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4.3.2. 3-Bromo-5-phenyl-4-(4,4,5,5-tetramethyl[1,3,2]-
dioxa-borolan-2-yl)-isoxazole (20). The same general
procedure was carried out with 2-phenylethynyl-4,4,5,5-
tetramethyl-[1,3,2]dioxaboralane (3) (0.25 g, 1.08 mmol,
1.0 equiv), purification by flash column chromatography
(eluting solvent petroleum ether/ethyl acetate 10:1 ratio)
gave the title compound (20) as a colourless solid, 0.26 g,
tetramethyl-2-(3-phenylsulfanyl-prop-1-ynyl)-[1,3,2]dioxa-
borolane (6) (0.25 g, 0.91 mmol, 5.0 equiv), purification by
flash column chromatography (eluting solvent petroleum
ether/ethyl acetate 10:1 ratio) gave the title compound (25)
as a colourless oil, 0.035 g, 48% yield; 1H NMR (250 MHz,
CDCl3): d 1.27 (12H, s, 4!CH3), 4.31 (2H, s, CH2SPh),
7.22–7.40 (5H, m, Ar-H); 13C NMR (62.9 MHz, CDCl3) d
24.8, 29.9, 84.2, 127.7, 129.4, 131.1, 132.9, 144.5, 178.9;
FTIR nmax/CHCl3, 3060 (w), 2980 (m), 2931 (w), 1589 (s),
1481 (m), 1440 (m), 1407 (m) cmK1; HRMS (EIC) calcd
for C16H19BNO3BrS: 395.0362, found: 395.0379.
1
69% yield. Mp: 76–79 8C; H NMR (250 MHz, CDCl3): d
1.30 (12H, s, 4!CH3), 7.33–7.47 (3H, m, Ar-H), 7.84–7.93
(2H, m, Ar-H); 13C NMR (62.9 MHz, CDCl3) d 24.7, 84.5,
127.3, 128.0, 128.5, 131.0, 145.2, 176.5; FTIR nmax/CHCl3,
2195(m),2360(m),2932(s),2979(s)1725(m) cmK1;HRMS
calcd for C15H17BBrNO3: 349.0485, found: 349.0490.
4.4. General procedure for the [3C2] cycloaddition
reaction of chloronitrile oxide with alkynylboronates
4.3.3. 3-Bromo-5-methyl-4-(4,4,5,5-tetramethyl[1,3,2]-
dioxa-borolan-2-yl)-isoxazole (22). The same general
procedure was carried out with 2-propy-1-nyl-4,4,5,5-
tetramethyl-[1,3,2]dioxaboralane (14) (0.20 g, 1.18 mmol,
1.0 equiv), purification by flash column chromatography
(eluting solvent petroleum ether/ethyl acetate 10:1 ratio)
gave the title compound (22) as a colourless solid, 0.15 g,
4.4.1. 3-Chloro-5-butyl-4-(4,4,5,5-tetramethyl[1,3,2]-
dioxa-borolan-2-yl)-isoxazole (26).
A solution of
N-chlorosuccinimide (1.34 g, 3.86 mmol) and glyoxylic
acid aldoxime (0.46 g, 3.86 mmol) in DME (5 ml) were
heated under reflux for 10 min (gas evolved). The mixture
was then cooled to room temperature and stirred until gas
evolution had stopped, at which point chlorination was
assumed complete. The resulting solution was used directly
for further reaction. A solution of 2-hexy-1-nyl-4,4,5,5-
tetramethyl-[1,3,2]dioxaboralane (2) (1.73 g, 7.72 mmol) in
DME (0.5 ml) was added via cannula to the chloronitrile
oxide solution, followed by the addition of KHCO3 (1.54 g,
15.4 mmol) and the mixture stirred for 16 h at 50 8C, then
the residual solid was removed by vacuum filtration and
solvent was removed in vacuo followed by extraction into
ethyl acetate (3!20 ml) and conc. in vacuo. Purification by
flash column chromatography (eluting solvent petroleum
ether/ethyl acetate 20:1 ratio) gave the title compound (26)
1
43% yield. Mp 86–87 8C; H NMR (250 MHz, CDCl3): d
1.26 (12H, s, 4!CH3), 2.50 (3H, s, CH3); 13C NMR
(62.9 MHz, CDCl3) d 13.1, 24.8, 83.9, 144.6, 179.7; FTIR
nmax/CHCl3, 2982 (s), 1593 (s) cmK1; HRMS calcd for
C10H15NO3Br 287.0328, found: 287.0332. Anal. Calcd for
C10H15BNO3Br: C, 41.71; H, 5.25; N 4.86; Br, 27.75.
Found: C, 41.99; H, 5.06; N, 4.86; Br, 27.87.
4.3.4. 3-Bromo-4-(4,4,5,5-tetramethyl[1,3,2]-dioxaboro-
lan-2-yl)-5-trimethylsilanyl-isoxazole (23). The same
general procedure was carried out with 2-trimethylsilyl-
4,4,5,5-tetramethyl-[1,3,2]dioxaboralane (4) (0.20 g,
0.89 mmol, 1.0 equiv), purification by flash column chro-
matography (eluting solvent petroleum ether/ethyl acetate
20:1 ratio) gave the title compound (23) as a colourless
solid, 0.18 g, 58% yield. Mp 108–110 8C; 1H NMR
(250 MHz, CDCl3): d 0.32 (12H, s, 4!CH3), 1.27 (9H, s,
Si(CH3)3); 13C NMR (62.9 MHz, CDCl3) d K1.82, 24.8,
84.2, 143.9, 189.1; FTIR nmax/CHCl3, 3392 (w), 2979 (s),
1541 (s), 1480 (m), 1374 (s), 1328 (s) cmK1; HRMS calcd
for C12H21BNO3SiBr: 345.0567, found: 345.0569. Anal.
Calcd for C12H21BNO3BrSi: C, 41.64; H, 6.12; N, 4.05; Br,
23.09. Found: C, 41.48; H, 5.85; N, 4.13; Br, 23.31.
1
as a yellow oil, 0.48 g, 44% yield; H NMR (250 MHz,
CDCl3): d 0.91 (3H, t, JZ7.0 Hz, CH2CH3), 1.27–1.41 (2H,
m, CH2CH2CH3), 1.31 (12H, s, 4!CH3), 1.59–1.73 (2H, m,
CH2CH2CH3), 2.92 (2H, t, JZ7.5 Hz, OCCH2); 13C NMR
(100.6 MHz, CDCl3) d 13.6, 22.0, 24.8, 27.0, 30.0, 83.8,
156.9, 184.0; FTIR nmax/CHCl3, 2979 (m), 2934 (m), 2874
(w), 1595 (s), 1455 (s), 1436 (m), 1414 (s), 1345 (s) cmK1
;
HRMS calcd for C13H21BNOCl: 285.1303, found: 285.1296.
4.4.2. 1-[4-(4,4,5,5-Tetramethyl[1,3,2]dioxaborolan-2-
yl)-5-trimethylsilanyl-isoxazol-3-yl]-ethanone (27). A
solution of 4,4,5,5-tetramethyl-2-trimethylsilanylethynyl-
[1,3,2]-dioxaborolane (4) (0.10 g, 0.45 mmol), ammonium
cerium (IV) nitrate (0.24 g, 0.45 mmol), formic acid (0.21 g,
4.50 mmol) in acetone (3 ml) were stirred under reflux for
10 h. The mixture was cooled to room temperature then
extracted with diethyl ether (5 ml) and washed with sodium
hydrogen carbonate {(2!2 ml) CARE: effervescence was
observed!}, brine (2!2 ml) and distilled water (2!2 ml).
The organics were dried (Na2SO4), filtered and conc. in
vacuo, then purified by flash column chromatography
(eluting solvent petroleum ether/ethyl acetate 5:1 ratio) to
give the title compound (27) and its regioisomer as a yellow
oil, 0.097 g, 69% yield as a 5:1 mixture of regioisomers.
4.3.5. 5-Benzyloxymethyl-3-bromo-4-(4,4,5,5-tetra-
methyl[1,3,2]dioxaborolan-2-yl)-isoxazole (24). The
same general procedure was carried out with 2-(3-
benzyloxy-prop-1-ynyl)-4,4,5,5-tetramethyl[1,3,2]dioxa-
borolane (5) (0.20 g, 0.74 mmol, 2.5 equiv), purification by
flash column chromatography (eluting solvent petroleum
ether/ethyl acetate 10:1 ratio) gave the title compound (24)
as a colourless oil, 0.046 g, 40% yield; 1H NMR (250 MHz,
CDCl3): d 1.29 (12H, s, 4!CH3), 4.60 (2H, s, CH2), 4.77
(2H, s, CH2), 7.31–7.38 (5H, m, Ar-H); 13C NMR
(100.6 MHz, CDCl3) d 24.8, 65.4, 73.1, 84.3, 127.7,
128.2, 128.6, 137.3, 144.5, 177.8; FTIR nmax/CHCl3, 3419
(br), 2977 (m), 2926 (w), 2867 (w), 1596 (s) cmK1; HRMS
calcd for C17H21BNO4Br: 393.0747, found: 393.0741.
1
(27) (minor): H NMR (250 MHz, CDCl3): d 0.23 (9H, s,
Si(CH3)3), 1.31 (12H, s, 4!CH3), 2.61 (3H, s, COCH3);
1
(27) (major): H NMR (250 MHz, CDCl3): d 0.32 (9H, s,
4.3.6. 3-Bromo-5-phenylsulfanylmethyl-4-(4,4,5,5-tetra-
methyl[1,3,2]dioxaborolan-2-yl)-isoxazole (25). The
same general procedure was carried out with 4,4,5,5-
Si(CH3)3), 1.34 (12H, s, 4!CH3), 2.59 (3H, s, COCH3); 13
C
NMR (100.6 MHz, CDCl3) Major regioisomer only d K1.8,
25.0, 28.4, 84.5, 163.6, 184.5, 193.1; FTIR nmax/CHCl3, 2995