Aryl nitrile oxide cycloaddition reactions in the presence of pinacol boronic acid ester

Article abstract of DOI:10.3762/bjoc.8.67

An aryl substrate with dual functionality consisting of a nitrile oxide and a pinacolyl boronate ester was prepared by mild hypervalent iodine oxidation (diacetoxyiodobenzene) of the corresponding aldoxime, without decomposition of the boronate functionality. The nitrile oxide was trapped in situ with a variety of dipolarophiles to yield aryl isoxazolines with the boronate ester function intact and available for subsequent reaction.

Full text of DOI:10.3762/bjoc.8.67

Aryl nitrile oxide cycloaddition reactions in the
presence of pinacol boronic acid ester
Sarah L. Harding1, Sebastian M. Marcuccio2 and G. Paul Savage*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 606–612.
1CSIRO Materials Science and Engineering, Private Bag 10, Clayton
South MDC, Vic 3169, Australia and 2Advanced Molecular
Technologies Pty Ltd, Unit 1, 7–11 Rocco Drive, Scoresby, VIC 3179,
Australia
doi:10.3762/bjoc.8.67
Received: 15 February 2012
Accepted: 29 March 2012
Published: 19 April 2012
Email:
G. Paul Savage* - paul.savage@csiro.au
Associate Editor: D. Y.-K. Chen
* Corresponding author
© 2012 Harding et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
dipolar cycloaddition; heterocycle; nitrile oxide; hypervalent iodine
oxidation; pinacol boronic acid esters
Abstract
An aryl substrate with dual functionality consisting of a nitrile oxide and a pinacolyl boronate ester was prepared by mild hyperva-
lent iodine oxidation (diacetoxyiodobenzene) of the corresponding aldoxime, without decomposition of the boronate functionality.
The nitrile oxide was trapped in situ with a variety of dipolarophiles to yield aryl isoxazolines with the boronate ester function
intact and available for subsequent reaction.
Introduction
Metal-mediated coupling reactions to form carbon–carbon compound could in turn be coupled with heterocycles or aryl
bonds, and 1,3-dipolar cycloaddition reactions to construct five- groups to give insecticidal [5] derivatives of type 3 (Scheme 1).
membered heterocycles are both powerful tools for assembling
organic molecules. Used in combination, these tools offer great The utility of arylboronic acids and esters in organic synthesis is
flexibility for strategies such as diversity-oriented synthesis [1], demonstrated by their use as key intermediates in transition-
solution-phase combinatorial libraries [2], and continuous-flow metal-catalysed bond-forming reactions [6], which include the
processes [3,4]. An important consideration when using these Miyaura–Suzuki coupling reaction [7], copper-catalysed
reactions for multistep syntheses is whether they are chemi- heteroatom arylation [8], allylboration [9], and the Petasis reac-
cally compatible, without having to resort to protection/depro- tion [10]. Aryl boronic esters also undergo many of these
tection systems. For example, to generate a library of coupling reactions and Miyaura’s protocol for the palladium-
3-bi(hetero)aryl isoxazoline analogues 3 a convenient substrate catalysed cross-coupling of bis(pinacolato)diboron with aryl
would be the arylboronate nitrile oxide 1, which would undergo and vinyl halides or triflates has become one of the most
1,3-dipolar cycloaddition to give isoxazolines 2. This latter popular methods for preparing arylboronic esters under mild
606

Beilstein J. Org. Chem. 2012, 8, 606–612.
genation of aldoximes have been reported, including the use of
lead tetraacetate [22,23], mercury(II) acetate [24], hypervalent
iodine [25,26], and manganese(IV) oxide [27]. We were inter-
ested in developing a mild method for the introduction of a
nitrile oxide functionality in the presence of an arylboronic
ester, allowing subsequent elaboration. We herein report the
synthesis and 1,3-dipolar cycloaddition reactions of 4-pinacola-
toboron benzonitrile oxide 1.
Results and Discussion
Several reports of 1,3-dipolar cycloaddition reactions of nitrile
oxides to vinylboronate esters [28,29] and alkynylboronate
esters [30,31] have recently appeared. In each case the nitrile
oxide was either isolated first (this procedure is limited to
hindered nitrile oxides, such as 2,4,6-trimethylbenzonitrile
oxide) or generated in situ by dehydrohalogenation of a
preformed hydroximoyl chloride. This is presumably to avoid
Scheme 1: Concept for library generation by dipolar cycloaddition fol-
lowed by boronate coupling.
conditions [11]. The resulting pinacolyl boronate esters have the
advantage of being stable, readily handled compounds.
The Huisgen 1,3-dipolar cycloaddition reaction is a powerful the competing oxidative side reactions that would be expected
and versatile method for constructing five-membered hetero- at the boronate ester if the nitrile oxide were generated oxida-
cycles [12-14]. Nitrile oxide 1,3-dipoles react with tively from the aldoxime [32]. The same thermodynamic bias
carbon–carbon dipolarophiles, such as alkenes [15], alkynes favouring the oxidation of carbon–boron bonds, which makes
[16,17], and benzyne [18,19], to give Δ2-isoxazolines and isox- boronic ester chemistry chemoselective, is a constraint that
azoles. These are interesting sources of bioactive compounds in potentially limits the utility of nitrile oxide cycloadditions in the
their own right, but isoxazoles are particularly valuable for their presence of a boronic acid ester.
latent functionality as β-hydroxyketones, β-aminoalcohols, 1,3-
diols, and a range of other 1,3-disubstituted compounds, 4-Formylphenylboronic acid pinacol ester 4 is commercially
through N–O bond cleavage [20]. Nitrile oxides are reactive available or easily prepared from the corresponding boronic
intermediates that are usually generated in situ and react imme- acid, via the bromide [33]. Reaction with 50% aqueous hydrox-
diately with the dipolarophile. There have been many methods ylamine gives the aldoxime 5 in good yield (Scheme 3). Only
reported for the generation of nitrile oxides, but the most one geometric isomer of the aldoxime was observed in the 1H
common one for alkyl nitrile oxides involves the dehydration of and 13C NMR spectra, and this was assigned as the Z isomer
primary nitro compounds [21]. Aryl nitrile oxides are more based on the 8.17 ppm chemical shift of the C(H)=N proton
commonly prepared by chlorination of aldoximes followed by [34]. Attempted methods for the chlorination of aryl aldoximes
dehydrohalogenation of the resulting hydroximoyl chlorides, or to aryl hydroximoyl chloride include the use of N-chlorosuccin-
by direct oxidative dehydrogenation of the aldoximes imide [35], chloramine-T [36], biphasic sodium hypochlorite
(Scheme 2) [17].
[37,38], and tert-butyl hypochlorite [39]. These methods led to
either no reaction or, when forced, decomposition of the inter-
While the halogenation–dehydrohalogenation process is most
common, several methods involving direct oxidative dehydro-
Scheme 3: Formation of 4-(aldoxime)phenylboronic acid pinacol
ester 5.
Scheme 2: General formation of alkyl (R) and aryl (Ar) nitrile oxides.
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Beilstein J. Org. Chem. 2012, 8, 606–612.
mediate 5 with no detectible hydroximoyl chloride 6. Given that (DIB) [26,40] could cleanly convert the aldoxime to the corres-
these reagents are oxidants such a result is perhaps not ponding nitrile oxide without decomposition of the boronate
surprising.
ester function. The nitrile oxide was trapped in situ with a
variety of dipolarophiles to yield isoxazolines (Table 1).
We then turned our attention to the direct oxidation of
aldoximes using the mild conditions of hypervalent iodine oxi- For mono-substituted or 1,1-disubstituted alkenes the regio-
dation and were pleased to discover that diacetoxyiodobenzene chemistry of the nitrile oxide cycloaddition followed the
Table 1: Cycloaddition of 4-pinacolatoboron benzonitrile oxide 1 with dipolarophiles.
entry
dipolarophile
product (7)
purified yield
(%)
a
b
c
d
styrene
74
59
56
49
methyl acrylate
tert-butyl acrylate
N,N-dimethylacrylamide
e
methyl methacrylate
82
f
1-heptene
36
48
g
α-methylstyrene
h
i
trans β-methylstyrene
36
71
1:2
4-bromostyrene
j
48
608

Beilstein J. Org. Chem. 2012, 8, 606–612.
expected outcome in which the oxygen of the nitrile oxide (Z)-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzalde-
became attached to the more substituted end of the double bond hyde oxime (5): To a stirred solution of 4-formylphenyl-
[41]. This regiochemical orientation was established from the boronic acid pinacol ester 4 (100 g, 0.43 mol) in diethyl ether
1H NMR chemical shifts for the cycloadduct isoxazoline ring (400 mL) was added 50% hydroxylamine in H2O (25.9 mL,
protons. The resonances of protons on C4 of the isoxazoline 0.43 mol) in one portion. The reaction mixture immediately
ring appear 1–2 ppm upfield from those of C5 protons on the became warm and was stirred for a further 10 min, then dried
isoxazoline rings [42], and hence 5-substituted isoxazolines are (MgSO4) and filtered, and the ether was removed under reduced
easily distinguished from 4-substituted isoxazolines. All of the pressure to yield a pale yellow oil (95 g, 89%) that crystallized
monosubstituted and 1,1-disubstituted alkenes led to 5-substi- upon standing. A sample of the crude material was kept as a
tuted and 5,5-disubstituted isoxazolines, respectively. In the slurry in hexane (140 mL) overnight, collected by filtration and
case of trans-β-methylstyrene (Table 1, entry h), an inseparable dried at room temperature in a vacuum oven, mp 115–117 °C;
mixture of regioisomers was obtained in a 2:1 ratio favouring 1H NMR (CDCl3, 400 MHz) δ 8.22 (s, 1H), 8.17 (s, 1H), 7.84
the addition of the nitrile oxide oxygen to the phenyl-substi- (d, J = 8.0 Hz, 2H), 7.59 (d, J = 8.0 Hz, 2H), 1.35 (s, 12H);
tuted end of the carbon–carbon double bond. This is consistent 13C NMR (CDCl3, 100 MHz) δ 150.3, 135.1, 134.4, 126.2,
with previously reported benzonitrile oxide cycloaddition reac- 84.0, 24.8; HRMS–EI (m/z): calcd for C13H18BNO3, 247.1374;
tions with trans-β-methylstyrene [43]. For both regioisomers found, 247.1373.
the coupling between the C4 and C5 protons (approximately
General procedure for cycloaddition reac-
5–6 Hz) indicated a retention of the configuration of the trans
geometry in the cycloadduct, which is consistent with the tions
concerted nature of the 1,3-dipolar cycloaddition reaction. To a stirred solution of the appropriate dipolarophile
(0.55 mmol) and diacetoxyiodobenzene (177 mg, 0.55 mmol) in
For the cycloaddition reaction with the hydantoin compound, methanol (5 mL), at 0 °C, was added 4-(4,4,5,5-tetramethyl-
3-methyl-1-(2-tert-butylphenyl)-5-methyleneimidazol-2,4-dione 1,3,2-dioxaborolan-2-yl)benzaldehyde oxime (5) (125 mg,
(Table 1, entry j), only a single diastereomer was detected. We 0.5 mmol) in methanol (3 mL), dropwise over 10 min followed
have previously observed that nitrile oxide cycloadditions to by three drops of trifluoroacetic acid. The pale yellow solution
this hydantoin and related compounds can show facial selec- was allowed to warm to room temperature and stirred for 2 h
tivity based on atropisomerism around the N-aryl bond [38,44]. then concentrated under reduced pressure. The residue was
With benzonitrile oxide the facial selectivity was 30:1 favouring purified by column chromatography on silica as stated.
addition anti to the tert-butyl group; however, with the boronate
ester benzonitrile oxide 5 only the anti cycloadduct was isolated 5-Phenyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
and no syn cycloadduct was detected.
yl)phenyl)-4,5-dihydroisoxazole (7a): Isolated as a white solid
(130 mg, 74%) after purification by column chromatography
(20% Et2O in petroleum ether, Rf 0.41, 50% Et2O in petroleum
Conclusion
Aryl nitrile oxides can be prepared oxidatively in the presence ether); 1H NMR (CDCl3, 400 MHz) δ 7.85 (d, J = 8.1 Hz, 2H),
of boronate esters by using the hypervalent iodine reagent, 7.69 (d, J = 8.1 Hz, 2H), 7.42–7.29 (m, 5H), 5.75 (dd, J = 11.0,
diacetoxyiodobenzene. Nitrile oxides prepared in this way 8.4 Hz, 1H), 3.79 (dd, J = 16.6, 11.0 Hz, 1H), 3.35 (dd, J =
undergo 1,3-dipolar cycloaddition to yield substituted isoxazo- 16.6, 8.4 Hz, 1H), 1.36 (s, 12H); 13C NMR (CDCl3, 100 MHz)
lines.
δ 156.1, 140.7, 134.95, 131.7, 128.65, 128.1, 125.8, 125.7, 83.8,
82.6, 42.9, 24.75; IR (KBr) ν/cm−1: 2979 (w), 1755 (w), 1607
(w), 1396 (m), 1358 (st), 1325 (m), 1269 (m), 1142 (st);
HRMS–EI (m/z): calcd for C21H24BNO3, 349.1844; found,
Experimental
General experimental procedures
Melting points were determined on a Büchi B-545 instrument 349.1839.
and are uncorrected. 1H and 13C NMR spectra were recorded on
a Bruker AV400 spectrometer at 400 and 100 MHz, respective- Methyl 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
ly, by using CDCl3 as solvent and internal reference. Electron yl)phenyl)-4,5-dihydroisoxazole-5-carboxylate (7b): Isolated
impact (EI) mass spectra were run on a ThermoQuest as a white solid (98 mg, 59%) after purification by column
MAT95XP mass spectrometer with an ionization energy of chromatography (20% Et2O in petroleum ether, Rf 0.18, 50%
70 eV. Accurate mass measurements were obtained on the same Et2O in petroleum ether); 1H NMR (CDCl3, 400 MHz) δ 7.83
instrument with a resolution of 5000–10000 by using perfluoro- (d, J = 8.2 Hz, 2H), 7.65 (d, J = 8.2 Hz, 2H), 5.19 (dd, J = 10.8,
kerosene (PFK) as the reference compound. Accurate masses 7.5 Hz, 1H), 3.81 (s, 3H), 3.79 (m, 2H), 1.34 (s, 12H);
were measured on the 11B ions.
13C NMR (CDCl3, 100 MHz) δ 170.6, 156.1, 135.1, 130.8,
609

Beilstein J. Org. Chem. 2012, 8, 606–612.
126.05, 84.05, 78.0, 52.8, 38.8, 24.8; IR (KBr) ν/cm−1: 2916 = 7.9, 7.3, 1.5 Hz, 1H), 3.87 (d, J = 17.8 Hz, 1H), 3.37 (d, J =
(w), 1761 (m), 1607 (w), 1392 (m), 1353 (st), 1325 (m), 1268 17.5 Hz, 1H), 3.20 (s, 3H), 1.36 (s, 9H), 1.32 (s, 12H);
(m), 1208 (m), 1140 (st); HRMS–EI (m/z): calcd for 13C NMR (CDCl3, 100 MHz) δ 156.4, 134.95, 132.3, 125.7,
C17H22BNO5, 331.1586; found, 331.1588.
83.95, 81.6, 39.8, 35.3, 31.6, 25.15, 24.8, 22.5, 13.95; IR (KBr)
ν/cm−1: 2929 (w), 1611 (w), 1397 (m), 1357 (st), 1323 (m),
tert-Butyl 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- 1268 (m), 1142 (st), 1091 (st), 858 (m); HRMS–EI (m/z): calcd
yl)phenyl)-4,5-dihydroisoxazole-5-carboxylate (7c): Isolated for C20H30BNO3, 343.2313; found, 343.2311.
as a white solid (105 mg, 56%) after purification by column
chromatography (10% Et2O in petroleum ether); 1H NMR 5-Methyl-5-phenyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
(CDCl3, 400 MHz) δ 7.78 (d, J = 8.2 Hz, 2H), 7.64 (d, J = 8.2 borolan-2-yl)phenyl)-4,5-dihydroisoxazole (7g): Isolated as a
Hz, 2H), 5.03 (t, J = 9.4 Hz, 1H), 3.56 (d, J = 9.4 Hz, 2H), 1.47 colourless oil (88 mg, 48%) after purification by column chro-
(s, 9H), 1.32 (s, 12H); 13C NMR (CDCl3, 100 MHz) δ 169.25, matography (10% Et2O in petroleum ether, Rf 0.54, 50% Et2O
156.1, 135.2, 131.3, 126.2, 84.2, 82.9, 79.0, 38.8, 28.1, 25.0; IR in petroleum ether); 1H NMR (CDCl3, 400 MHz) δ 7.80 (d, J =
(KBr) ν/cm−1: 2966 (m), 1742 (m), 1726 (m), 1611 (w), 1390 8.3 Hz, 2H), 7.63 (d, J = 8.3 Hz, 2H), 7.47 (m, 2H), 7.35 (m,
(m), 1356 (st), 1323 (m), 1142 (st); HRMS–EI (m/z): calcd for 2H), 7.27 (m, 1H), 3.49 (AB quartet, J = 16.5 Hz, 2H), 1.79 (s,
C20H28BNO5, 373.2055; found, 373.2048.
3H), 1.33 (s, 12H); 13C NMR (CDCl3, 100 MHz) δ 156.4,
145.6, 135.2, 132.4, 128.7, 127.5, 125.9, 124.8, 88.4, 84.2, 48.8,
N,N-dimethyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan- 28.5, 25.0; HRMS–EI (m/z): calcd for C22H26BNO3, 363.2000;
2-yl)phenyl)-4,5-dihydroisoxazole-5-carboxamide (7d): found, 363.1979.
Isolated as a colourless oil (85 mg, 49%) after purification by
column chromatography (80% Et2O in petroleum ether, Rf 0.43, 4-Methyl-5-phenyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
50% Et2O in petroleum ether); 1H NMR (CDCl3, 400 MHz) δ borolan-2-yl)phenyl)-4,5-dihydroisoxazole (7ha) and
7.82 (d, J = 8.3 Hz, 2H), 7.67 (d, J = 8.2 Hz, 2H), 5.38 (dd, J = 5-methyl-4-phenyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
11.2, 7.7 Hz, 1H), 4.19 (dd, J = 16.8, 7.7 Hz, 1H), 3.38 (dd, J = borolan-2-yl)phenyl)-4,5-dihydroisoxazole (7hb): Isolated as
16.8, 11.3 Hz, 1H), 3.21 (s, 3H), 3.00 (s, 3H), 1.34 (s, 12H); a colourless oil (65 mg, 36%) after purification by column chro-
13C NMR (CDCl3, 100 MHz) δ 167.2, 157.4, 135.0, 131.3, matography (10% Et2O in petroleum ether, Rf 0.57, 50% Et2O
126.0, 84.0, 78.3, 37.3, 36.8, 36.1, 24.8; IR (KBr) ν/cm−1: 2966 in petroleum ether) to give an inseparable 2:1 mixture of regio-
(m), 1742 (m), 1726 (m), 1611 (w), 1390 (m), 1356 (st), 1323 isomers a and b; 1H NMR (CDCl3, 400 MHz) regioisomer a, δ
(m), 1142 (st); HRMS–EI (m/z): calcd for C18H25BN2O4, 7.84 (d, J = 8.3 Hz, 2H), 7.66 (d, J = 8.3 Hz, 2H), 7.39–7.28 (m,
344.1902; found, 344.1899.
5H), 5.31 (d, J = 5.6 Hz, 1H), 3.70 (dq, J = 7.1, 5.7 Hz, 1H),
1.45 (d, J = 7.1 Hz, 3H), 1.35 (s, 12H); 1H NMR (CDCl3, 400
Methyl 5-methyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxa- MHz) regioisomer b, δ 7.70 (d, J = 8.3 Hz, 2H), 7.56 (d, J = 8.3
borolan-2-yl)phenyl)-4,5-dihydroisoxazole-5-carboxylate Hz, 2H), 7.25–7.19 (m, 5H), 4.66 (apparent qn, J = 6.3 Hz, 1H),
(7e): Isolated as a pale yellow oil (141 mg, 82%) after purifica- 4.33 (d, J = 6.0 Hz, 1H), 1.49 (d, J = 6.3 Hz, 3H), 1.31 (s, 12H);
tion by column chromatography (15% Et2O in petroleum ether, 13C NMR (CDCl3, 100 MHz) resonances that could be assigned
Rf 0.36, 50% Et2O in petroleum ether); 1H NMR (CDCl3, 400 to specific regioisomers by using proton–carbon 2D correlation
MHz) δ 7.81 (d, J = 8.1 Hz, 2H), 7.63 (d, J = 8.2 Hz, 2H), 3.88 spectroscopy are designated a or b, δ 160.2, 158.1, 140.8, 139.0,
(d, J = 17.0 Hz, 1H), 3.79 (s, 3H), 3.21 (d, J = 17.0 Hz, 1H), 135.1, 134.8, 131.5, 131.2, 130.4, 129.2, 128.7, 128.1, 127.6,
1.70 (s, 3H), 1.33 (s, 12H); 13C NMR (CDCl3, 100 MHz) δ 127.45, 126.3, 126.2, 125.4, 90.15a, 86.95b, 84.0, 83.9, 61.0b,
172.5, 156.3, 135.0, 131.3, 125.85, 86.2, 84.0, 53.0, 44.65, 24.8, 50.75a, 24.8, 24.75, 20.5b, 18.2a; HRMS–EI (m/z): calcd for
23.6; IR (KBr) ν/cm−1: 2975 (w), 1744 (m), 1609 (w), 1516 C22H26BNO3, 363.2000; found, 363.1988.
(w), 1394 (m), 1349 (st), 1326 (st), 1268 (m), 1146 (st);
HRMS–EI (m/z): calcd for C18H24BNO5, 345.1742; found, 5-(4-Bromophenyl)-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
345.1740.
borolan-2-yl)phenyl)-4,5-dihydroisoxazole (7i): Isolated as a
white solid (152 mg, 71%) after purification by column chroma-
5-Pentyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- tography (10% Et2O in petroleum ether, Rf 0.35, 50% Et2O in
yl)phenyl)-4,5-dihydroisoxazole (7f): Isolated as a colourless petroleum ether); 1H NMR (CDCl3, 400 MHz) δ 7.82 (d, J =
oil (61 mg, 36%) after purification by column chromatography 8.3 Hz, 2H), 7.65 (d, J = 8.3 Hz, 2H), 7.48 (d, J = 8.5 Hz, 2H),
(20% Et2O in petroleum ether, Rf 0.40, 50% Et2O in petroleum 7.26 (d, J = 8.5 Hz, 2H), 5.68 (dd, J = 11.0, 8.2 Hz, 1H), 3.77
ether); 1H NMR (CDCl3, 400 MHz) δ 8.32 (d, J = 8.3 Hz, 2H), (dd, J = 16.7, 11.1 Hz, 1H), 3.28 (dd, J = 16.7, 8.2 Hz, 1H),
7.50 (m, 4H), 7.31 (ddd, J = 8.2, 7.2, 1.6 Hz, 1H), 7.19 (ddd, J 1.33 (s, 12H); 13C NMR (CDCl3, 400 MHz) δ 156.1, 139.9,
610

Beilstein J. Org. Chem. 2012, 8, 606–612.
9. Roush, W. R.; Adam, M. A.; Walts, A. E.; Harris, D. J.
135.1, 131.9, 131.5, 127.55, 125.9, 122.1, 84.0, 81.9, 43.0,
24.85; HRMS–EI (m/z): calcd for C21H23BBrNO3, 427.0949;
found, 427.0936.
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spiro[4.4]non-2-ene-7,9-dione (7j): Isolated as a white solid
(121 mg, 48%) after purification by column chromatography
(50% Et2O in petroleum ether, Rf 0.38, Et2O); 1H NMR
(CDCl3, 400 MHz) δ 8.32 (d, J = 8.3 Hz, 2H), 7.50 (m, 4H),
7.31 (ddd, J = 8.2, 7.2, 1.6 Hz, 1H), 7.19 (ddd, J = 7.9, 7.3, 1.5
Hz, 1H), 3.87 (d, J = 17.5 Hz, 1H), 3.37 (d, J = 17.5 Hz, 1H),
3.20 (s, 3H), 1.36 (s, 9H), 1.32 (s, 12H); 13C NMR (CDCl3, 100
MHz) δ 169.1, 156.1, 155.4, 148.3, 135.1, 132.2, 130.3, 130.2,
129.8, 128.15, 127.5, 125.7, 97.1, 84.1, 36.7, 36.5, 32.5, 25.3,
24.8; IR (KBr) ν/cm−1: 2975 (m), 1791 (m), 1732 (st), 1489
(m), 1443 (m), 1367(st), 1367 (st), 1353 (st); HRMS–EI (m/z):
calcd for C28H34BN3O5, 503.2586; found, 503.2576.
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Supporting Information
Supporting Information File 1
1H and 13C NMR spectra, 2D spectra where required, and
mass spectra for all compounds.
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Moses, J. E. Chem. Commun. 2010, 46, 1272–1274.
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The authors wish to thank Dr Roger Mulder of CSIRO for
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