Lithium enolates from a (-)-quinic acid-derived cyclohexanone with a β-alkoxy leaving group: Regioselective preparation and evaluation of enolate stability towards β-elimination

Article abstract of DOI:10.1016/j.tetlet.2004.01.148

Deprotonation of a (-)-quinic acid-derived ketone {(2S,3S,4aR,8R,8aS)-8- [(tert-butyl(dimethyl)silyl)oxy]-2,3-dimethoxy-2,3-dimethylhexahydro-1, 4-benzodioxin-6(5H)-one} using lithium hexamethyldisilazide (LHMDS) at -78°C gave one regioisomeric enolate. The regiocontrol is governed by the axial β-silyloxy substituent and the resulting lithium enolate is stable towards β-elimination at temperatures up to -40°C. It was found that the axial β-silyloxy group could be conveniently eliminated using 2.1equiv of LHMDS at 0°C for 1h and that an equatorial β-alkoxy group was much more resistant to β-elimination. A chiral lithium amide base was used to overturn the inherent regioselectivity of ketone deprotonation with LHMDS.

Full text of DOI:10.1016/j.tetlet.2004.01.148

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Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2597–2601
Lithium enolates from a ())-quinic acid-derived cyclohexanone with
a b-alkoxy leaving group: regioselective preparation and evaluation
of enolate stability towards b-elimination
*
€
Lynne M. Murray, Peter OꢀBrien, Richard J. K. Taylor and Stefan Wunnemann
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Received 11 October 2003; revised 18 January 2004; accepted 30 January 2004
Abstract—Deprotonation of a ())-quinic acid-derived ketone {(2S,3S,4aR,8R,8aS)-8-[(tert-butyl(dimethyl)silyl)oxy]-2,3-dimethoxy-
2,3-dimethylhexahydro-1,4-benzodioxin-6(5H)-one} using lithium hexamethyldisilazide (LHMDS) at )78 ꢀC gave one regioisomeric
enolate. The regiocontrol is governed by the axial b-silyloxy substituent and the resulting lithium enolate is stable towards b-
elimination at temperatures up to )40 ꢀC. It was found that the axial b-silyloxy group could be conveniently eliminated using
2.1 equiv of LHMDS at 0 ꢀC for 1 h and that an equatorial b-alkoxy group was much more resistant to b-elimination. A chiral
lithium amide base was used to overturn the inherent regioselectivity of ketone deprotonation with LHMDS.
ꢁ 2004 Elsevier Ltd. All rights reserved.
())-Quinic acid is a very useful and versatile chiral pool
starting material for natural product synthesis1 and
many groups have developed elegant syntheses based on
stereoselective reactions of ())-quinic acid derivatives.2
Highlights over the last few years include the prepara-
tion of some gabosines by Shinada et al.3 and the total
synthesis of ())-brunsvigine by Sha et al.4 Recently, we
reported a range of highly stereoselective reactions of
enone 1 (prepared in three steps from ())-quinic acid)
culminating in a new entry to the core of scyphostatin,5
an inhibitor of neutral sphingomyelinase.
LHMDS) did not eliminate the potential b-alkoxy
leaving group (equatorial orientation due to the con-
formational lock imparted by the Ley bis-ketal)6 at
)78 ꢀC over 30 min. In a related study, we decided to
investigate the deprotonation and subsequent elimina-
tion of ketone 2, also prepared from ())-quinic acid.
Several interesting features are presented by studying
ketone 2: (i) being unsymmetrical, the regioselectivity of
deprotonation needed to be determined and for regio-
control, we imagined making use of chiral lithium amide
bases;7;8 (ii) the enolates generated from 2 possess either
an axial b-silyloxy group or an equatorial b-alkoxy
group allowing us to investigate stereoelectronic pref-
erences for elimination in the same substrate and (iii)
it was hoped that ketone 2 could itself become an
established and useful synthetic intermediate if regio
and stereoselective functionalisation processes could be
developed. In this paper, we report our findings on the
deprotonation and subsequent elimination of ketone 2.
O
O
HO CO2H
axial
equatorial
equatorial
O
TBSO
O
OMe
OMe
HO
OH
O
O
OH
OMe
OMe
(–)-Quinic acid
1
2
During the course of our work, full details of the
preparation and characterisation of ketone 2 were
reported by Whitehead and co-workers.9 Based on their
route, we modified our own methods and optimised a
very convenient way of preparing multi-gram quantities
of ketone 2. First of all, ())-quinic acid was converted
into bis-ketal 3 (with concomitant methyl ester forma-
tion) in 99% yield (after chromatography). Then, methyl
ester reduction using NaBH4 in MeOH at room tem-
perature (18 h) gave a 1,2-diol that was cleaved (without
In our previous work, it was shown that the lithium
enolate generated from 1 (by deprotonation with
Keywords: Cyclohexanones; Deprotonation; Elimination reactions;
Regiocontrol; ())-Quinic acid.
* Corresponding author. Tel.: +44-(0)-1904-432535; fax: +44-(0)-1904-
432516; e-mail: paob1@york.ac.uk
0040-4039/$ - see front matter ꢁ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.148

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L. M. Murray et al. / Tetrahedron Letters 45 (2004) 2597–2601
O
OSiMe3
isolation or work-up) to hydroxy ketone 4 after adding
water and NaIO4 (5.5 equiv) and stirring at room tem-
perature for 3 h. Work-up generated crude hydroxy
1. LHMDS, THF
Hax
Heq
–78 C, 30 min
1
˚
2
2. Me3SiCl
OMe
1
TBSO
O
TBSO
O
O
ketone 4 of high purity (as judged by H NMR spec-
troscopy), which was TBS protected under standard
OMe
O
>98 : 2
20
D
conditions to give the desired ketone 2 {½aꢀ +99.6 (c
OMe
OMe
1.0, CH2Cl2) (lit.,9 ½aꢀ +93.5 (c 0.86, CH2Cl2))} in 67%
2
5
D
yield (after purification by column chromatography)
over the three steps from bis-ketal 3. Instead of TBS
protecting, crude hydroxy ketone 4 was also eliminated
(using well documented conditions)10 to give known5;10
20
D
The regioselective preparation of a lithium enolate with
an axial b-silyloxy group enabled a detailed investiga-
tion of the elimination process. Thus, we generated the
lithium enolate by deprotonation of ketone 2 using
LHMDS at )78 ꢀC over 30 min.15 The solution was then
stirred at a particular temperature for a given length of
enone 1 {½aꢀ +70.4 (c 1.0, CH2Cl2) (lit.,10 ½aꢀ +64.4 (c
D
0.39, CH2Cl2))} in 90% overall yield from bis-ketal 3.
These protocols for preparing enone 1 and ketone 2 are
the most convenient synthetic routes reported to date11
and, by using Whiteheadꢀs NaBH4 reduction of 3,
avoid the use of a problematic DIBAL-H reduction
step.5;10
time and quenched with saturated NH4Cl . After
ðaqÞ
work-up, the crude reaction mixture was analysed by 1H
NMR spectroscopy to determine the amount of ketone
2 and enone 1 (elimination product). The full results are
presented in Table 1. In all cases, the crude product
mixture was obtained in essentially quantitative yield.
Initially, reactions were conducted using 1.05 equiv of
LHMDS (entries 1–3). At )78 ꢀC, the lithium enolate
from 2 was stable to elimination of the b-silyloxy group
for 1 h and probably much longer (entry 1). On raising
Table 1. Study on the deprotonation–elimination of ketone 2
O
OLi
LHMDS (1.05 or
2.1 eq), THF
–78 C, 30 min
OMe
˚
TBSO
O
TBSO
O
OMe
O
O
Initially, the regioselectivity of deprotonation of ketone
2 using LHMDS was determined. Treatment of a THF
solution of ketone 2 with LHMDS at )78 ꢀC for 30 min
followed by enolate trapping with Me3SiCl furnished a
single silyl enol ether 5 in quantitative crude yield. The
identity of 5 was established from its 1H NMR spectrum
(vide infra) and none of the regioisomeric silyl enol ether
was generated under these conditions. The regioselec-
tivity is presumably controlled by the acidifying effect on
the C-2 proton Hax by the axially disposed b-silyloxy
group (antiperiplanar arrangement of the axial rC–H
and axial r*C–O orbitals). We have found one previous
report of the use of an axial b-alkoxy group controlling
the regioselectivity of deprotonation in a cyclohexanone
(although not commented on by the authors): an axial b-
silyloxy group led to complete regiocontrol of silyl enol
OMe
OMe
2
T ( C) time (min)
˚
O
O
O
+
OMe
O
TBSO
O
OMe
O
OMe
1
OMe
2
Entry
LHMDS (equiv) T (ꢀC)a
Time (min)a
1:2b
1
2
3
4
5
6
7
8
9
1.05
1.05
1.05
2.10
2.10
2.20
2.20
2.10
0.90
)78
)40
0
60
60
0:100
15:85
45:55
2:98
60
60
)78
)40
)40
)40
0
ether formation in a route from
D-glucose to (+)-caly-
60
90
65:35
75:25
85:15
90:10
70:30
stegine B2.12 In this example, and in the generation of
silyl enol ether 5, it is notable that no elimination of the
axial b-silyloxy group occurred. Similar results have
been obtained by a number of groups using chiral bases
at )78 ꢀC in related systems.7;13 However, of particular
interest, Wipf et al. have reported that the enolates
of cyclohexenones with a b-N-CO2R group are only
susceptible to elimination at temperatures above
)20 ꢀC.14
120
60
0
180
a The lithium enolate derived from ketone 2 was formed at )78 ꢀC
using LHMDS (equivalent indicated) and it was then stirred at T (ꢀC)
for a given time (min).
b The ratio of 1:2 was determined by 1H NMR spectroscopy on the
crude product mixture (essentially quantitative yield in each case).

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L. M. Murray et al. / Tetrahedron Letters 45 (2004) 2597–2601
2599
the temperature (entries 2 and 3), we observed some b-
elimination (up to 45% at 0 ꢀC over 1 h, entry 3). Much
more surprising was our observation when 2.10 or
2.20 equiv of LHMDS were utilised, as this led to con-
siderably more b-elimination under otherwise identical
conditions (compare entries 2 with 5 and 3 with 8). The
highest amount of b-elimination (90%) was achieved
with 2.10 equiv of LHMDS at 0 ꢀC for 1 h (entry 8).
Surprisingly, we were not able to force the b-elimination
to completion using extended reaction times: some
ketone 2 (up to 5–10%) always remained unaffected.
(S)-6
Ph
N
Li
CF3
HB
O
OMe
HA
Me
Me
O
O
H
LHMDS
or
O
H
OMe
TBS
(R)-6
Ph
N
CF3
2
Li
Figure 1.
)78 ꢀC with 1.2 equiv of chiral base (S)-6/LiCl (gener-
ated from 1.2 equiv of the amine hydrochloride salt and
2.4 equiv of n-BuLi) for 30 min. Subsequent trapping
with Me3SiCl gave a 90:10 mixture of silyl enol ethers
The conditions reported in entry 8 (2.10 equiv LHMDS,
THF, 0 ꢀC, 1 h) represent a new set of conditions for the
elimination of an alkoxy group b to the carbonyl in a
cyclohexanone and this could have synthetic utility.16
Once the b-elimination has occurred, it seems likely that
the excess LHMDS will deprotonate the enone 1 to give
an enolate that appears to be resistant to further b-
elimination at 0 ꢀC. To verify this interesting result,
enone 1 was deprotonated at )78 ꢀC with LHMDS15
(1.1 equiv, 30 min) and then stirred at 0 ꢀC for 1 h. After
quench and work-up, enone 1 was recovered unchanged
in quantitative yield confirming that the lithium enolate
from 1 is indeed stable to b-elimination under these
conditions.
723 and 5 (by H NMR spectroscopy) in crude quanti-
1
tative yield, that is, a virtually complete reversal of
regioselectivity was achieved. Essentially the same result
(87:13 of 7 and 5) was obtained under internal quench
conditions.24 In order to obtain silyl enol ethers 7 and 5
free from the chiral amine precursor to (S)-6, it was
necessary to purify by column chromatography resulting
in a 41% isolated yield of a 90:10 mixture of 7 and 5.
Thus, by using LHMDS or chiral base (S)-6 regio-
selective access to synthetically useful silyl enol ethers
5 and 7, respectively, was accomplished.
(S)-6
N CF3
1.
With ketone 2, it is interesting that 2.10 equiv of
LHMDS gives far more elimination compared to the use
of 1.05 equiv. This may be due to the formation of an
aggregate (e.g., a dimer between the lithium enolate and
LHMDS)17 in solution that favours b-elimination.18
Speculating that the 0.05 equiv excess of LHMDS in the
1.05 equiv conditions (entries 2 and 3) may be catalysing
the b-elimination process, we carried out a reaction with
a deficiency of LHMDS (0.9 equiv) at 0 ꢀC for an ex-
tended time of 180 min. In this case, 70% elimination
occurred (entry 9) indicating that the lithium enolate
from 2 is prone to b-elimination of the axial OTBS
group at 0 ꢀC even without an additional equivalent of
LHMDS present. As described above, this is in contrast
to the lithium enolate from 1, which did not undergo
elimination of the equatorial-alkoxy group at 0 ꢀC.
O
OSiMe3
Ph
Li
+
5
LiCl, THF, –78 C
2. Me3SiCl
˚
TBSO
O
TBSO
O
O
OMe
OMe
O
90:10
7:5
external
quench
OMe
OMe
2
7
In summary, we have described different conditions for
the regioselective generation of silyl enol ethers 5 and 7
from unsymmetrical ())-quinic acid-derived ketone 2.
For example, chiral lithium amide base (S)-6 was used
to preferentially prepare silyl enol ether 7 from ketone 2.
In contrast, LHMDS deprotonation-trapping of 2 gave
silyl enol ether 5 exclusively, presumably due to an
activating effect of the b-silyloxy group. The lithium
enolate derived from 2 and LHMDS (1.05 equiv) was
completely stable to elimination of the b-silyloxy group
at )78 ꢀC for 1 h. However, at elevated temperatures
some b-elimination was observed (e.g., 15 % at )40 ꢀC
and 45% at 0 ꢀC; both over 1 h) and this process
occurred to a much greater extent in the presence of
2.10 equiv of LHMDS. In contrast, the lithium enolate
derived from enone 1 and 1.1 equiv of LHMDS did not
eliminate the equatorial b-alkoxy group at 0 ꢀC. These
observations should prove useful for other researchers
utilising b-functionalised cyclohexanones in organic
synthesis.
In order to overturn the inherent regioselectivity of
deprotonation presented by ketone 2, our attention
switched to the use of chiral bases. The regioselective
preparation of silyl enol ethers using chiral lithium
amide bases has previously been reported by Koga
and co-workers8a and Simpkins and co-workers8b We
selected chiral base (S)-6, originally introduced by Aoki
and Koga,19 as it is easy to synthesise the amine pre-
cursor,20;21 it does not require the use of HMPA for high
enantioselectivity and it performs well (up to 89% ee)
even at )78 ꢀC. In addition, based on Kogaꢀs results,19
chiral base (S)-6 should exhibit the required regioselec-
tivity for removal of proton HA (Fig. 1).
The deprotonation of ketone 2 using chiral base (S)-6
was carried out under external and internal quench
conditions. For externally quenched reactions, Simpkins
and Majewski recommend the use of LiCl as an addi-
tive.22 Thus, ketone 2 was deprotonated in THF at
Acknowledgements
We thank the BBSRC and Hoffmann-La Roche for a
CASE award (to L.M.M.), the EU for Erasmus funding

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L. M. Murray et al. / Tetrahedron Letters 45 (2004) 2597–2601
12. Deprotonation of ketone 8 (with an equatorial b-silyloxy
group) with LDA at )70 ꢀC followed by reaction with
(of S.W.) and Prof. M. Majewski (University of Sas-
katchewan) for exchange of unpublished results.
Complete preference
4:1 preference
with LDA at –78 C
˚
with LDA at –78 C
˚
R
R
H
H
H
O
O
H
O
References and notes
RO
RO
TBSO
OR
OR
H
O
H
1. Barco, A.; Benetti, S.; De Risi, C.; Marchetti, P.; Pollini,
G. P.; Zanirato, V. Tetrahedron: Asymmetry 1997, 8,
3515–3545.
TBSO
equatorial
8 (R = Bn)
axial
9 (R = Bn)
2. For recent examples, see: (a) Yoshida, A.; Ono, K.;
Suhara, Y.; Saito, N.; Takayama, H.; Kittaka, A. Synlett
2003, 1175–1179; (b) Ohtani, Y.; Shinada, T.; Ohfune, Y.
Me3SiCl gave a 4:1 mixture of regioisomeric silyl enol
ethers. In contrast, ketone 9 (with an axial b-silyloxy
group) gave a single regioisomeric silyl enol ether under
the same conditions. See: Boyer, F.-D.; Lallemand, J.-Y.
Tetrahedron 1994, 50, 10443–10458.
ꢀ
Synlett 2003, 619–622; (c) Gonzalez, C.; Carballido, M.;
Castedo, L. J. Org. Chem. 2003, 68, 2248–2255.
3. Shinada, T.; Fuji, T.; Ohtani, Y.; Yoshida, Y.; Ohfune, Y.
Synlett 2002, 1341–1343.
4. Sha, C.-K.; Hong, A.-W.; Huang, C.-M. Org. Lett. 2001,
3, 2177–2179.
5. Murray, L. M.; OꢀBrien, P.; Taylor, R. J. K. Org. Lett.
2003, 5, 1943–1946.
6. Hense, A.; Ley, S. V.; Osborn, H. M. I.; Owen, D. R.;
Poisson, J.-F.; Warriner, S. L.; Wesson, K. E. Chem. Rev.
2001, 101, 53–80.
7. For reviews, see: (a) Cox, P. J.; Simpkins, N. S. Tetra-
hedron: Asymmetry 1991, 2, 1–26; (b) OꢀBrien, P. J. Chem.
Soc., Perkin Trans. 1 1998, 1439–1457.
8. (a) Sobukawa, M.; Nakajima, M.; Koga, K. Tetrahedron:
Asymmetry 1990, 1, 295–298; (b) Bambridge, K.; Simp-
kins, N. S.; Clark, B. P. Tetrahedron Lett. 1992, 33, 8141–
8144.
9. Begum, L.; Box, J. M.; Drew, M. G. B.; Harwood, L. M.;
Humphreys, J. L.; Lowes, D. J.; Morris, G. A.; Redon, P.
M.; Walker, F. M.; Whitehead, R. C. Tetrahedron 2003,
59, 4827–4841.
10. Barros, M. T.; Maycock, C. D.; Ventura, M. R. J. Chem.
Soc., Perkin Trans. 1 2001, 166–173.
11. Our optimised routes to 1 and 2 start from 3 and proceed
without purification of any of the synthetic intermediates.
The protocol for the synthesis of ketone 2 is representa-
tive: (2S,3S,4aR,8S,8aS)-8-{[tert-Butyl(dimethyl)silyl]oxy}-
2,3-dimethoxy-2,3-dimethylhexahydro-1,4-benzodioxin-
6(5H)-one 2: To a stirred solution of bis-ketal 3 (1.88 g,
5.9 mmol) in MeOH (25 mL) at 0 ꢀC under N2 was added
NaBH4 (1.61 g, 42.5 mmol) in portions. After effervescence
had ceased, the mixture was allowed to warm to rt and
stirred at rt for 18 h. Then, water (40 mL) and NaIO4
(6.95 g, 32.5 mmol) were added and the resulting mixture
13. (a) Momose, T.; Toyooka, N.; Hirai, Y. Chem. Lett. 1990,
1319–1322; (b) Bunn, B. J.; Cox, P. J.; Simpkins, N. S.
Tetrahedron 1993, 49, 207–208; (c) Newcombe, N. J.;
Simpkins, N. S. J. Chem. Soc., Chem. Commun. 1995, 831–
832; (d) Nowakowski, M.; Hoffmann, H. M. R. Tetrahe-
dron Lett. 1997, 38, 1001–1004; (e) Hunt, K. W.; Grieco,
P. Org. Lett. 2002, 4, 245–248; (f) Honda, T.; Ono, S.;
Mizutani, H.; Hallinan, K. O. Tetrahedron: Asymmetry
1997, 8, 181–184; (g) Honda, T.; Endo, K.; Ono, S. Chem.
Pharm. Bull. 2000, 48, 1545–1548; (h) Honda, T.; Endo, K.
J. Chem. Soc., Perkin Trans. 1 2001, 2915–2919.
14. (a) Wipf, P.; Kim, Y. Tetrahedron Lett. 1992, 33, 5477–
5480; (b) Goldstein, D. M.; Wipf, P. Tetrahedron Lett.
1996, 37, 739–742.
15. We are confident that the enolates from 1 and 2 form upon
treatment with LHMDS at )78 ꢀC for 30 min since they
have been trapped with a range of aldehydes to give the
expected aldol adducts in 23–80% yields.
16. For an example where others have previously encountered
difficulty in eliminating a b-silyloxy group in a related
cyclohexanone, see: Hareau, G.; Koiwa, M.; Hanazawa,
T.; Sato, F. Tetrahedron Lett. 1999, 40, 7493–7496.
17. (a) Zhao, P.; Collum, D. B. J. Am. Chem. Soc. 2003, 125,
4008; (b) Zhao, P.; Collum, D. B. J. Am. Chem. Soc. 2003,
125, 14411.
18. An alternative explanation for the requirement of
2.10 equiv of LHMDS is as follows: if b-elimination of
the enolate of 2 is slow and enone 1 is readily deproto-
nated then as enone 1 is formed, it could be deprotonated
by the enolate of 2. In this way, 2.0 equiv LHMDS would
be required to complete the elimination process. We are
grateful to a referee for this suggestion.
19. Aoki, K.; Koga, K. Tetrahedron Lett. 1997, 38, 2505–
2506.
20. The amine precursor to (S)-5 was prepared by (i) acylation
of (S)-a-methylbenzylamine using (CF3CO)2O and (ii)
amide reduction using BH3ÆMe2S. The hydrochloride salt
was stirred at rt for 3 h. Saturated NH4Cl
(20 mL) was
ðaqÞ
added and the mixture was extracted with CH2Cl2
(5 · 20 mL). The combined organic extracts were dried
(Na2SO4) and evaporated under reduced pressure to give
crude 4 as a white solid. To a stirred solution of crude 4 in
DMF (13.5 mL) at rt under N2 were added imidazole
(1.54 g, 22.3 mmol) and tert-butyldimethylsilyl chloride
(2.53 g, 16.7 mmol). The resulting mixture was stirred at rt
for 16 h. Then, EtOAc (20 mL) was added and the layers
were separated. The aqueous layer was extracted with
EtOAc (3 · 20 mL) and the combined organic extracts
of the amine was prepared by bubbling HCl through a
ðgÞ
solution of the amine in Et2O. Majewski, M. Personal
communication.
21. (a) Aoki, K.; Koga, K. Chem. Pharm. Bull. 2000, 48, 571–
€
574; (b) Pache, S.; Botuha, C.; Franz, R.; Kundig, E. P.;
Einhorn, J. Helv. Chim. Acta 2000, 83, 2436–2451.
22. (a) Bunn, B. J.; Simpkins, N. S.; Spavold, Z.; Crimmin, M.
J. J. Chem. Soc., Perkin Trans. 1 1993, 3113–3116; (b)
Majewski, M.; Lazny, R.; Nowak, P. Tetrahedron Lett.
1995, 36, 5465–5468.
were washed with 1 M HCl
(20 mL) and saturated brine
ðaqÞ
(3 · 20 mL). The organic extracts were dried (Na2SO4) and
evaporated under reduced pressure to give crude 2.
Purification by flash chromatography on silica with
23. 1H NMR spectroscopic data for silyl enol ethers 5 and 7:
petrol–EtOAc (4/1) as eluent gave ketone 2 (1.48 g, 67%)
as a white solid, mp 93–94 ꢀC (lit.,9 100–101 ꢀC); ½aꢀ
d
H (400 MHz, CDCl3) for 5: 4.88 (dd, 1H, J ¼ 2:0, 6.0 Hz),
20
D
4.25–4.15 (m, 2H), 3.46 (dd, 1H, J ¼ 3:5, 10.5 Hz), 3.26 (s,
3H), 3.23 (s, 3H), 2.33 (dd, 1H, J ¼ 6:5, 16.5 Hz, CHAHB),
2.21 (ddd, 1H, J ¼ 2:0, 10.5, 16.5 Hz, CHAHB), 1.29 (s,
3H), 1.28 (s, 3H), 0.88 (s, 9H), 0.20 (s, 9H), 0.09 (s, 3H),
+99.6 (c 1.0, CH2Cl2) (lit.,9 ½aꢀ +93.5 (c 0.86, CH2Cl2)).
D
Spectroscopic data was identical to that reported in the
literature.9