Palladium(II)-catalyzed oxidative cyclization of allylic tosylcarbamates: Scope, derivatization, and mechanistic aspects

Article abstract of DOI:10.1002/chem.201202359

A highly selective oxidative palladium(II)-catalyzed (Wacker-type) cyclization of readily available allylic tosylcarbamates is reported. This operationally simple catalytic reaction furnishes tosyl-protected vinyl-oxazolidinones, common precursors to syn-1,2-amino alcohols, in high yield and excellent diasteroselectivity (>20:1). It is demonstrated that both stoichiometric amounts of benzoquinone (BQ) as well as aerobic reoxidation (molecular oxygen) is suitable for this transformation. The title reaction is shown to proceed through overall trans-amidopalladation of the olefin followed by β-hydride elimination. This process is scalable and the products are suitable for a range of subsequent transformations such as: kinetic resolution (KR) and oxidative Heck-, Wacker-, and metathesis reactions.

Full text of DOI:10.1002/chem.201202359

FULL PAPER
DOI: 10.1002/chem.201202359
Palladium(II)-Catalyzed Oxidative Cyclization of Allylic Tosylcarbamates:
Scope, Derivatization, and Mechanistic Aspects
Antoine Joosten,[a] Andreas K. ꢀ. Persson,[a] Renaud Millet,[a] Magnus T. Johnson,[b] and
Jan-E. Bꢁckvall*[a]
Abstract: A highly selective oxidative
palladium(II)-catalyzed (Wacker-type)
cyclization of readily available allylic
tosylcarbamates is reported. This op-
erationally simple catalytic reaction
furnishes tosyl-protected vinyl-oxazoli-
dinones, common precursors to syn-1,2-
amino alcohols, in high yield and excel-
lent diasteroselectivity (>20:1). It is
demonstrated that both stoichiometric
amounts of benzoquinone (BQ) as well
as aerobic reoxidation (molecular
oxygen) is suitable for this transforma-
tion. The title reaction is shown to pro-
ceed through overall trans-amidopalla-
dation of the olefin followed by b-hy-
dride elimination. This process is scala-
ble and the products are suitable for a
range of subsequent transformations
such as: kinetic resolution (KR) and
oxidative Heck-, Wacker-, and meta-
thesis reactions.
Keywords: amidopalladation · cyc-
lization · oxazolidinones · oxidation ·
palladium
Introduction
The palladium(II)-catalyzed intramolecular amination of
homoallylic carbamates, rendering oxazolidinones, has re-
cently attracted considerable attention.[9,10] This approach
has also been extended to an intermolecular variant furnish-
ing allylic N-carbamates.[11] In addition to these contribu-
tions, several efficient palladium-catalyzed cyclizations of al-
lylic tosylcarbamates involving for example amino-halogena-
tion,[12] amino-acetoxylation,[13] amino-alkynylation[14] and
amino-carbonylation[15] have also been reported. In general,
allylic and homoallylic substrates dominate this particular
catalytic approach, however, oxidative examples involving
carbamates with pending allenes[16] and alkynes[17] have also
proven successful in previous studies. Although many intra-
molecular amidopalladation reactions were known at the
outset of this work, there were, to the best of our knowl-
edge, no palladium(II)-catalyzed oxidative procedures in
which allylic carbamates are cyclized to form the unsaturat-
ed (vinyl) oxazolidinones by b-hydride elimination.[18] Re-
lated reactions in which allylic alcohols and amines are
used to tether the nucleophiles for a Wacker-type cycliza-
tion reactions have been reported for the synthesis of imi-
dazolidines,[19] oxazolidines,[20] thiadiazolidines,[21] or isoxa-
zolidine.[22] Ideally, this synthetic approach would be effec-
tive starting from cheap starting materials (allylic alcohols)
and readily available palladium catalysts. In this study we
report on an intramolecular palladium-catalyzed oxidative
cyclization of allylic tosylcarbamates leading to unsaturated
oxazolidinones (Scheme 1). Different features of this trans-
formation are presented, such as scalability, further derivati-
zation, and mechanistic aspects.
Oxidation reactions are of fundamental importance in
Nature and play a central role in organic synthesis.[1] Among
them, Wacker-type oxidative cyclizations provide efficient
access to oxygen and nitrogen heterocycles, and have been
extensively studied.[2] Following our interest in the carbon–
carbon bond-forming palladium(II)-catalyzed oxidative cyc-
lization of dienallenes,[3] enallenes,[4] and aza-enallenes,[5] we
were particularly interested in developing a new methodolo-
gy leading to the construction of unsaturated (vinyl) oxazoli-
dinones (Scheme 1). Oxazolidinones are common precursors
for 1,2-aminoalcohols, a key structural component in many
pharmaceutical compounds[6] and asymmetric ligands.[7,8]
Scheme 1. Outline and further perspectives of this study.
[a] Dr. A. Joosten, Dr. A. K. ꢀ. Persson, Dr. R. Millet,
Prof. J.-E. Bꢁckvall
Department of Organic Chemistry
Arrhenius Laboratory, Stockholm University
106 91, Stockholm (Sweden)
E-mail: jeb@organ.su.se
[b] M. T. Johnson
Department of Chemistry, Organic Chemistry
Lund University
P.O. Box 124, 22100 Lund (Sweden)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202359.
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Results and Discussion
the double bond. When the allylic alcohol (E)-5 was subject-
ed to the reaction conditions described in Table 1, a mixture
of the desired oxazolidinone 6 (59%) and tosylamine 7
(20%), originating from the decarboxylative Overman-type
isomerization, was obtained (Scheme 2).
Model reaction and optimization: The model tosylcarba-
mate 1 was prepared in one step by the treatment of crotyl
alcohol (E/Z 12:1) 4 with tosylisocyanate (TsNCO) in THF
at room temperature. After considerable experimentation
and screening for conditions furnishing acceptable amounts
of the desired oxazolidinone, we found that PdACTHNUTRGNEUNG(OAc)2
(1 mol%) in THF/DMSO (9:1) and in presence of benzo-
quinone (BQ, 1.5 equiv) provided the oxazolidinone in ac-
ceptable yield. In addition, we investigated the influence of
sodium acetate and acetic acid on the outcome of the reac-
tion (Table 1). Introduction of 0.5 equivalents of AcOH to
Scheme 2. Effects observed on oxazolidinone formation using E- or Z al-
lylic carbamates.
Table 1. Optimization of the palladium(II)-catalyzed cyclization of allylic
tosylcarbamate 1.[a]
On the contrary, when (Z)-5 was subjected to the same re-
action conditions, the desired isolated oxazolidinone was ob-
tained in 95% yield (Scheme 2). The transition states for
the Overman-rearrangement leading to 7 are different for
the E- and Z isomers. The transition state for the (E)-allylic
alcohol will be lower in energy compared with that of the
(Z)-allylic alcohol because of the pseudoaxial methyl group
in the latter (Scheme 3). This will favor formation of the
Overman side product from the E isomer.
Entry
HOAc
[equiv]
NaOAc
[equiv]
Conversion
[%][c]
Distribution
G
G
2/3/4[c]
1
2
3
4
5
6
7
0.5
0
2
1
7
0
0.5
1
2
1
100
100
100
65
45:5:50
40:20:40
75:20:5
55:35:10
80:10:10
85:10:5
85:10:5
100
7
3.5
0.5
0.5
100 (83)[b]
100
[a] 1 mol% PdACHTUNGTRENNUNG(OAc)2, 1.5 equiv of BQ, HOAc/NaOAc, THF/DMSO
(9:1), 508C, 12 h. [b] Yield of the isolated product. [c] Determined by
1H NMR spectroscopy of crude reaction mixtures.
Scheme 3. Proposed transition states for the Overman rearrangement.
the reaction mixture provided the expected oxazolidinone
Reaction scope: Having established that (Z)-allylic alcohols
provided better results than the corresponding (E)-isomer
we went on to further explore the scope of this reaction.
The readily available allylic alcohols were submitted to the
reaction conditions described in Scheme 2. The allylic alco-
hols (Z)-5 and 8 provided oxazolidinones as crystalline
solids in excellent yields (Table 2, entries 2 and 3), accompa-
nied by very small amounts of the corresponding Overman
side product. The trisubstituted prenyl-alcohol 10 proved to
be a much less effective substrate, and only 40% yield of
the corresponding isolated oxazolidinone 11 was obtained
after increasing the catalytic loading to 10 mol% and ex-
tending the reaction time to 48 h (Table 2, entry 4). Clearly,
terminally disubstituted allylic alcohols were reluctant to un-
dergo this palladium(II)-catalyzed amidation.
together with substantial amounts of allylic alcohol
4
(Table 1, entry 1). In this case, a small amount of a tosyla-
mine 3 derived from a decarboxylative Overman-type iso-
merization was also observed. Using 0.5 equivalents of
NaOAc provided more of the Overman-product 3 without
substantially affecting the formation of the oxazolidinone
(Table 1, entry 2).[23] The best reaction conditions were
found to be those in which both acetic acid and sodium ace-
tate were added in 7 and 0.5 equivalents, respectively
(Table 1, entry 6). Although the same product ratio was ob-
tained with a smaller amount of acetic acid (3.5 and
0.5 equiv, respectively, Table 1, entry 7), it appeared that
yields were systematically better with the conditions of
entry 6 (7 and 0.5 equiv, respectively). Under these condi-
tions, oxazolidinone 2 was isolated in 83% yield (Table 1,
entry 6). Evidently, buffered reaction conditions were crucial
for the selectivity between the desired 5-exo-trig and the un-
desired 6-endo-trig reaction.
Starting from the secondary, phenyl-substituted allylic al-
cohol 12, syn-1,2-amidoalcohol 13 was obtained in 61%
yield after hydrolysis of the corresponding oxazolidinone
under basic conditions (Table 2, entry 5). The additional hy-
drolysis step was required due to purification difficulties of
the oxazolidinone.
Influence of E/Z-isomerism: Since our model reaction was
run with an E/Z-mixture of allylic alcohol (12:1), we decid-
ed to investigate the effect of the stereochemistry around
We were pleased to find that secondary allylic alcohols
(Table 2, entries 6–9) produced trans-oxazolidinones with
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Palladium(II)-Catalyzed Oxidative Cyclization
FULL PAPER
Table 2. Scope and limitations of the cyclization of allylic tosylcarbama-
tes.[a]
Entry Alcohol
Product
d.r.
Yield [%][b]
1
–
83
E/Z=12/1
2
3
–
95[c]
Figure 1. X-ray structure of oxazolidinones 19 (left) and 21 (right).
possible to obtain oxazolidinones bearing different alkyl
chains in good yields and with excellent diastereoselectivi-
ties (Table 2, entries 6–8). The diastereoselectivity of the ox-
azolidines is higher than that previously obtained with relat-
ed protocols.[9a,c]
The developed catalytic process was also compatible with
cyclic allylic alcohols. With some modifications, mainly in-
creasing the catalytic loading and extending the reaction
time, the isolated bicyclic cis-oxazolidinone 21 was obtained
in 81% yield after 48 h (Table 2, entry 9). These bicyclic ox-
azolidinones have previously been prepared through palladi-
um(0) catalysis.[24] Finally, 1,3-amidoalcohol 23 was obtained
from homoallylic alcohol 22 by the same procedure and sub-
sequent hydrolysis. In this particular case, a higher catalytic
loading and a longer reaction time was required (Table 2,
entry 10).
As we were interested in some further reactivity of the ac-
quired oxazolidinones (see below) we decided to evaluate a
scale up of this reaction. Starting with 2 g (27.7 mmol) of al-
lylic alcohol 4, the reaction with TsNCO at 08C furnished
the desired allylic tosyl carbamate 1 (not isolated). Treat-
ment of the latter species under the reaction conditions de-
scribed in Table 2 produced 5.56 g (75%) of the desired vi-
nyloxazolidinone 2 as a colorless solid in a one-pot proce-
dure. Similarily, 7.64 g (78%) of 9 was obtained starting
from 3 g (34.8 mmol) of allylic alcohol 8 (Scheme 4). In the
–
–
81[c]
4
5
40[d]
>20:1 61[e,f]
6
7
8
>20:1 79[f]
>20:1 78[f]
>20:1 83[f]
9
>25:1 81[d]
10
–
46[c,d,e]
[a] Allylic alcohol (1 equiv) and TsNCO (1 equiv) were stirred in THF at
RT for 1 h. DMSO was added to give THF/DMSO 9:1 and this carba-
mate solution was used for the cyclization. Unless noted otherwise the
following reaction conditions were employed for the cyclization: Pd-
ACHTUNGTRENNUNG
Scheme 4. Scale up of the palladium-catalyzed oxidative cyclization of al-
lylic tosylcarbamates.
AHCTUNGTRENNUNG
product. [f] Same conditions as in [a] but with 5 mol% of Pd
48 h reaction time.
ACHTUNGRTEN(NUNG OAc)2 and
latter case, the pure product was isolated by precipitation of
the oxazolidinone after extraction. Unfortunately, the same
approach was not possible for 2, as precipitation failed to
separate the Overman product from the oxazolidinone.
excellent diastereoselectivity (see Figure 1 for X-ray struc-
tures) in yields in the range of 80%. For example, the
TBDMS-protected allylic alcohol 16 provided the isolated
trans-oxazolidinone 17 in 78% yield with excellent diastero-
selectivity (>20:1). Following the same procedure, it was
Biomimetic reoxidation procedure: Among the Wacker type
oxy- and azapalladation, a number of catalyst systems have
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J.-E. Bꢁckvall et al.
been reported for an aerobic version. The most widely used
[19,20,25]
systems involve the use of Pd
U
or Pd-
[21]
(OOCCF3)2
with DMSO or PdAHCTUNGTRENNUNG
Unfortunately, with our allylic tosylcarbamates, these sys-
tems gave poor yields and selectivities and we observed
large amounts of Overman and overoxidation products. To
further enhance the synthetic utility, as well as evaluating a
green(er) process for this transformation, an aerobic (biomi-
metic) reoxidation system was investigated. In this ap-
proach, catalytic amounts of BQ is used in combination with
an oxygen-activating cobalt complex (24), which allows the
use of molecular oxygen as terminal oxidant instead of BQ.
We have previously reported such procedures for a variety
of palladium-catalyzed reactions, in which O2 is serving as a
terminal oxidant.[4b,c,27]
Scheme 6. Deprotection of oxazolidinone 9 using magnesium in metha-
nol.
the tosyl-protected amidoalcohol 26 (Scheme 6). The devel-
opment of an efficient and simple deprotection is the first
step towards our final goal of attempting N-allenylation and
subsequent oxidative carbocyclization of these interesting
substrates (see Scheme 1).
Vinyl-substituted b-aminoalcohols and its protected ana-
logues have been widely used as building blocks for the syn-
thesis of unnatural amino acids,[30] pharmaceutically relevant
compounds,[31] and natural products.[32] We thus decided to
investigate an enzymatic kinetic resolution (KR) strategy to
obtain enantioenriched derivatives. The tosyl-protected vi-
nyloxazolidinone 2, in racemic form, was hydrolyzed and
subsequently subjected to an enzymatic KR in the presence
of 2,2,2-trifluoroethyl 3-(4-(trifluoromethyl)phenyl)propa-
noate as acyldonor and lipase AK “Amano” 20 as biocata-
lyst[33] (Scheme 7). Fortunately, the lipase showed a decent
selectivity for this substrate (E=60). At 51% conversion
the alcohol (S)-27 was isolated in 48% yield and with an
enantiomeric excess (ee) of 93%, whereas the ester (R)-28
was obtained in 42% yield and 89% ee.
To demonstrate the efficiency of this aerobic protocol, al-
lylic alcohol 4 was, after derivatization with TsNCO, subject-
ed to Pd
ACHTUNGTRENNUNG
balt(II)-salophen
Figure 2), BQ (10 mol%) and
an ambient pressure of molecu-
lar oxygen, under otherwise
similar reaction conditions as
those employed in the stoichio-
metrically, BQ-mediated reac-
tion (Scheme 5).
Figure 2. Structure of oxygen
activating CoII-salophen.
Scheme 5. Biomimetic, PdACHTNURTGEN(UNG OAc)2-catalyzed oxidative cyclization of allylic
tosylcarbamates.
Scheme 7. Enzymatic kinetic resolution of (rac)-27.
Using these catalytic conditions, oxazolidinone 2 was ob-
tained in 60% yield after 72 h. Similarily, oxazolidinone 6
was obtained in 75% yield starting from allylic alcohol (Z)-
5. It is worth pointing out that these conditions are not yet
optimized and further investigations on this and other alter-
native aerobic reoxidation protocols (ligand modulation) are
currently being investigated in our laboratories.
The vinyl moiety of the oxazolidinone 2 can also be func-
tionalized leading to interesting precursors of 1,2-aminoalco-
hols. As an example, classical palladium-catalyzed oxida-
tions were conducted. A Wacker type oxidation of the
double bond in oxazolidinone 2 led to the selective forma-
tion of the ketone 29 in 75% yield (95:5 ketone to aldehyde
selectivity), using conditions previously reported.[34]
Furthermore, a cross metathesis reaction between methyl
acrylate and oxazolidinone 2 afforded product 30 in 69%
isolated yield in the presence of 5 mol% of Hoveyda-
Grubbs 2nd generation catalyst (HG 2nd-gen, Scheme 8).
Finally, inspired by the recent interest in oxidative-Heck
reactions,[35] oxazolidinone 2 was allowed to react with Pd-
Further derivatization: Removal of the tosyl protecting
group is generally a quite impractical and time-consuming
process involving, for example, reductive cleavage using
sodium/lithium and naphthalene.[28] On the basis of an earli-
er report by Ragnarsson and co-workers,[29] we set out to in-
À
vestigate the possibility of cleaving the N Ts bond using
magnesium under ultrasonic conditions. It turned out that
the tosyl group was readily removed using this protocol.
Treatment of 9 with magnesium in dry methanol at 08C
under ultrasonic irradiation provided oxazolidinone 25 in
82% isolated yield, accompanied by approximately 10% of
AHCTUNGRTEGNUN(N OAc)2 (5 mol%), 1,10-phenanthroline (5 mol%, L9), and
1.5 equivalents of an arylboronic acid in a 1:1 mixture of
DMSO and THF at 508C for 24 h to furnish the arylated ox-
azolidinones (31) (Scheme 9).
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Palladium(II)-Catalyzed Oxidative Cyclization
FULL PAPER
of the oxazolidinone (Scheme 12).[2i] First, the reaction could
proceed through a (p-allyl)palladium(II)-intermediate, much
similar to procedures reported by White and Fraunhoffer.[9a]
The other two possibilities are cis- or trans-amidopalladation
of the olefin, followed by syn-b-hydride elimination.
(Scheme 12). To probe the mechanistic process that was op-
erating in our catalytic reaction, we first attempted to cy-
clize the homoallylic alcohol 35 under the conditions descri-
1
bed in Scheme 2. Interpretation of the crude H NMR spec-
trum from this reaction revealed that only trace amounts of
the desired oxazolidinone 2 was formed (Scheme 10) Based
on this observation we conclude that the cyclization most
Scheme 8. Functionalization of the vinyl moiety of oxazolidinone 2.
À
likely does not proceed through syn C H abstraction to give
a (p-allyl)palladium intermediate (M1). The small amount
of product observed in this reaction was considered to origi-
nate from terminal/internal olefin isomerization, followed
by amidopalladation.
Scheme 9. Palladium(II)-catalyzed oxidative Heck reaction.
Indeed, these catalytic conditions rendered predominantly
trans-arylated tosyl-oxazolidinones in excellent yield and
with high E/Z selectivity (>15:1) (Scheme 9). Initially we
intended to perform this reaction in a one-pot procedure
coupled to the cyclization, but we were unable to obtain sat-
isfactory conversions with this strategy.[36] Nevertheless, the
two-step procedure tolerated electronically diverse aryl bor-
onic acids.
Scheme 10. Attempt to observe formation of a p-allyl complex in the cyc-
lization of homoallylic tosylcarbamates.
Although we initially started this project with the purpose
of gaining access to substrates suitable for a copper(I)-cata-
lyzed allenylation, we are also currently looking into apply-
ing this cyclization startegy in natural product synthesis. Pri-
marily, we are investigating a synthesis of Kainic acid
(Figure 3, 34) starting from oxazolidinone 2. Other potential
targets, are for example sphingosine or pachastrissamine
(jaspine B) (Figure 3, 32 and 33).
Having excluded the above pathway we turned our focus
to oxidative amidopalladation processes. It has been shown
on previous occasions that the stereochemistry of true ami-
nopalladations are trans.[37] On the contrary, in the case of
amidopalladation, sulfonamidopalladation and sulfoacetami-
dopalladation, the addition across the double bond can
occur with either cis[38] or trans stereochemistry.[2i,13,38a–c,39]
Stahl and co-workers carried out an elegant and systematic
study on the aza-Wacker addition of different sulfonamides
to double bonds,[38c] which is similar to the work of Stoltz
and co-workers for oxypalladation.[26b] This study revealed
that both the substrate and the additives have a strong influ-
ence on the stereochemical outcome of the reaction. Using
a similar strategy, we decided to analyze the stereochemistry
of the amidopalldation step in our catalytic reaction. For
this purpose we synthesized the deuterium-labeled allylic al-
cohol trans-20[D1] (Scheme 11).[40] The synthesis started with
a stereoselective cis-1,4-chloroacetoxylation of 1,3-cyclohex-
adiene 36.[41] The acetyl group in the acetoxychlorinated
product 37 was then carefully deprotected using DIBAL-H
at 08C to give 38, avoiding the formation of the epoxycyclo-
hexene byproduct. Chloroalcohol 38 was then protected as a
silyl ether with TBDMSCl to furnish 39 in 81% yield. Incor-
poration of the deuterium was achieved by SN2-type nucleo-
philic substitution of the chloride by a deuterium (LiAlD4)
Figure 3. Possible target molecules.
Mechanistic investigation: Palladium(II)-catalyzed amina-
tions of olefins can potentially occur through a variety of
different mechanisms. Normally, the mechanism has to be
studied for each individual reaction as the choice of solvent,
additives and amine/amide-nucleophile can potentially
affect the mechanistic outcome. For our oxidative cycliza-
tion, three mechanisms were considered for the formation
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J.-E. Bꢁckvall et al.
Scheme 13. Cyclization of deuterium-labeled carbamate trans-41[D1].
of the deuterium atom. This result clearly demonstrates that
the reaction proceeds through a trans-amidopalladation. In
our case, the large excess of acetic acid may favor coordina-
tion of the alkene to palladium, which would lead to trans-
amidopalladation.
Scheme 11. Synthesis of the trans-deuterated cyclic alcohol 20.
Conclusion
In conclusion, we have developed a new and highly diaster-
eoselective cyclization of allylic carbamates. This reaction
involves inexpensive starting materials, in the form of allylic
alcohols, together with a readily available catalyst (Pd-
AHCTUNGRTEGNUN(N OAc)2) and oxidant (BQ). The method also proved scala-
ble, highly diastereoselective, and quite general. In this
study, we have also shown that the stereochemistry of this
sulfonylcarbamatopalladation is trans. This stereochemical
pathway opens up routes for asymmetric versions of this re-
action catalyzed by chiral PdII complexes.
Acknowledgements
Financial support from the European Research Council (ERC AdG
247014), the Berzelius Center EXSELENT, and the Swedish Research
Council is gratefully acknowledged. We thank Jan Deska (Universitꢁt zu
Kçln) for fruitful discussions and assistance with the metathesis reac-
tion.
Scheme 12. Three possible mechanistic scenarios.
to give trans-40[D1]. Finally, deprotection of the silyl group
using TBAF afforded the desired deuterium-labeled cyclo-
hexenol trans-20[D1] in 61% yield (ꢀ97% deuterium incor-
poration).
[1] J. E. Bꢁckvall, Modern Oxidation Methods, 2nd Edition, Wiley-
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Using trans-20[D1] as starting material, we argued that it
would be possible to determine whether the reaction occurs
through trans- or cis-amidopalladation (Scheme 12). The
first mechanism involves initial binding of a palladium(II)
species to the double bond on the opposite side (trans) of
the carbamate, followed by attack of the nitrogen and b-
deuteride elimination leading to exclusive formation of 21
(Scheme 12, upper pathway). The second pathway
(Scheme 12, lower pathway), cis-amidopalladation, occurs if
the tosyl carbamate coordinates to palladium(II) followed
by cis-migratory insertion of the olefin and subsequent b-hy-
dride elimination.[2i] This process would lead to formation of
the deuterated bicyclic oxazolidinone 21[D1].
[3] a) J. Lçfstedt, J. Franzꢃn, J. E. Bꢁckvall, J. Org. Chem. 2001, 66,
8015; b) J. Piera, A. Persson, X. Caldentey, J. E. Bꢁckvall, J. Am.
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Reaction of the allylic alcohol trans-20[D1] with tosyl iso-
cyanate produced the corresponding carbamate, trans-
41[D1]. Subjecting this carbamate to the catalytic conditions
described in Scheme 13 afforded 21, with complete removal
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Chem. Eur. J. 2012, 18, 15151 – 15157

Palladium(II)-Catalyzed Oxidative Cyclization
FULL PAPER
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Received: July 2, 2012
Published online: October 2, 2012
Chem. Eur. J. 2012, 18, 15151 – 15157
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15157