Total synthesis of a novel 2-thiabicyclo[3.2.0]heptan-6-one analogue of penicillin N

Article abstract of DOI:10.1016/j.tet.2003.08.004

A route has been developed which allows synthesis of novel cyclobutanone analogues of penicillin. This is illustrated by the synthesis of (1R,4R,5R,5′R,7S)-(1b) and (1S,4S,5S,5′R,7R)-7-[5′ -amino-5′-carboxy]pentanamido]-2-thiabicyclo[3.2.0] heptan-6-one-4-carboxylate (1a), an analogue of penicillin N. The key steps in the synthesis were the formation of the bicyclic structure via a [2+2] cycloaddition and the introduction of nitrogen at C7 via an intramolecular nitrene insertion.

Full text of DOI:10.1016/j.tet.2003.08.004

Tetrahedron 59 (2003) 8233–8243
Total synthesis of a novel 2-thiabicyclo[3.2.0]heptan-6-one
analogue of penicillin N
Amanda C. Ferguson,a Robert M. Adlington,a Domnic H. Martyres,b Peter J. Rutledge,c
Andrew Cowleyd and Jack E. Baldwina
,
*
aThe Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QY, UK
bBoehringer-Ingelheim Pharma, Biberach, Germany
cDepartment of Chemistry, The Centre for Synthesis and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
dChemical Crystallography Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QY, UK
Received 26 June 2003; revised 9 July 2003; accepted 7 August 2003
Abstract—A route has been developed which allows synthesis 0of novel cyclobutanone analogues of penicillin. This is illustrated by the
synthesis of (1R,4R,5R,50R,7S)-(1b) and (1S,4S,5S,50R,7R)-7-[5 -amino-50-carboxy]pentanamido]-2-thiabicyclo[3.2.0]heptan-6-one-4-car-
boxylate (1a), an analogue of penicillin N. The key steps in the synthesis were the formation of the bicyclic structure via a [2þ2]
cycloaddition and the introduction of nitrogen at C7 via an intramolecular nitrene insertion.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
the carbocyclic analogue (1) would be a suitable and
hydrolytically stable mimic, since PM3 calculations pre-
dicted close structural similarity (Fig. 1).4 It is noteworthy
that related fully carbocyclic bicyclo[3.2.0]heptan-6-ones
have recently been reported by Fairlamb et al., for use as a
squalene synthase enzyme probe.5
The biosynthesis of penicillins and cephalosporins have
been the subject of extensive investigation, as it is these
processes which permit viable production of these anti-
biotics on a commercial scale.1 In cephalosporin-producing
organisms, the enzyme isopenicillin N/penicillin N epimer-
ase converts isopenicillin N to penicillin N. This biosyn-
thesis proceeds towards cephalosporin C by a ring
expansion and subsequent hydroxylation. These biotrans-
formations occur differently in prokaryotes and eukaryotes.
In eukaryotes, the two steps are carried out by a single
bifunctional enzyme deacetoxycephalosporin C/deacetyl-
cephalosporin C synthase (DAOC/DACS) whereas in
prokaryotes, the enzymes are distinct; deacetoxycephalo-
sporin C synthase (DAOCS)2 and deacetylcephalosporin C
synthase (DACS) respectively.
The synthesis of 2-thiabicyclo[3.2.0]heptan-6-ones bearing
a 4-carboxylic acid functionality has previously been
reported by Dimentriko et al.,6 and Tomczuk,7 but these
attempts were unsuccessful in the installation of an
acylamino side chain at C7, and a gem-dimethyl group at
C3. We recently reported the synthesis of a 2-thiabicy-
clo[3.2.0]heptan-6-one derivative (2) bearing a carboxyl
group at C4 and a carbamate side chain at C7 (Fig. 2).8
Herein we report an extension of our methodology which
has enabled further incorporation of both a gem-dimethyl
group at C3 and the D-a-aminoadipoylamino group at C7 to
provide a cyclobutanone analogue of penicillin N.
The mechanism of the ring-expansion has been investigated
and a free radical pathway proposed.3 Ongoing studies
would be greatly enhanced by a crystal structure of an
DAOCS–substrate complex, which would represent an
extremely useful mechanistic probe. The b-lactam moiety
of penicillin N has proved to be hydrolytically unstable to
the aqueous crystallisation conditions and as such was
unsuitable for this purpose. It was therefore envisaged that
Keywords: cycloaddition; ketene; b-lactam; nitrene; penicillin; DAOCS.
*
Corresponding author. Tel.: þ44-1865-275-671; fax: þ44-1865-275632;
e-mail: jack.baldwin@chem.ox.ac.uk
Figure 1. 1a (1b—1R,4R,5R,50R,7S diastereomer).
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.08.004

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A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
carbamate which could be hydrolysed to a cis amino alcohol
(Scheme 2).
To this end, reductive dechlorination was effected by
heating the bicyclic adduct (10) in a suspension of zinc and
acetic acid, under reflux to afford the ketone (11) in 82%
yield.10 This was used without purification owing to
instability on silica. The endo-cyclobutanol (12) was
accessed stereoselectively using sodium borohydride,
which delivers hydride from the more accessible exo
face.10 The yields were increased to 83% by quenching
the reaction with the silica onto which it was adsorbed for
flash chromatography, bypassing extraction. This was
presumably due to the alcohol having some solubility in
water. Treatment of the endo-cyclobutanol (12) with
triphosgene and pyridine in CCl4 furnished the chlorofor-
mate (13), which proved unstable to silica and therefore was
converted into the azidoformate without purification.
Reaction with sodium azide in DMF afforded the azido-
formate (14) in 67% yield. X-Ray crystallography provided
explicit means of proving the relative stereochemistry
(Fig. 3).
Figure 2. (1R,4R,5R,7S)- and (1S,4S,5S,7R)-7-(tert-butyoxycarbonyl-
amino)-2-thiabicyclo[3.2.0]heptan-6-one-4-carboxylic acid ethyl ester.
2. Results and discussion
Our synthetic strategy was developed from our previous
investigations.8 The key steps in the synthesis proved to be
formation of the bicyclic nucleus by a [2þ2] cycloaddition
between dichloroketene and a dihydrothiophene, and
installation of an acylamino moiety at C7, which was
effected by an intramolecular nitrene insertion reaction.
The dihydrothiophene structure was produced according to
literature procedures8 (Scheme 1). Assembly of the alkene
(6) commenced with the alkylation of commercially
available triethylphosphono acetate (3) with 2-iodopropane
in 87% yield.9 The alkylated product (4) was then subjected
to a selenation/selenoxide elimination sequence in 77 and
83% yield, respectively.10 Synthesis of dihydrothiophene
(7) was achieved by a modified Horner–Wadsworth–
Emmons cyclisation in 78% yield,10,11 the mercaptaldehyde
generated in situ from its dimer, 1,4-dithiane-2,5-diol.
Isomerisation to the kinetic isomer (9) was achieved by
saponification to the lithium salt (8) in 98% yield, followed
by the formation of a mixed anhydride/ketene intermediate
which undergoes a decarboxylative elimination and sub-
sequent readdition of ethoxide to afford the kinetic isomer
(9) in 74% yield. The bicyclic structure racemic (^)-(10)
was then accessible in 96% yield via a [2þ2] cycloaddition;
the dichloroketene was generated in situ by the slow
addition of triethylamine to a solution of the dihydrothio-
phene and dichloroacetyl chloride over 4 days (Scheme 1).5,10
Azidoformates are known to thermally decompose to o-acyl
nitrenes, e.g. (15), which can insert into available C–H
bonds. We have previously reported that an intramolecular
nitrene insertion was possible in 29% yield in the synthesis
of the desmethyl analogue (2). This yield was low and it was
of concern that the 3,3-gem-dimethyl group may hinder the
reaction in this synthesis, as a result of its increased steric
bulk. In addition to this, it was anticipated that insertion at
C4 might be a strongly competing reaction. Nonetheless, the
reaction was optimised and the most efficient conditions
found to be highly dilute, to prevent intermolecular reaction,
rigorously anhydrous and anaerobic to prevent competing
nitrene reactions and with the dropwise introduction of the
azidoformate to the solvent at 1478C. As a result of these
optimised conditions the tricyclic structure (16) was isolated
in 28% yield, comparable to the yields obtained by Lowe
et al. on an oxygen analogue of penicillin G.12 A crystal
structure of (16) clearly indicated that the insertion had
occurred on the endo face, as required (Fig. 4).
Having constructed the first key bicyclic intermediate, the
second key step was introduction of the acylamino side
chain at C7. A route was proposed involving an intra-
molecular o-acyl nitrene insertion to provide a cyclic
After protection of the nitrogen functionality using di-tert-
butyl dicarbonate, triethylamine and a catalytic amount of
4-dimethyl amino pyridine (DMAP) in 79% yield, the cyclic
Scheme 1. (i) KOtBu, 2-iodopropane, DMSO, 608C, 87%. (ii) NaH, PhSeCl, THF, 77%. (iii) H2O2, DCM, 83%. (iv) LiCl, 1,4-dithiane-2,5-diol, DBU, MeCN,
78%. (v) LiOH·H2O, THF, H2O, 98%. (vi) Ethyl chloroformate, Et3N, 74%. (vii) Et3N, dichloroacetylchloride, CCl4, 5 days, 96%.

A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
8235
Scheme 2. (i) Zn, AcOH, reflux, 82%. (ii) NaBH4, MeOH 83%. (iii) Triphosgene, pyridine, carbon tetrachloride, 508C, quantitative. (iv) NaN3, DMF, 508C,
67%. (v) TCE, 1478C, 15 min, 28%.
carbamate (17) was hydrolysed using caesium carbonate in
ethanol to afford the cis-amino alcohol (18) in 90% yield.
Having obtained the tosylate salt it was then necessary to
couple the amino acid side chain. It was decided to use the
tosylate salt directly in the coupling reaction and liberate the
amine in situ, to maximise efficiency on the small scale.
Activation with EDCI and HOBt proved effective in the
acylation of the free amino function and coupling to the
protected D-a-aminoadipic acid. This afforded the fully
protected cyclobutanol (23) as a mixture of diastereomers,
in 82% yield. (Compounds0 (23), (24), (25) and (1) are
depicted as the (1S,4S,5S,5 R, (6R),7R) isomer.) At this
stage the cyclobutanol (23) was oxidised with o-iodoxy-
benzoic acid (IBX) in 90% yield to the cyclobutanone (24).
Deprotection was achieved by stirring in a solution of TFA/
anisole/toluene (3:1:20) for 0.5 h at ambient temperature to
afford the target compound in 71% yield (Scheme 4).
A careful protecting group strategy was required in order to
couple the D-(a)-aminoadipic acid side chain efficiently.
The final selection was made on the evidence of model
studies (Fig. 5). Deprotection of the para-nitrobenzyl
protected molecule13 (22a) and alloc protected molecule14
(22b) proved unsuccessful owing to decomposition under
deprotection conditions. It was proposed that the palladium
involved in the deprotection was coordinating to the sulfur,
causing structural degradation. However viable deprotec-
tion of the acid labile diphenylmethyl protecting group13
could be achieved, leaving the ring system structurally
intact. On this basis the diphenylmethyl protecting group
was utilised in protection of the 4-carboxyl functionality,
and th0e BOC and PMB groups used to protect the 50 amino
and 5 carboxyl functionalities of D-a-aminoadipic acid
respectively.15 There remained concern however, that on
final deprotection of (24) enolisation of the cyclobutanone
may result in racemistion at C7.12
1
The product (1a/1b) was identified by H and 13C NMR.
The stereochemistry was proven to be as required by
synthesising the deprotected cyclobutanol (25) by two
different methods. The first was via borohydride reduction
of the cyclobutanone (1), and the second via deprotection of
the cyclobutanol (23) (Scheme 5). An NMR doping
experiment proved the products to be identical.
The synthetic route proceeded with saponification of the
ethyl ester (18) to the carboxylic acid (19) with 1 equiv. of
LiOH·H2O. The reaction did not proceed to completion,
however the product was isolated by acid/base extraction in
75% yield and the residual starting material recycled. The
4-carboxyl group was then protected to afford the
diphenylmethyl ester (20) in 66% yield by stirring with
diphenyl diazomethane13 in acetonitrile at ambient tem-
perature. The amine functionality was then revealed by
removal of the BOC protecting group, stirring with 1 equiv.
of p-toluene sulfonic acid monohydrate at room temperature
to afford the tosylate salt (21) in 59% yield (Scheme 3).
Deuterium exchange at C7 was observed when (1a/1b) was
left standing in D2O for 5 days, clearly indicating that
enolisation of the cyclobutanone was occurring. However
1
the H NMR spectrum did not show any additional set of
signals which would have resulted from an additional pair of
diastereomers. It followed, therefore that the enolisation and
reprotonation was proceeding with total retention to give
(1a/1b).
Figure 4. The crystal structure of (1R,2S,6S,7R,10R)- and
(1S,2R,6R,7S,10S)-10-ethoxycarbonyl-5-aza-9,9-dimethyl-3-oxo-8-thiatri-
cyclo[5.3.0.0]decan-4-one (16).
Figure 3. The crystal structure of (1S,4R,5R,6R)- and (1R,4S,5S,6S)-3,3-
dimethyl-4-carboxethyl-2-thiabicyclo[3.2.0]heptane-6-azidoformate (14).

8236
A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
Figure 5. Model studies to select a protecting group strategy.
Scheme 3. (i) Di-tert-butyl dicarbonate, Et3N, DMAP, THF, 79%. (ii) Cs2CO3, EtOH, 90%. (iii) LiOH·H2O, DMF, H2O, 708C, 75%. (iv) Ph2CN2, MeCN,
66%. (v) p-TsOH, MeCN, 59%.
Scheme 4. (i) EDCI, HOBt, N-tert-butyloxycarbonyl-D-a-aminoadipic acid-a-p-methoxybenzyl ester, DCM, Et3N, 82%. (ii) IBX, DMSO, 90%. (iii)
TFA/anisole/toluene (3:1:20) 71%.
Table 1. Comparison of NOE integrals for (1S,4S,5S,50R,6R,7R)- and
(1R,4R,5R,50R,6S,7S)-7-[50-amino-50-carboxypentanamido]-3,3-dimethyl-
2-thiabicyclo[3.2.0]heptan-6-ol-4-carboxylate (25)
Quantitative NOE studies of (25a/25b) were consistent with
the structure drawn. The NOEs between Hb and Hc, Hd and
Hb, Hc and He, and Hd and He are large, i.e integrals of 6 or
greater, indicating that these protons are all orientated cis to
each other. The integral between Ha and Hc is less than 50%
of that expected for a cis relationship, suggesting that this
relationship is trans. NOEs between protons across the
diagonals of the cyclobutanone were not observed, but it is
probable that the conformation of the molecule is such that
the through-space distance is too large (Table 1).
Proton irradiated
NOE integrals
Ha
Hb
Hc
Hd
He
Ha
Hb
Hc
Hd
He
–
0
2.5
0
0
0
–
9.0
11.2
0
2.4
7.4
–
0
9.2
0
6.8
0
–
7.9
0
0
6.9
7.7
–

A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
8237
Scheme 5. Synthesis of (1S,4S,5S,50R,6R,7R)- and (1R,4R,5R,50R,6S,7S)-7-[50-amino-50-carboxypentanamido]-3,3-dimethyl-2-thiabicyclo[3.2.0]heptan-6-ol-
4-carboxylate (25). (i) IBX, DMSO. (ii) TFA/anisole/toluene (3:1:20). (iii) NaBH4, D2O.
3. Conclusion
triethylphosphono acetate (150.0 g, 0.67 mol). This was
stirred at room temperature for 1 h. The mixture was cooled
to 08C and 2-iodopropane (108.5 mL, 1.09 mol) was added
dropwise. The reaction was then stirred at ambient
temperature for 18 h, quenched with pH 7 phosphate buffer
(550 mL) and extracted into DCM (5£300 mL). The organic
extract was washed with brine (200 mL), dried and
concentrated in vacuo to yield a pale orange oil. This was
purified by distillation under reduced pressure to yield the
title compound as a colourless oil (155.1 g, 87%): bp 92–
938C (0.6 Torr); Rf 0.3 (EtOAc/PE 40–60 2:1); IR (NaCl
plates) nmax 2928 (m, CH), 1732 (s, CvO); dH (400 MHz,
CDCl3) 0.94 (3H, dd, J¼6.5, 1.5 Hz, methyl of CH(CH3)2),
1.09 (3H, dd, J¼6.5, 1.5 Hz, methyl of CH(CH3)2),
1.21–1.41 (9H, m, P(O)(OCH2CH3)2 and CO2CH2CH3),
To our knowledge, this is the first synthesis of the
2-thiabicyclo[3.2.0]heptan-6-one analogue of penicillin
N. The synthesis produced the target compound as a
diasteromeric pair. However, it is not anticipated that this
will be a problem for biological testing, as it is hoped that
the enzyme will select for the correct diastereomer. A
positive a-ketogluterate binding assay16 indicated that this
compound binds in the active site of DAOCS which leaves
us in a strong position to commence crystallisation trials.
4. Experimental
4.1. General
2
2.25–2.36 (1H, m, CH(CH3)2), 2.66 (1H, dd, JP¼10.0,
9.5 Hz, CHCH(CH3)2), 4.05–4.18 (6H, m, P(O)(OCH2-
CH3)2 and CO2CH2CH3); dC (100.63 MHz, CDCl3) 14.06
(CO2CH2CH3), 16.30 P(O)(OCH2CH3)2, 21.45 (one of
CH(CH3)2, 21.55 (one of CH(CH3)2, 28.20 (CH(CH3)2),
Chemical reagents were used as supplied by commercial
suppliers unless otherwise stated. Where necessary, solvents
were dried and purified according to recommended
methods.17 TCE was prepared freshly by distillation from
a suspension of phosphorus pentoxide. Ether refers to
diethyl ether. PE 40–60 refers to light petroleum in the
boiling range 40–608C. DCM refers to dichloromethane.
Organic solutions were dried over MgSO4 unless otherwise
stated. Flash chromatography refers to the method of Still
et al.18 Solvents were evaporated at 408C or below using a
Buchi R114 Rotavapor equipped with a water condenser,
Buchi B480 water bath and a Vacubrand MZ 2C pump fitted
with a CVC2 vacuum controller. Analytical TLC was
performed on Merck silica gel 60 F254 precoated
aluminium-backed plates. Proton (1H) and carbon (13C)
nuclear magnetic resonance (NMR) spectra were recorded
on Bruker DPX 200, DPX 250, Bruker DQX 400 and Bruker
AMX 500 spectrometers.
1
52.66 (d, JP¼132.0 Hz, CHCH(CH3)2), 61.02 (CO2CH2-
CH3), 62.29 (P(O)(OCH2CH3)2), 169.17 (CvO); m/z
(APCIþ) 289 ([MNa]þ, 15%), 267 ([MH]þ, 100%), 221
([M2OEt]þ, 6%).
4.1.2. (2RS)-Ethyl 2-(diethoxyphosphoryl)-2-phenylsele-
nyl-3-methyl-butanoate (5).10 To a stirred solution of
sodium hydride (7.94 g of a 60% dispersion, 199 mmol) in
THF (50 mL) at 2788C was added a solution of (2RS)-ethyl
2-(diethoxyphosphoryl)-3-methylbutanoate (4) (33.0 g,
124 mmol) in THF (50 mL) dropwise over 1 h. This was
allowed to warm to ambient temperature and stirred for
20 h. The solution was cooled to 2788C and a solution of
phenyl selenenyl chloride (25.0 g, 131 mmol) in THF
(100 mL) was added to the reaction mixture. The reaction
was again allowed to warm to ambient temperature and stir
for 15 h. The reaction was quenched with acetic acid (10%,
300 mL) and extracted with EtOAc (4£400 mL). The
organic extract was washed with saturated NaHCO3(aq)
4.1.1. (2RS)-Ethyl 2-(diethoxyphosphoryl)-3-methylbu-
tanoate (4).9 To a stirred solution of potassium tert-
butoxide (105.3 g, 0.94 mol) in DMSO (380 mL) was added

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A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
(100 mL), brine (100 mL) and concentrated in vacuo to
yield a brown oil. Flash chromatography on silica afforded
the title compound as a pale green oil (40.3 g, 77%): Rf 0.3
(EtOAc/PE 40–60 1:1); IR (NaCl plates) nmax 2980 (s, CH),
1721 (s, CvO); dH (200 MHz, CDCl3) 1.08–1.38 (15H, m,
P(O)(OCH2CH3)2, CO2CH2CH3 and CH(CH3)2), 2.49–
2.66 (1H, dquin, J¼26.0, 7.0 Hz, CH(CH3)2), 3.90 (2H,
qd, J¼7.0, 3.5 Hz, CO2CH2CH3), 4.09–4.32 (4H, m,
P(O)(OCH2CH3)2), 7.26–7.39 (3H, m, ArH), 7.75–7.81
(2H, m, ArH); dC (100.63 MHz, CDCl3) 13.73 (one of
CH(CH3)2), 16.40 (P(O)(OCH2CH3)2), 19.47 (CO2CH2-
CH3), 20.37 (one of CH(CH3)2), 34.08 (CH(CH3)2), 61.54
J¼3.0 Hz, SCH2CHvCH); dC (100.63 MHz, CDCl3) 14.16
(OCH2CH3), 31.35 (SC(CH3)2), 35.12 (SCH2), 59.24
(SC(CH3)2), 60.44 (OCH2CH3), 139.91 (SCH2CH),
142.54 (CHvCCO2CH2CH3), 163.50 (CvO); m/z
(APCIþ) 187 ([MH]þ, 10%), 157 ([M2Et]þ, 50%), 113
([M2CO2Et]þ, 100%).
4.1.5. 2,5-Dihydrothiophene-2,2-dimethyl-3-carboxylic
acid lithium salt (8). To a stirred solution of 2,5-
dihydrothiophene-2,2-dimethyl-3-carboxylic acid ethyl
ester (7) (18.8 g, 0.10 mmol) in anhydrous THF (200 mL)
was added LiOH·H2O (4.23 g, 0.10 mmol) in water
(200 mL). This was heated under reflux for 24 h. The
solvent was removed and the product dried in a dessicator
for 48 h to yield the title compound, isolated as the lithium
salt, as a yellow solid (16.2 g, 98%): Rf (acid) 0.25 (EtOAc/
PE 40–60 2:1); IR (NaCl plates) nmax 3434 (s, br, OH), 1642
(s, CvO); dH (200 MHz, CD3OD) 1.60 (6H, s, C(CH3)2),
3.63 (2H, d, J¼2.5 Hz, SCH2CH), 6.52 (1H, t, J¼2.5 Hz,
SCH2CH); dC (100.63 MHz, CD3OD) 31.32(S(CH3)2), 33.98
(SCH2), 67.17 (SC(CH3)2), 132.17 (SCH2CH), 150.27
(CHvCCO2H), 171.56 (CvO); m/z (APCI2) 157
([M2H]2, 78%), 113 (98%).
(CO2CH2CH3),
63.27
(P(O)(OCH2CH3)2),
127.39
(P(O)CSePh), 128.44, 129.35, 138.11 (CAr), 168.70
(CvO); m/z (APCIþ) 445 ([M(80Se)Na]þ, 100%), 443
([M(78Se)Na]þ, 43%), 442 ([M(77Se)Na]þ, 20%), 441
([M(76Se)Na]þ, 15%), 289 ([MNa2SePh]þ, 90%), 273
(85%), 199 (35%), 160 (37%).
4.1.3. Ethyl 2-(diethoxyphosphoryl)-3-methyl-but-2-eno-
ate (6).10 To a stirred solution of (2RS)-ethyl 2-(diethoxy-
phosphoryl)-2-phenylselenyl-3-methyl-butanoate (5) (27.6 g,
65.0 mmol) in DCM (200 mL) at 08C was added H2O2
(18.8 mL, 30% aq, 170 mmol) dropwise. The solution was
allowed to warm to ambient temperature and after 2 h the
reaction was quenched with saturated NaHCO3(aq) solution
(100 mL) and the organic layer separated and washed with
water (100 mL), dried and concentrated in vacuo to yield a
yellow oil. Flash chromatography on silica (EtOAc/PE 40–
60 2:1) afforded the title compound as a colourless oil
(14.2 g, 83%): Rf 0.35 (EtOAc/PE 40–60 2:1); IR (NaCl
plates) nmax 2983 (m, CH), 1723 (s, br, CvO), 1624 (m,
CvC); dH (200 MHz, CDCl3) 1.21–1.41 (9H, m,
P(O)C(OCH2CH3)2 and CO2CH2CH3), 1.81 (3H, d,
J¼2.0 Hz, one of CvC(CH3)2), 2.07 (3H, d, J¼3.0 Hz,
one of CvC(CH3)2), 3.97–4.22 (6H, m, P(O)(OCH2CH3)2
and CO2CH2CH3); dC (100.63 MHz, CDCl3) 13.99 (CO2-
CH2CH3), 16.27 (P(O)(OCH2CH3)2), 22.65 (d, J¼8.0 Hz,
P–CvC(CH3)2), 24.37 (d, J¼18.0 Hz, P–CvC(CH3)2),
61.37 (d, J¼22.0 Hz, P(O)(OCH2CH3)2), 62.11 (d,
J¼5.0 Hz, CO2CH2CH3), 135.63 (d, J¼9.0 Hz, C(CH3)2),
157.95 (d, J¼6.0 Hz, P–C), 167.26 (d, J¼4.0 Hz, CvO);
m/z (APCIþ) 287 ([MNa]þ, 100%).
4.1.6. (3RS)-2,3-Dihydrothiophene-2,2-dimethyl-3-car-
boxylic acid ethyl ester (9). To a solution of the lithium
salt (8) (4.00 g, 24.4 mmol) in THF (200 mL) was added
triethylamine (8.32 mL, 59.6 mmol). This solution was
stirred at ambient temperature for 0.5 h. The solution was
then cooled to 08C and ethyl chloroformate (6.31 mL,
65.9 mmol) was added dropwise. The solution was allowed
to warm to ambient temperature and stirred for a further
24 h. The solvent was removed and water (50 mL) was
added. The solution was acidified to pH 6 with HCl (5%, aq)
and the product extracted with EtOAc (4£75 mL). The
combined organic layers were then washed with saturated
NaHCO3(aq) (50 mL), brine (50 mL), dried and concentrated
in vacuo to yield a crude brown residue. This was purified
by flash chromatography on silica (PE 40–60/Et2O 10:1) to
yield the title compound as a fluorescent green oil (3.35 g,
74%): Rf 0.25 (PE 40–60/Et2O 10:1); IR (NaCl plates) nmax
3978 (m, CH), 1735 (s, CvO); dH (400 MHz, CDCl3) 1.29
(3H, t, J¼6.5 Hz, OCH2CH3), 1.40 (3H, s, one methyl of
C(CH3)2), 1.70 (3H, s, one methyl of C(CH3)2), 3.69 (1H, t,
J¼2.5 Hz, CHCO2Et), 4.12 (2H, q, J¼6.5 Hz, OCH2CH3),
5.52 (1H, dd, J¼6.5, 2.5 Hz, SCHvCH), 6.32 (1H, dd,
J¼6.5, 2.5 Hz, SCHvCH); dC (100.63 MHz, CDCl3) 14.24
(OCH2CH3), 25.80 (one of SC(CH3)2), 29.35 (one of
SC(CH3)2), 58.38 (SC(CH3)2), 60.79 (OCH2CH3), 63.14
(CHCO2Et), 120.42 (SCHvCH), 127.83 (SCHvCH),
171.01 (CvO); m/z (APCIþ) 203 ([MNH4]þ, 100%), 157
([M2Et]þ, 57%), 113 ([M2CO2Et]þ, 20%).
4.1.4. 2,5-Dihydrothiophene-2,2-dimethyl-3-carboxylic
acid ethyl ester (7).11 To a suspension of flame dried
lithium chloride (4.86 g, 115 mmol) in MeCN (210 mL)
were added 1,4-dithiane-2,5-diol (8.75 g, 57.5 mmol) and
then ethyl 2-(diethyoxyphosphoryl)-3-methyl-but-2-enoate
(6) (30.6 g, 116 mmol). This mixture was cooled to 08C and
1,8-diazabicyclo[5.4.0]undec-7-ene (17.6 mL, 117 mmol)
was added. This solution was allowed to warm to ambient
temperature and stirred for 8 h. The mixture was acidified to
pH 6 with acetic acid (25% aq) and extracted into EtOAc
(8£75 mL). The combined organic extracts were washed
with saturated NaHCO3(aq) (50 mL), brine (50 mL), dried
and concentrated in vacuo. The residue was purified by flash
chromatography on silica (PE 40–60) to afford the title
compound as a yellow oil (16.8 g, 78%): Rf 0.25 (PE 40–60/
Et2O 15:1); IR (NaCl plates) nmax 2967 (m, CH), 1716 (s,
CvO); dH (400 MHz, CDCl3) 1.28 (3H, t, J¼6.5 Hz,
OCH2CH3), 1.70 (6H, s, C(CH3)2), 3.78 (2H, d, J¼3.0 Hz,
SCH2CH), 4.22 (2H, q, J¼6.5 Hz, OCH2CH3), 6.85 (1H, t,
4.1.7. (1R,4R,5R)- and (1S,4S,5S)-7,7-Dichloro-3,3-
dimethyl-2-thiabicyclo[3.2.0]heptan-6-one-4-carboxylic
acid ethyl ester (10).12 To a stirred solution of (3RS)-2,3-
dihydrothiophene-2,2-dimethyl-3-carboxylic acid ethyl
ester (9) (13.5 g, 72.5 mmol) and dichloroacetyl chloride
(10.41 mL, 108 mmol) in carbon tetrachloride (1100 mL)
was added a solution of triethylamine (15.0 mL, 108 mmol)
in carbon tetrachloride (360 mL) dropwise over 4 days.
After a further 24 h the reaction was quenched with water
(100 mL) and the product extracted into carbon tetrachloride

A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
8239
(6£150 mL). The organic layer was washed with saturated
NaHCO3(aq) (200 mL), brine (200 mL), dried and concen-
trated in vacuo to yield the title compound as a crude brown
oil. This was purified by flash chromatography on silica
(DCM/Et2O 4:1) to afford the title compound as an orange
oil (20.6 g, 96%): Rf 0.25 (DCM/Et2O 4:1); IR (NaCl plates)
H), 2979, 2932 (C–H), 1728 (CvO); dH (400 MHz, CDCl3)
1.27 (3H, t, J¼7.0 Hz, OCH2CH3), 1.29 (3H, s, exo-methyl
of C(CH3)2), 1.64 (3H, s, endo-methyl of C(CH3)2), 2.11–
2.21 (1H, dtd, J¼12.0, 7.5, 1.0 Hz, endo-H of SCHCH2),
2.49 (1H, d, J¼4.0 Hz, OH), 2.86–2.92 (1H, ddd, J¼12.0,
7.5, 2.0 Hz, exo-H of SCHCH2), 3.46 (1H, q, J¼7.5 Hz,
SCH), 3.58 (1H, d, J¼10.0 Hz, CHCO2Et), 3.73–3.82 (1H,
m, SCHCH), 4.09–4.29 (2H, m, OCH2CH3), 4.40–4.49
(1H, qd, J¼7.5, 4.0 Hz, CHOH); dC (100.63 MHz, CDCl3)
14.25 (OCH2CH3), 28.18 (endo-methyl of C(CH3)2), 28.55
(exo-methyl of C(CH3)2), 34.36 (SCH), 42.43 (SCHCH2),
54.48 (SCHCH), 57.22 (CHCO2Et), 60.08 (SC(CH3)2),
60.89 (OCH2CH3), 62.24 (CHOH), 172.09 (CvO); m/z
(APCIþ) 231 ([MH]þ, 43%), 213 ([M2H2O]þ, 100%), 187
([MH22OEt]þ, 85%).
n
max 2950 (w, CH), 1802 (s, cyclobutanone CvO), 1736 (s,
ester CvO); dH (400 MHz, CDCl3) 1.26 (3H, t, J¼7.0 Hz,
OCH2CH3), 1.39 (3H, s, endo-methyl of SC(CH3)2), 1.59
(3H, s, exo-methyl of SC(CH3)2), 3.34 (1H, d, J¼4.5 Hz,
CHCO2Et), 4.11–4.28 (2H, m, OCH2CH3), 4.62 (1H, d,
J¼8.0 Hz, SCH), 5.04 (1H, dd, J¼8.0, 4.5 Hz,
CHC(O)CCl2);
dC
(100.63 MHz, CDCl3) 14.20
(OCH2CH3), 27.68 (exo-methyl of SC(CH3)2), 28.31
(endo-methyl of SC(CH3)2), 57.65 (SCH), 60.19
(OCH2CH3), 63.08 (C(CH3)2), 69.07 (CHC(O)CCl2),
85.02 (CCl2), 169.23 (ester CvO), 193.85 (cyclobutanone
4.1.10. (1R,4S,5S,6S)- and (1S,4R,5R,6R)-3,3-Dimethyl-
4-ethoxycarbonyl-2-thiabicyclo[3.2.0]heptane-6-chloro-
formate (13). To a stirred solution of (1R,4S,5S,6S)- and
(1S,4R,5R,6R)-3,3-dimethyl-2-thiabicyclo[3.2.0]heptan-6-
ol-4-carboxylic acid ethyl ester (12) (3.00 g, 13.0 mmol) in
carbon tetrachloride (60 mL) was added a suspension of
triphosgene (1.58 g, 5.32 mmol) in pyridine (1.20 mL,
14.9 mmol) and carbon tetrachloride (60 mL). This solution
was stirred at 508C for 3 h. The reaction mixture was cooled
to ambient temperature and DCM (100 mL) added. This was
washed with water (2£50 mL), brine (50 mL), dried and
evaporated to afford the title compound as a yellow brown
oil in quantitative yield. This was used without further
purification: Rf 0.30 (PE 40–60/Et2O 3:1); IR (NaCl Plates)
35
CvO); m/z (EIþ) 298 ([M37Clþ Cl]þ, 6%), 296
([M35Cl2]þ, 19%), 201 (36%), 163 (78%), 113 (75%).
4.1.8. (1S,4R,5R)- and (1R,4S,5S)-3,3-Dimethyl-2-thiabi-
cyclo[3.2.0]heptan-6-one-4-carboxylic acid ethyl ester
(11). To a stirred solution of (1R,4R,5R) and (1S,4S,5S)-
7,7-dichloro-3,3-dimethyl-2-thiabicyclo [3.2.0] heptan-6-
one-4-carboxylic acid ethyl ester (10) (19.5 g, 65.7 mmol)
in acetic acid (600 mL) was added zinc powder (18.0 g,
275 mmol). This mixture was heated under reflux for 15 h.
The mixture was filtered and the zinc residue washed with
acetic acid (100 mL). The filtrate was concentrated in
vacuo. The residue was dissolved in EtOAc (600 mL) and
washed with saturated NaHCO3(aq) (100 mL), brine
(100 mL), dried and concentrated in vacuo to afford the
title compound as an orange oil which was used without
further purification, (12.3 g, 82%): Rf 0.25 (DCM/Et2O 4:1);
IR (NaCl plates) nmax 2980 (w, CH), 1802 (s, cyclobutanone
CvO), 1735 (s, ester CvO); dH (400 MHz, CDCl3) 1.27
(3H, t, J¼7.0 Hz, OCH2CH3), 1.31 (3H, s, endo-methyl of
C(CH3)2), 1.57 (3H, s, exo-methyl of C(CH3)2), 2.94 (1H,
dt, J¼18.0, 3.0 Hz, endo-H of SCHCH2), 3.21 (1H, d,
J¼6.0 Hz, CHCO2Et), 3.44 (1H, ddd, J¼18.0, 7.5, 3.0 Hz,
exo-H of SCHCH2), 4.09–4.25 (3H, m, OCH2CH3, SCH),
4.72 (1H, m, SCHCH); dC (100.63 MHz, CDCl3) 14.22
(OCH2CH3), 27.47 (endo-methyl of C(CH3)2), 27.99 (exo-
methyl of C(CH3)2), 36.04 (SCH), 52.51 (SCHCH2), 60.94
(CHCO2Et), 61.25 (OCH2CH3), 64.27 (SC(CH3)2), 73.34
(SCHCH), 169.85 (ester CvO), 207.38 (cyclobutanone
CvO); m/z (APCIþ) 229 ([MH]þ, 5%), 183 ([M2OEt]þ,
10%), 155 ([M2CO2Et]þ, 5%), 201 (36%), 163 (78%), 113
(75%); HRMS: calculated for C11H17O3S [MH]þ:
229.0898; found: 229.0893.
n
max 2987 (s, CH), 1778 (s, CvO, chloroformate), 1731 (s,
CvO ester); dH (200 MHz, CDCl3) 1.30 (3H, t, J¼7.0 Hz,
OCH2CH3) 1.32 (3H, s, exo-methyl of C(CH3)2), 1.63 (3H,
s, endo-methyl of C(CH3)2), 2.43 (1H, dtd, J¼15.0, 7.0,
2.0 Hz, exo-H of SCHCH2), 2.97 (1H, dddd, J¼15.0, 8.0,
7.0, 3.0 Hz, endo-H of SCHCH2), 3.45 (1H, d, J¼9.5 Hz,
CHCO2Et), 3.53 (1H, q, J¼7.0 Hz, SCH), 3.92–4.07 (1H,
m, SCHCH), 4.21 (2H, q, J¼7.0 Hz, OCH2CH3), 5.26 (1H,
q, J¼8.0 Hz, SCHCH2CH); dC (400 MHz, CDCl3) 14.23
(OCH2CH3), 27.93 (endo-methyl of C(CH3)2), 28.46 (exo-
methyl of C(CH3)2), 34.78 (SCH), 40.52 (SCHCH2), 53.00
(SCHCH), 60.70 (SC(CH3)2), 61.12 (OCH2CH3), 70.88
(SCHCH2CH), 149.41 (chloroformate CvO), 170.12 (ester
CvO).
4.1.11. (1R,4S,5S,6S)- and (1S,4R,5R,6R)-3,3-Dimethyl-
4-ethoxycarbonyl-2-thiabicyclo[3.2.0]heptane-6-azido-
formate (14). To a stirred solution of (1R,4S,5S,6S)- and
(1S,4R,5R,6R)-3,3-dimethyl-4-carbonyl-2-thiabicyclo[3.2.0]
heptane-6-chloroformate (13) (3.80 g, 13.0 mmol) in DMF
(20 mL) was added NaN3 (4.23 g, 65.1 mmol). This solution
was stirred at 508C for 2 h, then allowed to cool to ambient
temperature and then concentrated in vacuo. The product
was dissolved in EtOAc (75 mL) and the organic solution
was washed with water (2£20 mL) and brine (20 mL), dried
and concentrated in vacuo. The residue was purified by flash
chromatography on silica (PE 40–60/Et2O 15:1) to afford
the title compound as a yellow oil which crystallised on
standing. (2.6 g, 67%): Rf 0.65 (PE 40–60/Et2O 3:1); mp
31.5–32.08C; IR (NaCl plates) nmax 2978 (s, CH), 2186,
2136 (N3), 1731 (s, br, CvO azidoformate and CvO ester);
dH (500 MHz, CDCl3) 1.28 (3H, t, J¼7.5 Hz, OCH2CH3),
1.31 (3H, s, exo-methyl of C(CH3)2), 1.61 (3H, s,
4.1.9. (1S,4R,5R,6R)- and (1R,4S,5S,6S)-3,3-Dimethyl-2-
thiabicyclo[3.2.0]heptan-6-ol-4-carboxylic acid ethyl
ester (12). To a stirred solution of (1S,4R,5R) and
(1R,4S,5S)-3,3-dimethyl-2-thiabicyclo[3.2.0]heptan-6-one-
4-carboxylic acid ethyl ester (11) (12.6, 55.2 mmol) in
MeOH (200 mL) at 08C was added sodium borohydride
(2.28 g, 60.2 mmol). The solution was allowed to warm to
ambient temperature and stirred for a further 5 h. The
product was adsorbed onto silica by solvent evaporation and
purified by flash chromatography (DCM/Et2O 8:1) to afford
the title compound as a pale yellow oil (10.5 g, 83%): Rf
0.25 (DCM/Et2O 8:1); IR (NaCl plates) nmax 3448 (s, br, O–

8240
A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
endo-methyl of C(CH3)2), (1H, dtd, J¼13.0, 8.0, 1.0 Hz,
exo-H of SCHCH2), 3.03 (1H, dtd, J¼13.0, 7.5, 3.0 Hz,
endo-H of SCHCH2), 3.44 (1H, d, J¼10.5 Hz, CHCO2Et),
3.48 (1H, q, J¼7.5 Hz, SCH), 3.99 (1H, m, SCHCH), 4.13–
4.24 (2H, q, J¼7.5 Hz, OCH2CH3), 5.18 (1H, q, J¼8.0 Hz,
SCHCHCH); dC (100.63 MHz, CDCl3) 14.1 (OCH2CH3),
27.1 (endo-methyl of C(CH3)2), 28.4 (exo-methyl of
C(CH3)2), 34.9 (SCH), 40.6 (SCHCH2), 52.8 (SCHCH),
60.8 (SC(CH3)2), 61.1 (OCH2CH3), 67.4 (SCHCH2CH),
156.0 (azidoformate CvO), 170.2 (ester CvO); m/z
(CI(NH3)) 317 ([MNH4]þ, 100%), 300 ([MH]þ, 15%),
213 (80%), 113 (30%).
4.43–4.48 (1H, ddd, J¼9.0, 5.0, 2.0 Hz, SCHCHCHCO2Et),
4.79–4.83 (1H, ddd, J¼7.0, 5.0, 2.0 Hz, CHNBOC), 5.12–
5.16 (1H, ddd, J¼7.0, 6.5, 2.0 Hz, CHOC(O)); dC
(100.63 MHz, (CD3)2C(O)) 14.02 (OCH2CH3), 27.4 (exo-
methyl of C(CH3)2), 27.91 (C(CH3)3), 28.6 (endo-methyl of
C(CH3)2), 47.0 (SCH), 50.3 (SCHCHCHCO2Et), 55.2
(SCHCHN), 57.4 (CHCHCO2Et), 59.7 (SC(CH3)2), 61.2
(OCH2CH3), 69.6 (OCH), 82.9 (C(CH3)3), 148.8 (BOC
CvO), 153.2 (CvO), 170.0 (CvO); m/z (CIþ) 389
([MNH4]þ, 10%), 333 (15%), 289 (100%), 272 (35%), 186
(27%), 113 (23%); HRMS calculated for C17H29N2O6S
[MNH4]þ: 389.1746; found 389.1752.
4.1.12. (1S,2R,6R, 7S,10S)- and (1R,2S,6S,7R,10R)-10-
Ethoxycarbonyl-5-aza-9,9-dimethyl-3-oxo-8-thiatricy-
clo[5.3.0.0]decan-4-one (16). A stirred volume of dry TCE
(700 mL) was degassed using a flow of argon through a
glass pipette for 0.5 h. This was heated under reflux at
4.1.14. (1S,4S,5S,6R,7R)- and (1R,4R,5R,6S,7S)-7-(tert-
Butyloxycarbonylamino)-3,3-dimethyl-2-thiabicyclo
[3.2.0]heptan-6-ol-4-carboxylic acid ethyl ester (18). To a
stirred solution of (1R,2S,6S,7R,10R)- and (1S,2R,6R,
7S,10S)-10-ethoxycarbonyl-9,9-dimethyl-5-(tert-butyloxy-
1478C. To this
a
solution of (1R,4S,5S,6S)- and
carbonyl-3-oxo-8-thiatricyclo[5.3.0.0]decan-4-one
(17),
(1S,4R,5R,6R)-3,3-dimethyl-4-ethoxycarbonyl-2-thiabicyclo
[3.2.0]heptane-6-azidoformate (14) (250 mg, 0.84 mmol) in
dry TCE (10 mL) was added dropwise over 3 min and the
solution was stirred for a further 15 min. The solution was
then cooled in an ice bath and concentrated in vacuo to yield
a brown oil. This was purified by flash chromatography on
silica (DCM/Et2O 4:1) to afford the title compound as a
white solid (63 mg, 28%): Rf 0.25 (PE 40–60/Et2O 1:1); mp
135–135.58C; IR (NaCl plates) nmax 3361 (m, NH), 2970
(m, CH), 1744 (CvO), 1717 (s, CvO); dH (500 MHz,
CDCl3) 1.29 (6H, t and s, J¼7.0 Hz, OCH2CH3 and exo-
methyl of C(CH3)2), 1.70 (3H, s, endo-methyl of C(CH3)2),
3.40 (1H, d, J¼6.0 Hz, CHCO2Et), 4.05–4.37 (4H, m, SCH,
SCHCHCHCO2Et, OCH2CH3), 4.32 (1H, dddd J¼7.5, 4.0,
2.0, 0.5 Hz, CHNH), 5.17 (1H, dt, J¼6.0, 2.0 Hz,
CHO(CO)NH), 5.21 (1H br s, NH); dC (100.63 MHz,
CDCl3) 14.2 (OCH2CH3), 27.4 (exo-methyl of C(CH3)2),
28.5 (endo-methyl of C(CH3)2), 47.0 (SCH), 51.2
(SCHCHCHCO2Et), 52.7 (SCHCHNH), 57.0 (CHCHCO2-
Et), 60.3 (SC(CH3)2), 61.1 (OCH2CH3), 72.6 (OCH), 159.9
(urethane CvO), 170.0 (ester CvO); m/z (APCIþ) 272
([MH]þ 100%), 226 (40%) 198 (25%); HRMS calculated
for C12H17NO4S [MH]þ: 271.0878; found 271.0877.
(50 mg, 0.135 mmol) in ethanol (5 mL) was added caesium
carbonate (9 mg, 27.6 mmol). This solution was stirred at
ambient temperature for 3 h. The volatile components were
removed in vacuo and the residue dissolved in water (2 mL)
and neutralised to pH 7 with citric acid (5%, aq). The
product was extracted into DCM (3£15 mL), dried and
concentrated in vacuo. The residue was purified by flash
chromatography on silica (DCM/Et2O 3:1) to afford the title
compound (42.0 mg, 90%): Rf 0.45 (DCM/Et2O 3:1); IR
(NaCl plates) nmax 3429 (br, NH, OH) 2980 (w, CH), 2961,
2925, (s, CH), 1729 (s, CvO ester) 1694 (s, urethane
CvO); dH (400 MHz, CDCl3) 1.28 (6H, t, J¼7.0 Hz,
OCH2CH3 and s, exo-methyl of C(CH3)2), 1.50 (9H, s,
C(CH3)3), 1.59 (3H, s, endo-methyl of C(CH3)2), 2.89 (1H,
d, J¼4.5 Hz, CHOH), 3.33 (1H, d, J¼8.5 Hz, CHCO2Et),
3.74 (1H, q, J¼8.5 Hz, SCHCHCHCO2Et), 3.93– 4.16 (4H,
m, OCH2CH3, CHNH, SCH), 4.52 (1H, q, J¼6.0 Hz,
CHOH), 5.12–5.16 (1H, br s NH); dC (100.63 MHz, CDCl3)
14.22 (OCH2CH3), 27.82 (exo-methyl of C(CH3)2) 28.31
(endo-methyl of C(CH3)2), 28.47 (C(CH3)3), 43.75 (SCH),
50.03 (CHNH), 50.88 (SCHCH), 57.98 (CHCO2Et), 59.45
(SC(CH3)2), 61.27 (OCH2CH3), 65.75 (HOCH), 79.84
(C(CH3)3), 155.86 (urethane CvO), 170.91 (ester CvO);
m/z (CIþ) 346 ([MH]þ, 10%), 290 (100%), 246 (40%),
120 (28%); HRMS calculated for C16H28N1O5S
[MH]þ:346.1671; found 346.1688.
4.1.13. (1S,2R,6R,7S,10S)- and (1R,2S,6S,7R,10R)-10-
Ethoxycarbonyl-9,9-dimethyl-5-(tert-butyloxycarbonyl-
3-oxo-8-thiatricyclo[5.3.0.0]decan-4-one (17). To a stirred
solution of (1R,2S,6S,7R,10R)- and (1S,2R,6R,7S,10S)-
10-ethoxycarbonyl-5-aza-9,9-dimethyl-3-oxo-8-thiatricyclo
[5.3.0.0]decan-4-one (16) (238 mg, 0.88 mmol) in THF
(24 mL) were added di-tert-butyl dicarbonate (396 mg,
1.81 mmol), triethylamine (0.25 mL, 1.79 mmol) and
4-(dimethylamino)pyridine (33.4 mg, 0.28 mmol). This
solution was stirred at ambient temperature for 4 h and then
concentrated in vacuo. The residue was dissolved in EtOAc
(40 mL), washed with water (10 mL), brine (10 mL), dried
and concentrated in vacuo to afford the title compound as a
brown solid. This was triturated with hexane to yield an off-
white solid (258 mg, 79%): Rf 0.25 (PE 40–60/Et2O 1:1); IR
(NaCl plates) nmax 2980 (m, CH), 1809 (s, urethane CvO)
1734 (s, ester CvO); dH (500 MHz, (CD3)2C(O) 1.28 (6H, t,
J¼7.0 Hz, OCH2CH3, exo–methyl of C(CH3)2), 1.50 (9H, s,
C(CH3)3), 1.59 (3H, s, endo-methyl of C(CH3)2), 3.12 (1H, d,
9.0 Hz, CHCO2Et), 4.28–4.06 (3H, m, SCH, OCH2CH3),
4.1.15. (1R,4R,5R,6S,7S)- and (1S,4S,5S,6R,7R)-7-(tert-
Butyloxycarbonylamino)-3,3-dimethyl-2-thiabicyclo
[3.2.0]heptan-6-ol-4-carboxylic acid (19). To a stirred
solution of (1S,4S,5S,6R,7R)- and (1R,4R,5R,6S,7S)-7-(tert-
butyloxycarbonylamino)-3,3-dimethyl-2-thiabicyclo[3.2.0]
heptan-6-ol-4-carboxylic acid ethyl ester (18) (88.0 mg,
0.255 mmol) in DMF (2 mL) was added a solution of
LiOH·H2O (18.0 mg, 0.43 mmol) in water (2 mL). This
solution was stirred at 708C for 18 h and was then acidified
to pH 2 with HCl (1 M, aq) and extracted into EtOAc
(3£10 mL). The organic layer was extracted into saturated
NaHCO3(aq) (3£10 mL). This was then acidified with HCl
(1 M, aq) and extracted into EtOAc (3£10 mL). The organic
layer was dried and concentrated in vacuo to afford the title
compound as a white solid (60.7 mg, 75%): Rf 0.75 (Et2O);
mp (dec. 1508C); IR (NaCl plates) nmax 3382 (m, NH, OH),
2974, 2931, (s, CH), 1667 (br, s, urethane CvO and CvO

A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
8241
acid); dH (500 MHz, CD3OD) 1.28 (3H, s, exo-methyl of
C(CH3)2), 1.43, (9H, s, C(CH3)3), 1.60 (3H, s, endo-methyl
of C(CH3)2), 3.43 (1H, d, J¼7.0 Hz, CHCO2H), 3.75 (1H, q,
J¼7.0 Hz, SCHCHCHCO2H), 4.02 (1H, t, J¼7.0 Hz, SCH),
4.17 (1H, t, J¼6.0 Hz, CHNH), 4.56 (1H, t, J¼7.0 Hz,
CHOH); dC (100.63 MHz, CD3OD) 26.99 (endo-methyl of
C(CH3)2), 27.62 (C(CH3)3), 28.02 (exo-methyl of C(CH3)2),
45.05 (SCH), 51.19 (CHNH), 51.70 (SCHCH), 58.61
(CHCO2H), 60.3 (SC(CH3)2), 64.19 (CHOH), 79.53
(C(CH3)3), 156.39 (urethane CvO), 173.49 (acid CvO);
m/z (APCI2) 316.4 ([M2H]2, 100%) 242 (10%), 216
(10%), 113 (35%); HRMS calculated for C14H23NO5S
[MH]þ: 318.1375; found 318.1369.
s, CH3C6H4), 3.86 (1H, d, J¼7.5 Hz, CHCO2CHPh2), 3.94
(1H, td, J¼7.0, 2.0 Hz, CHNHþ3 ), 4.03 (1H, qd, J¼7.5,
2.0 Hz, SCHCH), 4.12 (1H, dt, J¼7.0, 2.0 Hz, SCH), 4.69
(1H, dt, J¼7.0, 2.0 Hz, CHOH), 6.89 (1H, s, CHPh2), 7.20
(2H, d, J¼8.0 Hz, CH3(C6H4)SO3), 7.20–7.50 (10H,
CHPh2), 7.72 (2H, d, J¼8.0 Hz, CH3(C6H4)SO3); dC
(100.63 MHz, CD3OD) 20.0 (CH3(C6H4), 26.15 (endo-
methyl of C(CH3)2), 27.0 (exo-methyl of C(CH3)2), 40.35
(SCH), 50.79 (SCHCH), 53.14 (CHNH), 56.94 (CHCO2-
CHPh2), 61.55 (CHOH), 78.0 (CHPh2), 125.97, 126.69,
127.76, 127.85, 128.26, 128.47, 128.57, 128.79 (CAr),
170.1 ester (CvO); Found C, 62.28%; H, 6.02%.
C29H33NO6S2 requires C, 62.28%, H, 5.99%; m/z (ESþ)
384 ([M2pTsO2]þ, 3%), 167 (100%); HRMS calculated
for C22H26NO3S [M2pTsO2]þ: 384.1633; found
384.1633.
4.1.16. (1S,4S,5S,6R,7R)- and (1R,4R,5R,6S,7S)-7-(tert-
Butyloxycarbonylamino)-3,3-dimethyl-2-thiabicyclo
[3.2.0]heptan-6-ol-4-carboxylic acid diphenylmethyl
ester (20). To a stirred solution of (1S,4S,5S,6R,7R)-
and (1R,4R,5R,6S,7S)-7-(tert-butyloxycarbonylamino)-3,3-
dimethyl-2-thiabicyclo[3.2.0]heptan-6-ol-4-carboxylic acid
(19) (149 mg, 0.47 mmol) in acetonitrile (3 mL) was added
a solution of diphenyldiazomethane (91.2 mg, 0.47 mmol)
in acetonitrile (3 mL). This solution was stirred at ambient
temperature for 1 h. The volatile components were removed
in vacuo. The residue was taken up into EtOAc (20 mL) and
acetic acid (0.1 mL) added to quench any residual
diphenyldiazomethane. The mixture was washed with
water (2£2 mL), saturated NaHCO3(aq) (2 mL), brine
(2 mL), dried and concentrated in vacuo. The residue was
purified by flash chromatography on silica (DCM/Et2O 3:1)
to yield the title compound as a pale yellow oil (150 mg,
66%): Rf (DCM/Et2O) 0.3; IR (NaCl plates) nmax 3389 (s,
OH), 2975 (CH), 1713 (ester CvO), 1689 (urethane CvO);
dH (400 MHz, CDCl3) 1.12 (3H, s, exo-methyl of C(CH3)2),
1.44 (9H, s, C(CH3)3), 1.61 (3H, s, endo-methyl of
C(CH3)2), 2.55–2.62 (1H, br s, OH), 3.55 (1H, d,
J¼9.0 Hz, CHCO2CHPh2), 3.82 (1H, qd, J¼9.0, 1.0 Hz,
SCHCH), 4.07 (1H, br t, J¼7.0 Hz, SCH), 4.12 (1H, br m,
CHNH), 4.57 (1H, t, J¼7.0 Hz, CHOH), 5.50 (1H, br d,
J¼4.5 Hz, NH), 6.89 (1H, s, CHPh2), 7.30–7.40 (10H, m,
CHPh2); dC (400 MHz, CDCl3) 27.75 (endo-C of C(CH3)2),
28.32 (exo-C of C(CH3)2 and C(CH3)3), 43.71 (SCH), 51.11
(CHNH), 53.72 (SCHCH), 57.17 (CHCO2CHPh2), 59.63
(C(CH3)2), 65.79 (CHOH), 77.66 (CHPh2), 126.72, 127.64,
127.83, 128.15, 128.47, 128.54 (CAr), 139.85 (urethane
CvO), 170.02 (ester CvO); m/z (ESþ) 506 ([MNa]þ,
100%), 484 ([MH]þ, 25%), 428 (7%); HRMS calculated for
C27H33NO5S23Na: 506.1977; found 506.1990.
4.1.18. (1S,4S,5S,50R,6R,7R)- and (1R,4R,5R,50R,6S,7S)-
7-[50(tert-Butyloxycarbonylamino)-50-(p-methoxybenzyl-
oxycarbonyl)pentanamido]-3,3-dimethyl-2-thiabicyclo
[3.2.0]heptan-6-ol-4-carboxylic acid diphenylmethyl
ester (23). To a stirred solution of (1S,4S,5S,6R,7R)- and
(1R,4R,5R,6S,7S)-7-amino-3,3-dimethyl-2-thiabicyclo[3.2.0]
heptan-6-ol-4-diphenylmethyl ester p-toluenesulfonic acid
(21) (160 mg, 0.29 mmol) in DCM (1 mL) was added a
solution of N-tert-butyloxycarbonyl-D-a-aminoadipic acid
a-p-methoxybenzyl ester13 (113 mg, 0.29 mmol) in DCM
(2 mL) and a solution of 1-(3-dimethylaminopropyl)-3-
ethylcarbodiimide hydrochloride (EDCI) (71.6 mg,
0.37 mmol) and 1-hydroxybenzotriazole (HOBt) (65 mg,
0.48 mmol) in DCM (2 mL). Triethylamine (101 mL,
0.72 mmol) was added dropwise and the mixture stirred at
ambient temperature for 15 h. The volatile components
were removed in vacuo and the residue dissolved in EtOAc
(10 mL). This was washed with HCl (5%, aq, 2 mL),
saturated NaHCO3(aq) (2 mL), brine (2 mL), dried and
concentrated in vacuo to yield a pale yellow oil which was
purified by flash chromatography on silica (DCM/Et2O 3:1)
to yield a white foam (177 mg, 82%): Rf 0.3 (DCM/Et2O
3:1); IR (NaCl plates) nmax 3365 (m, NH), 2972 (s, CH),
1731 (s, br, ester CvO), 1651 (amide CvO); dH (400 MHz,
CDCl3) 1.10 (6H, s, exo-methyl of C(CH3)2 diastereomers A
and B), 1.40 (18H, s, C(CH3)3 diastereomers A and B), 1.61
(6H, s, endo-methyl of C(CH3)2, diastereomers A and B),
1.62–1.73 (6H, m, NHCHCH2 diastereomers A and B and
NHCHCH2CH2 diastereomer A), 1.74–1.90 (2H, br m,
NHCHCH2CH2 diastereomer B) 2.20–2.42 (4H, m,
NHCHCH2CH2CH2 diastereomers A and B) 3.56 (2H, 2d,
J¼9.0, 7.0 Hz, CHCO2CHPh2 diastereomers A and B) 3.80
(6H, s, OCH3 diastereomers A and B) 3.87 (2H, q, 8.0 Hz,
SCHCH diastereomers A and B), 4.03 (2H, t, J¼8.0 Hz,
4.1.17. (1S,4S,5S,6R,7R)- and (1R,4R,5R,6S,7S)-7-
Amino-3,3-dimethyl-2-thiabicyclo[3.2.0]heptan-6-ol-4-
carboxylic acid diphenylmethyl ester p-toluenesulfonic
acid salt (21). To a stirred solution of (1R,4R,5R,6S,7S)-
and (1S,4S,5S,6R,7R)-7-(tert-butyloxycarbonylamino)-3,3-
dimethyl-2-thiabicyclo[3.2.0]heptan-6-ol-4-carboxylic acid
diphenylmethyl ester (20) (149 mg, 0.31 mmol) in MeCN
(3 mL) was added p-toluene sulfonic acid monohydrate
(59 mg, 0.31 mmol). This solution was stirred at ambient
temperature for 10 h. Ether (5 mL) was added and the
tosylate salt filtered and dried in a dessicator to yield the title
compound as a white solid (102 mg, 59%): Rf (DCM/EtOAc
3:1) 0.05; dH (400 MHz, CD3OD) 1.11 (3H, s, exo-methyl of
C(CH3)2), 1.60 (3H, s, endo-methyl of C(CH3)2), 2.35 (3H,
t
SCH diastereomers A and B), 4.30 (2H, br m, BuO-
CONHCH diastereomers
A and B), 4.42 (2H, m,
SCHCHNH diastereomers A and B), 4.60 (2H, br m, CHOH
diastereomers A and B), 5.06 (4H, m, pMeO(C6H4)CH2
diastereomers A and B), 5.23 (2H, m, NHCHCH2, diastereo-
mers A and B), 6.46 (2H, m, NHCHCHOH diastereomers A
and B), 6.88 (6H, m, 2H from CHPh2 diastereomers A and B
and 4H from pMeO(C6H4)CH2 diastereomers A and B),
7.21–7.40 (24H, m, 20H from CHPh2 and 4H from
pMeO(C6H4)CH2 diastereomers A and B); dC (400 MHz,
CDCl3) 21.41 (NHCHCH2CH2), 27.87 (endo-methyl of
C(CH3)2), 28.31 (exo-methyl of C(CH3)2 and C(CH3)3), 31.85

8242
A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
(NHCHCH2CH2CH2), 35.36, 35.46 (NHCHCH2CH2CH2),
43.07, 43.44 (SCH), 49.26 (SCHCHNH), 50.97, 51.20
(NHCHCH2), 53.23 (SCHCHCHCO2CHPh2), 55.27
(OCH3), 57.17 (CHCO2CHPh2) 59.67, 59.72 (C(CH3)2),
65.28 (pMeO(C6H4)CH2) 65.40 (CHOH), 66.94 (C(CH3)3),
77.66 (CHPh2), 113.95 (two of pMeO(C6H4)CH2), 126.69,
127.48, 127.63, 127.82, 128.15, 128.47, 130.19, 139.79,
139.85 (CAr), 159.72 (urethane CvO), 169.77 (amide CvO),
172.47, 172.56 (2£ester CvO); m/z (ESþ) 747 ([MH]þ,
100%), 610 (10%); HRMS calculated for C41H51N2O9S:
747.3315; found 747.3298.
4.1.20. (1S,4S,5S,50R,7R)- and (1R,4R,5R,50R,7S)-7-[50-
Amino-50-carboxypentanamido]-3,3-dimethyl-2-thiabi-
cyclo[3.2.0]heptan-6-one-4-carboxylate (1a/1b). To a
so0lution of (1S,4S,5S,50R,7R)- and (1R,4R,5R,50R,7S)-7-
[5 (tert-butyloxycarbonylamino)-50-( p-methoxybenzyloxy-
carbonyl)pentanamido]-3,3-dimethyl-2-thiabicyclo[3.2.0]-
heptan-6-one-4-carboxylic acid diphenylmethyl ester (24)
(19 mg, 0.026 mmol) in TFA/anisole/toluene (3:1:20)
(7 mL) was stirred for 0.5 h at ambient temperature. This
solution was concentrated in vacuo and the residue
dissolved in H2O (10 mL). The aqueous solution was
washed with EtOAc (3£2 mL) and lyophilised to yield the
title compound as a white solid (6.60 mg, 71%): dH
(400 MHz, D2O) 1.28 (2£3H, s, exo-methyl of C(CH3)2
diastereomers A and B), 1.47 (2£3H, s, endo-methyl of
C(CH3)2 diastereomers A and B), 1.50–1.68 (2£2H, m,
NHCHCH2CH2 diastereomers A and B), 1.68–1.87 (2£2H,
m, NHCHCH2CH2 diastereomers A and B), 2.24–2.34
(2£2H, m, NHCHCH2CH2CH2 diastereomers A and B),
3.10 (2£1H, 2d, J¼3.5 Hz, CHCO2H diastereomers A and
B), 3.80 (2£1H, t, J¼7.0 Hz, CH(NH)CH2 diastereomers A
and B), 4.47–4.58 (2£2H, m, SCHCHCO2H diastereomers
A and B), 5.32–5.41 (2£1H, 2d, J¼2.5 Hz, SCHCHNH
diastereomers A and B); dC (100.63 MHz, D2O) 21.1
(NHCHCH2CH2), 27.2 (exo-C of C(CH3)2), 28.2, (endo-C
of C(CH3)2), 29.8 (NHCHCH2CH2CH2), 34.6 (NHCHCH2-
CH2CH2), 44.8 (SCH), 54.20 (CH(NH)CH2), 60.1
(CHCO2H), 62.7 (C(CH3)2), 63.1 (SCHCHNH), 67.2
(SCHCHCHCO2H), 174.3 (amide CvO), 175.7 (2£acid
CvO), 208.4, 208.9 (cyclobutanone CvO); m/z (ES-) 357
([M2H]2, 100%), 283 (9%), 279 (10%), 255 (10%); HRMS
calculated for C15H21N2O6S: 357.1120; found 357.1137.
4.1.19. (1R,4R,5R,50R,7S)- and (1S,4S,5S,50R,7R)-7-[50-
(tert-Butyloxycarbonylamino)-50-(p-methoxybenzyloxy-
carbonyl)pentanamido]-3,3-dimethyl-2-thiabicyclo-
[3.2.0]heptan-6-one-4-carboxylic acid diphenylmethyl
ester (24). To a stirred solu0tion of (1S,4S,5S,50R,6R,7R)-
and (1R,4R,5R,50R,6S,7S)-7-[5 (tert-butyloxycarbonylamino)-
50-( p-methoxybenzyloxycarbonyl)pentanamido]-3,3-di-
methyl-2-thiabicyclo[3.2.0]heptan-6-ol-4-carboxylic acid
diphenylmethyl ester (23) (89 mg, 0.12 mmol) in DMSO
(2 mL) was added o-iodoxybenzoic acid (IBX) (36.7 mg,
0.13 mmol) in one portion. This was stirred at room
temperature for 1.5 h. Water (5 mL) was added and the
product extracted into ethyl acetate (3£20 mL). The organic
layer was washed with saturated NaHCO3(aq) (3£5 mL) and
brine (2£5 mL). The organic layer was dried over sodium
sulfate and then concentrated in vacuo to afford the title
compound as a pale yellow foam. This was purified by flash
chromatography on silica (DCM/Et2O 4:1) to afford the title
compound as an off-white foam (80 mg, 90%): Rf 0.65
(DCM/Et2O 3:1); IR (NaCl plates) nmax 3315 (m, NH),
2965, 2950 (s, CH), 1789 (s, cyclobutanone CvO), 1731 (s,
br, ester CvO); dH (400 MHz, C6D6) 1.05 (6H, s, exo-
methyl of C(CH3)2, diastereomers A and B), 1.40 (18H, s,
C(CH3)3, diastereomers A and B), 1.43 (6H, s, endo-methyl
of C(CH3)2, diastereomers A and B), 1.48–1.52 (12H, br m,
NHCHCH2CH2CH2 diastereomers A and B), 3.30 (6H, s,
OCH3 diastereomers A and B), 3.47 (2H, 2d, J¼4.0 Hz,
CHCO2CHPh2 diastereomers A and B), 3.86–3.98 (2H, br t,
SCH diastereomers A and B), 4.28–4.31 (1H, br m,
SCHCHCHCO2), 4.42–4.55 (2H, m NHCHCH2 diastereo-
mers A and B), 4.80–5.10 (4H, br, m, pMeO(C6H4)CH2
diastereomers A and B), 5.09–5.18 (2H, q, J¼10.0 Hz,
CH2NHCCO2 diastereomers A and B), 5.40–5.51 (2H, q,
J¼9.0 Hz, SCHCHNH diastereomers A and B), 5.77–5.93
(2H, 2d, J¼8.5, 9.0 Hz, NHCHCH diastereomers A and B),
6.75 (6H, m, 2H from CHPh2 diastereomers A and B and 4H
from pMeO(C6H4)CH2 diastereomers A and B), 6.91–7.43
(24H, m, 20H from CHPh2 and 4H from pMeO(C6H4)CH2
diastereomers A and B); dC (100.63 MHz, CDCl3) 21.14
(NHCHCH2CH2), 27.51, 28.25, 28.32 (exo and endo-C of
C(CH3)2 and C(CH3)3), 32.01 (NHCHCH2CH2CH2), 35.09
(NHCHCH2CH2CH2), 45.24 (SCH), 55.27 (NHCHCH2),
59.57 (SCHCHCHCO2), 60.4 (C(CH3)2), 62.46 (C(CH3)3),
62.52 (SCHCHNH), 66.98 (pMeO(C6H4)CH2), 67.19
(SCHCHCHCO2CHPh2), 78.26 (CHPh2), 113.96 (two of
pMeO(C6H4)CH2), 126.67, 127.43, 127.78, 128.02, 128.40,
128.56, 128.63, 130.17 (16 CAr), 159.73 (CvO urethane),
168.97(CvO amide), 172.01, 172.46(2£ester CvO), 205.36
(cyclobutanone CvO); m/z (ESþ) 745 ([MH]þ, 100%), 689
(10%), 645 (5%), 610 (5%), 536 (8%); HRMS calculated for
C41H49N2O9S: 745.3152; found 745.3159.
4.1.21. (1S,4S,5S,50R,6R,7R)- and (1R,4R,5R,50R,6S,7S)-
7-[50-Amino-50-carboxypentanamido]-3,3-dimethyl-2-
thiabicyclo[3.2.0]heptan-6-ol-4-carboxylate (25). Method
1: To
a
solution of (1S,4S,5S,50R,7R)- and
(1R,4R,5R,50R,7S)-7-[50-amino-50-carboxypentanamido]-3,3-
dimethyl-2-thiabicyclo[3.2.0]heptan-6-one-4-carboxylate
(1a/1b) (6 mg, 0.017 mmol) in D2O (5 mL) was added
sodium borohydride (0.7 mg, 0.019 mmol). The product
was observed directly by NMR.
Method 2: (1S,4S,5S,50R,6R,7R)- an0d (1R,4R,5R,50R,6S,7S)-
7-50(tert-butyloxycarbonylamino)-5 -(p-methoxybenzyloxy-
carbonyl)pentanamido]-3,3-dimethyl-2-thiabicyclo[3.2.0]-
heptan-6-ol-4-carboxylic acid diphenylmethyl ester (23)
(5 mg, 0.007 mmol) in a TFA/anisole/toluene (3:1:20)
(0.5 mL) was stirred for 0.5 h at ambient temperature. This
solution was concentrated in vacuo and the residue dissolved
in H2O(2 mL).ThisaqueoussolutionwaswashedwithEtOAc
(1£0.5 mL) and lyophilised to yield the title compound as a
white solid (1.2 mg, 48%).
For (25): dH (400 MHz, D2O) 1.12 (2£3H, s, exo-methyl of
C(CH3)2 diastereomers A and B), 1.48 (2£3H, s, endo-
methyl of C(CH3)2 diastereomers A and B), 1.50–1.73
(4£2H, m, NHCHCH2CH2 diastereomers A and B and
NHCHCH2CH2 diastereomers A and B), 2.02–2.37 (2£2H,
m, NHCHCH2CH2CH2 diastereomers A and B), 2.97
(2£1H, 2d, J¼4.5 Hz, CHCO2H diastereomers A and B),
3.30 (2£1H, br m, CH(NH)CH2 diastereomers A and B),
3.56 (2£1H, q, J¼8.0 Hz, SCHCHCO2H diastereomers A

A. C. Ferguson et al. / Tetrahedron 59 (2003) 8233–8243
8243
6. Dmitrienko, G. I.; Viswannatha, T.; Savard, M. E.; Lange, G.
Tetrahedron Lett. 1985, 26, 1791–1794.
and B), 3.97 (2£1H, t, J¼5.0 Hz, SCHCHCO2H diastereo-
mers A and B), 4.53 (2£1H, t, J¼7.0 Hz, SCHCHNH
diastereomers A and B).
7. Tomczuk, B. E. Diss. Abst. Int. B 1980, 41(2), 576–577.
8. Martyres, D. H.; Baldwin, J. E.; Adlington, R. M.; Lee, V.
Tetrahedron 2001, 57(23), 4999–5007.
9. Wuts, P. G. M.; Putt, S. R.; Ritter, A. R. J. Org. Chem. 1988,
53, 4503–4508.
Acknowledgements
We would like to thank the BBSRC for the studentships
(A. C. F.) and (D. H. M.). In addition, we would like to thank
Dr N. Oldham for mass spectrometry, Dr B. Odell for high
field NMR experiments.
10. Kobayashi, Y.; Taguchi, T.; Hosoda, A. Tetrahedron Lett.
1985, 26, 6209–6212.
11. McIntosh, J. M.; Hayes, I. E. E. Can. J. Chem. 1987, 65,
110–113.
12. Lowe, G.; Swain, S. J. Chem. Soc., Perkin Trans. 1 1985,
391–398.
13. Bodansky, M.; Bodansky, A. The practise of peptide synthesis.
Reactivity and Structure Concepts in Organic Chemistry;
Hafner, K., Rees, C. W., Trost, B. M., Lehn, J. M., Schleyer, R.
v. R., Zahruchik, R., Eds.; Springer: Berlin, 1984; Vol. 21.
14. Lau, R. M.; Van Eupen, J. T. H.; Schipper, D.; Tesser, G. I.;
Verweij, J.; Vroom, E. V. Tetrahedron 2000, 56, 7601–7606.
15. Rutledge, P. J.; D.Phil. Thesis, University of Oxford, 1999.
16. Lee, H.-J.; Lloyd, M. D.; Harlos, K.; Schofield, C. J. Biochem.
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