The reaction of 3,4-dihydro-2H-pyran with oxalyl chloride: Formation and crystal structure analysis of an unexpected bicyclic product

Article abstract of DOI:10.1002/jhet.456

(Chemical Equation Presented) 3,4-Dihydro-2-H-pyran and oxalyl chloride react, depending on the conditions, to keto esters, a pyran-3-carboxylic acid or derivatives thereof, or to an hitherto unknown bicyclic acetal containing a vinyl chloride moiety. The structure of the latter product has been unambiguously elucidated by single-crystal X-ray structure analysis. A mechanism for its formation is proposed.

Full text of DOI:10.1002/jhet.456

September 2010
The Reaction of 3,4-Dihydro-2H-Pyran with Oxalyl Chloride:
Formation and Crystal Structure Analysis of an Unexpected
Bicyclic Product
1171
Bernd Schmidt,a* Frank Werner,a Alexandra Kelling,b and Uwe Schildeb
a
¨
Institut fu¨r Chemie–Organische Synthesechemie, Universitat Potsdam, Karl-Liebknecht-Straße
24-25, Haus 25, D-14476 Golm
b
¨
Institut fu¨r Chemie–Anorganische Chemie, Universitat Potsdam, Karl-Liebknecht-Straße
24-25, Haus 26, D-14476 Golm
*E-mail: bernd.schmidt@uni-potsdam.de
Received November 24, 2009
DOI 10.1002/jhet.456
Published online 26 July 2010 in Wiley Online Library (wileyonlinelibrary.com).
3,4-Dihydro-2-H-pyran and oxalyl chloride react, depending on the conditions, to keto esters, a
pyran-3-carboxylic acid or derivatives thereof, or to an hitherto unknown bicyclic acetal containing
a vinyl chloride moiety. The structure of the latter product has been unambiguously elucidated by
single-crystal X-ray structure analysis. A mechanism for its formation is proposed.
J. Heterocyclic Chem., 47, 1171 (2010).
INTRODUCTION
indeed conveniently accessible via basic hydrolysis of
the trichloromethyl ketone 2, less satisfactory results
were obtained for the methanolysis. However, the ester
3b is more conveniently synthesized by converting 3a to
its chloride, which is subsequently treated with metha-
nol. We thought that the strongly basic conditions used
for the conversion of 2 to 3a and the additional steps
required to obtain the ester 3b are disadvantageous and
therefore sought for a more straightforward alternative.
Despite their structural simplicity, tetrahydropyran-3-
carboxylic acid and its esters display interesting biologi-
cal activities as attractants for cockroaches [1–3]. One
approach toward these compounds proceeds via hydro-
genation of dihydropyran-3-carboxylic acid derivatives
3, which are in turn available by trichloroacetoxylation
of dihydropyran (1) and subsequent methanolysis or
basic hydrolysis of the intermediate trichloromethyl
ketones 2, as outlined in Scheme 1 [4–6]. A Cobalt-cata-
lyzed methoxycarbonylation of vinyl bromides has also
been investigated, but is less commonly used [7].
RESULTS AND DISCUSSION
5,6-Dihydro-4H-pyran-3-carboxylic acid (3a) has also
been used as an intermediate in the synthesis of pharma-
cologically active heterocycles, for example, pyrimidines
with activity against cancer [8], anti-inflammatory pep-
tide conjugates [9,10], or fungicides [11]. The corre-
sponding carboxaldehyde is a useful intermediate in the
synthesis of renin inhibitors [12] and anti-inflammatory
agents [13].
The reaction of alkyl vinyl ethers with oxalyl chloride
was investigated by Effenberger. Thus, by adjusting the
appropriate stoichiometry, symmetrical 1,2-diketones
were synthesized in good yields [14]. Later, Tietze et al.
demonstrated that the intermediate a-keto acid chlorides
undergo a clean decarbonylation at temperatures above
100ꢀC to give acyclic E-3-alkoxy acryloyl chlorides
[15], which are useful intermediates in the synthesis of
heterocycles [16]. This approach should also be applica-
ble to dihydropyran-3-carboxylic acid (3a) and its deriv-
atives, with dihydropyran (1). In contrast to the estab-
lished two- to four-step route depicted in Scheme 1, this
In light of the relevance of dihydropyran-3-carboxylic
acid and its derivatives, in particular for the synthesis of
heterocycles, we investigated the route described in
Scheme 1 in detail. While the carboxylic acid 3a is
C
V
2010 HeteroCorporation

1172
B. Schmidt, F. Werner, A. Kelling, and U. Schilde
Vol 47
Scheme 1. Established routes to tetra- and dihydropyran-3-carboxylic
acids.
Table 1
ꢀ
˚
Selected bond lengths (A) and bond angles ( ) for 5.
Cl1A–C2A
O1A–C1A
O1A–C7A
O2A–C1A
O3A–C6A
O3A–C7A
C3A–C7A
C2A–C3A
1.704(2)
1.362(3)
1.433(3)
1.201(3)
1.450(4)
1.396(3)
1.492(3)
1.320(4)
Cl1B–C2B
O1B–C1B
O1B–C7B
O2B–C1B
O3B–C6B
O3B–C7B
C3B–C7B
C2B–C3B
1.709(2)
1.362(3)
1.425(3)
1.197(3)
1.456(3)
1.391(3)
1.501(3)
1.314(4)
Cl1A–C2A–C1A 120.62(19) Cl1B–C2B–C1B 120.77(19)
O1A–C1A–O2A
O1A–C1A–C2A
C1A–C2A–C3A
C2A–C3A–C4A
C6A–O3A–C7A
O1A–C7A–C3A
122.2(2)
107.6(2)
110.1(2)
134.6(2)
109.1(2)
105.9(2)
O1B–C1B–O2B
O1B–C1B–C2B
C1B–C2B–C3B
C2B–C3B–C4B
C6B–O3B–C7B
121.8(2)
107.8(2)
110.0(2)
135.7(2)
108.7(2)
synthesis might potentially be conducted as a one-pot
reaction. A literature search revealed that the reaction of
dihydropyran (1) with oxalyl chloride followed by treat-
ment with methanol gives exclusively the a-keto ester 4,
which was used as a substrate in hetero-Diels-Alder
reactions [17,18]. However, this transformation was not
documented in full detail in the literature, and we there-
fore decided to start at this point. We could indeed
obtain 4 in fair yield if 1 was treated with 1.5 equiva-
lents of oxalyl chloride at low-temperature, followed by
the addition of methanol. Next, we tried to synthesize
methyl ester 3b by inducing a decarbonylation before
methanolysis. To this end, the reaction mixture was
heated to 120ꢀC, after the addition of the oxalyl chlo-
ride. To our surprise, the desired ester 3b was only
obtained as a minor product. The major product showed
an Mþ signal at m/z ¼ 174, with the characteristic iso-
topic pattern for compounds with one chlorine atom. In
the IR spectrum absorptions at 1793 cmꢁ1 and 1771
cmꢁ1 suggested the presence of a lactone moiety. On
the basis of the information gathered from NMR- and
mass spectra, a molecular formula of C7H7O3Cl was
O1B–C7B–C3B 105.62(19)
observed. The latter was interpreted as a cyclic acetal
carbon. The proton that is attached to this carbon atom
was observed at 5.59 ppm as a singlet. Notably, no
methanol was incorporated in the molecule. From this
information, we tentatively assigned the bicyclic struc-
ture 5 to the new compound (Scheme 2).
Unambiguous proof for this structural assignment
came from a single crystal X-ray structure analysis.
Compound 5 has the structure of a bicyclic acetal. One
chlorine atom is attached to a C-C-double bond, which
is in conjugation to the carbonyl group. As the com-
pound crystallizes in the centrosymmetric space group
P1, both enantiomers are found in the cell. Two mole-
cules are present in the asymmetric unit, which are sym-
metry independent and therefore show slight differences
in their structural parameters. Representative bond
lengths and angles for both symmetry independent mole-
cules (denoted as A and B) are listed in Table 1. A rep-
resentation of both symmetry-independent molecules is
given in Figure 1.
1
deduced. From the H-NMR spectra, it became obvious
that the six-membered oxacycle was still intact and that
the protons of the CH2-groups are no longer chemically
equivalent, but are diastereotopic. This is indicative for
the formation of a new stereogenic center in the course
of the reaction. In the 13C-NMR-spectrum two signals
arising from quaternary olefinic carbon atoms at 166
ppm and 118 ppm, and a signal at 98 ppm were
Scheme 2. Formation of a-keto ester 4 and the unexpected bicyclic
acetal product 5 from dihydropyran.
Figure 1. Molecular structure of compound 5 with thermal ellipsoids
drawn at the 50% probability level.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

September 2010
The Reaction of 3,4-Dihydro-2H-Pyran with Oxalyl Chloride: Formation
and Crystal Structure Analysis of an Unexpected Bicyclic Product
1173
Scheme 4. Formation of bicyclic product 5 from keto ester 4.
philic attack of the carboxylate at the anomeric carbon,
resulting in the product 5 (Scheme 3).
It should be noted that the crucial step of this
sequence, the formation of the vinyl chloride moiety
from a ketone, is not without precedence. A somewhat
related reaction was reported for 7-keto steroids, which
react with oxalyl chloride in refluxing toluene to vinylic
7-chloro steroids [28]. We do not have definite evidence
that the individual steps leading to the formation of
5 indeed proceed in this order. It is, however, quite
unlikely that the chlorination of the ketone moiety
occurs after methanolysis of the acid chloride, because a
considerable amount of the oxalyl chloride would also
be destroyed. Strong support for the final demethylation
step proposed by us comes from an experiment, in
which pure keto ester 4 was treated with oxalyl chloride.
After heating the mixture to 120ꢀC, an 80% conversion
to the expected bicyclic product 5 was observed by
NMR-spectroscopy of the crude reaction mixture. The
only other component that could be detected was
unreacted starting material 4 (Scheme 4).
From the results discussed above, we concluded that
the selective formation of the desired dihydropyran-3-
carboxylic acid derivatives 3 requires careful removal of
excess oxalyl chloride before the reaction mixture is
heated to induce the decarbonylation. This was achieved
by applying vacuum at moderately high-temperatures
before the mixture is heated to 120ꢀC to induce the
decarbonylation. After completion of the decarbonyla-
tion step, different nucleophiles were added to obtain
the desired derivatives 3. Thus, with an aqueous solution
of Na2CO3 the carboxylic acid 3a was obtained,
whereas addition of methanol and pyridine gave the
ester 3b. By using diisopropyl amine as a nucleophile,
Figure 2. Structures of spicatolide A and myxostiolide.
A literature search revealed, that there is ample prece-
dence for bicyclic saturated c-butyrolactones annellated
to six-membered oxacycles. In most cases, these products
result from an addition of 1,3-dicarbonyl compounds to
cyclic enol ethers via a radical pathway [19–25]. In con-
trast, only comparatively few 3,4-unsaturated bicyclic
c-butyrolactones such as 5 have been described in the
literature. Interestingly, this structural pattern is found in
some biologically active natural products such as the
anti-inflammatory germacranolide spicatolide A [26] or
the plant-growth regulator myxostiolide (Fig. 2) [27].
Elucidation of the mechanism for the formation of 5
is hampered by a lack of isolable intermediates. We pro-
pose the following tentative mechanism: in the first step,
oxalyl chloride will most likely attack at carbon atom
C3 to give the a-keto acid chloride 6. We assume that
now excess oxalyl chloride reacts with one carbonyl
oxygen of 6 to give the intermediate vinyl ester 7. Upon
heating, 7 undergoes a fragmentation to CO, CO2, and a
vinyl chloride 8, which is eventually trapped by metha-
nol in the presence of pyridine. Most likely, the
sequence is terminated by demethylation of the methyl
ester 9 with a chloride ion and intramolecular nucleo-
Scheme 3. Mechanistic proposal for the formation of the unexpected
bicyclic acetal 5.
Scheme 5. Synthesis of dihydropyran-3-carboxylic acid derivatives 3
via decarbonylation of a-keto acid chlorides.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

1174
B. Schmidt, F. Werner, A. Kelling, and U. Schilde
Vol 47
less solid, mp 49ꢀC. 1H-NMR (300 MHz, CDCl3): d 5.58
(s, 1H, H-7a), 4.10 (dddm, 1H, J ¼ 1.9, 4.1, 12.2 Hz, H-6),
3.76 (dt, 1H, J ¼ 2.4, 12.2 Hz, H-6), 2.98 (dm, 1H, J ¼ 14.5
Hz, H-4), 2.47 (ddm, 1H, J ¼ 6.4, 14.5 Hz, H-4), 1.95 (m,
1H, H-5), 1.77 (ddm, 1H, J ¼ 4.8, 12.4 Hz, H-5); 13C-NMR
(75 MHz, CDCl3): d 165.6 (C-2), 156.5 (C-3a), 118.4 (C-3),
98.1 (C-7a), 65.3 (C-6), 25.7, 23.9 (C-4, C-5); IR: 1793, 1771,
1086, 990 cmꢁ1; MS: m/z 176/174 (Mþ), 145/147; Anal. calcd.
for C7H7ClO3: C, 48.2; H, 4.0. Found: C, 47.9; H, 4.1.
Single crystal X-ray structure determination of 5. Suitable
crystals were obtained by dissolving a sample of 5 in dichloro-
methane and slowly evaporating the solvent at 4ꢀC in an open
vessel. Single-crystal diffraction data were measured at 210 K
on an imaging plate diffraction system IPDS-II (Stoe) using
the expected amide 3c was obtained in nearly quantita-
tive yield (Scheme 5).
CONCLUSION
The reaction of oxalyl chloride with dihydropyran,
followed by thermal decarbonylation and trapping with
an appropriate nucleophile, is a useful alternative syn-
thesis of dihydropyran-3-carboxylic acid derivatives. An
unexpected reaction pathway was observed in the course
of this synthesis, leading to an interesting bicyclic vinyl
chloride. The structure of this product was unambigu-
ously elucidated by X-ray crystal structure analysis.
˚
graphite-monochromated Mo-Ka radiation (k ¼ 0.71073 A).
One hundred eighty frames were collected with x scan widths
of 0.5ꢀ and 3 min exposure times. The data were corrected by a
spherical absorption correction using the program X-Area [30]
as well as for Lorentz, polarization and extinction effects. Crys-
tal data: C7H7ClO3, Mr ¼ 174.58, space group P1, a ¼
EXPERIMENTAL
All reactions were run under an atmosphere of dry nitrogen
in dried glassware. Commercial reagents were used as
received. Methanol was obtained dried and degassed in a sep-
˚
˚
˚
8.4324(16) A, b ¼ 8.6447(17) A, c ¼ 10.8716(19) A, a ¼
77.477(15)ꢀ, b ¼ 78.056(15)ꢀ, c ¼ 76.460(15)ꢀ, V ¼ 741.9(2)
1
tum bottle under argon and used as received. H-NMR-spectra
A , Z ¼ 4, dcalc ¼ 1.563 gꢂcmꢁ3, colorless block, 0.55 ꢃ 0.31
3
˚
were obtained at 300 MHz in CDCl3 with CHCl3 (d ¼ 7.26
ppm) as an internal standard. Coupling constants are given in
Hz. 13C-NMR spectra were recorded at 75 MHz in CDCl3
with CDCl3 (d ¼ 77.0 ppm) as an internal standard. IR spectra
were recorded as films on NaCl or KBr plates or as KBr-discs.
The peak intensities are defined as strong (s), medium (m), or
weak (w). Mass spectra were obtained at 70 eV.
ꢃ 0.19 mm, l ¼ 0.464 mmꢁ1, 4819 total reflections (2ymax
¼
50.00ꢀ), 2452 independent (Rint ¼ 0.0571), 1854 observed [I <
2r(I)], 256 parameters. Final R1 [I < 2r(I)] ¼ 0.0393, wR2
(all data) ¼ 0.0947, S ¼ 0.955, largest difference peak and hole
3
˚
0.272 and ꢁ0.220 eꢂA . The structure was solved by direct
methods using the SHELXS-97 [31] program and refined by
full-matrix least-squares of F2 using the program SHELXL-97
[32]. All nonhydrogen atoms were refined with anisotropic dis-
placement parameters. The hydrogen atoms were located in a
difference Fourier map. CCDC 755365 contains the supplemen-
tary crystallographic data for this compound. These data can be
obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Methyl 2-(5,6-dihydro-4H-pyran-3-yl)-2-oxoacetate
(4). Oxalyl chloride (2.20 mL, 24.9 mmol) was cooled to
ꢁ10ꢀC. 3,4-Dihydro-2H-pyran (1, 1.50 mL, 16.6 mmol) was
slowly added, and the mixture was warmed to ambient temper-
ature. Stirring at ambient temperature was continued for 12 h,
and the mixture was then recooled to 0ꢀC. Triethyl amine
(4.60 mL, 33.2 mmol) followed by methanol (1.40 mL, 33.2
mmol) were slowly added. Water was added to the reaction
mixture, which was then extracted with dichloromethane, dried
with Na2SO4, filtered, and evaporated. The residue was puri-
fied by chromatography on silica (eluent hexane/MTBE mix-
tures of increasing polarity) to give the title compound 4 (1.32
g, 47%) as a colourless oil. Analytical data match those
5,6-Dihydro-4H-pyran-3-carboxylic acid (3a). Oxalyl
chloride (3.60 mL, 41.5 mmol) was cooled to 0ꢀC, and 3,4-
dihydro-2H-pyran (1, 2.50 mL, 27.7 mmol) was added. The
solution was slowly warmed to ambient temperature and stir-
ring was continued for 1 h. Excess oxalyl chloride was evapo-
rated in vacuo (10 mbar) at 30ꢀC. The mixture was then
heated to 120ꢀC for 0.5 h, cooled to ambient temperature, and
poured into an ice-cold aqueous solution of Na2CO3. The alka-
line solution was extracted with dichloromethane, and then
acidified with hydrochloric acid (6 M). The aqueous layer was
extracted with dichloromethane, and the organic solution was
dried with MgSO4, filtered, and evaporated to yield the title
compound 3a (2.76 g, 78%) as a colourless solid, mp 72–
74ꢀC. Analytical data match those reported in the literature
[5]. 1H-NMR (300 MHz, CDCl3): d 10.42 (bs, 1H, COOH),
7.69 (s, 1H, H-2), 4.07 (t, 2H, J ¼ 5.2 Hz, H-6), 2.23 (dt, 2H,
J ¼ 6.5, 1.2 Hz, H-4), 1.86 (m, 2H, H-5); 13C-NMR (75 MHz,
CDCl3): d 173.4 (C¼¼O), 157.4 (C-2), 105.1 (C-3), 66.8 (C-6),
20.9, 18.8 (C-4-5); IR: 1661 (s), 1623 (s), 1431 (s), 1175 (s)
cmꢁ1; MS (EI): m/z 128 (Mþ), 83 (Mþ-CO2H), 55; Anal.
calcd. for C6H8O3: C, 56.3; H, 6.3. Found: C, 56.3; H, 6.2.
Methyl 5,6-dihydro-4H-pyran-3-carboxylate (3b). Oxalyl
chloride (3.60 mL, 41.5 mmol) was cooled to 0ꢀC, and 3,4-
dihydro-2H-pyran (1, 2.50 mL, 27.7 mmol) was added. The so-
lution was slowly warmed to ambient temperature and stirring
1
reported in the literature [29]. H-NMR (500 MHz, CDCl3): d
7.82 (s, 1H, H-2), 4.14 (t, 2H, J ¼ 5.2 Hz, H-6), 3.83 (s, 3H,
OMe), 2.28 (t, 2H, J ¼ 6.2 Hz, H-4), 1.87 (m, 2H, H-5); 13C-
NMR (75 MHz, CDCl3): d 184.3 (O¼¼CACOOMe), 163.9
(COOMe), 163.4 (C-2), 114.1 (C-3), 67.8 (C-6), 52.4 (OMe),
20.5, 17.4 (C-4, C-5); IR: 1731 (s), 1653 (m), 1599 (s), 1172
(s) cmꢁ1; MS (EI): m/z 170 (Mþ), 111 (Mþ-CO2Me), 83.
3-Chloro-5,6-dihydro-4H-furo[2,3-b]pyran-2-(7aH)-one
(5). Oxalyl chloride (2.00 mL, 23.2 mmol) was cooled to
ꢁ10ꢀC. 3,4-Dihydro-2H-pyran (1, 1.40 mL, 15.5 mmol) was
slowly added, and the mixture was warmed to ambient temper-
ature. Stirring at ambient temperature was continued for 12 h.
The solution was then heated to 120ꢀC for 0.5 h, cooled to
ambient temperature, and then to 0ꢀC. Pyridine (1.30 mL) and
methanol (0.7 mL) were added and stirring was continued at
ambient temperature for 2 h. All volatiles were removed in
vacuo, and the residue was purified by chromatography on
silica (eluent hexane/MTBE mixtures of increasing polarity).
The title compound 5 (1.11 g, 42%) was obtained as a colour-
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

September 2010
The Reaction of 3,4-Dihydro-2H-Pyran with Oxalyl Chloride: Formation
and Crystal Structure Analysis of an Unexpected Bicyclic Product
1175
[3] Szori, K.; Szollosi, G.; Bartok, M. N. J Chem 2008, 32,
was continued for 1 h. Excess oxalyl chloride was evaporated
in vacuo (10 mbar) at 30ꢀC. The mixture was then heated to
120ꢀC for 0.5 h, and subsequently cooled to ambient tempera-
ture. A mixture of methanol (2.60 mL, 64.2 mmol) and pyri-
dine (5.00 mL) was added, and the solution was stirred at am-
bient temperature for 12 h. All volatiles were evaporated, and
the residue was purified by chromatography on silica (eluent
hexane/MTBE mixtures of increasing polarity) to give the title
compound 3b (2.52 g, 64%) as a colourless liquid. The com-
pound was found to be sufficiently pure by H-NMR spectros-
copy, although repeated attempts to obtain microanalytical
data within the usual limits were unsuccessful. Analytical data
1
match those reported in the literature [6]. H-NMR (300 MHz,
1354.
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[8] Finlay, M. R. V. Astrazeneca Patent WO 2009007749,
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CDCl3): d 7.52 (s, 1H, H-2), 3.99 (t, 2H, J ¼ 5.3 Hz, H-6),
3.65 (s, 3H, OMe), 2.21 (t, 2H, J ¼ 6.4 Hz, H-4), 1.82 (m,
2H, H-5); 13C-NMR (75 MHz, CDCl3): d 168.1 (C¼¼O), 155.3
(C-2), 105.7 (C-3), 66.5 (C-6), 50.9 (OMe), 21.0, 19.1 (C-4-5);
IR: 1700 (s), 1628 (s), 1261 (s), 1171 (s) cmꢁ1; MS: m/z 142
(Mþ), 111 (Mþ-OMe), 83 (Mþ-CO2Me), 55.
[10] Piccariello, T.; Kirk, R.; Olon, L. P. New River Pharma-
ceuticals Inc., WO 2003101476, 2003.
[11] Plowman, R. E. Imperial Chemical Industries Ltd., DE
2245511, 1973.
[12] Spoors, P. G.; Kallander, L. S.; Claremon, D. A. Vitae
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Matan, T. T.; Kulkarni-Almeida, A.; Balachandran, S.; Junjappa, H.;
Balakrishnan, A.; Vishwakarma, R. A. Bioorg Med Chem 2008, 16,
7167.
N,N-Diisopropyl-5,6-dihydro-4H-pyran-3-carboxamide
(3c). Oxalyl chloride (1.40 mL, 16.6 mmol) was cooled to
0ꢀC, and 3,4-dihydro-2H-pyran (1, 1.00 mL, 11.0 mmol) was
added. The solution was slowly warmed to ambient tempera-
ture and stirring was continued for 1 h. Excess oxalyl chloride
was evaporated in vacuo (10 mbar) at 30ꢀC. The mixture was
then heated to 120ꢀC for 0.5 h, and subsequently cooled to
ambient temperature. A mixture of diisopropyl amine (3.00
mL, 21.2 mmol) and triethyl amine (3.00 mL) was added, and
the solution was stirred at ambient temperature for 12 h. All
volatiles were evaporated, and the residue was purified by
chromatography on silica (eluent hexane/MTBE mixtures of
increasing polarity) to give the title compound 3c (2.25 g,
96%) as a highly viscous oil. The compound was found to be
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1
sufficiently pure by H-NMR spectroscopy. Repeated attempts
to purify the compound by crytallization were unsuccessful.
1H-NMR (300 MHz, CDCl3): d 6.46 (s, 1H, H-2), 3.88 (t, 2H,
J ¼ 5.1 Hz, H-6), 3.73 (sept, 2H, J ¼ 6.8 Hz, CH-Me2), 2.12
(t, 2H, J ¼ 6.2 Hz, H-4), 1.79 (m, 2H, H-5), 1.16 (d, 12 H,
J ¼ 6.8 Hz, Me); 13C-NMR (75 MHz, CDCl3): d 170.3
(C¼¼O), 143.6 (C-2), 111.3 (C-3), 65.3 (C-6), 47.8 (2*CH-
Me2), 21.5, 21.1 (C-4-5), 20.8 (4*Me); IR: 1612 (s), 1434 (s),
1157 (s)1029 (s) cmꢁ1; MS: m/z 211 (Mþ), 111 [Mþ-CO(NPri)2].
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet