Synthesis of the pleuromutilin antibiotic SB-268091: A new practical and efficient synthesis of quinuclidine-4-thiol

Article abstract of DOI:10.1021/op300263w

A synthesis of the pleuromutilin antibiotic SB-268091 is described which includes a new and improved route to the quinuclidine-4-thiol ligand. This chemistry has been run on multikilo scale and involved a reductive double debenzylation using sodium in liquid ammonia. Alternative conditions have been developed to prepare quinuclidine-4-thiol which avoid the use of sodium in liquid ammonia. The generality of this process to prepare differentially S-protected quinuclidine-4-thiols is also discussed and exemplified by the preparation of a range of analogues.

Full text of DOI:10.1021/op300263w

Article
pubs.acs.org/OPRD
Synthesis of the Pleuromutilin Antibiotic SB-268091: A New Practical
and Efficient Synthesis of Quinuclidine-4-thiol
Bernie Choudary,† Robert G. Giles,†,∥ Florence Jovic,† Norman Lewis,† Steve Moore,†
and Michael Urquhart*,†,§
†Formerly from GlaxoSmithKline, Old Powder Mills, Leigh, Tonbridge, Kent TN11 9AN, United Kingdom
S
* Supporting Information
ABSTRACT: A synthesis of the pleuromutilin antibiotic SB-268091 is described which includes a new and improved route to
the quinuclidine-4-thiol ligand. This chemistry has been run on multikilo scale and involved a reductive double debenzylation
using sodium in liquid ammonia. Alternative conditions have been developed to prepare quinuclidine-4-thiol which avoid the use
of sodium in liquid ammonia. The generality of this process to prepare differentially S-protected quinuclidine-4-thiols is also
discussed and exemplified by the preparation of a range of analogues.
of 1.5 g L−1 and this culture was likely to meet the requirements
INTRODUCTION
■
Pleuromutilin 1 is a naturally occurring diterpenoid1 antibiotic
for phases II and III.
which can be obtained from various Basidiomycetes fungi.2
The quinuclidinethiol 4 is not commercially available
although it is a literature compound6 and the synthesis of 4,
Modification of the glycolic ester side chain in 1 has been
shown to give derivatives with improved antimicrobial activity3
as its perchlorate salt, is depicted in Scheme 2.
particularly those containing an α-alkylthioester function.
Scheme 2. Literature preparation of quinuclidine-4-thiol
Tiamulin 2a contains this type of subunit and was marketed
by Sandoz for use as a veterinary antibiotic.4 More recently
GlaxoSmithKline marketed retapamulin 2b for use as a topical
antibiotic for the treatment of bacterial skin infections and this
product also has efficacy against certain Gram-positive bacteria
including MRSA.5 The alkylthioester 5 (SB-268091) was in
development for the treatment of sinusitis and otitis media.
The synthesis of 5 was semi-synthetic from the natural
product and was prepared from reaction of the methanesulfo-
nate 3 with quinuclidine-4-thiol 4 (Scheme 1). Extensive
Scheme 1. Synthesis of pleuromutilin SB-268091
The original medicinal chemistry route to the quinuclidine
ligand 4 was based on the above route, but various problems
were highlighted. The conjugate addition of methylmercaptan
did not go to completion, it was difficult to remove HCl after
the nitrile hydrolysis, and the presence of divalent sulfur leads
to a very slow hydrogenation of intermediate 7a. The hydrogen
iodide mediated demethylation was particularly hazardous
because pyrophoric phosphine gas was produced from
degradation of the hypophosphoric acid stabilizer, and this
had significant process safety implications.
development studies were undertaken so that a robust
fermentation and extraction procedure for pleuromutilin 1
was achieved which was capable of supplying sufficient material
for preproject and phase I. The result of this work showed that
pleuromutilin 1 could be fermented from the fungus Octojuga
pseudopinsitus in 3000 L batches and titres of approximately
0.74 g L−1 were obtained. Later investigations identified the
culture Clitopilus passackerianus which produced titres in excess
Received: September 20, 2012
© XXXX American Chemical Society
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The above synthesis was long, required two chromatographic
purifications, and was too hazardous for scale-up. A rapid
development was required for 5, so it was clear that any new
route to 4 should follow a similar rationale to the above
procedure. Incorporation of sulfur by conjugate addition
worked well although methyl or n-alkyl groups are seldom
used as protecting groups for thiols as they are notoriously
difficult to cleave.7 N-Benzylpiperidone 6 is cheap and readily
available, and there would be potential for the removal of both
benzyl protecting groups in the corresponding intermediate 7b
within a single step using a dissolved metal reduction i.e.
sodium or lithium in liquid ammonia.8 Conjugate addition to
an acrylate rather than an acrylonitrile would remove the need
for a hydrolysis step as well as the problems associated with it.
The conjugate addition of benzylmercaptan to ethyl cyclo-
hexylideneacetate has been reported;9 hence, we decided to
pursue this type of approach for the synthesis of quinuclidine-4-
thiol 4.
reflux, and now a 2 h reaction time was achieved. A cleaner and
more rapid reaction was achieved, however, by changing the
reaction solvent to the less coordinating dichloromethane, and
the salt 7b was isolated in 91% yield11 under these conditions.12
Reductive bis-debenzylation of 7b was achieved using sodium
(5 equiv) in liquid ammonia (30 vol) and gave quinuclidine-4-
thiol 4 in 76% yield as its hydrochloride salt (4a) after an
ammonium chloride workup. A direct crystallisation of 4a from
the quenched mixture was essential as earlier extraction
procedures had demonstrated that the product had a
propensity to dimerise to the disulfide 12 particularly when
in solution (Scheme 4).13,14
The synthesis of 4a to date involved the isolation of each
intermediate from 6 to 4a which enabled the potential for
further purification by crystallization of the intermediates 9 (as
it is HCl salt) and 10.15 Given the process from 6 to 7b was
inherently high yielding and the crystallization of 7b gave highly
pure product, it was decided to further streamline the process
by combining the first four steps. This key modification would
greatly improve processing times by removing the need for
isolation of viscous oils and intermediate crystalline solids. The
first isolated intermediate was now 7b, and the overall yield
from 6 was 57% as opposed to 63% where the intermediates
had previously been isolated. This modification included using
toluene for the extraction solvent for stages up to 10 so that
efficient azeotropic drying of all product solutions would be
achieved and simplified longer-term solvent recovery options.
The reduction of 9 was also carried out in a toluene/THF
mixture so that a commercial 3.5 M solution of LiAlH4 in
toluene could be used rather than the alternative 1 M solution
in THF. This was particularly advantageous with respect to
throughput. This process was subsequently transferred to small-
scale pilot-plant equipment where a total of 4.3 kg of the
quinuclidinium salt 7b was obtained. This material was
successfully converted to the target 4-mercaptoquinuclidin-1-
ium chloride 4a using sodium in liquid ammonia in two batches
at −50 to −40 °C at a 45 L scale to give a total of 1.1 kg of
product which averaged out at 53% for both batches.16
This new synthesis of 4 is shorter than the original Grob
procedure6 and offers significant advantages. All stages provide
clean products, and thus, chromatographic isolation/purifica-
tion of the intermediates is not required. The resulting purity of
4 derived from this route is typically high (96−99% based on
GC assay) albeit as the hydrochloride salt (4a). This new route
delivers 4a in 48% overall yield from benzylpiperidone 6 which
is significantly higher yielding than the original Grob procedure
which delivered 4 as its perchlorate salt in 20% overall yield
from 6.
Reaction of piperidone 6 with triethyl phosphonoacetate
gave the ester 8 in 97% yield, and reaction with
benzylmercaptan following the conditions of Huffman and
Yim9 gave the conjugate adduct 9 in 84% yield (Scheme 3).
Scheme 3. Conjugate addition of benzyl mercaptan to ethyl
2-(1-benzylpiperidin-4-ylidene)acetate
Safety assessment of the conjugate addition chemistry
showed that there was an uncontrollable gas evolution which
was not seen as an issue on small scale but could cause
problems on further scale-up. The published procedure
describes the addition of benzylmercaptan to sodium hydride
(0.1 equiv) in toluene followed by the addition of a solution of
8 in toluene and DMF. No effervescence was observed during
the mercaptan addition, but vigorous effervescence ensued at
the beginning of the addition of the solution of 8 after a short
induction period. Simply adding the DMF solvent to the
benzylmercaptan resulted in an addition rate-controlled gas
evolution and a far safer process.10
Reduction of ester 9 using lithium aluminium hydride
proceeded smoothly to give the alcohol 10 in 90% yield.
Methanesulfonylation of 10 in THF gave a very good recovery
of the salt 7b after an extended reaction (6 days) at room
temperature. A much more rapid conversion to 7b was
achieved if the intermediate solution of 11 was heated to
The next stages of the synthesis required the preparation of
the methanesulfonate 3 from pleuromutilin 1 followed by
formation of the thioether 5.
Scheme 4. Preparation of 4-mercaptoquinuclidin-1-ium chloride
B
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Scheme 5. Formation of ring-contracted by-product 14
Scheme 6. Alternative debenzylation conditions for the conversion of 11 to 4
The medicinal chemistry method for preparing SB-268091
was inefficient in two ways. It produced significant levels of by-
products so that chromatography was required for product
isolation. Also, crystallisation of the final product was
inefficient, requiring 60 vol of solvent, seeding and 5 days to
reach completion. This was attributed to the low purity of
product derived from this processing.
thus avoiding the formation of 15. Removal of solvent followed
by IPA crystallisation of the residue gave 5 with an
unacceptably high level of residual IPA (4−6% w/w). An
aqueous acid/base rework was carried out which gave 5 with
undetectable levels of IPA and a purity of >98%. This method
was used to prepare 175 g of 5 which satisfied the first supplies
requirement.
The minimisation of impurities during these stages was
considered vital to facilitate the final crystallisation of the drug
substance.
Further development showed that methanol and sodium
hydroxide could be used as solvent and base, respectively, for
the coupling of 3 with 4, and as a result, dilution with water
resulted in the direct crystallisation of 5 in 91% yield (96%
purity). It was further demonstrated that an aqueous methanol
recrystallisation of 5 increased the product purity to 98% with
no residual solvent issues. These crystallization conditions
removed the need for an acid/base rework, and this chemistry
was run on the pilot plant where a total of 1.8 kg of 5 was
prepared.20
The liquid ammonia was distilled off into sulfuric acid for this
small-scale pilot-plant campaign to prepare 4-mercaptoquinu-
clidinium chloride 4a, but the longer-term use of sodium in
liquid ammonia for the conversion of 7b to 4a could have
proved problematic. Clearly there is potential for recycling the
ammonia used for this step, but this would be difficult to
engineer, and because of the low site emission limits for
ammonia (10 kg per annum), it was prudent to evaluate
alternative methods to debenzylate 7b.
Selective N-debenzylation was achieved in 78% yield using
thiophenolate conditions21 to give 16. Apart from dissolved
metal reductions (i.e. sodium in liquid ammonia or alcohols),
there are relatively few methods available for the debenzylation
of thioethers. Alternative methods include HF,22 aluminium
trichloride,23 and HBr,24 but HF was discounted because of
potential handling issues with this reagent on larger scale.
Removal of the S-benzyl function was achieved using
Strict control on stoichiometry of methanesulfonyl chloride
(MsCl) as well as addition temperature was required for the
conversion of 1 to 3. Exhaustive methanesulfonylation
ultimately results in the formation of the ring-contracted
product 14 possibly via the mechanism depicted in Scheme 5.
This type of Wagner−Meerwein rearrangement/dehydration
with pleuromutilins is known.17 Simply conducting this
reaction under cold conditions (<−15 °C) with 1.16 equiv of
MsCl resulted in a much cleaner reaction18 and 3 could be
crystallised directly by concentration of the mixture followed by
diluting with hexane.
The alkylthioester 5 is generated from the reaction between
the sodium salt of 4 (generated from sodium methoxide in
ethanol) and 3. There was mass spectroscopic evidence that
performing the coupling of 3 with a deficiency of 4a led to the
formation of by-products derived from the reaction of 5 with 3.
These impurities were completely removed when the reaction
was conducted with a slight excess of 4a.19 Residual 4a was
readily removed during the aqueous workup of 5 as it is
completely water-soluble. The initial workup method involved
an aqueous quench and extraction into DCM, although the
product solution had to be kept cold to limit the quaternisation
of 5 to give 15. Later studies demonstrated that ethyl acetate,
toluene, or TBME could all be used as extraction solvents for 5,
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Figure 1. Preparation of differentially protected quinuclidine-4-thiols.
aluminium trichloride, but the initial yield of 4a was only 33%.
Thus, this chemistry was not further optimised. A superior
method involved heating 16 with hydrobromic acid under Dean
and Stark conditions25 such that the benzyl bromide that
formed during the reaction was effectively removed.26 In this
way, the hydrobromide 4b was formed in 78% yield and was of
purity comparable to that of the corresponding hydrochloride
salt 4a produced from the dissolved metal conditions (Scheme
6).27
To gauge the generality of the above approach to
quinuclidine-4-thiol, it was decided to explore a range of
alternatively protected thiols within this chemistry. With this in
mind, the unsaturated ester 8 was reacted with cyclo-
hexanethiol, thiophenol,28 4-methylbenzylmercaptan, 4-me-
thoxybenzylmercaptan, and 2,4,6-trimethylbenzylmercaptan;
the corresponding conjugate adducts 17 were prepared in
good to excellent yield. Reduction to the alcohol 18 also
proceeded smoothly as did the two-step one-pot methane-
sulfonylation/cyclisation chemistry to give the quinuclidinium
salts 19. Finally, selective deprotection of the N-benzyl function
using basic thiophenol conditions led to the corresponding
protected quinuclidine-4-thioethers 20 (Figure 1). The above
sequence of reactions certainly demonstrates the generality of
this method to prepare differentially protected quinuclidine-4-
thiols, and it is anticipated that subsequent deprotection to
quinuclidine-4-thiol 4 would be readily achieved either using
the methods already described above for 16 or using
established literature methods for the corresponding thio-
ethers.7
synthesis of this compound has been developed which is both
efficient and safe. Sodium in ammonia was used to remove two
benzyl protecting groups in a single step, and this was
demonstrated on 45-L scale. An alternative two-step protocol
was also developed using thiophenol for N-debenzylation and
HBr for S-debenzylation which does not require a dissolved
metal reduction. The generality of this approach was further
demonstrated by preparing a range of differentially protected 4-
quinuclidinethiols 20. A useful synthesis of the pleuromutilin
methanesulfonate 3 is discussed together with the development
of its reaction with the quinuclidine-4-thiol 4 to deliver the
antibiotic 5. The control of pleuromutilin by-products is also
described which obviated column chromatography, and the
resulting chemistry was successfully run on the pilot plant and
delivered 1.8 kg of 5.
EXPERIMENTAL SECTION
■
General Procedures. Reactions requiring anhydrous
conditions were performed using oven-dried glassware and
conducted under nitrogen. Anhydrous solvents were purchased
from Fisher Scientific and were used without further
purification. Melting points were determined on a Buchi 510
apparatus and are uncorrected. IR spectra were recorded on a
Nicolet 710 FT-IR spectrometer. NMR spectra were recorded
on a JEOL GSX400 instrument operating at 400 MHz for
proton NMR and 100 MHz for carbon NMR and were
performed in DMSO-d6 solutions using tetramethylsilane as the
internal reference except where indicated otherwise. Mass
spectra were recorded on a Micromess Platform II, Sciex API
III and a VG Trio-2. High resolution mass spectra were
recorded on a 70-VSEQ instrument. Flash chromatography was
performed using Fisher Matrix silica gel (60, 40−63 μm)
Conclusions. A synthesis of the pleuromutilin derived
antibiotic 5 has been developed. This synthesis used
quinuclidine-4-thiol 4 as a pharmaceutical ligand, and a new
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according to the published procedure.29 TLC was performed
13.5 h and quenched by the dropwise addition of water (250
mL); the organic phase was separated and the aqueous
extracted with toluene (2 × 250 mL). The organic phases
were combined and dried via repeated azeotropic vacuum
distillation, and the solvent was removed under vacuum to give
9 as a yellow/orange oil (97.7 g, 100%).
on glass backed plates precoated with silica (0.2 mm, 60F254
)
and developed using standard visualising agents: UV
fluorescence (254 and 366 nm), potassium permanganate and
iodine.
Ethyl 1-Benzylpiperidin-4-ylideneacetate, 8. Potassium tert-
butoxide (124.5 g, 1.11 mol) was added portionwise to a
solution of triethyl phosphonoacetate (248.9 g, 1.11 mol) in
DMF (600 mL) at such a rate that the internal temperature did
not exceed 50 °C. The mixture was stirred at 40−50 °C for 1 h
and heated to 50 °C; a solution of N-benzylpiperidone (200 g,
1.06 mol) in DMF (400 mL) was added dropwise. The mixture
was stirred for 10 min at 60 °C, cooled to 20 °C over 1 h,
quenched by the dropwise addition of water (1.5 L), and
extracted with ethyl acetate (2 × 700 mL). The combined
extracts were washed with water (2 × 1 L) and dried (Na2SO4),
and the solvent was removed under vacuum to afford the title
compound as an orange oil (264.7 g, 97%). Rf 0.43 (1:1
EtOAc/lt. petrol 40:60); IR (Nujol) 3062, 3027, 2979, 2942,
2903, 2800, 1713, 1652, 1601, 1494, 1467, 1454, 1395, 1380,
1364, 1343, 1308, 1291, 1268, 1253, 1197, 1169, 1152, 1128,
1096, 1071, 1041, 991, 909, 865, 817, 783, 740, 699, 605, 540,
458; 1H NMR (400 MHz, CDCl3) δH 1.26 (3H, t, J = 7.1), 2.32
(2H, t, J = 5.5), 2.52 (4H, t, J = 5.7), 2.99 (2H, t, J = 5.5), 3.52
(2H, s), 4.15 (2H, q, J = 7.1), 5.63 (1H, s), 7.25 (1H, m), 7.30
(5H, m); 13C NMR (100 MHz, CDCl3) δC 14.70, 29.83, 37.19,
54.49, 54.92, 59.99, 62.98, 114.44, 127.49, 128.64, 129.47,
138.68, 159.94, 166.98; MS (EI, 70 eV), m/z 260 (M + H),
232, 214, 168, 153, 125, 120, 91.
2-(1-Benzyl-4-benzylthiopiperidin-4-yl)ethanol, 10. A sol-
ution of ester 9 (320.1 g, 0.835 mol) in THF (600 mL) was
added dropwise to a stirred solution of lithium aluminium
hydride (31.7 g, 0.835 mol) in THF (1.3 L) at 25 °C under
nitrogen maintaining an addition temperature of below 35 °C.
The mixture was stirred at 25 °C for 1 h then very carefully
quenched by the dropwise addition of water (30 mL) over 1 h.
The mixture was sequentially treated with 1 M NaOH (aq)
(150 mL) and water (150 mL), stirred for 1.5 h then diluted
with ethyl acetate (2L). The mixture was filtered through
Celite, the residue was washed with ethyl acetate (500 mL), the
filtrate and washings were combined, washed with brine (1 L)
and dried (Na2SO4). The solvent was removed under vacuum
to give the title compound 10 as a white solid which was dried
(256.4 g, 90%). An analytical sample of 10 was prepared by
dissolving 10 (6.0 g, 17.6 mmol) in methanol (30 mL), diluting
with water (25 mL) dropwise and stirring for 1 h. Filtration
gave 10 as white crystals (4.69 g, 78%), mp 85−86 °C. Rf 0.03
(1:1 EtOAc/lt. petrol 40:60); IR (Nujol) 3215, 3057, 3025,
2924, 2810, 2766, 1493, 1464, 1454, 1377, 1367, 1342, 1325,
1262, 1150, 1115, 1096, 1079, 1067, 1035, 1006, 782, 743, 715,
696; 1H NMR (400 MHz, CDCl3) δH 1.72 (2H, m), 1.82 (2H,
m), 1.90 (2H, t, J = 6.3), 2.57 (4H, m), 3.53 (2H, s), 3.71 (2H,
s), 3.87 (2H, t, J = 6.3), 7.27 (10H, m); 13C NMR (100 MHz,
CDCl3) δC 31.74, 35.94, 47.76, 63.13, 126.97, 127.07, 128.16,
128.19, 128.56, 129.02, 129.07, 137.59, 138.48; MS (EI, 70 eV),
m/z 342 (M + H), 218, 91. Found: C, 73.88; H, 8.18; N, 4.06;
S, 9.45%. C21H27NOS requires C, 73.86; H, 7.97; N, 4.10, S,
9.39.
Ethyl 1-Benzyl-4-benzylthiopiperidin-4-ylacetate, 9. A
solution of benzyl mercaptan (138 mL, 1.173 mol) in toluene
(500 mL) and DMF (60 mL) was slowly added to a suspension
of sodium hydride (60% oil suspension, 8.16 g, 0.204 mol) in
toluene (1 L), and the resulting mixture was stirred at ∼25 °C
for 30 min. A solution of 8 (264.7, 1.02 mol) in toluene (1 L)
was added, and the reaction mixture was stirred at 25−30 °C
for 20 h. The reaction mixture was quenched with water (1 L),
and the organic layer was separated. The aqueous layer was
washed with toluene (500 mL). The combined toluene extracts
were further washed with water (2 × 1 L) and dried via
repeated azeotropic vacuum distillation, and the solvent was
removed under vacuum to give the title compound as a yellow/
orange oil (330 g, 84%). Rf 0.22 (1:1 EtOAc/lt. petrol 40:60);
IR (Nujol) 3084, 3061, 3027, 2978, 2935, 2811, 2768, 1731,
1652, 1601, 1494, 1464, 1453, 1393, 1367, 1332, 1316, 1251,
1229, 1196, 1182, 1153, 1103, 1071, 1034, 976, 913, 865, 780,
738, 714, 698, 579, 463; 1H NMR (400 MHz, CDCl3) δH 1.28
(3H, t, J = 7.2), 1.89 (4H, m), 2.58 (4H, m), 2.65 (2H, s), 3.53
1-Benzyl-4-benzylthioquinuclidinium Chloride, {1-Azonia-
1-(phenylmethyl)-bicyclo[2.2.2]octane chloride}, 7b. A sol-
ution of methanesulfonyl chloride (61.4 mL, 0.792 mol) in
DCM (400 mL) was added dropwise to a stirred solution of
alcohol 10 (245.54 g, 0.720 mol) and N,N-diisopropyl-N-
ethylamine (138 mL, 0.792 mol) in DCM (1.6 L) at 15 °C
under nitrogen such that a steady reflux was achieved. The
resulting solution was heated under reflux for 1.5 h, cooled to
35 °C and treated with brine (1 L) and water (500 mL). The
biphasic mixture was stirred for 10 min, allowed to stand at 20
°C for 14 h; then the aqueous was removed and washed with
DCM (500 mL). The combined organic phases were washed
with brine (1 L) and dried (Na2SO4), and the solvent was
removed under vacuum to give a cream-colored solid. The solid
was diluted with THF (1 L), heated to reflux with stirring,
stirred for 30 min, and then cooled to ambient. Filtration under
vacuum gave 7b as a cream-colored solid which was washed
with THF (2 × 100 mL) and dried (248.45 g, 91%);30 mp 136
to 137 °C. Rf 0.00 (EtOAc); IR (Nujol) 3405, 3028, 2928,
2854, 1601, 1496, 1461, 1378, 1343, 1254, 1216, 1076, 1045,
(2H, s), 3.71 (2H, s), 4.17 (2H, q, J = 7.1), 7.27 (10H, m); 13
C
NMR (100 MHz, CDCl3) δC 14.68, 32.30, 36.11, 47.23, 49.47,
49.69, 60.81, 63.36, 127.37, 128.59, 128.87, 129.51, 138.05,
138.86, 170.68; MS (EI, 70 eV), m/z 384 (M + H), 356, 338,
260, 232, 170, 168, 120, 91. HCl salt: white powder from
EtOAc, mp 150−154 °C. Found: C, 65.74; H, 7.39; N, 3.32; S,
7.69; Cl, 8.30. C23H29NO2S·HCl requires C, 65.77; H, 7.2; N,
3.33, S, 7.63, Cl, 8.44.
Alternative Synthesis of Ethyl 1-Benzyl-4-benzylthiopiper-
idin-4-ylacetate, 9. Benzyl mercaptan (35 mL, 292.5 mmol)
was added dropwise to a stirred suspension of powdered
sodium hydroxide (2.0 g, 51 mmol) in DMF (14 mL) and
toluene (270 mL). This mixture was stirred for 30 min then a
solution of 8 (66.2 g, 255 mmol) in toluene (270 mL) was
added dropwise. The resulting solution was stirred at 20 °C for
1
1029, 1007, 931, 843, 783, 775, 713, 697, 634, 486, 470; H
NMR (400 MHz, CDCl3) δH 2.11 (6H, t, J = 7.7), 3.72 (2H, s),
3.89 (6H, t, J = 7.8), 5.05 (2H,s), 7.22 (2H, m), 7.28 (3H, m),
7.39 (3H, m), 7.65 (2H, m); 13C NMR (100 MHz, CDCl3) δC
30.74, 32.80, 38.50, 54.72, 67.02, 127.61, 127.71, 129.05,
129.18, 129.52, 130.87, 133.75, 137.48; MS (EI, 70 eV), m/z
323 (M +), 233, 91. Found: C, 66.65; H, 3.70; N, 7.65; S,
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8.62%. C21H26NS.Cl.H2O requires C, 66.57; H, 7.97; N, 3.69, S,
8.46%.
solution. The product was filtered, washed with THF (250
mL), and dried at 50 °C in vacuo (53.9 g, 57% from 6).
Pilot-Scale Preparation of 1-Benzyl-4-benzylthioquinucli-
dinium Chloride, {1-Azonia-1-(phenylmethyl)-bicyclo[2.2.2]-
octane chloride}, 7b. Potassium tert-butoxide (3.1 kg, 27.6
mol) was cautiously charged in two lots with intermediate
cooling to a solution of triethyl phosphonoacetate (6.3 kg, 28.1
mol) in DMF (14.2 kg), keeping the temperature below 35 °C.
The reaction mixture was warmed to 48 °C and stirred for 30
min. A solution of N-benzyl-4-piperidone 6 (5.0 kg, 26.4 mol)
in DMF (7.6 kg) was charged to the reaction mixture over 25
min, keeping the temperature of the reaction mixture below 70
°C. The transfer line was rinsed by charging DMF (2.8 kg) to
the reaction mixture. The reaction mixture was stirred at 70 °C
for 1 h. TLC and HPLC indicated that the reaction was
complete. The reaction mixture was cooled to 24 °C.
Demineralised water (50.0 L) was added. The mixture was
extracted with toluene (15.6 kg), and the aqueous layer was
extracted twice with toluene (2 × 15.6 kg). The organic layers
were combined and washed twice with demineralised water (2
× 25.0 L). The organic product solution was dried by distilling
out 40 L of solvent, charging toluene (43.3 kg), and distilling
out 40 L of solvent. Toluene (5.3 kg) was added to give a
solution of 8 in toluene (22.6 kg).
A solution of benzyl mercaptan (3.65 kg, 29.4 mol) in DMF
(1.4 kg) and toluene (4.4 kg) was added over 25 min to a
suspension of sodium hydride (0.204 kg, 5.1 mol) in toluene
(21.7 kg) at 21 to 25 °C. The transfer line was rinsed by
charging toluene (8.1 kg) to the reaction mixture. The mixture
was stirred at 20−25 °C for 15 min. The solution of 8 in
toluene from above (22.6 kg) was charged to the reaction
mixture over 25 min, maintaining the temperature below 25 °C.
The transfer line was rinsed by charging toluene (4.4 kg) to the
reaction mixture. The reaction mixture was stirred at 25−30 °C
for 9.5 h. NMR indicated that the reaction had stopped at about
85% conversion. The reaction mixture was cautiously quenched
with demineralised water (25.0 L), and the resulting phases
were separated. The aqueous phase was extracted with toluene
(10.8 kg). The organic layers were combined and washed twice
with demineralised water (2 × 25.0 L). The solution of 9 was
dried by distilling out 61 L of solvent, charging toluene (21.7
kg), distilling out 30 L of solvent, charging toluene (43.4 kg),
and finally distilling out 45 L of solvent. The solution was
cooled to 22 °C and discharged from the reactor to give 17.2 kg
of a solution of 9 in toluene. The solution was not assayed, but
used directly in the next stage of the process.
Streamlined Preparation of 1-Benzyl-4-benzylthioquinu-
clidinium chloride, {1-Azonia-1-(phenylmethyl)-
bicyclo[2.2.2]octane chloride}, 7b. Solid potassium tert-
butoxide (Assay ∼97%, 31.0 g, 0.285 mol) was added in
portions to a solution of triethyl phosphonoacetate (63.0 g,
0.281 mol) in DMF (150 mL) so that the temperature did not
exceed 50 °C. The reaction mixture was maintained at 50 °C
for 30 min after the addition was complete. A solution of N-
benzyl-4-piperidone 6 (50 g, 0.264 mol) in DMF (80 mL) was
added at a rate so that the reaction temperature did not exceed
75 °C. When complete reaction was achieved, the reaction
mixture was cooled to 25 °C, quenched with water (500 mL)
and extracted into toluene (3 × 180 mL), and the combined
organic extracts were washed with water (2 × 250 mL). The
toluene solution was dried via repeated azeotropic vacuum
distillation until a dry solution of 8 in toluene (approximately
250 mL) was obtained.
A solution of benzyl mercaptan (36.5 g, 0.293 mol) in
toluene (125 mL) and DMF (1.5 L) was slowly added to a
suspension of sodium hydride (60% oil suspension, 2.04 g, 51
mmol) in toluene (250 mL), and the resulting mixture was
stirred at ∼25 °C for 15 min. The previously prepared solution
of 8 in toluene (approximately 250 mL) was added, and the
reaction mixture was stirred at 25 to 30 °C for 20 h. The
reaction mixture was quenched with water (250 mL), and the
organic layer was separated. The aqueous layer was washed
with toluene (125 mL). The combined toluene extracts were
further washed with water (2 × 250 mL) before being dried via
repeated azeotropic vacuum distillation, until a dry solution of 9
in toluene (approximately 250 mL) was obtained.
The toluene solution of 9 was added to a 3.5 M solution of
lithium aluminium hydride in toluene/THF (75 mL, 264
mmol) in toluene (230 mL) over 1 h between 15 and 50 °C.
The resulting reaction mixture was stirred at 40 °C for 1 h. A
mixture of water (10 mL) and THF (90 mL) was carefully
added to the reaction mixture at 0 °C, followed by 1 M aqueous
sodium hydroxide (40 mL), and water (40 mL). The resulting
precipitate of aluminium salts was removed via filtration, and
the filtrate was extracted with toluene (250 + 125 mL). The
combined organic extracts were washed with water (2 × 250
mL) and dried via repeated azeotropic vacuum distillation until
a dry solution of 10 in toluene (approximately 90 mL) was
obtained.
A solution of methanesulfonyl chloride (23 mL, 0.29 mol) in
DCM (150 mL) was added to the previously prepared toluene
solution of 10 admixed with DCM (600 mL) and Hunig’s base
[N,N-diisopropylethylamine] (51 mL, 0.29 mol). The addition
rate was controlled by the rate of reflux. The resulting mixture
was kept at reflux temperature for 2 h. Saturated brine solution
(400 mL) was added to the cooled reaction mixture, and it was
stirred for 20 h at 25 °C. The organic layer was removed, and
the aqueous phase was extracted with DCM (180 mL). The
combined organic extracts were washed with saturated brine
(350 mL). The organic layer was concentrated via atmospheric
distillation to a volume of 320 mL, and THF (750 mL) was
added to the residue. Distillation was repeated until 500 mL of
distillate had been removed and further THF (750 mL) was
added. Distillation was repeated until 500 mL of distillate had
been removed, and further THF (750 mL) was added. The
resulting THF solution was cooled, and 7b crystallised out of
Toluene (19.9 kg) and a 3.5 M solution of lithium aluminum
hydride in THF/toluene (6.7 kg, 26.3 mol) were charged into
the reactor. The transfer line was rinsed by charging THF (4.5
kg) to the suspension. The suspension was cooled to 20 °C,
and the solution of 9 in toluene (17.2 kg) was added in three
lots with intermediate cooling over 45 min, maintaining the
temperature below 30 °C. Toluene (4.5 kg) was added, and the
reaction mixture was stirred at 40−45 °C for 1 h. TLC and MS
indicated that the reaction was complete. The mixture was
cooled to 12 °C. A mixture of THF (8.0 kg) and water (1.0 kg)
was added to the reaction mixture over 16 min, followed by a
slow addition of a solution of sodium hydroxide (0.16 kg) in
demineralised water (4.0 L), and demineralised water (4.0 L).
The temperature of the mixture was kept below 25 °C. The
resulting suspension was warmed to 75 °C and stirred for 30
min. Finally, the suspension was cooled to 45 °C. The attempt
to filter the suspension using a pressure plate filter was not
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successful because blocking of the filter occurred. The contents
of the filter were recharged into the reactor. Toluene (30 L)
was used to wash the lines. The suspension was heated, and 25
L of solvent was distilled out. Toluene (10 L) was charged, and
20 L of solvent were distilled out. Toluene (8.7 kg) was
charged, and the suspension was cooled to 50 °C. The
suspension was filtered using the Ellerwerk centrifuge, which
was precoated with Celite (1.0 kg). The solids in the centrifuge
were washed with toluene (32.5 kg). Demineralised water (25.0
L) was added to the filtrate, and the mixture was stirred for 10
min. After settling for 2 h, only partial separation had occurred.
The aqueous phase that had separated was drained off. The
mixture was warmed to 25−30 °C and allowed to settle for 1.5
h. The lower aqueous phase was separated off. The organic
mixture (49.4 kg) was drummed up and recharged into the
reactor via an in-line filter. The drum and transfer line were
rinsed by charging toluene (4.5 kg) to the mixture. The mixture
was dried by distilling out 40.0 L of solvent, charging toluene
(43.4 kg), distilling out 50 L of solvent, charging toluene (43.4
kg), and finally distilling out 50 L of solvent. The solution was
cooled to 25 °C, and DCM (40 kg) was added. It was
discharged from the reactor to give 49.6 kg of a solution of 10
in toluene/DCM. The solution was not assayed, but used
directly in the next stage of the process.
DMF (3.78 kg, 29.3 mol), DCM (66.2 kg), and the solution
of 10 in toluene/DCM (49.6 kg) were charged into a reactor. A
solution of methanesulfonyl chloride (3.42 kg, 29.7 mol) in
DCM (20.0 kg) was added in two lots over 30 min, maintaining
the temperature below 25 °C. The transfer line was rinsed by
transferring DCM (6.6 kg) to the reaction mixture. The
mixture was stirred for 7 h at reflux. TLC and MS indicated that
the reaction profile was satisfactory. The reaction mixture was
cooled to 23 °C and transferred to another reactor. The
transfer line was rinsed by charging DCM (6.6 kg) to the
mixture. A solution of sodium chloride (23.6 kg) in
demineralised water (70.8 kg) was added, and the mixture
was stirred for 17.5 h at 20−30 °C. The mixture was allowed to
settle for 30 min, and the lower organic layer was separated off.
The aqueous layer was washed with DCM (23.9 kg). The
organic phases were combined and extracted with a solution of
sodium chloride (11.9 kg) in demineralised water (35.7 kg).
The solvent of the organic phase, DCM, was replaced with
THF in a “put-and-take” distillation step by distilling out 95.0 L
of solvent, charging THF (66.7 kg), distilling out 50 L of
solvent, charging THF (66.7 kg), distilling out 50 L of solvent,
and finally charging THF (66.7 kg). The resulting suspension
was cooled to 30 °C and stirred for 2 h at 25−30 °C. The
product was isolated in a centrifuge and washed with THF
(22.2 kg). The wet product (4.1 kg) was dried for 73.5 h under
vacuum at 50 °C to give 2.7 kg of 7b (28.4% yield from 6).31
4-Thioquinuclidinium Chloride, {1-Azoniabicyclo[2.2.2]-
octane-4-thiol chloride}, 4a. Ammonia (220 mL, 12.94 mol)
was condensed into a cooled (−40 °C) three-neck, 500-mL
flask under nitrogen equipped with a coldfinger reflux head
(cardice/propan-2-ol) and a glass-coated magnetic follower.
The contents were kept cold by means of a cardice/propan-2-ol
bath. Sodium metal (3.1g, 0.135g atoms) was added in portions
to the stirred mixture over ∼30 min, and the resulting dark-blue
(almost black) solution was stirred for 30 min. The salt 7b
(10.0g, 27.82 mmol) was added in portions over ∼30 min via a
powder funnel to the stirred mixture. The dark mixture was
stirred at −33 °C for 2.5 h and then very carefully quenched by
the dropwise addition of methanol (200 mL).32 Solid
ammonium chloride (10.19 g, 0.19 mol) was added, the
mixture was stirred for 20 min, and then the ammonia was
allowed to distill out of the mixture.33 Once the internal
temperature had reached 25 °C, the stirred mixture was heated
to reflux and sirred at reflux for 1 h. Filtration under vacuo
removed inorganic insolubles, and the residue was washed with
methanol (20 mL). The filtrate was evaporated in vacuo to give
a white solid residue which was suspended in toluene (400 mL)
and stirred at 25 °C for 30 min. Filtration under vacuum gave a
cream-colored solid which was washed with toluene (100 mL)
and pulled dry.
The solid (9.27 g) was suspended in propan-2-ol (120 mL),
heated to reflux, and stirred for 30 min. Hot filtration under
vacuum gave a white solid and a pale-yellow filtrate from which
a colourless solid rapidly precipitated. The residue was washed
with hot propan-2-ol (20 mL) and pulled dry. The combined
filtrate and washings were cooled to −8 °C, allowed to stand
for 16 h at this temperature, and filtered under vacuum. The
residue was washed with cold propan-2-ol (20 mL) and pulled
dry to give 4a as colourless/grey crystals (wt = 3.77 g, 76%),
mp >250 °C; IR (Nujol) 3392, 2924, 2779, 2727, 2552, 2470,
1644, 1463, 1438, 1377, 1333, 1281, 1224, 1171, 1038, 1005,
1
918, 839, 770, 722, 664, 555, 536, 526, 419; H NMR (400
MHz, CDCl3) δH 2.01 (6H, t, J = 7.9), 3.25 (6H, t, J = 7.9),
3.50 (1H, s), 11.15 (1H, s); 13C NMR (100 MHz, CDCl3) δC
32.50, 35.57, 46.41; MS (EI, 70 eV), m/z 144 (M + 1), 111, 83.
Found: C, 46.83; H, 8.33; N, 7.79; S, 17.78. C7H13NS·HCl
requires C, 46.78; H, 7.85; N, 7.79, S, 17.84.
Pilot-Scale Preparation of 4-Thioquinuclidinium Chloride,
{1-Azoniabicyclo[2.2.2]octane-4-thiol chloride}, 4a. THF
(88.9 kg) was charged into the reactor and cooled to −50 °C
using the liquid nitrogen heat exchanger. The cold contents of
the reactor were discharged. Sodium sticks (0.62 kg, 27.0 mol)
were charged into the reactor via the charge-hole. Liquid
ammonia (35.2 kg) was added, which resulted in a total volume
of the contents of the reactor of 45.0 L. The reactor was cooled
to −44 °C, and 7b (2.0 kg, 5.6 mol) was charged in five lots at
−45 to −40 °C. The reaction mixture was stirred for 1 h
between −45 and −40 °C. Methanol (4.0 kg) was cautiously
charged over 20 min, allowing the temperature to rise to −32
°C. Methanol (27.7 kg) was added over 30 min. The
temperature had risen to −18 °C. Ammonium chloride (2.04
kg, 38.1 mol) was added in four lots. The reaction mixture was
warmed gradually from −18 to 55 °C over 3.5 h using
consecutively cooling water, hot water, and steam. During the
heat-up, 2.0 L of distillate was collected. The reaction mixture
was stirred for 1 h at 55 to 60 °C and then cooled to 29 °C.
The reaction mixture was filtered via a pressure plate filter,
which was precoated with Celite (2.0 kg). The filter system was
washed with methanol (5.4 kg). The filtrate (37.2 kg) was
subjected to a “put-and-take” distillation by distilling out 32 L
of solvent, charging 2-propanol (31.4 kg), distilling out 35 L of
solvent, charging 2-propanol (31.4 kg) and finally distilling out
35 L of solvent. The hot product mixture was transferred to
another reactor via a filter system, which was rinsed with hot 2-
propanol (3.9 kg). The product solution was cooled to −3 °C
and stirred for 2 h between −5 and 0 °C. The product was
isolated using the Rosenmund pressure plate filter and washed
with cold 2-propanol (3.9 kg). The wet product (1.2 kg) was
dried under vacuum at 50 °C for 60 h to give 0.552 kg of 4a
(0.520 kg at 100%, 52.0% of theory from 7b) (weight-based
purity calculated as 94.1% pure).16
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4-Quinuclidinium Disulfide Dichloride, {4-(1-
Azoniabicyclo[2.2.2]octane)disulfide chloride}, 12. Sulfuryl
chloride (0.45 mL, 5.56 mmol) was added dropwise to a stirred
suspension of 4a (2.0 g, 11.13 mmol) in pyridine (20 mL) at 25
°C such that the internal temperature did not exceed 53 °C.
The suspension was stirred at room temperature for 24 h, and
the solvent was removed under vacuum to afford a cream-
colored solid. This solid was suspended in propan-2-ol (40 mL)
and filtered under vacuum. The residue was washed with
propan-2-ol (2 × 10 mL) to give the title compound 12 as a
cream-colored solid (wt = 1.97 g, 99%). Recrystallisation from
methanol and propan-2-ol gave 12 as cream-colored crystals; IR
(thin film) 3390, 2924, 2853, 2780, 2727, 2698, 2624, 2573,
2471, 2351, 2089, 1643, 1492, 1465, 1436, 1417, 1377, 1334,
1305, 1281, 1253, 1169, 1162, 1046, 1027, 1008, 1004, 997,
contents of the reactor were stirred for 30 min and allowed to
settle for 15 min. The lower organic layer was separated off, and
DCM (8.1 kg) was added to the aqueous layer. After the
mixture stirred for 5 min and settled for 15 min, the organic
phase was separated. A large amount of interface was observed,
which was drummed up and returned into the reactor via an in-
line filter. DCM (8.1 kg) was charged via the in-line filter into
the reactor, and the mixture was warmed to 23 °C. The mixture
was stirred for 5 min and allowed to settle for 15 min. The
organic layer was separated. The combined organic phases were
heated, and 25 L of solvent was distilled off. Whilst continuing
the distillation, n-hexane (22.1 kg) was slowly added over 1.5 h
until the temperature of the mixture had risen to 46.3 °C and a
further 10.7 L of distillate had been collected. Seed crystals of 3
(0.01 kg, 0.022 mol) were added during the hexane addition
when the mixture appeared to have become cloudy. After the
addition was complete, a sample was taken which indicated that
the product had oiled out. The contents of the reactor were
cooled to 34 °C, and seed crystals of 3 (0.01 kg, 0.022 mol)
were added. The mixture was stirred for 40 min between 34
and 31 °C after which the product had crystallised. The mixture
was cooled to 1 °C and stirred for 2 h between 0 and 5 °C. The
product was isolated using the Rosenmund pressure plate filter
and washed twice with n-hexane (8.0 and 3.2 kg). The wet
product (2.6 kg) was dried under vacuum at room temperature
for 13.5 h to give 2.1 kg of 3 (2.05 kg at 100%, 60% of theory
from 1) (weight-based purity calculated as 97.9% pure).
14-Deoxy-14-(quinuclidine-4-thioacetoxy)mutilin,
{(3aS,4R,5S,6S,8R,9R,9aR,10R)-6-Ethenyldecahydro-5-hy-
droxy-4,6,9,10-tetramethyl-1-oxo-3a,9-propano-3aH-cyclo-
pentacycloocten-8-yl-[1-azoniabicyclo[2.2.2]octane-4-thio]-
acetate}, 5. Sodium hydroxide (1.54 g, 38.5 mmol) was added
in portions to a stirred solution of 4a (3.16 g, 17.59 mmol) in
methanol (60 mL) at 15 °C under nitrogen, maintaining an
internal temperature of below 20 °C. The mixture was stirred at
20 °C for 1 h, cooled to 15 °C, and 3 (6.99 g, 15.31 mmol) was
added in portions so that the internal temperature did not
exceed 20 °C. The mixture was stirred at 20 °C for 1.5 h and
cooled to 2 °C; water (150 mL) was added, maintaining an
internal temperature below 5 °C. After stirring at 5 °C for 1 h,
the product was collected by filtration, washed with water (100
mL), and dried to give 5 as a white solid (wt = 7.21 g, 94%). An
analytical sample of 5 was prepared by dissolving 5 (1.0 g, 1.99
mmol) in methanol (5 mL) at 60 °C and diluting with water (5
mL) dropwise such that the internal temperature was
maintained above 60 °C. The mixture was cooled to 5 °C
over 2 h and stirred at 5 °C for 1 h. The product 5 was
collected by filtration, washed with water (5 mL) and dried
(0.93 g, 93%). This material had spectroscopic properties
identical to those reported.34
1
917, 839, 791, 721, 659, 532, 471, 426, 419; H NMR (400
MHz, CD3OD) δH 2.18 (6H, t, J = 7.9), 3.50 (6H, t, J = 7.9);
13C NMR (100 MHz, CD3OD) δC 30.57, 42.80, 48.91; MS
(FIA/ms/ms, 70 eV), m/z 285 (M + H), 176, 144, 110, 96, 82,
68.
Pleuromutilin Methanesulfonate, {(3aS,4R,5S,6S,8R,9-
R,9aR,10R)-6-Ethenyldeca hydro-5-hydroxy-4,6,9,10-tetra-
methyl-1-oxo-3a,9-propano-3aH-cyclopentacyclo-octen-8-yl
Methanesulfonyloxyacetate}, 3. Triethylamine (2.19 mL,
15.71 mmol) was added to a stirred solution of pleuromutilin
1 (5.31 g, 14.03 mmol) in DCM (54 mL) at 15 °C under
nitrogen. The mixture was cooled to 15 °C and treated
dropwise with a solution of methanesulfonyl chloride (1.27 mL,
16.40 mmol) in DCM (3 mL), maintaining an addition
temperature of below −15 °C. The mixture was stirred below
−15 °C for 1 h and warmed to 5 °C; water (50 mL) was added,
and the mixture was stirred for 30 min. The organic phase was
removed and the aqueous extracted with DCM (2 × 20 mL);
the combined organic extracts were washed with brine (50
mL), and a quantity of DCM (60 mL) was removed by
distillation. n-Hexane (66 mL) was added to the mixture slowly
(over 30 min) whilst the distillation was continued, and a
further 80 mL of distillate was collected. The white suspension
was cooled to 5 °C and stirred for 1 h, and the solid was
collected by filtration, washed with n-hexane (2 × 12 mL), and
dried to give the title compound 3 as a white solid (wt = 5.10 g,
1
80%). H NMR (400 MHz, CDCl3) δH 0.73 (3H, d, J = 6.8),
0.90 (3H, d, J = 6.8), 1.23 (4H, m), 1.47 (7H, m), 1.72 (5H,
m), 2.22 (5H, m), 3.19 (3H, s), 3.37 (1H, d, J = 6.4), 4.65 (2H,
s), 5.22 (1H, d, J = 17.6), 5.35 (1H, d, J = 10.8), 5.84 (1H, d, J
= 8.4), 6.45 (1H, dd, J = 17.6, 11.2); 13C NMR (100 MHz,
CDCl3) δC 11.54, 14.74, 16.61, 24.83, 26.44, 26.80, 30.34,
34.40, 36.07, 36.53, 39.17, 41,84, 44.02, 44.62, 45.41, 58.02,
65.12, 70.54. 74.57, 77.23, 117.53, 138.65, 165.76, 216.74; MS
(FIA-MS/MS, 70 eV), m/z 474 (M + NH4), 303, 285, 245.
Pilot-Scale Preparation of Pleuromutilin Methanesulfo-
nate, {(3aS,4R,5S,6S,8R,9R,9aR,10R)-6-Ethenyldecahydro-5-
hydroxy-4,6,9,10-tetramethyl-1-oxo-3a,9-propano-3aH-cy-
clopentacyclo-octen-8-yl methanesulfonyloxyacetate}, 3.
DCM (39.2 kg), pleuromutilin (1, 2.69 kg, 7.1 mol), and
triethylamine (0.81 kg, 8.0 mol) were charged into the reactor,
and the mixture was cooled to −17 °C. A solution of
methanesulfonyl chloride (0.95 kg, 8.3 mol) in DCM (5.0 kg)
was added over 1 h, maintaining the temperature below −15
°C. The transfer line was rinsed by charging DCM (3.0 kg) to
the reaction mixture. The mixture was stirred for 25 min
between −15 and −20 °C. The reaction mixture was heated to
13 °C, and demineralised water (24.0 L) was added. The
Pilot-Scale Preparation of 14-Deoxy-14-(quinuclidine-4-
thioacetoxy)mutilin, {(3aS,4R,5S,6S,8R,9R,9aR,10R)-6-Ethe-
nyldecahydro-5-hydroxy-4,6,9,10-tetramethyl-1-oxo-3a,9-
p r o p a n o - 3 a H - c y c l o p e n t a c y c l o o c t e n - 8 - y l [ 1 -
azoniabicyclo[2.2.2]octane-4-thio]acetate}, 5. Methanol
(6.79 kg) and 4a (0.472 kg, 2.63 mol) were charged into the
reactor, and the mixture was cooled to 13 °C. Sodium
hydroxide (0.221 kg, 5.53 mol) was added over 5 min,
maintaining the temperature below 20 °C. The mixture was
stirred for 1 h between 18 and 22 °C and cooled to 14 °C.
Methanesulfonate 3 (1.00 kg, 2.19 mol) was added over 10
min, and the reaction mixture was stirred for 2 h between 18
and 22 °C. HPLC indicated that the reaction had gone to
H
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completion. The mixture was cooled to 3 °C, and
demineralised water (21.5 L) was added in 1.0 L portions,
maintaining the temperature below 5 °C. The suspension was
stirred for 2 h between 4 and 6 °C. The product was isolated
using the Rosenmund pressure plate filter and washed with
demineralised water (15.0 L). The wet product (1.64 kg) was
dried under vacuum at 50 °C for 22 h to give 1.03 kg of crude 5
(1.01 kg at 100%, 91.6% of theory from 3).
Methanol (7.9 kg) and crude 5 (2.08 kg, 4.1 mol) were
charged into the reactor. The solution was heated to 60 °C and
stirred for 10 min between 60 to 65 °C. The solution was
discharged into a preheated 20-L container, and the reactor was
cleaned by boiling out with methanol (40.0 kg). The product
solution was charged back into the reactor via an in-line filter.
Methanol (3.4 kg) was heated to 50 °C and charged to the
reaction mixture via the in-line filter. The reaction mixture was
heated, and 3.0 L of solvent was distilled off. Demineralised
water (10.5 L) was slowly added, maintaining the temperature
of the reaction mixture at 60 to 65 °C. The suspension was
gradually cooled over 2 h to 0−5 °C. The suspension was
stirred for 2 h at 0 to 5 °C. The product was isolated using a
Rosenmund pressure plate filter and washed with demineralised
water (10.0 L). The wet product (2.4 kg) was dried under
vacuum at 50 °C for 109 h to give 1.8 kg of 5 (1.76 kg at 100%,
84.8% of theory from crude 5) (weight-based purity calculated
as 98.0% pure).
stirred for 1h. The reaction mixture was cooled to 0 °C and
quenched by the dropwise addition of methanol (50 mL)
followed by water (100 mL). The mixture was washed with
ethyl acetate (100 mL), and the organic phase was washed with
water (100 mL). The combined aqueous phases were washed
with ethyl acetate (25 mL), and the water was removed by
evaporation to give a white solid. Recrystallisation from propan-
2-ol (40 mL) with a hot filtration gave 4a as white crystals (wt
= 0.4 g, 33%). This material had physical and spectroscopic
properties identical to those described above.
Conversion of 4-Benzylthioquinuclidine, {1-Aza-4-
benzylthiobicyclo[2.2.2]octane}, 16 to 4-Thioquinuclidinium
Bromide, {1-Azoniabicyclo[2.2.2]octane-4-thiol bromide}, 4b
Using Aqueous HBr. A suspension of 16 (2.0 g, 8.57 mmol) in
48% hydrobromic acid in water (30 mL) was heated under
reflux under Dean and Stark conditions for 19 h such that the
benzyl bromide formed was removed from the mixture. The
mixture was allowed to cool to ambient and the hydrobromic
acid removed by evaporation. The resulting gum was diluted
with toluene (30 mL), this was removed under vacuum (repeat
3×), and the grey solid was suspended in toluene (30 mL) and
propan-2-ol (30 mL). Filtration gave 4b as a white solid which
was washed with propan-2-ol (10 mL) and toluene (10 mL)
and dried (wt = 1.50 g, 78%); IR (Nujol) 2921.5, 2853.8,
2769.8, 2719.8, 2631.9, 2578.4, 2466.9, 2341.1, 1958.2, 1491.3,
1459.6, 1423.9, 1377.3, 1343.4, 1327.1, 1306.2, 1283.2, 1253.2,
1175.1, 1164.2, 1048.0, 1032.6, 1010.2, 1004.9, 929.3, 918.7,
4-Benzylthioquinuclidine, {1-Aza-4-(phenylmethylthio)-
bicyclo[2.2.2]octane}, 16. Thiophenol (18.5 mL, 184.6
mmol) was added to a stirred suspension of 7b (32.6 g, 91.2
mmol) in 20% sodium hydroxide solution (55 mL). The
mixture was heated to 90 to 100 °C, stirred for 1 h and cooled
to 20 °C. The mixture was quenched with water (200 mL),
extracted with ethyl acetate (2 × 150 mL) and washed with
water (100 mL). The combined organic extracts were washed
with 1 M hydrochloric acid (2 × 100 mL), the acidic aqueous
was washed with ethyl acetate (100 mL) then basified with 1 M
sodium hydroxide solution (220 mL). The basic aqueous phase
was extracted with ethyl acetate (2 × 150 mL), washed with
water (100 mL) and dried (Na2SO4). The solvent was removed
under vacuum to give crude 16 as a grey solid (16.59 g, 78%).
An analytical sample of 16 (3.0 g, 12.88 mmol) was prepared by
dissolving in methanol (20 mL), stirring and adding water (25
mL) dropwise. An oil was precipitated which solidified with
continued stirring and after 30 min the solid was colected by
filtration under vacuum. The residue was washed with water (2
× 10 mL) and dried to give 16 as a grey solid (2.35 g, 78%);
mp = 94−95 °C. Rf 0.05 (1:9 MeOH: DCM); IR (thin film)
2925, 2858, 1600, 1582, 1495, 1452, 1377, 1348, 1314, 1264,
1240, 1200, 1168, 1149, 1065, 1052, 1028, 1013, 970, 917, 824,
1
875.0, 855.8, 837.8, 722.0, 662.2, 531.7; H NMR (400 MHz,
CD3OD) δH 2.22 (6H, t, J = 7.9), 3.48 (6H, t, J = 7.9); 13C
NMR (100 MHz, CD3OD) δC 33.87, 35.98, 48.90.
General Method for the Preparation of Ethyl 1-
Benzyl-4-alkylthiopiperidin-4-ylacetate, 17. The alkyl or
phenyl mercaptan (46.0 mmol) was added dropwise to a stirred
suspension of sodium hydride (60% oil suspension, 0.30 g, 15.4
mmol) in toluene (50 mL) and DMF (3 mL), and the resulting
mixture was stirred at ambient for 30 min. A solution of 8 (10.0
g, 38.0 mmol) was added dropwise, and the reaction mixture
was stirred at 25−30 °C for 20 h. The reaction mixture was
quenched with 1 M hydrochloric acid (200 mL) and the
suspension was stirred for 30 min. Filtration gave crude 17
which was washed with toluene or ethyl acetate (2 × 40 mL).
The following compounds were prepared according to this
procedure.
1-Benzyl-4-(cyclohexylthio)-4-(2-ethoxy-2-oxoethyl)-
piperidin-1-ium Chloride, 17a. Reaction of 8 with cyclo-
hexanethiol gave the title compound as a white solid (78%).
Mp 187−189 °C; IR (Nujol) 2662, 2504, 2449, 1731, 1498,
1460, 1423, 1406, 1377, 1303, 1271, 1249, 1230, 1216, 1193,
1176, 1152, 1115, 1038, 1001, 984, 952, 940, 888, 751, 743,
698, 601; 1H NMR (400 MHz, CDCl3) δH 1.25 (4H, m), 1.40
(5H, m), 1.62, (4H, m), 1.83 (2H, m), 2.05 (2H, d, J = 15.4),
2.75 (5H, m), 3.19 (3H, m), 4.14 (4H, m), 7.44 (3H, m), 7.69
(2H, m); 13C NMR (100 MHz, CDCl3) δC 14.17, 25.25, 26.17,
33.14, 35.82, 41.20, 45.40, 47.06, 47.94, 60.93, 61.01, 128.08,
129.28, 130.06, 131.33, 169.14; MS (EI, 70 eV), m/z 376 (M +
H), 260, 232, 188, 172, 141, 120, 113, 91.
1
807, 781, 715, 699, 651, 568, 486; H NMR (400 MHz,
DMSO) δH 1.64 (6H, t, J = 7.6), 2.79 (6H, t, J = 7.6), 3.76
(2H,s), 7.22 (2H, m), 7.31 (3H, m); 13C NMR (100 MHz,
DMSO) δC 31.20, 32.17, 40.71, 48.06, 126.51, 128.21, 128.85,
138.83; MS (EI, 70 eV), m/z 234 (M + H), 143, 115, 110, 91.
Found: C, 72.03; H, 8.34; N, 5.95; S, 13.77. C14H19NS requires
C, 72.05; H, 8.21; N, 6.00, S, 13.74.
Conversion of 4-Benzylthioquinuclidine, {1-Aza-4-
(phenylmethylthio)bicyclo[2.2.2]octane}, 16, to 4-Thioquinu-
clidinium Chloride, {1-Azoniabicyclo[2.2.2]octane-4-thiol
chloride}, 4a, Using AlCl3. A solution of 16 (2.0 g, 8.5
mmol) in toluene (50 mL) was added to a stirred suspension of
AlCl3 (2.28 g, 17 mmol) in toluene (50 mL) at 20 °C. The
mixture was stirred at 20 °C for 2h then heated to 50 °C and
17a was basified with 1 M sodium hydroxide, extracted into
DCM, and dried (MgSO4), and the solvent was removed under
vacuum to give ethyl 2-(1-benzyl-4-(cyclohexylthio)piperidin-4-
yl)acetate (83%) which was used directly in the next stage to
prepare 18a.
I
dx.doi.org/10.1021/op300263w | Org. Process Res. Dev. XXXX, XXX, XXX−XXX

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Article
1-Benzyl-4-(2-ethoxy-2-oxoethyl)-4-(phenylthio)piperidin-
1-ium Chloride, 17b. Reaction of 8 with thiophenol gave the
title compound as a pale-yellow solid (71%).
This product was not further characterised, but rather
basified with 1 M sodium hydroxide, extracted into DCM, and
dried (MgSO4). The solvent was removed under vacuum to
give ethyl 2-(1-benzyl-4-(phenylthio)piperidin-4-yl)acetate as a
pale-yellow oil (80%) which was used directly in the next stage
to prepare 18b.
under vacuum to give ethyl 2-(1-benzyl-4-((2,4,6-
trimethylbenzyl)thio)piperidin-4-yl)acetate (80%) which was
used directly in the next stage to prepare 18e.
General Method for the Preparation of 2-(1-Benzyl-4-
alkylthiopiperidin-4-yl)ethanols, 18. A solution of the ester
17 (73.3 mmol) in THF (100 mL) was added dropwise to a
stirred suspension of lithium aluminium hydride (4.50 g, 0.1
mmol) in THF (60 mL) under nitrogen at ambient. The
reaction mixture was quenched by the dropwise addition of 1
M sodium hydroxide (200 mL) after which ethyl acetate (120
mL) was added, and the reaction mixture was stirred at 25−30
°C for 1 h. The reaction mixture was filtered through Celite,
and the residue was washed with ethyl acetate (2 × 80 mL).
The organic phase was separated, washed with brine (100 mL),
and dried (Na2SO4), and the solvent was removed under
vacuum to give the title compound.
1-Benzyl-4-(2-ethoxy-2-oxoethyl)-4-((4-methylbenzyl)-
thio)piperidin-1-ium Chloride, 17c. Reaction of 8 with 4-
methylbenzylmercaptan gave the title compound as a white
solid (80%). Mp 167−169 °C; IR (Nujol) 3337, 2921, 2854,
2663, 2497, 1727, 1514, 1499, 1462, 1409, 1377, 1343, 1299,
1255, 1218, 1196, 1189, 1180, 1148, 1121, 1071, 1038, 1013,
986, 953, 920, 838, 827, 784, 748, 742, 700, 664, 602, 566, 518,
495, 479; 1H NMR (400 MHz, CDCl3) δH 1.27 (3H, t, J = 7.1),
2.05 (2H, d, J = 15.2), 2.32 (3H, s), 2.58 (2H, t, J = 12.8), 2.70
(2H, s), 3.11 (4H, m), 3.67 (2H, s), 4.09 (2H, s), 4.17 (2H, q, J
= 7.1), 7.12 (4H, m), 7.43 (3H, m), 7.63 (2H, m), 12.4 (1H,
bs); 13C NMR (100 MHz, CDCl3) δC 14.20, 21.05, 32.19,
32.58, 45.27, 46.44, 47.97, 60.78, 60.94, 128.07, 128.75, 129.26,
129.38, 130.08, 131.33, 133.59, 137.25, 169.14; MS (EI, 70 eV),
m/z 398 (M + H), 260, 232, 168, 141, 120, 113, 105, 91.
17c was basified with 1 M sodium hydroxide, extracted into
ethyl acetate, and dried (MgSO4), and the solvent was removed
under vacuum to give ethyl 2-(1-benzyl-4-((4-methylbenzyl)-
thio)piperidin-4-yl)acetate (85%) which was used directly in
the next stage to prepare 18c.
1-Benzyl-4-(2-ethoxy-2-oxoethyl)-4-((4-methoxybenzyl)-
thio)-1-methylpiperidin-1-ium Chloride, 17d. Reaction of 8
with 4-methoxybenzylmercaptan gave the title compound as a
white solid (83%). Mp 105−127 °C; IR (Nujol) 3328, 2924,
2853, 2549, 1726, 1631, 1609, 1584, 1511, 1496, 1462, 1376,
1341, 1318, 1301, 1243, 1218, 1195, 1184, 1176, 1145, 1112,
1095, 1032, 1006, 972, 944, 931, 866, 842, 780, 759, 742, 706,
699, 675, 603, 569, 529, 513; 1H NMR (400 MHz, CDCl3) δH
1.26 (3H, t, J = 7.2), 2.03 (2H, d, J = 15.0), 2.59 (2H, m), 2.70
(2H, s), 3.13 (4H, m), 3.67 (2H, s), 3.78 (3H, s), 4.08 (2H, s),
4.17 (2H, q, J = 7.2), 6.82 (2H, m), 7.21 (2H, m), 7.43 (3H,
m), 7.63 (2H, m); 13C NMR (100 MHz, CDCl3) δC 14.20,
31.37, 32.58, 45.23, 46.44, 47.96, 55.26, 60.79, 60.94, 114.1,
128.07, 128.47, 129.27, 129.97, 130.08, 131.32, 158.94, 169.16;
MS (EI, 70 eV), m/z 414 (M + H), 260, 232, 211, 188, 168,
141, 121, 113, 91, 30. Found: C, 62.07; H, 7.29; N, 3.03.
C24H32NO3SCl requires C, 62.02; H, 7.24; N, 3.01.
1-Benzyl-4-(2-ethoxy-2- oxoethyl )-4 -((2, 4, 6 -
trimethylbenzyl)thio)piperidin-1-ium Chloride, 17e. Reaction
of 8 with 2,4,6-trimethylbenzylmercaptan36 gave the title
compound as a white solid (88%). Mp 186−188 °C; IR
(Nujol) 3450, 2919, 2853, 2644, 2315, 1727, 1610, 1584, 1511,
1457, 1377, 1346, 1305, 1279, 1238, 1223, 1174, 1154, 1117,
1088, 1066, 1031, 1006, 959, 948, 932, 873, 850, 772, 759, 711,
570, 458; 1H NMR (400 MHz, CDCl3) δH 1.28 (3H, t, J = 7.2),
2.24 (12H, m), 2.75 (4H, m), 3.24 (4H, m), 3.66 (2H, s), 4.24
(4H, m), 6.83 (2H, s), 7.44 (3H, m), 7.61 (2H, m) 12.4 (1H,
bs); 13C NMR (100 MHz, CDCl3) δC 14.21, 19.38, 20.91,
26.69, 32.51, 34.78, 44.61, 46.47, 47.90, 60.97, 127.88, 128.36,
129.25, 129.38, 130.23, 131.44, 136.98, 137.30, 169.27; MS (EI,
70 eV), m/z 426 (M + H), 294, 260, 202, 188, 168, 141, 133,
120, 91.
The following compounds were prepared according to this
procedure.
2-(1-Benzyl-4-(cyclohexylthio)piperidin-4-yl)ethanol, 18a.
Reduction of 17a (free base) gave the title compound as a
white solid (86%). Mp 84 °C; IR (Nujol) 3195, 3028, 2926,
2854, 1497, 1463, 1377, 1366, 1350, 1334, 1307, 1252, 1223,
1196, 1140, 1117, 1100, 1088, 1078, 1063, 1023, 991.2, 969,
940, 909, 885, 810, 775, 741, 693, 617, 571, 536, 501; 1H NMR
(400 MHz, CDCl3) δH 1.23 to 1.43 (5H, m), 1.55 (1H, m),
1.74 (6H, m), 1.91 (4H, m), 2.59 (6H, m), 3.52 (2H, s), 3.86
(2H, t, J = 6.2), 7.21 to 7.31 (5H, m); 13C NMR (100 MHz,
CDCl3) δC 25.47, 26.38, 36.03, 36.42, 40.36, 47.72, 49.14,
59.30, 63.16, 126.97, 128.18, 129.08, 138.45; MS (EI, 70 eV),
m/z 334 (M + H), 252, 218, 188, 175, 128, 120, 110, 98, 91, 56,
42.
2-(1-Benzyl-4-(phenylthio)piperidin-4-yl)ethanol, 18b. Re-
duction of 17b (free base) gave the title compound as a white
solid (92%). IR (Nujol) 3353, 2853, 2781, 1465, 1377, 1350,
1306, 1133, 1085, 1077, 1066, 1024, 978, 922, 775, 751, 741,
1
700, 696; H NMR (400 MHz, CDCl3) δH 1.70 to 1.78 (6H,
m), 2.56 (2H, m), 2.70 (2H, m), 3.57 (2H, s), 3.98 (2H, t, J =
6.6), 7.24 to 7.34 (8H, m), 7.53 (2H, m); 13C NMR (100 MHz,
CDCl3) δC 35.45, 49.2, 50.59, 59.27, 59.58, 63.12, 127.1,
128.23, 128.79, 128.98, 129.19, 130.66, 137.50; MS (EI, 70 eV),
m/z 328 (M + H), 218, 188, 175, 160, 142, 132, 128, 120, 110,
106, 98, 91, 82, 56, 42.
2-(1-Benzyl-4-((4-methylbenzyl)thio)piperidin-4-yl)-
ethanol, 18c. Reduction of 17c (free base) gave the title
compound as a cream-colored solid (92%). Mp 84−86 °C; IR
(Nujol) 3190, 3064, 3020, 2855, 2827, 2780, 2724, 1914, 1513,
1498, 1474, 1454, 1431, 1399, 1377, 1364, 1344, 1307, 1285,
1260, 1225, 1214, 1189, 1164, 1141, 1110, 1100, 1082, 1057,
1023, 1002, 990, 949, 925, 916, 900, 886, 816, 797, 744, 698,
1
662, 608, 584, 524, 513, 481, 464; H NMR (400 MHz,
CDCl3) δH 1.71 (2H, m), 1.81 (2H, m), 1.88 (2H, t, J = 6.3),
2.31 (3H, s), 2.55 (4H, m), 3.52 (2H, s), 3.63 (2H, s), 3.86
(2H, t, J = 6.3), 7.09 (2H, d, J = 7.8), 7.20 (2H, d, J = 8.0), 7.24
to 7.31 (5H, m); 13C NMR (100 MHz, CDCl3) δC 21.06,
31.39, 35.86, 47.66, 48.93, 59.24, 63.12, 126.97, 128.16, 129.09,
129.11, 129.30, 134.34, 136.72, 138.46; MS (EI, 70 eV), m/z
356 (M + H), 218, 188, 174, 160, 128, 120, 110, 105, 98, 91,
56, 30.
2-(1-Benzyl-4-((4-methoxybenzyl)thio)piperidin-4-yl)-
ethanol, 18d. Reduction of 17d37 gave the title compound as a
white solid (79%). Mp 83−85 °C; IR (Nujol) 3193, 3021,
2922, 2853, 2781, 1880, 1608, 1581, 1511, 1497, 1467, 1457,
1440, 1401, 1377, 1366, 1348, 1330, 1299, 1247, 1184, 1174,
17e was basified with 1 M sodium hydroxide, extracted into
ethyl acetate, and dried (MgSO4). The solvent was removed
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1140, 1127, 1119, 1100, 1088, 1077, 1065, 1029, 991, 970, 940,
911, 831, 814, 777, 745, 693, 618, 546, 514, 501, 441, 421, 405;
1H NMR (400 MHz, CDCl3) δH 1.72 (2H, m), 1.81 (2H, m),
1.90 (2H, t, J = 6.3), 2.56 (4H, m), 3.54 (2H, s), 3.63 (2H, s),
3.79 (3H, s), 3.86 (2H, t, J = 6.3), 6.84 (2H, m), 7.25 (3H, m),
7.29 to 7.32 (3H, m); 13C NMR (100 MHz, CDCl3) δC 31.05,
35.79, 55.24, 59.14, 63.08, 114.00, 127.00, 128.23, 129.13,
129.32, 130.06, 138.29, 158.67; MS (EI, 70 eV), m/z 372 (M +
H), 218, 211, 207, 188, 174, 160, 128, 121, 110, 98, 91, 56.
2-(1-Benzyl-4-((2,4,6-trimethylbenzyl)thio)piperidin-4-yl)-
ethanol, 18e. Reduction of 17e (free base) gave the title
compound as a cream-colored solid (98%). Mp 115−116 °C;
IR (Nujol) 3131, 2930, 1611, 1455, 1377, 1334, 1312, 1266,
1235, 1200, 1150, 1118, 1105, 1090, 1055, 1002, 977, 964, 921,
1458, 1417, 1377, 1348, 1307, 1252, 1213, 1153, 1074, 1038,
1003, 940, 930, 891, 844, 825, 816, 771, 752, 743, 711, 635,
583, 520, 482, 467; 1H NMR (400 MHz, CDCl3) δH 2.09 (6H,
m), 2.28 (3H, s), 3.67 (2H, s), 3.83 (6H, m), 5.00 (2H, s), 7.05
(2H, d, J = 7.9), 7.14 (2H, d, J = 8.0), 7.33 to 7.41 (3H, m),
7.61 (2H, m); 13C NMR (100 MHz, CDCl3) δC 21.00, 30.30,
31.55, 37.97, 54.36, 66.71, 127.16, 128.63, 129.10, 129.31,
130.43, 133.32, 133.90, 136.98; MS (EI, 70 eV), m/z 338 (M+),
233, 205, 172, 110, 105, 91.
1-Benzyl-4-((4-methoxybenzyl)thio)quinuclidin-1-ium
Chloride, 19d. Reaction of 18d gave the title compound as a
white solid (67%) after trituration with THF. Mp 121−123 °C;
IR (Nujol) 3575, 3339, 3266, 3033, 2924, 2853, 2099, 1902,
1655, 1608, 1582, 1513, 1491, 1461, 1446, 1424, 1380, 1344,
1305, 1253, 1242, 1218, 1181, 1136, 1101, 1070, 1036, 1014,
929, 839, 821, 777, 755, 736, 714, 700, 632, 600, 483, 523, 473,
429; 1H NMR (400 MHz, CDCl3) δH 2.09 (6H, m), 3.70 (2H,
s), 3.75 (3H, s), 3.86 (6H, m), 5.01 (2H, s), 6.78 (2H, m), 7.21
(2H, m), 7.37 (3H, m), 7.61 (2H, m); 13C NMR (100 MHz,
CDCl3) δC 30.29, 31.78, 37.93, 54.36, 55.22, 66.71, 114.05,
127.14, 128.82, 129.11, 129.87, 130.44, 133.31, 158.77; MS (EI,
70 eV), m/z 354 (M+), 233, 205, 172, 121, 110, 91.
1
853, 826, 777, 746, 709, 619, 576, 470; H NMR (400 MHz,
CDCl3) δH 1.78 (2H, m), 1.88 (2H, m), 1.96 (2H, t, J = 6.4),
2.24 (3H, s), 2.37 (6H, s), 2.59 (4H, m), 3.54 (2H, s), 3.63
(2H, s), 3.95 (2H, t, J = 6.4), 6.83 (2H, s), 7.25 to 7.32 (5H,
m); 13C NMR (100 MHz, CDCl3) δC 19.47, 20.87, 25.89,
35.73, 47.05, 49.08, 59.35, 63.16, 127.01, 128.20, 129.09,
129.63, 136.69, 136.99, 138.30; MS (EI, 70 eV), m/z 384 (M +
H), 252, 218, 207, 188, 174, 160, 133, 128, 116, 110, 91, 56.
General Method for the Preparation of 1-Benzyl-4-
(alkylthio)quinuclidin-1-ium Chlorides, 19. Methanesul-
fonyl chloride (6.0 mL, 79 mmol) was added dropwise to a
stirred solution of the alcohol 18 (70.4 mmol) and DIPEA
(15.4 mL, 88 mmol) in DCM (200 mL) under nitrogen at
ambient. The reaction mixture was heated to reflux and stirred
for 5 h. The reaction mixture was cooled to room temperature,
and brine (200 mL) was added. The organic phase was
separated, and the aqueous was washed with DCM (100 mL).
The combined organic phases were washed with brine (100
mL) and dried (Na2SO4), and the solvent was removed under
vacuum to give the crude title compound.
1-Benzyl-4-((2,4,6-trimethylbenzyl)thio)quinuclidin-1-ium
Chloride, 19e. Reaction of 18e gave the title compound as a
white solid (92%) after trituration with THF. Mp 239−242 °C;
IR (Nujol) 3380, 2925, 2725, 1612, 1582, 1461, 1377, 1342,
1
1213, 1158, 1076, 1038, 932, 851, 765, 709, 633, 561, 495; H
NMR (400 MHz, DMSO) δH 2.18 (9H, m), 2.29 (6H, s), 3.70
(2H, s), 3.55 (6H, m), 3.75 (2H, s), 4.52 (2H, s), 6.80 (2H, s),
7.54 (5H, bs); 13C NMR (100 MHz, CDCl3) δC 18.86, 25.71,
29.28, 37.76, 54.12, 65.97, 127.63, 128.67, 128.92, 129.25,
130.17, 133.00, 135.98, 136.52; MS (EI, 70 eV), m/z 366 (M+),
233, 205, 172, 148, 133, 110, 91.
General Method for the Preparation of 1-Benzyl-4-
(alkylthio)quinuclidines, 20. A suspension of the quinucli-
dinium salt 19 (56.8 mmol) in 20% sodium hydroxide (50 mL)
and thiophenol (11.5 mL, 115 mmol) was heated to 90−100
°C and stirred until reaction was complete (6 h). The reaction
mixture was cooled to ambient, diluted with water (200 mL),
and extracted with ethyl acetate (2 × 200 mL). The combined
organic extracts were washed with water (150 mL) followed by
1 M hydrochloric acid (2 × 120 mL). The organic phase was
discarded, the acidic aqueous phase was basified with 1 M
sodium hydroxide (300 mL) and extracted with ethyl acetate (2
× 200 mL). The combined organic extracts were dried
(Na2SO4), and the solvent was removed under vacuum to
afford the crude product 20.
The following compounds were prepared according to this
procedure.
1-Benzyl-4-(cyclohexylthio)quinuclidin-1-ium Chloride,
19a. Reaction of 18a gave the title compound as a white
solid (87%) after column chromatography (40% MeOH in
DCM). Mp 78−81 °C; IR (Nujol) 2926, 2850, 1494, 1457,
1376, 1345, 1263, 1215, 1073, 1039, 1000, 928, 884, 842, 817,
771, 709, 631, 475; 1H NMR (400 MHz, CDCl3) δH 1.17 (1H,
m), 1.27 (4H, m), 1.51 (1H, m), 1.66 (2H, m), 1.83 (2H, m),
2.10 (6H, m), 2.65 (1H, m), 3.83 (6H, m), 4.98 (2H, s), 7.33−
7.41 (3H, m), 7.61 (2H, m); 13C NMR (100 MHz, CDCl3) δC
25.21, 26.01, 30.90, 35.88, 37.99, 40.69, 54.37, 66.73, 127.19,
129.25, 130.76. 133.31; MS (EI, 70 eV), m/z 316 (M+), 234,
200, 110, 91.
The following compounds were prepared according to this
procedure.
1-Benzyl-4-(phenylthio)quinuclidin-1-ium Chloride, 19b.
Reaction of 18b gave the title compound as a white solid
(67%) after triturating with THF. IR (Nujol) 3384, 3324, 3229,
2924, 2856, 1619, 1584, 1571, 1499, 1464, 1410, 1376, 1343,
1306, 1066, 1053, 1032, 1004, 974, 919, 842, 774, 758, 710,
704, 696, 651, 632, 582, 546, 507, 466, 424, 412; 1H NMR (400
MHz, CDCl3) δH 2.05 (6H, m), 3.83 (6H, m), 5.06 (2H, s),
7.31−7.42 (8H, m), 7.60 (2H, m); 13C NMR (100 MHz,
CDCl3) δC 30.40, 40.22, 54.54, 66.74, 127.13, 128.20, 129.15,
129.28, 130.05, 130.51, 133.36, 137.58; MS (EI, 70 eV), m/z
310 (M+), 110, 91, 56.
4-(Cyclohexylthio)quinuclidine, 20a. Reaction of 19a gave
the title compound as a pale-green oil (55%) which crystallised
on standing. Mp 49−52 °C; IR (Nujol) 2901, 1451, 1377,
1350, 1312, 1259, 1198, 1171, 1118, 1052, 1009, 997, 971, 924,
1
883, 855, 825, 818, 763, 743, 698, 647, 517; H NMR (400
MHz, CDCl3) δH 1.22 (1H, m), 1.33 (4H, m), 1.54 (1H, m),
1.71 (8H, m), 1.89 (2H, m), 2.76 (1H, m), 2.91 (6H, m); 13C
NMR (100 MHz, CDCl3) δC 25.46, 26.28, 33.36, 36.28, 39.08,
40.51, 48.66; MS (EI, 70 eV), m/z 226 (M + 1), 144, 115, 87.
4-(Phenylthio)quinuclidine, 20b. Reaction of 19b gave the
title compound as a white solid (64%). Mp 75−76 °C, IR
(Nujol) 3073, 3057, 2938, 2866, 1581, 1571, 1472, 1452, 1436,
1377, 1353, 1313, 1259, 1168, 1150, 1064, 1053, 1024, 1007,
998, 970, 922, 826, 784, 758, 705, 696, 648, 513, 435; 1H NMR
1-Benzyl-4-((4-methylbenzyl)thio)quinuclidin-1-ium Chlor-
ide, 19c. Reaction of 18c gave the title compound as a pale
brown solid (90%) after trituration with toluene. Mp 213−218
°C; IR (Nujol) 3472, 3387, 2928, 2855, 1614, 1515, 1496,
K
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(400 MHz, CDCl3) δH 1.69 (6H, m), 2.89 (6H, m), 7.35 (3H,
m), 7.48 (2H, m);38 MS (EI, 70 eV), m/z 220 (M + 1), 163,
123, 111, 96, 83, 68, 56, 42.
REFERENCES
■
(1) (a) Arigoni, D. Gazz. Chim. Ital. 1962, 92, 884−901. (b) Birch, A.
J.; Cameron, D. W.; Holzapfel, C. W.; Richards, R. W. Chem. Ind.
(London) 1963, 1963, 374−375.
4-((4-Methylbenzyl)thio)quinuclidine, 20c. Reaction of 19c
gave the title compound as a pale-brown solid (74%). Mp 97−
99 °C; IR (Nujol) 2857, 1512, 1454, 1377, 1315, 1255, 1237,
1210, 1172, 1111, 1053, 1012, 973, 821, 784, 739, 693, 649,
523, 477; 1H NMR (400 MHz, CDCl3) δH 1.73 (6H, m), 2.31
(3H, s), 2.93 (6H, m), 3.71 (2H, s), 7.09 (2H, d, J = 7.8), 7.27
(2H, d, J = 7.6); 13C NMR (100 MHz, CDCl3) δC 21.02, 30.89,
32.56, 40.42, 48.63, 128.69, 129.10, 135.31, 136.40; MS (EI, 70
eV), m/z 248 (M + H), 143, 128, 115, 110, 82.
(2) Kavanagh, F.; Hervey, A.; Robbins, W. J. Proc. Natl. Acad. Sci.
U.S.A. 1951, 37, 570−574.
(3) Riedl, K. J. Antibiot. 1976, 29, 132−139.
(4) Egger, H. Reinshagen, H. U.S. Patent 4,208,326, 1977.
(5) Jones, R. N.; Fritsche, T. R.; Sader, H. S.; Ross, J. E. Antimicrob.
Agents Chemother. 2006, 50, 2583−2586.
(6) Eckhardt, W.; Grob, C. A. Helv. Chem. Acta 1974, 57, 2339−
2345.
(7) Greene, T. W. Protective Groups in Organic Synthesis; Wiley-
Interscience: Canada, 1981.
(8) (a) Corrie, J. E. T.; Hlubuceck, J. R.; Lowe, G. J. Chem. Soc.,
Perkin Trans. 1 1977, 1421−1425. (b) Yardley, J. P.; Rees, R. W.;
Smith, H. J. Med. Chem. 1967, 10, 1088−1091. (c) du Vigneaud, V.;
Behrens, O. K. J. Biol. Chem. 1937, 117, 27−36.
(9) Huffman, W. F. Yim, N. P. U. S. Patent 4,299, 969, 1981.
(10) Later work demonstrated that sodium hydride could be replaced
for sodium hydroxide which represents a far safer and cheaper
alternative.
(11) Product was isolated as a hydrate, and corrected yield for 7b is
86%.
4-((4-Methoxybenzyl)thio)quinuclidine, 20d. Reaction of
19d gave the title compound as a white solid (68%). Mp 97−98
°C; IR (Nujol) 2936, 1608, 1583, 1508, 1462, 1417, 1377,
1351, 1312, 1303, 1247, 1229, 1183, 1170, 1096, 1053, 1024,
1
1009, 968, 875, 837, 831, 782, 745, 695, 647, 553, 513; H
NMR (400 MHz, CDCl3) δH 1.71 (6H, m), 2.94 (6H, m), 3.73
(2H, s), 3.80 (3H, s), 6.83 (2H, m), 7.25 (2H, m); 13C NMR
(100 MHz, CDCl3) δC 30.58, 32.52, 38.61, 48.64, 55.25,
113.90, 129.95, 130.31, 158.51; MS (EI, 70 eV), m/z 264 (M +
H), 143, 128, 121, 115, 110, 100, 86, 82.
4-((4-Trimethylbenzyl)thio)quinuclidine, 20e. Reaction of
19e gave the title compound as a white solid (54%). Mp 106−
107 °C; IR (Nujol) 2725, 1612, 1578, 1458, 1378, 1352, 1312,
1262, 1203, 1172, 1053, 1032, 1018, 972, 853, 828, 784, 723,
695, 650, 560; 1H NMR (400 MHz, CDCl3) δH 1.81 (6H, m),
2.34 (3H, s), 2.38 (6H, s), 2.98 (6H, m), 3.71 (2H, s), 7.01
(2H, s); 13C NMR (100 MHz, CDCl3) δC 19.22, 19.43, 25.13,
32.18, 39.88, 48.69, 128.96, 130.17, 136.42, 136.83; MS (EI, 70
eV), m/z 276 (M + H), 143, 115, 100, 86, 82, 71, 58.
(12) Additional studies showed that triethylamine can be used in
place of Hunigs base (N,N-diisopropyl-N-ethylamine) which repre-
sents a significant cost saving.
(13) Compound 4a can be recrystallised from IPA (22 vol) in
excellent yield.
(14) An authentic sample of disulfide 12 was prepared in excellent
yield by reacting 4a with sulfuryl chloride in pyridine using reported
conditions for disulfide formation: Derbesy, G.; Harpp, D. N.
Tetrahedron Lett. 1994, 35, 5381−5384.
(15) The HCl salt of 9 can be purified by slurrying within a 3:1
heptane/toluene mixture which removes unreacted ester 8 as well as
benzyl mercaptan. Compound 10 can be purified by crystallisation
from aqueous denatured alcohol (1:1).
(16) This yield was significantly lower than expected following on
from the small-scale investigational work, and this was attributed to
physical losses of the substrate 7b up the reactor riser during the
addition process and was caused by localized boiling. Future work
would have evaluated adding the substrate as a slurry within a suitable
solvent to minimize such losses.
(17) Birch, A. J.; Holzapfel, C. W.; Rickards, R. W. Tetrahedron 1966,
359−387.
(18) Purity of methansulfonate 3 derived under these conditions was
approximately 98%.
(19) Purity of product 5 from using equimolar equivalents of 3 with
4a was 87%, whereas using 1.2 equiv of 4a resulted in an increase in
product purity to 98%. Quinuclidine-4-thiol 4a was charged on the
basis of its assay.
(20) To generate a longer term specification for the 4-
mercaptoquinuclidinium salt (4a or 4b) that is appropriate for its
reaction with methanesulfonate 3, it would be necessary to understand
acceptable levels of disulfide 12, as well as any by-products derived
from incomplete debenzylation of 7b, that could be contained within
4. This understanding would be gained through appropriate doping
work of such impurities within the coupling reaction of 3 with 4 to
prepare 5.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 13C NMR, and MS spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Michael.W.Urquhart@gsk.com
Present Address
§Glaxo Group Limited, Medicines Research Centre, Gunnels
Wood Road, Stevenage, Hertfordshire, SG1 2NY, United
Kingdom.
Notes
The authors declare no competing financial interest.
∥Deceased.
ACKNOWLEDGMENTS
■
We are grateful to Shaun Mustoe and Alex Smoleskis for
analytical support, Jeff Richards, Duncan Bryant and Paul
Blackler for spectroscopic support, as well as Emma Griva and
Deirdre Bermingham for process engineering, Jeff Soden and
Ruud Watervaal for process chemistry. We are also indebted to
Andre Wanders for process safety evaluation studies and Jolanta
Thirkill for environmental input into this work. Duncan Bryant
passed away in 2005.
(21) Kametani, T.; Kigasawa, K.; Hiiragi, M.; Wagatsuma, N.;
Wakisaka, K.; Kusama, O. J. Med. Chem. 1969, 12, 694−696.
(22) Sakakibara, S.; Shimonishi, Y.; Kishida, Y.; Okada, M.; Sugihara,
H. Bull. Chem. Soc. Jpn. 1967, 40, 2164−2167.
(23) (a) Clarke, K.; Hughes, C. G.; Scrowston, R. M. J. Chem. Soc.,
Perkin Trans. 1 1973, 356−359. (b) Kaji, K.; Kuzuya, M. Chem. Pharm.
Bull. 1970, 18, 970−981.
DEDICATION
■
(24) Panigot, M. J.; Fesik, S. W.; Curley, R. W., Jr. J. Labelled Compd.
Radiopharm. 1995, 36, 439−444.
This manuscript is dedicated to the memory of both Robert
(Rob) Giles and Duncan Bryant both of whom died in 2005.
(25) Sheehan, J. C. Tishler, M. U.S. Patent 2,477,149, 1949.
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(26) Collie, N.; Schryver, S. B. J. Chem. Soc. 1890, 57, 767−782.
(27) It was observed through informal stability evaluations that both
crystalline products 4a and 4b showed no appreciable degradation
(oxidation to 12) within a reasonable time frame under standard
storage conditions.
(28) Mairanovsky, V. G. Angew. Chem., Int. Ed. Engl. 1976, 15, 281−
292.
(29) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923−
2925.
(30) Karl Fischer inspection showed this material contained 5%
water, and is thus a mono-hydrate (MW = 378.03).
(31) The very poor yield observed on scale-up compared to those
from laboratory runs (averaging at 73% of 10 from 6) was attributed to
the formation of gelling of the reaction mixture during the quench of
the lithium aluminium hydride reduction. Subsequent removal of the
inorganic residues from the product solution became very difficult as
the filters became clogged, and therefore yield was lost in order to
deliver inorganic free product for use in the next stage. Future
modifications would be needed to address this gelling issue by using an
agitated filtration kit, by using Rochelle’s salt (aqueous sodium
potassium tartrate) to break up the emulsion, or by developing
conditions to avoid the gelling in the first place.
(32) Vigorous reaction ensues during this quench so that greater care
is required at the beginning of the quench.
(33) The exit port for the ammonia gas was situated such that the gas
passed through a dilute hydrochloric acid reservoir (2.5 L conc HCl in
9.5 L water; conc ≈ 2.3 M).
(34) Berry, V.; Dabbs, S.; Frydrych, C. H.; Hunt, E.; Woodnutt, G.;
Sanderson, F. D. PCT Int. Appl. WO/1999/21855 A1 19990506,
1999, .
(35) This material can be effectively recrystallised from toluene (15
vol) to provide an analytically pure sample.
(36) Further development had shown that powdered sodium
hydroxide could be used as a replacement for sodium hydride at this
stage, and this reaction used sodium hydroxide rather than sodium
hydride. This modification has significant processing and safety
advantages.
(37) The HCl salt of 17d was used for this reaction, and reaction
with 3 mol equiv of lithium aluminium hydride proved very slow in 15
vol of THF. This was a consequence of the poor solubility of 17d;
therefore, triethylamine (1 mol equiv) was added which led to
solvation of the substrate and complete reaction after stirring overnight
at ambient.
(38) 13C NMR data were not collected for this compound.
M
dx.doi.org/10.1021/op300263w | Org. Process Res. Dev. XXXX, XXX, XXX−XXX