Amidine catalysed O- to C-carboxyl transfer of heterocyclic carbonate derivatives

Article abstract of DOI:10.1039/b805850d

The structural requirements of amidines necessary to act as efficient O- to C-carboxyl transfer agents are delineated and the scope of this process outlined through its application to a range of oxazolyl, benzofuranyl and indolyl carbonates. The Royal Society of Chemistry.

Full text of DOI:10.1039/b805850d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Amidine catalysed O- to C-carboxyl transfer of heterocyclic carbonate
derivatives†
Caroline Joannesse, Carmen Simal, Carmen Concello´n, Jennifer E. Thomson, Craig D. Campbell,
Alexandra M. Z. Slawin and Andrew D. Smith*
Received 9th April 2008, Accepted 8th May 2008
First published as an Advance Article on the web 16th June 2008
DOI: 10.1039/b805850d
The structural requirements of amidines necessary to act as efficient O- to C-carboxyl transfer agents
are delineated and the scope of this process outlined through its application to a range of oxazolyl,
benzofuranyl and indolyl carbonates.
Introduction
Bicyclic amidines such as DBU are commonly used in syn-
thesis for a variety of base-mediated transformations including
eliminations,1 esterifications,2 condensations,3 and the formation
of trichlorocarbinols.4 A common misconception regarding the
reactivity of these species is their low nucleophilicity, however,
they have been implicated as nucleophilic catalysts in a number
of reactions5 including ring opening polymerisations,6 silylation
of alcohols,7 esterifications with dimethyl carbonate,8 and the
acylation of alcohols.9 Despite their widespread use, only limited
applications employing amidines as catalysts for C–C bond
forming reactions have been delineated. For example, Zhang and
Shi have shown that DBU may be employed in cyanoacylation
reactions,10 while Aggarwal and Mereu have shown that DBU
is a versatile catalyst for Baylis–Hillman reactions.11 Nearly
forty years ago Steglich and Ho¨fle showed that both DMAP and
4-(pyrrolidino)pyridine (PPY) could promote the rearrangement
of 5-oxazolyl carbonate derivatives 1 to their corresponding 4-
carboxyazlactones 2 (Fig. 1).12 A number of elegant asymmetric
Fig. 1 Catalysts for O- to C-carboxyl transfer.
approaches to this transformation using catalysts such as 5–8
have been developed,13 and this O-to C-carboxyl transfer reaction
has been applied to a range of heterocyclic (oxazolyl, indolyl
and benzofuranyl) carbonate derivatives in both the racemic
11 and 12 respectively (Fig. 2).13c,16 While this product formally
and asymmetric series. To date, only limited catalysts other than
corresponds to protonation of the enolate postulated to occur
DMAP or PPY derivatives are known to promote this synthetically
during the catalytic cycle, this product is often present in crude
useful transformation. Vedejs et al. have shown that both achiral
product mixtures prior to work-up and when strictly anhydrous
conditions are employed. It has been proposed that this product
and chiral phosphines such as 4 and 6 are catalytically active in
this reaction,13c while we have shown that N-heterocyclic carbene
may arise from an internal proton-transfer pathway due to the
3 is an efficient catalyst of this transformation.14,15
presence of potentially acidic C–H bonds adjacent to phosphorus
in the intermediate ion pair, although conclusive evidence for this
Despite the vast advances made within this area of research,
limitations of the methodology developed to date include the
low reactivity of C(4)-a-branched oxazolyl carbonates such as 9
in this reaction, presumably due to steric hindrance,13c and the
formation of significant quantities (typically >20%) of the parent
benzofuranones ( )-15 and ( )-16 when employing PEt3 or PBu3
4 upon the rearrangement of benzofuranyl carbonates such as
pathway has not been delineated.16 Furthermore, to the best of our
knowledge, the corresponding product arising from the carbonate
functionality upon generation of the parent benzofuranone has
not been isolated or characterised.
As part of a research programme concerned with the devel-
opment and applications of nucleophilic organocatalysts,17 we
demonstrate herein that amidines can act as efficient O- to C-
carboxyl transfer agents and show that the catalytic efficiency and
product distributions of these reactions is markedly affected by
catalyst structure.18 The optimal catalyst identified from an initial
screen is subsequently applied to the rearrangement of a wide
range of oxazolyl, benzofuranyl and indolyl carbonates.
EaStCHEM, School of Chemistry, University of St Andrews, North Haugh,
St Andrews, UK KY16 9ST. E-mail: ads10@st-andrews.ac.uk
† Electronic supplementary information (ESI) available: Spectroscopic
data (1H and 13C) for all new products as well as 1H data for all
known compounds prepared by a non-literature procedure. See DOI:
10.1039/b805850d
2900 | Org. Biomol. Chem., 2008, 6, 2900–2907
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
(entry 2). Although none of the desired C-carboxyazlactone ( )-
18 was formed in this reaction, the isolation of azlactone ( )-
19 and diphenyl carbonate implied that DBU was involved as
a nucleophilic catalyst in this transformation. Treatment of an
authentic sample of ( )-18 with DBU returned only starting
material, implying that product ( )-18 is not an intermediate
in the formation of ( )-19 in this reaction. Simply changing
the geometric constraints of the amidine from DBU to DBN
showed a remarkable change in product distribution, giving, at
full conversion of 17, an 80 : 20 mixture of the desired C-
carboxyazlactone ( )-18 : azlactone ( )-19 (entry 3).
Cognisant of the recent seminal O-acylation studies employing
amidine catalysis by both Birman and Okamoto et al.,20 further
investigations sought to develop this amidine promoted O- to
C-carboxyl transfer reaction into a viable synthetic protocol.
Systematic variation of the amidine catalyst structure and reaction
solvent was investigated in order to minimise the formation of
undesired azlactone ( )-19 and diphenyl carbonate 20 from 17.
The effect of changing the solvent from THF to CH2Cl2 was
first investigated, with DBU (10 mol%) giving an identical 40%
conversion of 17 to ( )-19 and diphenyl carbonate 20 (Table 2,
entry 2), although DBN (10 mol%) in CH2Cl2 gave an improved
90 : 10 ratio of ( )-18 : ( )-19 (entry 4). The incorporation of a
sulfur atom conjugated to the catalytic site was next investigated,
with thioamidine 24 and its benzannulated derivative 25 giving
essentially exclusive conversion of carbonate 17 to ( )-18 (>98%),
affording ( )-18 in 63% and 87% isolated yield respectively (entries
5 and 6). Further variation in catalyst ring size using 26 or 27 was
investigated, with only starting material returned upon treatment
of 17 with imidazo-fused 26 (10 mol%), while 27 (10 mol%) gave
an 88 : 12 mixture of ( )-18 : ( )-19 (entries 7 and 8). The effect
of altering the migrating group showed that methyl carbonates
are also tolerated in this reaction. Treatment of methyl carbonate
22 with amidine 24 gave a 90 : 10 mixture of ( )-23 : ( )-19,
giving ( )-23 in 59% isolated yield, while benzannulated amidine
25 again gave full conversion to exclusively ( )-23, giving ( )-23
in 87% isolated yield (entries 9 and 10). Addition of either 26 or
27 to 22 returned only starting material in both cases (entries
11 and 12). These findings indicate the critical importance of
the size and substitution of both heterocyclic rings within these
amidines in terms of catalyst activity and product distribution
and follow a similar trend to the studies of both Birman and
Okamoto for alcohol O-acylation.20,21 These results also indicate
that benzannulated amidine 25 is the optimal catalyst tested in this
initial screen to promote the desired O- to C-carboxyl transfer.
The formation of the parent azlactone ( )-19 and diphenyl
carbonate 20 from phenyl carbonate 17 under anhydrous reaction
conditions in these reactions is noteworthy, as this is analogous to
the formation of benzofuranones upon reaction of benzofuranyl
carbonates with phosphine catalysts.13c,16 The generation of these
by-products in variable amounts upon the reaction of carbonate
17 with amidines DBU, DBN, 24, 26 and 27 indicates that
competitive mechanistic pathways are available in these reactions.
The formation of the desired C-carboxyazlactone ( )-18 from 17
presumably follows the established mechanistic pathway of the
Steglich rearrangement, with nucleophilic attack of the amidine at
the carbonate carbonyl and subsequent collapse of the tetrahedral
intermediate 28 generating carboxyl transfer intermediate 29 and
enolate 30. Recombination generates tetrahedral intermediate 31,
Fig. 2 Limitations of the O- to C-carboxyl transfer reaction methodology.
Results and discussion
The identification of an efficient amidine catalysed O-to
C-carboxyl transfer protocol
Our interest in the ability of amidines to catalyse the rearrange-
ment of oxazolyl carbonates developed from screening a range
of organic bases for the generation of the corresponding NHC
from triazolium salt 21.19 Relatively efficient catalysis of the
rearrangement of oxazolyl carbonate 17 to ( )-18 was noted
using DBU as the base for the deprotonation of salt 21 in THF,
proceeding to give ( )-18 in ∼85% conversion after 2 hours
(Table 1, entry 1). However, background experiments showed that
treatment of 17 with DBU (10 mol%) alone in THF promoted
40% conversion to the parent azlactone ( )-19, with diphenyl
carbonate 20 isolated as a by-product of this transformation
Table 1 Identification of amidines as O- to C-carboxyl transfer catalysts
Amidine
(mol%)
Product ratio
(
)-18 : ( )-19a
Entry
Salt
Conversiona
1
2
3
21
—
—
DBU (9)
DBU (10)
DBN (10)
85%
40%
>98%
100 : 0
0 : 40
80 : 20
a All reaction conversions and product distributions were judged by 1H
NMR spectroscopic analysis of the crude reaction product.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2900–2907 | 2901
©

View Article Online
Table 2 Variation of product distribution with amidine catalyst structure
Product ratio
(
(
)-18 or
Entry
Amidine
R
Solvent
Conversiona
)-23 : ( )-19a
1
2
3
4
5
6
7
8
9
DBU
DBU
DBN
DBN
24
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
THF
CH2Cl2
THF
40%
40%
>98%
>98%
>98%
>98%
0
>98%
>98%
>98%
0
0 : 40
0 : 40
Fig. 3 Plausible mechanistic pathway for the O- to C-carboxyl transfer
reaction.
80 : 20
90 : 10
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
>98:<2; (63%)b
25
>98:<2; (87%)b
scope of this transformation (Table 3). Variation of the carbonate
functionality within the oxazole structure was first probed. Treat-
ment of C(4)-benzyl substituted Ph, Me, Bn and C(Me)2CCl3 car-
bonate derivatives 17, 22, 32 and 3322 with amidine 25 (2–10 mol%)
gave exclusively the corresponding ( )-C-carboxyazlactones
( )-18, ( )-23, ( )-38 and ( )-39, isolated in 76–92% yield in each
case (entries 1–4). C(4)-Isobutyl substituted carbonates 34–37 also
readily rearranged upon treatment with 25 (entries 5–8), giving the
desired products ( )-40–( )-43 in 61–93% yield. In both series,
rearrangement of the phenyl carbonate derivatives proved the most
26
—
27
88 : 12
24
90 : 10; (59%)b
10
11
12
25
>98:<2; (87%)b
26
—
—
27
0
a All product conversions and distributions were judged by 1H NMR
spectroscopic analysis of the crude reaction product. b Isolated yield
of homogeneous product after aqueous work-up or chromatographic
purification.
with elimination of the amidine giving ( )-18 and regenerating
the catalyst (Fig. 3). Although a complete and unambiguous
mechanistic hypothesis to account for the formation of diphenyl
carbonate 20 and azlactone ( )-19 in these reactions has yet to be
fully developed, it seems reasonable to assume in situ formation of
phenoxide as a necessity for the formation of 20. Phenoxide could
be generated from a number of sources in the assumed catalytic
cycle, including tetrahedral intermediates 28 or 31, or alternatively
from in situ hydrolysis of 17, ( )-18 or 29 from adventitious
water. While not conclusive, preliminary investigations indicate
that treatment of 17 with KOPh (1 eq) and 18-crown-6 (1 eq)
generates azlactone ( )-19 and diphenyl carbonate as the major
reaction products, while addition of KOPh (10 mol%) and 18-
crown-6 (10 mol%) to ( )-18 returns only starting material.
Ongoing investigations are aimed towards developing a full
mechanistic understanding of the product distributions of these
transformations.
Table 3 Variation of carbonate functionality
Entry
R
Rꢀ
Product
25 (mol%)
Yielda
1
2
3
4
5
6
7
8
17
22
32
33
34
35
36
37
Bn
Bn
Bn
Bn
i-Bu
i-Bu
i-Bu
i-Bu
Ph
Me
Bn
C(Me)2CCl3
Ph
Me
Bn
(
(
(
(
(
(
(
(
)-18
)-23
)-38
)-39
)-40
)-41
)-42
)-43
2
10
10
10
2
10
10
10
87%
87%
76%
92%b
61%
61%
93%
92%
Probing the generality of amidine catalysed O- to C-carboxyl
transfer: oxazolyl carbonates
C(Me)2CCl3
a Isolated yield of homogeneous product after aqueous work-up or
chromatographic purification. b Reaction time of one hour.
Having established benzannulated thioamidine 25 as the optimal
catalyst screened, further investigations sought to explore the
2902 | Org. Biomol. Chem., 2008, 6, 2900–2907
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
efficient, with catalyst loadings of 2 mol% employed to give full
conversion to the corresponding ( )-C-carboxyazlactone within
15 minutes. The catalyst loadings required for the rearrangement
of the other carbonate derivatives could also be lowered, but
required significantly longer reaction times to achieve reasonable
product conversions.
Extensive C(4)-substituent modification within the oxazole
skeleton was next investigated using the most reactive phenyl
carbonate derivatives (Table 4). Rearrangement of the C(4)-
methyl and C(4)-phenyl oxazolyl carbonates 44 and 45 to their
corresponding carboxyazlactones ( )-50 and ( )-51 using 2 and
5 mol% of amidine 25 respectively proceeded within 15 minutes
(entries 3 and 4). Importantly, a C(4)-isopropyl substituent is
tolerated (entry 5), a reaction that has previously proven difficult
with chiral DMAP derivatives.13c This procedure also tolerates
heteroatom containing C(4)-substituents with good reaction ef-
ficiency (entries 6–8), and allows chemoselective rearrangement
of tyrosine derived dicarbonate 48 to give ( )-54 in 92% isolated
yield.
from N-benzylisatin, gave good conversion to ( )-57 within
one hour, affording ( )-57 in 88% isolated yield (Scheme 1). The
molecular structure of ( )-57 was unambiguously proven by X-
ray analysis (Fig. 4). This procedure proved equally applicable
to benzofuranyl carbonates, as treatment of carbonate 58 with
10 mol% of 25 gave full conversion to ( )-59 in 15 minutes,
affording ( )-59 in 92% isolated yield.
Table 4 Variation of C(4)-substitution within the oxazolyl carbonate
Scheme 1 Thioamidine promoted rearrangement of indolyl and benzo-
furanyl carbonates 56 and 58.
Entry
R
Product
25 (mol%)
Yielda
1
2
3
4
5
6
7
8
17
34
44
45
46
47
48
49
Bn
i-Bu
Me
Ph
i-Pr
(
(
(
(
(
(
(
(
)-18
)-40
)-50
)-51
)-52
)-53
)-54
)-55
2
2
2
5
5
2
2
2
87%
61%
65%
84%
58%
87%
92%
92%
4-BnOC6H4CH2
4-PhOCO2C6H4CH2
MeSCH2CH2
a Isolated yield of homogeneous product after aqueous work-up or
chromatographic purification.
Amidine catalysed O- to C-carboxyl transfer: indolyl and
benzofuranyl carbonates
Fig. 4 Molecular representation of the X-ray crystal structure of ( )-57.
Further studies focused upon extending the substrate scope of
this transformation through probing the ability of thioamidine
25 to catalyse the O- to C-carboxyl rearrangement of a number
of benzofuranyl23 and indolyl carbonates. Both Fu and Vedejs
et al. have demonstrated the asymmetric rearrangement of these
heterocyclic carbonates, although reactions in the indolyl series
typically employ relatively long reaction times (typically 48–
96 hours with 5–10 mol% catalyst at rt or 35 ◦C) for full reaction
conversion.13c,f While this low reactivity may be countered by
the incorporation of an electron withdrawing substituent within
the indolyl skeleton, the ability of benzannulated thioamidine 25
to promote this rearrangement at rt upon unactivated indolyl
carbonates was tested. Addition of 25 (5 mol%) to N-benzyl
protected indolyl carbonate 56, readily prepared in three steps
Conclusion
In conclusion, a number of amidines can promote the rear-
rangement of oxazolyl carbonates to their corresponding ( )-C-
carboxyazlactones, with benzannulated thioamidine 25 proving
optimal. Variation of the carbonate functionality and the C(4)-
substituent within the oxazole skeleton is readily accommodated
and this protocol has also been successfully extended to ben-
zofuranyl and indolyl carbonates. Current studies are focused
upon probing fully the mechanism of this transformation and
developing applications of enantiomerically pure amidines in
asymmetric catalysis.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2900–2907 | 2903
©

View Article Online
poured into aqueous 0.1 M HCl, extracted with Et2O and dried
(MgSO4) or chromatographic purification (silica).
Experimental procedures and analytical data
General information
General procedure B—synthesis of carbonates
All reactions involving moisture sensitive reagents were performed
under an inert atmosphere via standard vacuum line techniques
and with freshly dried solvents. All glassware was flame dried
and allowed to cool under vacuum. Tetrahydrofuran (THF) and
dichloromethane were obtained dry from a solvent purification
system (MBraun, SPS-800). Petrol is defined as petroleum ether
40–60 ◦C. All solvents and commercial reagents were used as
supplied without further purification unless stated otherwise.
Room temperature refers to 20–25 ◦C. Temperatures of 0 ◦C were
obtained using an ice–water bath and reaction reflux conditions
using an oil bath equipped with a contact thermometer. In vacuo
refers to the use of a Bu¨chi Rotavapor R-2000 rotary evaporator
with a Vacubrand CVC2 vacuum controller or a Heidolph
Laborota 4001 rotary evaporator with a vacuum controller. An-
alytical thin layer chromatography was performed on aluminium
sheets coated with 60 F254 silica. TLC visualisation was carried
out with ultraviolet light (254 nm), followed by staining with 1%
aqueous KMnO4 solution. Flash column chromatography was
Based upon the procedure described by Fu et al.,13a triethylamine
(1.10 eq) was added to a stirred solution of azlactone (1.00 eq) in
◦
THF at 0 C, followed by addition of the desired chloroformate
(1.06 eq) and stirred at 0 ◦C for 30 minutes before warming to
room temperature and stirring overnight. The resulting solution
was poured into H2O and the aqueous phase extracted with Et2O
(× 3). The organic extracts were combined, washed with aqueous
0.1 M HCl, saturated aqueous NaHCO3 solution, brine, dried
(MgSO4), filtered and concentrated in vacuo.
Phenyl 4-benzyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydrooxazole-
4-carboxylate ( )-18
Following general procedure A, oxazolyl carbonate 1713b
(0.20 mmol, 81 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg) gave,
after 15 minutes and an acidic work-up, ( )-18 (71 mg, 87%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.13b
1
performed on Kieselgel 60 silica in the solvent system stated. H
and 13C nuclear magnetic resonance (NMR) spectra were acquired
on either a Bruker Avance 300 (300 MHz 1H, 75.4 MHz 13C) or a
Bruker Avance II 400 (400 MHz 1H, 100 MHz 13C) spectrometer
and in the deuterated solvent stated. Coupling constants (J)
are reported in Hz. Multiplicities are indicated by: s (singlet), d
(doublet), dd (doublet of doublets), sept (septet) and m (multiplet).
The abbreviation Ar is used to denote aromatic. Infrared spectra
(mmax) were recorded on a Perkin-Elmer Spectrum GX FT-IR
spectrometer using either thin films on NaCl plates (thin film) or
KBr discs (KBr disc) as stated. Only the characteristic peaks are
quoted. Microanalyses were carried out on a Carlo Erba CHNS
analyser. Melting points were recorded on an Electrothermal
apparatus and are uncorrected. Mass spectrometric (m/z) data
was acquired by electrospray ionisation (ESI), electron impact
(EI) or chemical ionisation (CI), either at the University of St
Andrews Mass Spectrometry facility or from the EPSRC National
Mass Spectrometry Service Centre, Swansea. At the University of
St Andrews, low and high resolution ESI MS were carried out
on a Micromass LCT spectrometer and low and high resolution
CI MS were carried out on a Micromass GCT spectrometer.
At the EPSRC National Mass Spectrometry Service Centre, low
resolution CI MS was carried out on a Micromass Quattro II
spectrometer and high resolution EI MS on a Finnigan MAT 900
XLT spectrometer.
Phenyl 2-(4-methoxyphenyl)-4-methyl
5-oxo-4,5-dihydrooxazole-4-carboxylate ( )-23
Following general procedure A, oxazolyl carbonate 2215
(0.20 mmol, 68 mg), CH2Cl2 (2 mL) and 25 (5 mol%, 1.9 mg) gave,
after 15 minutes and an acidic work-up, ( )-23 (59 mg, 87%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.15
3,4-Dihydro-2H-pyrimido [2,1-b] benzothiazole 25
Following a modified procedure described by Birman et al.,20b
a mixture of aminopropanol (5.30 mmol, 4.00 mL) and 2-
chlorobenzothiazole (5.80 mmol, 0.730 mL) was heated at 130 ◦C
overnight. After cooling, toluene (23 mL) and thionyl chloride
(11.7 mmol, 0.850 mL) were added and the reaction mixture was
refluxed at 120 ◦C for 4 hours. The cooled mixture was then
poured onto ice cold 20% aqueous KOH (10 mL) and extracted
with CH2Cl2 (× 3). The organic extracts were combined, washed
with saturated aqueous NaHCO3, dried (MgSO4), filtered and
concentrated under vacuum. KOH (13.3 mmol, 0.744 g) and
methanol (50 mL) were added to the resultant residue and the
solution was refluxed for 3 hours. After cooling, the solution was
poured into water and extracted with CH2Cl2 (× 3). The organic
extracts were combined, washed with brine, dried (MgSO4),
filtered and concentrated under vacuum. Chromatographic pu-
rification (EtOH–CH2Cl2 10 : 90) gave the product as an orange
solid (0.445 g, 44%) with spectroscopic data (1H NMR and 13C
NMR) in accordance with the literature.24
Literature procedures were used for the preparation of known
compounds 24, 26 and 27;24 32 and 36;13a 17, 34, 44, 45 and 58;13b
22, 35, 46 and 4915 and 58,13c and all gave spectroscopic data
consistent with the literature.
General experimental procedures
4-Benzyl 2-(4-methoxyphenyl)-oxazol-5-yl
General procedure A—rearrangement of carbonates
1,1,1-trichloro-2-methylpropan-2-yl carbonate 33
The selected catalyst was added to a solution of the desired
carbonate in CH2Cl2 at room temperature. After the specified time,
the solution was concentrated in vacuo and the desired product was
obtained by either an acidic work-up in which the solution was
Following general procedure B, triethylamine (3.91 mmol,
0.540 mL), 4-benzyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydro-
oxazole (3.55 mmol, 1.00 g) (prepared from DL-leucine following
a procedure described in the literature),13c THF (30 mL) and
2904 | Org. Biomol. Chem., 2008, 6, 2900–2907
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
2,2,2-trichloro-1,1-dimethyl chloroformate (3.76 mmol, 0.900 g)
gave, after chromatography (petrol–Et2O 75 : 25), oxazole 33
(1.34 g, 78%) as a colourless solid. Mp 62–64 ◦C; Found C,
54.51; H, 4.14; N, 2.92%; C22H20Cl3NO5 requires C, 54.51; H, 4.16;
163.5 (C), 163.5 (C), 133.1 (C), 130.4 (ArH), 130.1 (ArH), 128.3
(ArH), 127.6 (CH), 117.3 (C), 114.3 (ArH), 105.2 (C), 91.0 (C),
78.2 (C), 55.6 (CH3), 39.1 (CH2) and 21.3 (CH3); m/z (CI+) 282.1
(100, M − (COOCMe2CCl3) + H+), 484.0 (5, (35Cl)M + H+), 486.0
(5, (37Cl)M + H+); HRMS (ES+) C22H21Cl3NO5 requires 484.0480,
found 484.0478 (−0.3 ppm).
N, 2.89%; tmax (KBr disc)/cm−1 2996 (C–H), 1783 (C O), 1611
=
=
(C N), 1246 (C–O) and 802 (C–Cl); dH (300 MHz, CDCl3) 7.89
(2H, d, J 9.0, 4-OMe(2,6)ArH), 7.31–7.29 (5H, m, ArH), 6.93 (2H,
d, J 9.0, 4-OMe(3,5)ArH), 3.88 (2H, s, CH2), 3.85 (3H, s, OCH3)
and 1.92 (6H, s, 2 × CH3); dC (75 MHz, CDCl3) 161.5 (C), 155.4
(C), 148.4 (C), 145.7 (C), 137.5 (C), 129.0 (ArH), 128.6 (ArH),
127.8 (ArH), 126.7 (ArH), 123.3 (C), 120.0 (C), 114.2 (ArH), 104.6
(C), 92.3 (C), 55.5 (CH3), 31.8 (CH2) and 21.1 (CH3); m/z (CI+)
282.0 (100, M − (COOCMe2CCl3) + H+), 484.0 (6, (35Cl)M + H+),
486.0 (6, (37Cl)M + H+); HRMS (ES+) C22H21Cl3NO5 requires
484.0477, found 484.0480 (−0.5 ppm).
Phenyl 4-isobutyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydrooxazole-
4-carboxylate ( )-40
Following general procedure A, oxazolyl carbonate 3413b
(0.20 mmol, 73 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg)
gave, after 15 minutes and chromatography (petrol–Et2O 80 : 20),
( )-40 (45 mg, 61%) as a colourless oil with spectroscopic data
(1H NMR) in accordance with the literature.13b
Methyl 4-isobutyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydrooxazole-
4-carboxylate ( )-41
2-(4-Methoxyphenyl)-4-isobutyloxazol-5-yl
1,1,1-trichloro-2-methylpropan-2-yl carbonate 37
Following general procedure A, oxazolyl carbonate 3515
(0.20 mmol, 70 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg)
gave, after 15 minutes and chromatography (petrol–Et2O 80 : 20),
( )-41 (43 mg, 61%) as a colourless oil with spectroscopic data
(1H NMR) in accordance with the literature.15
Following general procedure B, triethylamine (4.40 mmol,
0.610 mL), 4-isobutyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydro-
oxazole (4.00 mmol, 0.995 g) (prepared from DL-leucine as
described in the literature),13a THF (30 mL) and 2,2,2-trichloro-
1,1-dimethyl chloroformate (4.24 mmol, 1.02 g) gave, after chro-
matography (petrol–Et2O 75 : 25), oxazole 37 (1.35 g, 74%) as
a colourless solid. Mp 68–70 ◦C; Found: C, 50.40; H, 5.09; N,
3.13%; C19H22Cl3NO5 requires C, 50.63; H, 4.92; N, 3.11%; tmax
Benzyl 4-isobutyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydrooxazole-
4-carboxylate ( )-42
(KBr disc)/cm−1 2960 (C–H), 1781 (C O), 1613 (C N), 1242
(C–O) and 802 (C–Cl); dH (300 MHz, CDCl3) 7.89 (2H, d, J 9.0, 4-
OMe(2,6)ArH), 6.93 (2H, d, J 9.0, 4-OMe(3,5)ArH), 3.85 (3H, s,
OCH3), 2.34 (2H, d, J 6.9, CH2), 2.05 (1H, sept, J 6.9, CH), 2.01
(6H, s, (CH3)2) and 0.96 (6H, s, (CH3)2); dC (75 MHz, CDCl3)
161.4 (C), 155.2 (C), 148.8 (C), 145.9 (C), 127.7 (ArH), 123.6
(C), 120.1 (C), 114.2 (ArH), 104.6 (C), 92.3 (C), 55.5 (CH3), 34.0
(CH2), 27.7 (CH), 22.5 (CH3) and 21.2 (CH3); m/z (CI+) 248.1
(100, M − (COOCMe2CCl3) + H+), 450.1 (2, (35Cl)M + H+), 452.1
(2, (37Cl)M + H+); HRMS (ES+) C19H23Cl3NO5 requires 450.0636,
found 450.0633 (−0.8 ppm).
Following general procedure A, oxazolyl carbonate 3613a
(0.20 mmol, 76 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg) gave,
after 15 minutes and an acidic work-up, ( )-42 (70 mg, 92%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.13a
=
=
1,1,1-Trichloro-2-methylpropan-2-yl 4-isobutyl-2-(4-
methoxyphenyl)-5-oxo-4,5-dihydrooxazole-4-carboxylate ( )-43
Following general procedure A, oxazolyl carbonate 37 (0.20 mmol,
90 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg) gave, after
15 minutes and an acidic work-up, ( )-43 (83 mg, 92%) as a
colourless solid. Mp 64–66 ◦C; tmax (KBr disc)/cm−1 2958 (C–H),
Benzyl 4-benzyl-2-(4-methoxyphenyl)-5-oxo-4,5-dihydrooxazole-4-
carboxylate ( )-38
=
=
=
1818 (C O), 1762 (C O), 1647 (C N), 1261 (C–O) and 791 (C–
Cl); dH (300 MHz, CDCl3) 7.96 (2H, d, J 9.0, 4-OMe(2,6)ArH),
6.97 (2H, d, J 9.0, 4-OMe(3,5)ArH), 3.87 (3H, s, OCH3), 2.36 (1H,
dd, J 14.4, 5.7, CH2), 2.03 (1H, dd, J 14.4, 7.5, CH2), 1.90 (3H, s,
CH3), 1.87 (3H, s, CH3), 1.71 (1H, sept, J 6.6, CH), 0.94 (3H, d,
J 6.6, CH3) and 0.89 (3H, d, J 6.6, CH3); dC (75 MHz, CDCl3)
175.1 (C), 163.7 (2 × C), 163.2 (C), 130.2 (ArH), 117.7 (C), 114.4
(ArH), 105.2 (C), 90.8 (C), 77.1 (C), 55.6 (CH3), 41.5 (CH2), 24.7
(CH), 23.9 (CH3), 23.1 (CH3) and 21.2 (CH3); m/z (CI+) 248.1
(100, M − (COOCMe2CCl3) + H+), 450.1 (7, (35Cl)M + H+), 452.1
(7, (37Cl)M + H+); HRMS (ES+) C19H23Cl3NO5 requires 450.0636,
found 450.0631 (−1.1 ppm).
Following general procedure A, oxazolyl carbonate 3213a
(0.20 mmol, 83 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg) gave,
after 15 minutes and an acidic work-up, ( )-38 (63 mg, 76%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.13a
1,1,1-Trichloro-2-methylpropan-2-yl 4-benzyl 2-(4-
methoxyphenyl)-5-oxo-4,5-dihydrooxazole-4-carboxylate ( )-39
Following general procedure A, oxazolyl carbonate 33 (0.20 mmol,
97 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg) gave, after 1 hour
and an acidic work-up, ( )-39 (89 mg, 92%) as a colourless oil.
4-((4-Benzyloxy)benzyl)-2-(4-methoxyphenyl)-oxazolyl-5-yl
phenyl carbonate 47
tmax (thin film)/cm−1 2933 (C–H), 1821 (C O), 1760 (C O), 1642
=
=
=
(C N), 1262 (C–O) and 793 (C–Cl); dH (300 MHz, CDCl3) 7.80
(2H, d, J 9.0, 4-OMe(2,6)ArH), 7.21–7.16 (5H, m, ArH), 6.90
(2H, d, J 9.0, 4-OMe(3,5)ArH), 3.84 (3H, s, OCH3), 3.59 (1H,
d, J 13.8, CH2), 3.48 (1H, d, J 13.8, CH2), 1.95 (3H, s, CH3)
and 1.93 (3H, s, CH3); dC (75 MHz, CDCl3) 173.7 (C), 163.6 (C),
Following general procedure B, triethylamine (2.84 mmol,
0.400 mL), 4-((4-benzyloxy)benzyl)-2-(4-methoxyphenyl)-5-oxo-
4,5-dihydrooxazole (2.58 mmol, 1.00 g) (prepared from DL-
tyrosine following a procedure described in the literature),13a THF
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2900–2907 | 2905
©

View Article Online
(30 mL) and phenyl chloroformate (2.71 mmol, 0.310 mL) gave 47
as a colourless solid after recrystallisation with Et2O (1.10 g, 84%),
mp 116–117 ◦C; tmax (KBr disc)/cm−1 3033 (Ar-H), 2911 (Alk-H),
Phenyl 2-(4-methoxyphenyl)-5-oxo-4-isopropyl-4,5-
dihydrooxazole-4-carboxylate ( )-52
Following general procedure A, oxazolyl carbonate 4615
(0.20 mmol, 70 mg), CH2Cl2 (2 mL) and 25 (5 mol%, 1.9 mg)
gave, after 15 minutes and chromatography (petrol–Et2O 80 : 20),
( )-52 (41 mg, 58%) as a colourless oil with spectroscopic data
(1H NMR) in accordance with the literature.15
=
=
1800 (C O), 1613 (C N) and 1214 (C–O); dH (400 MHz, CDCl3)
7.96 (2H, d, J 9.0, 4-OMe(2,6)ArH), 7.48–7.30 (10H, m, 2 × PhH),
7.25 (2H, m, 4-OBn(3,5)ArH), 6.95 (4H, d, 4-OMe(3,5)ArH and
4-OBn(2,6)ArH), 5.07 (2H, s, CH2), 3.93 (2H, s, CH2) and 3.90
(3H, s, OCH3); dC (75 MHz, CDCl3) 161.5 (C), 157.7 (C), 155.5
(C), 150.8 (C), 150.1 (C), 145.6 (C), 137.2 (C), 130.1 (ArH), 129.8
(ArH), 128.7 (ArH), 128.0 (ArH), 127.9 (ArH), 127.5 (ArH), 126.9
(ArH), 123.8 (C), 120.6 (ArH), 119.9 (C), 115.0 (ArH), 114.2
(ArH), 70.1 (CH2), 55.5 (CH3) and 31.1 (CH2); m/z (ES+) 530.2
(100, M + Na), HRMS (ES+) C31H25NO6Na requires 530.1580,
found 530.1584 (+0.9 ppm).
Phenyl 4-(4-(benzyloxy)benzyl)-2-(4-methoxyphenyl)-5-oxo-4,5-
dihydrooxazole-4-carboxylate ( )-53
Following general procedure A, oxazolyl carbonate 47 (0.20 mmol,
101 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg) gave, after
15 minutes and an acidic work-up, ( )-53 (88.0 mg, 87%) as a
yellow oil with spectroscopic data (1H NMR) in accordance with
the literature.15
2-(4-Methoxyphenyl)-4-((4-phenoxycarbonyloxy)benzyl)oxazolyl-
5-yl phenyl carbonate 48
Phenyl 2-(4-methoxyphenyl)-5-oxo-4-(4-phenoxy-
Triethylamine (14.0 mmol, 1.94 mL) was added to a stirred
solution of DL-N-(4-methoxybenzoyl)tyrosine (3.17 mmol, 1.00
g) (prepared from DL-tyrosine following a procedure described in
the literature)13a followed by the addition of phenyl chloroformate
(13.8 mmol, 1.50 mL) and stirred at room temperature overnight.
The resulting solution was poured into aqueous 1 M HCl and
the aqueous phase extracted with Et2O (× 3). The organic
extracts were combined, washed with saturated aqueous NaHCO3
solution, dried (MgSO4), filtered and concentrated in vacuo.
Chromatographic purification (petrol–Et2O 80 : 20) afforded the
oxazole 48 (1.46 g, 86%) as a colourless solid. Mp 71–74 ◦C; tmax
carbonyloxy)benzyl)-4,5-dihydrooxazole-4-carboxylate ( )-54
Following general procedure A, oxazolyl carbonate 48 (0.20 mmol,
108 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg) gave, after
15 minutes and an acidic work-up, ( )-54 (99.0 mg, 92%) as a
yellow oil with spectroscopic data (1H NMR) in accordance with
the literature.15
Phenyl 2-(4-methoxyphenyl)-4-(2-(methylthio)ethyl)-5-oxo-4,5-
dihydrooxazole-4-carboxylate ( )-55
Following general procedure A, oxazolyl carbonate 4915
(0.20 mmol, 77 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg) gave,
after 15 minutes and an acidic work-up, ( )-55 (72 mg, 93%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.15
(KBr disc)/cm−1 3065 (Ar-H), 2934 (Alk-H), 1798 (C O), 1779
=
=
=
(C O), 1614 (C N) and 1235 (C–O); dH (300 MHz, CDCl3) 7.92
(2H, d, J 9.0, 4-OMe(2,6)ArH), 7.45–7.21 (14H, m, ArH), 6.95
(2H, d, J 9.0, 4-OMe(3,5)ArH), 3.93 (2H, s, CH2) and 3.85 (3H, s,
OCH3); dC (75 MHz, CDCl3) 161.6 (C), 155.7 (C), 152.1 (C), 151.1
(C), 150.8 (C), 150.1 (C), 149.8 (C), 145.8 (C), 135.7 (C), 130.1
(ArH), 129.9 (ArH), 129.7 (ArH), 127.96 (ArH), 126.4 (ArH),
126.4 (ArH), 126.4 (ArH), 123.0 (C), 121.0 (CH), 120.6 (ArH),
119.8 (C), 114.3 (ArH), 55.5 (CH3) and 31.0 (CH2); m/z (CI+)
538.1 (62, M + H+), 418.1 (17, M − (COOPh) + H+); HRMS (ES+)
C31H23NO8Na requires 560.1321, found 560.1323 (+0.2 ppm).
1-Benzyl-3-phenylindole-2-yl phenyl carbonate 56
Following general procedure B, triethylamine (0.77 mL,
5.52 mmol), N-benzyl-3-phenyloxindole24 (1.50 g, 5.02 mmol) and
phenyl chloroformate (0.67 mL, 5.32 mmol) in THF (23 mL) gave,
after recrystallisation (CH2Cl2–MeOH), carbonate 56 (1.82 g,
87%) as a pale yellow solid. Mp 116–118 ◦C; tmax (KBr disc)/cm−1
=
=
3060 (C–H), 3033 (C–H), 2925 (C–H), 1793 (C O), 1620 (C C)
and 1228 (C–O); dH (300 MHz; CDCl3) 7.80–7.74 (1H, m, ArH),
7.63–7.59 (2H, m, ArH), 7.44–7.38 (2H, m, ArH), 7.28–7.09 (12H,
m, ArH), 6.77–6.72 (2H, m, ArH) and 5.30 (2H, s, CH2Ph); dC
(75 MHz; CDCl3) 150.7 (C), 150.2 (C), 138.7 (C), 136.6 (C), 132.9
(C), 132.4 (C), 129.6 (ArH), 128.9 (2 × ArH), 128.4 (ArH), 127.8
(ArH), 126.9 (ArH), 126.6 (ArH), 126.5 (ArH), 125.0 (C), 122.6
(ArH), 120.9 (ArH), 120.5 (ArH), 119.8 (ArH), 110.0 (ArH), 103.8
(C) and 46.2 (CH2); m/z (CI) 420.2 (100, M + H+); HRMS (CI)
C28H22NO3 requires 420.1600, found 420.1589 (−2.5 ppm).
Phenyl 2-(4-methoxyphenyl)-4-methyl-5-oxo-4,5-dihydrooxazole-
4-carboxylate ( )-50
Following general procedure A, oxazolyl carbonate 4413b
(0.20 mmol, 66 mg), CH2Cl2 (2 mL) and 25 (2 mol%, 0.8 mg)
gave, after 15 minutes and chromatography (petrol–Et2O 80 : 20),
( )-50 (43 mg, 65%) as a colourless oil with spectroscopic data
(1H NMR) in accordance with the literature.13b
Phenyl 2-(4-methoxyphenyl)-5-oxo-4-phenyl-4,5-dihydrooxazole-
carboxylate ( )-51
Phenyl 1-benzyl 2-oxo-3-phenyl-2,3-dihydroindole-3-carboxylate
( )-57
Following general procedure A, oxazolyl carbonate 4513b
(0.20 mmol, 74 mg), CH2Cl2 (2 mL) and 25 (5 mol%, 1.9 mg) gave,
after 15 minutes and an acidic work-up, ( )-51 (62 mg, 83%) as
a colourless oil with spectroscopic data (1H NMR) in accordance
with the literature.13b
Following general procedure A, indole 56 (0.20 mmol, 84 mg),
CH2Cl2 (2 mL) and 25 (5 mol%, 1.9 mg) gave, after 1 hour and
an acidic work-up, ( )-57 (74 mg, 88%) as a yellow solid. Mp
85–87 ◦C; tmax (KBr disc)/cm−1 3061 (C–H), 3032 (C–H), 2936
2906 | Org. Biomol. Chem., 2008, 6, 2900–2907
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
=
=
=
(C–H), 1768 (C O), 1714 (C O), 1608 (C C) and 1189 (C–O);
dH (300 MHz; CDCl3) 7.65–7.61 (1H, m, ArH), 7.54–7.19 (15H, m,
ArH), 7.09–7.05 (2H, m, ArH), 6.87–6.84 (1H, m, ArH), 5.15 (1H,
ABq, J 15.8, CHAHBPh) and 4.86 (1H, ABq, J 15.8, CHAHBPh);
dC (75 MHz; CDCl3) 172.7 (C), 167.8 (C), 150.6 (C), 143.7 (C),
135.4 (2 × C), 130.0 (ArH), 129.5 (ArH), 128.9 (ArH), 128.8
(ArH), 128.6 (ArH), 128.2 (ArH), 127.8 (ArH), 127.2 (ArH), 126.7
(C), 126.3 (ArH), 126.0 (ArH), 123.3 (ArH), 121.2 (ArH), 110.2
(ArH), 64.4 (C) and 44.2 (CH2); m/z (ESI+) 442.1 (100, M + Na+)
HRMS (ESI+) C28H21NO3Na requires 442.1419, found 442.1422
(+0.7 ppm).‡
Birman and H. Jiang, Org. Lett., 2005, 7, 3445; V. B. Birman and X. Li,
Org. Lett., 2006, 8, 1351; V. B. Birman and L. Guo, Org. Lett., 2006, 8,
4859; V. B. Birman, H. Jiang, X. Li, L. Guo and E. W. Uffman, J. Am.
Chem. Soc., 2006, 128, 6536.
10 W. Zhang and M. Shi, Org. Biomol. Chem., 2006, 4, 1671.
11 V. K. Aggarwal and A. Mereu, Chem. Commun., 1999, 2311.
12 W. Steglich and G. Ho¨fle, Tetrahedron Lett., 1970, 4727.
13 For asymmetric versions of this reaction see: (a) J. C. Ruble and G. C.
Fu, J. Am. Chem. Soc., 1998, 120, 11532; (b) S. A. Shaw, P. Aleman
and E. Vedejs, J. Am. Chem. Soc., 2003, 125, 13368; (c) S. A. Shaw, P.
Aleman, J. Christy, J. W. Kampf, P. Va and E. Vedejs, J. Am. Chem. Soc.,
2006, 128, 925; (d) H. V. Nguyen, D. C. D. Butler and C. J. Richards,
Org. Lett., 2006, 8, 769; (e) J. G. Seitzberg, C. Dissing, I. Søtofte, P.-O.
Norrby and M. Johannsen, J. Org. Chem., 2005, 70, 8332; (f) For the
application of this methodology to the preparation of oxindole and
benzofuran derivatives see: I. D. Hills and G. C. Fu, Angew. Chem., Int.
Ed., 2003, 42, 3921.
Phenyl 3-benzyl-2-oxo-2,3-dihydrobenzofuran-3-carboxylate
( )-59
14 J. E. Thomson, K. Rix and A. D. Smith, Org. Lett., 2006, 8,
3785.
Following general procedure A, benzofuran 5813c (0.20 mmol,
69 mg), CH2Cl2 (2 mL) and 25 (10 mol%, 3.8 mg) gave, after
15 minutes and an acidic work-up, ( )-59 (64 mg, 92%) as a
yellow oil with spectroscopic data (1H NMR) in accordance with
the literature.13c
15 J. E. Thomson, C. D. Campbell, C. Concello´n, N. Duguet, K. Rix,
A. M. Z. Slawin and A. D. Smith, J. Org. Chem., 2008, 73, 2784; J. E.
Thomson, A. F. Kyle, C. Concello´n, K. A. Gallagher, P. Lenden, L. C.
Morrill, A. J. Miller, C. Joannesse, A. M. Z. Slawin and A. D. Smith,
Synthesis, 2008, DOI: 10.1055/s-2008-1077890.
16 G. Peris and E. Vedejs, J. Org. Chem., 2008, 73, 1158.
17 N. Duguet, C. D. Campbell, A. M. Z. Slawin and A. D. Smith, Org.
Biomol. Chem., 2008, 6, 1108.
18 While this manuscript was in preparation, a related paper detailing
a single example describing the use of a chiral thioamidine catalyst
for a modestly enantioselective acetyl rearrangement (63% e.e. at 80%
conversion using 32 mol% of catalyst, absolute configuration not
determined) was published; see: F. R. Dietz and H. Gro¨ger, Synlett,
2008, 663.
19 For the use of NEt3 as a base to promote this transformation with
azolium salt 21 and its application to the tandem multi-step synthesis of
C-carboxyazlactones see: C. D. Campbell, N. Duguet, K. A. Gallagher,
J. E. Thomson, A. G. Lindsay, A. O’Donoghue and A. D. Smith, Chem.
Commun., 2008, DOI: 10.1039/b806816j.
Acknowledgements
The authors would like to thank the Royal Society for a University
Research Fellowship (ADS), the EPSRC (CJ and CC), Ministerio
de Educacio´n y Ciencia (CS), EaStCHEM and the University of
St Andrews (JET) for funding and the EPSRC National Mass
Spectrometry Service Centre (Swansea). The authors would like
to thank Katherine A. Gallagher for the preparation of a number
of carbonate substrates.
20 (a) M. Kobayashi and S. Okamoto, Tetrahedron Lett., 2006, 47, 4347;
(b) V. B. Birman, X. Li and Z. Han, Org. Lett., 2007, 9, 37.
21 It is noteworthy that substitution adjacent to the assumed reactive
amino group is tolerated within the amidine substructure, as 2-alkyl
substitution of DMAP generally markedly decreases its reactivity as a
nucleophilic catalyst; for a review of the reactivity of DMAP derivatives
see: A. C. Spivey and S. Arseniyadis, Angew. Chem., Int. Ed., 2004, 43,
5436.
References
1 For example see: H. Oediger and F. Mo¨ller, Angew. Chem., Int. Ed.
Engl., 1967, 6, 76; B. A. Otter, A. Taube and J. J. Fox, J. Org. Chem.,
1971, 36, 1251.
2 N. Ono, T. Yamada, T. Saito, K. Tanaka and A. Kaji, Bull. Chem. Soc.
Jpn., 1978, 51, 2401.
22 All oxazolyl carbonates were prepared using standard procedures from
the known corresponding a-amino acids based upon the previous work
of Fu13a and Vedejs.13b,c
23 For the seminal work of Black upon the rearrangement of benzofuranyl
carbonates with DMAP see: T. H. Black, S. M. Arrivo, J. S. Schumm
and J. M. Knobeloch, J. Chem. Soc., Chem. Commun., 1986, 1524;
T. H. Black, S. M. Arrivo, J. S. Schumm and J. M. Knobeloch, J. Org.
Chem., 1987, 52, 5425; C. J. Moody, K. J. Doyle, M. C. Elliott and T. J.
Mowlem, J. Chem. Soc., Perkin Trans. 1, 1997, 2413.
3 H. Yamanaka, M. Yokoyama, T. Sakamoto, T. Shiraishi, M. Sagi and
M. Mizugaki, Heterocycles, 1983, 20, 1541.
4 V. K. Aggarwal and A. Mereu, J. Org. Chem., 2000, 65, 7211.
5 N. Ghosh, Synlett, 2004, 574.
6 B. G. G. Lohmeijer, R. C. Pratt, F. Leibfarth, J. W. Logan, D. A. Long,
A. P. Dove, F. Nederberg, J. Choi, C. Wade, R. M. Waymouth and J. L.
Hedrick, Macromolecules, 2006, 39, 8574.
7 S. Kim and H. Chang, Bull. Chem. Soc. Jpn., 1985, 58, 3669.
8 W.-C. Shieh, S. Dell and O. Repic, J. Org. Chem., 2002, 67, 2188.
9 For asymmetric acylations see: V. B. Birman, E. W. Uffman, H. Jiang,
X. Li and C. J. Kilbane, J. Am. Chem. Soc., 2004, 126, 12226; V. B.
24 B. M. Trost and M. U. Frederiksen, Angew. Chem., Int. Ed., 2005, 44,
308.
‡ CCDC reference number 684299. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b805850d
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 2900–2907 | 2907
©