Multiselective enzymatic reactions for the synthesis of protected homochiral cis- and trans-1,3,5-cyclohexanetriols

Article abstract of DOI:10.1016/S0957-4166(00)00368-2

For the synthesis of the potentially antipsoriatic vitamin D derivative 3 (Ro 65-2299) an efficient and multiselective enzymatic step had been developed in which the easily accessible trans-1,3,5-triacetoxy-cyclohexane 5 was selectively monohydrolyzed in the presence of the cis-isomer 8 to give (1R,3R)-1,3-diacetoxy-5-hydroxy-cyclohexane 6 in high enantiomeric excess (>99%) and yield (84%). Furthermore, for the synthesis of the enantiomer of 3, a simple and efficient enzymatic procedure for the asymmetric acetylation of cis-1,5-dihydroxy-3-(tert-butyldimethylsilanoxy)-cyclohexane 10 in an anhydrous organic solvent providing (1R,3S,5S)-1-acetoxy-3-hydroxy-5-(tert-butyldimethylsilanoxy)-cyclohexane 11 in >99% ee and quantitative yield is described. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00368-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4171–4178
Multiselective enzymatic reactions for the synthesis of
protected homochiral cis- and trans-1,3,5-cyclohexanetriols†
Beat Wirz,a,* Hans Idinga and Hans Hilpertb
aPharmaceutical Research Basel-Biological Sciences, F. Hoffmann-La Roche Ltd, Basel, Switzerland
bNon-clinical Development-Process Research, F. Hoffmann-La Roche Ltd, Basel, Switzerland
Received 5 September 2000; accepted 13 September 2000
Abstract
For the synthesis of the potentially antipsoriatic vitamin D derivative 3 (Ro 65-2299) an efficient and
multiselective enzymatic step had been developed in which the easily accessible trans-1,3,5-triacetoxy-
cyclohexane 5 was selectively monohydrolyzed in the presence of the cis-isomer 8 to give (1R,3R)-1,3-
diacetoxy-5-hydroxy-cyclohexane 6 in high enantiomeric excess (>99%) and yield (84%). Furthermore, for
the synthesis of the enantiomer of 3, a simple and efficient enzymatic procedure for the asymmetric
acetylation of cis-1,5-dihydroxy-3-(tert-butyldimethylsilanoxy)-cyclohexane 10 in an anhydrous organic
solvent providing (1R,3S,5S)-1-acetoxy-3-hydroxy-5-(tert-butyldimethylsilanoxy)-cyclohexane 11 in >99%
ee and quantitative yield is described. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
For the treatment of psoriasis, one of the most common genetically determined skin diseases,
novel orally active vitamin D analogues1 have attracted much attention in recent years. Retiferol
3 (Ro 65-2299), a compound that lacks the C,D-ring,2 was selected as a clinical candidate. In
order to facilitate material supply a novel and very efficient synthesis suitable for the multi-kg
scale was developed by Hilpert et al.3 This new route features an efficient Julia coupling of the
key precursors ketone 1 and sulfone 2 (Scheme 1).
The synthetic concept starting from the trans-cyclohexanetriol 4 (Scheme 2) represents a
highly attractive and short route to ketone 1 but depends on a very demanding multiselective
enzymatic hydrolysis of the triacetate 5. To our knowledge no stereoselective hydrolysis of a cis-
or trans-triacyloxycyclohexane has been described in the literature to date, though several such
examples for the hydrolysis of 1,2-, 1,3- or 1,4-diacyloxycyclohexanes exist. The presence of a
third identical acetyl group in the substrate demands additional discrimination, namely regio-
selectivity as well as enantioselectivity and limitation to monohydrolysis.
* Corresponding author. E-mail: beat.wirz@roche.com
†
Dedicated to Dr. Hans Georg Leuenberger on the occasion of his 60th birthday.
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00368-2

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Scheme 1. Julia coupling of ketone 1 and sulfone 2 to retiferol Ro 65-2299
Scheme 2. Chemoenzymatic route to the trans-ketone 1
The starting material trans-triol 4 is commercially available from Tokyo Kasei as a 1:1
mixture of 4 and the cis-isomer 7 which is converted to the corresponding 1:1 mixture of trans-
and cis-triacetates 5 and 8, respectively, by conventional means. For reasons of technical
relevance we tried to avoid the necessity of completely removing the cis-isomer even though the
presence of the cis-triacetate 8 increased the already demanding selectivity requirements for this
enzymatic step considerably. Following the enzymatic step, alcohol 6 is converted to the
diacetoxy ketone 1 by a bleach oxidation.3 An even more direct alternative access to 6, the
selective enzymatic diacylation of trans-triol 4 in the presence of its cis-isomer 7 in anhydrous
organic solvents, failed.
To enable clinical evaluation of the retiferol 3, an analytical method was required to quantify
various isomers. Therefore, we also developed a route to the enantiomeric ketone 16 and the
cis-ketone 15 (Scheme 3); both intermediates can then be converted to the target isomers
according to Scheme 1. Meso-diol 10 as substrate for the enzymatic acetylation reaction was
obtained by a highly selective monosilylation of commercially available cis-cyclohexanetriol 7.3
The corresponding enzymatic step, the desymmetrization of diol 10, was considered to be less
demanding with respect to the required selectivity as the desymmetrization of numerous
prochiral diols by asymmetric acylation has already been described in the literature. In the

B. Wirz et al. / Tetrahedron: Asymmetry 11 (2000) 4171–4178
4173
Scheme 3. Chemoenzymatic routes to the trans-ketone 16 and cis-ketone 15
context of the synthesis of vitamin D analogues we should mention Huang et al.,4 who started
an approach from cis-cyclohexane-1,3,5-triol which was diacetylated in vinyl acetate with lipase
SAM (78% yield). The residual hydroxy group was tosylated followed by methanolysis of the
acetyl groups. The resulting prochiral diol was then asymmetrically monoacylated with lipase
SAM to the (1R,3S,5S)-1-acetoxy-3-hydroxy-5-tosyloxy-cyclohexane in >95% ee and 92% yield.
In a reverse hydrolytic approach Suemune et al.5 prepared the closely related (1S,3R,5S)-1-acet-
oxy-3-benzyloxy-5-hydroxy-cyclohexane from the corresponding diacetate by means of pig liver
esterase (87% ee; 62% yield) or various lipases (1R,3S,5R in up to 84% ee; 16% yield). For the
same reaction Carda et al.6 reported 85% ee in 70% yield with pig liver esterase.
2. Results and discussion
2.1. Enzymatic preparation of (1R,3R)-1,3-diacetoxy-5-hydroxy-cyclohexane 6
The screening for the regio- and enantioselective monohydrolysis of trans-triacetate 5 with
numerous lipases (EC 3.1.1.3) and esterases (EC 3.1.1.1) provided four commercially available
lipases from Candida rugosa (formerly classified as C. cylindracea) which met all discrimination
requirements and enabled a highly efficient solution of this problem. In addition, all four
enzymes, namely lipases AY, MY, OF and Chirazyme L-3, exhibited a rather good trans/cis-
selectivity so that a 1:1-mixture of the trans/cis-isomers 5 and 8 could be used as an easily
accessible starting material. The four enzymes showed comparable selectivities with respect to
the enantiomeric excess of 6, the formation of the undesired trans-meso-diacetate 9 and
over-hydrolysis: after ꢀ95% conversion, product 6 was generated in roughly 36% (GLC area %)
and >99% ee, besides ꢀ0.5% meso-diacetate 9, ꢀ5% cis-diacetate 14, <10% monoacetates/triols
and the remaining substrates 8 (ꢀ50%) and 5 (ꢀ6%). Lipase MY exhibited the best overall-per-
formance but Lipase OF was preferred because it showed an approximately six times faster
reaction rate than the three other enzymes (on a weight basis). Optimization of lipase OF
revealed that the use of a nonpolar biphasic cosolvent (cyclohexane), low temperature (6°C,
freezing point of cyclohexane) and pH 7.0 enhanced the overall performance.

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The mono- and triacetate isomers still present in the crude diacetate product 6 could be
readily removed by a chromatographic filtration step (crude product/silica gel 1:10); however,
neither the trans-meso-diacetate 9 nor the cis-diacetate 14 were depleted under these conditions.
In order to reduce the latter and further improve the efficiency of the chromatographic filtration
step the enzyme reaction was carried out with an enriched trans-isomer which could be obtained
by a crystallization step.3 Under optimized conditions and using a trans-enriched substrate
(5/8=81:19) crude product 6 was generated, after conversion of 1.07 ester equivalents and
before the chromatographic filtration step, in 71.1% purity (GLC area %) containing 1.9%
cis-diacetate 14, 0.5% meso-diacetate 9, 0.5% monoacetate(s) and the remaining substrates 8
(18.5%) and 5 (2.2%). After chromatographic filtration diacetate 6 was obtained in 95.7% purity
(99.5% ee) and 84% yield containing 2.6% 14 and 0.6% 9 (see Section 4.4).
Hydrolysis experiments with the corresponding tributyrate with lipase OF were unselective
(formation of monobutyrate), as were butanolysis tests with the mentioned Candida rugosa
enzymes in tert-butyl methyl ether/butanol 9:1. Also the experiments for an even shorter
approach, the selective acylation of 4/7 mixtures with various acyl donors, were unsuccessful
(essentially due to over-acylation or insufficient cis/trans selectivity).
2.2. Enzymatic preparation of (1R,3S,5S)-1-acetoxy-3-hydroxy-5-(tert-butyldimethylsilanoxy)-
cyclohexane 11
A broad panel of commercially available lipases was tested for the asymmetric acetylation of
meso-diol 10 in vinyl acetate. It turned out that an amazingly high number of enzymes exhibited
excellent selectivity with respect to both enantiomeric excess and diacetate formation (Table 1).
Table 1
Asymmetric acetylation of diol 10 with various lipases
Lipasea
Conversion (GLC area %) % ee
Lipasea
Conversion (GLC area %) % ee
11
12
1.1
11
11
12
11
QL
PL
100
99
99
99
97
96
95
92
91
88
87
86
86
84
\99
\99
\99
\99
\99
\99
\99
\99
\99
\99
\99
\99
\99
\99
SAM-2
B1
Chirazyme L-2
M-AP
80
77
76
76
74
74
73
69
68
67
63
60
58
0.2
0.4
14
\99
\99
97
0.7
1.2
0.7
0.5
2.7
0.4
0.3
0.3
0.2
0.3
0.5
–
Chirazyme L-1
Chirazyme L-6
Chirazyme L-3
Chirazyme L-4
AK
0.2
0.6
0.2
0.1
0.1
0.4
0.1
–
99
G
\99
\99
\99
\99
99
\99
96
\99
99
C. utilis
B
AP
IM-20
F13
AY
D
PS
Chirazyme L-5
MY
C. cylindracea
OF
LIP
F5
0.1
0.2
Chirazyme L-7
0.5
a Conditions: 4 mg of 10 dissolved in 1 ml of vinyl acetate was slightly agitated at rt in the presence of 0.5–3 mg
of suspended enzyme.

B. Wirz et al. / Tetrahedron: Asymmetry 11 (2000) 4171–4178
4175
The fastest enzyme, lipase QL from Meito Sangyo, was employed in a preparative experiment
(s/e=10) using vinyl acetate/ethyl acetate 1:9 as both solvent and acylating agent. After
termination of the reaction the enzyme was filtered off (for further reuse), and the product was
obtained in high yield (99%) and purity (98%). Further optimization of the reaction towards
higher substrate concentration was not carried out but is considered to be feasible.
2.3. Enzymatic preparation of cis-1,3-diacetoxy-5-hydroxy-cyclohexane 14
In contrast to Huang et al.,4 who used the relatively expensive lipase SAM for the preparation
of cis-diacetate 14, we employed lipase QL which recommended itself by virtue of its perfor-
mance in the enzyme screening directed to the selective acylation of 4/7 mixtures for the
preparation of 6. In this screening, carried out in tert-butanol/vinyl acetate 10:1, only a few
enzymes were found to be capable of diacetylating cis-triol 7 with reasonable activity. Among
them lipase QL again afforded the best results. In a 50 g experiment the mono-, di- and
triacetates were formed in a ratio of 1:13:1 (GLC area %). Removal of the side components by
chromatographic filtration on silica gel yielded pure product 14 in 70% yield.
2.4. Determination of the absolute configuration of the alcohols 6 and 11
The absolute configuration of the alcohols 6 and 11 was established as follows. The
configuration of the remaining hydroxy group in alcohol 11 was inverted under Mitsunobu
conditions to give compound 12. Deprotection of the silyl-group afforded alcohol 13. Com-
pounds 6 and 13 showed identical 1H NMR, IR and MS spectra, [h]D values with opposite signs,
and they could be base-line separated on a chiral GC-column (BGB-174), thus corroborating the
enantiomeric relationship of the two compounds. The absolute configuration of the alcohol 6
(and thus also of 11) was unambiguously established by an X-ray analysis of the tert-butyl ester
17, which was prepared from ketone 1 by Peterson olefination.
3. Conclusion
The two versatile protected homochiral 1,3,5-cyclohexanetriol building blocks 6 and 11 could
be efficiently prepared by two highly selective enzymatic desymmetrizations of meso-compounds:
the trans-isomer 6 by asymmetric monohydrolysis of the corresponding triacetate 5 in the
presence of the cis-isomer 8, and the cis-isomer 11 by asymmetric acylation of the monosilylated
cis-triol 10 in an anhydrous organic solvent. Especially the first reaction represents an extremely
attractive biotransformation with unusually high multiple selectivity.
The homochiral building blocks open, for example, an efficient way to the A-ring moiety of
vitamin D derivatives, which become accessible in both enantiomeric forms.
4. Experimental
4.1. General
NMR spectra were recorded on a Bruker AC (250 MHz) or a Bruker AM (400 MHz), and
IR spectra on a Nicolet-FTIR 20 SXB. MS-EI were measured on an SSQ 7000 Finnigan MAT.

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Optical rotations were determined on a Perkin–Elmer Polarimeter 241, and melting points
(uncorrected) on a Tottoli apparatus. GC determinations were carried out on a Perkin–Elmer
AutoSystem and an HP5890-II. BGB-capillary columns were purchased from BGB-Analytik
AG, Anwil, Switzerland.
4.2. Enzymes
Lipases MY, OF, PL and QL were a kind gift from Meito Sangyo Co. (Tokyo), lipases AP,
AK, AY, CE, D, F-AP, G, M-AP, PS from Amano Pharmaceutical Co. (Nagoya, Japan) and
lipase IM-20 from Novo Nordisk (Bagsvaerd, Denmark). Lipases B1, F5 and F13 were
purchased from Enzymatix (Cambridge, UK), lipase B from Wako Pure Chemical Industries
(Osaka), lipase LIP from Asahi Chemical Industry Co. (Tokyo), lipase from C. cylindracea from
Sigma and lipase SAM-2 and lipase from C. utilis from Fluka. Chirazymes were from Roche
Diagnostics (Mannheim).
4.3. Preparation of enriched trans-1,3,5-triacetoxy-cyclohexane 5
A suspension of 44.00 g (333.0 mmol) cyclohexanetriol (enriched to a 76:18 mixture of
trans/cis as described by Hilpert et al.3) in 110 ml pyridine was treated at 22°C with 157 ml
acetic anhydride (1665 mmol) over 45 min, after which time the temperature rose to 45°C. The
yellow solution was stirred at 45°C until completion of the reaction (4 h). The mixture was
evaporated to dryness at 52°C/10 mbar, the residue was dissolved in 300 ml dichloromethane
and the organic layer washed with 300 ml 0.2N HCl and 200 ml deionized water. The aqueous
layers were extracted with 200 ml dichloromethane, the combined organic layers were dried over
magnesium sulfate and evaporated to dryness to give 85.73 g (331.9 mmol) of crude 5 as a pale
yellow oil: GLC (area %): 77.0% trans-triacetate 5, 17.7% cis-triacetate 8. IR (neat): 2961 m,
1
1736 s br (CꢀO), 1438 m, 1370 s, 1229s br, 1131 s, 1029 s, 980 s. H NMR (400 MHz, CDCl3):
1.35–1.65 and 2.07–2.35 (m each, 6H, 3×CH2), 2.04 and 2.07 (ratio 2.8:1, s each, 9H, 3×CH3),
4.78 (m, 0.8 H, 3×HꢁCO of cis-isomer), 5.09 and 5.39 (ratio 2:1, m each, 3H, 3×HꢁCO of trans
isomer). MS (EI): 199/7 (M+−OAc), 138/20, 96/90, 78/35, 43/100. C12H18O6 requires: C, 55.81;
H, 7.03%; found: C, 55.94; H, 7.10.
4.4. Enzymatic preparation of (1R,3R)-1,3-diacetoxy-5-hydroxy-cyclohexane 6
Crude 5 (81.74 g, 316.5 mmol; 77.0% GLC) obtained above was taken up in 200 ml
cyclohexane and to the suspension added 1.3 l 0.1 M sodium chloride solution and 50 ml 0.1 M
sodium phosphate buffer pH 7.0 under vigorous stirring. The resulting suspension/solution was
cooled to 5–6°C and the pH readjusted to 7.0 with a few drops of 1N sodium hydroxide
solution. The reaction was started by adding 1.60 g lipase OF dissolved in 15 ml 0.1 M sodium
chloride solution and the pH kept constant at 7.0 by the controlled addition (pH stat) of 1.0N
sodium hydroxide solution under vigorous stirring at 5–6°C. After 21.5 h, after the consumption
of 260 ml (1.07 equiv.), the reaction mixture (having become homogeneous) was extracted with
1.5 l of dichloromethane. The organic phase was very turbid and was clarified by filtration
through Dicalite Speedex (100 g, prewashed with water). The aqueous phase additionally
obtained by this step was combined with the first and extracted once more with 1.5 l

B. Wirz et al. / Tetrahedron: Asymmetry 11 (2000) 4171–4178
4177
dichloromethane (passed through the same filter bed). The combined organic phases were dried
over sodium sulfate, concentrated and dried on HV. The residue (66.1 g; 71% GLC) was
chromatographed on 700 g of silica (40–63 mm) with hexane/ethyl acetate 3:2 to give 46.3 g
(214.1 mmol; 95.7% GLC; 84%) trans-diacetate 6 as a colourless oil: GLC (area %): 95.7%
trans-diacetate 6, 2.6% cis-diacetate 14, 0.6% trans-meso-diacetate 9. 99.5% ee. [h]D=−14.4
(CHCl3, 1%). IR (neat): 3452m br (OH), 2957m, 1735 s br (CꢀO), 1464m, 1438m, 1371m, 1236m
1
br, 1122m, 1073m, 1026m, 977m. H NMR (400 MHz, CDCl3): 1.43–1.65, 2.05–2.31 (m each,
4H and 3H, 3×CH2 and OH), 2.04 and 2.05 (s each, 3H each, 2×CH3), 4.04 (m, 1H, HꢁC(5)),
5.05 and 5.29 (m each, 1H each, HꢁC(1) and HꢁC(3)). MS (EI): 156/2 (M+−OAc), 96/75, 43/100.
C10H16O5 requires: C, 55.55; H, 7.46%; found: C, 55.76; H, 7.37.
4.5. Preparation of all-cis-5-(tert-butyldimethyl-silanyloxy)-cyclohexane-1,3-diol 10
To a suspension of 74.01 g (560.0 mmol) of dry cis-triol 7 in 1480 ml of THF was added
subsequently at 22°C 95.72 g of tert-butyldimethylsilyl chloride and 62.33 g of triethylamine,
and the suspension was treated in one portion with 24.44 g of NaH (60% in oil) whereby the
temperature rose slowly to 45°C within 30 min. After 2 h at 40°C the suspension was cooled to
10°C and filtered. The filtrate was evaporated and the residue triturated at 22°C with 750 ml of
hexane. Filtration of the suspension and drying of the residue afforded 128.0 g (519.4 mmol;
93%) of pure (GLC) diol 10 as a white solid, mp 121–122°C. IR (nujol): 3418m, 3337m and
1
3256m (OH), 2929s, 2856s, 1466m, 1461m, 1360m, 1257m 1012s. H NMR (250 MHz, CDCl3):
0.08 (s, 6H, (CH3)2Si), 0.89 (s, 9H, (CH3)3C), 1.55 and 1.93–2.28 (m each, 3H and 5H, 3×CH2
and 2×OH), 3.78 (m, 3H, 3×HꢁCO). MS (EI): 189/2 (M+−C(CH3)3), 171/45, 129/30, 119/28,
75/100. C12H26O3Si requires: C, 58.49; H, 10.64%; found: C, 58.53; H, 10.51.
4.6. Enzymatic preparation of (1R,3S,5S)-1-acetoxy-3-hydroxy-5-(tert-butyldimethylsilanoxy)-
cyclohexane 11
Diol 10 (8.95 g, 36.3 mmol) was dissolved in 90 ml vinyl acetate and 810 ml ethyl acetate.
Lipase QL (895 mg) was added and the reaction mixture stirred at rt. After termination of the
reaction (46 h) the enzyme was filtered off, the filtrate concentrated and the residue dried
overnight on HV to give 10.55 g (36.5 mmol; 100%) of 11 as a yellowish oil: GLC (area %):
98.5%, >99% ee; [h]D=+5.0 (CHCl3, 1%); [h]365=+18.7 (CHCl3, 1%). IR (neat): 3431m (OH),
1
2953s, 2858s, 1738s (CꢀO), 1472m, 1365m, 1249s, 1110s, 1052s, 1028s, 873m, 838s, 777m. H
NMR (250 MHz, CDCl3): 0.06 and 0.07 (s each, 3H each, (CH3)2Si), 0.88 (s, 9H, (CH3)3C),
1.37–1.54 (m, 3H, HꢁC(2,4,6)), 2.03–2.22 (m and s br, 4H, HꢁC(2,4,6) and OH), 2.04 (s, 3H,
COCH3), 3.73 (m, 2H, HꢁC(3,5)), 4.76 (m, 1H, HꢁC(1)). MS (EI): 289/1 (M+H+), 171/100,
129/33, 117/55, 79/45, 75/60, 43/47. C14H28O4Si requires: C, 58.29; H, 9.78%; found: C, 57.93; H,
9.48.
4.7. Enzymatic preparation of cis-1,3-diacetoxy-5-hydroxy-cyclohexane 14
cis-Triol 7 (51.6 g, 390.4 mmol) was suspended in 1.0 l tert-butanol and 0.1 l vinyl acetate.
After addition of 2.58 g of lipase QL the reaction mixture was stirred at rt (in the course of the
reaction the substrate dissolved). After termination of the reaction (93 h) the enzyme was filtered
off and the filtrate concentrated to dryness to give crude 14 (81.4% GLC, 6.5% cis-monoacetate,

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B. Wirz et al. / Tetrahedron: Asymmetry 11 (2000) 4171–4178
6.2% 8). This material (88.0 g) was chromatographed on 1.5 kg of silica (40–63 mm) with
hexane/ethyl acetate 3:2 to give 59.1 g (273.4 mmol; 70%) of the cis-diacetate 14 as a white solid,
mp 71.6–72.4°C. GLC (area %): 98.6%. IR (nujol): 3494m (OH), 2949s, 2864s, 1736s br (CꢀO),
1
1466s, 1450m, 1420m, 1376s, 1246s br, 1134s, 1024s, 977s. H NMR (400 MHz, CDCl3): 1.43
and 2.28 (m each, 3H each, 3×CH2), 1.88 (s br, 1H, OH), 2.04 (s, 6H, 2×CH3), 3.78 (m, 1H,
HꢁC(5)), 4.78 (m, 2H, HꢁC(1) and HꢁC(3)). MS (EI): 217/10 (M+H+), 96/45, 43/100. C10H16O5
requires: C, 55.55; H, 7.46%; found: C, 55.47; H 7.29.
4.8. GLC analysis
GLC purity of 6: GLC: OV-1-OH (25 m×0.32 mm; H2; 100 kPa), 100–250°C with 10°C/min;
Inj.: 240°C. Det.: 320°C. Retention times (min): 6.32 cis- and trans-monoacetate, 7.50 trans-
diacetate 6, 7.85 cis-diacetate 14, 8.04 trans-meso-diacetate 9, 8.74 trans-triacetate 5 and 9.33
cis-triacetate 8. Enantiomeric excess of 6: BGB-174 (15 m×0.25 mm; He; 80 kPa), 100–200°C
with 2°C/min; Inj.: 210°C. Det.: 210°C. Retention times (min): 29.8 ent-6, 30.6 6.
GLC purity of 11: GLC: OV-1-OH (25 m×0.32 mm; H2; 100 kPa), 100–250°C with 10°C/min;
Inj.: 240°C. Det.: 320°C. Retention times (min): 9.1 diol 10, 10.4 monoacetate 11, 11.4 diacetate
12. Enantiomeric excess of 11: BGB-172 (15 m×0.25 mm; H2; 80 kPa), 120–160°C with 1°C/min;
Inj.: 180°C. Det.: 200°C. Retention times (min): 32.0 11, 33.2 ent-11.
Acknowledgements
We express our thanks to Patrik Stocker, Jean-Pierre Ga¨rtner and Maya Zurfluh for their
skillful technical assistance and to our colleagues from the service labs for analyzing numerous
samples, especially to Willy Walther and Sabine Ulrike Mayer for the determination of the
enantiomeric excess of our products and Michael Hennig for establishing an X-ray structure.
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