Preparation and allylation of enamides and enecarbamates generated via iron(0) reduction of oximes and derivatives

Article abstract of DOI:10.1055/s-2006-942513

Reductive acylation of oximes and oxime carbonates can be effected with iron powder in the presence of an acid chloride or a chloroformate to produce enamides or enecarbamates. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-942513

PAPER
3585
Preparation and Allylation of Enamides and Enecarbamates Generated via
Iron(0) Reduction of Oximes and Derivatives
Preparationand
A
u
llylationof
i
Enamid
x
es andEnec
i
a
rbam
a
a
tes ng Sun, Steven M. Weinreb*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
E-mail: smw@chem.psu.edu
Received 30 March 2006; revised 11 April 2006
Dedicated for the 65th Birthday of Marty Semmelhack
Fe, PhMe
Ac2O, 70 °C
40–85%
Abstract: Reductive acylation of oximes and oxime carbonates can
be effected with iron powder in the presence of an acid chloride or
a chloroformate to produce enamides or enecarbamates.
NOH
NHAc
R'
R'
R
R
or
Fe, DMF
Ac2O, r.t.
TMSCl (cat.)
Key words: acylations, N-acyliminium ions, addition reactions, al-
lylations, reductions
Scheme 1
Enamides have been widely used as intermediates in or-
ganic synthesis. For example, enamides are valuable pre-
cursors for generation of N-acyliminium ions,1,2 and have
also been utilized recently in asymmetric hydrogenation
reactions to prepare N-acyl derivatives of chiral amines.3
Enamides have been synthesized by a variety of methods,
including the Curtius rearrangment,4 by direct addition of
amides to alkynes,5 and by transition-metal-promoted
coupling of vinyl halides with amides.6 Another common
method for the preparation of enamides is simply to N-
acylate an imine with an acid halide or an acid anhydride.7
However, this methodology is only practicable for synthe-
sis of tertiary enamides derived from N-alkyl- or N-aryl-
substituted imines due to the difficulty in condensing am-
methodology has only been applied to ketoximes derived
from aryl or hindered ketones.3g,10
As part of some ongoing projects which involve the use of
enamides and enecarbamates as N-acyliminium ion pre-
cursors,2 we became interested in possibly extending the
scope of this methodology to acylating agents other than
acetic anhydride. Moreover, since acetamides can be dif-
ficult to cleave, especially in the presence of sensitive
functionality, we considered the possibility of effecting
this transformation to enable generation of enecarbamates
bearing more readily removable acyl protecting groups. In
this paper are described some of our studies in this area.
monia with aldehydes or ketones to form simple unsubsti- Initial exploratory experiments were conducted with a-te-
tuted imines.8
tralone oxime (1; Scheme 2). It was found that treatment
of 1 with iron powder and benzoyl chloride (2 equiv) in
tetrahydrofuran at room temperature led to the desired
enamide 2 in good yield. Similarly, with ethyl chlorofor-
mate and benzyl chloroformate the enecarbamates 3 and
4, respectively, were generated. Since aldehyde oximes
had not previously been used in this type of reductive acy-
lation, aldoxime 5 was treated under the same conditions
with ethyl chloroformate and benzyl chloroformate to
produce enecarbamates 6 and 7, respectively, in moderate
isolated yields. We believe some loss of product is proba-
bly occurring here via enecarbamate hydrolysis on chro-
matography (vide infra).
In 1975 Barton et al. demonstrated that ketoximes, as well
as their O-acyl and O-alkyl derivatives, could be convert-
ed into secondary acetyl enamides in excellent yields by
heating at reflux with acetic anhydride in pyridine.9 A free
radical mechanism was postulated for this transformation.
In addition, it was found that treatment of ketoximes with
acetic anhydride in the presence of stoichiometric
amounts of metals such as Cr(II) and Ti(III) also affords
the corresponding enamides. More recently, Burk and co-
workers reported that acetyl enamides can be prepared in
moderate to good yields from ketoximes by heating at 70
°C in toluene–acetic anhydride in the presence of inexpen-
sive iron powder (Scheme 1).10 In an improvement of this
methodology, Zhang and Casalnuovo found that these re-
actions can be effected at room temperature if N,N-di-
We also investigated applying this methodology to ke-
toxime systems, which would produce non-conjugated
enamide and enecarbamate products. For example, when
methylformamide is used as solvent, and also that the 4-phenylcyclohexanone oxime (8) was subjected to the
reaction is initiated by addition of a catalytic amount of standard reduction conditions using benzoyl chloride, the
chlorotrimethylsilane.3g It might be noted that to date the enamide 10 was produced in only low irreproducible iso-
lated yields (19–48%; Scheme 3). We speculated that the
problem here might be due the HCl generated in the pro-
cess, which may accelerate the in situ hydrolysis of the
acid-sensitive product 10 by adventitious moisture.
SYNTHESIS 2006, No. 21, pp 3585–3588
Advanced online publication: 25.07.2006
x
x.
x
x
.
2
0
0
6
DOI: 10.1055/s-2006-942513; Art ID: Z06706SS
© Georg Thieme Verlag Stuttgart · New York

3586
C. Sun, S. M. Weinreb
PAPER
ion 12, which undergoes alkylation to afford product 13 in
60–69% isolated overall yields for the two steps. Similar-
ly, carbonate 9 could be converted to Cbz-protected
allylation product 14 in 60–64% yields using benzyl chlo-
roformate.
O
NOH
HN
R
Fe, THF
RCOCl
r.t.
We have also examined the possibility of using oxime
methyl ethers in this process. The reaction of methoxime
11 with iron powder/ethyl chloroformate at room temper-
ature as done with 9 proved to be sluggish, and heating at
50 °C was therefore required. Allylation of the crude ene-
carbamate product then afforded ethyl carbamate 13 in
somewhat lower yields than those obtained with the
oxime carbonate 9 (49–55% for two steps).
2 R = Ph (89%)
3 R = OEt (94%)
4 R = OBn (59%)
1
Fe, THF
ROCOCl
r.t.
H
N
OR
Ph
Ph
NOH
O
6 R = Et (42%)
7 R = Bn (47%)
5
O
Scheme 2
1) Fe, THF
R'OCOCl
+
HN
NOR
OR'
NHCOOR'
TMSCl (cat.)
r.t. or 50 °C
O
NOR
2) BF3·Et2O
CH2Cl2
TMS
Fe, THF
PhCOCl
r.t.
HN
Ph
Ph
Ph
12
Ph
–78 °C to r.t.
see
text
9 R = COOEt
11 R = Me
13 R' = Et (60–69% from 9)
(49–55% from 11)
14 R' = Bn (60–64% from 9)
Ph
Ph
10
8 R = H
9 R = COOEt
Scheme 4
Scheme 3
Finally, an acyclic ketoxime system was also examined in
this two-step operation. Oxime carbonate 15 derived from
2-decanone was found to undergo reductive acylation
with ethyl chloroformate under the standard conditions,
although in this case it was advantageous to conduct the
reaction at 50 °C (Scheme 5). Allylation of the crude ene-
carbamate afforded 16 in 62–69% overall isolated yields
for the two steps. The transformation with 15 could also
be carried out with benzyl chloroformate to generate the
Cbz-substituted allylic system 17 in 65–68% yields.
Therefore, in an attempt to alleviate this difficulty, oxime
8 was first converted to oxime carbonate 9 with ethyl
chloroformate. Reaction of carbonate 9 with benzoyl
chloride and iron powder using a catalytic amount of
TMSCl as initiator3g gave enamide 10 in a more reproduc-
ible 45% yield. Unfortunately, none of the corresponding
enecarbamate could be isolated from the reductive acyla-
tion of 9 with ethyl chloroformate. During the course of
these experiments, however, it became clear to us that the
main problem is the general tendency of non-conjugated
secondary (NH) enamides like 10 to hydrolyze upon chro-
matography to the corresponding ketone. It should also be
noted that in our experience tertiary (N-alkyl and N-aryl)
enamides generated from aliphatic ketones have been
found to be significantly more resistant to hydrolysis, and
are amenable to chromatographic purification.2
1) Fe, THF
ROCOCl
TMSCl (cat.)
50 °C
NOCOOEt
NHCOOR
Me
Me(H2C)7
2) BF3·Et2O
CH2Cl2
Me(H2C)7
Me
16 R = Et (62–69%)
17 R = Bn (65–68%)
15
TMS
–78 °C to r.t.
In order to make use of the non-conjugated ketone-de-
rived enamide systems generated by this methodology it
became apparent that chromatography should be avoided.
Thus, we explored effecting direct alkylations of the N-
acyliminium ions generated from the crude enamides.
Treatment of oxime carbonate 9 with iron powder and eth-
yl chloroformate at room temperature in tetrahydrofuran
containing a catalytic amount of TMSCl produced the cor-
responding enecarbamate. After a brief aqueous workup,
the product was immediately exposed to allyltrimethyl-
silane–boron trifluoride etherate in dichloromethane
(Scheme 4). The latter operation leads to N-acyliminium
Scheme 5
In conclusion, we have developed reductive acylation
methodology, which allows one to convert readily avail-
able oximes and oxime carbonates to a variety of en-
amides and enecarbamates using inexpensive reagents.
Isolation of products in the case of conjugated systems is
feasible by chromatography. However, with non-conju-
gated compounds it is preferable to simply use the crude
material for further transformations.
Synthesis 2006, No. 21, 3585–3588 © Thieme Stuttgart · New York

PAPER
Preparation and Allylation of Enamides and Enecarbamates
3587
Preparation of Oxime Ethyl Carbonates; General Procedure
To a solution of the oxime (3.17 mmol) in THF (10 mL) were added
TEA (2.21 mL, 15.9 mmol) and ethyl chloroformate (6.34 mmol) at
0 °C. The reaction mixture was stirred at 0 °C until TLC showed no
starting material remained. The mixture was then diluted with water
and the aqueous layer was extracted with CH2Cl2. The extract was
dried over MgSO4 and evaporated. The residue was purified by sil-
ica gel flash column chromatography to provide the O-acylated
oxime.
chloroformate (0.71 mL, 4.96 mmol) at r.t. After stirring overnight,
the reaction mixture was diluted with sat. NaHCO3 solution and
subsequently extracted with Et2O. The combined organic layers
were dried over MgSO4 and concentrated in vacuo. Basic alumina
(II–III) chromatography (hexanes–EtOAc) of the residue afforded
the enecarbamate or enamide product.
Compounds 6 and 7 have been previously prepared.11
N-(3,4-Dihydronaphthalen-1-yl)benzamide (2)
Yield: 89%; white solid; mp 147–148 °C.
4-Phenylhexanone Oxime Ethyl Carbonate (9)
Yield: 96%; white solid; mp 43–44 °C.
1H NMR (300 MHz, CDCl3): d = 7.11–7.26 (m, 5 H), 4.27 (q, J =
7.0 Hz, 2 H), 3.31–3.37 (m, 1 H), 2.68–2.78 (m, 2 H), 2.26 (dt, J =
4.9, 13.7 Hz, 1 H), 1.92–2.12 (m, 3 H), 1.57–1.73 (m, 2 H), 1.29 (t,
J = 7.0 Hz, 3 H).
1H NMR (400 MHz, CDCl3): d = 7.89–7.91 (m, 2 H), 7.50–7.60 (m,
4 H), 7.21–7.24 (m, 4 H), 6.66 (br s, 1 H), 2.85 (t, J = 7.6 Hz, 2 H),
2.46 (dt, J = 5.3, 7.6 Hz, 2 H).
13C NMR (90 MHz, CD2Cl2): d = 166.7, 137.3, 135.3, 132.4, 132.1,
132.0, 129.0, 128.2, 127.9, 127.4, 126.8, 121.3, 120.6, 28.0, 22.7.
HRMS (APCI+): m/z [MH+] calcd for C17H16NO: 250.1231; found:
250.1242.
13C NMR (90 MHz, CDCl3): d = 167.0, 154.1, 144.8, 128.4, 126.5,
64.4, 53.3, 43.1, 33.6, 32.8, 31.6, 26.3, 14.2.
HRMS (ESI+): m/z [MH+] calcd for C15H20NO3: 262.1443; found:
262.1472.
(3,4-Dihydronaphthalen-1-yl)carbamic Acid Ethyl Ester (3)
Yield: 94%; white solid.
1H NMR (400 MHz, CDCl3): d = 7.17–7.22 (m, 4 H), 6.33 (br s, 1
H), 6.12 (br s, 1 H), 4.21 (q, J = 7.1 Hz, 2 H), 2.78 (t, J = 7.6 Hz, 2
H), 2.35–2.40 (m, 2 H), 1.31 (t, J = 7.1 Hz, 3 H).
2-Decanone Oxime Ethyl Carbonate (15)
Yield: 85%; light yellow oil (mixture of geometric isomers).
13C NMR (100 MHz, CDCl3): d = 154.7, 136.9, 131.6, 131.3, 127.8,
Major Geometric Isomer
1H NMR (300 MHz, CDCl3): d = 4.23 (q, J = 7.1 Hz, 2 H), 2.26 (t,
J = 7.3 Hz, 2 H), 1.92 (s, 3 H), 1.49 (m, 2 H), 1.20–1.31 (m, 13 H),
0.81 (t, J = 6.4 Hz, 3 H).
127.5, 126.4, 120.4, 116.3, 61.1, 27.7, 22.1, 14.5.
HRMS (APCI+): m/z [MH+] calcd for C13H16NO2: 218.1181; found:
218.1181.
13C NMR (75 MHz, CDCl3): d = 166.3, 153.9, 64.263, 35.5, 31.7,
29.1, 29.0, 28.99, 26.1, 22.5, 14.9, 14.2, 13.9.
(3,4-Dihydronaphthalen-1-yl)carbamic Acid Benzyl Ester (4)
Yield: 59%; white solid.
Minor Geometric Isomer
1H NMR (360 MHz, CDCl3): d = 7.35–7.41 (m, 5 H), 7.16–7.22 (m,
4 H), 6.36 (br s, 1 H), 6.28 (br s, 1 H), 5.20 (s, 2 H), 2.79 (t, J = 7.6
Hz, 2 H), 2.26 (dt, J = 5.1, 7.6 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 154.4, 136.8, 136.2, 131.5, 131.3,
128.5, 128.2, 127.8, 127.5, 126.4, 120.4, 116.5, 66.9, 27.7, 22.1.
HRMS (ESI+): m/z [MH+] calcd for C18H18NO2: 280.1338; found:
280.1358.
1H NMR (300 MHz, CDCl3): d = 4.24 (q, J = 7.1 Hz, 2 H), 2.36 (t,
J = 8.0 Hz, 2 H), 1.96 (s, 3 H), 1.41–1.49 (m, 2 H), 1.22–1.32 (m,
13 H), 0.83 (t, J = 6.9 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 166.8, 154.0, 64.3, 31.7, 30.1, 29.3,
29.1, 29.0, 25.6, 22.5, 19.8, 14.2, 14.0.
HRMS (ESI+): m/z [MH+] calcd for C13H26NO3: 244.1913; found:
244.1907.
Styrylcarbamic Acid Ethyl Ester (6)
Yield: 42%; white solid.
1H NMR (300 MHz, CDCl3): d = 7.11–7.38 (m, 6 H), 6.67 (d, J =
10.8 Hz, 1 H), 5.92 (d, J = 14.5 Hz, 1 H), 4.17 (q, J = 6.7 Hz, 2 H),
1.17 (t, J = 6.4 Hz, 3 H).
4-Phenylcyclohexanone O-Methyloxime (11)
To a solution of 4-phenylcyclohexanone (4.00 g, 22.96 mmol) in
EtOH (60 mL) were added pyridine (2.80 mL, 34.34 mmol) and
methoxylamine hydrochloride (2.30 g, 27.54 mmol) at r.t. The reac-
tion mixture was stirred for 1 h until TLC showed no starting mate-
rial remained. Sat. NH4Cl was then added and the aqueous phase
was extracted with CH2Cl2. The extract was dried over MgSO4 and
evaporated. The residue was purified by flash column chromatogra-
phy (25–50% EtOAc–hexanes) to provide the product 11 (4.48 g,
96%) as a colorless oil.
1H NMR (360 MHz, CDCl3): d = 7.34–7.38 (m, 2 H), 7.25– 7.28 (m,
3 H), 3.93 (s, 3 H), 3.43–3.48 (m, 1 H), 2.81 (tt, J = 3.0, 12.1 Hz, 1
H), 2.58–2.62 (m, 1 H), 2.29 (dt, J = 4.7, 13.6 Hz, 1 H), 2.05–2.15
(m, 2 H), 1.91 (dt, J = 5.1, 13.8 Hz, 1 H), 1.61–1.82 (m, 2 H).
Styrylcarbamic Acid Benzyl Ester (7)
Yield: 47%; white solid.
1H NMR (300 MHz, CDCl3): d = 7.14–7.37 (m, 11 H), 6.74 (d, J =
10.8 Hz, 1 H), 5.96 (d, J = 14.6 Hz, 1 H), 5.18 (s, 2 H).
13C NMR (75 MHz, CDCl3): d = 153.5, 136.1, 135.8, 128.6, 128.4,
128.2, 126.3, 125.3, 123.9, 110.9, 67.4.
Addition of Allyltrimethylsilane to Enamides; General Proce-
dure
13C NMR (90 MHz, CDCl3): d = 158.5, 145.5, 128.3, 126.5, 126.1,
60.8, 43.5, 33.9, 32.8, 31.7, 24.6.
HRMS (APCI+): m/z [MH+] calcd for C13H18NO: 204.1388; found:
204.1369.
To a solution of O-acyloxime (0.30 mmol) in THF (3 mL) were add-
ed iron powder (51 mg, 0.91 mmol) and ethyl chloroformate (344
mL, 3.58 mmol) or benzyl chloroformate (0.514 mL, 3.61mmol).
One drop of TMSCl was then added to the reaction mixture, which
was stirred (at r.t. for 9 or at 50 °C for 11 and 15) until all the starting
material was consumed. The mixture was diluted with sat. NaHCO3
solution and was extracted with Et2O. The combined organic layers
were dried over Na2SO4, and concentrated. The residue was dis-
solved in CH2Cl2 (3 mL) and cooled to –78 °C. Allyltrimethylsilane
Preparation of Enamides and Enecarbamates; General Proce-
dure
To a solution of oxime (0.62 mmol) in THF (3 mL) were added iron
powder (173 mg, 3.10 mmol) and ethyl chloroformate (0.49 mL,
4.96 mmol), or benzoyl chloride (0.58 mL, 4.96 mmol), or benzyl
Synthesis 2006, No. 21, 3585–3588 © Thieme Stuttgart · New York

3588
C. Sun, S. M. Weinreb
PAPER
(190 mL, 1.19 mmol) and BF3·Et2O (36 mL, 0.286 mmol) were add-
ed to the solution. After stirring at –78 °C for 5 min and at r.t. over-
night, the mixture was diluted with sat. NH4Cl solution and the
aqueous phase was extracted with CH2Cl2. The organic extract was
dried over MgSO4 and concentrated in vacuo. The residue was pu-
rified by silica gel flash column chromatography to provide the ad-
duct.
HRMS (APCI+): m/z [MH+] calcd for C21H34NO2: 332.2590; found:
332.2596.
Acknowledgment
We are grateful to the National Institutes of Health (CA-034303) for
financial support of this research.
(1-Allyl-4-phenylcyclohexyl)carbamic Acid Ethyl Ester (13)
Yield: 60–69% for two steps from 9; pale yellow oil.
References
1H NMR (360 MHz, CDCl3): d = 7.20–7.33 (m, 5 H), 5.79–5.86 (m,
1 H), 5.06–5.12 (m, 2 H), 4.45 (br s, 1 H), 4.12 (q, J = 6.9 Hz, 2 H),
2.47–2.53 (m, 3 H), 2.25 (br d, J = 13.2 Hz, 2 H), 1.63–1.80 (m, 4
H), 1.43 (dt, J = 3.6, 13.6 Hz, 2 H), 1.27 (t, J = 7.1 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 154.9, 146.7, 133.7, 128.3, 126.7,
126.0, 118.1, 60.1, 53.9, 43.9, 43.7, 34.8, 29.0, 14.6.
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HRMS (APCI+): m/z [MH+] calcd for C18H26NO2: 288.1964; found:
288.1953.
(1-Allyl-4-phenylcyclohexyl)carbamic Acid Benzyl Ester (14)
Yield: 60–64% for two steps; white solid; mp 57–58 °C.
1H NMR (360 MHz, CDCl3): d = 7.22–7.41 (m, 10 H), 5.79–5.87
(m, 1 H), 5.06–5.17 (m, 4 H), 4.61 (br s, 1 H), 2.49–2.55 (m, 3 H),
2.28 (d, J = 13.2 Hz, 2 H), 1.60–1.80 (m, 4 H), 1.45 (dt, J = 3.4, 13.6
Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 154.6, 146.6, 136.7, 133.5, 128.5,
128.3, 128.0, 127.99, 126.7, 126.1, 118.3, 66.1, 54.1, 44.0, 43.7,
34.8, 29.0.
HRMS (APCI+): m/z [MH+] calcd for C23H28NO2: 305.2120; found:
305.2124.
(1-Methyl-1-octylbut-3-enyl)carbamic Acid Ethyl Ester (16)
Yield: 62–69% for two steps; pale yellow oil.
1H NMR (300 MHz, CDCl3): d = 5.66–5.80 (m, 1 H), 5.01–5.07 (m,
2 H), 4.46 (br s, 1 H), 4.01 (q, J = 7.0 Hz, 2 H), 2.24–2.45 (m, 2 H),
1.45–1.68 (m, 2 H), 1.15–1.28 (m, 18 H), 0.84 (t, J = 6.9 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 154.8, 133.9, 118.2, 60.0, 54.7,
43.0, 38.7, 31.8, 29.9, 29.5, 29.2, 24.4, 23.5, 22.6, 14.6, 14.1.
HRMS (ESI+): m/z [MH+] calcd for C16H32NO2: 270.2433; found:
270.2408.
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1975, 1237.
(1-Methyl-1-octylbut-3-enyl)carbamic Acid Benzyl Ester (17)
Yield: 65–68% for two steps; pale yellow oil.
1H NMR (360 MHz, CDCl3): d = 7.27–7.36 (m, 5 H), 5.68–5.78 (m,
1 H), 5.03–5.07 (m, 4 H), 4.58 (br s, 1 H), 2.28–2.46 (m, 2 H), 1.50–
1.68 (m, 2 H), 1.19–1.30 (m, 15 H), 0.87 (t, J = 7.0 Hz, 3 H).
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Chem. 1984, 49, 821. (b) Anilkumar, R.; Chandrasekhar, S.;
Sridhar, M. Tetrahedron Lett. 2000, 41, 5291.
13C NMR (75 MHz, CDCl3): d = 154.5, 136.8, 133.7, 128.4, 128.0,
127.9, 118.4, 65.9, 54.9, 43.0, 38.7, 31.8, 29.9, 29.5, 29.2, 24.3,
23.5, 22.6, 14.1.
Synthesis 2006, No. 21, 3585–3588 © Thieme Stuttgart · New York