Acyloxylactonisations mediated by lead tetracarboxylates

Article abstract of DOI:10.1016/j.tet.2009.01.042

The reaction of lead(IV) tetracarboxylates with carboxylic acids containing unsaturated side chains has been found to give acyloxy lactone products in a diastereoselective process; the reaction can be extended to lead(IV) tetrazolates to give the analogous outcome. Mechanistic implications of these results are discussed.

Full text of DOI:10.1016/j.tet.2009.01.042

Tetrahedron 65 (2009) 2537–2550
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Acyloxylactonisations mediated by lead tetracarboxylates
,
Ian F. Cottrell b, Andrew R. Cowley a, Laura J. Croft a, Lauren Hymns a, Mark G. Moloneya
*
,
Ewan J. Nettleton a, H. Kirsty Smithies a, Amber L. Thompson a
a The Department of Chemistry, Chemistry Research Laboratory, The University of Oxford, Mansfield Road, Oxford OX1 3TA, UK
b Department of Process Research, Merck Sharp and Dohme Research Laboratories, Hertford Rd, Hoddesdon, Herts. EN11 9BU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 August 2008
Received in revised form 19 December 2008
Accepted 9 January 2009
Available online 15 January 2009
The reaction of lead(IV) tetracarboxylates with carboxylic acids containing unsaturated side chains has
been found to give acyloxy lactone products in a diastereoselective process; the reaction can be extended
to lead(IV) tetrazolates to give the analogous outcome. Mechanistic implications of these results are
discussed.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Lead(IV)
Lactonisation
Cyclisation
Carboxylic acids
1. Introduction
to aminophosphonic acids by decarboxylation using lead tetra-
acetate (LTA).17 Considerable effort has been expended upon de-
The pioneering work of Criegee in the 1960s firmly established
the synthetic utility of lead(IV) reagents for a variety of oxidative
processes of hydroxy, amino, alkene and carboxylate groups1–7 and
this reagent has found extensive application, for example, in high
yielding processes in alkaloid chemistry.8 The utility of these re-
agents comes from a potent thermodynamic driving force in the
reduction of lead(IV) to lead(II), as indicated by a very high oxi-
dising potential9 and facile ligand exchange in solution, particularly
for carboxylic acids.10,11 It is perhaps surprising, therefore, that
vigorous conditions are required for these oxidative processes,
which usually involve refluxing acetic acid or benzene solutions of
the substrate with, most commonly, lead tetraacetate; presumably
this is a consequence of a kinetically unfavourable rather than an
intrinsically thermodynamically difficult process. The synthetic
utility of the potent leaving group ability of lead(IV) is illustrated in
the well-known oxidative decarboxylation process of carboxylic
acids,12 a reaction which is known to be catalysed by copper(II) and
proceeds by a radical mechanism, once again driven by the re-
duction of lead(IV) to lead(II).13,14 The ease of lead(IV)-mediated
processes and in particular the potent leaving group ability of lea-
d(IV) is further illustrated by the facile Grob fragmentation ob-
served upon treatment of dicarboxylic acids,15,16 and a more recent
variation of this process is the conversion of aminocarboxylic acids
fining the mechanistic course of these lead(IV) mediated reactions,
with the most accepted process involving cationic or radical in-
termediates.14 More recently, the development of ligand coupling
processes,18 which give synthetically useful carbon–carbon bond
forming reactions19–24 has illustrated a more sophisticated di-
mension to organolead chemistry, and cyclisation reactions are
beginning to emerge, which are driven by this reagent.25,26 Fur-
thermore, LTA-mediated hetero-domino processes27 and oxidative
cleavage of vicinal diols, 28 have been shown to provide direct and
rapid access to high molecular complexity under mild conditions.
Our interest in this area has been in the development of novel
reaction processes mediated by lead(IV) and particularly for car-
bon–carbon bond formation. Lead tetracarboxylates derived from
either monocarboxylic acids or dicarboxylic acids by metathesis of
lead tetraacetate are readily accessible, and have generally been
found to be stable amorphous powders, which can be successfully
applied to the Pinhey arylation procedure, and for the trans-
metallation of allylstannanes leading to the formation of allylic
esters.29 In this earlier work, we examined ligands, principally
substituted benzoates, which would be chemically inert to lead(IV),
but of interest was examination of carboxylic acids substituted with
potentially reactive
p-functionality. Reactions of such compounds
with LTA are not unknown, and were first reported by Alder and
Schneider30 who indicated that plumbolactonisation of bicyclic
dicarboxylic acid 1 could be used to generate bislactone 2 in good
(60%) yield. Later, other conformationally restricted dicarboxylic
acids (e.g., 3) were found to be applicable, although yields were not
* Corresponding author.
E-mail address: mark.moloney@chem.ox.ac.uk (M.G. Moloney).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.01.042

2538
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
Scheme 1.
always high and the reactions were not developed further.31–34
Nothing further was done on this reaction until 1980, when Corey
reported35 that the reaction could be applied to a small range of
enedioic acid substrates (e.g., 4a,b) to give bislactone products (e.g.,
5, 6) in a stereospecific manner (Scheme 1), and he suggested that
the first step involves plumbolactonisation followed by either SN1
or SN2 displacement of the resulting organolead(IV) intermediate,
depending on the reaction conditions. Similar processes are of
course well known for halo(iodo,bromo)lactonisation36–39 and
halolactamisation,40–42 mercurilactonisation and selenolactonisa-
tion43,44 as well as other electrophilic cyclisation processes,37,45,46
which are enjoying a renaissance of interest. The synthetic value of
these types of processes has been significantly expanded by recent
developments in palladium-catalysed oxygenations,56 amina-
tions57,58 and carbocyclisations59 of alkenes. In order to pursue the
reactivity of lead(IV) in this context, we examined lactonisation
processes mediated by lead(IV), particularly to establish their
generality and stereospecificity; a preliminary report of this work
has appeared.47
Lead(IV) tetracarboxylates 7a–j were effectively prepared by
metathesis of commercially available lead tetraacetate with a range
of carboxylic acids, by azeotropic removal of acetic acid in toluene
using our published method48,29 (Scheme 2); these compounds are
amorphous colourless or pale yellow powders, freely soluble in
chloroform, with the exception of lead tetrathiophenecarboxylate
7i, which is a bright yellow and very insoluble powder. They were
used without further purification. Reaction of lead(IV) tetra-
carboxylates 7a–j with 4-pentenoic acid gave the lactone products
8a–j in variable but in some cases excellent yield; the formation of
butyrolactone ring had been confirmed previously in a closely re-
lated system by single crystal X-ray analysis.49 Notable was that
although lead tetraacetate 7a could give a high yield of product, the
reaction was unreliable. On the other hand, lead tetrathiophene-
carboxylate 7i gave a similarly high yield of the lactone product
although it was not freely soluble in the reaction mixture; progress
of this reaction could be easily monitored visually, beginning as
a yellow slurry, and finishing as a white slurry. The 2,6-dimethoxy-
benzoate 8g (60%) was formed in substantially better yield than the
corresponding benzoate 8b (45%) and the 2,6-dichlorobenzoate 8h
(22%), consistent with an electronic reactivity correlation, but the
lower yields of lactones 8c,d, which lack ortho-substitution point to
the activating effect of bulk at this position; such reactivity en-
hancement resulting from the presence of o-groups is well known
in the reactivity of aryllead triacetates.22 Purification of products
from low yielding reactions (e.g., 8c) proved to be problematic,
principally as a result of the large excess of benzoate by-product
generated in the course of the reaction. Of interest is that an at-
tempt to isolate lead tetra(4-pentenoate) by direct metathesis of
lead tetraacetate with 4-pentenoic acid gave an unstable product,
from which only lactone 9 could be isolated.
In view of the convenience of handling and the high yielding
reaction of lead(IV) tetra(2-thiophenecarboxylate), this reagent
was examined with a variety of substituted 4-pentenoic acids 10 to
assess the scope of the reaction (Scheme 3 and Table 1); the re-
quired acids were available commercially, or were prepared by
allylation of methyl isobutyrate followed by ester hydrolysis (entry
Scheme 2.

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2539
Scheme 3.
Table 1
Formation of substituted 4-pentenoic acids 10 and their reaction with lead(IV)
tetra(2-thiophenecarboxylate) to give lactones 11 (Scheme 2)
Entry R1
R2
R3 R4 R5 R6 Methoda Acid
Lactone
Yield (%) Yield (%)
1
2
3
4
5
6
7
8
Me
Ph
Me
Me
Ph
Ph
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
Ph
Ph
H
H
H
A
B
B
B
C
10a (64) 11a (58)
10b (80) 11b (69)
10c (94) 11c (73)
10d (78) 11d (75)
10e (32) 11e (0)
10f (73) 11f (0)
10g (48) 11g (48)
10h (37) 11h (53b)
CH2CH2CH2CH2CH2
H
Me
H
H
H
Me
H
H
H
H
H
H
H
H
C
D
D
D
d
d
9
10
11
Me Me
H
H
10i (6)
d
11i (53)
11j (34)
11k (0)
H
H
H
Me
d
a
Method A: allylation of methyl isobutyrate followed by ester hydrolysis; Method
B: allylation of substituted acetic acids using 2.5 equiv of LDA; Method C: allylation
of malonate followed by ester hydrolysis and decarboxylation; Method D: modifi-
cation of Johnson orthoester Claisen chemistry.
b
Mixture of cis/trans diastereomers (1:3.6).
1, Table 1),50 allylation of substituted acetic acids using 2.5 equiv of
LDA (entries 2–4, Table 1),51 allylation of malonate followed by
ester hydrolysis and decarboxylation (entries 5 and 6, Table 1), or
a modification of Johnson orthoester Claisen chemistry52,53 repor-
ted by Ja¨ger and Gu¨nther (entries 7–9, Table 1).54 2-Vinylbenzoic
acid 12 was prepared using a method reported by Dale et al.55 by
reaction of the Grignard derived from 2-bromostyrene followed by
reaction with crushed dry ice. The substituted 4-pentenoic acids
10a–i (Scheme 3 and Table 1) were reacted with lead(IV) tetra(2-
thiophenecarboxylate) as a solution or suspension in toluene under
an argon atmosphere, and heated for 3 h, giving the lactone prod-
ucts 11 of 5-exo cyclisation usually in moderate to good yield
(Scheme 3, Table 1, entries 1–11), but none of the six-membered
ring lactones, formed via 6-endo cyclisation, were isolated. Lactones
11c and 11j were obtained as single diastereomers, and the five-
membered ring structure of all of the products 11 was assigned on
the basis of IR spectrometry (1766–1785 cmꢀ1), 1H NMR spectros-
copy as well as X-ray crystallography where possible (lactones 11c,
11g and 11j) (Fig. 1).56 Noteworthy was that the yield of lactone 8i
derived from 4-pentenoic acid was 60% if the reaction was con-
ducted with microwave irradiation (90 ꢁC, at 150 W and 100 psi),
although lactone 9 was also obtained in this case in 20% yield. Good
Figure 1. Thermal ellipsoid plots (ORTEP-3) at 40% probability level for compounds
11c, 11g, 11j and 17.
formation of spirocyclic lactones (entry 4, Table 1), demonstrated
that the methodology can be used to generate fused and spirocyclic
ring systems (Scheme 4). However, 2-cyclopenten-1-acetic acid 14
(n¼1) and 2-cyclohex-2-enylacetic acid 14 (n¼2)57–59 failed to react
with lead(IV) tetra(2-thiophenecarboxylate) to give the expected
products 15 (n¼1 and 2) (Scheme 5). These results show that
substitution in positions 2-, 3- and 4- can all be tolerated under the
general reaction conditions, but that substitution at the terminal
position is not, and this may be a result of unfavourable steric in-
teractions with the eight-coordinate tetaracarboxylate lead(IV)
cation, shown by X-ray crystallography of LTA60 and lead(IV) tet-
ra(o-benzoylbenzoate).61,62
It became apparent that the yields of the lactones in these re-
actions were compromised by the formation of benzaldehyde by
oxidation of the solvent, toluene, which was readily detected in the
yields of products with substituents a- to the carboxylic acid were
obtained (entries 1–4, 10) preferentially giving the cis outcome
(entry 10, Table 1) but this contrasted with complete loss of re-
activity if substituents were located at the terminus of the double
bond (entries 5, 6 and 11, Table 1). Substituents along the length of
the chain were tolerated giving good yields of product (entries 7–9,
Table 1). Of interest was that 2-vinylbenzoic acid 12 cyclised effi-
ciently to give 3-oxo-1,3-dihydro-isobenzofuran-1-yl methyl 2-
thiophenecarboxylate 13 in a 43% yield, and this along with the
Scheme 4.

2540
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
Scheme 5.
NMR spectra of crude reaction products, and by formation of lac-
tone 9 in variable amounts; for example, in the reaction of 4-pen-
tenoic acid, this amounted to 15%. When the reaction of
4-pentenoic acid and lead(IV) tetra(2-thiophenecarboxylate) was
performed in the non-oxidisable solvents,
a,a,a-trifluorotoluene,
chlorobenzene, dimethoxymethane, the yields of corresponding
lactone 8i were 62, 67, and 53%, respectively; the convenient re-
moval of
a,a,a-trifluorotoluene made this the reagent of choice.
Similar efficacy of trifluorotoluene in lead tetraacetate-mediated
diol cleavages, which induce subsequent domino reactions has
been reported.63 Reduction of the concentration of the reaction
improved yields further; for example, in the reaction of 2-methyl-
4-pentenoic acid with lead(IV) tetra(2-thiophenecarboxylate) at
90 ꢁC for 20 min, at 50 and 25 mmol Lꢀ1, the yield of lactone 8i
amounted to 77 and 75% and lactone 9 to 17 and 13%, respectively.
Thus, dilute reaction conditions improved yields, and the optimal
Scheme 6.
lactones, respectively, 18 and 19 (Scheme 6), assigned on the basis
of the carbonyl stretch in their IR spectra (1734 and 1728 cmꢀ1
,
respectively), in agreement with the reported absorption frequen-
cies for six-membered and larger ring lactones.64
The mechanism of these reactions deserves some comment:
oxidative decarboxylation of carboxylic acids by lead tetraacetate is
well known,14 and the available evidence supports a free-radical
chain mechanism.13,65–67 However, in the plumbolactonisation
process reported here, no products of decarboxylation derived from
either the unsaturated ester or thiophenecarboxylic acid were ob-
served, and neither were any products from radical trapping when
the reaction between lead(IV) tetra(2-thiophenecarboxylate) and
4-pentenoic acid was carried out in the presence of 1,1-diphenyl-
ethylene; this approach has been successfully applied to demon-
strate radical intermediacy in related processes.68,13 Although
yields of lactone 8i were lower in these experiments (20%), 54% of
the 1,1-diphenyethylene was recovered and no other products were
isolated. Furthermore, if the reaction was conducted in the pres-
ence of radical inhibitors (galvinoxyl and benzoquinone), lactone
formation was not prevented.47 Even if the reaction was carried out
under an oxygen atmosphere, lactone formation was still achieved
in a 53% yield, and when reaction of lead(IV) tetra(2-thio-
reaction conditions were 25 mmol Lꢀ1 in
a,a,a-trifluorotoluene
solvent at 90 ꢁC for 20 min, and the reaction was stopped once all
the Pb(IV) had been reduced to Pb(II), indicated by the change in
colour of the reaction mixture from bright yellow to white. Simi-
larly, acid 16 was readily converted to lactone 17 in 43% yield,
whose stereochemistry was unequivocally established by X-ray
crystallographic analysis.54 Using these optimised conditions at
135–145 ꢁC, even previously unreactive substrates could be con-
verted to the lactones, albeit in low yield; thus, 4-hexenoic acid
gave lactone 11k in 10% yield. Similarly, by reacting 2-cyclopenten-
1-acetic acid with lead(IV) tetra(2-thiophenecarboxylate) at 135 ꢁC,
a 9% yield of the bicyclic lactone 15 (n¼1) was isolated. The ste-
reochemistry of the fused ring system of this compound was
established by NOE studies (irradiation of (H-8endo) gives en-
hancement at (H-3endo); of (H-3exo) gives enhancement at (H-4) (H-
5) and (H-8exo); of (H-4) gives enhancement at (H-3exo) and (H-5);
of
d
4.93 (H-5) gives enhancement at
d
2.78 (H-3exo),
d
3.13 (H-4); of
2.16
d
5.45 (H-6) gives enhancement at (H-8endo),
d
2.00 (H-7exo),
d
phenecarboxylate) with 4-pentenoic acid in a,a,a-trifluorotoluene
(H-7endo)), confirming the reaction had given the cis-fused rings
and the 2-thiophenecarboxylate resides in an exo-position to the
ring junction.
Other higher homologous unsaturated acids were found to be
applicable to this sequence; both 5-hexenoic acid and 6-heptenoic
acid successfully underwent plumbolactonisation with lead(IV)
tetra(2-thiophenecarboxylate) to give six- and seven-membered
at 90 ꢁC in the dark was attempted, lactone 8i was obtained in a 74%
yield. All this evidence together strongly suggests that these re-
actions of 4-pentenoic acids with lead(IV) tetra(2-thio-
phenecarboxylate) does not occur via a radical pathway. On the
other hand, direct interaction of the lead(IV) species with the
system does not appear to be operating either; thus, although the
reaction of lead tetraacetate with cyclohexene in -trifluorotoluene
p-
a,a,a

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2541
Scheme 7.
at 90 ꢁC under argon (normally carried out in refluxing benzene or
acetic acid69) gave products 20 and 21 as detected by GC–MS
analysis (Scheme 7), when the analogous reaction was carried out
using lead(IV) tetra(2-thiophenecarboxylate), no substitution or
addition products were formed, and after 3 h, only starting mate-
that addition to the double bond followed by intramolecular
opening by the carbomethoxy group is strongly indicated for the
norbornene system. Taken together, the evidence suggests initial
carboxylate exchange in the reactions of 4-pentenoic acids, rather
than an initial PbIV–
p interaction. Direct evidence for rapid car-
rials were present. The lack of Pb–
p
interaction was further dem-
boxylate ligand exchange at lead(IV) came from an examination of
the 104.4 MHz 207Pb spectra for the metathesis of lead tetra-
benzoate (PbB4) with 4-pentenoic acid (P); at 298 K, there was only
a broad resonance, but at 225 K, this resolved into a pentet of sig-
nals approximately in a ratio of 0.3:2.8:6:4.7:1.32, indicative of
a free ligand exchange equilibrium (Fig. 2) but weighted in favour of
the benzoate containing species PbPB3 and PbB4, with lead tetra-
onstrated because no cyclisation of lead(IV) tetra(2-
thiophenecarboxylate) with ethyl-3-oxohept-6-enoate 22 oc-
curred, and the only products, which could be isolated were the
mono- and di-(2-thiophenecarboxylate) esters 23a and 23b
(Scheme 7), arising from a reaction analogous to the well known a-
acetoxylation of
b-dicarbonyl compounds by lead tetracarbox-
ylates.70 This contrasts with the work of Ferraz et al.71 who found
that substrate 22 could be readily cyclised with iodine to give enol
ether 25 in good yield (Scheme 7). Furthermore, reaction of methyl
benzoate located at one extremity (
d
ꢀ1896.8 ppm) and lead
tetra(4-pentenoate) at the other (
d
ꢀ1877.0).
A ligand coupling mechanism74,18 has been proposed for vinyl-
ations, phenylations and alkynylations mediated by organolead(IV)
tricarboxylates22 by reductive elimination at the lead(IV) centre,
and the possibility of the existence of radicals under the reaction
conditions appears to be excluded. Since the available evidence
indicates that reaction of lead(IV) tetra(2-thiophenecarboxylate) is
with the carboxylic acid functionality rather than the double bond
of pentenoic acids, it seems likely that the first step of the reaction
with 4-pentenoic acid is ligand exchange to give a mixed lead
tetracarboxylate 27 (Scheme 9). Rather than decarboxylation of the
carboxylic acid via a radical pathway, the lead(IV) centre directs
4-pentenoate with lead(IV) tetra(2-thiophenecarboxylate) in a,a,a-
trifluorotoluene at 90 ꢁC for 3 h gave no colour change and only
unreacted methyl ester was detected by GC–MS analysis of the
crude reaction mixture; no other products were detected. This
contrasts with the work of Moriarty, who showed that the reaction
of methyl endo-5-carboxybicyclo[2.2.1]hept-2-ene 25 gave acetoxy
lactone 26 (Scheme 8), while the saturated analogue, methyl endo-
5-carboxy bicycle[2.2.1]heptane, was completely unreactive under
the oxidation conditions.72,73 For these results, Moriarty concluded
cyclisation, possibly by the formation of a
rangement could then occur to give a transient Pb–
which then undergoes ligand coupling to give the observed prod-
uct. Attempts to trap the putative Pb– complex 29 using an ex-
p-complex 28. A rear-
s
complex 29,
s
ternal nucleophile, such as methyl Meldrum’s acid or sodium azide,
have not been successful, and no products other than the lactone
Scheme 8.
ester 8i was isolated. This may be due to the fact that such Pb–
s
Figure 2. Ligand exchange between lead tetrabenzonate (PbB4) and 4-pentenoic acid (P).

2542
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
Scheme 9.
(conversions 34–72%) containing mainly tetrazole 31a along with
ester 31b (Scheme 10) in a ratio of about 2:1 regardless of the
stoichiometry of the starting lead(IV) compound, consistent with
a higher migratory aptitude for the tetrazolyl ligand. Of interest was
that these reactions were much faster than the analogous processes
involving lead tetracarboxylates, consistent with the lower stability
of tetrazolyl lead(IV) derivatives. In an effort to fine tune this pro-
cess further, the reaction of a range of benzoate-tetrazolyl lead(IV)
complexes 32 was examined, in which the benzoates were chosen
for their variation in steric and electronic modifying substituents.
Again, these were readily prepared by the metathesis process de-
scribed earlier with lead tetraacetate and equal stoichiometries of
the benzoate and 5-phenyl-(1H)-tetrazole units; of interest was
that all p-substituted benzoate lead(IV) carboxylates were much
more moisture stable than the corresponding o-substituted ones,
and that all were substantially more stable than lead(IV) tetra(5-
phenyl-(1H)-tetrazolide) 30 (n¼4). These ditetrazole dicarboxylate
Scheme 10.
complexes, if they exist, are very unstable and rapidly undergo
reductive elimination to liberate the lead(II) salt and the lactone
product before any exchange with external nucleophiles is possible.
In an effort to further extend the scope of the plumbolactoni-
sation process, systems containing tetrazole ligands were exam-
ined; the tetrazolyl unit, with a pKa of 4.5, is of similar acidity to
carboxylic acids, but has not been previously examined as a ligand
for lead(IV). In the event, preparation of mixed ligand complexes 30
(mþn¼4) was readily achieved using the metathesis process de-
scribed earlier with lead tetraacetate and different stoichiometries
of 2-thiophenecarboxylic and 5-phenyl-(1H)-tetrazole, although
the products were found to be of low solubility in common sol-
vents, and were moisture sensitive, with lead(IV) tetra(5-phenyl-
(1H)-tetrazolide) 30 (n¼4) decomposing to brown lead(II) oxide in
a closed container within a week; complexes containing 1 equiv or
more of 2-thiophenecarboxylic acid were markedly more stable.
Lead tetra(5-phenyltetrazol-1-ide) was found to react effectively
lead(IV) complexes 32 were reacted with 4-pentenoic acid in a,a,a-
trifluorotoluene (Scheme 11) under both thermal conditions and
microwave conditions; overall, yields were generally better and the
reaction faster in cases in which microwave conditions were used
(reaction times reduced to 10 min), and in some cases these con-
ditions gave very good conversions of product (66, 61 and 61%,
respectively, entries 2–4, Table 2). Each reaction yielded a mixture
of products due to the transferral of the tetrazole ligand 33a, the
benzoate ligand 34a–i or the 4-pentenoic acid 33b to the lactone
product (Scheme 11 and Table 2). In the majority of cases, prefer-
ence for the transferral of the tetrazole, over the carboxylate, to the
with pentenoic acid in a,a,a-trifluorotoluene to give only the tet-
razolylmethyl lactone 31a, which could be isolated in 53% yield,
although the mixed ligand complexes returned product mixtures
Scheme 11.

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2543
Table 2
Reaction times, conversions and product ratios for lactones 33a, 34a–i and 33b for the reactions of lead(IV) ditetrazole complexes 32 with 4-pentenoic acida according to
Scheme 11
Entry
Benzoic acid
derivative in lead(IV)
ditetrazole complex 32
Lactone product
Time to decolouration
under thermal conditions
Conversion (%) (product ratio)
Thermal conditions
Microwave conditions
1
2
3
4
5
6
7
8
9
Benzoic acid
34a
34b
34c
34d
34e
34f
34g
34g
34i
20 min
3 h
3 h
30 min
15 min
15 min
10 min
5 min
10 min
51 (4:2:1)
31 (4:1:1)
41 (5:2:1)
31 (5:2:1)
48 (5:2:1)
38 (4:1:1)
50 (6:8:1)
50 (4:7:1)
42 (1:1:1)
55 (3:1:1)
66 (4:1:1)
61 (5:1:1)
61 (3:1:1)
37 (3:1:1)
50 (3:1:1)
40 (5:4:1)
43 (5:6:1)
52 (4:4:1)
p-Methoxybenzoic acid
o-Methoxybenzoic acid
p-Methylbenzoic acid
o-Methylbenzoic acid
p-Chlorobenzoic acid
o-Chlorobenzoic acid
2-Benzoylbenzoic acid
2,6-Dimethoxybenzoic acid
a
4-Pentenoic acid (1 equiv) and a lead(IV) complex (1 equiv) were reacted with microwave irradiation (ramp time: 5 min, power: 300 W). All products were identified by 1
H
NMR and mass spectrometric analysis.
Scheme 12.
lactone product was observed; some exceptions were found for
which almost no selectivity of tetrazole over carboxylate transfer
was observed (entries 7–9, Table 2). One interesting case is that of
lead(IV) di(o-methoxybenzoate) di(5-phenyl(1H)-tetrazole), which
reacts very slowly under thermal conditions, and shows a high
conversion of the tetrazole-containing product 33a compared with
the carboxylate-containing product 34c when the reaction is per-
formed under microwave conditions (entry 3, Table 2). This may be
due to coordination between the o-methoxy group and the Pb(IV),
which has been observed spectroscopically in related systems.75
However, the addition of a second o-methoxy group (entry 9, Table
2) does not benefit from similar selectivity, and a near 1:1 ratio of
products 33a/34i is obtained, as it is in the case of o-chloro and o-
benzoyl groups, which also do not benefit from the possibility of
coordination (entries 7–9, Table 2).
Hydroxy lactonisations are known and have been achieved with
hydrogen peroxide catalysed by methyltrioxorhenium,78 acidic hy-
drogen peroxide79 or by asymmetric dihydroxylation.80 However,
direct acyloxy lactonisation processes have been rarely reported
previously; such processes have been achieved using ammonium
persulfate and trifluoromethanesulfonic acid in acetic acid,81 or in
two-step sequences by selenolactonisations of dienylcarboxylic acids
followed by oxidative elimination.82 The plumbolactonisation process
described in this paper represents a one-step method for efficient
access to hydroxy lactones. The behaviour of the plumbolactonisation
process contrasts with the outcome using hypervalent iodine re-
agents, in which lactonisation with rearrangement is obtained.83
2. Conclusion
Since the synthesis of fused tetrazole and imidazole derivatives
of medicinal chemistry relevance by iodocyclisation has been of
recent interest,76 we wondered if the plumbolactonisation se-
quence might be extended to include the analogous aza one. The
required butenylphenyl-1H-tetrazole 35 was prepared by a modi-
fication of the literature procedure76 using n-BuLi as the base,
although the desired product 35 was obtained as a mixture along
with butyl derivative 36. Reaction of the butenylphenyl-1H-tet-
razole 35,76 with lead tetracetate, lead(IV) tetra(2-thio-
phenecarboxylate), and lead(IV) tetra(5-phenyl-(1H)-tetrazolide)
gave complex mixtures 37a–c, in which the expected products
could be detected by mass spectrometric analysis, but from which
only product 38 could be isolated; noteworthy is that detailed
NMR analysis indicated this to be the 2H-tetrazolyl system
(Scheme 12).
We have demonstrated that plumbolactonisations using lead tet-
racarboxylates, giving acyloxy lactone products, are viable processes,
experimentally simple to execute, and which can proceed with high
diastereocontrol. Extension to tetrazolyl containing analogues is
possible, giving tetrazolyl lactone products, although in these cases
the reaction is limited by the stability of the lead(IV) reagent.
3. Experimental
3.1. Formation of lead(IV) carboxylates 19a–j: general
method48
Lead(IV) tetraacetate (1.0 equiv) and
a
carboxylic acid
(4.0 equiv) were dissolved in dry toluene (150 mL) and stirred at
room temperature for 20 min. The solvent was then removed in

2544
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
vacuo (40 ꢁC, 65 mbar). The resulting solid was then re-dissolved in
dry toluene (150 mL), stirred for a further 20 min and the solvent
was removed in vacuo. This procedure was repeated for a third time
to ensure all acetic acid had been removed.
3.1.9. Lead(IV) tetra(thiophene-2-carboxylate)48 7i
Lead(IV) tetraacetate (3.11 g, 7.0 mmol) and 2-thiophene-
carboxylic acid (3.59 g, 28.0 mmol) were reacted according to the
General Method to give carboxylate 7i as a yellow powder (4.90 g,
98%). Mp 208 ꢁC; [Found: C, 33.35; H, 1.93; S, 17.71. C20H12O8S4Pb
requires: C, 33.56; H,1.69; S,17.92.] nmax (CDCl3)/cmꢀ1 2360s, 2341s,
1525m, 1420m; dH(200 MHz; CDCl3) 7.98 (1H, d, J 3.4, H-3), 7.69
(1H, d, J 4.8, H-5), 7.22 (1H, m, H-4).
3.1.1. Lead(IV) tetrabenzoate 7a
Lead(IV) tetraacetate (2.00 g, 4.5 mmol) and benzoic acid
(2.20 g, 18.0 mmol) were reacted according to the General Method
to give carboxylate 7a as a yellow powder (3.05 g, 98%). Mp 186–
189 ꢁC (lit.70 176 ꢁC); nmax(Nujol)/cmꢀ1 1599w, 1412s; dH(200 MHz;
CDCl3) 8.14 (2H, d, J 7.2, ArH o- to CO2), 7.63 (1H, t, J 7.6, ArH p- to
CO2), 7.47 (2H, dd, J 7.7 and 7.4, ArH m- to CO2).
3.1.10. Lead(IV) tetrafuroate48 7j
Lead(IV) tetraacetate (2.00 g, 4.5 mmol) and 2-furoic acid
(2.02 g, 18.0 mmol) were reacted according to the General Method
1 to give carboxylate 7j as a pale brown powder (4.44 g, 97%). Mp
86–87 ꢁC; nmax(CDCl3)/cmꢀ1 1764s, 1682s, 1480m, 1304s;
dH(400 MHz; CDCl3) 7.66 (1H, dd, J 0.8 and 1.8, H-3), 7.34 (1H, dd, J
0.8 and 3.6, H-5), 6.57 (1H, dd, J 1.8 and 3.6, H-4).
3.1.2. Lead(IV) tetracinnamate 7b
Lead(IV) tetraacetate (2.97 g, 6.8 mmol) and cinnamic acid
(4.01 g, 27.1 mmol) were reacted according to the General Method
to give carboxylate 7b as a yellow/brown powder (5.19 g, 96%). Mp
139–140 ꢁC; nmax(film)/cmꢀ1 2361s, 1718w, 1564w, 1433m;
dH(200 MHz; CDCl3) 7.93 (1H, d, J 15.9, CH]CHPh), 7.41–7.58 (5H,
m, ArH), 6.51 (1H, d, J 15.9, CH]CHPh).
3.2. Cyclisation of 4-pentenoic acid by lead(IV) carboxylates:
general method
4-Pentenoic acid (1.0 equiv) and a lead(IV) tetracarboxylate
(1.0 equiv) were dissolved in dry toluene (15 mL). The reaction
mixture was stirred at 90 ꢁC under argon for 3 h. The excess solvent
was removed in vacuo at 40 ꢁC (usually 77 mbar). The residue was
dissolved/suspended in ethyl acetate (100 mL) and filtered. The
filtrate was then washed with 5% sodium carbonate (aq) (150 mL),
5% sodium chloride (aq) (100 mL), dried over magnesium sulfate,
filtered and concentrated in vacuo. The residue was purified by
flash column chromatography. The eluting solvent is specified in
each case.
3.1.3. Lead (IV) tetra(4-bromobenzoate) 7c
Lead(IV) tetraacetate (4.49 g, 10.13 mmol) and 4-bromobenzoic
acid (8.15 g, 40.53 mmol) were reacted according to the General
Method to give carboxylate 7c as a brown powder (10.81 g, 99%).
nmax(film)/cmꢀ1 1586s, 1422s; dH(200 MHz; CDCl3) 7.95 (2H, d, o- to
CO2), 7.65 (2H, d, m- to CO2).
3.1.4. Lead(IV) tetra(4-methoxybenzoate) 7d
Lead (IV) tetraacetate (3.55 g, 8.01 mmol) and p-anisic acid
(4.87 g, 32.03 mmol) were reacted according to the General
Method to give carboxylate 7d as a yellow powder (6.35 g, 98%).
nmax(film)/cmꢀ1 2359s, 2339s, 668s; dH(200 MHz; CDCl3) 8.07 (2H,
d, J 5.0, ArH o- to CO2), 6.92 (2H, d, J 5.0, ArH m- to CO2), 3.86 (3H, s,
OCH3).
3.2.1. (5-Oxotetrahydrofuran-2-yl)methyl acetate 8a
4-Pentenoic acid (200 mg, 2 mmol) and lead(IV) tetraacetate 7a
(0.87 g, 2.0 mmol) were reacted according to the General Method.
The crude product was purified using flash column chromatogra-
phy, eluting with dichloromethane to give lactone 8a (269 mg,
85%), a colourless oil. [Found: C, 53.05; H, 6.6. C7H10O4 requires: C,
53.16; H, 6.37.] nmax(film)/cmꢀ1 1777s, 1742s; dH(200 MHz; CDCl3)
4.71–4.77 (1H, m, H-2), 4.10–4.40 (2H, m, CH2O), 2.11 (3H, s, CH3),
1.95–2.47 (4H, m, 2ꢂH-3,-4); dC(50.3 MHz; CDCl3) 177.0, 170.8, 77.3,
65.2, 28.0, 23.6, 20.5; m/z (CI, NH3) 176 (96%, MþNHþ4 ), 159 (100,
MþHþ), 85 (42).
3.1.5. Lead(IV) tetra(3,4-dimethoxybenzoate) 7e
Lead(IV) tetraacetate (4.50 g, 10.0 mmol) and 3,4-dimethoxy-
benzoic acid (7.397 g, 40.0 mmol) were reacted according to the
General Method to give carboxylate 7e as an orange powder (9.02 g,
95%). nmax(film)/cmꢀ1 2361w, 1684w, 1597m, 1518m, 1456m, 1419s,
1386m, 1275s; dH(200 MHz; CDCl3) 7.8 (1H, dd, H-5), 7.6 (1H, s, H-
2), 6.90 (1H, d, H-5), 3.95 (3H, s, OMe), 3.91 (3H, s, OMe).
3.2.2. (5-Oxotetrahydrofuran-2-yl)methyl benzoate 8b
3.1.6. Lead(IV) tetra(2,4-dimethoxybenzoate) 7f
4-Pentenoic acid (200 mg, 2 mmol) and lead(IV) tetrabenzoate
7a (1.38 g, 2 mmol) were reacted according to the General Method.
The crude product was purified using flash column chromatogra-
phy, eluting with 2:1 (40–60) petroleum ether/ethyl acetate to give
lactone 8b (196 mg, 45%), a colourless oil. Rf¼0.37 (2:1 (40–60)
petroleum ether/ethyl acetate); [Found: C, 65.32; H, 5.66. C12H12O4
requires: C, 65.32; H, 5.66.] nmax(film)/cmꢀ1 1776s, 1719s, 1272s;
dH(200 MHz; CDCl3) 7.98 (2H, d, J 7.2, ArH o- to CO2), 7.33–7.61 (3H,
m, ArH), 4.75–4.89 (1H, m, H-2), 4.30–4.55 (2H, m, CH2O),1.95–2.64
(4H, m, 2ꢂH-3,-4); dC(50.3 MHz; CDCl3) 177.1, 166.4, 133.6, 129.8,
129.5, 128.7, 77.4, 65.7, 28.1, 23.8; m/z (CI, NH3) 238 [MþNH4]þ, 221
[MþH]þ. HRMS (ESþ) found 221.0815 (MHþ), C12H13O4 requires
221.0814.
Lead(IV) tetraacetate (3.00 g, 6.77 mmol) and 2,4-dimethoxy-
benzoic acid (4.93 g, 27.07 mmol) were reacted according to the
General Method to give carboxylate 7f as a bright yellow powder
(6.05 g, 96%), which was unstable in CDCl3 solution, turning brown
immediately preventing a 1H NMR spectrum being obtained. Mp
159 ꢁC; nmax/cmꢀ1 1610s, 1387w.
3.1.7. Lead(IV) tetra(2,6-dimethoxybenzoate) 7g
Lead(IV) tetraacetate (3.00 g, 6.77 mmol) and 2,6-dimethoxy-
benzoic acid (4.93 g, 27.07 mmol) were reacted according to the
General Method to give carboxylate 7g as an orange powder
(5.88 g, 93%). Mp 161 ꢁC; nmax(film)/cmꢀ1 1597s, 1434w;
dH(200 MHz; CDCl3) 7.37 (1H, t, ArH-4), 6.63 (2H, d, ArH-3,-5), 3.91
(6H, s, 2ꢂOMe).
When this reaction was carried out in the presence of the radical
inhibitor benzoquinone (22 mg, 0.2 mmol) a colourless oil (0.131 g,
29%) was obtained; in the presence of the radical inhibitor galvi-
noxyl (98 mg, 0.23 mmol) resulting in a colourless oil (0.277 g, 67%)
was obtained.
3.1.8. Lead(IV) tetra(2,6-dichlorobenzoate) 7h
Lead(IV) tetraacetate (3.50 g, 7.89 mmol) and 2,6-dimethoxy-
benzoic acid (6.031 g, 31.57 mmol) were reacted according to the
General Method to give carboxylate 7h as an yellow powder (7.50 g,
98.2%). nmax(film)/cmꢀ1 2360s,1718br,1564m,1433m; dH(200 MHz;
CDCl3) 7.19–7.52 (3H, m, ArH-3,-4,-5).
3.2.3. (5-Oxotetrahydrofuran-2-yl)methyl 3-phenylacrylate 8c
4-Pentenoic acid (200 mg, 2 mmol) and lead(IV) tetracinnamate
7b (1.62 g, 2 mmol) were reacted according to the General Method.

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2545
The crude reaction mixture was purified by flash column chroma-
tography eluting with dichloromethane to give lactone 8c (26 mg,
7%) as a yellow oil. Rf¼0.28 (dichloromethane); [Found: C, 68.56; H,
5.93. C14H14O4 requires: C, 68.28; H, 5.73.] nmax(film)/cmꢀ1 1631s,
1409s; dH(200 MHz; CDCl3) 7.75 (1H, dd, J 16.0, CH]CHPh), 7.32–7.61
(5H, m, ArH), 6.44 (1H, d, J 16.0, CH]CHPh), 4.70–4.89 (1H, m, H-2),
4.21–4.46 (2H, m, CH2O), 1.96–2.72 (4H, m, 2ꢂH-3,-4); dC(50.3 MHz;
CDCl3) 176.7, 166.5, 145.9, 134.0, 130.7, 128.9, 128.3, 115.7, 77.5, 65.3,
28.5, 23.9; m/z (CI, NH3) 247 ([MþH]þ, 10%), 131 (100); HRMS (ESþ)
found 246.0894 (Mþ), C14H14O4 requires 246.0892.
dC(50.3 MHz; CDCl3) 131.5, 77.4, 65.6, 56.0, 28.1, 23.8; m/z (CI, NH3)
281 ([MþH]þ, 7%), 166 (10), 165 (100); HRMS (ESþ) found 281.1026
(MþH), C14H16O6 requires 281.1025.
3.2.8. (5-Oxotetrahydrofuran-2-yl)methyl 2,6-chlorobenzoate 8h
4-Pentenoic acid (197 mg, 1.97 mmol) and lead(IV) tetra(2,6-
dichlorobenzoate) 7h (1.91 g, 1.97 mmol) were reacted according
to the General Method. The crude product was purified by flash
column chromatography eluting with 1:1 ethyl acetate:(40–60)
petroleum ether to give the product (126 mg, 22%), a pale yellow
oil. Rf¼0.33 (1:1 (30–40) petroleum ether/ethyl acetate); nmax
-
3.2.4. (5-Oxotetrahydrofuran-2-yl)methyl 4-methoxybenzoate 8d
4-Pentenoic acid (167 mg, 1.66 mmol) and lead(IV) tetra(4-
methoxybenzoate) 7d (1.35 g, 1.66 mmol) were reacted according
to the General Method except that acetonitrile was used as the
solvent. The crude product was purified using flash column chro-
matography eluting with 1:1 (30–40) petroleum ether/ethyl ace-
tate. The product crystallised out to give lactone 8d as white
needles (0.21 g, 39%). Mp 76 ꢁC; Rf¼0.44 (1:1 (30–40) petroleum
ether/ethyl acetate); [Found: C, 62.07; H, 5.64. C13H14O5 requires: C,
62.39; H, 5.64.] dH(200 MHz; CDCl3) 7.98 (2H, dd, J 4.8 and 2.1, ArH-
2,-6), 6.90 (2H, dd, J 4.8 and 2.1, ArH-3,-5), 4.80–4.92 (1H, m, H-2),
4.36–4.58 (2H, m, CH2O), 3.85 (3H, s, OMe), 2.04–2.73 (4H, m, 2ꢂH-
3, 2ꢂH-4); HRMS (ESþ) found 251.0921 ([MþH]þ) C13H14O5, re-
quires 251.0919; m/z (CI, NH3) 251 [MþH]þ, 153 (12%), 136 (15), 135
(100).
(film)/cmꢀ1 1780m, 1742m, 1274m, 1143m; dH(200 MHz; CDCl3)
7.26–7.40 (3H, m, ArH-3,-4,-5), 4.83–4.92 (1H, m, H-2), 4.44–
4.68 (2H, m, CH2O), 2.04–2.64 (4H, m, 2ꢂH-3, 2ꢂH-4);
dC(50.3 MHz; CDCl3) 131.5, 128.0, 76.8, 66.5, 28.1, 23.9; m/z
(APCI) 289, 291, 292 [MþH]þ, 177 (12%), 175 (65) 173 (100);
HRMS (ESþ) found 306.0297 (MþNH4), C12H10Cl2O4, requires
306.0300.
3.2.9. (5-Oxo-tetrahydrofuran-2-yl)methyl thiophene-2-
carboxylate 8i
4-Pentenoic acid (200 mg, 2.00 mmol) and lead(IV) tetra(thio-
phene-2-carboxylate) 7i (1.43 g, 2.00 mmol) were reacted accord-
ing to the General Method. After work-up, the crude product was
purified by flash column chromatography eluting with dichloro-
methane to give lactone 8i (0.38 g, 84%). Rf¼0.14 (dichloro-
methane); mp 42–45 ꢁC; nmax(film)/cmꢀ1 1777s, 1711s, 1261s,
1163m; dH(200 MHz; CDCl3) 7.83–7.86 (1H, dd, J 1.3 and 3.8, thio-
phene H-2), 7.61–7.63 (1H, dd, J 1.3 and 5.0, thiophene H-5), 7.12–
7.16 (1H, m, thiophene H-4), 4.85–4.89 (1H, m, H-2), 4.45 (H, m,
CH2O), 2.20–2.71 (4H, m, 2ꢂH-3 and 2ꢂH-4); dC(50.3 MHz; CDCl3)
134.2, 133.2. 128.0, 77.3, 65.9. 28.2, 24.0; HRMS (ESþ) found
227.0378 (Mþ), C10H11O4S requires 227.0378.
3.2.5. (5-Oxotetrahydrofuran-2-yl)methyl 3,4-dimethoxy-
benzoate 8e
4-Pentenoic acid (200 mg, 2.00 mmol) and lead(IV) tetra(3,4-
dimethoxybenzoate) 7e (1.86 g, 2.00 mmol) were reacted according
to the General Method. The crude product was purified by flash
column chromatography eluting with dichloromethane followed
by 10% methanol/dichloromethane to give lactone 8e (170 mg,
45%),
a
yellow oil. Rf¼0.11 (10% methanol/dichloromethane);
3.2.10. (5-Oxo-tetrahydrofuran-2-yl)methyl furan-2-carboxylate 8j
4-Pentenoic acid (200 mg, 2.0 mmol) and lead(IV) tetrafuroate
7j (1.30 g, 2.0 mmol) were reacted according to General Method 3.
A brown precipitate was seen to form during the 3 h of heating
under argon. After the general work-up the crude product (a yellow
oil) was purified by flash column chromatography eluting with 8:1
(40–60) petroleum ether/ethyl acetate to give lactone 8j as a col-
ourless oil (0.13 g, 31%). Rf¼0.1 (8:1 (30–40) petroleum ether/ethyl
acetate); nmax(film)/cmꢀ1 1810s, 1739s, 1291m, 1041s; dH(400 MHz;
CDCl3) 7.68 (1H, m, furan H-2), 7.33 (1H, m, furan H-5), 5.59 (1H, m,
furan H-4), 5.76–5.92 (1H, m, H-2), 5.03–5.18 (2H, m, CH2O), 2.36–
2.62 (4H, m, 2ꢂH-3 and 2ꢂH-4); dC(100.6 MHz; CDCl3) 168.1, 153.4,
143.2, 135.7, 121.4, 112.6. 34.4, 28.1.
dH(200 MHz; CDCl3) 7.67 (1H, dd, J 2.0 and 8.4, ArH-6), 7.52 (1H, d, J
2.0, ArH-2), 6.90 (1H, d, J 8.5, ArH-5), 4.86–4.91 (1H, m, H-2), 4.4–
4.6 (2H, m, CH2O), 3.94 (6H, s, 2ꢂOMe), 1.92–2.67 (4H, m, 2ꢂH-3,-
4); m/z (APCI) 281 ([MþH]þ). HRMS (EI) found 280.0952 (Mþ),
C14H16O6 requires 280.0947.
3.2.6. (5-Oxotetrahydrofuran-2-yl)methyl 2,4-dimethoxy-
benzoate 8f
4-Pentenoic acid (200 mg, 2.00 mmol) and lead(IV) tetra(2,4-
dimethoxybenzoate) 7f (1.86 g, 2.00 mmol) were reacted according
to the General Method. The crude product was purified by flash
column chromatography eluting with 1:1 (30–40) petroleum ether/
ethyl acetate, to give lactone 8f (267 mg, 48%), a yellow oil. Rf¼0.13
(1:1 (30–40) petroleum ether/ethyl acetate); nmax(film)/cmꢀ1
1777s, 1733s, 1269s; dH(200 MHz; CDCl3) 7.85 (1H, d, J 7.3, ArH-6),
6.5–6.6 (2H, m, ArH), 4.1–4.5 (3H, m, CH2O and H-2), 3.84 and 3.86
(6H, 2ꢂs, 2ꢂOMe), 2.0–2.8 (4H, m, 2ꢂH-3,-4); dC(50.3 MHz; CDCl3)
136.3, 134.2. 115.8, 104.7, 98.9, 77.6, 65.4, 55.8. 55.5, 28.2, 24.0; m/z
(APCI) 281 ([MþH]þ). HRMS (EI) found 298.1292 (MþNHþ4 ),
C14H16O6 requires 298.1291.
3.2.11. Carboxylate ligands exchange in lead(IV) complexes by 207Pb
NMR spectroscopy
Lead(IV) tetrabenzoate (0.034 g, 0.05 mmol) was dissolved in
CDCl3 and transferred to an NMR tube. 4-Pentenoic acid (20 mL,
0.20 mmol) was then added to the NMR tube directly. A spectrum
was taken at 295 K on the Amx500 NMR spectrometer to give
a 104.4 MHz 207Pb NMR spectrum showing five sharp peaks rep-
resenting PbP4, PbP3B, PbP2B2, PbPB3 and PbB4 in a ratio of
0.3:2.8:6:4.7:1.32.
3.2.7. (5-Oxotetrahydrofuran-2-yl)methyl 2,6-dimethoxy-
benzoate 8g
4-Pentenoic acid (200 mg, 2.00 mmol) and lead(IV) tetra(2,6-
dimethoxybenzoate) 7g (1.86 g, 2.00 mmol) were reacted accord-
ing to the General Method. The crude product was purified by flash
column chromatography eluting with dichloromethane to give
a yellow oil 8g (0.36 g, 60%). Rf¼0.1 (dichloromethane); nmax(film)/
cmꢀ1 1773m, 1734m, 1257w, 1112m; dH(200 MHz; CDCl3) 7.32 (1H,
d, J 8.5, ArH-4), 6.57 (2H, d, J 8.4, ArH-3,-5), 4.87 (1H, m, H-2), 4.56
(2H, m, CH2O), 3.83 (6H, s, 2ꢂOMe), 2.20–2.71 (4H, m, 2ꢂH-3,-4);
3.3. Cyclisation of substituted 4-pentenoic acid by lead(IV)
carboxylates
3.3.1. (4,4-Dimethyl-5-oxo-tetrahydrofuran-2-yl)methyl
thiophene-2-carboxylate 11a
2,2-Dimethyl-4-pentenoic acid (210 mg, 1.6 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.17 g, 1.6 mmol) were reacted
according to the General Method. After work-up, the crude product

2546
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
was obtained as a yellow oil, which was then recrystallised from
(40–60) petroleum ether and ethyl acetate to give lactone 11a as
a white needles (242 mg, 58%). Mp 95–97 ꢁC; nmax(film)/cmꢀ1
1772s, 1712s, 1260s; dH(200 MHz; CDCl3) 7.84 (1H, dd, J 1.2 and 3.7,
thiophene H-3), 7.59 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.10 (1H,
dd, J 3.7 and 5.0, thiophene H-4), 4.77 (1H, m, H-2), 4.45 (2H, m,
CH2O), 2.20 (1H, dd, J 12.8 and 6.5, H-3 trans- to CH2), 1.95 (1H, dd, J
9.8 and 12.8, H-3 cis- to CH2), 1.33 (3H, s, Me), 1.31 (3H, s, Me);
dC(50.3 MHz; CDCl3) 181.2, 161.8, 134.1, 133.1, 127.9, 74.0, 65.6, 40.0,
39.0, 25.1, 24.7; HRMS (ESþ) found 255.0687, C12H15O4S requires
255.0691.
3.3.5. (2-Methyl-5-oxo-tetrahydro-furan-2-yl)methyl thiophene-2-
carboxylate 11g
4-Methyl 4-pentenoic acid (228 mg, 2.0 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were reacted
according to the General Method. After work-up, the crude product
(a yellow oil) was purified by flash column chromatography eluting
with 2:1 (40–60) petroleum ether/ethyl acetate to give lactone 11g
as colourless plates with crystal structure details given below
(231 mg, 48%). Rf¼0.23 (2:1 (40–60) petroleum ether/ethyl ace-
tate); nmax(film)/cmꢀ1 1775s, 1712s, 1258m, 1093w; dH(400 MHz;
CDCl3) 7.82 (1H, dd, J 1.2 and 4.9, thiophene H-3), 7.65 (1H, dd, J 1.2
and 4.9, thiophene H-5), 7.17 (1H, dd, J 3.9 and 4.9, thiophene H-4),
4.39 (2H, s, CH2O), 2.83 (1H, m, H-4 trans- to Me), 2.70 (1H, m, H-4
cis- to Me), 2.39 (1H, m, H-3 trans- to Me), 2.14 (1H, m, H-3 cis- to
Me), 1.56 (3H, s, Me); dC(100.6 MHz; CDCl3) 176.3, 161.5, 134.1, 133.1,
132.5, 128.0, 83.8, 60.5, 30.6, 29.9, 23.8; HRMS (ESþ) found
241.0530, C11H13O4S requires 241.0535.
3.3.2. (5-Oxo-4,4-diphenyl-tetrahydrofuran-2-yl)methyl
thiophene-2-carboxylate 11b
2,2-Diphenyl 4-pentenoic acid (495 mg, 2.0 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were reacted
according to the General Method. After work-up, the crude product
(a yellow oil) was purified by flash column chromatography eluting
with dichloromethane to give lactone 11b as a pale yellow oil,
which crystallised on standing (504 mg, 69%). Mp 125–127 ꢁC;
Rf¼0.3 (dichloromethane); nmax(film)/cmꢀ1 1769s, 1712s;
dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.2 and 3.8, thiophene H-3), 7.60
(1H, dd, J 1.2 and 5.0, thiophene H-5), 7.24–7.44 (10H, m, ArH), 7.11
(1H, dd, J 3.8 and 5.0, thiophene H-4), 4.65 (1H, dd, J 3.1 and 12.2,
CH2O), 4.71 (1H, m, H-2), 4.43 (1H, dd, J 5.9 and 12.2, CH2O), 3.09
(1H, dd, J 5.3 and 13.1, H-3), 2.87 (1H, dd, J 10.5 and 13.1, H-3);
dC(100.6 MHz; CDCl3) 176.5,161.7,141.6,139.2,127.3, 74.4, 64.9, 57.8,
39.4; m/z (APCI) 378.5 (Mþ, 100%); HRMS (ESþ) found 396.1268,
C22H17O4S [MþNH4]þ requires 396.1270.
3.3.6. (3-Methyl-5-oxo-tetrahydro-furan-2-yl)methyl thiophene-2-
carboxylate 11h
3-Methyl 4-pentenoic acid (301 mg, 2.6 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.88 g, 2.6 mmol) were reacted
according to the General Method. After work-up, the crude product
(a brown oil) was purified by flash column chromatography eluting
with 3:1 (40–60) petroleum ether/ethyl acetate to give lactone 11h
as a pale yellow oil (255 mg, 53%). Rf¼0.15 (3:1 (40–60) petroleum
ether/ethyl acetate); nmax(film)/cmꢀ1 1783s, 1712s, 1260s;
dH(400 MHz; CDCl3) major diastereomer 7.81 (1H, m, thiophene H-
3), 7.58 (1H, m, thiophene H-5), 7.12 (1H, m, thiophene H-4), 4.52
(1H, m, CH2O), 4.41 (1H, m, CH2O), 4.36 (1H, m, H-2), 2.80 (1H, dd, J
8.6 and 17.7, H-4), 2.48 (1H, m, H-3), 2.25 (1H, dd, J 8.6 and 17.4, H-
4), 1.24 (3H, d, J 6.7, Me); minor diastereomer 7.81 (1H, m, thio-
phene H-3), 7.58 (1H, m, thiophene H-5), 7.12 (1H, m, thiophene H-
4), 4.78 (1H, m, H-2), 4.52 (1H, m, CH2O), 4.41 (1H, m, CH2O), 2.70
(1H, m, H-4), 2.48 (1H, m, H-3), 2.40 (1H, m, H-4), 1.17 (3H, d, J 7.1,
Me); dC(100.6 MHz; CDCl3) major diastereomer 175.8, 134.1, 133.1,
132.6, 127.7, 83.8, 64.5, 36.6, 32.3, 18.1; minor diastereomer 175.8,
134.1, 133.1, 132.6, 127.7, 79.6, 63.5, 36.5, 32.1, 14.0; HRMS (ESþ)
found 241.0523, C11H12O4S [MþH]þ requires 241.0535, found
258.0799, [MþNH4]þ requires 258.0800.
3.3.3. (4-Methyl-5-oxo-4-phenyl-tetrahydrofuran-2-yl)methyl
thiophene-2-carboxylate 11c
2-Methyl 2-phenyl 4-pentenoic acid (380 mg, 2.0 mmol) and
lead(IV) tetra(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were
reacted according to the General Method. After work-up, the crude
product (a yellow oil) was purified by flash column chromatog-
raphy eluting with dichloromethane to give lactone 11c (460 mg,
73%) as colourless blocks (crystal structure details given below).
Mp 79–82 ꢁC; Rf¼0.23 (dichloromethane); nmax(film)/cmꢀ1 1774s,
1712s, 1260s; dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.3 and 3.8,
thiophene H-3), 7.60 (1H, dd, J 1.3 and 5.0, thiophene H-5), 7.34–
7.41 (4H, m, o-, m- ArH), 7.28–7.34 (1H, m, p-ArH), 7.12 (2H, m,
thiophene H-4), 4.51–4.61 (2H, m, H-2 and CH2O), 4.38 (1H, m,
CH2O), 2.24 (1H, m, H-3), 2.80 (1H, m, H-3), 1.63 (3H, s, Me);
dC(100.6 MHz; CDCl3) 178.7, 161.7, 140.4, 134.1, 133.1, 132.7, 129.1,
127.9, 127.7, 125.8, 74.3, 65.6, 49.5, 39.9, 26.3; m/z (APCI) 316.6
(Mþ, 100%); HRMS (ESþ) found 334.1109, C17H16O4S [MþNH4]þ
requires 334.1113.
3.3.7. (3,3-Dimethyl-5-oxo-tetrahydro-furan-2-yl)methyl
thiophene-2-carboxylate 11i
3,3-Dimethyl 4-pentenoic acid (180 mg,1.40 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.00 g, 1.4 mmol) were reacted
according to the General Method. After work-up, the crude product
was purified by flash column chromatography eluting with
dichloromethane to give lactone 11i as a yellow oil (190 mg, 53%).
Rf¼0.33 (dichloromethane); nmax(film)/cmꢀ1 1785s, 1714s, 1260m;
dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.2 and 3.8, thiophene H-3), 7.60
(1H, dd, J 1.2 and 5.0, thiophene H-5), 7.11 (1H, dd, J 3.8 and 5.0,
thiophene H-4), 4.54 (1H, m, H-2), 4.39 (2H, m, CH2O), 2.40–2.54
(2H, dd, J 17.1 and 32.5, 2ꢂH-4), 1.30 (3H, s, Me), 1.18 (3H, s, Me);
dC(100.6 MHz; CDCl3) 175.5, 161.7, 84.9, 63.1, 44.0, 38.5, 26.7, 21.6;
m/z (APCI) 254.29 (Mþ, 100%); HRMS (ESþ) found 255.0688,
C21H15O4S requires 255.0691.
3.3.4. (1-Oxo-2-oxa-spiro[4.5]dec-3-yl)methyl thiophene-2-
carboxylate 11d
2-Cyclohexane 4-pentenoic acid (336 mg, 2.0 mmol) and
lead(IV) tetra(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were
reacted according to the General Method. After work-up, the
resulting yellow oil crystallised to give lactone 11d (0.44 g, 75%).
Mp 59–62 ꢁC; nmax(film)/cmꢀ1 2933s, 1766s, 1713s, 1261s;
dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.2 and 5.0, thiophene H-3),
7.60 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.12 (1H, dd, J 3.8 and
5.0, thiophene H-4), 4.73 (1H, m, H-2), 4.56 (1H, dd, J 3.1 and
12.1, CH2O), 4.34 (1H, dd, J 6.1 and 12.2, CH2O), 2.39 (1H, dd, J
6.9 and 13.0, H-3), 1.20–1.89 (11H, m, cyclohexane CH2 and H-
3); dC(100.6 MHz; CDCl3) 180.8, 161.8, 134.1, 133.1, 132.7, 127.9,
74.3, 65.7, 44.5, 35.1, 34.1, 31.9, 25.2, 22.0; m/z (APCI) 294.66
(Mþ, 100%); HRMS (ESþ) found 295.0095, [MþH]þ requires
295.1004; found 312.1261, C15H18O4S [MþNH4]þ requires
312.1270.
3.3.8. cis-(4-Methyl-5-oxo-tetrahydrofuran-2-yl)methyl thiophene-
2-carboxylate 11j
2-Methyl-4-pentenoic acid (287 mg, 2.5 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.80 g, 2.5 mmol) were reacted
according to the General Method. After work-up, the crude product
was purified by flash column chromatography eluting with
dichloromethane to give lactone 11j as colourless plates with
crystal structure details given below (203 mg, 30%). Rf¼0.39
(dichloromethane); mp 72–76 ꢁC; [Found: C, 54.99; H, 5.03. Calcd

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2547
for C11H12O4S: C, 54.99; H, 5.04%.] nmax(film)/cmꢀ1 1773s, 1712s,
1262s; dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.2 and 3.7, thiophene H-
3), 7.60 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.12 (1H, dd, J 3.7 and
5.0, thiophene H-4), 4.70 (1H, m, H-2), 4.55 (1H, dd, J 12.2 and 3.1,
CH2O), 4.35 (1H, dd, J 11.4 and 4.2, CH2O), 2.78 (1H, m, H-4), 2.25
(1H, m, H-3), 1.71 (1H, dd, J 9.8 and 12.8, H-3), 1.35 (1H, d, J 3.7, Me);
dC(50.3 MHz; CDCl3) 178.7, 134.1, 133.1, 127.9, 75.3, 65.4, 35.3, 32.8,
15.3; m/z (APCI) 240.92 (Mþ, 20%), 110.76 (100); HRMS (ESþ) found
258.0795, C11H12O4S [MþNH4]þ requires 258.0800. The yield im-
(257 mg, 43%). Mp 79–82 ꢁC; nmax(film)/cmꢀ1 1765s, 1712s, 1260m;
dH(400 MHz; CDCl3) 7.97 (1H, m, thiophene H-3), 7.75 (2H, m, ArH),
7.61 (2H, m, ArH, thiophene H-5), 7.10 (1H, dd, J 3.9 and 5.0, thio-
phene H-4), 5.79 (1H, m, H-2), 4.75 (1H, dd, J 3.9 and 12.0, CH2O),
4.63 (1H, dd, J 5.8 and 12.0, CH2O); dC(100.6 MHz; CDCl3) 183.1,
161.5, 145.9, 132.4, 130.4, 134.3, 134.1, 133.2, 129.9, 127.9, 126.0,
122.4, 78.4, 64.7; HRMS (ESþ) found 292.0642, C14H10O4S
[MþNH4]þ requires 292.0644.
proved to 63% when this reaction was done in
trifluorotoluene.
a
,a
,a
-
3.4.2. (2-Oxohexahydro-2H-cyclopenta[b]furan 6-yl)thiophene-2-
carboxylate 15 (n¼1)
2-Cyclopenten-1-acetic acid (505 mg, 4.0 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (3.44 g, 4.8 mmol) were suspended
3.3.9. 1-(5-Oxotetrahydrofuran-2-yl)ethyl thiophene-
2-carboxylate 11k
in a,a,a
-trifluorotoluene (50 mL) and heated at 90 ꢁC under argon
4-Hexenoic acid (607 mg, 5.3 mmol), lead(IV) tetra(thiophene-
2-carboxylate) (3.82 g, 5.3 mmol) were suspended in -tri-
for 30 min. The temperature was then increased to 135 ꢁC and after
1 h the reaction mixture had turned from bright yellow to white.
The solvent was removed in vacuo to leave a white solid, which was
dissolved in ethyl acetate (150 mL) and filtered. The filtrate was
washed with 5% sodium carbonate (aq) (2ꢂ150 mL) then satd brine
(2ꢂ100 mL). The aqueous layer was then extracted with ethyl ac-
etate (100 mL) and the combined organic layers were dried over
magnesium sulfate, filtered and concentrated in vacuo to give an
orange oil. Purification by flash column chromatography eluting
with 4:1 (40–60) petroleum ether/ethyl acetate to give lactone 15
(n¼1) crystals (86 mg, 9%). Rf¼0.11 (4:1 (40–60) petroleum ether/
ethyl acetate); dH(400 MHz; CDCl3) 7.82 (1H, dd, J 1.2 and 3.7,
thiophene H-4), 7.57 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.12 (1H,
dd, J 3.7 and 5.0, thiophene H-3), 5.45 (1H, s, H-6), 4.93 (1H, d, J 6.9,
H-5), 3.13 (1H, m, H-4), 2.78 (1H, dd, J 18.6 and 10.3, H-3), 2.40 (1H,
dd, J 3.0 and 18.6, H-3), 2.31 (1H, m, H-8), 2.16 (1H, m, H-7), 2.00
(1H, m, H-7), 1.68 (1H, m, H-8); dC(100.6 MHz; CDCl3) 176.6, 161.0,
133.9, 133.1, 132.9, 127.9, 87.5, 79.0, 36.5, 35.3, 31.0, 29.5; HRMS
(ESþ) found 253.0523, C12H12O4S [MþH]þ requires 253.0535; found
270.0808, [MþNH4]þ requires 270.0800.
a,a,a
fluorotoluene (20 mL) and heated at 145 ꢁC under argon overnight
during, which time the colour changed from bright yellow to pale
brown. After general work-up the crude product (a brown oil) was
purified by flash column chromatography eluting with 5:2 (40–60)
petroleum ether/ethyl acetate to give lactone 11k as a colourless oil
(123 mg, 10%). Rf¼0.14 (5:2 (40–60) petroleum ether/ethyl acetate);
nmax(film)/cmꢀ1 1778s, 1710s, 1260s; dH(400 MHz; CDCl3) 7.84 (1H,
dd, J 1.2 and 3.7, thiophene H-3), 7.60 (1H, dd, J 1.2 and 5.0, thio-
phene H-5), 7.12 (1H, dd, J 3.8 and 5.0, thiophene H-4), 5.30 (1H, m,
CHMe), 4.62 (1H, m, H-2), 2.48 (2H, m, 2ꢂH-4), 2.20–2.42 (2H, m,
2ꢂH-3), 1.38 (3H, d, J 6.8, Me); dC(100.6 MHz; CDCl3) 133.9, 132.8,
127.9, 80.8, 71.6, 28.0, 22.8, 15.6; m/z (ESI) 241.18 (MþH, 38%);
HRMS (ESþ) found 241. 0529, C11H12O4S [MþH]þ requires 241.0535;
found 258.0807, [MþNH4]þ requires 258.0800.
3.4. Microwave-assisted cyclisation of 4-pentenoic acid by
lead(IV) tetra(thiophene-2-carboxylate)
4-Pentenoic acid (1 equiv, 0.5 mmol, 0.05 mL) and a lead(IV)
tetra(thiophene-2-carboxylate) 3 (1 equiv, 0.5 mmol, 320 mg) were
suspended in dry a,a,a-trifluorotoluene (5 mL). The reaction mix-
3.4.3. (3,4-Dimethyl-5-oxo-tetrahydrofuran-2-yl)methyl
thiophene-2-carboxylate 17
ture was stirred under argon and irradiated with microwaves in
a closed container using the following microwave settings: power
(max): 150 W, ramp time (max): 2 min, hold time: 10 min, tem-
perature: 90 ꢁC, pressure (max): 100 psi. The required temperature
was obtained within the ramp time. The power supplied reached
a maximum of 150 W in the first 100 s then was constant at w50 W.
After 10 min hold time the lead(IV) tetra(thiophene-2-carboxylate)
had completely decolourised. The excess solvent was removed in
vacuo at 40 ꢁC (usually at 77 mbar) using the rotary evaporator. The
resulting residue was then suspended in ethyl acetate (50 mL) and
filtered. The filtrate was washed with 10% sodium carbonate (aq)
(50 mL), brine (50 mL), dried over magnesium sulfate and filtered.
The excess solvent was again removed in vacuo to yield a brown oil
(75 mg, 82%) containing a mixture of lactone 11l and lactone 9, in
the ratio of 3:1.
2,3-Dimethyl 4-pentenoic acid (256 mg, 2.0 mmol) and lead(IV)
tetra(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were reacted
according to General Method 6 in a,a,a-trifluorotoluene (15 mL).
After work-up, the crude product was purified by flash column
chromatography eluting with 4:1 (40–60) petroleum ether/ethyl
acetate to give lactone 17 as colourless blocks with crystal structure
details given below (220 mg, 43%). Rf¼0.10 (4:1 (40–60) petroleum
ether/ethyl acetate); nmax(film)/cmꢀ1 1777s, 1713s, 1261s;
dH(400 MHz; CDCl3) 7.83 (1H, dd, J 1.2 and 3.7, thiophene H-3), 7.61
(1H, dd, J 1.2 and 5.0, thiophene H-5), 7.12 (1H, dd, J 3.7 and 5.0,
thiophene H-4), 4.71 (1H, m, H-2), 4.53 (1H, m, CH2O), 4.42 (1H, m,
CH2O), 2.85 (1H, m, H-4), 2.72 (1H, m, H-3), 1.20 (3H, d, J 7.1, H-4),
0.96 (3H, d, J 7.2, Me-3); dC(100.6 MHz; CDCl3) 134.09; 133.03,
127.87, 78.19, 63.52, 40.11, 36.13, 9.68, 8.53; HRMS (ESþ) found
255.0690, C12H14O4S [MþH]þ requires 255.0691; found 272.0957,
[MþNH4]þ requires 272.0957.
3.4.1. (3-Oxo-1,3-dihydro-isobenzofuran-1-yl)methyl thiophene-
2-carboxylate 13
3.4.4. (6-Oxo-tetrahydropyran-2-yl)methyl thiophene-
2-Vinylbenzoic acid (296 mg, 2.0 mmol) and lead(IV) tetra-
(thiophene-2-carboxylate) (1.43 g, 2.0 mmol) were suspended in
2-carboxylate 18
5-Hexenoic acid (228 mg, 2.0 mmol) and lead(IV) tetra(thio-
phene-2-carboxylate) (1.43 g, 2.0 mmol) were reacted according
to the General Method. The reaction mixture was seen to change
from yellow to white within 1.25 h of heating. After general
work-up the crude product (a yellow oil) was purified by flash
column chromatography eluting with 1:1 (40–60) petroleum
ether/ethyl acetate to give lactone 18 (230 mg, 48%). Rf¼0.34 (1:1
(40–60) petroleum ether/ethyl acetate); nmax(film)/cmꢀ1 1734s,
1712s, 1418s, 1262s; dH(400 MHz; CDCl3) 7.82 (1H, m, thiophene
H-3), 7.58 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.10 (1H, dd, J
a,a,a
-trifluorotoluene (15 mL) and heated at 90 ꢁC under argon for
5 h (until the colour had changed from bright yellow to cream). The
solvent was removed in vacuo to leave a pale brown solid, which
was dissolved in ethyl acetate (100 mL) and filtered. The filtrate was
washed with 5% sodium carbonate (aq) (150 mL) then satd brine
(2ꢂ100 mL). The organic layer was then dried over magnesium
sulfate, filtered and concentrated in vacuo to give a yellow oil. Pu-
rification by flash column chromatography eluting with 2:1 (40–
60) (petroleum ether/ethyl acetate) gave crystalline lactone 13

2548
I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
1.2 and 3.8, thiophene H-4), 4.67 (1H, m, H-2), 4.43 (2H, m,
CH2O), 2.64 (1H, m, H-5), 2.50 (1H, m, H-5), 1.86 (2H, m, 2ꢂH-4),
1.71 (2H, m, 2ꢂH-3); dC(100.6 MHz; CDCl3) 161.8, 134.0, 133.0,
132.9, 127.9, 77.6, 66.0, 29.5, 24.4, 18.2; HRMS (ESþ) found
258.0808, C11H12O4S [MþNH4]þ requires 258.0800. The same
1H NMR spectrum. Attempts to crystallise these complexes were
unsuccessful due to their low solubility and stability in the organic
solvents.
3.6.1. Lead(IV) tri(thiophene-2-carboxylate)mono(5-phenyl-(1H)-
tetrazole) 30 (m¼3, n¼1)
reaction carried out in
product in a 43% yield.
a,a,a-trifluorotoluene gave the same
LTA (2.00 g, 4.5 mmol), thiophene-2-carboxylic acid (1.73 g,
13.5 mmol) and 5-phenyl-(1H)-tetrazole (0.60 g, 4.1 mmol) were
reacted according to the above general method to give 30 (m¼3,
n¼1) as a pale yellow powder (2.48 g, 75%): mp 155 ꢁC dec; nmax
(KBr disk)/cmꢀ1 1526s, 1423s; dH(200 MHz; DMSO-d6) 8.08 (2H, dd,
J 4.3 and 8.0, ArH), 7.84 (3H, dd, J 1.2 and 4.9, thiophene H-3), 7.68
(3H, dd, J 1.2 and 3.7, thiophene H-5), 7.48–7.63 (3H, m, ArH), 7.18
(3H, dd, J 3.7 and 4.9, thiophene H-4); dC(400 MHz; DMSO-d6)
165.8, 132.9, 130.3, 129.8, 128.8, 127.3.
3.4.5. (7-Oxo-oxepan-2-yl)methyl thiophene-2-carboxylate 19
6-Heptenoic acid (256 mg, 2.0 mmol) and lead(IV) tetra(thio-
phene-2-carboxylate) (1.43 g, 2.0 mmol) were reacted according
to the General Method. The reaction mixture was seen to change
from yellow to white within 2 h of heating. After the general
work-up the crude product (a yellow oil) was purified by flash
column chromatography eluting with 1:1 (40–60) petroleum
ether/ethyl acetate to give lactone 19 (130 mg, 26%). Rf¼0.45 (1:1
(40–60) petroleum ether/ethyl acetate); nmax(film)/cmꢀ1 1794s,
1728s, 1258s; dH(400 MHz; CDCl3) 7.81 (1H, dd, J 1.2 and 3.7,
thiophene H-4), 7.56 (1H, dd, J 1.2 and 5.0, thiophene H-5), 7.10
(1H, dd, J 3.7 and 5.0, thiophene H-3), 4.58 (1H, m, H-2), 4.38–
4.45 (1H, m, CH2O), 4.30–4.35 (1H, m, CH2O), 2.63–2.74 (1H, m,
H-6), 2.56–2.63 (1H, m, H-6), 1.90–2.09 (3H, m, H-3, 2ꢂH-5),
1.52–1.71 (3H, m, H-3, 2ꢂH-4); dC(100.6 MHz; CDCl3) 134.0,
132.9, 127.8, 77.4, 66.4, 34.8, 31.1, 27.9, 22.8; HRMS (ESþ) found
255.0703, C12H15O4S requires 255.0691. The same reaction car-
3.6.2. Lead(IV) di(thiophene-2-carboxylate)di(5-phenyl-(1H)-
tetrazole) 30 (m¼2, n¼2)
LTA (2.00 g, 4.5 mmol), thiophene-2-carboxylic acid (1.15 g,
9.0 mmol) and 5-phenyl-(1H)-tetrazole (1.20 g, 8.2 mmol) were
reacted according to the above general method to give 30 (m¼2,
n¼2) as a pale yellow powder (3.13 g, 99%): mp 165 ꢁC dec; nmax
(KBr disk)/cmꢀ1 1421s, 1609m, 1164m; dH(200 MHz; DMSO-d6) 8.06
(4H, dd, J 4.3 and 8.0, ArH), 7.87 (2H, dd, J 1.0 and 4.9, thiophene H-
3), 7.68 (2H, dd, J 1.0 and 3.8, thiophene H-5), 7.45–7.60 (6H, m,
ArH), 7.19 (2H, dd, J 3.8 and 4.9, thiophene H-4); dC(400 MHz;
DMSO-d6) 127.3, 127.5, 128.7, 129.9, 130.8, 132.3, 132.4, 141.5, 157.8,
166.9.
ried out in
yield.
a,a,a-trifluorotoluene gave the same product in a 23%
3.5. Reaction of ethyl-3-oxohept-6-enoate 22 with lead(IV)
tetra(thiophene-2-carboxylate)
3.6.3. Lead(IV) mono(thiophene-2-carboxylate)tri(5-phenyl-(1H)-
tetrazole) 30 (m¼1, n¼3)
Lead(IV) tetra(thiophene-2-carboxylate) (1.57 g, 2.2 mmol) and
ethyl-3-oxohept-6-enoate (0.34 g, 2.0 mmol) were suspended in
toluene (15 mL) under argon. The reaction mixture was then heated
at 90 ꢁC for 3 h, however, after 3 min the reaction had changed from
bright yellow to white. Following the general work-up a precipitate
formed, which was filtered to give ester 23b (50.7 mg, 9%). nmax-
(film)/cmꢀ1 1782s, 1736s, 1414s, 1262s, 1098s; dH(400 MHz; CDCl3)
7.97 (2H, dd, J 1.2 and 3.8, thiophene H-3), 7.68 (2H, dd, J 1.2 and 4.9,
thiophene H-5), 5.89 (1H, m, H-6), 7.15 (2H, dd, J 3.3 and 4.9,
thiophene H-4), 5.08 (2H, dd, J 1.2 and 2.7, 2ꢂH-7), 4.35 (4H, q, J 7.1,
OCH2CH3), 3.21 (2H, m, 2ꢂH-4), 2.49 (2H, m, 2ꢂH-5), 1.34 (6H, t, J
7.1, OCH2CH3); dC(100.6 MHz; CDCl3) 198.7, 162.7, 159.2, 136.7,135.8,
134.7, 131.0, 128.2, 115.5, 63.4, 37.6, 27.0, 13.8; m/z (APCI) 423.56
(MþH, 30%), 297.64 (10).
LTA (2.00 g, 4.5 mmol), thiophene-2-carboxylic acid (0.57 g,
4.5 mmol) and 5-phenyl-(1H)-tetrazole (1.80 g, 12.3 mmol) were
reacted according to the above general method to give 30 (m¼1,
n¼3) as a pale tan coloured powder (3.00 g, 98%): mp 119 ꢁC dec;
nmax (KBr disk)/cmꢀ1 1527s, 1419s, 1609m, 1563s, 1163m;
dH(400 MHz; DMSO-d6) 8.06 (6H, dd, J 4.3 and 8.0), 7.76 (1H, dd, J
1.2 and 4.9, thiophene H-3), 7.62 (1H, dd, J 1.2 and 3.7, thiophene H-
5), 7.47–7.59 (9H, m), 7.15 (1H, dd, J 3.7 and 4.9, thiophene H-4);
dC(400 MHz; DMSO-d6) 132.2, 130.4, 129.8, 128.7, 128.1, 127.3.
3.6.4. Lead(IV) tetra(5-phenyl-(1H)-tetrazole) 30 (m¼0, n¼4)
LTA (2.00 g, 4.5 mmol) and 5-phenyl-(1H)-tetrazole (2.41 g,
18 mmol) were reacted according to the above general method to
give 30 (m¼0, n¼4) as a tan coloured powder, which turned deep
brown over several days (3.50 g, 97%): mp 114 ꢁC dec; nmax (KBr
disk)/cmꢀ1 1609m, 1563s, 1163m; dH(400 MHz; DMSO-d6) 8.03 (8H,
dd, J 4.3 and 8.0), 7.46–7.64 (12H, m); dC(400 MHz; DMSO-d6) 130.6,
127.4, 128.1, 129.9.
The filtrate gave ester 23a (0.33 g, 56%). dH(400 MHz; CDCl3)
7.97 (2H, dd, J 1.2 and 3.8, thiophene H-3), 7.68 (2H, m, thiophene
H-5), 7.15 (2H, m, thiophene H-4), (2H, m, thiophene H-3), 5.80 (1H,
m, H-6), 5.65 (1H, s, H-2), 4.95–5.11 (2H, m, 2ꢂH-7), 4.12–35 (2H, m,
OCH2CH3), 2.85 (1H, t, J 7.3, H-4), 2.62 (1H, t, J 7.3, H-4), 2.30–2.48
(2H, m, 2ꢂH-5),1.34 (3H, m, OCH2CH3); dC(100.6 MHz; CDCl3) 199.1,
164.4,162.6,160.5,159.2, 136.7,135.8,134.7,131.0,128.2, 115.7,115.5,
89.1, 62.5, 61.3, 42.0, 38.9, 27.4, 27.0, 14.0; m/z (APCI) 297.62 (MþH,
15%).
3.6.5. Reaction of 4-pentenoic acid with lead(IV) tetra-
(5-phenyltetrazol-1-ide) 30 (m¼0, n¼4)
4-Pentenoic acid (0.2 mL, 2.0 mmol) and lead(IV) tetra(5-phe-
nyltetrazol-1-ide) 30 (m¼0, n¼4) (1.56 g, 2.0 mmol) were reacted
according to the General Method to yield a brown oil (230 mg, 53%)
containing a mixture of products, 33a and 8b, in the ratio of 6:1.
5-(5-Phenyltetrazol-1-ylmethyl)dihydrofuran-2-one 33a (106 mg,
21%) was isolated as a white solid by flash column chroma-
tography eluting with 1:1 ethyl acetate/(40–60) petroleum ether
(Rf¼0.2 33a, 0.45 8b). Mp 82–84 ꢁC; nmax(KBr disk)/cmꢀ1 1773s,
1450s, 1182s; dH(400 MHz; CDCl3) 8.09–8.18 (2H, m, ArH), 7.46–
7.55 (3H, m, ArH), 5.10–5.17 (1H, m, H-2), 4.98 (1H, dd, J 3.1
and 14.1, CH2N), 4.86 (1H, dd, J 5.0 and 14.1, CH2N), 2.14–2.65 (4H, m,
H-3, 4); dC(400 MHz; CDCl3) 130.6,128.9,126.9, 76.3, 55.6, 27.7, 25.0;
m/z (CI, NH3) 245.11 ([MþH]þ, 100%), 262.13 ([MþNa]þ, 45); HRMS
(ESþ) found 245.1033 [MþH]þ, requires 245.1039.
3.6. General method for (5-phenyl-(1H)-tetrazole) containing
complexes
The required stoichiometry (between 0 and 4 equiv) of thio-
phene-2-carboxylic acid was dissolved in 50 mL of dry toluene. To
the resulting solution, the required stoichiometry (between 0 and
4 equiv) of 5-phenyl-(1H)-tetrazole in dry THF (20 mL) was added,
then 1 equiv of LTA. The mixture was stirred for 20 min, and the
solvent was removed in vacuo at 40 ꢁC (usually at 77 mbar). This
procedure was repeated twice to remove the acetic acid formed
shown by the disappearance of the singlet at dH (DMSO-d6) in the

I.F. Cottrell et al. / Tetrahedron 65 (2009) 2537–2550
2549
3.7. Reaction of lead(IV) tetraacetate with an excess of 5-(2-
(but-3-enyl)phenyl)tetrazole 35
Z¼4,
m
¼0.226 mmꢀ1 Dcalcd¼1.370 Mg/m3, T¼150(2) K, 15,011 re-
flections collected, 3475 independent [R(int)¼0.036], R1¼0.0515,
wR2¼0.0585 [I>3
s(I)].
Lead(IV) tetraacetate (0.22 g, 0.5 mmol) was reacted with 5-(2-
(but-3-enyl)phenyl)tetrazole 35 (0.40 g, 2 mmol, containing the
3.8.2. Single crystal X-ray diffraction data for 11g (CCDC 693950)
C11H12O4S1, Mr¼240.28, monoclinic (P2(1)/n), a¼11.0732(2) Å,
tetrazole 36) in dry
a,a,a-trifluorotoluene. Work-up in the usual
way and attempted purification of the mixture by flash column
chromatography on silica gel gave only azepine 38 (10 mg). Rf¼0.2
(2:1 heptane/ethyl acetate); dH(400 MHz; CDCl3) 8.23 (1H, d, J 7.2,
ArH), 7.86 (1H, dd, J 7.6 and 1.6, ArH), 7.36–7.52 (Ar–H), 7.28–7.34
(Ar–H), 5.49–5.56 (1H, m, CH2CHN), 5.34–5.39 (1H, m, NCH), 5.16–
5.22 (1H, m, NCH), 2.93–3.07 (2H, m, C6H4CH2), 2.89 (2H, t, J 7.9,
ArCH2), 2.51–2.59 (1H, m, CH), 2.20–2.29 (1H, m, CH), 1.47–1.55
(2H, m, CH2), 1.25–1.33 (4H, m, CH2CH2), 0.82–0.91 (3H, m, CH3);
dC(100.6 MHz; CDCl3) 166.0, 154.2, 142.4, 139.5, 132.0, 130.6, 130.4,
130.2, 129.9, 129.6, 127.8, 126.0, 125.3, 122.7, 58.4, 55.2, 34.0, 31.7,
30.9, 30.1, 22.5, 14.1; m/z (ESþ) 473.30 ([MþMeCNþNH4]þ, 100%).
b¼7.7705(2) Å, c¼13.4673(3) Å,
b
¼103.9786(11)ꢁ, V¼1124.47(4) Å3,
Z¼4,
m
¼0.283 mmꢀ1 Dcalcd¼1.419 Mg/m3, T¼150(2) K, 11,110 re-
flections collected, 2546 independent [R(int)¼0.021], R1¼0.0277,
wR2¼0.0304 [I>3
s(I)].
3.8.3. Single crystal X-ray diffraction data for 11j (CCDC 693951)
C11H12O4S1, Mr¼240.28, triclinic (Pꢀ1), a¼6.9926(3) Å,
b¼7.4493(4) Å, c¼11.5072(6) Å,
a
¼97.881(2)ꢁ,
Z¼4,
b
¼98.856(3)ꢁ,
g
¼103.2016(17)ꢁ,
V¼567.29(5) Å3,
m
¼0.281 mmꢀ1
Dcalcd¼1.407 Mg/m3, T¼150(2) K, 7414 reflections collected, 2573
independent [R(int)¼0.034], R1¼0.0390, wR2¼0.0474 [I>3
s(I)].
3.8. X-ray crystal structure determination
3.8.4. Single crystal X-ray diffraction data for 17 (CCDC 693952)
C12H14O4S, Mr¼254.31, monoclinic (P2(1)/c), a¼7.0552(3) Å,
Crystals of 11c, 11g, 11j and 16 were grown by slow diffusion.
Single crystal X-ray diffraction data were collected using graphite
b¼19.0298(9) Å, c¼9.4200(4) Å,
b
¼105.478(3)ꢁ, V¼1218.85(9) Å3,
Z¼4,
m
¼0.265 mmꢀ1 Dcalcd¼1.386 Mg/m3, T¼150(2) K, 9800 re-
monochromated Mo K
a
radiation (
l
¼0.71073 Å) on an Enraf-Non-
flections collected, 2748 independent [R(int)¼0.036], R1¼0.0474,
ius KappaCCD diffractometer. The diffractometer was equipped
wR2¼0.0593 [I>3s(I)].
with a Cryostream N2 open-flow cooling device,84 and the data
were collected at 150(2) K. Series of
u-scans were performed in
Supplementary data
such a way as to cover a sphere of data to a maximum resolution of
0.77 Å. Cell parameters and intensity data were processed using the
DENZO-SMN package.85 The structures were solved by direct
methods86 and refined by full-matrix least squares on F using the
CRYSTALS suite.87 Intensities were corrected for absorption effects
by the multi-scan method, based on multiple scans of identical and
Laue equivalent reflections. In general, non-hydrogen atoms were
refined with anisotropic displacement parameters, however, where
there was disorder in the structures for 11g, 11j and 17. For these
compounds, examination of the resulting model showed the car-
bon atoms of the thiophene ring to have large thermal parameters,
and a difference Fourier map showed a nearby peak of residuak
electron density. These observations were taken to be due to dis-
order of this group over two orientions related by a 180ꢁ rotation
about the bond joing it to the remainder of the molecule. Cordi-
nates and site occupancies were refined for the disordered non-
hydrogen atoms. For compounds 11g and 11j anisotropic thermal
parameters of the atoms of the major orientation of the ring and
isotropic thermal parameters of the minor orientation were refined,
however, for 17 the refinement was unstable, so isotropic thermal
parameters were used for both components. For the disordered
thiophene rings, geometric restraints were applied (the S–C bond
lengths were restrained to 1.71(1) Å, C]C bonds to 1.36(1) Å, C–C
bonds to 1.42(1) Å and chemically-equivalent bond angles to their
common mean) and the total of the occupancies of the two ori-
entations was constrained to unity. In general, the hydrogen atoms
were all visible in a difference map, but were repositioned geo-
metrically, then refined with soft restraints on the bond lengths and
angles to regularise their geometry after which the positions were
refined with riding constraints. For structure 17 hydrogen atoms
were positioned geometrically after each cycle of refinement. In
each case, a 3-term Chebychev polynomial weighting scheme was
applied.88,89 Crystallographic data for the structure have been de-
posited with the Cambridge Crystallographic Data Centre. Copies of
the data can be obtained free of charge from http://www.ccdc.ca-
m.ac.uk/products/csd/request/ (CCDC 693949–693952).
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.01.042.
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