Reversal of enantioselectivity on protonation of enol(ate)s derived from 2-methyl-1-tetralone using C2-symmetric sulfonamides

Article abstract of DOI:10.1016/j.tetlet.2004.10.087

The synthesis of enantiomerically enriched (R)-2-methyl-1-tetralone 1 (64% e.e.) was achieved through protonation of its lithium enolate 3 using a C ₂-symmetrical bis-sulfonamide 5d as an internal proton source. Access to the complementary (S)-enantiomer 1 (45% e.e.) can be achieved using an external quench strategy involving acetic acid as the external proton source.

Full text of DOI:10.1016/j.tetlet.2004.10.087

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 9465–9468
Reversal of enantioselectivity on protonation of enol(ate)s
derived from 2-methyl-1-tetralone using C2-symmetric sulfonamides
Ewan Boyd,a Gregory S. Coumbarides,b Jason Eames,b,* Alastair Hay,a
Ray V. H. Jones,a Rachel A. Stensonc and Michael J. Suggateb
aSyngenta, Grangemouth Manufacturing Centre, Earls Road, Grangemouth, Scotland FX3 8XG, UK
bDepartment of Chemistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
cSyngenta, Global Specialist Technology, PO Box A38, Leeds Road, Huddersfield HD2 1FF, UK
Received 1 September 2004; revised 6 October 2004; accepted 14 October 2004
Available online 5 November 2004
Abstract—The synthesis of enantiomerically enriched (R)-2-methyl-1-tetralone 1 (64% e.e.) was achieved through protonation of its
lithium enolate 3 using a C2-symmetrical bis-sulfonamide 5d as an internal proton source. Access to the complementary (S)-enanti-
omer 1 (45% e.e.) can be achieved using an external quench strategy involving acetic acid as the external proton source.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
from 2-methyl tetralone 1 and compare the use of these
bis-sulfonamides as chiral scaffolds for internal and
external enantioselective protonation. We were inter-
ested in the use of chiral bis-sulfonamides as proton do-
nors due to their mild acidity.6 For this study, we
focussed on the use of chiral bis-sulfonamides7 derived
from the conformationally rigid C2-symmetric 1,2-
diaminocyclohexane (R,R)-4 as this chiral scaffold has
shown to lead to good levels of facial selectivity for a
variety of enantioselective processes.8
The synthesis of optical active a-substituted carbonyl
compounds by enantioselective protonation of enolic
and enolate species, derived from racemic carbonyl com-
pounds is well documented.1 However, there are a lim-
ited number of reports into the use of chiral proton
donors that exhibit good levels of enantiocontrol for a
wide range of structurally related ketones.2 Within this
area, there are two general approaches; the more com-
mon has focussed on the use of an internal chiral proton
source, such as enantiomerically pure chiral acids,3
whereas, the use of an external achiral proton source
in the presence of a chiral ligand has attracted much less
attention.4 Reports into the comparison between these
two strategies are rare.5
We decided to use ( )-2-methyl-1-tetralone 1 as our par-
ent ketone due to its predictable enolate stereochemistry
and known enantiomeric separability,9 and the corre-
sponding enol acetate 210 as our pro-enolate precursor
due to improved thermal stability over other related enol
derivatives (Scheme 1).11 This enol acetate 2 was effi-
ciently synthesised by addition of acetic anhydride and
HClO4 to a solution of ( )-2-methyl-1-tetralone 1 and
CCl4 at room temperature (Scheme 1).10 The required
We now report our study into the use of C2-symmetrical
bis-sulfonamides as potential chiral proton donors for
the enantioselective protonation of enol(ate)s derived
OLi-t-BuOLi
O
OAc
H
Me
Me
Me
MeLi.LiBr (2 equiv.)
THF, -78 oC
Ac2O, HClO4
CCl4
2 × LiBr
3
2; 94%
(rac)-1
Scheme 1. Synthesis of enol acetate 2 and conversion to the lithium enolate 3.
*
Corresponding author. Tel./fax: +44 2078825251; e-mail addresses: j.eames@qmul.ac.uk; jason.eames@ntlworld.com
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.10.087

9466
E. Boyd et al. / Tetrahedron Letters 45 (2004) 9465–9468
O
S
N
lone 1 in moderate yield (Scheme 3). The overall levels of
enantioselectivity varied considerably and was evidently
dependent on the structural nature of the bis-sulfona-
mide used.
O
H
R
H
NH2
NH2
RSO2Cl
H
H
Et3N, CH2Cl2
N
S
O
H
H
O
R
The highest enantioselectivity was obtained using the bis-
2-naphthyl sulfonamide 5d, which gave the (R)-enantio-
mer of 2-methyl tetralone 1 with 64% enantiomeric
excess (Scheme 3: entry 4). It appears that the structural
nature of the sulfonamide was very important on the
facial outcome of protonation and the overall level of
enantioselectivity. For the simplest unsubstituted bis-
aryl sulfonamide 5a and the bis-toluyl-sulfonamide 5b
formation of the other complementary (S)-enantiomer
1 was preferred although with low enantiomeric excess
(Scheme 3: entries 1 and 2). Whereas, the constitutional
isomer, bis-1-naphthyl sulfonamide 5c surprisingly gave
no enantioselectivity presumably due to equally
favoured facial protonation pathways (Scheme 3: entry
3). By comparison, the sterically demanding 2,4,6-tri-iso-
propylphenyl bis-sulfonamide 5e favoured formation of
the (R)-2-methyl tetralone 1 with 24% enantiomeric
excess. From these facial selectivities it appears that the
aryl substituents present within the bis-sulfonamides
5a–e influence the facial protonation more than the
(R,R)-1,2-diaminocyclohexane sub-structure itself. This
may be a consequence of differing aggregates present
within each lithium enolate sub-structure and the associ-
ated lithium enolate complex in solution.
(R,R)-4
(R,R)-5a-e
Entry
Aryl sulfonyl chloride
R = C6H5-
Bis-sulfonamide
% Yield
1
2
(R,R)-5a[7a]
(R,R)-5b[7b]
(R,R)-5c[7c]
71
79
R = 4-Me-C6H4-
R = 1-naphthyl-
R = 2-naphthyl-
R = 2,4,6-(i-Pr)3-C6H2-
72
84
3
4
5
(R,R)-5d[7d]
(R,R)-5e[7e]
94
Scheme 2. Synthesis of C2-symmetric sulfonamides (R,R)-5a–e.
lithium enolate complex 3 was liberated from the enol
acetate 2 using House and Trostꢀs approach,12 by simple
addition of MeLiÆLiBr (2equiv).13
We initially screened a series of structurally related C2-
symmetric bis-sulfonamides 5a–e7 as potential chiral
Brønsted acids against the lithium enolate complex 3
to determine their relative facial protonation preference.
The required bis-sulfonamides 5a–e7 were efficiently
synthesised by addition of a variety of substituted aryl
sulfonyl chlorides to a solution of (R,R)-1,2-diaminocy-
clohexane 4 and Et3N in CH2Cl2 (Scheme 2). Addition
of these sulfonamides in THF to a stirred solution of
the prostereogenic lithium enolate 3 in THF at ꢀ78ꢁC
gave, after quenching the resulting solution after 1h
with Me3SiCl (to remove any unreacted lithium enolate
3 as its silyl enol ether 6 in order to prevent in situ race-
misation), the enantiomerically enriched 2-methyl-tetra-
Our attention next turned to probing the use of these
bis-sulfonamides 5a–e as potential chiral scaffolds for
the enantioselective protonation of enol(ate)s derived
from 2-methyl-1-tetralone 1 using acetic acid14 as an
external proton source. We chose to deprotonate these
bis-sulfonamides 5a–e using MeLi to form the required
bis-lithium-sulfonamides 7a–e (Scheme 4). Construction
of the chiral enolate complex was achieved by simple
O
S
N
O
H
R
H
H
(R,R)-5a-e
N
S
O
H
O
R
O
OSiMe3
Me
OAc
Me
1. MeLi.LiBr
2. (R,R)-5a-e, -78 o
3. Me3SiCl
H
Me
C
2
(R)-1
6
Bis-sulfonamides
Entry
Ketone
% e.e.
12
% Yield
(R,R)-5a
R = C6H5-
36
69
(S)-1
(S)-1
1
2
22
(R,R)-5b
(R,R)-5c
R = 4-Me-C6H4-
R = 1-naphthyl-
R = 2-naphthyl-
0
(rac)-1
(R)-1
49
57
3
4
5
(R,R)-5d
64
24
R = 2,4,6-(i-Pr)3-C6H2- (R,R)-5e
(R)-1
51
Scheme 3. Enantioselective protonation of enol acetate 2 using proton sources (R,R)-5a–e.

E. Boyd et al. / Tetrahedron Letters 45 (2004) 9465–9468
9467
addition of a solution of bis-lithium-sulfonamides 7a–e
in THF at ꢀ78ꢁC to a solution of lithium enolate com-
plex 3 in THF at ꢀ78ꢁC (derived from addition of MeLi
to the enol acetate 2) (Scheme 5). The resulting solution
was allowed to stir for 30min at ꢀ78ꢁC, before being
quenched by the addition of acetic acid. Using this
approach, the highest level of enantioselectivity achieved
was 63% e.e. favouring formation of the (R)-enantiomer
of 2-methyl tetralone 1 using the bis-toluyl sulfonamide
7b. From this study, it is very interesting to note that rel-
ative levels of facial protonation in all these examples
have been reversed, most notably for bis-2-toluyl sulfon-
amide 5b and bis-2-naphthyl sulfonamide 5d from (S)-1;
22% e.e. ! (R)-1; 63% e.e. and (R)-1; 64% e.e. ! (S)-1;
45% e.e., respectively (Scheme 3: entries 2 and 4 vs
Scheme 5: entries 2 and 4). This is presumably a conse-
quence of internal proton delivery (from the bis-sulfon-
amide) for internal quench versus competitive external
protonation on the other face of the enolate away from
the bis-sulfonamide ligand. This type of complementary
stereocontrol has previously been documented;4,15 Koga
has elegantly shown that internal enantioselective proto-
nation of a related lithium enolate (derived from 2-
methyl tetralone 1) can lead to one relative enantiomer,
whereas, external alkylation of the same enolate with
benzyl bromide leads to the other relative enantio-
mer.4,15 However, it appears to our knowledge that this
is the first report of moderate reversal of facial control
through the use of two complementary enantioselective
protonation strategies. Evidently both internal and
external proton transfer processes are different since
they lead to reversed facial selectivity. This fine balance
between these processes is particularly important when
considering the generation of lithium enolates derived
from enol equivalents [e.g., silyl enol ethers (e.g., 6)
and enol acetates (e.g., 2)] and organolithium reagents
(e.g., MeLi and MeLiÆLiBr). The use of an excess of
MeLi to maximise lithium enolate 1 formation, can
potentially lower the enantiomeric excess for internal
enantioselective protonation due to competing external
protonation (through direct deprotonation of the chiral
proton donor with the remaining MeLiÆLiBr complex,
and subsequent external protonation on aqueous
work-up). This competitive protonation pathway can
lead to the other unwanted enantiomeric product and
will consequently lower the overall enantiomeric excess.
In conclusion, we have reported two complementary
protonation processes, which allow access to both
enantiomers of 2-methyl tetralone 1 via an internal
and external enantioselective protonation strategy using
substituted bis-sulfonamides as the chiral proton media-
tor. Both these protonation pathways lead to opposite
facial control. Previous reports into the enantioselective
protonation of 2-methyl-1-tetralone have mostly relied
on the use of internal chiral proton donors (e.g., chiral
alcohols,3a,b,d ammonium salts,10c and amides).3e How-
ever, there are a few reports which have given excellent
enantiocontrol for a chiral ligand and external achiral
proton donor combination.16 The associated levels of
stereocontrol have been shown to be good to excellent
(up to 94% e.e.,3a,b,d 40% e.e.,10c and 64% e.e.,3e respec-
tively) for a wide range of acids.
2. Representative experimental procedures
2.1. (R)-2-methyl tetralone 1
O
S
N
O
S
N
O
H
R
O
H
R
Typical procedure for an internal quench: A solution of
MeLiÆLiBr (0.20mL, 1.50Min ether, 0.30mmol) was
added dropwise to the enol acetate 2 (32mg, 0.15mmol)
at room temperature. The resulting solution was stirred
for 30min and then cooled to ꢀ78ꢁC. A pre-cooled solu-
tion of bis-sulfonamide 5d (74mg, 0.15mmol) in THF
(1mL) at ꢀ78ꢁC was slowly added, and the resulting
solution was stirred for 1h before being quenched with
Me3SiCl (0.1mL). A saturated solution of NaHCO3was
added and the resulting solution was extracted with
ether (3 · 10mL). The organic phase was washed again
with a saturated solution of NaHCO3 and the solvent
was removed under vacuum. The residue was purified
by flash chromatography on silica gel eluting with light
petroleum/ether (19:1) to give (R)-2-methyl-1-tetralone
15 (14mg, 57%) as a colourless oil with 64% enantio-
meric excess (determined by chiral HPLC using a Chi-
ralcel OD column9 solvent hexane/isopropyl alcohol
(98:2): flow rate: 0.7mL/min; retention time (S)-enantio-
mer 10.8min, (R)-enantiomer 11.6min); RF [light petro-
leum (40–60ꢁC)/ether (9:1)] 0.5; vmax(film)/cmꢀ11686
MeLi
THF
H
H
Li
Li
N
S
O
N
S
O
H
O
H
O
R
R
(R,R)-5a-e
(R,R)-7a-e
Scheme 4. Synthesis of C2-symmetric bis-lithium sulfonamides (R,R)-
7a–e.
O
OAc
Me
1. MeLi.LiBr
2. 7a-e, -78 o
3. acetic acid
H
Me
C
4. Me3SiCl
(R)-1
2
Bis-sulfonamides
R = C6H5-
Entry
Ketone
% e.e.
4
% Yield
63
50
(R,R)-7a
(S)-1
(R)-1
(R)-1
1
2
63
R = 4-Me-C6H4-
R = 1-naphthyl-
(R,R)-7b
(R,R)-7c
12
45
0
47
54
3
4
5
3
(CO); dH (250MHz, CDCl3) 8.00 (1H, d, J = 7.7, CH;
R = 2-naphthyl-
(S)-1
(R,R)-7d
(R,R)-7e
Ar), 7.47 (1H, dd, 3J = 7.7 and 7.6, CH; Ar), 7.25
R = 2,4,6-(i-Pr)3-C6H2-
(rac)-1
3
3
53
(1H, t, J = 7.7, CH; Ar), 7.22 (1H, d, J = 7.6, CH;
Ar), 3.00 (2H, m, CH2C@C), 2.60 (1H, m, CHMe),
2.20 (1H, dt, J = 13.2 and 4.4, CHAHB), 1.87 (1H, m,
3
Scheme 5. Enantioselective protonation of enol acetate 2 using acetic
acid as an external proton sources.
CHAHB) and 1.28 (3H, d, 3J = 7.3, MeCH); dC

9468
E. Boyd et al. / Tetrahedron Letters 45 (2004) 9465–9468
Henin, F.; Aboulhoda, S. J. Tetrahedron: Asymmetry
1997, 8, 381.
4. Yasukata, T.; Koga, K. Tetrahedron: Asymmetry 1993, 4,
35.
(62.5MHz, CDCl3) 200.8, 144.2, 133.1, 132.4, 128.7,
127.4, 126.6, 42.0, 31.3, 28.8 and 15.3 (Found M+,
160.0882. C11H12O requires M+, 160.0882); m/z 160.1
(100%, M). The purity was >99% determined by HPLC.
5. (a) Eames, J.; Weerasooriya, N. Tetrahedron Lett. 2000,
41, 521; (b) Eames, J.; Weerasooriya, N. Chirality 1999,
787.
6. For a related example: pKa (tetralone in DMSO) = 25 and
pKa (phenylsulfonamide in DMSO) = 16. For further
information see: (a) Bordwell, F. G.; Algrim, D. J. Org.
Chem. 1976, 41, 2507; (b) Bordwell, F. G.; Fried, H. E.;
Hughes, D. L.; Lynch, T.-Y.; Satish, A. V.; Whang, Y. E.
J. Org. Chem. 1990, 55, 3300; (c) Bordwell, F. G.; Fried,
H. E. J. Org. Chem. 1991, 56, 4218.
Typical procedure for an external quench: A solution of
MeLiÆLiBr (0.20mL, 1.50Min ether, 0.30mmol) was
added dropwise to the enol acetate 2 (32mg, 0.15mmol)
at room temperature. A solution of pre-formed lithium
sulfonamide 7b [sulfonamide 5b 74mg, 0.15mmol and
MeLi (0.20mL, 1.50M in ether, 0.30mmol)] in THF
(1mL) was added. The resulting solution was stirred
for 30min at ꢀ78ꢁC. The reaction was quenched, by
the dropwise addition of glacial acetic acid (0.2mL), fol-
lowed by Me3SiCl (0.2mL). A saturated solution of
NaHCO3 was added and this solution was extracted
with ether (3 · 10mL). The organic phase was washed
again with a saturated solution of NaHCO3 and the sol-
vent was removed under vacuum. The residue was puri-
fied by flash chromatography on silica gel eluting with
light petroleum/ether (9:1) to give (R)-2-methyl-1-tetra-
lone 15(12mg, 50%) as a colourless oil with 63% enantio-
meric excess (determined by chiral HPLC using a
Chiralcel OD column9 solvent hexane/isopropyl alcohol
(98:2): flow rate: 0.7mL/min; retention time (S)-enantio-
mer 10.8min, (R)-enantiomer 11.6min). The purity was
>99% determined by HPLC.
7. (a) Takahashi, H.; Yoshioka, M.; Shibasaki, M.; Ohno,
M.; Imai, N.; Kobayashi, S. Tetrahedron 1995, 51, 12013;
(b) Takahashi, H.; Kawakita, T.; Ohno, M.; Yoshioka,
M.; Kobayashi, S. Tetrahedron 1992, 48, 5691; Alfonso, I.;
Rebolledo, F.; Gotor, V. Tetrahedron: Asymmetry 1999,
10, 367; Pulacchini, S.; Sibbons, K. F.; Shastri, K.;
Motevalli, M.; Watkinson, M.; Wan, H.; Whiting, A.;
Lightfoot, A. P. Dalton Trans. 2003, 10, 2043; (c)
Denmark, S. E.; Christenson, B. L.; OꢀConnor, S. P.
Tetrahedron Lett. 1995, 36, 2219; (d) Alvaro, G.; Grilli, S.;
Martelli, G.; Savoia, D. Eur. J. Org. Chem. 1999, 1523; (e)
Gorlitzer, H. W.; Spiegler, M.; Anwander, R. Eur. J.
Inorg. Chem. 1998, 1009; Denmark, S. E.; Christenson, B.
L.; OꢀConnor, S. P. Tetrahedron Lett. 1995, 36, 2219.
8. For representative examples, see: (a) Lee, E. K.; Kim,
S.-H.; Jung, B.-H.; Anh, W.-S.; Kim, G.-J. Tetrahedron
Lett. 2003, 44, 1971; (b) Mimoun, H.; de Saint Laumer, J.
Y.; Giannini, L.; Scopellita, R.; Floriani, C. J. Am. Chem.
Soc. 1999, 121, 6158; (c) Hoffmann, R. W.; Klute, W.;
Dress, R. K.; Wenzel, A. J. Chem. Soc., Perkin Trans. 2
1995, 1721; (d) Costa, A. M.; Jimeno, C.; Gavenonis, J.;
Carroll, P. J.; Walsh, P. J. J. Am. Chem. Soc. 2002, 124,
6929; (e) Vasse, J.-L.; Stranne, R.; Zalubovskis, R.; Gayet,
C.; Moberg, C. J. Org. Chem. 2003, 68, 3258; (f) Kizirian,
J.-C.; Caille, J.-C.; Alexakis, A. Tetrahedron Lett. 2003,
44, 8893; (g) Cobb, A. J. A.; Marson, C. M. Tetrahedron:
Asymmetry 2001, 12, 1547; (h) Mimoun, H.; de Saint
Laumer, J. Y.; Giannini, L.; Scopellita, R.; Floriani, C. J.
Am. Chem. Soc. 1999, 121, 6158; (i) Davies, S. G.;
Mortlock, A. A. Tetrahedron 1993, 49, 4419; (j) Corey, E.
J.; Sarshar, S.; Azimioara, M. D.; Newbold, R. C.; Noe,
M . C.J. Am. Chem. Soc. 1996, 118, 7851.
9. By HPLC using a Chiralcel OD column, see: Muzart, J.;
He´nin, F.; Aboulhoda, S. A. Tetrahedron: Asymmetry
1997, 8, 381, for details. For 2-methyltetralone 1; solvent
hexane/isopropyl alcohol (98:2): flow rate: 0.5mL/min;
retention time (R)-enantiomer 14.4min, (S)-enantiomer
13.1min.
10. (a) Cuenca, A.; Medio-Simon, M.; Aguilar, G. A.; Weibel,
D.; Beck, A. K.; Seebach, D. Helv. Chim. Acta 2000, 83,
3153; (b) Takahashi, T.; Nakao, N.; Koizumi, T. Tetra-
hedron: Asymmetry 1997, 8, 3293; (c) Fuji, K.; Tanaka, K.;
Miyamoto, H. Tetrahedron: Asymmetry 1993, 4, 247.
11. This is due to their reduced basicity as a consequence of
competitive conjugation of the enol oxygen to the neigh-
bouring C@O group.
Acknowledgements
We are grateful to The Royal Society (to J.E.), The Uni-
versity of London Central Research Fund (to J.E.) and
Syngenta (to M.J.S.) for their financial support, and the
EPRSC National Mass Spectrometry Service (Swansea)
for accurate mass determinations.
References and notes
1. (a) Eames, J.; Weerasooriya, N. Tetrahedron: Asymmetry
2001, 12, 1; (b) Eames, J.; Weerasooriya, N. J. Chem. Res.
(S) 2001, 1; (c) Fehr, C. Angew. Chem., Int. Ed. 1996, 35,
2566, and references cited therein.
2. (a) Yangagisawa, A.; Kikuchi, T.; Kuribayashi, T.;
Yamamoto, H. Tetrahedron 1998, 54, 10253; (b) Yanag-
isawa, A.; Ishihara, K.; Yamamoto, H. Synlett 1997, 411;
(c) Yanagisawa, A.; Watanabe, T.; Kikuchi, T.; Yama-
moto, H. J. Org. Chem. 2000, 65, 2979; (d) Yangagisawa,
A.; Kikuchi, T.; Watanabe, T.; Yamamoto, H. Bull.
Chem. Soc. Jpn. 1999, 72, 2337; (e) Yangagisawa, A.;
Kikuchi, T.; Yamamoto, H. Synlett 1998, 174; (f)
Nakamura, Y.; Takeuchi, S.; Ohgo, Y.; Yamaoka, M.;
Yoshida, A.; Mikami, K. Tetrahedron Lett. 1997, 38, 2709;
(g) Mikami, K.; Yamaoka, M.; Yoshida, A. Synlett 1998,
607; (h) Nakamura, Y.; Takeuchi, S.; Ohgo, Y.; Yamaoka,
M.; Yoshida, A.; Mikami, K. Tetrahedron 1999, 55, 4595.
3. (a) Asensio, G.; Aleman, P.; Cuenca, A.; Gil, J.; Medio-
12. House, H. O.; Trost, B. M. J. Org Chem. 1965, 30, 2502.
13. Eames, J.; Coumbarides, G. S.; Weerasooriya, N. J.
Labelled Compd. Radiopharm. 2001, 44, 871.
14. Riviere, P.; Koga, K. Tetrahedron Lett. 1997, 38, 7859.
15. (a) Murakata, M.; Nakajima, M.; Koga, K. J. Chem. Soc.,
Chem. Commun. 1990, 1657; (b) Fehr, C. Angew. Chem.,
Int. Ed. 1996, 35, 2573.
´
Simon, M. Tetrahedron: Asymmetry 1998, 9, 4073; (b)
Asensio, G.; Aleman, P.; Cuenca, A.; Gil, J.; Domingo, L.
´
R.; Medio-Simon, M. J. Org. Chem. 1998, 63, 9342; (c)
Yangagisawa, A.; Kikuchi, T.; Yamamoto, H. Synlett
1998, 174; (d) Fuji, K.; Kawabata, T.; Kuroda, A. J. Org.
Chem. 1995, 60, 1914; (e) Fujihara, H.; Tomioka, K. J.
Chem. Soc., Perkin Trans. 1 1999, 2377; (f) Muzart, J.;
16. Yamashita, Y.; Emura, Y.; Odashima, K.; Koga, K.
Tetrahedron Lett. 2000, 41, 209.