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12 The X-ray structure of pyran 8 is available in supplementary materials
(Figure
[1]
).
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16 For selected experimental procedures
and characterizations:
General Procedure
for Lewis Acid Mediated Allylations Using Pyran 8: To a solution
of 317.7 mg of pyran 8 in 65 mL of freshly
distilled CH2Cl2 at -78 °C
under N2 were added 503.9 mg of SnBr4 (1.5
equiv, 1.16 mmol) and 0.488 mL of allyltrimethylsilane (4 equiv,
3.06 mmol). The reaction was vigorously stirred for 12 h and allowed
to slowly warm to r.t. The solvent was removed under reduced pressure
and purification using silica gel column chromatography afforded
157.6 mg of 9a and 9b as
a mixture in addition to recovery of the Close’s auxiliary
in 70-90% recovery range when attempted. The ratio
of 9a:9b was found
to be 4:1 from the crude 1H NMR. Preparative thin
layer chromatography (1% Et2O in hexanes) was
useful to separate the major isomer 9a from
the minor isomer 9b.
9a (major). Rf = 0.30
(10% Et2O in hexanes). 1H
NMR (500 MHz, CDCl3): δ = 1.06 (d, J = 7.0 Hz,
3 H), 1.70 (m, 1 H), 1.78 (m, 1 H), 2.03 (m, 2 H), 2.18 (m, 1 H),
2.47 (ddd, J = 1.0,
10.0, 18.0 Hz, 2 H) 3.43 (ddd, J = 6.0,
7.0, 7.0 Hz, 1 H), 4.96 (dd, J = 1.0, 10.0 Hz, 1 H),
5.04 (dd, J = 2.0,
15.0 Hz, 1 H), 5.52 (dd, J = 4.0,
8.0 Hz, 1 H), 5.74 (m, 1 H), 7.27-8.31 (m, 7 H). 13C
NMR (125 MHz, C6D5CD3): δ = 137.9, 135.8,
134.2, 128.8, 128.5, 127.8, 125.4, 125.2, 125.1, 124.8, 124.2, 115.6,
76.8, 69.6, 36.8, 32.2, 27.4, 27.3, 17.8. IR (thin film): 3052 (m),
2932 (m), 1641 (m) cm-1. MS (EI): m/e (% relative
intensity) = 266.2 (25) [M+],
249.6 (100). Optical rotation was not pursued because samples of 9a still contained some 9b.
9b (minor). Rf = 0.30
(10% Et2O in hexanes). 1H
NMR (500 MHz, CDCl3): δ = 1.06 (d, J = 7.0 Hz,
3 H), 1.55 (m, 3 H), 1.74 (m, 1 H), 2.03 (m, 2 H), 2.65 (m, 1 H),
4.01 (dd, J = 4.0, 10.0
Hz, 1 H), 4.97 (d, J = 10.0 Hz, 1 H), 5.11
(d, J = 17.0 Hz,
1 H), 5.33 (dd, J = 1.0,
10.0 Hz, 1 H), 5.88 (m, 1 H), 7.07-8.25 (m, 7 H). 13C
NMR (125 MHz, C6D5CD3): δ = 136.0,
128.9, 128.8, 127.8, 127.5, 125.4, 123.8, 123.6, 115.7, 76.8, 68.1,
33.3, 32.6, 30.5, 27.6, 16.7 (3 signals are missing overlap with
solvent). IR (thin film): 3064 (m), 2945 (m), 1635 (m)cm-1.
MS (EI): m/e (% relative
intensity) = 284.2(50) [M + NH4]+,
264.1(10), 247.1(100); m/e calculated for C19H26NO:
284.2015. Found: 284.2015.
General
Procedure for Lewis Acid Mediated Allylations Using Pyran 20: To
a solution of pyran 20 (5.0 mg, 0.011 mmol)
in 1.0 mL of anhyd CH2Cl2 at -78 °C
were added 7.63 mg SnBr4 (1.5 equiv, 0.0174 mmol) and
7.50 µL of allyltrimethylsilane (4 equiv. 0.0464 mmol).
The mixture was warmed to r.t. and stirred at r.t. for 12 h before
it was quenched with sat. aq NH4Cl (2 mL). The resultant
mixture was extracted with CH2Cl2 (3 × 3
mL), and the combined extracts were dried over Na2SO4,
filtered, and concentrated under reduced pressure. Silica gel flash
chromatography (gradient eluent: 0-10% EtOAc in
hexanes) of the crude gave 2.32 mg (combined yield 70%)
of pyran 21a and 22b with
a 7:1 diastereomeric ratio.
Pyran 21a (major):
Rf = 0.30 (10% EtOAc
in hexanes). 1H NMR (500 MHz, CDCl3): δ = 0.99
(d, J = 7.0
Hz, 3 H), 1.36 (ddd, J = 6.0,
12.0, 18.0 Hz, 1 H), 1.75 (dd, J = 12.0,
22.0 Hz, 1 H), 1.86 (m, 1 H), 2.27 (ddd, J = 6.5,
14.0, 15.0 Hz, 1 H), 2.42 (ddd, J = 4.5,
11.0, 16.5 Hz, 1 H), 2.71 (d, J = 1.5 Hz,
1 H), 3.68 (ddd, J = 4.5,
7.0, 15.5 Hz, 1 H), 4.46 (ddd, J = 3.5,
8.5, 15.5 Hz, 1 H), 5.13 (ddd, J = 9.5,
15.5, 17.5 Hz, 2 H), 5.84 (d, J = 2.0
Hz, 1 H), 5.94 (m, 1 H), 7.08-8.20 (m, 7 H). 13C
NMR (75 MHz, CDCl3): δ = 134.9, 128.7,
127.5, 125.8, 125.4, 125.2, 122.8, 122.6, 116.7, 86.0, 80.6, 69.7, 39.0,
38.5, 33.5, 16.2 (missing 3 peaks). IR (thin film): 3432 (s), 3072
(w), 3063 (w), 2971 (s), 2962 (s), 2953 (s) cm-1. MS
(EI): m/e (% relative
intensity) = 282.2 (15) [M+], 125.1(100); m/e calcd
for C19H22O2: 282.1620. Found: 282.1618.
Pyran 21b (minor): Rf = 0.32
(10% EtOAc in hexanes). 1H NMR (500
MHz, CDCl3): δ = 0.98 (d, J = 7.0 Hz,
3 H), 1.33 (ddd, J = 11.0,
20.5, 22.5 Hz, 1 H), 1.64 (dd, J = 5.5,
15.5 Hz, 1 H), 2.28 (m, 3 H), 2.82 (s, 1 H), 4.05 (ddd, J = 10.0, 13.0,
22.5 Hz, 1 H), 4.36 (ddd, J = 5.5,
10.5, 16.0 Hz, 1 H), 5.22 (m, 2 H), 5.82 (d, J = 4.5
Hz, 1 H), 5.98 (m, 1 H), 7.30-8.20 (m, 7 H). MS (EI): m/e (% relative
intensity) = 282.2(15) [M+],
125.1(100); m/e calcd
for C19H22O2: 282.1620. Found:
282.1617.
Hydroboration Reactions: To
a solution of 9.0 mg of pyran 16 (0.0218
mmol) in 2.0 mL of anhyd THF at r.t. was added 0.25 mL of BH3〈THF
complex (2.5 equiv, 1 M solution in THF). The resultant mixture
was stirred for 1 h and warmed to 55 °C for 1
h, and then, the mixture was cooled to r.t. and excess 30% aq
H2O2 and 15% aq NaOH was added dropwise carefully.
The resultant mixture was warmed to 60 °C for
10 min and vigorously stirred at r.t. for 1 h. The mixture was extracted
with Et2O (2 × 5 mL) and EtOAc
(2 × 5 mL). The combined extracts were
dried over Na2SO4, filtered, and concentrated
under reduced pressure. Silica gel flash chromatography (50% EtOAc
in hexanes) of the crude provided 6.62 mg of the desired alcohol 20 (70% yield) as a colorless
oil.
Pyran 20: Rf = 0.35
(50% EtOAc in hexanes). [α]
d
20 =
-77.0
(c 0.35, CHCl3). 1H
NMR (500 MHz, CDCl3): δ = 0.68 (d, J = 7.0 Hz,
3 H), 0.94 (d, J = 6.5
Hz, 3 H), 2.01 (ddd, J = 3.5,
7.0, 10.0 Hz, 2 H), 2.25 (m, 1 H), 2.67 (s, 3 H), 3.72 (dq, J = 7.0, 8.5
Hz, 1 H), 4.02 (dd, J = 7.5,
16.0 Hz, 1 H), 4.84 (d, J = 8.0
Hz, 1 H), 5.04 (d, J = 9.0
Hz, 1 H), 5.63 (d, J = 2.0
Hz, 1 H), 7.20-8.41 (m, 12 H). 13C
NMR (125 MHz, CDCl3): δ = 162.5, 139.0,
134.9, 133.8, 131.7, 128.8, 128.5, 128.2, 127.5, 126.1, 125.7, 125.3,
125.0, 124.4, 86.3, 82.4, 67.3, 58.9, 57.1, 40.2, 32.6, 28.6, 15.1,
13.9 (missing 3 signals). IR (thin film): 3391 (m), 3029 (w), 2980
(w), 2930 (m), 2878 (m), 1684 (s), 1435 (m) cm-1.
MS (EI): m/e (% relative
intensity) = 430.3(5) [M+],
273.2(100); m/e calcd
for C27H30N2O3: 430.2256.
Ffound: 430.2234.
Pyran 22 [C6-epimer]:
Rf = 0.35 (50% EtOAc
in hexanes). 1H NMR (500 MHz, CDCl3): δ = 0.80
(d, J = 7.0
Hz, 3 H), 1.03 (d, J = 6.0
Hz, 3 H), 1.15 (ddd, J = 3.0,
5.0, 12.0 Hz, 1 H), 1.67 (dd, J = 12.0, 23.0 Hz, 1
H), 2.46 (m, 1 H), 2.81 (s, 3 H), 3.51 (ddd, J = 3.5,
5.0, 10.5 Hz, 1 H), 3.79 (dq, J = 6.5, 8.5
Hz, 1 H), 4.69 (d, J = 8.5
Hz, 1 H), 5.37 (d, J = 9.0
Hz, 1 H), 5.45 (d, J = 3.0
Hz, 1 H), 7.20-8.25 (m, 12 H). 13C
NMR (75 MHz, CDCl3): δ = 167.6, 136.9,
134.9, 133.2, 128.6, 128.5, 128.3, 127.9, 127.8, 127.4, 125.6, 125.3,
125.1, 122.8, 90.9, 80.1, 68.4, 60.7, 55.7, 35.0, 32.2, 29.6, 28.5, 16.0,
14.5 (missing 2 peaks). MS (EI): m/e (% relative intensity) = 430.3
(5) [M+], 273.2 (100); m/e calcd
for C27H30N2O3: 430.2256.
Found: 430.2232.
17
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21 When substituents at C5 and C6 are trans in these pyranyl heterocycles,
we observed strong NOE (see pyrans 21 and 22) between protons at C2 and C3 presumably
because these two protons are not necessarily locked in a diaxial relationship
unlike those in 20.