Efficient route to 2H-1,3-oxazines through ring expansion of isoxazoles by rhodium carbenoids

Article abstract of DOI:10.1016/j.tet.2008.03.010

Studies related to the total synthesis of elisabethin C led to the discovery of a rhodium-catalyzed cascade sequence involving isoxazole ring expansion and a [4+3] cycloaddition. The scope of the isoxazole ring expansion was explored, resulting in the synthesis of a range of 2H-1,3-oxazines in 47-96% yield.

Full text of DOI:10.1016/j.tet.2008.03.010

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 6901e6908
www.elsevier.com/locate/tet
Efficient route to 2H-1,3-oxazines through ring expansion
of isoxazoles by rhodium carbenoids
*
James R. Manning, Huw M.L. Davies
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 14260-3000, USA
Received 18 January 2008; received in revised form 28 February 2008; accepted 3 March 2008
Available online 7 March 2008
Dedicated to Professor John Hartwig on his receipt of the 2007 Tetrahedron Young Investigator Award
Abstract
Studies related to the total synthesis of elisabethin C led to the discovery of a rhodium-catalyzed cascade sequence involving isoxazole ring
expansion and a [4þ3] cycloaddition. The scope of the isoxazole ring expansion was explored, resulting in the synthesis of a range of 2H-1,3-
oxazines in 47e96% yield.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Isoxazole ring expansion; NeO insertion; Rhodium carbenoid; Aryldiazoacetates
1. Introduction
natural products by differentiating between the enantiomers of
the racemic dihydronaphthalene derivative 1. One enantiomer
of the substrate undergoes the combined CeH activation/Cope
rearrangement while the other is cyclopropanated (Scheme 1).
In seeking to broaden the scope of this methodology, we
focused our attention on the marine bisnor-diterpenoid elisa-
bethin C (6),6,7 using the fused isoxazole 8 as the substrate
for the combined CeH activation/Cope rearrangement
(Scheme 2). The isoxazole subunit would be used as a protect-
ing group for the diketone functionality of elisabethin C, to be
unmasked at a late stage of the synthesis.
The metal-catalyzed reactions of diazo compounds are ca-
pable of a diverse array of transformations.1 With the advent
of new catalysts and the recognition that different classes of
carbenoids can open up new vistas of reactivity, the field
continues to expand.2 We have had a long-standing interest
in developing new synthetic methods derived from the chemi-
stry of donor/acceptor-substituted carbenoids. This paper de-
scribes the discovery of an unexpected but highly efficient ring
expansion of isoxazoles by rhodium carbenoid intermediates.
A recent focus of our group has been the synthesis of bio-
logically active marine natural products utilizing the enantio-
divergent combined CeH activation/Cope rearrangement
methodology.3 This reaction occurs during allylic CeH func-
tionalization using rhodium-stabilized vinylcarbenoids. Recent
total syntheses completed using this methodology include
2. Results and discussion
The synthesis of fused isoxazole 8 was achieved using a rel-
atively straightforward approach starting from b-diketone 9,
which in turn was readily prepared by literature procedures8,9
from the commercially available mono-protected 1,4-cyclo-
hexadione (Scheme 3). If the isoxazole forming reaction was
allowed to run to completion, the resulting regioisomeric mix-
ture of isoxazoles proved inseparable by chromatography at all
subsequent steps. Fortunately, if the reaction was stopped after
10 min, the regioisomeric oxime intermediates 10 and 11
could be chromatographically separated. Treatment of each
(þ)-erogorgiaene (3),4 (ꢀ)-elisapterosin
B
(4),5 and
(ꢀ)-colombiasin A (5).5 This strategy has been successful at
rapidly introducing three of the stereocenters common in these
* Corresponding author. Tel.: þ1 716 645 6800x2186; fax: þ1 716 645
6547.
E-mail address: hdavies@acsu.buffalo.edu (H.M.L. Davies).
0040-4020/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.03.010

6902
J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
R1 Me
isomer separately with warm acetic acid furnished the corre-
N2
R2
Me
sponding isoxazoles in isomerically pure form. Regioisomer
11 was chosen initially to carry through the sequence to test
the key carbenoid reaction. Cyclization to form isoxazole 12
and subsequent acetal deprotection to ketone 13 proceeded
smoothly. The remaining two steps to form vinyl triflate 15 us-
ing Comins’ reagent 1410 followed by a palladium-catalyzed
reduction11 gave 8 in good overall yield.
The Rh2(R-DOSP)4-catalyzed reaction12 of the fused isox-
azole 8 with vinyldiazoacetate 2 (3 equiv) gave a most unusual
result (Scheme 4). Instead of the expected reaction to form 7,
the major isolable product was an unprecedented tricyclic de-
rivative 16, which was formed as a 1:1 mixture of diastereo-
mers in 62% yield. Each diastereomer was formed with low
enantioinduction (19% ee). This material has incorporated
into isoxazole 8 2 equiv of the vinylcarbenoid derived from
2. Compound 16 can be considered as formally derived from
a carbenoid insertion into the isoxazole NeO bond and a tan-
dem cyclopropanation/Cope rearrangement between another
carbenoid and the diene component of the substrate.
Rh2(R-DOSP)4
Me
CO2Me
Me
2
R3
1
R1
R1 Me
R2
Me
R2
H
Me
Me
H
Me
R3
R3
H
MeO2C
MeO2C
>90% ee, >95% de
1 = R2 = R3 = H
(4 steps)
R
1 = R3 = OTBS
R
R2 = OMe
(7 or 8 steps)
Me
Me
Me
O
O
OH
O
HO
or
Me
H
Me
O
Me
H
H
Me
Me
Me
Me
H
H
Me
Me
3
4
5
Me
(+)-erogorgiaene
(−)-elisapterosin B
(−)-colombiasin A
O
CO2Me
Me
N
O
Scheme 1. Synthesis of marine natural products.
N2
N
H
Me
MeO2C
2
8
7
Rh2(R-DOSP)4
Me
Me
Me
O
CO2Me
Me
O
O
Rh(II)
O
Me
N2
Me
N
N
O
O
MeO2C
H
Me
H
Me
[3,3]
MeO2C
N
N
MeO2C
Me
6
7
8
2
Me
elisabethin C
CO2Me
CO2Me
Me
16
Scheme 2. Retrosynthesis of elisabethin C.
Me
possible intermediate
Scheme 4. Reaction of 8 with a vinylcarbenoid.
The formation of 16 is an unusual transformation and we
were intrigued by this novel carbenoid reactivity. Formal
[4þ3] cycloadditions between vinylcarbenoids and dienes by
a tandem cyclopropanation/Cope rearrangement are well pre-
cedented,13 but the carbenoid insertion into the isoxazole
NeO bond is not an established process. Rhodium carbenoids
containing isoxazoles have been used in intermolecular cyclo-
propanations without any side reaction on the isoxazole ring.14
Additionally, intramolecular CeH insertion reactions have
been successfully achieved on substrates containing an isox-
azole ring.15 It is known, however, that isoxazolium ylides,
typically generated by deprotonation of an isoxazolium salt,
undergo rearrangements to generate either 2H-1,3-oxazines
or 3-imino-2-en-1-ones, depending on the substitution of the
isoxazolium salt.16 Consequently, we decided to explore fur-
ther the scope of the NeO insertion chemistry using rhodium
carbenoids and various isoxazole substrates.
HO
OH
N
O
O
N
OH
O
Me
Me
Me
.
NH2OH HCl, NaOAc
H2O/THF, reflux (10 min)
O
O
O
O
O
O
O
9
10
11
39%
55%
Me
Me
O
1 N HCl
AcOH
65 °C
100%
N
N
O
11
acetone
reflux
95%
O
O
12
13
Cl
Me
Me
Pd(PPh3)4, LiCl
Et3SiH, THF, reflux
N
14
N(Tf)2
O
O
N
N
NaHMDS
THF, -78 °C
86%
92%
OTf
15
8
The first series of experiments studied the effect of the car-
benoid structure on the efficiency of the NeO insertion, using
Scheme 3. Synthesis of dihydrobenzoisoxazole 8.

J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
6903
Table 1
Reaction of rhodium carbenoids with 3,5-dimethylisoxazole
Further studies were performed with a range of isoxazoles
(Table 2). All four isoxazoles 20aed produced the 2H-1,3-
oxazine products 21aed, respectively, in high yield (67e
96%). These studies demonstrate that siloxy, halo, and even
ester functionalities are compatible with this chemistry. The
ester derivative 20d gave a tautomeric mixture of the ring ex-
pansion products 21d and 22d (entry 5). Compound 21d was
formed cleanly in the carbenoid reaction, but is prone to isom-
erization to 22d during silica gel chromatography, illustrating
the relative mildness of the carbenoid reaction conditions.
Having discovered that isoxazoles could be effectively ring-
expanded, it became of interest to determine if the reaction
could be extended to other heterocyclic systems. Benzisoxa-
zoles were found to be similarly reactive with carbenoids, al-
though further transformations occurred in certain cases
(Scheme 5). The reaction with 1,2-benzisoxazole 23 produced
the ring expansion product 24 cleanly in 72% yield if 23 was
used in excess, and aziridine 25 in 94% yield if 3 equiv of the
diazo component was used. In the case of anthranil 26, alde-
hyde 27, formally the result of a 6p-electrocyclic ring-opening
of the expected NeO insertion product, was isolated along
with epoxide 28. Compound 28 could be obtained exclusively
if 3 equiv of the diazoacetate was used in the reaction. 5-
Chloro-3-phenylanthranil (29) cleanly produced ketone 30,
which did not readily undergo epoxide formation.
R1
N
CO2R2
O
Rh2(OAc)4
(2 mol%)
N2
N
O
R1
CO2R2
DCM, reflux
17a-c
18
19a-c
Entry
R1
R2
19
Yielda (%)
1
2
3
a
H
Et
19a
19b
19c
56
47
88
CO2Me
Ph
Me
Me
Reported yields are of isolated products.
3,5-dimethylisoxazole (18) as a reference substrate (Table 1).
In recent years we have shown that donor/acceptor-substituted
carbenoids are capable of higher selectivity than the conven-
tional carbenoids lacking a donor group. In this case all three
of the prototypical types of carbenoids, derived from 17aec,
induced an NeO insertion into the isoxazole, although the re-
action with the donor/acceptor-substituted carbenoid (entry 3)
was the most efficient (88% yield).
The next series of reactions were conducted to determine
what types of functionality on the isoxazole would be compat-
ible with the NeO insertion (Table 2). Methyl phenyldiazo-
acetate (17c) was used as the carbenoid source because it had
resulted in the highest yield of product in the initial evaluation.
MeO2C
Table 2
Reaction of diazoacetate 17c with isoxazoles
Ph
Ph
N
N2
O
N
N
23
(1)
N2
N
O
Ph
CO2Me
CO2Me
Ph
Rh2(OAc)4
DCM, reflux
product
21a-d
Rh2(OAc)4
O
O
R1
R3
17c
DCM, reflux
CO2Me
Ph
CO2Me
17c
24
25
0.5 eq
2.2 eq
R2
72% yield
-
-
94% yield
Entry
1
Isoxazole
Product
Yielda (%)
Ph
CO2Me
O
O
Ph
N
CO2Me
O
N
O
O
H
N2
83
93
20a
21a
N
26
N
OTBS
OTBS
(2)
(3)
Ph
CO2Me
N
17c
27
Ph
28
Rh2(OAc)4
1,2-DCE, reflux
CO2Me
Ph
N
CO2Me
O
Ph
CO2Me
N
O
67% yield
16% yield
97% yield
0.5 eq
3 eq
2
3
Cl
20b
21b
-
Cl
Ph
O
Ph
Cl
Ph
N
CO2Me
O
Cl
N
O
O
N2
20c
N
96
21c
29
N
Ph
CO2Me
17c
Rh2(OAc)4
DCM, reflux
Br
30
Br
Ph
CO2Me
98% yield
Ph
N
CO2Me
O
21d
CO2Me
N
O
Scheme 5. Reaction of diazoacetate 17c with benzisoxazoles.
20d
CO2Me
4
67b
Another system worthy of study was benzoisothiazoles
(Scheme 6). Previously it had been shown that the related
2-substituted isothiazol-3(2H)-ones undergo NeS insertion
with carbenoids lacking an electron-donating group.17 The re-
action of 17c with 3-chloro[d]benzoisothiazole (31) was a very
efficient process resulting in the formation of 2H-1,3-benzo-
thiazine 32 in 88% yield.
a
Reported yields are of isolated products.
b
During attempted purification, product formed a 1:1 inseparable mixture
with its tautomer 22d.
Ph
HN
CO2Me
O
CO2Me
The reaction mechanism for the transformations described
abovelikelyproceedsthrough an isoxazolium ylideintermediate
22d

6904
J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
Cl
respective literature procedures. The synthesis of compounds
9e13, 15, and 8 is described in Supplementary data. Unless
otherwise noted, all other reagents were obtained from com-
mercial sources and used as received. Mass spectral determi-
nations were carried out by LCeMS (ESI), or electron
impact ionization (EI). Melting points are uncorrected. Flash
Cl
N
N2
Rh2(OAc)4
N
CO2Me
Ph
CO2Me
DCM, reflux
S
31
S
32
Ph
17c
88% yield
Scheme 6. Ring expansion of an isothiazole.
˚
column chromatography was performed on silica gel 60 A
(230e400 mesh) using a pentane/diethyl ether mixture as
the eluent unless otherwise specified.
33, formed by attack of the isoxazole nitrogen onto the rhodium
carbenoid (Scheme 7). At this point, two reasonable pathways
could lead to the ring-expanded product 35. The ylide 33 could
undergo a 1,2-shift to generate 35 directly, as previously pro-
posed for the ring expansion of 2-substituted isothiazol-3(2H)-
ones.17 Alternatively, ylide 33 could undergo a ring-opening to
34, followed by a 6p-electrocyclization to give 35 as proposed
for the ring expansion of isoxazolium ylides derived from depro-
tonation of isoxazolium salts.16c
4.2. Synthesis of ring expansion/[4þ3]-cycloaddition
product 16
4.2.1. Compound 16
To a flame dried 25 mL round bottom flask under argon and
charged with a stir bar were added 8 (0.0500 g, 0.306 mmol),
Rh2(R-DOSP)4 (0.017 g, 0.0092 mmol, 0.03 equiv), and 2,2
DMB (2 mL). A reflux condenser was attached to the flask
and the solution was heated to reflux. A solution of 2
(0.172 g, 1.23 mmol) in 2,2-DMB (3 mL) was added via sy-
ringe pump addition over 20 min. The solution was refluxed
for another 5 min, then allowed to cool to ambient tempera-
ture. The mixture was concentrated in vacuo and the residue
was purified by flash chromatography (silica gel, 5:1e2:1 pen-
tane/diethyl ether) to give the diastereomeric products 16a
(0.038 g, 32% yield) and 16b (0.036 g, 30% yield) as clear
oils.
N
O
R5
CO2R2
R3
R1 CO2R2
R1
R3
R4
Rh(II)
-N2
N2
N
O
N
O
1,2-shift
CO2R2
R1
CO2R2
R5
R3
R5
R4
35
R4
33
ring
6π
opening
R1
N
O
R3
R5
R4
34
4.2.2. Compound 16a
Rf 0.16 (2:1 pentane/diethyl ether); FTIR (neat): 2930, 1746,
1711, 1640, 1436, 1236, 1193, 1066, 1028, 1006, 970 cmꢀ1; 1H
NMR (500 MHz, CDCl3) d 6.89 (d, J¼8.0 Hz, 1H), 6.69 (d,
J¼4.5 Hz, 1H), 5.77 (d, J¼15.5 Hz, 1H), 5.55 (dq, J¼15.5,
6.5 Hz, 1H), 3.81 (s, 3H), 3.74 (s, 3H), 3.55 (m, 1H), 2.67e
2.60 (m, 3H), 2.29e2.20 (m, 2H), 1.68 (d, J¼6.5 Hz, 3H),
1.24 (t, J¼7.5 Hz, 3H), 1.19e1.15 (m, 1H), 0.91 (d,
J¼6.5 Hz, 3H), 0.84 (d, J¼7.0 Hz, 3H); 13C NMR (125 MHz,
CDCl3) d 170.8 (C), 167.3 (C), 165.4 (C), 145.5 (CH), 135.9
(CH), 132.9 (CH), 129.9 (C), 127.9 (CH), 126.1 (C), 89.0
(C), 77.6 (C), 52.9 (CH3), 52.1 (CH3), 48.5 (CH), 38.6 (CH2),
37.8 (CH), 31.2 (CH), 27.7 (CH2), 21.5 (CH3), 17.6 (CH3),
14.6 (CH3), 12.1 (CH3); LRMS (EI) m/z (relative intensity):
387 (24) [M]þ, 328 (100) [MꢀCO2CH3]þ; HRMS (EI) calcd
for [C22H29NO5]þ 387.2040, found 387.2041; HPLC analysis:
19% ee (Chiralpak AD-H, 1% i-PrOH in hexanes, 0.8 mL/min,
l¼254 nm, tR¼17.3 min, major; 18.4 min, minor).
Scheme 7. Possible mechanistic pathways for isoxazole ring expansion.
3. Conclusion
In summary, the reactions of various isoxazoles with rhodium
carbenoids have been examined and found to produce 2H-1,3-
oxazines through a ring expansion in good to excellent yields.
These reactions are likely to proceed via ylide intermediates,
which then either expand through a 1,2-shift or open up to 3-
imino-2-en-1-ones, which subsequently undergo a 6p-electro-
cyclization to 2H-1,3-oxazines. Studies toward applications of
this transformation in heterocycle synthesis are currently
underway.
4. Experimental
4.1. General
4.2.3. Compound 16b
Rf 0.31 (2:1 pentane/diethyl ether); FTIR (neat): 2919,
All experiments were performed under anhydrous condi-
tions in an atmosphere of argon except where stated, using
flame dried glassware. 2,2-Dimethylbutane (DMB) was puri-
fied by distillation over sodium. DCM and THF were dried
by a solvent purification system (passed through activated alu-
mina). The vinyldiazoacetate 2,4 dimethyl diazomalonate
17b,18 methyl phenyldiazoacetate 17c,19 and methyl 2-(3-
methylisoxazol-5-yl)acetate 20d8 were prepared by their
1741, 1710, 1641, 1435, 1234, 1065, 1026, 1003, 970 cmꢀ1
;
1H NMR (500 MHz, CDCl3) d 6.90 (d, J¼7.5 Hz, 1H), 6.69
(d, J¼5.0 Hz, 1H), 6.11 (dq, J¼15.5, 6.5 Hz, 1H), 5.91 (d,
J¼15.5 Hz, 1H), 3.74 (s, 3H), 3.68 (s, 3H), 3.57 (m, 1H),
2.72e2.58 (m, 2H), 2.54e2.48 (m, 1H), 2.27e2.21 (m, 2H),
1.78 (d, J¼6.5 Hz, 3H), 1.27e1.20 (m, 4H), 0.80 (d,
J¼6.0 Hz, 3H), 0.78 (d, J¼7.5 Hz, 3H); 13C NMR
(125 MHz, CDCl3) d 170.2 (C), 167.3 (C), 165.0 (C), 145.3

J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
6905
(CH), 135.7 (CH), 131.6 (CH), 130.1 (C), 128.5 (CH), 126.0
(C), 87.8 (C), 77.9 (C), 52.3 (CH3), 52.1 (CH3), 48.4 (CH),
38.6 (CH2), 38.4 (CH), 31.2 (CH), 27.5 (CH2), 18.8 (CH3),
17.7 (CH3), 15.0 (CH3), 11.6 (CH3); LRMS (ESI) m/z (relative
intensity): 388 (100) [MþH]þ; HRMS (ESI) calcd for
[C22H30NO5]þ 388.2118, found 388.2121; HPLC analysis:
19% ee (Regis R,R-Whelk, 0.5% i-PrOH in hexanes, 1.0 mL/
min, l¼254 nm, tR¼20.8 min, major; 24.7 min, minor).
over 45 min. The solution was refluxed another 15 min and
then cooled to ambient temperature. The reaction mixture
was concentrated in vacuo and the residue was purified by
flash chromatography to give the product.
4.4.1. Ethyl 4,6-dimethyl-2H-1,3-oxazine-2-carboxylate
(19a)
The reaction was performed with 3,5-dimethylisoxazole 18
(0.146 g, 1.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol), and
ethyl diazoacetate 17a (0.057 g, 0.5 mmol) in 1,2-DCE. Puri-
fied by flash chromatography (silica gel, 1:1 pentane/diethyl
ether) to give 19a as a clear oil (0.051 g, 56% yield). Rf
0.13 (1:1 pentane/diethyl ether); FTIR (neat): 2983, 1743,
4.3. Synthesis of isoxazole 20a
4.3.1. 2-(3-Methylisoxazol-5-yl)ethanol
To a flame dried round bottom flask under argon and charged
with a stir bar were added methyl 2-(3-methylisoxazol-5-yl)-
acetate 20d8 (0.62 g, 4.0 mmol) and THF (10 mL). The solution
was cooled to 0 ꢁC in an ice bath and a solution of LAH (2.0 M
in THF, 1 mL) was added by syringe. The solution was stirred
for 10 min and then slowly quenched with water. The solution
was then extracted with ether (3ꢂ15 mL) and the combined
organic extracts dried over anhydrous MgSO4, filtered, and
concentrated in vacuo. The residue was purified by flash chro-
matography (silica gel, diethyl ether as eluent, visualized
with KMnO4 stain) to give 2-(3-methylisoxazol-5-yl)ethanol
as a clear oil (0.252 g, 50% yield). The characterization data
were in agreement with the literature values.20
1
1619, 1575, 1443, 1385, 1202, 1096, 1023 cmꢀ1; H NMR
(500 MHz, CDCl3) d 5.59 (s, 1H), 5.37 (s, 1H), 4.36e4.26
(m, 2H), 2.04 (s, 3H), 1.98 (s, 3H), 1.34 (t, J¼7.5 Hz, 3H);
13C NMR (75 MHz, CDCl3) d 168.1 (C), 165.8 (C), 162.9
(C), 100.7 (CH), 85.9 (CH), 61.8 (CH2), 23.7 (CH3), 19.0
(CH3), 14.0 (CH3); LRMS (ESI) m/z (relative intensity): 206
(100) [MþNa]þ, 184 (36) [MþH]þ; HRMS (ESI) calcd for
[C9H14NO3]þ 184.0968, found 184.0970.
4.4.2. Dimethyl 4,6-dimethyl-2H-1,3-oxazine-2,2-
dicarboxylate (19b)
The reaction was performed with 3,5-dimethylisoxazole 18
(0.049 g, 0.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol), di-
methyl diazomalonate 17b18 (0.111 g, 0.70 mmol), and 1,2-
DCE. The solution was refluxed for 2 h after the diazo addition
was complete. Purified by flash chromatography (silica gel, 1:5
pentane/diethyl ether) to give 19b as a clear oil (0.053 g, 47%
yield). Rf 0.24 (diethyl ether); FTIR (neat): 2950, 1748, 1654,
4.3.2. 5-(2-(tert-Butyldimethylsilyloxy)ethyl)-3-methyl-
isoxazole (20a)
To a flame dried round bottom flask under argon and
charged with a stir bar were added 2-(3-methylisoxazol-5-yl)-
ethanol (0.105 g, 0.83 mmol), TBSCl (0.149 g, 0.99 mmol),
DMAP (0.003 g, 0.025 mmol), and DCM (10 mL). Imidazole
(0.062 g, 0.91 mmol) was then added and the solution stirred
overnight. The solution was then washed with brine, dried
over anhydrous MgSO4, filtered, and concentrated in vacuo.
The residue was purified by flash chromatography (silica
gel, 7:1 pentane/diethyl ether) to give the product as a clear
oil (0.107 g, 54% yield). Rf 0.25 (7:1 pentane/diethyl ether);
FTIR (neat): 2929, 1607, 1472, 1417, 1255, 1101, 835,
1574, 1435, 1294, 1259, 1134, 1056, 784, 706 cmꢀ1 1H
;
NMR (500 MHz, CDCl3) d 5.39 (s, 1H), 3.86 (s, 6H), 2.10 (s,
3H), 2.02 (s, 3H); 13C NMR (75 MHz, CDCl3) d 166.6 (C),
166.2 (C), 162.1 (C), 100.5 (CH), 53.5 (CH3), 24.1 (CH3),
19.2 (CH3), missing C attributed to excessive peak broaden-
ing/quadrupolar effect due to N; LRMS (ESI) m/z (relative in-
tensity): 477 (39) [2MþNa]þ, 250 (51) [MþNa]þ; HRMS
(ESI) calcd for [C10H13NO5Na]þ 250.0686, found 250.0680.
1
776 cmꢀ1; H NMR (300 MHz, CDCl3) d 5.89 (s, 1H), 3.88
(t, J¼6.5 Hz, 2H), 2.92 (t, J¼6.5 Hz, 2H), 2.26 (s, 3H), 0.87
(s, 9H), 0.02 (s, 6H); 13C NMR (75 MHz, CDCl3) d 170.4
(C), 159.5 (C), 102.5 (CH), 60.6 (CH2), 30.4 (CH2), 25.7
(CH3), 18.1 (C), 11.3 (CH3), ꢀ5.6 (CH3); LRMS (EI) m/z (rel-
4.4.3. Methyl 4,6-dimethyl-2-phenyl-2H-1,3-oxazine-2-
carboxylate (19c)
The reaction was performed with 3,5-dimethylisoxazole 18
(0.049 g, 0.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol), 17c
(0.132 g, 0.75 mmol), and DCM. Purified by flash chromatog-
raphy (silica gel, 1.5:1 pentane/diethyl ether) to give 19c as
a white solid (0.109 g, 89% yield). Mp 72e73 ꢁC, Rf 0.19
(1:1 pentane/diethyl ether); FTIR (neat): 2954, 1742, 1659,
t
ative intensity): 226 (9) [M]þ, 184.100 [Mꢀ Bu]þ; HRMS (EI)
calcd for [MꢀCH3]þ [C11H20NO2Si]þ 226.1258, found
226.1249.
1
4.4. General procedure for the rhodium-catalyzed isoxazole/
isothiazole ring expansion reactions
1574, 1235, 729 cmꢀ1; H NMR (500 MHz, CDCl3) d 7.74
(dd, J¼8.0, 1.5 Hz, 2H), 7.40e7.35 (m, 3H), 5.36 (s, 1H),
3.72 (s, 3H), 2.13 (s, 3H), 2.01 (s, 3H); 13C NMR (75 MHz,
CDCl3) d 170.3 (C), 164.7 (C), 162.2 (C), 138.8 (C), 128.6
(CH), 127.9 (CH), 126.2 (CH), 100.7 (CH), 52.7 (CH3), 23.9
(CH3), 19.2 (CH3), missing C attributed to excessive broaden-
ing/quadrupolar effect due to N; LRMS (ESI) m/z (relative in-
tensity): 246 (100) [MþH]þ. Anal. Calcd for C14H15NO3: C,
68.56; H, 6.16; N, 5.71. Found: C, 68.42; H, 6.17; N, 5.60.
To a flame dried 25 mL round bottom flask under argon and
charged with a stir bar were added the isoxazole substrate,
Rh2(OAc)4, and solvent (DCM or 1,2-DCE, 5 mL). A water-
cooled condenser was attached to the flask and the solution
was heated to reflux. A solution of diazoacetate in solvent
(DCM or 1,2-DCE, 5 mL) was then added by syringe pump

6906
J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
1
4.4.4. Methyl 6-(2-(tert-butyldimethylsilyloxy)ethyl)-4-
and DCM. H NMR analysis of the crude reaction mixture
in CD2Cl2 did not contain 22d. Purified by flash chromatogra-
phy (silica gel, 1:1 pentane/diethyl ether) to give a 1:1 mixture
of 21d and 22d as a sticky white solid (0.102 g, 67% yield). Rf
methyl-2-phenyl-2H-1,3-oxazine-2-carboxylate (21a)
The reaction was performed with 20a (0.073 g, 0.30 mmol),
Rh2(OAc)4 (4.0 mg, 0.009 mmol), 17c (0.064 g, 0.36 mmol),
and DCM. Purified by flash chromatography (silica gel, 4:1e
3:1 pentane/diethyl ether) to give 21a as a clear oil (0.098 g,
83% yield). Rf 0.11 (5:1 pentane/diethyl ether); FTIR (neat):
2954, 2928, 2856, 1745, 1658, 1574, 1235, 1098, 1006, 833,
1
0.17 (1:1 pentane/diethyl ether); H NMR (500 MHz, CDCl3)
d 7.73e7.71 (m, 2H), 7.57e7.56 (m, 2H), 7.40e7.39 (m, 6H),
6.45 (s, 1H), 6.00 (s, 1H), 5.45 (s, 1H), 5.19 (s, 1H), 3.78 (s,
3H), 3.73 (s, 3H), 3.72 (s, 3H), 3.65 (s, 3H), 3.33 (s, 2H),
2.24 (s, 3H), 2.03 (s, 3H); LRMS (ESI) m/z (relative intensity):
629 (85) [2MþNa]þ, 326 (100) [MþNa]þ; HRMS (EI) calcd
for [C16H17NO5]þ 303.1101, found 303.1111.
1
776, 729, 696 cmꢀ1; H NMR (500 MHz, CDCl3) d 7.73 (dd,
J¼8.0, 1.5 Hz, 2H), 7.40e7.35 (m, 3H), 5.43 (s, 1H), 3.87e
3.85 (m, 2H), 3.70 (s, 3H), 2.49 (t, J¼7.0 Hz, 2H), 2.15 (s, 3H),
0.86 (s, 9H), 0.03 (s, 6H); 13C NMR (75 MHz, CDCl3) d 170.5
(C), 164.8 (C), 162.9 (C), 138.8 (C), 128.8 (CH), 128.1 (CH),
126.4 (CH), 101.3 (CH), 78.9 (C), 59.4 (CH2), 52.8 (CH3), 37.0
(C), 25.7 (CH3), 24.1 (CH3), 18.1 (CH2), ꢀ5.5 (CH3); LRMS
(ESI) m/z (relative intensity): 390 (100) [MþH]þ; HRMS (ESI)
calcd for [C21H32NO4Si]þ 390.2095, found 390.2091.
4.4.8. Methyl 2-phenyl-2H-benzo[e][1,3]oxazine-2-
carboxylate (24)
The reaction was performed with freshly purified 1,2-
benzisoxazole 23 (0.065 g, 0.55 mmol, purified by passing
through a silica gel pipet column eluted with 5:1 pentane/
diethyl ether), Rh2(OAc)4 (7 mg, 0.016 mmol), 17c (0.044 g,
0.25 mmol), and DCM. Purified by flash chromatography (sil-
ica gel, 2:1e1:1 pentane/diethyl ether) to give 24 as a clear oil
(0.048 g, 72% yield). Rf 0.23 (1:1 pentane/diethyl ether);
FTIR (neat): 2950, 1745, 1633, 1608, 1229, 1050, 1035,
4.4.5. Methyl 4-(chloromethyl)-6-methyl-2-phenyl-2H-1,3-
oxazine-2-carboxylate (21b)
The reaction was performed with 3-chloromethyl-5-meth-
ylisoxazole 20b (0.066 g, 0.5 mmol), Rh2(OAc)4 (6.6 mg,
0.015 mmol), 17c (0.115 g, 0.65 mmol), and DCM. Purified
by flash chromatography (silica gel, 5:1 pentane/diethyl ether)
to give 21b as a clear oil (0.130 g, 93% yield). Rf 0.14 (5:1
pentane/diethyl ether); FTIR (neat): 1743, 1655, 1572, 1434,
1003, 759, 727, 697 cmꢀ1 1H NMR (400 MHz, CDCl3)
;
d 8.41 (s, 1H), 7.76 (d, J¼7.0 Hz, 2H), 7.42e7.32 (m, 4H),
7.22 (dd, J¼7.5, 1.5 Hz, 1H), 7.08 (d, J¼8.5 Hz, 1H), 6.98
(appt t, J¼7.0 Hz, 1H), 3.76 (s, 3H); 13C NMR (75 MHz,
CDCl3) d 169.1 (C), 157.5 (CH), 153.2 (C), 138.6 (C),
134.5 (CH), 128.8 (CH), 128.1 (CH), 127.4 (CH), 126.4
(CH), 122.0 (CH), 116.7 (C), 116.5 (CH), 91.7 (C), 53.1
(CH3); LRMS (ESI) m/z (relative intensity): 268 (27)
[MþH]þ; HRMS (ESI) calcd for [C16H14NO3]þ 268.0968,
found 268.0969.
1369, 1238, 1031, 727, 697 cmꢀ1 1H NMR (500 MHz,
;
CDCl3) d 7.73 (dd, J¼8.0, 1.5 Hz, 2H), 7.41e7.38 (m, 3H),
5.66 (s, 1H), 4.22 (s, 2H), 3.73 (s, 3H), 2.08 (s, 3H); 13C
NMR (75 MHz, CDCl3) d 169.8 (C), 164.7 (C), 163.0 (C),
138.1 (C), 129.1 (CH), 128.2 (CH), 126.3 (CH), 98.0 (CH),
53.1 (CH3), 45.5 (CH2), 19.7 (CH3), missing C attributed to
excessive broadening/quadrupolar effect due to N; LRMS
(ESI) m/z (relative intensity): 581 (17) [2MþNa]þ, 302
(100) [MþNa]þ; HRMS (ESI) calcd for [C14H14NO3ClNa]þ
302.0554, found 302.0557.
4.4.9. Aziridine 25
The reaction was performed with freshly purified 1,2-
benzisoxazole 23 (0.065 g, 0.55 mmol, purified by passing
through a silica gel pipet column eluted with 5:1 pentane/
diethyl ether), Rh2(OAc)4 (7 mg, 0.016 mmol), 17c
(0.291 g, 1.65 mmol), and DCM. Purified by flash chroma-
tography (silica gel, 3:1e2:1 pentane/diethyl ether) to give
25 as a white solid (0.215 g, 94% yield). Mp 147e149 ꢁC;
Rf 0.36 (1:1 pentane/diethyl ether); FTIR (neat): 2950,
1742, 1491, 1281, 1208, 1128, 1074, 1045, 1031, 986, 759,
4.4.6. Methyl 5-bromo-4,6-dimethyl-2-phenyl-2H-1,3-
oxazine-2-carboxylate (21c)
The reaction was performed with 4-bromo-3,5-dimethyl-
isoxazole (0.088 g, 0.5 mmol), Rh2(OAc)4 (4.4 mg,
0.01 mmol), 17c (0.115 g, 0.65 mmol), and DCM. Purified
by flash chromatography (silica gel, 7:1 pentane/diethyl ether)
to give 21c as a clear oil (0.155 g, 96% yield). Rf 0.21 (5:1
pentane/diethyl ether); FTIR (neat): 1746, 1638, 1564, 1432,
727, 696 cmꢀ1 1H NMR (500 MHz, CDCl3) d 7.84 (d,
;
1376, 1309, 1241, 1137, 1011, 908, 726, 696 cmꢀ1
;
1H
J¼7.0 Hz, 2H), 7.80 (d, J¼7.0 Hz, 2H), 7.44e7.36 (m,
3H), 7.29e7.15 (m, 5H), 7.04 (d, J¼8.0 Hz, 1H), 6.95e
6.92 (m, 1H), 3.83 (s, 3H), 3.60 (s, 3H), 3.36 (s, 1H); 13C
NMR (75 MHz, CDCl3) d 168.6 (C), 167.4 (C), 149.5 (C),
137.5 (C), 137.0 (C), 128.9 (CH), 128.6 (CH), 128.3 (CH),
128.0 (CH), 127.7 (CH), 127.0 (CH), 122.8 (CH), 120.3
(C), 118.5 (CH), 89.5 (C), 53.0 (CH3), 52.7 (C), 52.3
(CH3), 44.4 (CH), 2 missing CH resonances attributed to
overlapping signals; LRMS (ESI) m/z (relative intensity):
853 (100) [2MþNa]þ, 438 (36) [MþNa]þ. Anal. Calcd for
C25H21NO5: C, 72.28; H, 5.10; N, 3.37. Found: C, 71.92;
H, 5.09; N, 3.33.
NMR (500 MHz, CDCl3) d 7.72 (dd, J¼7.5, 1.5 Hz, 2H),
7.40e7.38 (m, 3H), 3.72 (s, 3H), 2.34 (s, 3H), 2.22 (s, 3H);
13C NMR (75 MHz, CDCl3) d 169.9 (C), 163.1 (C), 160.2
(C), 137.9 (C), 129.0 (CH), 128.8 (C), 128.2 (CH), 126.3
(CH), 97.4 (C), 53.1 (CH3), 24.4 (CH3), 19.2 (CH3); LRMS
(ESI) m/z (relative intensity): 324 (10) [MþH]þ; HRMS
(ESI) calcd for [C14H15BrNO3]þ 324.0230, found 324.0234.
4.4.7. Compounds 21d and 22d
The reaction was performed with 20d (0.078 g, 0.5 mmol),
Rh2(OAc)4 (6.6 mg, 0.015 mmol), 17c (0.132 g, 0.75 mmol),

J.R. Manning, H.M.L. Davies / Tetrahedron 64 (2008) 6901e6908
6907
4.4.10. Methyl 2-(2-formylphenylimino)-2-phenyl-
acetate (27)
LRMS (ESI) m/z (relative intensity): 777 (100) [2MþNa]þ,
400 (100) [MþNa]þ, 378 (18) [MþH]þ; HRMS (EI) calcd
for [C22H16NO3Cl]þ 377.0813, found 377.0826.
The reaction was performed with anthranil 26 (0.179 g,
1.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol), 17c (0.085 g,
0.5 mmol), and 1,2-DCE. Purified by flash chromatography
(silica gel, 5:1 pentane/diethyl ether) to give 27 (0.089 mg,
67% yield) and 28 (0.034 g, 16% yield) as yellow oils. 27:
Rf 0.30 (2:1 pentane/diethyl ether); FTIR (neat): 1733, 1690,
1624, 1592, 1450, 1302, 1273, 1226, 1192, 1174, 1155,
4.4.13. Methyl 4-chloro-2-phenyl-2H-benzo[e][1,3]-
thiazine-2-carboxylate (32)
The reaction was performed with 3-chloro-1,2-benzisothi-
azole 31 (0.085 g, 0.5 mmol), Rh2(OAc)4 (6.6 mg,
0.015 mmol), 17c (0.115 g, 0.65 mmol), and DCM (5 mL).
Purified by flash chromatography (silica gel, 3:1 pentane/di-
ethyl ether) to give 32 as a clear oil (0.140 mg, 88% yield).
Rf 0.25 (2:1 pentane/diethyl ether); FTIR (neat): 2966, 1736,
1009, 764, 689, 669 cmꢀ1 1H NMR (400 MHz, CD2Cl2)
;
d 10.20 (s, 1H), 7.90 (d, J¼7.5 Hz, 2H), 7.86 (d, J¼7.5 Hz,
1H), 7.60e7.49 (m, 4H), 7.26 (t, J¼7.5 Hz, 1H), 6.85 (d,
J¼8.0 Hz, 1H), 3.58 (s, 3H); 13C NMR (75 MHz, CD2Cl2)
d 190.3 (CH), 164.7 (C), 161.7 (C), 152.7 (C), 135.1 (CH),
133.5 (C), 132.8 (CH), 129.2 (CH), 128.6 (CH), 128.3 (CH),
127.0 (C), 125.5 (CH), 119.4 (CH), 52.4 (CH3); LRMS
(ESI) m/z (relative intensity): 557 (52) [2MþNa]þ, 290
(100) [MþNa]þ, 268 (68) [MþH]þ; HRMS (EI) calcd for
[C16H13NO3]þ 267.0890, found 267.0903.
1
1446, 1434, 1234, 1007, 819, 764, 733, 693 cmꢀ1; H NMR
(500 MHz, CDCl3) d 7.65e7.62 (m, 2H), 7.55e7.54 (m,
2H), 7.51e7.46 (m, 2H), 7.34 (m, 3H), 3.85 (s, 3H); 13C
NMR (75 MHz, CDCl3) d 167.6 (C), 137.8 (CH), 136.8 (C),
133.7 (CH), 133.2 (C), 132.3 (CH), 130.3 (CH), 129.2 (CH),
128.2 (CH), 127.0 (CH), 120.5 (C), 117.0 (C), 82.3 (C),
54.3 (CH3); LRMS (EI) m/z (relative intensity): 317 (5)
[M]þ, 121 (100); HRMS (EI) calcd for [C16H12ClNO2S]þ
317.0272, found 317.0265.
4.4.11. (E)-Methyl 3-(2-(2-methoxy-2-oxo-1-phenyl-
ethylideneamino)phenyl)-2-phenyloxirane-
2-carboxylate (28)
Acknowledgements
The reaction was performed with anthranil 26 (0.060 g,
0.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol), 17c (0.255 g,
1.5 mmol), and 1,2-DCE. Purified by flash chromatography
(silica gel, 5:1 pentane/diethyl ether) to give 28 as a yellow
oil (0.201 g, 97% yield). Rf 0.24 (2:1 pentane/diethyl ether);
FTIR (neat): 1734, 1449, 1434, 1298, 1234, 1197, 1171,
The National Institutes of Health (GM080337) is gratefully
acknowledged. J.R.M. thanks the National Institutes of Health
for a Ruth L. Kirchstein predoctoral fellowship (DA019287).
Supplementary data
1009, 763, 739, 691 cmꢀ1 1H NMR (400 MHz, CD2Cl2)
;
d 7.87 (d, J¼7.0 Hz, 2H), 7.57e7.54 (m, 1H), 7.49e7.45
(m, 5H), 7.31e7.17 (m, 5H), 6.80 (d, J¼8.0 Hz, 1H), 4.33
(s, 1H), 3.64 (s, 3H), 3.48 (s, 3H); 13C NMR (75 MHz,
CD2Cl2) d 167.6 (C), 165.3 (C), 160.6 (C), 148.9 (C), 135.8
(C), 133.9 (C), 132.5 (CH), 129.2 (CH), 129.1 (CH), 128.9
(CH), 128.52 (CH), 128.48 (CH), 127.5 (CH), 126.5 (CH),
126.2 (C), 125.3 (CH), 117.4 (CH), 66.8 (C), 63.1 (CH),
52.4 (CH3), 52.3 (CH3); LRMS (ESI) m/z (relative intensity):
438 (100) [MþNa]þ, 416 (41) [MþH]þ; HRMS (EI) calcd for
[C25H21NO5]þ 415.1414, found 415.1416.
Synthesis of compounds 9e13, 15, and 8, and the method
for structural assignment of 13 and its regioisomer. gHSQC
and gHMBC correlations for 16. Supplementary data associ-
ated with this article can be found in the online version, at
doi:10.1016/j.tet.2008.03.010.
References and notes
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4.4.12. Methyl 2-(2-benzoyl-4-chlorophenylimino)-2-
phenylacetate (30)
2. (a) Taber, D. F.; Joshi, P. V. Modern Rhodium-Catalyzed Organic Reac-
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The reaction was performed with 5-chloro-3-phenylanthra-
nil 29 (0.115 g, 0.5 mmol), Rh2(OAc)4 (6.6 mg, 0.015 mmol),
17c (0.106 g, 0.6 mmol), and DCM. Purified by flash chroma-
tography (silica gel, 5:1 pentane/diethyl ether) to give 30 as
a sticky yellow oil (0.185 g, 98% yield). Rf 0.24 (5:1 pen-
tane/diethyl ether); FTIR (neat): 1734, 1664, 1449, 1283,
1226, 1195, 1173, 1009, 688 cmꢀ1 1H NMR (500 MHz,
;
C6D6) d 7.81 (d, J¼7.0 Hz, 2H), 7.54 (d, J¼7.5 Hz, 2H),
7.50 (d, J¼2.5 Hz, 1H), 7.05e6.95 (m, 5H), 6.92e6.89 (m,
2H), 6.66 (d, J¼8.5 Hz, 1H), 3.10 (s, 3H); 13C NMR
(75 MHz, CD2Cl2) d 195.1 (C), 164.7 (C), 160.4 (C), 147.2
(C), 137.7 (C), 133.6 (C), 133.3 (CH), 132.9 (C), 132.5
(CH), 131.8 (CH), 130.7 (C), 130.1 (CH), 129.9 (CH), 128.9
(CH), 128.7 (CH), 128.3 (CH), 120.8 (CH), 52.5 (CH3);
7. Previous total synthesis: Miyaoka, H.; Honda, D.; Mitome, H.; Yamada, Y.
Tetrahedron Lett. 2002, 43, 7773.
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Kamiya, J.; Maruyama, M. J. Med. Chem. 1989, 32, 351.
10. Comins, D. L.; Dehghani, A. Tetrahedron Lett. 1992, 33, 6299.

6908
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