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4791
the aqueous layer reached pH 10. The organic layer was
separated, dried (MgSO4), and evaporated to afford
23.63 g of crude product, which was suspended in hot
hexane, filtered and dried to afford 5 (17.75 g, 79% ):
m.p. 238–240 ꢁC; IR (nujol) (inter alia) 3067, 2976,
(1H, d, J 7.5 Hz); 13C NMR (125 MHz, CDCl3) 20.45,
20.66, 25.10, 45.94, 51.11, 83.99, 124.81, 127.58,
130.80, 135.76, 145.24, 171.49; dB (CDCl3, 96 MHz)
29.98; MS (EI+) m/z 330 (61%, M+ ꢀ H), 273 (30),
188 (67), 130 (49), 83 (100); Acc. MS calc. for
C19H30BNO3 m/z 331.2319; found 331.2324.
2933, 1632 (s, CO.N), 1608, 1564 cmꢀ1 1H NMR
;
(500 MHz, CDCl3) d 1.07 (d, J 6.5 Hz, 6H), 1.27 (d, J
6.5 Hz, 6H), 3.39 (sept, J 6.5 Hz, 1H), 4.10 (sept, J
6.5 Hz, 1H), 7.24 (d, J 7.5 Hz, 1H), 7.33–7.42 (m, 2H),
8.04 (d, J 7.5 Hz, 1H); 13C NMR (126 MHz, CDCl3) d
20.46, 20.54, 47.10, 50.90, 125.00, 129.11, 129.43,
134.93, 140.5 (br s, ArC-B), 141.64, 172.52, 141.61; 11B
NMR (96 MHz, CDCl3) d 21 (br); MS (EI+) m/z 693
(7%, M+), 105 (51), 57 (100); Anal. calc. for
C39H54B3N3O6 requires C, 67.56; H, 7.85; N, 6.06;
found: C, 67.38; H, 7.98; N, 5.96%.
3.4. Conversion boroxine 3 into ester 7
A mixture of boroxine 3 (0.55 g, 2.40 mmol), pinacol
(0.28 g, 2.4 mmol), dilute aqueous hydrochloric acid
(2.40 ml of a 1 M solution), diethyl ether (50 ml) and
water (5 ml) was stirred for 48 h, followed by adjustment
of the pH to 9 (aqueous saturated sodium carbonate).
After separation, the aqueous layer was re-extracted
with diethyl ether (3·), the combined organic extracts
were dried (MgSO4) and evaporated to afford pinacol
ester 7 (0.73 g, 92%) which was spectroscopically and
analytically identical to that prepared in the previous
experiment.
3.3. Preparation of pinacol ester 7 via diisopropylester 6
TMEDA (7.50 ml, 49.6 mmol) and n-butyllithium
(35.9 ml of a 1.38 M solution in hexanes) were stirred
in dry tetrahydrofuran (500 ml) at ꢀ78 ꢁC under argon.
N,N-diisopropylbenzamide (15.0 g, 45.0 mmol) was
added in dry tetrahydrofuran (100 ml) dropwise over
10 min. After 45 min, triisopropylborate (10.4 ml,
45.0 mmol) was added in dry tetrahydrofuran (100 ml)
over 15 min. After a further 2 h the mixture was allowed
to warm to room temperature and the solvent was re-
moved by evaporation to give a sticky solid, which
was then re-dissolved in diethyl ether (500 ml). The ethe-
real solution was washed with dilute aqueous hydro-
chloric acid (49.6 ml of a 1 M solution), and water
(100 ml). After separation, the aqueous was re-extracted
with diethyl ether (2·), the combined organic extracts
were dried (MgSO4), and evaporated to afford crude
diisopropylester 7 (9.79 g, 40%) which was used without
purification. The crude ester 6 (6.29 g, 18.9 mmol) was
dissolved in Et2O (60 ml) and treated with water
(40 ml), followed by pinacol (3.55 g, 18.9 mmol) and di-
lute aqueous hydrochloric acid (9.0 ml of a 1 M solu-
tion). After 72 h, the reaction pH was adjusted 9
(aqueous saturated sodium carbonate), separated, and
the aqueous layer was re-extracted (3·) with diethyl
ether. The combined organic extracts were dried
(MgSO4) and evaporated to afford pinacol ester 7
(5.63 g, 90%) as a light brown sticky solid, which slowly
crystallised on stand ing. Pure crystals of 7 were isolated
by suspension in hexane to afford 4.01 g (64% yield) as
white crystals. Further slow re-crystallisation (hexane/
Et2O diffusion) provided crystals suitable for single crys-
tal X-ray analysis: m.p. 108–110 ꢁC; IR (film) (inter alia)
3061, 2977, 2930, 1635 (s, CO.N), 1612, 1596, 1563,
3.5. Reduction of diisopropylamide 7 to give
benzylamine 8
To a stirred suspension of sodium borohydride
(12.14 g, 320 mmol) in dry tetrahydrofuran (350 ml) un-
der argon was added chlorotrimethylsilane (81.23 ml,
636 mmol) and the mixture heated at reflux for 2 h.
After cooling to RT, the pinacol ester 7 (16.58 g,
23.9 mmol) was added as a suspension in tetrahydrofu-
ran (30 ml) and the mixture heated at reflux for 64 h.
After cooling to room temperature, the reaction mixture
was quenched carefully with methanol (480 ml) over
30 min (Caution. evolves H2.), followed by water
(50 ml). The solvent was partially evaporated, saturated
aqueous ammonium chloride (40 ml) was added, fol-
lowed by aqueous hydrochloric acid (80 ml of a 3 M
solution), taking the aqueous layer to pH 1. Dichloro-
methane (250 ml) was added, followed by solid sodium
carbonate with vigorous stirring until the aqueous layer
reached pH 9. The organic layer was separated, the
aqueous layer was re-extracted with dichloromethane
(2 · 100 ml), the combined organic extracts were dried
(MgSO4) and evaporated to afford benzylamine 8
(15.88 g, 96%) as a mixture of boronic acid and anhy-
dride. Slow crystallisation from water provided crystals
of boronic acid 8a which were suitable for single crystal
X-ray analysis: m.p. 152.0–152.9 ꢁC; IR (nujol) (inter
alia) 970, 2352, 2333, 1376, 752 cmꢀ1; UV (CH3CN)
1
196 (e 46,085), 220 (e 8512) nm; H NMR (400 MHz,
CDCl3) d 1.14 (d, J 6.8, 12H), 3.15 (septet, J6.8, 2H),
3.86 (s, 2H), 7.26–7.22 ( m, 1H), 7.38–7.29 (m, 2H),
7.99–7.96 (m, 1H), 9.58 (br, 2H) (addition of D2O
caused the peak at d 9.58 to disappear); 13C NMR
(101 MHz, CDCl3) d 19.74, 47.52, 51.89, 127.01,
130.10, 130.70, 136.72, 142.30; 11B NMR (128 MHz,
1
1448, 1432 cmꢀ1; H NMR (500 MHz, CDCl3) 1.12 (d,
J 6.5 Hz, 6H), 1.32 (s, 12H), 1.58 (d, J 7 Hz, 6H), 3.51
(sep, J 6.5 Hz, 1H), 3.75 (sep, J 6.5 Hz, 1H), 7.16 (d, J
7.5 Hz, 1H), 7.32 (1H, m), 7.34–7.44 (1H, m), 7.81