Cyclobutanone mimics of penicillins: Effects of substitution on conformation and hemiketal stability

Article abstract of DOI:10.1021/jo801274m

(Chemical Equation Presented) The tendency for carbocyclic analogues of penicillins to undergo hydrate and hemiketal formation is central to their ability to function as β-lactamase inhibitors. 2-Thiabicyclo[3.2.0]heptan- 6-one-4-carboxylates with alkoxy functionality at C3 have been prepared through two complementary diastereoselective substitution reactions following a highly stereoselective chlorination with sulfuryl chloride. We have found that carbocyclic analogues with 3β substituents favor an endo envelope conformation in solution, the solid state, and the gas phase, whereas those with 3α substituents adopt an exo envelope. Evidence from X-ray crystal structures and ab initio calculations suggests that an anomeric effect contributes to the large conformational preference of the tetrahydrothiophene ring that favors the C3 substituent in an axial orientation. In addition, the envelope conformation of the bicycle, which is determined by the stereochemistry of the C3 substituent, has a dramatic effect on the ability of the cyclobutanone to undergo hemiketal formation in methanol-d₄.

Full text of DOI:10.1021/jo801274m

Cyclobutanone Mimics of Penicillins: Effects of Substitution on
Conformation and Hemiketal Stability
Jarrod W. Johnson, Darryl P. Evanoff, Marc E. Savard, Gerald Lange,
Timothy R. Ramadhar, Abdeljalil Assoud, Nicholas J. Taylor,† and Gary I. Dmitrienko*
Department of Chemistry, UniVersity of Waterloo, 200 UniVersity AVenue West, Waterloo,
Ontario, Canada N2L 3G1
dmitrien@uwaterloo.ca
ReceiVed June 13, 2008
The tendency for carbocyclic analogues of penicillins to undergo hydrate and hemiketal formation is
central to their ability to function as ꢀ-lactamase inhibitors. 2-Thiabicyclo[3.2.0]heptan-6-one-4-
carboxylates with alkoxy functionality at C3 have been prepared through two complementary diastereo-
selective substitution reactions following a highly stereoselective chlorination with sulfuryl chloride. We
have found that carbocyclic analogues with 3ꢀ substituents favor an endo envelope conformation in
solution, the solid state, and the gas phase, whereas those with 3R substituents adopt an exo envelope.
Evidence from X-ray crystal structures and ab initio calculations suggests that an anomeric effect
contributes to the large conformational preference of the tetrahydrothiophene ring that favors the C3
substituent in an axial orientation. In addition, the envelope conformation of the bicycle, which is
determined by the stereochemistry of the C3 substituent, has a dramatic effect on the ability of the
cyclobutanone to undergo hemiketal formation in methanol-d4.
Introduction
ꢀ-Lactam antibiotics have been the most widely used
antibacterial agents for several decades, but the continued
effectiveness of these agents is threatened by increasing bacterial
resistance.1 This has become a serious problem worldwide and
is especially worrisome in hospital-acquired infections. The most
significant mode of resistance is the expression of ꢀ-lactamases,
enzymes that have the ability to efficiently inactivate ꢀ-lactams
through hydrolysis.2
These enzymes are divided into four classes on the basis of
sequence homology.3 The class A, C, and D ꢀ-lactamases rely
FIGURE 1. Hydrolysis of a penicillin by ꢀ-lactamases.
† Deceased November 2006.
(1) (a) Fisher, J. F.; Meroueh, S. O.; Mobashery, S. Chem. ReV. 2005, 105,
395–424. (b) Theuretzbacher, U.; Toney, J. H. Curr. Opin. InVest. Drugs 2006,
7, 158–166.
(2) (a) Livermore, D. M. Clin. Microbiol. ReV. 1995, 8, 557–584. (b) Jacoby,
G.; Bush, K. In Frontiers in Antimicrobial Resistance; White, D. G., Alekshun,
M. N., McDermott, P. F., Eds.; ASM Press: Washington, DC, 2005; pp 53-65.
(3) Ambler, R. P. Philos. Trans. R. Soc. London, Ser. B 1980, 289, 155–
165.
on an active-site serine residue as a nucleophile and achieve
turnover through the formation of an acyl enzyme intermediate
and subsequent hydrolytic deacylation (Figure 1). The zinc-
dependent class B enzymes, which have been termed metallo-
ꢀ-lactamases (MBLs), do not form a covalently bound acyl
6970 J. Org. Chem. 2008, 73, 6970–6982
10.1021/jo801274m CCC: $40.75 2008 American Chemical Society
Published on Web 08/19/2008

Cyclobutanone Mimics of Penicillins
FIGURE 2. Cyclobutanones as potential broad-spectrum inhibitors of ꢀ-lactamases.
enzyme intermediate since the active-site nucleophile is a zinc-
bound water (or hydroxide).4
ꢀ-lactamase or R61 transpeptidase.17 Meth-Cohn et al. synthe-
sized cyclobutanone 4 but did not disclose any biochemical or
biological data.18
One strategy for overcoming ꢀ-lactamase-mediated resistance
has involved the use of a ꢀ-lactamase inhibitor such as
clavulanate, sulbactam, or tazobactam, in combination with the
ꢀ-lactam antibiotic.5,6 Another successful approach has involved
the development of ꢀ-lactams, such as the extended-spectrum
penicillins and cephalosporins and the carbapenems, which are
less susceptible to ꢀ-lactamase hydrolysis and maintain broad
antibiotic activities. While therapies employing these ꢀ-lactams
have been successful against bacteria producing class A and
class C serine ꢀ-lactamases, they are less effective against those
producing the class D “OXA” enzymes7 and not effective
against the class B metallo-ꢀ-lactamase producers.8 More
importantly, the MBLs,9 some OXA-type ꢀ-lactamases,10 and
a clinically important group of class A enzymes called KPCs11
possess the ability to hydrolyze carbapenems,12 the ꢀ-lactams
that have been used as the last line of defense against multidrug
resistant organisms.13 Currently, there are no inhibitors of
metallo-ꢀ-lactamases available for clinical use.14
A subsequent publication by Cocuzza and Boswell described
the synthesis of several cyclobutanone derivatives, including 5
and 6, but none of the free acids demonstrated anti-ꢀ-lactamase
activity.19,20
Cyclobutanones. In the 1980s, several research groups
proposed independently that analogues of ꢀ-lactams in which
the ꢀ-lactam ring is replaced by a cyclobutanone system might
inhibit serine ꢀ-lactamases and DD-transpeptidases by virtue of
their ability to form an enzyme-bound hemiketal15 with an
active-site serine residue (Figure 2).16 As early as 1981, Gordon
et al. reported the synthesis of cyclobutanones 1-3 for this
purpose but found no significant inhibition of either R-TEM
Studies by Lowe and Swain, however, indicated that
2-oxabicyclo[3.2.0]heptanones 7 and 8 showed slow, time-
dependent inhibition of the class A ꢀ-lactamases R-TEM-2 and
BcI, and 8 demonstrated activity against R61 transpeptidase.21
Our laboratory described the preparation of dichlorocyclobu-
tanone 9,22 and Kelly et al. reported that 9 was a weak
competitive inhibitor (Ki ) 1 mM) of R61 transpeptidase.23-25
Although the preliminary biochemical data served as a proof
of principle for the concept that cyclobutanones might be
reasonable mimics of the penicillins, other more impressive
advances in the inhibition of class A ꢀ-lactamases, which led
to the clinically very useful clavulanic acid and sulbactam,
discouraged further explorations of the cyclobutanones in this
context.
(4) Crowder, M. W.; Spencer, J.; Vila, A. J. Acc. Chem. Res. 2006, 39, 721–
728.
(5) (a) Miller, L. A.; Ratnam, K.; Payne, D. J. Curr. Opin. Pharmacol. 2001,
1, 451–458. (b) Lee, N.; Yuen, K.-Y.; Kumana, C. R. Drugs 2003, 63, 1511–
1524.
(6) For reviews on ꢀ-lactamase inhibition, see: (a) Pratt, R. F. In The
Chemistry of ꢀ-Lactams; Page, M. I., Ed.; Blackie Academic and Professional:
Glasgow, 1992; pp 229-271. (b) Page, M. G. P. Drug Resist. Updates 2000, 3,
109–125. (c) Georgopapadakou, N. H. Expert Opin. InVest. Drugs 2004, 13,
1307–1318.
(7) Brown, S.; Amyes, S. J. Antimicrob. Chemother. 2006, 57, 1–3.
(8) (a) Jones, R. N.; Biedenbach, D. J.; Sader, H. S.; Fritsche, T. R.; Toleman,
M. A.; Walsh, T. R. Diagn. Microbiol. Infect. Dis. 2005, 51, 77–84. (b) Walsh,
T. R. Clin. Microbiol. Infect. 2005, 11 (Suppl. 6), 2–9.
(9) (a) Rasmussen, B. A.; Bush, K. Antimicrob. Agents Chemother. 1997,
41, 223–232. (b) Livermore, D. M.; Woodford, N. Curr. Opin. Microbiol. 2000,
3, 489–495. (c) Walsh, T. R.; Toleman, M. A.; Poirel, L.; Nordmann, P. Clin.
Microbiol. ReV. 2005, 18, 306–325.
(10) Walther-Rasmussen, J.; Høiby, N. J. Antimicrob. Chemother. 2006, 57,
373–383.
(11) (a) Ke, W.; Bethel, C. R.; Thomson, J. M.; Bonomo, R. A.; van den
Akker, F. Biochemistry 2007, 46, 5732–5740. (b) Naas, T.; Cuzon, G.; Villegas,
M.-F.; Quinn, J. P.; Nordmann, P. Antimicrob. Agents Chemother. 2008, 52,
1257–1263.
(12) Queenan, A. M.; Bush, K. Clin. Microbiol. ReV. 2007, 20, 440–458.
(13) Edwards, J. R.; Betts, M. J. J. Antimicrob. Chemother. 2000, 45, 1–4.
(14) (a) Toney, J. H. Curr. Opin. InVest. Drugs 2003, 4, 115–116. (b) Toney,
J. H.; Moloughney, J. G. Curr. Opin. InVest. Drugs 2004, 5, 823–826.
(15) The term hemiketal, once abandoned, has been reinstated in IUPAC
nomenclature. Moss, G. P.; Smith, P. A. S.; Tavernier, D. Pure Appl. Chem.
1995, 67, 1307–1375.
(17) Gordon, E. M.; Plusˇcˇec, J.; Ondetti, M. A. Tetrahedron Lett. 1981, 20,
1871–1874.
(18) Meth-Cohn, O.; Reason, A. J.; Roberts, S. M. J. Chem. Soc., Chem.
Commun. 1982, 90–92.
(19) Cocuzza, A. J.; Boswell, G. A. Tetrahedron Lett. 1985, 26, 5363–5366.
(20) Some sulfoxide- and sulfone-substituted cyclobutanones (n ) 1, 2) did
show inhibition of ꢀ-lactamase and synergy with penicillin G against S. aureus,
when in the form of their benzhydryl esters 6b. Boswell, G. A.; Cocuzza, A. J.
U.S. Patent 4,505,905, 1985; Chem. Abstr. 1985, 103, 141731.
(21) (a) Lowe, G.; Swain, S. J. Chem. Soc., Chem. Commun. 1983, 1279–
1281. (b) Lowe, G.; Swain, S. J. Chem. Soc., Perkin Trans. 1 1985, 391–398.
(22) Lange, G.; Savard, M. E.; Viswanatha, T.; Dmitrienko, G. I. Tetrahedron
Lett. 1985, 26, 1791–1794.
(23) Kelly, J. A.; Knox, J. R.; Moews, P. C.; Hite, G. J.; Bartolone, J. B.;
Zhao, H. J. Biol. Chem. 1985, 260, 6449–6458.
(24) The 4 Å resolution crystallographic data reported by Kelly et al.23 is
consistent with the binding of cyclobutanone 9 to the active site but did not
define the detailed structure of the enzyme-bound inhibitor.
(25) The sample of 9 employed in the study by Kelly et al.23 was prepared
by Tomczuk, B. E. Ph.D. Thesis, University of Connecticut, 1980; Diss. Abstr.
Int. B 1980, 41, 576-577.
(16) For a review of non-ꢀ-lactam mimics of ꢀ-lactams: Jungheim, L. N.;
Ternansky, R. J. In The Chemistry of ꢀ-Lactams; Page, M. I., Ed.; Blackie
Academic and Professional: Glasgow, 1992; pp 306-324.
(26) Bush, K.; Mobashery, S. In ResolVing the Antibiotic Paradox; Rosen,
B. P., Mobashery, S., Eds.; Kluwer Academic/Plenum Publishers: New York,
1998; Vol. 456.
J. Org. Chem. Vol. 73, No. 18, 2008 6971

Johnson et al.
intermediate, followed by addition of ethanol to the unsaturated
ketene generated.29
The deconjugated dihydrothiophene 17 was then subjected
to a fully regioselective [2 + 2] cycloaddition with dichloro-
ketene,30-32 which was generated in situ from dichloroacetyl
chloride and triethylamine. As dichlorocyclobutanones are
known to be sensitive to base-induced ring cleavage,17,33
conversion of 18 to the carboxylic acid 9 was accomplished
with acid catalysis. The ester 18 and the acid 9 were readily
dechlorinated with excess zinc dust in acetic acid to give
cyclobutanones 19 and 20, respectively, in good yield.
The emergence of serine and metallo-ꢀ-lactamases with a
broad spectrum of activity encompassing all known classes of
ꢀ-lactam antibiotics requires renewed efforts to establish strate-
gies with potential for overcoming this growing threat through
development of broad spectrum ꢀ-lactamase inhibitors that are
effective against both the serine and the metallo-ꢀ-lactamases.26
Cyclobutanones of the type indicated above have the potential
for formation of not only an enzyme-bound hemiketal in the
active site of serine ꢀ-lactamases but also an enzyme-bound
hydrate in the active site of a metallo-ꢀ-lactamase and hence
represent a core structure that might form the basis for the design
of broad spectrum ꢀ-lactamase inhibitors. Previous studies have
yielded only a superficial understanding of the properties of such
systems, which provides little guidance for renewed efforts to
exploit this potential. We report herein the results of a systematic
experimental and theoretical exploration of the influence of the
structure of such bicyclic cyclobutanones on their conformations
and on their tendencies to form hemiketals and hydrates, which
should assist the rational design of more effective ꢀ-lactamase
inhibitors based on this general strategy.
The cyclobutanones 9 and 18-20 were all found to prefer
an endo envelope in solution as indicated by the lack of coupling
1
between H4 and H5 in the H NMR spectra. Examination of
molecular models indicated that the dihedral angle between H4
and H5 would be close to 90° in this conformation only. X-ray
crystallographic analysis and ab initio molecular orbital calcula-
tions (RHF, vide infra) later confirmed this conformational
preference for acids 9 and 20 in the solid state (Figure 3) and
gas phase, respectively. NMR analysis of cyclobutanones 7 and
8 led Lowe and Swain to recognize that an endo envelope is
also preferred by the 2-oxa ring system.21
Hydrate and Hemiketal Formation. Several biochemical
studies since the 1970s have shown that peptide aldehydes34
and R-fluorinated ketones35,36 are potent inhibitors of serine-,
cysteine-, and metalloproteases37 and, more recently, metallo-
ꢀ-lactamases.38 Enzyme-bound tetrahedral adducts of these so-
Results and Discussion
The synthetic route used for the preparation of 2-thiabicyclo-
[3.2.0]heptan-6-one-4-carboxylates involved a [2 + 2] cycload-
dition with dichloroketene as previously reported by our group22
and later used by the Baldwin group for the synthesis of 10
and 11 (Scheme 1).27
(30) It is not yet clear whether the cyclobutanone-forming reaction proceeds
a concerted process or via a stepwise pathway. In any event, the
via
regioselectivity of the cycloaddition is compatible with that expected for a dipolar
transition-state in which partial positive charge is developed at C5 of the
dihydrothiophene and stabilized by interaction with the sulfur atom.
(31) In addition to the [2 + 2] cycloaddition involving the ketene CdC bond,
which would lead directly to the cyclobutanone product, recent studies have
identified the possibility of a competing [2 + 2] cycloaddition, which involves
the ketene CdO bond. The initially formed oxetane product then rearranges
through a zwitterionic intermediate to provide the cyclobutanone product. (a)
Machiguchi, T.; Okamoto, J.; Takachi, J.; Hasegawa, T.; Yamabe, S.; Minato,
T. J. Am. Chem. Soc. 2003, 125, 14446–14448. (b) Machiguchi, T.; Okamoto,
J.; Morita, Y.; Hasegawa, T.; Yamabe, S.; Minato, T. J. Am. Chem. Soc. 2006,
128, 44–45.
(32) For a broader recent overview of ketene chemistry including cycload-
ditions, see: Tidwell, T. T. In Ketenes, 2nd ed.; Wiley: New York, NY, 2006.
(33) For examples of dichlorocyclobutanone ring openings: (a) Ghosez, L.;
Montaigne, R.; Mollet, P Tetrahedron Lett. 1966, 135–139. (b) Brook, P. R.;
Duke, A. J. J. Chem. Soc. 1971, 1764–1769.
(34) (a) Westerik, J. O.; Wolfenden, R. J. Biol. Chem. 1972, 247, 8195–
8197. (b) Thompson, R. C. Biochemistry 1973, 12, 47–51. (c) Gorenstein, D. G.;
Shah, D. O. Biochemistry 1982, 21, 4679–4686 and references therein. (d) Shah,
D. O.; Gorenstein, D. G. Biochemistry 1983, 22, 6096–6101. (e) Stein, R. L.;
Strimpler, A. M. Biochemistry 1987, 26, 2611–2615.
(35) (a) Gelb, M. H.; Svaren, J. P.; Abeles, R. H. Biochemistry 1985, 24,
1813–1817. (b) Imperiali, B.; Abeles, R. H. Biochemistry 1986, 25, 3760–3767.
(36) Reiter, L. A.; Martinelli, G. J.; Reeves, L. A.; Mitchell, P. G. Bioorg.
Med. Chem. Lett. 2000, 10, 1581–1584 and references therein.
(37) For a review of protease inhibitors, see: Leung, D.; Abbenante, G.;
Fairlie, D. P. J. Med. Chem. 2000, 43, 305–341.
(38) (a) Walter, M. W.; Felici, A.; Galleni, M.; Soto, R. P.; Adlington, R. M.;
Baldwin, J. E.; Fre`re, J.-M.; Gololobov, M.; Schofield, C. J. Bioorg. Med. Chem.
Lett. 1996, 6, 2455–2458. (b) Walter, M. W.; Adlington, R. M.; Baldwin, J. E.;
Schofield, C. J. Tetrahedron 1997, 53, 7275–7290 and references therein.
(39) Transition-state analogues: (a) Wolfenden, R. Acc. Chem. Res. 1972, 5,
10–18. (b) Lienard, G. E. Science 1973, 180, 149–154.
(40) (a) Brayer, G. D.; Delbaere, L. T. J.; James, M. N. G.; Bauer, C.-A.;
Thompson, R. C. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 96–100. (b) Chen, R.;
Gorenstein, D. G.; Kennedy, W. P.; Lowe, G.; Nurse, D.; Schultz, R. M.
Biochemistry 1979, 18, 921–926. (c) Christianson, D. W.; Lipscomb, W. N. Proc.
Natl. Acad. Sci. U.S.A. 1985, 82, 6840–6844. (d) Christianson, D. W.; Lipscomb,
W. N. J. Am. Chem. Soc. 1986, 108, 4998–5003. (e) Christianson, D. W.; David,
P. R.; Lipscomb, W. N. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 1512–1515. (f)
Liang, T.-C.; Abeles, R. H. Biochemistry 1987, 26, 7603–7608. (g) Christianson,
D. W.; Lipscomb, W. N. Acc. Chem. Res. 1989, 22, 62–69 and references therein.
(h) Brady, K.; Wei, A.; Ringe, D.; Abeles, R. H. Biochemistry 1990, 29, 7600–
7607.
Phosphonate 13, generated in high yield by the Michaelis-
Arbuzov reaction of 12 with triethylphosphite, was treated with
paraformaldehyde and piperidine before dehydration with p-
toluenesulfonic acid in toluene under Dean-Stark conditions.
The vinyl phosphonate 14 was combined with 2,5-dihydroxy-
1,4-dithiane and triethylamine, in a procedure developed by
McIntosh and Sieler,28 to achieve a one-pot conjugate addition
and intramolecular Horner-Wadsworth-Emmons cyclization.
Saponification of the ester 15, followed by treatment of the acid
16 with triethylamine and ethyl chloroformate provided ester
17 through a deconjugation procedure which likely proceeds
through deprotonation and decarboxylation of a mixed anhydride
(27) The Baldwin group synthesized carbocyclic analogues of penicillin N
for studies with deacetoxycephalosporin C synthase (DAOCS) using this route
(Scheme 1) in combination with the intramolecular nitrene C-H insertion method
developed by Lowe and Swain.21 (a) Martyres, D. H.; Baldwin, J. E.; Adlington,
R. M.; Lee, V.; Probert, M. R.; Watkin, D. J Tetrahedron 2001, 57, 4999–5007.
(b) Ferguson, A. C.; Adlington, R. M.; Martyres, D. H.; Rutledge, P. J.; Cowley,
A.; Baldwin, J. E. Tetrahedron 2003, 59, 8233–8243.
(28) (a) McIntosh, J. M.; Sieler, R. A. Can. J. Chem. 1978, 56, 226–231.
(b) McIntosh, J. M.; Sieler, R. A. J. Org. Chem. 1978, 43, 4431–4433.
(29) Deconjugation under these mild conditions is a method that has shown
some generality. Straight-chain as well as carbocyclic R,ꢀ-unsaturated acids can
also undergo deconjugation, but the process is slower and can generate significant
amounts of the conjugated ester. Ethyl chloroformate seems to be more effective
than methyl and benzyl chloroformate.
6972 J. Org. Chem. Vol. 73, No. 18, 2008

Cyclobutanone Mimics of Penicillins
SCHEME 1. Synthetic Route to 2-Thiabicyclo[3.2.0]heptan-6-one-4-carboxylate Derivativesa
a Mercaptoacetaldehyde is commercially available as its dimer, 2,5-dihydroxy-1,4-dithiane.
TABLE 1. Hydrate and Hemiketal Formation with 9 and 20a
FIGURE 3. X-ray crystal structures of 9 and 20. Color scheme: white,
gray, red, yellow, and green represent H, C, O, S, and Cl, respectively.
ketone
% hydrate
% hemiketal
9
20
74
0
88
24
a Percentage of hydrate formation was determined by 1H NMR in
D2O/acetone-d6,42 and hemiketal formation was determined in
methanol-d4.
called transition-state analogues39 have been observed spectro-
scopically and crystallographically.40
The superior inhibition of serine proteases by di- and
trifluoromethyl ketones relative to that of the mono- and
nonfluorinated ketones35 is a clear indication of the importance
of the stability of the enzyme-bound hemiketal (or hydrate).41
Interestingly, the Ki values reported by Gelb et al. for triflu-
oromethyl ketones with acetylcholinesterase (16 nM) and
carboxypeptidase A (0.2 nM) demonstrate that these electro-
philic carbonyl compounds can be potent and effective inhibitors
of both serine proteases and zinc-dependent enzymes despite
the fact that they are nearly entirely hydrated in aqueous
solution.35a
SCHEME 2. Chlorination of 18 and X-ray Structure of 21r
Similarly, the effectiveness of cyclobutanones as ꢀ-lactamase
inhibitors is expected to be highly dependent upon the stability
of the enzyme-bound tetrahedral intermediate mimic, and it
occurred to us that the extent of hydration and hemiketal
formation in water and alcohol could serve as an estimate of
the relative stability of the tetrahedral adducts and might be
used in the rational design of inhibitors.
SCHEME 3. Solvolysis of 21r and X-ray Structure of 25r
The tendency of the 2-thiabicyclo[3.2.0]heptan-6-one system
to undergo hydration and hemiketal formation was evaluated
by NMR experiments involving the free acids 9 and 20 in D2O42
or CD3OD (Table 1). The dichlorocyclobutanone 9 (δC6 198)
quickly formed the hydrate (δC6 99) in D2O and reached a
ketone:hydrate equilibrium ratio of 26:74 within 5 min, but the
dechlorinated acid 20 did not undergo any measurable hydration.
Similarly, the corresponding experiments in methanol-d4 showed
a greater extent of hemiketal formation with 9 than with 20
and that each ketone formed the hemiketal to a greater extent
than the hydrate.43
the regioselective chlorination of 18 with sulfuryl chloride,44
which furnished 21r in excellent yield and with high stereo-
selectivity (Scheme 2). The stereoselectivity of the process may
be a consequence of the initial chlorination of 18 occurring on
the exo face to generate the S-chlorosulfonium ion A. Elimina-
tion of HCl might then lead to the sulfur-stabilized carbocation
B with the chloride leaving group poised for subsequent attack
from the exo face of the ring system, which would provide the
3R-chloride 21r.
Derivatives. Access to C3-substituted variants of the
2-thiabicyclo[3.2.0]heptan-6-one system was achieved through
(41) Fluorination not only stabilizes the hemiketal (relative to the ketone)
but also reduces the pKa of the hemiketal (pKa ∼4.9 within chymotrypsin) and
results in a more effective transition-state analogue.40f
(42) A small amount of acetone-d6 was used to facilitate solubilization. The
ratio D2O:acetone-d6 was approximately 3:1.
(43) Several other ketones and aldehydes, including cyclobutanone, have also
been shown to form hemiketals more extensively than hydrates: Wiberg, K. B.;
Morgan, K. M.; Maltz, H. J. Am. Chem. Soc. 1994, 116, 11067–11077.
1
It was clear from the H NMR spectrum of 21r that the
conformation was different from that of 9 and 20 since the
(44) Bordwell, F. G.; Pitt, B. M. J. Am. Chem. Soc. 1955, 77, 572–577.
J. Org. Chem. Vol. 73, No. 18, 2008 6973

Johnson et al.
TABLE 2. Substitutions at C3 with Alcohols and AcOHa
solvent(s)
time (h) product
OR
Rb
ꢀ
21ꢀ 26
MeCN/H2O 1:1
MeCN/MeOH 1:1
MeCN/i-PrOH 1:1
MeCN/t-BuOH 1:1
AcOH
48
48
40
48
48
1
22
23
24
25
28
28
29
OHc
OMe
6
88
0
0
0
1
75 24
Oi-Prd 46 20 16 18
Ot-Bue 34
7
44 15
FIGURE 4. An equilibrium involving 22r, 22ꢀ, and 22c.
OAc
OAc
OTFE
3
4
5
52 43
79
76
2
11
19
AcOH, 80 °C
CF3CH2OH
6
0
coupling constant between H4 and H5 was found to be 6 Hz
(J4,5 ≈ 0 Hz for 9 and 20). The stereochemistry and exo
envelope conformation for 21r were confirmed by a single-
crystal X-ray diffraction study (see Scheme 2).
48
a Reactions were performed at room temperature unless otherwise
stated. b Product distributions were determined by 1H NMR of the crude
product. c Remaining 6% is attributed to 22c. d 85% conversion. e 42%
conversion.
Substitutions. It was found that the solvolysis of 21r could
be accomplished in either water or alcohol with the use of
acetonitrile as a polar and miscible cosolvent to provide the
3-alkoxy (S,O-acetal) substitution products (Scheme 3). Hy-
drolysis of 21r generated the thiolactols 22r and 22ꢀ in a ratio
of 1:14, respectively, which could not be separated by flash
chromatography. Interestingly, 1H and 13C NMR spectra of the
chromatographed thiolactol mixture showed a third material
(∼6%) that was identified as a tricyclic intramolecular hemiketal
22c (δC6 105.9) (Figure 4).45,46 Since the 22r:22ꢀ:22c ratio was
consistently close to 6:88:6 in several different preparations, it
is thought that the mixture may be part of an equilibrium that
allows the interconversion of 22r and 22ꢀ. In order to gain
insight into the mechanism of interconversion, an NMR experi-
ment was conducted in which the thiolactol mixture 22 was
subjected to acidic methanol-d4/acetonitrile-d3 (1:1).47 The lack
of any cross-over reactions under the reaction conditions (to
generate 23r or 23ꢀ) suggests that equilibration via ring-opened
aldehyde C is more likely than equilibration through a sulfur-
stabilized carbocation D.
and generated considerably more of the epimerization and
elimination byproduct, 21ꢀ and 26.48 The unsaturated ester 26
could be prepared separately through dehydration of the
thiolactol 22ꢀ with TsOH in refluxing toluene under Dean-Stark
conditions and was dechlorinated with zinc in acetic acid to
give 27.49
In contrast to the hydrolysis, the substitutions in MeOH,
i-PrOH, and t-BuOH selectively produced the R-OR retention
products. The results of additional control experiments in acidic
methanol-d4/acetonitrile-d3 discount the possibility that the R-OR
products are the result of the conjugate addition of ROH to 26,50
demonstrate that equilibration of the substitution products R-OR
and ꢀ-OR does not occur,51 and imply that the R/ꢀ ratios are
the consequences of kinetic and not thermodynamic control.
The high R-selectivity in the substitution with methanol led
us to speculate that a cyclobutanone hemiketal could be
generated that would block the endo face of C3 from subsequent
inversion (structures E and F, Figure 5). In this way, attack
from the exo (R) face of F would lead to the major isomer 23r
whereas the minor isomer 23ꢀ would be a result of attack from
the endo (ꢀ) face of D. In addition, we considered the possibility
that the hemiketal could provide higher R selectivity through a
double inversion resulting from neighboring group participation
(structures G and H). Work reported by Grudzinski and
Similar to the hydrolysis, the solvolysis reaction of 21r in
methanol/acetonitrile was complete within 48 h and cleanly
generated the substitution products 23r and 23ꢀ (Table 2).
However, the reactions with sterically demanding alcohols,
2-propanol and tert-butyl alcohol, showed incomplete conversion
(45) An oxatricyclo[3.2.1.03,6]octane has been observed previously: (a)
Grudzinski, Z.; Roberts, S. M. J. Chem. Soc., Perkin Trans. 1 1975, 1767–
1773. X-ray study: (b) Glen, R. C.; Murray-Rust, P.; Riddell, F. G.; Newton,
R. F.; Kay, P. B. J. Chem. Soc., Chem. Commun. 1982, 25–26. (c) Isaacs, N. S.;
Rzepa, H. S.; Sheppard, R. N.; Lobo, A. M.; Prabhakar, S. J. Chem. Soc., Perkin
Trans. 2 1987, 1477–1482.
(46) A related intramolecular attack by an alcohol has also been observed in
ꢀ-lactam systems: Baldwin, J. E.; Cobb, J. E.; Sheppard, L. N Tetrahedron 1987,
43, 1003–1012.
(49) Elimination of HCl from 21r could not be effected under basic
conditions with pyridine, Et3N, or DBU, in CH2Cl2, THF, or MeCN. Heating
21r in MeCN achieved partial elimination, but the epimer 21ꢀ was observed as
a major byproduct that showed even slower elimination (by NMR). The use of
AgCO3 resulted in a complex mixture with only a low yield of the elimination
product 26.
(50) Ester 26 was dissolved in CD3OD/CD3CN (1:1) with AcCl (1.3 equiv,
0.1 M). While a large amount of hemiketal was observed (93%), none of the
addition products 23r or 23ꢀ were detected after 6 d.
(47) The thiolactol mixture 22r:22ꢀ:22c was dissolved in acetonitrile-d3 and
methanol-d4 (1:1, 1 mL total) and combined with acetyl chloride (1.5 equiv,
0.05 M) to generate HCl in situ. Neither of the cross-over products 23r or 23ꢀ
were detected after 48 h at room temperature. Removal of the solvents provided
22 in the same 6:88:6 ratio.
(48) The 3ꢀ-Cl epimer 21ꢀ could not be made or isolated in useful quantities
for further studies since it was found to be unstable to chromatography. However,
21ꢀ was found to prefer an endo envelope conformation in solution, as indicated
by the multiplicity of H3 and H4 in the 1H NMR spectrum. As with 28ꢀ, H3
(51) Cyclobutanone 23r was dissolved in isopropanol-d8/acetonitrile-d3 (1:
1) and treated with AcCl (to give 0.1 M HCl). After 4 d at room temperature
none of the possible cross-over products 24r or 24ꢀ were detected and 23r
was recovered. Separately, 23ꢀ was subjected to the same conditions and no
reaction was observed after 4 d at room temperature.
and H4 in 21ꢀ appear as singlets and J3R,4 ) 0 Hz.
6974 J. Org. Chem. Vol. 73, No. 18, 2008

Cyclobutanone Mimics of Penicillins
SCHEME 4. Solvolysis of 21r with AcOH and TFEOH,
X-ray Structure of 28ꢀ, and Rationale for Selectivity
TABLE 3. Substitutions at C3 Promoted by Silver Triflatea
FIGURE 5. Possible intermediates in substitutions involving 21r with
ROH (E ) CO2Et).
Roberts52 involving the bromination of similar bicyclic sub-
strates indicates that this pathway is plausible.53
ROH
equiv ROH time (h) product
OR
OHd
OMe
OMe
Oi-Pr
Ot-Bue
Rb
ꢀ
21ꢀ 26
While the hypothesis involving intermediates E-H is reason-
able for methanol,54a hemiketal formation with a bulkier alcohol
such as tert-butanol is less significant.54b It is likely that the
retention product 25r is the result of attack by tert-butanol on
intermediate D, in which the exo face is more accessible than
endo face as a consequence of steric shielding by the 7ꢀ-
chlorine.55 The rationale that steric hindrance drives the 5:1 R:ꢀ
selectivity would be consistent with the lower conversion
observed in the reaction with tert-butanol.
The 2:1 R:ꢀ selectivity shown by 2-propanol could be
rationalized by a combination of its capability to slowly form
the hemiketal,54c which would lead to 24r from F or H, and
its ability to attack from the endo (ꢀ) face of C or D, which
would yield 24ꢀ.
The high selectivity in favor of inversion demonstrated by
acetic acid (28r:28ꢀ ≈ 1:17) and trifluoroethanol (29r:29ꢀ ≈
1:15) was unexpected as it clearly contrasts with that of the
aforementioned alcohols. This selectivity suggests that attack
from the endo face must be favored electronically since the
enhanced acidity and weaker nucleophilicity of AcOH and
TFEOH54d increase the SN1 character of the substitution relative
to reactions with ordinary alcohols (Scheme 4).
c
H2O
excess
4
6
22
23
23
24
25
6
85
0
2
0
2
3
4
2
MeOH
MeOH
i-PrOH
t-BuOH
5
22 72
26 61
16 71
10
5
10
6
12
8
11
5
42 30 22
a Reaction conditions: AgOTf (1.2 equiv) in CH2Cl2 was used with 4 Å
MS (3 Å MS for R ) Me). Reactions were begun at 0 °C and allowed to
warm to room temperature. b Product distributions were determined by 1H
NMR of the crude product. c Hydrolysis was performed at room temper-
ature in MeCN/H2O 1:1 without the use of molecular sieves. d Remaining
6% is attributed to 22c. e 92% conversion.
in TSr that are not present in TSꢀ. The high R selectivity
demonstrated by t-BuOH, however, indicates that the 7ꢀ-Cl
imposes such a large destabilizing steric interaction in TSꢀ that
TSr is preferred.
Substitutions Promoted by Silver Triflate. While the
solvolysis reactions using ROH/MeCN were useful for the
synthesis of 22ꢀ and 23r-25r and interesting from a mecha-
nistic point of view, they generally required relatively long
reaction times and difficult chromatographic separations and
generated large proportions of the epimerization and elimination
byproduct 21ꢀ and 26. Efforts were then made to develop a
complementary method that would be more selective for
inversion and improve upon the reaction times and the amount
of byproduct formed. The use of silver triflate was explored
(Table 3), as it has been used as a promoter in substitutions at
the anomeric position of chlorocarbohydrates.
Attack on the carbocation from the R (exo) face may be
disfavored as a result of two destabilizing eclipsing interactions
(52) The bromination of several bicyclo[3.2.0]hept-2-ene-6-ones in methanol
and water was studied by Grudzinski and Roberts.45a
Since the R/ꢀ selectivity of the hydrolysis was not a concern
(based on the proposed interconversion of the products 22r and
22ꢀ), acetonitrile was chosen as a water-miscible solvent.56
However, the use of MeCN with 21r was otherwise considered
problematic due to the spontaneous epimerization to 21ꢀ
observed previously49 and CH2Cl2 was used for reactions with
alcohols. Typically, 21r was slowly added as a solution in
CH2Cl2 to a suspension of AgOTf, ROH, and molecular sieves
in CH2Cl2 at 0 °C.
(53) The fact that a tricyclic methyl ketal (neutral form of H) was not
observed may indicate that either this is a minor pathway, or that H is very
reactive. Note that a full 1 equiv of HCl is generated in this solvolysis reaction
while the brominations by Grudzinski and Roberts using N-bromoacetamide were
nonacidic.
(54) Cyclobutanone 26 was dissolved in deuterated alcohols so that hemiketal
formation could be observed by NMR. (a) Methanol-d4: 96% hemiketal in 2 h;
96% after 4 d. (b) tert-Butanol-d10: less than 1% hemiketal after 24 h; 5% after
18 d. (c) Isopropanol-d8: 7% hemiketal in 5 h; 36% after 8 d. (d) Trifluoroethanol-
d3: less than 1% hemiketal after 4 h; 3% after 8 d.
(55) The five-membered ring is essentially flat in structure D (RHF/6-31G(d)).
See Table S4 (Supporting Information) for Cartesian coordinates.
(56) The spectra for the 22r:22ꢀ:22c mixture isolated from the AgOTf
reaction were identical to material obtained from previous preparations.
J. Org. Chem. Vol. 73, No. 18, 2008 6975

Johnson et al.
FIGURE 7. Bond lengths (Å) obtained from X-ray crystal structures.
The X-ray structure of 20 shows two crystallographically different
molecules in the unit cell.
of 4′-thionucleosides.59,60 In addition, it is possible that anomeric
FIGURE 6. Conformations of C3-substituted cyclobutanones. Coupling
patterns in the 1H NMR spectra shown (25ꢀ and 25r, Z ) Ot-Bu) are
representative of derivatives 21-25, 28, and 29.
effects are responsible for the axial arrangements that have been
1
recognized by H NMR analysis in several known R-halo and
R-alkoxy tetrahydrothiophenes.61
Since the anomeric effect is often characterized by a shorten-
ing of the O-C bond and lengthening of the C-X bond in the
O-C-X segment of carbohydrate derivatives, the bond lengths
obtained in the X-ray crystal structures of 21r, 25r, and 28ꢀ
were examined closely (Figure 7). Indeed, structural evidence
for the anomeric effect was discovered in 2-thiabicyclo[3.2.0]-
heptan-6-ones that possess electron-withdrawing substituents at
C3.62
The 3R-Cl derivative 21r clearly displays a shortened S-C3
bond (1.782 Å) and a lengthened C3-Cl bond (1.800 Å) in
comparison to structurally related tetrahydrothiophenes,59 dithianes
and dioxanes,63a,c chlorocarbohydrates,63d and the unsubstituted
cyclobutanones 9 and 20. Similarly, the solid-state structure of
28ꢀ exhibits a shorter S-C3 bond (1.801 Å) relative to 9 and
20 and C3-O bond (1.441 Å) of intermediate length relative
to acetoxythiopyranosides.63b
Indeed, the AgOTf-promoted reactions displayed improved
reaction times and an increased selectivity for inversion with
ROH. Interestingly, 10 equiv of methanol showed less selectivity
for inversion than 10 equiv of tert-butanol. Since a decrease to
5 equiv of MeOH improved the ꢀ selectivity, it is possible that
hemiketal formation remains significant at low concentrations
of MeOH and that leakage to 23r could be through structures
F-H (and to a lesser extent D).
The proportion of the elimination byproduct 26 remained a
concern in the AgOTf-catalyzed reactions with the hindered
nucleophiles i-PrOH and t-BuOH. This observation inspired an
investigation into whether the elimination could be effected
exclusively in the absence of a nucleophile. To our satisfaction,
the addition of 21r to AgOTf in refluxing CH2Cl2 cleanly
furnished 26 in 86% isolated yield.
Conformations of C3-Substituted Cyclobutanone Deri-
vatives. During the development of the solvolysis reactions and
silver triflate substitutions it became clear that cyclobutanones
with 3R substituents (Z ) Cl, O-alkyl) conformed to an exo
envelope, while those with 3ꢀ substituents (Z ) H, Cl, OAc,
O-alkyl) showed preference for the endo envelope. This was
apparent from 1H NMR spectroscopy (Figure 6), as the spectrum
of each derivative displayed a pattern similar to that of either
21r and 25r or 28ꢀ, the structures of which have been solved
by X-ray crystallographic studies.
(59) For examples of X-ray crystal structures of R-aza derivatives, see: Koole,
L. H.; Plavec, J.; Liu, H.; Vincent, B. R.; Dyson, M. R.; Coe, P. L.; Walker,
R. T.; Hardy, G. W.; Rahim, S. G.; Chattopadhyaya, J. J. Am. Chem. Soc. 1992,
114, 9936–9943.
(60) Watts, J. K.; Sadalapure, K.; Choubdar, N.; Pinto, M. B.; Damha, M. J.
J. Org. Chem. 2006, 71, 921–925.
We considered the possibility that the conformational prefer-
ences could be a result of an anomeric effect involving the sulfur
and the electron-withdrawing substituents at C3.57 Although the
conformational properties of sulfur-containing six-membered
rings are well-known,57,58 the anomeric effect has been studied
relatively little in five-membered rings since the conformational
consequences are much less dramatic and twisted conformations
are typically favored in which the substituents are in pseudoaxial
or pseudoequatorial orientations. While this is evident in the
crystal structures of numerous 4′-thionucleoside derivatives (R-
aza substituents),59 anomeric effects and gauche effects have
been argued previously for rationalization of the conformations
(61) (a) trans-2,3-Dichlorotetrahydrothiophene: Delaney, P. A.; Johnstone,
R. A. W. Tetrahedron 1985, 41, 3845–3851. (b) 2-Alkoxy-3-chlorotetrahy-
drothiophenes: Delaney, P. A.; Johnstone, R. A. W.; Leonard, P. A.; Regan, P.
J. Chem. Soc., Perkin Trans. 1 1991, 285–289. (c) trans-2,3-Dibromotetrahy-
drothiophene and 2-alkoxy-3-bromotetrahydrothiophenes: Wilson, G. E., Jr.;
Albert, R. J. Org. Chem. 1973, 38, 2156–2159.
(62) The anomeric effect is generally thought to be strongest with electron-
withdrawing substituents: halogen > OR > SR > OH > NR2.57c
(63) The X-ray structures of the following derivatives were considered useful
for comparison. Arrows indicate bond lengths (Å). (a) Kalff, H. T.; Romers, C.
Acta Crystallogr. 1965, 18, 164–168. (b) Adam, D.; Freer, A. A.; Isaacs, N. W.;
Kirby, G. W.; Littlejohn, A.; Rahman, M. S J. Chem. Soc., Perkin Trans. 1
1992, 1261–1264. (c) Altona, C.; Romers, C. Acta Crystallogr. 1963, 16, 1225–
1232. (d) Paulson, H.; Luger, P.; Heiker, F. R. In Anomeric Effect: Origin and
Consequences; Szarek, W. A., Horton, D., Eds.; ACS Symposium Series 87;
American Chemical Society: Washington, DC, 1979; Chapter 5 and references
therein.
(57) Anomeric effect: (a) Deslongchamps, P. Stereoelectronic Effects in
Organic Chemistry; Pergamon Press: New York, 1983. (b) Kirby, A. J. The
Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer Verlag:
Berlin, 1983. (c) Juaristi, E.; Cuevas, G. The Anomeric Effect; CRC Press: Boca
Raton, 1995. (d) Juaristi, E. Conformational BehaViour of Six-Membered Rings;
VCH Publishers: New York, 1995.
(58) (a) Romers, C.; Altona, C.; Buys, H. R.; Havinga, E. Top. Stereochem.
1969, 4, 39–97. (b) Pericas, M. A.; Riera, A.; Guilera, J. Tetrahedron 1986, 42,
2717–2724.
6976 J. Org. Chem. Vol. 73, No. 18, 2008

Cyclobutanone Mimics of Penicillins
TABLE 4. Calculated Conformational Preferences for
Interestingly, in contrast to 21r and 28ꢀ, the X-ray crystal
structure of the 3R-Ot-Bu derivative 25r shows a long S-C3
bond (1.844 Å) and a short C3-O bond (1.395 Å). This is
consistent with the general consensus that anomeric effects are
stronger with oxygen than sulfur.57,58,64
Cyclobutanone and ꢀ-Lactam Derivativesa
Computational Results. Ab initio (RHF/6-31G(d)) molecular
orbital (MO) calculations65 were carried out with several of the
cyclobutanone derivatives that had been prepared synthetically
(Table 4). As expected, the conformational preferences observed
by NMR (solution phase) and by X-ray (solid state) were also
found in the gas phase, and the preferred conformations favor
C3 substitutents axial orientations.
In order to gain additional insight into the origin of the
conformational preferences and putative stereoelectronic stabi-
lizations of axial arrangements, additional calculations were
performed with a series of simplified cyclobutanone derivatives
that lack the C4-carboxylate moiety.
Interestingly, these calculations show that the C4-carboxylate
has a minor effect on the conformations of unsubstituted (Z )
H) cyclobutanones, as each of the cyclobutanones 9 and 18-20
(1.4-1.9 kcal/mol) and 33 and 3466 (2 kcal/mol) have a
significant preference for the endo envelope, and suggest that
the eclipsing interactions between substituents at C4 and C5 in
the exo envelope are significant (Figure 8).67
Computational evidence of the anomeric effect is noted in
the comparison of cyclobutanones 33 and 34 to compounds with
R substituents at C3. The small preference for the exo envelope
(0.05 kcal/mol) shown by cyclobutanone 35r, for example,
indicates that the energy gained by stereoelectronic stabilization
in the exo envelope is enough to overcome the natural endo
preference (1.89 kcal/mol) of the fused bicycle. Thus, compari-
son of 33 with 35r-39r suggests that anomeric effects could
be worth up to 1.9, 3.3, 3.1, 1.5, and 1.1 kcal/mol for Z ) R-Cl,
R-OAc, R-OMe, R-Ot-Bu, and R-SMe, respectively. Similar
trends were found with the dichlorocyclobutanones 40r, 41r,
and 42r, which gave values of 1.9, 3.2, and 1.0 kcal/mol for Z
) R-Cl, R-OMe, and R-SMe, respectively, when compared to
34.
Calculations involving ꢀ-substituents at C3 also indicate that
an anomeric effect is present, as the ꢀ-alkoxy derivatives
36ꢀ-38ꢀ show increased preferences for the endo envelope
(2.8, 2.6, and 1.4 kcal/mol for Z ) ꢀ-OAc, ꢀ-OMe, and ꢀ-Ot-
Bu) relative to 33 (Z ) H). However, the ꢀ-Cl derivative 35ꢀ,
which contains a sterically larger substituent, shows only a
modest increase in endo preference (0.2 kcal/mol), and the
ꢀ-SMe analogue 39ꢀ shows a decrease in endo preference (1.0
kcal/mol). Since this pattern is magnified in the 7,7-dichloro
series 40ꢀ-42ꢀ (-0.8, +2.4, and -1.9 kcal/mol for ꢀ-Cl,
ꢀ-OMe, and ꢀ-SMe compared to 34), it seems reasonable to
conclude that large 3ꢀ-substituents experience a significant steric
interaction with the 7ꢀ-H or 7ꢀ-Cl (Figure 8).68
Given the large magnitudes of the calculated conformational
preferences (2-5 kcal/mol) for cyclobutanones 21-25, 28, and
32 and the relatively small preferences with derivatives
a Structure optimizations (RHF/6-31G(d)) used Gaussian-0365 and
comparison of the relative electronic energy of each conformer was
done following zero-point energy corrections. b Cyclobutanones 30-44
were used for calculations only and were not prepared in this study.
(64) (a) Schleyer, P. v. R.; Jemmis, E. D.; Spitznagel, G W J. Am. Chem.
Soc. 1986, 107, 6393–6394. (b) Salzner, U.; Schleyer, P. v. R. J. Am. Chem.
Soc. 1993, 115, 10231–10236. (c) Trapp, M. L.; Watts, J. K.; Weinberg, N.;
Pinto, B. M. Can. J. Chem. 2006, 84, 692–701.
(65) Frisch, M. J. et al., Gaussian-03, ReVision B.04.; Gaussian Inc.: Pittsburg,
PA, 2003.
(66) The structure of 34 has been solved previously by a single-crystal X-ray
study: Lange, G. M. Sc. Thesis, University of Waterloo, 1984.
(67) The endo envelope is also favored with 2-oxa and 2-carba analogues
(43-46) of 33 and 34. See Table S1 in Supporting Information.
35r-39r, and 40r-42r, it is clear that the C4-carboxylate
moiety significantly enhances the preference for the exo
conformation with R-substituents. It is likely that a disfavored
steric interaction between the 3R-Z function and the carboxylate
J. Org. Chem. Vol. 73, No. 18, 2008 6977

Johnson et al.
FIGURE 9. Two conformations of penicillin G.71 Figures of the endo
envelope (C3-puckered, axial) conformation I and the exo envelope
(S-puckered, equatorial) conformation J were adapted from X-ray
crystal structures of the potassium salt (CCDC: BPENK01) and the
procaine salt (CCDC: PRPENG), respectively.
studies by Dobson et al. demonstrate that these differences in
conformational preference are not present in solution. The use
of lanthanide ions as probes74 and 13C cross polarization magic
angle spinning (CP-MAS) NMR experiments75 indicate that
several penicillins (with different solid-state preferences) all
favor the exo envelope conformation J in aqueous solution with
ratios of endo:exo conformers ranging from 45:55 to 21:79,
respectively.76
While penicillin G has been found to favor the endo envelope
conformation I in the gas phase by several computational
studies77 including our own (2.2 kcal/mol, Table 4), molecular
dynamics (MD) studies by D´ıaz et al.77a show that solvent has
a significant energetic effect (∼1.5 kcal/mol) in stabilizing the
exo envelope conformation J, and the MD simulation that
predicts an endo:exo ratio of 70:30 in aqueous solution is in
reasonable agreement with Dobson’s experimental results.
In the context of biological activity, it is thought that
ꢀ-lactamases preferentially bind to the exo envelope conformer
J of penicillins. Through detailed MD simulations of penicillin
G complexed with the class A TEM-1 ꢀ-lactamase, D´ıaz et al.
have shown that H-bonding interactions between the C3-
carboxylate and Ser130, Ser235, and Arg244 are favored with
the equatorial conformer J and that a steric clash between the
2ꢀ-methyl group and Ala237 is also avoided.78 Additional
molecular modeling studies involving mechanisms of penicillin
acylation in TEM-1 (class A)79 and P99 (class C)80 ꢀ-lactamases
have also involved the exo envelope conformation J of the
penicillins, but analogous studies with the class B and class D
enzymes have yet to be reported.
FIGURE 8. Favored conformations and relevant steric interactions.
in the endo envelope accounts for much of this energy since a
significant rotation of the C4-CO2Et bond69 is observed in the
optimized endo envelope conformations of 23r, 25r, 28r, and
32r and not in the exo envelope conformations.70 Comparison
of the conformational preferences of cyclobutanones with
ꢀ-substituents, however, indicates that the carboxylate has a
minor effect, as 21ꢀ, 23ꢀ, and 32ꢀ have nearly identical
preferences as the decarboxy analogues 40ꢀ-42ꢀ.
In addition to the computational results described thus far,
the conformational preferences of several 2-oxa and 2-carba
(CH2) analogues have been calculated for the purpose of
comparison (Table S1, Supporting Information). It is worth
noting that the calculated conformational preferences for the
2-thia series are generally larger in magnitude than the prefer-
ences of the 2-oxa counterparts, and that this is likely a
consequence of increased steric interactions in the tetrahydro-
furan system due to the shorter endocyclic carbon-oxygen bond
lengths.
Conformations of Penicillins. The importance of the penicil-
lins to antibiotic therapy has led to numerous studies of their
three-dimensional structures by experimental and theoretical
methods, and particular attention has been paid to the confor-
mational properties of the thiazolidine ring. X-ray crystal
structures, which have been obtained for a variety of penicillins,
show that the bicycle can adopt an endo envelope conformation
I or an exo envelope conformation J (Figure 9).71 The endo
and exo envelopes are also referred to as the C3-puckered and
S-puckered conformations, based on the atom that is most out
of the plane of the ring, or the axial and equatorial conforma-
tions, respectively, based on the orientation of the 3R-carboxy-
late.
Of additional interest is the fact that a noncovalent complex
between the R61 transpeptidase and a peptidoglycan-mimetic
penicillin has been trapped crystallographically, and the X-ray
(74) Dobson, C. M.; Ford, L. O.; Summers, S. E.; Williams, R. J. P. J. Chem.
Soc., Chem. Commun. 1975, 71, 1145–1153.
(75) (a) Clayden, N. J.; Dobson, C. M.; Lian, L.-Y.; Twyman, J. M. J. Chem.
Soc., Perkin Trans 2 1986, 1933–1940. (b) Twyman, J. M.; Fattah, J.; Dobson,
C. M. J. Chem. Soc., Chem. Commun. 2 1991, 647–649.
Attempts have been made to correlate the solid-state confor-
mational preferences with biological activity,72,73 but NMR
(68) It should be noted, however, that the distances measured between the
3ꢀ-Z atom and the 7ꢀ-X atom were longer than van der Waals contacts in each
of the optimized structures. For van der Waals radii, see: Bondi, A. J. Phys.
Chem. 1964, 68, 441–451. Carbon 1.70 Å; oxygen 1.52 Å; sulfur 1.80 Å; chlorine
1.75 Å.
(69) The lowest energy conformations show a dihedral angle of close to 0°
between the C4-C5 bond and the CdO bond of the ester.
(70) A plot of potential energy vs dihedral angle, generated by a torsional
scan calculation, is provided in Supporting Information for cyclobutanone 18 in
each of the endo and exo conformations (Figure S1).
(71) Dexter, D. D.; van der Veen, J. M. J. Chem. Soc., Perkin Trans. 1 1978,
185–190.
(72) (a) Cohen, N. C. J. Med. Chem. 1983, 26, 259–264. (b) Balsamo, A.;
Domiano, P.; Macchia, B.; Nardelli, M. Eur. J. Med. Chem. 1980, 15, 559–562.
(73) The assumption that ꢀ-lactam solid-state conformations represent solution
phase conformations has been questioned: Koch, A.; Ku¨hne, R.; Franke, R.
Pharmazie 1990, 45, 694–695.
(76) A spin-labeled penicillin was also found to adopt an exo envelope
conformation in solution: Mustafi, D.; Makinen, M. W. J. Am. Chem. Soc. 1995,
117, 6739–6746.
(77) (a) D´ıaz, N.; Sua´rez, D.; Sordo, T. L. J. Comput. Chem. 2003, 24, 1864–
1873. (b) Pen˜a-Gallego, A.; Cabaleiro-Lago, E. M.; Ferna´ndez-Ramos, A.;
Hermida-Ramo´n, J. M.; Mart´ınez-Nu´n˜ez, E. J. Mol. Struct. (Theochem) 1999,
491, 177–185. (c) Frau, J.; Coll, M.; Donoso, J.; Mun˜oz, F. J. Mol. Struct.
(Theochem) 1991, 231, 109–124. (d) Joshi, N. V.; Virudachalam, R.; Rao,
V. S. R. Curr. Sci. 1978, 47, 933–936.
(78) D´ıaz, N.; Sordo, T. L.; Merz, K. M., Jr.; Sua´rez, D. J. Am. Chem. Soc.
2003, 125, 672–684.
(79) (a) Meroueh, S. O.; Fisher, J. F.; Schlegel, B.; Mobashery, S. J. Am.
Chem. Soc. 2005, 127, 15397–15407. (b) Hermann, J. C.; Hensen, C.; Ridder,
L.; Mulholland, A. J.; Ho¨ltje, H.-D. J. Am. Chem. Soc. 2005, 127, 4454–4465.
(c) Oliva, M.; Dideberg, O.; Field, M. J. Proteins 2003, 53, 88–100.
(80) Fenollar-Ferrer, C.; Frau, J.; Donoso, J.; Mun˜oz, F. Proteins 2003, 51,
442–452.
6978 J. Org. Chem. Vol. 73, No. 18, 2008

Cyclobutanone Mimics of Penicillins
TABLE 5. Cyclobutanone Hemiketal Formation in Methanol-d4
FIGURE 10. Conformations of cyclobutanone hemiketals as indicated
1
by H and 13C NMR (R ) Me, i-Pr, t-Bu).
with cyclobutanones 23ꢀ-25ꢀ and 28ꢀ (15-40%), which adopt
the endo envelope. It is reasonable that the unsaturated esters
26 and 27 show larger proportions of hemiketal than 18 and
19, respectively, since the C3 methylene unit could impose
additional steric congestion in the endo envelope.82
While it has been well established that nucleophilic attack
on the carbonyl group is highly favored from the exo face of
bicyclo[3.2.0]heptanones, the reversible nature of hemiketal
formation gave rise to a mixture of the ketone and each of the
R- and ꢀ-hemiketals that often required weeks to equilibrate.
Tabulation of the equilibrium ratios of R-hemiketal/ꢀ-hemiketal
revealed that the relative stability of the R- and ꢀ-hemiketals is
highly sensitive to steric hindrance in the same way that the
extent of hemiketal formation was found to be. Namely, the
cyclobutanone derivatives with the greatest steric bulk on
the endo face of the bicycle have the highest R/ꢀ-hemiketal
ratios.
a The extent of hemiketal formation was determined by 1H NMR
experiments in neat CD3OD. b The ratio was determined when the
system had reached equilibrium.
Interestingly, the coupling patterns in 1H NMR and the
chemical shifts observed in the 13C NMR indicate that the
ꢀ-hemiketals of 3ꢀ-alkoxy cyclobutanones 23ꢀ-25ꢀ and 28ꢀ
adopt the exo envelope conformation (Figure 10).83
structure shows that the penicillin adopts the exo envelope
conformation J.81
Rate of Hemiketal Formation. In contrast to the fast hydrate
formation observed with 9 (X ) Cl) in D2O and the classical
study by Wiberg et al. involving carbonyl reactions,43 hemiketal
formation involving 9 and 18 was surprisingly slow given the
high electrophilicity of the carbonyl carbon and required several
hours to equilibrate.84 The results reported by Wiberg et al.,
which show that hemiketal formation with ketones is fast, were
replicated by our laboratory for cyclobutanone itself85 and were
also reflected in the relatively fast hemiketal formation dem-
onstrated by the dechlorinated cyclobutanones 19 and 20.
Although the extent of hemiketal formation is greater for the
7,7-dichlorinated systems as compared with the nonchlorinated
compounds, our qualitative observations of relative rates of
hemiketal formation suggest that the process is somewhat
impeded by the halogen substituents.86
Cyclobutanone Hemiketal Formation. Given the extensive
computational evidence (above), which suggests that penicillins
bind to ꢀ-lactamase active sites in the exo envelope conforma-
tion, and the large magnitude of the conformational preferences
calculated in the present study for the 3-alkoxy cyclobutanone
derivatives (Table 4), we wondered whether the conformation
of the tetrahydrothiophene ring would have a significant effect
on the tendency of the cyclobutanones to undergo hydrate and
hemiketal formation.
Evaluation of hemiketal formation with cyclobutanones
involved simple NMR experiments in which the cyclobutanone
of interest was dissolved in neat methanol-d4 (Table 5). As
mentioned above, the dichlorocyclobutanones 9 and 18 (X )
Cl) underwent hemiketal formation to a much greater extent
than the dechlorinated ketones 19 and 20 (X ) H). As expected,
this pattern was also evident in the unsaturated system as
hemiketal formation occurred to a greater extent with 26 (X )
Cl) than with 27 (X ) H).
Hemiketal formation must involve either protonation of the
carbonyl oxygen or at least substantial H-bonding to the oxygen
from the solvent to activate the carbonyl for nucleophilic attack
at carbon. The chlorine atoms at C7 likely diminish electron
density on the carbonyl oxygen atom, making it less basic and
Exposure of the 3-alkoxy dichlorocyclobutanones 23-25 and
28ꢀ to methanol-d4 revealed that the extent of hemiketal
formation was highly dependent upon the steric environment
surrounding the carbonyl. Thus, hemiketal formation is highly
favored in cyclobutanones that prefer the exo envelope,
23r-25r (98%), whereas hemiketal formation is less significant
(82) Steric hindrance of hydration has been speculated previously for ketones
with nearby tert-leucine sidechains (ref 36 and references therein).
(83) For tables of selected 1H and 13C NMR data for cyclobutanones and
cyclobutanone hemiketals, see Tables S5-S7 (Supporting Information).
(84) Reaction rates were not determined quantitatively, but hemiketal
formation was followed by NMR and plots of % hemiketal vs time have been
supplied for each ketone in Supporting Information (Table S8).
(81) The noncovalent complex of the R61 transpeptidase with a so-called
perfect penicillin substrate was achieved by cross-linking the enzyme such that
Lys65 and Tyr159 are restrained (PDB code 1PW1). Silvaggi, N. R.; Josephine,
H. R.; Kuzin, A. P.; Nagarajan, R.; Pratt, R. F.; Kelly, J. A. J. Mol. Biol. 2005,
345, 521–533.
(85) Our laboratory also found that hemiketal formation with cyclobutanone
(25 mg) in CD3OD (1 g) is fast, as equilibrium was achieved in 15 min at ambient
temperature and 4.5% hemiketal was formed (by 1H NMR at 500 MHz).
(86) The approximate trend in relative rates of hemiketal formation is 23r-
25r > 27 ≈ 26 ≈ 19 > 18 > 25ꢀ > 24ꢀ > 23ꢀ.
J. Org. Chem. Vol. 73, No. 18, 2008 6979

Johnson et al.
less prone to protonation by an acid catalyst or to act as an
H-bond acceptor. Furthermore, the chlorines likely offer a degree
of steric hindrance to the solvation that might be required for
lowering the activation energy for the addition step in hemiketal
formation.
prove valuable in fine-tuning the properties of such compounds
for affinity for specific targets.
Experiments designed at exploiting these observations in the
context of design of ꢀ-lactamase inhibitors are under way in
this laboratory,87,88 but the potential for broader application of
these concepts in the specific design of related compounds
targeting other penicillin targets elsewhere is certainly encouraged.
Conclusions
The analysis presented above reveals a strong influence of
C3 substitution on the conformational preferences of the
2-thiabicyclo[3.2.0]heptan-6-one ring system and also on rate
and extent of hemiketal formation. The stereochemistry at C3
plays a crucial role in determining the conformational prefer-
ences of the five-membered ring, and steric influences, including
steric hindrance of solvation, have been implicated in controlling
the rate and extent of hemiketal formation. In addition, within
the observations outlined above are, to our knowledge, some
of the clearest demonstrations of anomeric effects in tetrahy-
drothiophene systems.
Experimental Section
7,7-Dichloro-2-thiabicyclo[3.2.0]heptan-6-one-4-carboxylic Acid
(9). Ethyl ester 18 (1.323 g, 4.916 mmol) was dissolved in dioxane
(10 mL) and stirred with 6 M HCl (20 mL) at 80 °C for 6 h. The
reaction mixture was cooled to room temperature, extracted with
CH2Cl2 (3 × 20 mL), and dried with Na2SO4 before the solvent
was removed in vacuo. The crude beige-colored solid was recrystal-
lized from PhMe to give dichlorocyclobutanone 9 (936.1 mg, 3.883
1
mmol, 79%) as light yellow needles. Mp 160-161 °C. H NMR
(300 MHz, acetone-d6): δ 3.06 (dd, Jgem ) 12.4 Hz, J3ꢀ,4 ) 5.9
Hz, 1H, H3ꢀ), 3.50 (d, Jgem ) 12.4 Hz, 1H, H3R), 3.81 (d, J4,3ꢀ
)
5.9 Hz, 1H, H4), 4.69 (d, J1,5 ) 8.5 Hz, 1H, H1), 5.14 (d, J5,1 )
8.5 Hz, 1H, H5). 13C NMR (75.5 MHz, acetone-d6): δ 36.3, 50.7,
59.5, 68.6, 90.2, 171.3, 196.0. IR (film, cm-1): br 3200-2600, 2951,
1810, 1700, 1453, 1417, 1262, 1190. LRMS (EI) m/z (relative
intensity): 244 (M[(37Cl2)]+, 5), 242 ([M(37Cl35Cl)]+, 27), 240
([M(35Cl2)]+, 38), 222 (10), 194 (20), 180 (60), 141 (40), 130 (40),
85 (100). HRMS (EI) m/z: 239.9415 calcd for C7H635Cl2O3S;
239.9421 obsd.
The design of cyclobutanones as potential ꢀ-lactamase
inhibitors (or for interaction with other penicillin-binding
proteins) might well benefit from an ability to predict the extent
to which the structure of the cyclobutanone best suits the nature
of the interactions of the biological target with the normal
ꢀ-lactam substrates. The detailed computational studies78-80 that
involve the acylation of class A and C serine ꢀ-lactamases
indicate that the reaction pathway involves exo attack of the
active-site serine on the ꢀ-lactam carbonyl with the thiazolidine
ring of the penicillin in the exo envelope conformation. The
analysis of the conformational preferences of the cyclobutanones
examined in the present study would suggest that a 2-thiabicyclo-
[3.2.0]heptan-6-one-4-carboxylate that either lacks a substituent
at C3 or one that incorporates a 3ꢀ substituent would be a poor
choice for the task of mimicking the tetrahedral intermediate-
forming step of the catalytic process since both of these systems
favor the endo envelope conformation. On the other hand, such
systems incorporating a 3R substituent exhibit a substantial
preference for an exo envelope conformation. Furthermore, these
systems exhibit a tendency to form hemiketals relatively rapidly
and completely, which are properties desirable for a potential
ꢀ-lactamase inhibitor functioning by the mechanism suggested
by the theoretical studies indicated above.
2-Thiabicyclo[3.2.0]heptan-6-one-4-carboxylic Acid (20). Zinc
dust (293.4 mg, 4.486 mmol) was added to a stirring solution of
dichlorocyclobutanone 9 (212.1 mg, 0.8798 mmol) in glacial acetic
acid (25 mL) at room temperature before it was heated to 80 °C.
An additional portion of zinc dust was added after 1.5 h (293.0
mg, 4.481 mmol), and the suspension was stirred for an additional
16 h before it was cooled to room temperature. The solution was
diluted with EtOAc (100 mL) and filtered through glass wool to
remove residual solid before concentration under reduced pressure.
The resulting oil was redissolved in EtOAc (50 mL) and washed
with 10% HCl (2 × 50 mL). The organic phase was then dried
over Na2SO4 and the solvent removed under reduced pressure.
Trituration with CH2Cl2/hexane provided 20 as a white solid (134.0
1
mg, 0.7782 mmol, 88%). Mp 98-100 °C. H NMR (300 MHz,
acetone-d6): δ 2.85 (ddd, Jgem ) 18.4 Hz, J7ꢀ,1 ) 3.2 Hz, J7ꢀ,5 )
3.2 Hz, 1H, H7ꢀ), 3.22 (dd, Jgem ) 12.1 Hz, J3ꢀ,4 ) 5.9 Hz, 1H,
H3ꢀ), 3.44 (d, Jgem ) 12.1 Hz, 1H, H3R), 3.56 (d, J4,3ꢀ ) 5.9 Hz,
1H, H4), 3.70 (ddd, Jgem ) 18.3 Hz, J7R,1 ) 8.2 Hz, J7R,5 ) 3.5
Hz, 1H, H7R) 4.17 (ddd, J1,5 ) 8 Hz, J1,7R ) 8.2 Hz, J1,7ꢀ ) 3.5
Hz, 1H, H1), 4.64 (m, 1H, H5). 13C NMR (75.5 MHz, acetone-d6):
δ 35.6, 37.5, 50.3, 56.6, 72.3, 172.4, 208.4. IR (film, cm-1): br
3200-2600, 2933, 1780, 1705, 1383, 1252. LRMS (EI) m/z
(relative intensity): 174 ([M + 2]+, 2.7), 173 ([M + 1]+, 4.4), 172
(M+, 49), 130 (65), 97 (10), 85 (100). HRMS (EI) m/z: 172.0194
calcd for C7H8O3S; 172.0191 obsd.
The details of the interactions of other penicillin-sensitive
enzymes with their targets is not entirely clear, and it is
conceivable that the preferences of these other targets for the
conformations of the substrate may differ so that an understand-
ing of the properties of potential cyclobutanone mimics might
(87) Hydrolysis of the ethyl ester functionality (in 23, for example) cannot
be achieved cleanly in basic (NaOH) or acidic (HCl) conditions due to the
sensitivity of the dichlorocyclobutanone ring and the S,O-acetal functionalities,
respectively. Our group is currently pursuing the synthesis of the free acids via
alternative esters that can be cleaved under mild conditions so that biochemical
assays may be conducted.
Ethyl 3r,7,7-Trichloro-2-thiabicyclo[3.2.0]heptan-6-one-4-car-
boxylate (21r). To a stirring solution of dichlorocyclobutanone 18
(1.005 g, 3.735 mmol) in dry CH2Cl2 (45 mL) at 0 °C was added
(88) It should be noted that the development of cyclobutanone analogues of
ꢀ-lactam antibiotics into therapeutically useful ꢀ-lactamase inhibitors will require
attention to numerous issues beyond those associated with the tendency of such
compounds to form hydrates or hemiketals in the active sites of metallo- or
serine ꢀ-lactamases, respectively. For example, drug candidates that rely on an
electrophilic functionality may be prone to inactivation by natural electrophile
scavengers such as glutathione and might exhibit toxic side effects arising from
nonspecific covalent modification of biomolecules in the host.89,90 Earlier in vitro
studies of inhibition of serine proteases by electrophilic aldehydes and ketones91
have revealed, however, that koff for such reversible inhibitors can be substantially
decreased and selectivity increased with appropriate structural modifications of
the inhibitors which increase their affinity for the enzyme through specific
favorable hydrogen bonds and van der Waals contacts with active-site residues.
A reviewer is thanked for constructive comments in this regard.
(89) For examples of drug leads which incorporate ketone functionalities,
see ref 37 and (a) Romero, F. A.; Hwang, I.; Boger, D. L. J. Am. Chem. Soc.
2006, 128, 14004–14005. (b) Hardouin, C.; Kelso, M. J.; Romero, F. A.; Rayl,
T. J.; Leung, D.; Hwang, I.; Cravatt, B. F.; Boger, D. L. J. Med. Chem. 2007,
50, 3359–3368. (c) Wegener, D.; Hildmann, C.; Riester, D.; Schober, A.; Myer-
Almes, F.-J.; Deubzer, H. E.; Oehme, I.; Witt, O.; Lang, S.; Jaensch, M.;
Makarov, V.; Lange, C.; Busse, B.; Schwienhorst, A. Biochem. J. 2008, 413,
143–150.
(90) For an overview of potential difficulties with drug candidates that
incorporate ketone functionalities, see: Rishton, G. M. Drug DiscoV. Today 2003,
8, 86–96.
(91) For examples of enhancement of inhibitory potency by structural
modifications of electrophilic ketone-based protease inhibitors, see refs 35b and
40h and Brady, K.; Abeles, R. H. Biochemistry 1990, 29, 7608–7617.
6980 J. Org. Chem. Vol. 73, No. 18, 2008

Cyclobutanone Mimics of Penicillins
a solution of SO2Cl2 (1 M in CH2Cl2, 4.50 mL, 4.50 mmol). The
solution was stirred at room temperature for 4 h before it was
concentrated under reduced pressure to give 21r as a white
crystalline solid (1.132 g, 3.729 mmol, 99.8%). Mp 84-87 °C. 1H
NMR (300 MHz, CDCl3): δ 1.30 (t, J ) 7.1 Hz, 3H, CO2CH2CH3),
3.97 (dd, J4,3 ) 4.8 Hz, J4,5 ) 5.8 Hz, 1H, H4), 4.23 (B of ABX3,
JAB ) 10.8 Hz, JBX ) 7.1 Hz, 1H, one of CO2CH2CH3), 4.26 (A
of ABX3, JAB ) 10.8 Hz, JAX ) 7.1 Hz, 1H, one of CO2CH2CH3),
4.94 (d, J1,5 ) 8.3 Hz, 1H, H1), 5.20 (dd, J5,1 ) 8.3 Hz, J5,4 ) 5.8
Hz, 1H, H5) 5.87 (d, J3ꢀ,4 ) 4.8 Hz, 1H, H3ꢀ). 13C NMR (75.5
MHz, CDCl3): δ 14.0, 59.8, 60.0, 62.5, 65.5, 74.1, 85.0, 166.2,
192.0. IR (film, cm-1): 2985, 1815, 1740, 1372, 1266, 1236, 1212.
LRMS (EI) m/z (relative intensity): 306 (M[(37Cl235Cl)]+, 1), 304
([M(37Cl35Cl2)]+, 2), 302 ([M(35Cl3)]+, 2), 267 (10), 259 (15), 207
(100), 169 (50), 131 (55), 99 (90). HRMS (EI) m/z: 301.9338 calcd
for C9H935Cl3O4S; 301.9336 obsd.
Ethyl 7,7-Dichloro-3ꢀ-hydroxy-2-thiabicyclo[3.2.0]heptan-6-one-
4-carboxylate (22ꢀ). A solution of the trichlorocyclobutanone 21r
(57.5 mg, 0.189 mmol) in MeCN (2 mL) was added slowly
dropwise to a stirring solution of AgOTf (60.1 mg, 0.234 mmol)
in MeCN (5 mL) and H2O (2 mL). The mixture was stirred for 4 h
at room temperature before it was concentrated under reduced
pressure, diluted with CH2Cl2 (25 mL) and filtered through Celite.
The resulting yellow oil was subjected to flash chromatography
(10% EtOAc/hexane), which provided a colorless oil (48.6 mg,
0.170 mmol, 90%) that was determined to be an 88:6:6 mixture of
the thiolactol 22ꢀ, epimer 22r, and an oxa-thia-tricyclo-octane 22c,
respectively.
3H, CO2CH2CH3), 3.32 (s, 3H, OCH3), 3.75 (app. t, J4,3ꢀ ) 4.5
Hz, J4,5 ) 5.5 Hz, 1H, H4), 4.17 (B of ABX3, JAB ) 10.8 Hz, JBX
) 7.1 Hz, 1H, one of CO2CH2CH3), 4.26 (A of ABX3, JAB ) 10.8
Hz, JAX ) 7.1 Hz, 1H, one of CO2CH2CH3), 4.66 (d, J1,5 ) 8.1
Hz, 1H, H1), 5.17 (dd, J5,4 ) 5.5 Hz, J5,1 ) 8.1 Hz, 1H, H5), 5.32
(d, J3ꢀ,4 ) 4.5 Hz, 1H, H3ꢀ). 13C NMR (75.5 MHz, CDCl3): δ 14.1,
56.7, 57.4, 58.7, 61.8, 66.4, 85.3, 97.4, 167.3, 193.4. IR (film, cm-1):
2986, 2930, 2831, 1813, 1739, 1464, 1335, 1265, 1215, 1085, 1021.
LRMS (EI) m/z (relative intensity): 302 (M[(37Cl2)]+, 0.4), 300
([M(37Cl35Cl)]+, 1.5), 298 ([M(35Cl2)]+, 2), 253 (20), 217 (10), 203
(20), 189 (20), 167 (60), 165 (100), 143 (50). HRMS (EI) m/z:
297.9833 calcd for C10H1235Cl2O4S; 297.9827 obsd.
Representative Procedure for Substitutions Promoted by Silver
Triflate (Table 3). Ethyl 7,7-Dichloro-3ꢀ-methoxy-2-thiabicyclo-
[3.2.0]heptan-6-one-4-carboxylate (23ꢀ). The 3R-chlorocyclobu-
tanone 21r (104.1 mg, 0.343 mmol) in CH2Cl2 (1 mL) was added
dropwise over 10 min to a suspension of AgOTf (118.3 mg, 0.460
mmol), MeOH (75 µL, 1.841 mmol), and 3 Å MS (1 g) in CH2Cl2
(8 mL) at 0 °C. The reaction mixture was allowed to warm to room
temperature over 2 h and stirred for an additional 4 h at room
temperature before dilution with CH2Cl2 and filtration through
Celite. The filtrate was concentrated under reduced pressure, and
1H NMR of the crude mixture showed a product distribution of
22:72:2:4 for R-OMe(23r):ꢀ-OMe(23ꢀ):ꢀ-Cl(21ꢀ):26, respectively.
Flash chromatography (5% EtOAc/hexane) provided 23ꢀ (29.7 mg,
0.099 mmol, 29%) and a 38:62 mixture of 23r:23ꢀ (32.0 mg, 0.107
mmol, 31%). 1H NMR (300 MHz, CDCl3): δ 1.28 (t, J ) 7.1 Hz,
3H, CO2CH2CH3), 3.34 (s, 3H, OCH3), 3.82 (s, 1H, H4), 4.19 (q,
J ) 7.1 Hz, 2H, CO2CH2CH3), 4.59 (d, J1,5 ) 8.6 Hz, 1H, H1)
5.02 (d, J5,1 ) 8.6 Hz, 1H, H5), 5.46 (s, 1H, H3R). 13C NMR (75.5
MHz, CDCl3): δ 14.0, 56.3, 57.9, 59.4, 62.2, 64.9, 90.4, 95.2, 167.8,
192.7. IR (film, cm-1): 2984, 2934, 2829, 1817, 1734, 1370, 1313,
1259, 1212, 1086. LRMS (EI) m/z (relative intensity): 302
(M[(37Cl2)]+, 1), 300 ([M(37Cl35Cl)]+, 5), 298 ([M(35Cl2)]+, 8), 263
(30), 252 (20), 217 (40), 203 (100), 189 (90), 169 (55), 165 (55).
HRMS (EI) m/z: 297.9833 calcd for C10H1235Cl2O4S; 297.9836 obsd.
Ethyl 7,7-Dichloro-2-thiabicyclo[3.2.0]hept-3-ene-6-one-4-car-
boxylate (26). A solution of cyclobutanone 21r (105.0 mg, was
0.346 mmol) in CH2Cl2 (2 mL) was slowly added to a stirring
solution of AgOTf (104.1 mg, 0.405 mmol) in refluxing CH2Cl2
(20 mL) dropwise over 10 min. After 2 h at reflux, the solution
was cooled to room temperature, diluted with CH2Cl2, filtered
through Celite, and concentrated in vacuo. Purification by flash
chromatography (10% EtOAc/hexane) furnished the elimination
product 26 as a colorless oil that crystallized under reduced pressure
(74.4 mg, 0.279 mmol, 81%). Mp 75-76 °C. 1H NMR (300 MHz,
CDCl3): δ 1.31 (t, J ) 7.1 Hz, 3H, CO2CH2CH3), 4.21 (B of ABX3,
JAB ) 10.7 Hz, JBX ) 7.1 Hz, 1H, one of CO2CH2CH3), 4.24 (A
of ABX3, JAB ) 10.7 Hz, JAX ) 7.1 Hz, 1H, one of CO2CH2CH3),
4.89 (d, J1,5 ) 10.0 Hz, 1H, H1) 5.43 (dd, J5,1 ) 10.0 Hz, J5,3 )
1.7 Hz, 1H, H5), 7.43 (d, J3,5 ) 1.7 Hz, 1H, H3). 13C NMR (75.5
MHz, CDCl3): δ 14.2, 59.6, 61.2, 71.3, 93.8, 122.2, 144.4, 161.1,
187.4. IR (film, cm-1): 3075, 2984, 1812, 1705, 1576, 1370, 1327,
1238, 1078. LRMS (EI) m/z (relative intensity): 268 ([M(37Cl35Cl)]+,
1.5), 266 ([M(35Cl2)]+, 2.0), 238 (20), 203 (100), 175 (40). HRMS
(EI) m/z: 265.9571 calcd for C9H835Cl2O3S; 265.9572 obsd.
Alternatively, the unsaturated ester 26 could be prepared from
thiolactol 22ꢀ. TsOH ·H2O (10.9 mg, 0.057 mmol) was stirred in
PhMe (40 mL) and heated at reflux under a Dean-Stark trap for
2 h.92 The thiolactol 22ꢀ (83.7 mg, 0.294 mmol) was then added
as a solution in PhMe (2 mL) and stirred at reflux for an additional
18 h. The solution was concentrated under reduced pressure and
purified by flash chromatography (10% EtOAc/hexane) to give a
colorless oil that crystallized under vacuum (73.8 mg, 0.276 mmol,
94%).
Thiolactol 22ꢀ. 1H NMR (500 MHz, CDCl3): δ 1.26 (t, J ) 7.1
Hz, 3H, CO2CH2CH3), 2.7-2.9 (brs, 1H, OH), 3.83 (s, 1H, H4),
4.18 (q, J ) 7.1 Hz, 2H, CO2CH2CH3), 4.67 (d, J1,5 ) 8.5 Hz, 1H,
H1) 5.05 (d, J5,1 ) 8.5 Hz, 1H, H5), 5.98 (s, 1H, H3R). 13C NMR
(125 MHz, CDCl3): δ 14.0, 59.3, 59.8, 62.3, 65.0, 86.0, 90.4, 167.9,
193.1. IR (film, cm-1): br 3600-3300, 2984, 1815, 1732, 1246,
1213, 1024. LRMS (EI) m/z (relative intensity): 288 (M[(37Cl2)]+,
0.4), 286 ([M(37Cl35Cl)]+, 5), 284 ([M(35Cl2)]+, 8.0), 238 (15), 203
(50), 195 (75), 151 (85), 115 (100). HRMS (EI) m/z: 283.9677
calcd for C9H1035Cl2O4S; 283.9681 obsd.
Epimer 22r. 1H NMR (500 MHz, CDCl3): δ 1.26 (3H,
CO2CH2CH3), 2.7-2.9 (1H, OH), 3.70 (dd, J4,3ꢀ ) 4.6 Hz, J4,5 )
4.8 Hz, 1H, H4), 4.18 (2H, CO2CH2CH3), 4.79 (d, J1,5 ) 8.4 Hz,
1H, H1), 5.08 (dd, J5,1 ) 8.4 Hz, J5,4 ) 4.8 Hz, 1H, H5), 5.84 (d,
J3ꢀ,4 ) 4.6 Hz, 1H, H3ꢀ). 13C NMR (125 MHz, CDCl3): δ 14.0,
57.0, 58.7, 61.6, 65.7, 86.2, 87.0, 166.8, 192.3.
1
Oxa-thia-tricyclo-octane 22c. H NMR (500 MHz, CDCl3): δ
1.26 (3H, CO2CH2CH3), 2.7-2.9 (1H, OH), 3.7 (m, 1H, H4), 3.97
(m, 1H, H5), 4.18 (m, 2H, CO2CH2CH3), 4.26 (m, 1H, H1), 5.87
(m, 1H, H3R). 13C NMR (125 MHz, CDCl3): δ 14.0, 50.1, 55.2,
56.4, 62.3, 86.8, 90.6, 105.7, 168.4.
Alternatively, 22ꢀ could be obtained by hydrolysis of 21r
without AgOTf. H2O (15 mL) was added to a solution of the 3R-
Cl cyclobutanone 21r (769.2 mg, 2.534 mmol) in MeCN (15 mL).
After stirring at room temperature for 48 h, the solution was
concentrated in vacuo to a yellow oil. Flash chromatography (10%
EtOAc/hexane) afforded a colorless oil (543.2 mg, 1.905 mmol,
75%) with spectral properties identical to those of the material
prepared in ROH/MeCN: an 88:6:6 mixture of 22ꢀ, 22r, and 22c.
Representative Procedure for Solvolysis of 21r in ROH/
MeCN (Table 2). Ethyl 7,7-Dichloro-3r-methoxy-2-thiabicyclo-
[3.2.0]heptan-6-one-4-carboxylate (23r). The 3R-chlorocyclobu-
tanone 21r (102.2 mg, 0.337 mmol) was dissolved in MeCN (5
mL) and stirred with MeOH (5 mL) at room temperature for 48 h.
The solution was concentrated under reduced pressure to give a
1
colorless oil that partially crystallized in vacuo. H NMR of the
crude mixture showed a product distribution of 75:24:1 for
R-OMe(23r):ꢀ-OMe(23ꢀ):26, respectively. Flash chromatography
(5% EtOAc/hexane) provided 23r (23.4 mg, 0.0782 mmol, 23%,
98% pure) and a 68:32 mixture of 23r and 23ꢀ (50.3 mg, 0.168
mmol, 50%). 1H NMR (300 MHz, CDCl3): δ 1.27 (t, J ) 7.1 Hz,
(92) Incomplete conversion was observed when PhMe and TsOH were not
predried.
J. Org. Chem. Vol. 73, No. 18, 2008 6981

Johnson et al.
Ethyl 2-Thiabicyclo[3.2.0]hept-3-ene-6-one-4-carboxylate (27).
Dichlorocyclobutanone 26 (171.3 mg, 0.641 mmol) was dissolved
in AcOH (15 mL), combined with zinc dust (216.0 mg, 3.303
mmol), and stirred at 80 °C for 5 h. The reaction mixture was cooled
to room temperature, diluted with EtOAc, and filtered through glass
wool to remove residual zinc dust. The organic solution (75 mL)
was washed with 10% HCl (2 × 75 mL) and brine (50 mL) before
it was dried over Na2SO4 and concentrated. Flash chromatography
(10% EtOAc/hexane) provided the dechlorinated cyclobutanone 27
and CHRP grants and by an operating grant from the Canadian
Institutes of Health Research (to G.I.D.). We thank Janet Venne
for assistance with NMR experiments and Professor Thammaiah
Viswanatha for valuable discussions.
Supporting Information Available: Experimental proce-
1
dures and characterization data; H and 13C NMR spectra for
18-19, new compounds, hydrate formation, and hemiketal
formation; additional tables of calculated conformational and
configurational preferences; total energies of modeled structures;
Cartesian coordinates for optimized structure D; plots of the
calculated energy of rotation around the C4-CO2Et bond in
18; tables of selected spectroscopic data for cyclobutanones and
cyclobutanone hemiketals; tables and plots of percent hemiketal
formation over time; crystallographic information files in CIF
format and tables of crystallographic data for 9, 20, 21r, 25r,
and 28ꢀ; complete reference 65. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
as a colorless oil (106.5 mg, 0.537 mmol, 84%). H NMR (300
MHz, CDCl3): δ 1.25 (t, J ) 7.1 Hz, 3H, CO2CH2CH3), 3.34 (ddd,
Jgem ) 18.7 Hz, J7ꢀ,1 ) 5.7 Hz, J7ꢀ,5 ) 3.3 Hz, 1H, H7ꢀ), 3.74
(ddd, Jgem ) 18.7 Hz, J7R,1 ) 8.3 Hz, J7R,5 ) 5.1 Hz, 1H, H7R),
4.11-4.22 (m, 3H, H1 and CO2CH2CH3), 5.05-5.14 (m, 1H, H5),
7.43 (s, 1H, H3). 13C NMR (75.5 MHz, CDCl3): δ 14.2, 37.0, 60.7
(2C), 76.0, 121.2, 143.2, 161.8, 200.0. IR (film, cm-1): 3069, 2983,
1790, 1700, 1567, 1370, 1328, 1305, 1226. LRMS (EI) m/z (relative
intensity): 200 ([M + 2]+, 2), 199 ([M+1]+, 5), 198 ([M]+, 40),
170 (10), 156 (40), 141 (40), 128 (60), 111 (100), 97 (25). HRMS
(EI) m/z: 198.0351 calcd for C9H10O3S; 198.0349 obsd.
Acknowledgment. Dedicated to the memory of Nicholas
Justin Taylor. This work was supported by NSERC Discovery
JO801274M
6982 J. Org. Chem. Vol. 73, No. 18, 2008