Synthesis and spectra of aldehyde- and ketone-O-(2,5-dihydro-2-methoxy-5,5- dimethyl-1,3,4-oxadiazol-2-yl)oximes - Thermal sources of some methoxy(oximino)carbenes

Article abstract of DOI:10.1139/v05-061

Ten new oxadiazoline sources of methoxy(oximino)carbenes were synthesized by exchanging the acetoxy group of 2-acetoxy-2-methoxy-Δ³-1,3, 4-oxadiazoline with an oximino group. The new compounds were characterized by means of spectroscopy and the formation of carbenes upon thermolysis of a few was demonstrated by means of interception with tert-butyl alcohol. The carbenes fragmented to form methoxycarbonyl and iminyl radicals.

Full text of DOI:10.1139/v05-061

443
Synthesis and spectra of aldehyde- and ketone-O-
(2,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-
oxadiazol-2-yl)oximes — Thermal sources of some
methoxy(oximino)carbenes
Arkadiusz Klys, Malgorzata Dawid, and John Warkentin
Abstract: Ten new oxadiazoline sources of methoxy(oximino)carbenes were synthesized by exchanging the acetoxy
group of 2-acetoxy-2-methoxy-∆3-1,3,4-oxadiazoline with an oximino group. The new compounds were characterized by
means of spectroscopy and the formation of carbenes upon thermolysis of a few was demonstrated by means of inter-
ception with tert-butyl alcohol. The carbenes fragmented to form methoxycarbonyl and iminyl radicals.
Key words: 2,2-dioxyoxadiazoline, methoxy(oximino)carbene, oxadiazole, radical pair, rearrangement.
Résumé : On a effectué la synthèse de 10 nouvelles oxadiazolines sources de méthoxy(oximino)carbènes en procédant
à l’échange du groupe acétoxy de la 2-acétoxy-2-méthoxy-∆3-1,3,4-oxadiazoline avec un groupe oximino. On a caracté-
risé les nouveaux composés par spectroscopie et la formation de carbènes par thermolyse de quelques-unes de ces sour-
ces a été mise en évidence par leur interception par l’alcool tert-butylique. Les carbènes se fragmentent avec formation
des radicaux méthoxycarbonyle et iminyle.
Mots clés : 2,2-dioxyoxadiazoline, méthoxy(oximino)carbène, oxadiazole, paire de radicaux, réarrangement.
[Traduit par la Rédaction] Klys et al. 448
Introduction
(Scheme 2), and show that carbenes 3d and 3e are indeed
generated by thermolysis of 2d and 2e. Some potential intra-
molecular reactions of a generalized carbene 3 are shown in
Scheme 3.
Some 2,5-dihydro-5,5-dimethyl-2,2-dioxy-1,3,4-oxadiazoles
(1a–1d) (also known as 5,5-dimethyl-2,2-dioxy-∆3-1,3,4-
oxadiazolines) have been shown to be versatile reagents for
the generation of carbenes including alkoxymethoxy-,
acetoxymethoxy-, alkoxyaryloxy-, and diaryloxycarbenes.
(1) Having shown that an alkyne functional group in the
carbene from 1e (2) and a carbonyl group in the carbene
from 1f (3) can intercept a dialkoxycarbene in an intra-
molecular reaction, leading to interesting products, and that
thiocarbonyl groups are effective interceptors in intermole-
cular reactions (4), we decided to try the oxadiazoline ap-
proach to generate carbenes containing other functional
groups (Scheme 1).
Methods, results, and discussion
Compounds 2 were prepared by the exchange method (5),
from 2-acetoxy-2-methoxy-2,5-dihydro-5,5-dimethyl-1,3,4-
oxadiazole (also known as 2-acetoxy-2-methoxy-5,5-
dimethyl-∆3-1,3,4-oxadiazoline) (4) and the appropriate
oxime (5) in the presence of catalytic p-toluenesulfonic or
trifluoroacetic acid (Scheme 4). All of the oximes used in
this work are known compounds and compounds 2, with the
exception of 2b, were the (E)-isomers. In the case of 2b, a
mixture of (E)- and (Z)-isomers was obtained. They were not
separated and the 13C NMR spectrum of the major isomer
was assigned on the basis of signal intensities.
We now report the preparation of some aldehyde and
ketone-O-(2,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-oxadiazol-
2-yl)oximes (2) (also known as corresponding oxadiazolinyl
oximes) that are precursors of carbenes of type
3
The oximes 5 were prepared by standard methods, includ-
ing the (Z)-oxime of benzaldehyde (6). Exchange with the
latter gave the same product (2c) as that obtained with the
(E)-oxime of benzaldehyde, indicating that the oxime
isomerizes under the acidic exchange conditions or that the
product isomerizes after exchange has occurred (or both). To
date we have been unable to prepare analogues of 2 with a
(Z)-aldoxime unit.
Received 11 November 2004. Published on the NRC
Research Press Web site at http://canjchem.nrc.ca on 6 May
2005.
A. Klys,1 M. Dawid,2 and J. Warkentin.3 Department of
Chemistry, McMaster University, Hamilton, ON L8S 4M1,
Canada.
Structural assignments are based on 1H and 13C NMR
spectra, as well as high-resolution mass spectra of more
complex samples. In some cases, the connectivity was
1Present address: Department of Organic Chemistry, The
University of ºódï, ºódï, Poland.
2Present address: Torcan Chemicals, Aurora, ON L4G 3H4,
Canada.
1
supported with gradient HSQC spectra, to correlate H and
3Corresponding author (e-mail: warkent@mcmaster.ca).
13C NMR chemical shifts. The 13C NMR spectra of 2,2-
Can. J. Chem. 83: 443–448 (2005)
doi: 10.1139/V05-061
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Can. J. Chem. Vol. 83, 2005
Scheme 1.
Scheme 2. For 2a and 3a: R1 = R2 = Me; 2b and 3b: R1 = Et,
R2 = Me (major), R1 = Me, R2 = Et (minor); 2c and 3c: R1 =
Ph, R2 = H; 2d and 3d: R1 = Ph, R2 = Me; 2e and 3e: R1 =
R2 = Ph; 2f and 3f: CR1R2 = 9-fluorenylidene; 2g and 3g:
R1R2 = (PhCH)4; 2h and 3h: R1R2 = (CH2)4; 2i and 3i: R1R2 =
(CH2)5; 2j and 3j: R1 = MeCO, R2 = Me.
Scheme 3.
Scheme 4.
family 2 with unsaturation in the group(s) attached to the
iminyl carbon.
Thermolysis of 2d in benzene containing tert-butyl alco-
hol (1.05 mol/L) gave 6 (35%) and 7 (30%) as major prod-
ucts. Acetophenone azine was also isolated. Capture of a
carbene intermediate by the alcohol would afford 6, while
intramolecular rearrangment, by 1,2-migration of the iminyl
function, or fragmentation to radicals and radical pair cou-
pling, would lead to 7 (Scheme 5). There is precedent for the
capture of carbenes by alcohols (10), and for rearrangement
of acyloxycarbenes (1f, 11), which are models for 3. Con-
certed rearrangement of carbene 3d to 7 need not necessarily
involve a large rate constant, even though rearrangement
competed with bimolecular capture by tert-butyl alcohol.
Capture of dimethoxycarbene by alcohols, in acetonitrile at
room temperature, occurs with pseudo-first-order rate con-
stants lying between about 104 s–1 (ethanol) and 105 s–1
(methanol) (10a). Capture occurs by protonation to form an
ion pair, and would be slowed in benzene, accelerated by the
higher temperature used in this work, and probably slowed
for tert-butyl alcohol, given that ethanol reacts more slowly
than methanol.
dialkoxy-5,5-dimethyl-∆3-1,3,4-oxadiazolines have the C-2
and C-5 signals near 137 and 119 δ, respectively (7). Values
of the 13C NMR chemical shift of iminyl carbons in 25
ketoximes fall between δ = 157.7 and 170.1 (8).
IR spectra were generally not informative enough to pro-
vide strong evidence of structure, with a weak band attrib-
uted to C=N stretching (9) near 1650 cm–1. The carbonyl
frequency of 2j, as well as strong fingerprint bands for a few
of the compounds 2 are included.
To test for the intermediacy of a carbene 3, the potential
precursor 2 was thermolysed in dry benzene, and in benzene
containing tert-butyl alcohol (1.05 mol/L). Compounds cho-
sen for initial study, namely 2d and 2e, are members of the
© 2005 NRC Canada

Klys et al.
445
Scheme 5.
Scheme 6.
Alternatively,
7
could arise by coupling of the
part, by the carbene homolysis – radical combination mecha-
nism.
methoxycarbonyl- and methylphenyliminyl radical pair.
Some dialkoxycarbenes have been shown to fragment to rad-
ical pairs at 110 °C (10d, 12). The fact that acetophenone az-
ine was isolated means that the radical-pair mechanism plays
some role because the azine is almost certainly derived from
coupling of the appropriate iminyl radicals. The fraction of 7
that arises by that route, as opposed to intramolecular rear-
rangement of 3d, is not known at this time.
There isn’t any evidence for or against intramolecular ad-
dition of carbenes 3 to the iminyl functional group to gener-
ate the bicyclic intermediate of Scheme 3. Survival of such
materials at 110 °C is unlikely, and alternative methods for
generating carbenes 3 at lower temperatures will need to be
investigated.
Compound 2e afforded four products (9–12) when ther-
molysed in benzene containing tert-butyl alcohol (1.05 mol/L).
Orthoformate 9 (32%) is the product of capture of carbene
3e with tert-butyl alcohol while compound 10 (12%) is the
carbene dimer and 11 (14%) appears to be the product of a
1,2-migration of the diphenyliminyl group to the carbene
site of 3e, either in a concerted intramolecular rearrangement
or by means of coupling of methoxycarbonyl radicals and
diphenyliminyl radicals (Scheme 6).
Analogous rearrangements, which are known for some
acyloxycarbenes (1f, 11), appear to be concerted, in general.
In the present case, the formation of diphenyliminyl radicals
is strongly implied by the finding of compound 12, which is
most likely the product of coupling of the latter radicals.
This conclusion was reinforced when thermolysis of 2e in
the presence of dimethyl acetylenedicarboxylate (DMAD)
gave a mixture of products from which 13 was isolated. Pre-
sumably, 13 arose from intramolecular, radical aromatic sub-
stitution following addition of the diphenyliminyl radical to
DMAD. Thus, the rearrangement of 3e occurred, at least in
Summary
Ten new oxadiazolines, with a methoxy and an oximino
substituent at C-2, were prepared and characterized. All of
them can be expected to afford methoxy(oximino)carbenes
upon thermolysis in benzene, as demonstrated for two of
them by means of trapping with tert-butyl alcohol. In the ab-
sence of the alcohol a carbene dimer, as well as products
from carbene fragmentation to a radical pair, were isolated
(Scheme 7).
Experimental
IR spectra were taken with a Bruker Tensor 27 FT-IR in-
strument equipped with a Harrick ATR accessory and a
ZnSe crystal. Neat samples in the solid or liquid state were
used. NMR spectra were obtained with Bruker AV 200 or
AV 600 instruments and, in one case, with a Varian Geminii
200BB machine. H NMR chemical shifts in CDCl3 and in
C6D6 were referenced to the signal from CHCl3 at δ =
1
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Can. J. Chem. Vol. 83, 2005
Scheme 7.
7.26 ppm and the C6HD5 signal at 7.16 ppm, respectively.
13C NMR shifts for solutions in CDCl3 were referenced to
the center line of the CDCl3 triplet at δ = 77.16 ppm. Mass
spectra were taken with a Micromass (Waters) GCT, TOF
mass spectrometer, with ammonia for CI spectra. Most of
the oxadiazolines gave (M + H)+ masses, as well as the base
peak corresponding to the 2-methoxy-5,5-dimethyl-∆3-1,3,4-
oxadiazolinyl species (C5H9N2O2). A few did not afford
(M + H)+ masses, but gave the base peak instead. The mass
corresponding to loss of N2 was not generally observed.
2-Butanone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-
oxadiazol-2-yl)oximes (2b) (E/Z mixture, not separated
except for 13C NMR of the major isomer)
Colourless oil. IR (most intense peaks, cm–1): 1165, 1112,
1
898. H NMR (600 MHz, CDCl3) (major : minor isomer ra-
tio ca. 2.5:1, integrations separate for each isomer) δ: (major
isomer) 1.028 (t, J = 7.6 Hz, 3H), 1.556 (s, 3H), 1.625 (s,
3H), 1.933 (s, 3H), 2.191 (q, J = 7.6 Hz, 2H), 3.629 (s, 3H);
(minor isomer) 1.087 (t, 3H), 1.558 (s, 3H), 1.611 (s, 3H),
1.845 (s, 3H), 2.418 (m, 2H), 3.608 (s, 3H). 13C NMR
(200 MHz, CDCl3) (major isomer) δ: 10.62, 14.67, 19.52,
23.21, 24.22, 29.46, 52.95, 120.50, 163.10. HR-MS (CI,
NH3) m/z: calcd. for C9H18N3O3 (M + H)+: 216.1348; found:
216.1345.
Synthesis
The standard procedure involved 20 mmol of the acetoxy-
methoxyoxadiazoline (4) in methylene chloride containing a
catalytic amount of toluenesulfonic acid and 20 mmol of the
relevant oxime, except in the case of the preparation of 2g,
where the oxime was used in fourfold excess. The solution
was kept at room temperature for 2 days before aq. KOH
(5 mL H2O plus ca. 0.4 g of KOH) was added and the result-
ing two-phase mixture was left for 1 day, except in the case
of 2d, which is rapidly destroyed by strong base. In that
case, the catalytic acid and acetic acid were removed by
means of a quick extraction with aq. NaHCO3. After separa-
tion of the phases, washing with water, and drying with
Na2SO4, the solvent was evaporated and the residue was
column chromatographed on SiO2 with 10% EtOAc in
hexane for elution. The first fraction to be eluted was always
the oxadiazoline. Yields (unoptimized) were about 40% in
all cases except for 2g, which was obtained in 15%–25%
yield.
Benzaldehyde-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-
1,3,4-oxadiazol-2-yl)(E)-oxime (2c)
1
White solid, mp 65 °C. H NMR (200.1 MHz, CDCl3) δ:
1.60 (s, 3H), 1.69 (s, 3H), 3.69 (s, 3H), 7.33-7.55 (m, 5H),
8.23 (s, 1H). 13C NMR (50.3 MHz, CDCl3) δ: 23.22, 24.87,
53.12, 121.22, 127.71, 128.88, 130.89, 131.09, 138.31,
152.51. HR-MS (CI, NH3) m/z calcd. for C12H16N3O3 (M +
H)+: 250.1192; found: 250.1171.
Acetophenone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-
1,3,4-oxadiazol-2-yl)oxime (2d)
Colourless oil. IR (most-intense peaks, cm–1): 1166.0,
1
1109.3, 904.8, 762.3, 693.5. H NMR (200.1 MHz, CDCl3)
δ: 1.59 (s, 3H), 1.65 (s, 3H), 2.36 (s, 3H), 3.71 (s, 3H),
7.33–7.36 (m, 3H), 7.53–7.58 (m, 2H). 13C NMR
(50.3 MHz, CDCl3) δ: 13.53, 23.12, 24.89, 53.13, 120.97,
126.52, 128.50, 129.90, 135.62, 138.51, 158.68. In the E-
oxime itself, the chemical shift of the iminyl carbon (CDCl3)
is 155.9 δ (8). HR-MS (CI, NH3) m/z calcd. for C13H18N3O3
(M + H)+: 264.1348; found: 264.1361.
Acetone-O-(2,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-
oxadiazol-2-yl)oxime (2a)
1
Colourless oil. H NMR (200.1 MHz, CDCl3) δ: 1.56 (s,
3H), 1.61 (s, 3H), 1.87 (s, 3H), 1.95 (s, 3H), 3.60 (s, 3H).
13C NMR (50.3 MHz, CDCl3) δ: 15.58, 21.40, 22.55, 24.13,
52.26, 119.91, 137.69, 158.65. MS (CI, NH3) m/z: calcd. for
C8H16N3O3 (M + H)+: 202.23; found: 202.2.
Benzophenone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-
1,3,4-oxadiazol-2-yl)oxime (2e)
Colourless solid, mp 63 to 64 °C. IR (strongest sig-
© 2005 NRC Canada

Klys et al.
447
1
nals, cm–1: 1028.26, 923.09, 700.03. H NMR (200.1 MHz,
CDCl3) δ: 1.58 (s, 3H), 1.68 (s, 3H), 3.54 (s, 3H), 7.29–7.44
(m, 10H). 13C NMR (50.3 MHz, CDCl3) δ: 23.21, 24.96,
53.16, 121.02, 128.17, 128.34, 128.47, 129.26, 130.16,
132.60, 135.59, 160.73. HR-MS (CI, NH3) m/z calcd. for
C18H21N3O3 (M + H)+: 326.1505; found: 326.1482.
53.35, 99.78, 147.40, 188.23, 220.02. MS (CI, NH3) m/z
calcd. for C9H16N3O4 (M + H)+: 230.24; found: 230.1.
Thermolysis procedure
The oxadiazoline (1.5 mmol) in 5 mL of benzene, added
to 5.25 mmol of tert-butyl alcohol, was heated in a sealed
vessel at 110 °C for 24 h. The tube was then opened, and the
solvent and volatile products were evaporated with a rotary
evaporator. The residue was column chromatographed on sil-
ica gel with petroleum ether – ethyl acetate as the eluent.
Some fractions had to be rechromatographed on a 2 mm sil-
ica plate in a Chromatotron apparatus, with the same sol-
vents for elution. Thermolysis of 2d afforded products 6–8.
The carbene dimer was not found.
Fluorenone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-
oxadiazol-2-yl)oxime (2f)
Pale yellow solid, mp 105 to 106 °C. IR (most intense
peaks, cm–1): 1166.18, 1098.28, 899.09. 1H NMR
(600 MHz, CDCl3) δ: 1.65 (s, 3H), 1.78 (s, 3H), 3.81 (s,
3H), 7.23 (t, J = 7.2 Hz, 1H), 7.36, 7.33 (t, J = 7.2 Hz, 1H),
7.38 (t, J = 7.2 Hz, 1H), 7.45 (t, J = 7.2 Hz, 1H), 7.58 (d,
J = 7.2 Hz, 1H), 7.60 (d, J = 7.2 Hz, 1H), 7.63 (d, J =
7.2 Hz, 1H), 8.33 (d, J = 7.2 Hz, 1H). 13C NMR (150 MHz,
CDCl3) δ: 23.27, 24.94, 53.47, 120.13, 120.17, 121.80,
122.41, 128.17, 128.59, 129.99, 130.35, 130.91, 132.01,
135.00, 138.80, 140.88, 142.13, 155.33. HR-MS (CI, NH3)
m/z calcd. for C18H18N3O3 (M + H)+: 324.1348; found:
324.1331.
Acetophenone-O-(tert-butoxy)(methoxymethylene)oxime
(6)
1
Colourless oil, yield 35%. H NMR (600 MHz, CDCl3) δ:
1.35 (s, 9H), 2.29 (s, 3H), 3.42 (s, 3H), 6.00 (s, 1H), 7.35–
7.36 (m, 3H), 7.65–7.67 (m, 2H). 13C NMR (150 MHz,
CDCl3) δ: 13.48, 14.27, 28.97, 31.74, 51.31, 113.90, 126.42,
128.51, 129.43, 156.07.
2,3,4,5-Tetraphenylcyclopentadienone-O-(1,5-dihydro-2-
methoxy-5,5-dimethyl-1,3,4-oxadiazol-2-yl)oxime (2g)
1
Reddish foam. H NMR (200 MHz, CDCl3) δ: 1.05 (s,
Methyl (phenylethylidene)carbamate (7)
Colourless oil, in 30% yield. H NMR (600 MHz, CDCl3)
δ: 2.39 (s, 3H), 3.92 (s, 3H), 7.39–7.45 (m, 3H), 7.72–7.73
(m, 2H). 13C NMR (150 MHz, CDCl3) δ: 13.88, 54.81,
126.53, 128.12, 130.15, 134.09, 153.99 (C=N), 162.22
(C=O).
1
3H), 1.38 (s, 3H), 3.03 (s, 3H), 6.73–6.85 (m, 4H), 6.99–
7.27 (m, 16H). 13C NMR (50.3 MHz, CDCl3) δ: 21.99,
25.10, 53.30, 121.60, 124.69, 126.75, 126.98, 127.21,
127.47, 127.61, 127.76, 130.01, 130.08, 130.66, 131.32,
132.66, 132.51, 132.66, 133.60, 134.05, 136.30, 138.16,
145.61, 151.57, 160.88. HR-MS (CI, NH3) m/z calcd. for
C34H29N3O3: 527.2209; found: 527.2213.
Acetophenone azine (8) (13)
1
Yield: 7%. H NMR (200.2 MHz, CDCl3) δ: 2.33 (s, 6H),
Cyclopentanone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-
7.42–7.45 (m, 6H), 7.91–7.93 (m, 4H). 13C NMR (150 MHz,
CDCl3) δ: 15.25, 126.82, 128.51, 129.83, 138.51, 158.15.
HR-MS (CI, NH3) m/z calcd. for C16H16N2: 236.1313;
found: 236.1313 and 237.1387 (M + H)+.
1,3,4-oxadiazol-2-yl)oxime (2h)
1
Colourless oil. H NMR (200 MHz, CDCl3) δ: 1.57 and
1.62 (2s, superimposed on m, total 10H), 2.18 (m, 2H), 2.53
(m, 2H), 3.60 (s, 3H). 13C NMR (150 MHz, CDCl3) δ:
23.19, 24.77, 25.48, 25.70, 25.99, 26.82, 32.18, 52.86,
120.34, 138.46, 164.34. MS (CI, NH3) m/z calcd. for
C5H9N2O2 (M – C5H8NO)+: 129.14; found: 129.1 (100%).
Thermolysis of 2e afforded benzophenone-O-(tert-
butoxy)(methoxymethylene)oxime (9), the carbene dimer
(10), methyl (diphenylmethylene)carbamate (MeOCON=CPh2,
11) (14), and benzophenone azine (12, 5%) (15).
Cyclohexanone-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-
1,3,4-oxadiazol-2-yl)oxime (2i)
Benzophenone-O-(tert-butoxy)(methoxymethylene)oxime
(9)
Colourless oil. IR (C=N and four most intense peaks, cm–1):
1
Compound 9 was very susceptible to hydrolysis and was
not obtained in a pure state. Yield: ca. 32%. 1H NMR
(200.2 MHz, CDCl3) δ: 1.29 (s, 9H), 3.40 (s, 3H), 5.99 (s,
1H), 7.31–7.52 (m, 10H). 13C NMR (50.3 MHz, CDCl3) δ:
28.92, 51.38, 114.01, 128.02–129.58 (eight signals visible in
this range, but not resolved). A meaningful mass spectrum
could not be obtained.
1763.2, 1164.3, 1101.8, 897.8, 862.3. H NMR (600 MHz,
CDCl3) δ: 1.57 (s, 3H), 1.61 (s, 3H), 1.70–1.80 (m, 6H),
2.36 (distorted t, 2H), 2.52 (distorted t, 2H), 3.59 (s, 3H).
13C NMR (150 MHz, CDCl3) δ: 23.33, 24.70, 25.23, 28.43,
31.31, 52.87, 120.49, 125.67, 138.29, 171.14. MS (CI, NH3)
m/z calcd. for C5H9N2O2 (M – C6H10NO)+: 129.14; found:
129.1 (100%).
Biacetyl-O-(1,5-dihydro-2-methoxy-5,5-dimethyl-1,3,4-
oxadiazol-2-yl)]monoxime (2j)
1,2-Dimethoxy-1,2-(diphenyliminoxy)ethene (10)
1
Colourless oil, yield 12%. H NMR (200.2 MHz, CDCl3)
Colourless oil. IR (neat, cm–1): 1702.4 (C=O), 913.3
(most intense fingerprint peak). 1H NMR (200.2 MHz,
CDCl3) δ: 1.59 (s, 3H), 1.67 (s, 3H), 2.05 (s, 3H), 2.28 (s,
δ: 3.66 (s, 6H), 7.33–7.27 (m, 10H). 13C NMR (50.3 MHz,
CDCl3) δ: 55.40, 128.45, 128.55, 129.03, 129.17, 129.94,
131.08, 132.20, 134.70, 154.67, 164.63. A correlation be-
tween the 1H and 13C NMR signals was established by
means of gradient HSQC spectra. The trans isomer was as-
sumed.
1
3H), 3.70 (s, 3H). H NMR (200.2 MHz, C6D6) δ: 1.27 (s,
3H), 1.40 (s, 3H), 1.76 (s, 3H), 1.90 (s, 3H), 3.44 (s, 3H).
13C NMR (50.3 MHz, CDCl3) δ: 9.54, 23.30, 24.81, 25.59,
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Can. J. Chem. Vol. 83, 2005
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Methyl (diphenylmethylene)carbamate (11) (14)
Pale yellow oil, yield 14%. 1H NMR (200.2 MHz, CDCl3)
δ: 3.86 (s, 3H), 7.33–7.59 (m, 10H). 13C NMR (50.3 MHz,
CDCl3) δ: 53.26, 128.46, 129.10, 131.30, 136.48, 163.31
(C=O), 171.55 (C=N). The signals at 128.46 and 129.10 are
much more intense than the other ArH signals, and probably
correspond to several coincident chemical shifts.
Dimethyl 1-phenylisoquinoline-3,4-dicarboxylate (13)
Oxadiazoline 2e (488 mg, 1.5 mmol) in benzene, heated
with DMAD (745 mg, 5.25 mmol) and worked up as de-
scribed previously, gave 85 mg (2.7 mmol, 18%) of 13.4
White solid, mp 126 to 127 °C. 1H NMR (600 MHz,
CDCl3) δ: 4.03 (s, 3H), 4.12 (s, 3H), 7.52–7.56 (m, 3H),
7.68–7.71 (m, 3H), 7.83 (t, J = 7.5 Hz, 1H), 8.01 (d, J =
8.4 Hz, 1H), 8.17 (d, J = 8.4 Hz, 1H). 13C NMR (150 MHz,
CDCl3) δ: 53.3, 53.4, 125.7, 127.6, 127.8, 128.3, 128.7,
129.5, 129.8, 130.3, 131.8, 132.2, 133.8, 138.5, 162.3,
166.1, 168.2. The structure of 13 was determined by means
of X-ray crystallography.
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Acknowledgements
The authors are grateful to the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) for generous
financial support and to Dr. Patton Giles, Chemical Ab-
stracts, Columbus Ohio, for assistance with some of the no-
menclature. The work of Professor J. Britten, who
determined the structure of 13 by means of X-ray crystallog-
raphy, is greatly appreciated. Preliminary work on the syn-
thesis of a few of the oxadiazolines by Dr. Paramashivappa
Rangappa is also acknowledged.
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