19 (d, 2J = 13.5, 1H, 2-CH2),
3.29 (d, 2J = 13.5, 1H, 2-CH2), 4.38 (dd, 3J1 = 7.7, 3J2 = 3.5, 1H,
5-CH); 13C NMR (CDCl3) 19.3 (10-CH3), 19.4 (10-CH3), 26.2 (C-8),
28.6 (C-9), 35.3 (C-6), 43.9 (C-7), 46.9 (C-10), 48.6 (C-2), 55.0
(C-1), 87.5 (C-5); HRMS (CI, MeOH) m/z calcd for C10H16O3S
[M+H]+: 217.0899; found: 217.0898.
18. Phenyl camphor-10-sulfonate was recently synthesized directly from (+)-
camphor-10-sulfonic acid 6 in 68% yield using diphenyliodonium salts, see:
Jalalian, N.; Petersen, T. B.; Olofsson, B. Chem. Eur. J. 2012, 18, 14140–14149.
19. The highest reported yield for the conversion of 6 to (+)-camphor-10-sulfonyl
chloride is 99%, see: Jung, J.-C.; Kache, R.; Vines, K. K.; Zheng, Y.-S.; Bijoy, P.;
Valluri, M.; Avery, M. A. J. Org. Chem. 2004, 69, 9269–9284.
Acknowledgments
We thank Ms Breege Masterson for some preliminary experi-
ments, Dr. John E. O’Brien and Dr. Manuel Reuther for obtaining
NMR spectra and Dr. Martin Feeney for obtaining mass spectra.
One of us (F.W.L.) received financial support from Trinity College
Dublin.
20. Edminson, S. R.; Hilditch, T. P. J. Chem. Soc. 1910, 223–231.