Chemo-enzymatic synthesis of (-)-epipentenomycin I

Article abstract of DOI:10.1016/j.tetasy.2003.10.003

A chemo-enzymatic synthesis of (-)-epipentenomycin I is reported using a lipase-catalysed kinetic resolution of the racemic pentacyclic alcohol 8. Flash vacuum pyroloysis of (-)-8 so obtained gave (-)-(4R)-4-hydroxy-5-methylene-2- cyclopentenone. Epoxidation of this compound with dimethyldioxirane followed by hydrolytic ring-opening of the resulting epoxide gave (-)-epipentenomycin I.

Full text of DOI:10.1016/j.tetasy.2003.10.003

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 3885–3889
Chemo-enzymatic synthesis of (−)-epipentenomycin I
Tawesin Klomklao,a Stephen G. Pyne,b,* Apiwat Baramee,a Brian W. Skeltonc and Allan H. Whitec
aDepartment of Chemistry, Chiang Mai University, Chiang Mai 50202, Thailand
bDepartment of Chemistry, University of Wollongong, Wollongong, NSW, 2522, Australia
cDepartment of Chemistry, University of Western Australia, Crawley, WA, 6009, Australia
Received 23 August 2003; accepted 1 October 2003
Abstract—A chemo-enzymatic synthesis of (−)-epipentenomycin I is reported using a lipase-catalysed kinetic resolution of the
racemic pentacyclic alcohol 8. Flash vacuum pyroloysis of (−)-8 so obtained gave (−)-(4R)-4-hydroxy-5-methylene-2-cyclopen-
tenone. Epoxidation of this compound with dimethyldioxirane followed by hydrolytic ring-opening of the resulting epoxide gave
(−)-epipentenomycin I.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
by hydrolytic ring-opening of the resulting epoxide.7c
We now report a chemo-enzymatic synthesis of (−)-
(4R,5S)-epipentenomycin I via (−)-(4R)-4-hydroxy-5-
methylene-2-cyclopentenone.
The pentenomycin group of natural products 1–4 have
antibacterial activities against both Gram-positive and
Gram-negative bacteria. Pentenomycins I 1 and II 2
were first isolated from Streptomyces eurythermus in
19731 while pentenomycin III 3 was isolated 3 years
later from Streptoverticillium eurocidicum.2 Epipenteno-
mycin I 4, however, was isolated in 19893a from car-
pophores of Perziza sp., much later than its first
synthesis in racemic form in 1980.4,5 This compound
was shown to have the (4S,5R) absolute configuration
by X-ray crystallographic analysis of its 2-bromo-O-tri-
acetate derivative,3b thus establishing that penteno-
mycin I 1 and epipentenomycin I 4 are epimeric at C-5.
Their biological activities and their highly oxygenated
structures have attracted several synthetic studies on
their total synthesis,6 the synthesis of their
racemates4,5,7 and their analogues,8 e.g. epipenteno-
mycin II 5 and III 6. We recently reported a
diastereoselective synthesis of ( )-epipentenomycin I 4
and III 6 from the diastereoselective epoxidation of
4-hydroxy- and 4-acetoxy-5-methylene-2-cyclopen-
tenone, respectively, with dimethyldioxirane followed
2. Results and discussion
The known racemic epoxide, rac-79 (a 1.6:1 mixture of
diastereomers), was converted to a mixture the known
racemic alcohols rac-8 and rac-10 (Scheme 1).9 These
isomers could be obtained pure after separated by
column chromatography and recrystallization. The rel-
ative stereochemistry of these compounds was deter-
mined by NOESY experiments and single crystal X-ray
analysis on their corresponding acetates rac-9 and rac-
11, respectively.10 NOESY studies on rac-9 showed
cross-peaks between the allylic proton Ha and the
benzylic methine Hb, while in rac-11, cross-peaks were
observed between the allylic proton Hb and one meth-
ylene proton Ha (Scheme 1). Kinetic resolution of rac-8
was achieved using PS-D ‘Amano’ I lipase on Celite
and vinyl acetate (6 equiv.) in acetonitile at 40°C. After
* Corresponding author. Fax: +612 4221 4287; e-mail: spyne@uow.edu.au
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.10.003

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T. Klomklao et al. / Tetrahedron: Asymmetry 14 (2003) 3885–3889
Scheme 1. * indicates NOESY cross peaks. Reagents and conditions: (a) Et3N, CH2Cl2, 0°C to rt, 16 h; (b) Ac2O, pyridine; (c)
PS-D ‘Amano’ I lipase on Celite, vinyl acetate, MeCN, 40°C, 101 h; (d) TFA, MeCN, H2O, 60°C 16 h.
101 h the reaction was terminated at 41% conversion.11
The reaction mixture was separated by column chro-
matography to provide acetate (−)-9 {[h]2D4=−189 (c 2.03,
CHCl3)}, in 37% yield and in greater than 98% enan-
tiomeric excess, and the alcohol (+)-8 in 53% yield and
68% enantiomeric purity, as determined on its acetate
(+)-9. The enantiomeric excesses of (−)-9 and that of (+)-9
were determined by 1H NMR using the chiral shift
reagent, europium tris[3-(heptafluoropropylhydroxy-
methylene)-(+)-camphorate] (Eu(hfc)3, see Section 3 (gen-
eral remarks) for specific details).
While base catalysed hydrolysis (K2CO3, MeOH) of
acetate (−)-9 resulted in Michael addition of methanol to
the enone of (−)-8, acid catalysed hydrolysis using
aqueous trifluoroacetic acid in acetonitile smoothly gave
the desired alcohol (−)-8 {[h]2D4=−120 (c 1.28, CHCl3)}
in excellent yield (96%) (Scheme 1).
Figure 1. Single-crystal X-ray crystallographic structure of
(−)-13.10
Flash vacuum pyrolysis of (−)-8 at 450°C and <0.1 mmHg
gave (−)-(4R)-4-hydroxy-5-methylene-2-cyclopentenone,
(−)-12, in 76% yield. Treatment of this compound with
freshly distilled dimethyldioxirane (DMDO) in acetone12
initially at −78°C and then at rt for 16 h produced an
83:17 mixture of the anti- and syn-epoxy-alcohols, respec-
tively. The crude mixture was separated by flash chro-
matography to give about a 90:10 mixture of these
isomers. Recrystallization of this mixture gave pure
(−)-(3S,7R)-13 in 39% yield (mp 94–95°C) from which
single crystals were obtained. X-Ray crystallographic
analysis (Fig. 1) confirmed our earlier NOESY NMR
experiments that the major diastereomer resulted from
epoxidation of (−)-12, anti to the allylic hydroxy
group.7c,10
Heating a solution of (−)-13 in water7c,13 in a sealed tube
1
at 80°C, with monitoring by H NMR spectroscopy,
Scheme 2. Reagents and conditions: (a) FVP (450°C, <0.1
mmHg); (b) dimethyldioxirane, acetone, −78°C to rt, 16 h; (c)
H2O, 80°C, 16 h; (d) Ac2O, pyridine, rt, 16 h.
cleanly led to ring-opening of the epoxide group. After
freeze-drying (−)-epipentenomycin I, (−)-4 was obtained
in86%yield(Scheme2). Thissamplehadidenticalspectral

T. Klomklao et al. / Tetrahedron: Asymmetry 14 (2003) 3885–3889
3887
characteristics to those reported in the literature for
(+)-4,3b,7a however its specific rotation was opposite in
sign {[h]2D6=−75 (c 1.15, MeOH), lit.3b ([h]2D3=+35.3 (c
2.6, MeOH)} and larger in magnitude than the litera-
ture value. This discrepancy in the magnitude of the
specific rotations may be due to the hydroscopic nature
of these compounds and therefore (−)-4 was converted
to the known and less hydroscopic triacetate (−)-14
(Scheme 2). This sample had identical spectral charac-
teristics to those reported in the literature for (+)-
14,3b,7a and the magnitude of its specific rotation was
similar to the literature value {[h]2D6=−55 (c 1.16,
MeOH), lit.3b ([h]2D3=+47 (c 0.21, MeOH)}.
Hz, 1H), 7.05–7.50 (m, 9H). 13C NMR l 34.3, 44.5,
52.1, 56.7, 78.2, 123.0, 124.2, 124.7, 125.9, 126.0, 126.3,
126.5, 126.8, 134.2, 139.8, 141.6, 144.5, 144.6, 160.0,
208.3. LRMS (CI +ve) (isobutane): 288 [M]+ (2%), 178
[C14H10]+ (100%). HRMS (CI +ve) calcd for C20H16O2
[M]+ 288.115082. Found: 288.115030. rac-10: White
crystals, mp 159–160°C (from CH2Cl2/hexane) (lit.14
159–160°C from CH2Cl2/hexane). IR (CHCl3), wmax
,
3450, 3030, 2933, 1710, 1680, 1440, 1325, 765 cm−1. H
NMR l 2.09 (br d, OH, 1H), 1.73, 2.04, 4.38 (ABX
system, JAB=12.3 Hz, JAX=JBX=2.7 Hz, 3H), 4.23–
4.25 (m, 1H), 4.35–4.37 (m, 1H), 6.19 (dd, J=6, 0.9 Hz,
1H), 7.06–7.43 (m, 9H). 13C NMR l 41.0, 44.4, 47.7,
56.3, 81.0, 122.9, 123.8, 125.2, 125.5, 125.7, 125.8,
126.0, 126.2, 134.7, 140.3, 141.7, 143.9, 145.0, 159.0,
207.3.
1
In conclusion we have developed an efficient chemo-
enzymatic synthesis of (−)-epipentenomycin I using a
lipase catalysed kinetic resolution of the racemic penta-
cyclic alcohol rac-8. The chiral chemical intermediates
described here should be valuable for the synthesis of
other bioactive molecules and we are currently investi-
gating their chemistry.
3.3. Kinetic resolution of 9%,10%-dihydro-5-hydroxy-
spiro[3-cyclopenten-1,11%-(9,10)ethanoanthracene]-2-one
rac-8
To a solution of rac-8 (1.000 g, 3.47 mmol) and vinyl
acetate (1.9 mL, 20.82 mmol) in acetonitrile (35 mL)
was added immobilized PS-D ‘Amano’ I Lipase on
Celite (2.000 g). After 101 h at 40°C, with stirring at
400 rpm, 41% conversion was calculated from the enan-
tiomeric excess of the starting alcohol 8 and the acetate
9 according to % conv.=68/(68+98)=41.11 The reac-
tion mixture was diluted in CH2Cl2, filtered through a
plug of Celite and evaporated to dryness. The crude
mixture was purified by column chromatography on
silica gel. Elution with 30% petroleum spirit in CH2Cl2
gave the acetate (−)-9 (0.42 g, 37%, ee >98%). The
column was then stripped with 20% EtOAc in CH2Cl2
to obtain the alcohol (+)-8 (0.53 g, 53%, ee=68%). The
alcohol (+)-8 (0.53 g) was recrystallized from CH2Cl2/
petroleum spirit to obtain (+)-8 (0.33 g, 33%, ee >98%).
(+)-8: White crystals, mp 166–167°C (from CH2Cl2/
petroleum spirit). [h]2D4=+119.7 (c 1.98; CHCl3), ee
>98%. Spectral data were identical to rac-8. (−)-9:
White crystals; mp 210–211°C (from CH2Cl2/petroleum
spirit), [h]2D4=−189.2 (c 2.03, CHCl3), ee >98%. 1H
NMR: l 1.74 (s, 3H), 1.89, 2.11, 4.38 (ABX system,
3. Experimental
3.1. General remarks
All NMR spectra were determined in solutions of
CDCl3 at 300 MHz (1H NMR) or 75 MHz (13C NMR)
unless otherwise stated. NMR assignments are based
upon 2D NMR spectral analysis (COSY and DEPT).
Lipase used in this experiment was PS-D Amano I
(immobilized PS-D ‘Amano’ I lipase on Celite) from
Amano Enzyme Inc., Japan. To determine the enan-
tiomeric excess of the enzymatically resolved alcohols,
they were converted in to their acetates 9 with acetic
anhydride/pyridine. The enantiomeric excess was deter-
mined by 1H NMR using the chiral shift reagent,
europium tris[3-(heptafluoropropylhydroxymethylene)-
(+)-camphorate]. The acetate 9 (5.0 mg) was dissolved
in 0.7 mL of a solution of Eu(hfc3) (14.8 mg) in CDCl3
(5.0 mL). The acetate signals of (+)-9 and (−)-9 were at
l 1.872 and 1.877, respectively.
J
AB=12.7 Hz, JAX=JBX=2.4 Hz, 3H), 3.99 (s, 1H),
3.2. 9%,10%-Dihydro-5-hydroxy-spiro[3-cyclopenten-1,11%-
(9,10)ethanoanthracene]-2-one rac-8 and rac-10
5.49 (dd, J=2.7, 1.2 Hz, 1H), 6.30 (dd, J=6.0, 0.6 Hz,
1H), 7.11–7.33 (m, 8H), 7.45 (dd, J=6.0, 2.7 Hz, 1H).
13C NMR l 20.6, 34.5, 44.5, 52.4, 55.3, 78.8, 123.1,
123.8, 125.2, 125.9, 126.3, 126.7, 126.8, 135.6, 139.6,
140.9, 144.3, 144.6, 156.6, 169.8, 207. LRMS (CI +ve)
(isobutane): 331 [M+H]+ (7%), 271 (65%), 179 [C14H10+
H]+ (100%). HRMS (CI +ve) calcd for C22H18O3 (M)+
330.124909. Found: 330.125595.
To a solution of the crude epoxide rac-79 (dr=1.6:1
1
was indicated by H NMR) (1.50 g, 5.21 mmol) in dry
CH2Cl2 (20 mL) at 0°C was added triethylamine (1.5
mL, 10.42 mmol) and the resulting mixture was stirred
at rt overnight. Solvents were evaporated to dryness
and the diastereomeric alcohols were separated by
column chromatography on silica gel (CH2Cl2:
petroleum spirit; 9:1 as the eluent). Recrystallization
from CH2Cl2 and petroleum spirit gave rac-8 (0.843 g,
2.927 mmol) and rac-10 (0.552 g, 1.9167 mmol). rac-8:
White crystals, mp 162–163°C (from CH2Cl2/petroleum
spirit) (lit.14 163–165°C, from CH2Cl2/hexane). IR
(CHCl3), wmax, 3415, 3025, 2933, 1709, 1460, 1170, 763
3.4. (−)-9%,10%-Dihydro-5-hydroxy-spiro[3-cyclopenten-
1,11%-(9,10)ethanoanthracene]-2-one (−)-8
To a solution of (−)-9 (372 mg, 1.125 mmol) in
CH3CN/H2O (3:1, 16 mL) was added TFA (2.9 mL,
3.94 mmol). The reaction mixture was then heated
overnight at 60°C. The aqueous layer was extracted
with CH2Cl2 (3×15 mL) and the organic layers were
washed with water, brine and dried (MgSO4). The
solvent was evaporated to dryness and the residue was
1
cm−1. H NMR l 0.86 (br d, OH, 1H), 1.78, 2.30, 4.40
(ABX system, JAB=12.7Hz, JAX=JBX=2.5 Hz, 3H),
3.99 (s, 1H), 4.57–4.60 (m, 1H), 6.19 (dd, J=7.5, 1.5

3888
T. Klomklao et al. / Tetrahedron: Asymmetry 14 (2003) 3885–3889
purified by flash column chromatography on silica gel
using 20% EtOAc in CH2Cl2 as the eluent to give the
alcohol (−)-8 (311 mg, 96%). (−)-8: white crystals; mp
162–163°C (CH2Cl2/pretoleum spirit), [h]2D7=−120 (c
1.28; CHCl3). Spectral data were identical to rac-8.
1H and 13C NMR of this compound was identical to
that reported in the literature.3 1H NMR (D2O): l 3.71
(d, J=12 Hz, 1H, CH
6 HOH), 3.82 (d, J=12 Hz, 1H,
CH
6
HOH), 4.86 (m, 1H, H-4), 6.41 (dd, J=6.3, 1.5 Hz,
H-2), 7.73 (dd, J=6.3, 2.1 Hz, H-3). 13C NMR (D2O+
MeCN as internal standard): l 63.6 (C-6), 77.5 (C-4),
82.4 (C-5), 132.5 (C-2), 163.8 (C-3), 208.5 (C-1). LRMS
(CI +ve): 145 [M+H]+ (100%), 127 [M−H2O]+ (26%).
HRMS (CI +ve) calcd for C6H9O4 (M+H)+ 145.050356.
Found: 145.050084.
3.5. (−)-(4R)-4-Hydroxy-5-methylene-2-cyclopentenone
(−)-12
Compound (−)-8 (390 mg, 1.354 mmol; 30 mg was used
each time) was placed in 10 mL round-bottom flask
connected to a gas phase pyrolysis apparatus and the
system was subjected to high vacuum (<0.1 mmHg).
The sample was carefully pyrolysed with heat gun and
the vapour passed through the heating column at
450°C. The crude product was purified by flash column
chromatography on silica gel, elution with petroleum
spirit gave (−)-12 (113.9 mg, 76%) that was homoge-
neous from TLC and NMR analysis. This compound
was used immediately in the next step or could be
stored for a few days in the freezer without noticeable
polymerization. (−)-12: viscous liquid, [h]2D6=−163.3 (c
1.24, CHCl3), IR (neat) wmax 3403, 2876, 1705, 1579,
3.8. (−)-(4R,5S)-Epipentenomycin I triacetate (−)-14
A solution (−)-4 (11.3 mg, 0.079 mmol) in pyridine (2
mL) was added excess acetic anhydride at room tem-
perature and the reaction mixture was left to stir
overnight. The solvents were then evaporated to dry-
ness. The crude product was purified by flash column
chromatography on silica gel using 30% EtOAc in
petroleum sprit as the eluent to obtain (−)-14 (18.7 mg,
88%). (−)-14: viscous liquid, [h]2D6=−55.1 (c 1.16;
MeOH) lit.3 [h]2D3=+47.5 (c 0.64; MeOH). IR (neat)
1
wmax 2923, 2360, 1735, 1374, 1214, 1054 cm−1. The H
1
1400, 895 cm−1. H NMR l 2.42–2.56 (br, OH, 1H),
and 13C NMR of this compound was identical to that
reported in the literature.3 1H NMR l 2.14, 2.13 and
2.04 (s, 3×3H, OAc), 4.07 (d, J=12 Hz, 1H,
5.22 (s, 1H, H-4), 5.76 (s, 1H, =CH6 H), 6.20 (s, 1H,
=CH6 H), 6.45 (dd, J=6.5, 0.9 Hz, 1H, H-2), 7.54 (ddd,
J=6.0, 2.4, 0.9 Hz, 1H, H-3). 13C NMR l 70.7 (C-4),
119.0 (C-6), 136.2 (C-2), 145.1 (C-5), 159.6 (C-3), 194.8
(C-1). CIMS (isobutane): 111 [M+H]+ (100%).
CH6 HOAc), 4.52 (d, J=12 Hz, 1H, CH6 HOAc), 6.26 (m,
1H, H-4), 6.49 (dd, J=6.3, 1.8 Hz, 1H, H-2), 7.38 (dd,
J=6.3, 2.4 Hz, 1H, H-3). 13C NMR l 197.3 (C-1),
170.2, 170.1 and 170.0 (CO-acetyls), 155.0 (C-3), 134.9
(C-2), 82.8 (C-5), 77.2 (C-4), 62.5 (C-6), 20.8 (Me-
acetyls). LRMS (CI +ve): 271 [M+H]+ (100%). HRMS
(CI +ve) calcd for C12H15O7 (M+H)+ 271.082469.
Found: 271.081778.
3.6. (−)-(3S,7R)-7-Hydroxy-1-oxaspiro[2.4]hept-5-en-4-
one (−)-13
To a solution of (−)-12 (62.1 mg, 0.565 mmol) in
acetone (5 mL) was added a solution of freshly distilled
dimethyldioxirane (21 mL, 1.491 mmol) in acetone at
−78°C and the reaction was left to stir overnight at
room temperature. The solvent was then evaporated to
Acknowledgements
1
dryness. The crude product (dr=83:17 from H NMR
We thank the University of Wollongong and the Grad-
uate School Chiang Mai University for partial financial
support and Toyoko Iwata from Amano Enzyme Inc.,
Nagoya, Japan for the generous gift of lipase enzymes.
analysis) was purified by flash column chromatography
on silica gel using 10% EtOAc in petroleum spirit as the
initial eluent followed by 20% EtOAc in petroleum
spirit. Recrystallization from CH2Cl2 in the freezer
(−20°C) gave pure (−)-13 (28 mg, 39%). (−)-13: white
crystals; mp 94–95°C (from CH2Cl2), [h]2D6=−159.6 (c
References
1
1.03; CHCl3), H NMR l 2.51 (br d, J=7.5 Hz, OH,
1H), 3.14 (d, J=6.6 Hz, 1H, CH
6
HO), 3.29 (d, J=6.9
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6
1.5 Hz, 1H, H-5), 7.70 (dd, J=6.3, 2.4 Hz, 1H, H-6).
13C NMR l 51.1 (C-2), 64.2 (C-3), 71.0 (C-7), 135.9
(C-5), 161.8 (C-6), 199.8 (C-4). LRMS (CI +ve) (isobu-
tane): 127 [M+H]+ (100%), 109 [M−H2O]+ (37%), 81
(15%). HRMS (CI +ve) calcd for C6H7O3 (M+H)+
127.040235. Found: 127.039519.
3.7. (−)-(4R,5S)-Epipentenomycin I (−)-4
4. Smith, A. B., III; Pilla, N. N. Tetrahedron Lett. 1980, 21,
4691.
A solution of (−)-13 (14.5 mg, 0.115 mmol) in H2O (0.5
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evaporated under vacuo to obtain (−)-4 (14.3 mg, 86%)
that was pure by TLC and NMR analysis. (−)-4: vis-
5. Shono, T.; Matsumura, Y.; Yamane, S.; Suzuki, M.
Chem. Lett. 1980, 1619.
6. (a) Elliott, J. D.; Hetmanski, M.; Palfreyman, M. N.;
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Palfreyman, M. N. J. Chem. Soc., Perkin Trans. 1 1984,
cous liquid [h]2D6=−75.3 (c 1.15; MeOH), IR (neat) wmax
,
3381, 2928, 2532, 1711, 1657, 1253, 1058, 840 cm−1. The

T. Klomklao et al. / Tetrahedron: Asymmetry 14 (2003) 3885–3889
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V=836 A , Z=2. R=0.050, Rw=0.051, for No=1671
CCD reflections (F>4|(F)), Mo Ka radiation) (chirality
indeterminate) (CCDCc 184071). rac-11: Monoclinic,
P21/c (i.e. racemic specimen), a=9.592(2), b=17.638(4),
3
,
,
c=10.078(2) A, i=103.265(4)°, V=1160 A , Z=4. R=
0.049, Rw=0.056, for No=1981 (CCDCc 184072)). (−)-
7. (a) Smith, A. B., III; Branca, S. J.; Pilla, N. N.; Guaciaro,
M. A. J. Org. Chem. 1982, 47, 1855; (b) Pohmakotr, M.;
Popuang, S. Tetrahedron Lett. 1991, 32, 275; (c) Phutd-
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8. (a) Verlaak, J. M. J.; Klunder, A. J. H.; Zwanenburg, B.
Tetrahedron Lett. 1982, 23, 5463; (b) Klunder, A. J. H.;
Houwen-Claassen, A. A. M.; Kooy, M. G.; Zwanenburg,
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9. Chantarasiri, N.; Dinprasert, P.; Thebtaranonth, C.;
Thebtaranonth, Y.; Yenjai, C. J. Chem. Soc., Chem.
Commun. 1990, 286.
10. rac-9 and rac-11 correspond to structures 12a (CCDCc
184071) and 12b (CCDCc 184072) of Ref. 7c (crystal/
refinement details not given in Ref. 7c). For these two
isomers of C22H18O3, Mr=330.4 at ca.0 153 K. rac-9:
Monoclinic, space group P21 (note that although this
space group is chiral, the bulk sample is racemic), a=
13 is monoclinic, P21, a=6.906(3), b=10.971(4),
3
,
,
c=7.212(3) A, i=95.157(6)°, V=544 A , Z=4. R=
0.086, Rw=0.010, for No=1176 (CCDCc 212132). The
chirality adopted is assigned from chemistry. The mate-
rial was ‘difficult’, presumably in consequence of the
packing of the molecules in strings, each made up of
symmetry-related sequences of the independent compo-
nents molecule 1 or molecule 2 of the asymmetric unit,
linked by hydrogen-bonds between the hydroxyl hydro-
gen of one molecule and the ketonic oxygen of the next.
11. The percentage conversion was calculated from the enan-
tiomeric excess of the starting alcohol 8 and acetate 9
according to % conv.=68/(68+98)=41. See: Chen, C.-S.;
Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J. Am. Chem.
Soc. 1982, 104, 7294.
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,
8.132(1), b=9.697(1), c=10.898(2) A, i=103.344(4)°,