Regiochemistry of an ambident cyclic ketene-N,O-acetal nucleophile and its anion toward electrophiles

Article abstract of DOI:10.1016/j.tetlet.2011.06.023

The five-membered cyclic ketene-N,O-acetal, 2-oxazolidin-2-ylidene-1- phenylethanone 1, and its anion 2, formed on deprotonation, are ambident nucleophiles. Compound 1 was synthesized by benzoylation of 2-methyl-2-oxazoline to give a ring-opened N,C,O-tri

Full text of DOI:10.1016/j.tetlet.2011.06.023

Tetrahedron Letters 52 (2011) 4507–4511
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Regiochemistry of an ambident cyclic ketene-N,O-acetal nucleophile
and its anion toward electrophiles
⇑
Yingquan Song , Hondamuni I. De Silva, William P. Henry, Guozhong Ye, Sabornie Chatterjee,
⇑
Charles U. Pittman Jr.
Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The five-membered cyclic ketene-N,O-acetal, 2-oxazolidin-2-ylidene-1-phenylethanone 1, and its anion
2, formed on deprotonation, are ambident nucleophiles. Compound 1 was synthesized by benzoylation
of 2-methyl-2-oxazoline to give a ring-opened N,C,O-trisbenzoylation product, 9, followed by N,O-double
debenzoylation using methanolic KOH. Compound 1 reacted with benzoyl chloride to give N,C,O-tris-
benzoylated 9, and reacted with phenyl chloroformate to give the similar ring-opened carbonic acid 3-
[(2-chloro-ethyl)-phenoxycarbonyl-amino]-3-oxo-1-phenyl-propenyl ester phenyl ester, 13. In contrast,
ambident anion 2 reacted with benzoyl chloride to give the b,b-bisbenzoylated cyclic ketene-N,O-acetal,
16, and reacted with phenyl chloroformate to give the novel heterocycle 3-(2-hydroxy-ethyl)-6-phenyl-
[1,3]oxazine-2,4-dione, 17.
Received 16 March 2011
Revised 2 June 2011
Accepted 7 June 2011
Available online 21 June 2011
Keywords:
Cyclic ketene acetal
Ambident nucleophile
Regiochemistry
Ó 2011 Elsevier Ltd. All rights reserved.
2-Oxazolidin-2-ylidene-1-phenylethanone 1, a five-membered
cyclic ketene-N,O-acetal, and its deprotonated form, anion 2, are
both ambident nucleophiles (Fig. 1). The ring nitrogen, exocyclic
b-carbon and carbonyl oxygen are nucleophilic in both 1 and 2.
Compound 1 has beenused in the syntheses of fused indolederiv-
atives,1 bicyclic pyridones,2 1,3-oxazoheterocycle-fused-[1,2-b]iso-
quino-lin-1(2H)-imines3 and 5H-oxazolo[3,2-a]pyridine deriva
tives.4 These reactions used 1 as an ambident nucleophile which re-
acted with ambident electrophiles to effect ring closures. The regio-
Ph
Ph
Ph
+
E
+
E
β
O
H
O
H
O
H
or
or
or
+
..
..
..
E
N
O
N
O
O
N
O
1
1
1
+
_
O
Ph
Ph
Ph
N
+
E
E
chemistry of
1 reacting with electrophiles having only one
_
β
O
O
nucleophilic site has not been reported.
or
_
N
During studies of cyclic ketene-N,O-acetal chemistry with vari-
ous electrophiles,5 interest grew in the regiochemistry of 1 and 2
reacting with acid chlorides and aryl chloroformates. Preliminary
results are reported here.
+
E
N
O
O
2
2
2
Figure 1. The five-membered cyclic ketene-N,O-acetal 1, and its deprotonated
form, anion 2, are ambident nucleophiles.
Results
Benzoylation of 2-methyl-2-oxazoline and 2-methyl-2-oxazine
Ph
S
Ph
Compound 1 was previously made from ketene dithioacetal 3
and 2-aminoethanol4 (Eq. 1). Starting material 3, in turn, was pre-
pared from acetophenone with NaH and CS2, followed by methyl-
ation with methyl iodide. In contrast, cyclic ketene-N,O-acetal 4, an
analog of 1 with two methyl groups at C-4, was
Na
THF
O
O
OH
H2N
+
ð1Þ
S
HN
O
3
1
synthesized via benzoylation of 2,4,4-trimethyl-2-oxazoline 5 to
give the N,O-bisbenzoylated cyclic ketene-N,O-acetal 6, followed
by N-debenzoylation with KOH/methanol (Eq. 2).6 Thus, an
⇑
Corresponding authors. Tel.: +1 662 325 7616; fax: +1 662 325 7611.
E-mail addresses: yingquan@gmail.com (Y. Song), cpittman@chemistry.mssta-
te.edu (C.U. Pittman Jr.).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.023

4508
Y. Song et al. / Tetrahedron Letters 52 (2011) 4507–4511
O
O
The combination of Eqs. 3 and 5 provides an easier and safer
route to 1 than from ketene dithioacetal and 2-aminoethanol (Eq.
1). With 1 available, nucleophilic reactions of 1 and its deproto-
nated form, anion 2, with the electrophiles, benzoyl chloride and
phenyl chloroformate, were studied.
Ph
O
Et3N
N
O
+
2 PhCOCl
Ph
N
CH3CN, reflux
5
Ph
O
6
ð2Þ
Benzoylation of 1
1) KOH, methanol, rt
+
2) H
HN
O
2-Oxazolidin-2-ylidene-1-phenylethanone
1 produced only
trisbenzoylated ring-opened product 9 (Eq. 7) when reacted with
benzoyl chloride in refluxing THF in the presence of Et3N. Thus, 1
equiv of benzoyl chloride, diluted in THF, was added over 5 h to
1/THF at 22–23 °C, in an attempt to generate N,C-bisbenzoylated
7 by kinetically favored N-benzoylation (Eq. 7) without further
reaction occuring to produce 9. However, 7 was still not formed.
A 2% yield of 9 was obtained and most of the starting material 1
was recovered.
4
alternative route to 1 would be N-debenzoylation of the bisbenzoy-
lated cyclic ketene-N,O-acetal 7 (Eq. 3), an analog of 6. 7 might form
by the N,O-bisbenzoylation of 2-methyl-2-oxazoline, 8. However,
repeated attempts to generate 7 by this route failed (Table 1). Sur-
prisingly, N,O,C-trisbenzoylation of 2-methyl-2-oxazoline occurred
instead, followed by ring-opening, giving only the vinyl ester imide,
9. Formation of 9 occurred employing only 2 equiv of benzoyl chlo-
ride in either CH3CN or THF. This same reaction pattern was followed
in benzoylation of 2-methyl-2-oxazine, 10, which produced only the
analogous ring-opened vinyl ester imide 11 where the crystal struc-
ture showed the (Z) geometrical isomer was made (Eq. 4).
O
Ph
Ph
O
O
2.1 eq.PhCOCl, Et3N
THF, 68oC
O
Ph
HN
O
Ph
N
O
O
1
Cl
9
ð7Þ
O
1) KOH/MeOH, rt
2) H2SO4
Ph
PhCOCl, Et3N
O
O
N
O
68%
Ph
CH3CN or THF
reflux
N
O
O
Ph
1.0 eq.PhCOCl, Et3N
THF, 22-23oC
8
7
Ph
9
1
N
+
+
O
76%
ð3Þ
ð4Þ
2%
Ph
O
recovered
7
0%
Ph
O
PhCOCl, Et3N
CH3CN or THF
reflux
O
Ph
HN
O
Ph
N
O
Reaction of 1 with phenyl chloroformate
1
9
Cl
Compound 1 did not react with phenyl chloroformate in
CH3CN/Et3N after 7.5 h at 22–23 °C. No new products were
detected by TLC analysis. After refluxing for 5 h, however,
ring-opened carbonic acid 3-[(2-chloro-ethyl)-phenoxycarbonyl-
amino]-3-oxo-1-phenyl-propenyl ester phenyl ester, 13, was iso-
lated and purified in 65% yield (Eq. 8). Two equivalents of phenyl
chloroformate were consumed generating 13, as was the case
during bisbenzoylation of 1 to form 9 (Eq. 7) with the stronger
benzoyl chloride electrophile. No N-phenoxycarbonylation prod-
uct 14 or C-phenoxycarbonylation product 15 were detected.
O
Ph
O
Et3N
THF, reflux
O
Ph
O
N
O
+ 2.1 eq. PhCOCl
Ph
N
10
Cl
11
37% yield
Preparation of cyclic ketene-N,O-acetal 1
O
Ph
Interestingly, 1 was prepared directly from 9. Subjecting ring-
opened 9 to methanolic KOH gave a 44% isolated yield of 1 (Eq.
5), the product originally sought through 7. However, subjecting
11 to methanolic KOH gave an unidentified product (Eq. 6) rather
than 12, the six-membered ring analog of 1.
PhO
O
O
O
2.4 eq.PhOC(O)Cl, Et3N
Ph
HN
O
7.5h rt + 5h reflux in CH3CN
PhO
O
N
O
65%
Ph
1
ð8Þ
Cl
13
O
Ph
Ph
Ph
O
O
O
OPh
O
H
PhO
KOH
O
Ph
or
ð5Þ
ð6Þ
N
O
14
HN
O
MeOH
O
N
O
Ph
Ph
15
N
O
1
44%
9
Cl
Benzoylation of ambident anion 2
Ph
O
Ph
O
O
Ph
O
Ambident anion 2, obtained by deprotonation of 1 by sodium
hydride in THF, was reacted with 1 equiv of benzoyl chloride to ex-
plore its regiochemistry. Mono-benzoylation occurred at the exo-
KOH
MeOH
H
N
O
N
O
Cl
cyclic b-carbon, giving the b,b-bisbenzoylated 16, in
a 52%
11
12
isolated yield (Eq. 9).

Y. Song et al. / Tetrahedron Letters 52 (2011) 4507–4511
4509
Table 1
Benzoylation of 2-methyl-2-oxazoline 8 and 2-methyl-2-oxazine 10a
O
O
O
O
Ph
Ph
PhCOCl
PhCOCl, Et3N
THF, reflux
Entry
Substrate
Equiv of PhCOCl
Solvent
Product
Yieldb (%)
N
O
Ph
O
O
N
Ph
1
2
8
8
8
10
2.2
2.2
3.3
2.1
CH3CN
THF
THF
9
9
9
11
60c
O
56c,d
60e
52c
8
Ph
N
O
7
3
Ph
4f
THF
_
18
Cl
O
Ph
a
Reactions were run in refluxing THF or CH3CN for 5 h. Et3N was used as the acid
Ph
scavenger. Column chromatography was used for purification (stationary phase:
silica gel, eluting solvent: acetone/hexanes or ethyl acetate/hexanes).
N
O
O
b
9
Cl
Isolated/purified yield.
Yield was based on PhCOCl (the limiting reagent).
A 89% yield (based on PhCOCl, the limiting reagent) was obtained using 1.80 g
O
Ph
c
d
O
Ph
O
Ph
PhCOCl
PhCOCl, Et3N
THF, reflux
N
O
of 8 and 2.3 equiv PhCOCl, freshly distilled from SOCl2, and flame-dried glassware.
Ph
Ph
N
O
e
N
O
Yield was based on 8 (the limiting reagent).
Reaction was run for 4.5 h.
f
19
5
_
6
Cl
Ph
O
O
O
Ph
Scheme 1. A mechanism for benzoylation of 2-methyl-2-oxazoline to N,C,O-
trisbenzoylated, ring-opened compound 9.
O
Ph
Ph
O
1.1 eq. PhCOCl
3.3 eq. NaH
THF
O
ð9Þ
_
N
O
HN
HN
Likewise, benzoylation of 2-methyl-2-oxazine 10 gave 11 (Eq. 4).
In contrast, benzoylation of 20 gave ring-retained N,C-bisbenzoyla-
tion product 21 (Eq. 11).6 This substituent effect has been observed
elsewhere.5l
52%
2
16
1
Reaction of phenyl chloroformate with ambident anion 2
O
Compound 2 was also reacted with 1 equiv of the weaker elec-
trophile, phenyl chloroformate in THF at room temperature (Eq.
10). Surprisingly, the expected C-alkoxycarbonylation adduct 15,
an analog of 16 (Eq. 9), was not isolated. Instead, the completely
unforseen 3-(2-hydroxy-ethyl)-6-phenyl-[1,3]oxazine-2,4-dione,
17, was formed in 25% yield. In addition, the starting material 1
was recovered in 12% yield.
O
Ph
Et3N
N
O
+ 2 PhCOCl
ð11Þ
Ph
N
O
CH3CN, reflux
11% yield
20
21
The reaction of N,C,O-trisbenzoylated, ring-opened 9 with
methanolic KOH reclosed the ring and formed cyclic ketene-N,O-
acetal 1. O-debenzoylation by hydroxide likely triggers a subsequent
intramolecular SN2 attack by an imide carbonyl oxygen on the meth-
ylene carbon bearing chlorine, giving rise to 77 (Scheme 2). N-
Debenzoylation then leads to cyclic ketene-N,O-acetal 1. Alterna-
tively, N-debenzoylation might occur prior to O-debenzoylation,
similar to the preparation of 2-oxazolines by dehydrohalogenation
of b-haloalkylamides with aqueous or alcoholic alkali.8
O
Ph
Ph
Ph
O
O
OPh
O
THF
rt
PhOC(O)Cl
+ NaH
_
N
HN
O
O
HN
O
PhOC(O)Cl
Ph
15
not isolated
2
1
ð10Þ
O
Ph
O
O
N
O
Ph
O
O
O
H
+ PhCOOH + KCl
17
+
H
O
Ph
25%
KOH
OH
N
O
1
MeOH
Ph
N
+
H
Discussion
9
Cl
O
_
O
O-debenzoylation
by KOH
Benzoylation of 2-methyl-2-oxazoline to N,C,O-trisbenzoylated,
ring-opened 9 presumably arises from N,C-bisbenzoylated com-
pound 7 (Scheme 1). The ketone oxygen of 7 may subsequently
have been O-benzoylated to form cation 18, followed by chloride
nucleophilic attack at C-5 to open the ring. O-Benzoylation would
be promoted by strong electron donation in 7 from the ring oxygen
and nitrogen atoms making this keto oxygen more negative and
nucleophilic.
The contrasting benzoylation behavior of 2-methyl-2-oxazoline
to 9 versus 2,4,4-trimethyl-2-oxazoline to 6 (Eq. 2 vs 3) highlights
the importance of the C-4 methyl substituents on these reaction
paths. These two methyl groups stop 2,4,4-trimethyl-2-oxazoline
benzoylation at the N,C-bisbenzoylation stage. Even if a third ben-
zoylation does occur at the ketone oxygen to form a 1,3-oxazolin-
ium cation 19, analogous to 18 (Scheme 1), the two C-4 methyl
groups hinder the chloride attack at C-5, so ring opening cannot oc-
cur. Thus, O-benzoylation will be reversible and 6 is the isolated
product.
Ph
OH
N
_
OH
O
O
Ph
Ph
O
O
O
Ph
Ph
O
O
_
OH
Ph
N
O
Ph
Cl
_
N
O
N-debenzoylation
by KOH
O
Ph
O
OH
O
Ph
Ph
O
O
Ph
N
O
Ph
N
Cl
7
Scheme 2. A suggested route from 9 to 1.

4510
Y. Song et al. / Tetrahedron Letters 52 (2011) 4507–4511
Ph
Benzoylation of 1 was slow at ambient temperature and formed
O
O
only 2% of trisbenzoylated, ring-opened 9. Most of 1 was recovered.
N,C-Bisbenzoylated 7 may have formed as an intermediate which
rapidly reacted with more benzoyl chloride to form 9 (Eq. 7).
Benzoylation of 2 is totally different than benzoylation of 1
(Scheme 3). The b-carbon site of 2 reacted with benzoyl chloride
to give adduct 22. Using excess NaH ensures removal of the acidic
b-proton of 22 to form stable anion 23. This is similar to the Claisen
condensation where a stoichiometric amount of base drives the
reaction to completion. Excess NaH also reduced the possibility
O
Ph
O
N-benzoylation
N of 2 attacks
amide carbonyl of 7
route a
_
7
+ 2
N
O
Ph
N
2
7
O
O
O
O
+
_
Ph
Ph
Ph
Ph
β-C of 2 attacks
H
7
2 +
_
amide carbonyl of 7
route b
N
O
N
O
of 22 being deprotonated by 2, which would regenerate
(Scheme 3). Aqueous workup transforms 23 to 16.
1
22
23
Scheme 4. The reason why N,C-dibenzoylated 7 is not observed.
N,C-Dibenzoylation product
7 could have formed via N-
benzoylation of 2 (Scheme 4). However, 7 could further react with
2 in two ways. The nitrogen of 2 could attack the amide carbonyl
carbon of 7 (route a in Scheme 4), regenerating 7 and 2 with no
net change. However, if the b-carbon of 2 attacks 7 (route b),
b,b-dibenzoyl product, 22, will form. Subsequent deprotonation
of 22 by sodium hydride gives stable anion 23, which gives 16 on
workup. This accounts for why benzoylation of 2 appears to occur
at the softer carbon site rather than the harder nitrogen site,
despite the fact that benzoyl chloride is a harder electrophile than
methyl iodide. For comparison, methyl iodide reacted with 4, the
C-4 dimethylated analog of 1, on the b-carbon.6
Ph
O
Ph
PhO
O
NEt3
Cl
O
PhO
PhOC(O)Cl, Et3N
CH3CN, reflux
N
O
HN
O
O
O
14
1
O
O
O
OPh
Ph
O
OPh
Ph
O
Reaction of 1 with phenyl chloroformate gave ring-opened
product 13 (Scheme 5). This route is similar to formation of 9
(Eq. 7). Nucleophilic attack by 1 on phenyl chloroformate probably
occurs via the ring nitrogen, giving rise to intermediate 14 (similar
to 7 in Eq. 7). Subsequent b-keto oxygen phenoxycarbonylation
leads to cation 28, which is ring-opened by chloride attack. This
ring-opening finds analogy in Scheme 1.
PhO
N
O
PhO
N
OH
Cl
13
28
Cl
Scheme 5. A suggested route to 13.
In contrast to the conversion of 1 to 9 (Eq. 7), 2 reacted with
phenyl chloroformate generating the substituted [1,3]-oxazine-
2,4-dione 17 (Scheme 6 and Eq. 10) instead of the expected
b,b-disubstituted cyclic ketene-N,O-acetal 15 from C-phenoxycarb-
onylation. Phenyl chloroformate, a harder electrophile than iodo-
methane, reacted with anion 2 at its nitrogen, which is a harder
nucleophilic site than its b-carbon. This produces 14. Compound
14 is expected to undergo an intramolecular nucleophilic attack
of the carbonyl oxygen on the carbamate carbonyl carbon to form
intermediate 29. Compound 29 loses phenoxide to generate the
fused ring oxazolinium cation 30 rather than cleaving the bond
to the ring nitrogen to generate the oxazoline 31.9 Hydrolysis of
cation 30 provides ring-opened product 17.
Ph
Ph
N
Ph
O
O
O
PhOC(O)Cl
PhO
+ NaH
_
N
O
O
HN
O
O
14
2
1
Ph
Ph
Ph
O
O
O
+
PhO
OH2
OH2
+
+
_
O
O
N
O
N
O
N
O
O
29
30
_
+
+
+
H ,
H
The differences when reacting 1 versus 2 with phenyl chlorofor-
mate may be due to differences in reaction conditions rather than
intrinsic reactivity differences. Intermediate 14, formed during
reaction of 1 with phenyl chloroformate, in the presence of Et3N
(Scheme 5), might have undergone b-keto oxygen phenoxycarb-
Ph
Ph
N
Ph
O
O
O
OH
+
O
O
N
PhO
O
O
O
N
OH
17
31
OH
Scheme 6. A suggested route to 17.
Ph
N
Ph
Ph
23
1
+
_
O
O
O
_
onylation faster than intramolecular nucleophilic attack of the ke-
tone’s oxygen on the carbamate carbonyl carbon to form
oxazolinium cation 29 (Scheme 6). Et3N may have activated phenyl
chloroformate for b-keto oxygen phenoxycarbonylation via nucle-
ophilic catalysis.10 Intermediate 14, formed by reaction of 2 with
phenyl chloroformate, in the absence of Et3N, simply undergoes
intramolecular nucleophilic attack to form 17, since keto oxygen
attack on unactivated phenyl chloroformate would be slower.
Similar [1,3]-oxazine-2,4-dione compounds have been synthe-
sized from b-oxo-esters.11 [1,3]-Oxazine derivatives oxazinomy-
cin and minimycin are reported to have antibiotic activity.11
Therefore, the preparation of [1,3]-oxazine-2,4-diones from
NaH
THF
O
N
O
Ph
HN
O
2
1
O
PhCOCl
O
_
N
O
O
O
O
O
O
O
NaH
Ph
H
2
workup
Ph
Ph
Ph
Ph
Ph
_
N
O
N
O
HN
23
16
22
Scheme 3. Benzoylation of ambident anion 2 gave 16.

Y. Song et al. / Tetrahedron Letters 52 (2011) 4507–4511
4511
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A.; Moses, N.; Pittman, C. U., Jr. Tetrahedron 2006, 62, 4093–4102; (g) Ye, G.;
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2-methyl-2-oxazoline via cyclic ketene-N,O-acetal chemistry may
have synthetical potential.
Acknowledgments
The authors acknowledge the educational and general funds of
Mississippi State University for financial support of this work.
Supplementary data
Complete experimental synthetic descriptions and character-
izations of all the compounds described here, including crystal
structures of 1, 11 and 17, can be found in the online version, at
doi:10.1016/j.tetlet.2011.06.023.
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