Construction of the tricyclic furanochroman skeleton of phomactin a via the Prins/Conia-ene cascade cyclization approach

Article abstract of DOI:10.1021/jo200644t

A substrate-controlled asymmetric Prins/Conia-ene cascade cyclization has been developed with In(OTf)₃ in CH₃CN from 0 to 70 °C. These conditions afforded very good yields of the 1-oxadecalin product in one pot and effectively suppre

Full text of DOI:10.1021/jo200644t

ARTICLE
pubs.acs.org/joc
Construction of the Tricyclic Furanochroman Skeleton of Phomactin A
via the Prins/Conia-Ene Cascade Cyclization Approach
Shuangping Huang, Guangyan Du, and Chi-Sing Lee*
Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen Graduate School,
Shenzhen University Town, Xili, Shenzhen 518055, China
S Supporting Information
b
ABSTRACT:
A substrate-controlled asymmetric Prins/Conia-ene cascade cyclization has been developed with In(OTf)3 in CH3CN from 0 to
70 °C. These conditions afforded very good yields of the 1-oxadecalin product in one pot and effectively suppressed the racemization
of the 1-oxadecalin product with almost no enantiomeric excess (ee) loss. This cascade cyclization has been successfully employed
for the construction of the highly functionalized 1-oxadecalin unit of phomactin A with an acyclic β-keto ester and an alkynal as the
substrates via a one-pot operation (66% yield, single diastereomer). The 1-oxadecalin moiety has been readily converted to the
tricyclic furanochroman skeleton of phomactin A via the epoxidation/dealkoxycarbonylation protocol under very mild conditions
with 52% yield in three steps.
’ INTRODUCTION
Prins reaction1,2 involves addition of an alkene to a Brønsted
or Lewis acid-activated aldehyde substrate followed by trapping
of a nucleophile (or a solvent molecule) or regeneration of an
alkene via deprotonation. This class of reactions has been
developed into a very useful cyclization method for construction
of highly functionalized tetrahydropyrans and tetrahydropyranones.3ꢀ10
Recently, we have reported an efficient Prins/Conia-ene cascade
cyclization for construction of highly functionalized 1-oxadeca-
lins using InCl3 with simple acyclic substrates.11,12 As shown in
Figure 1, condensation of the β-keto ester13ꢀ19 and alkynal
substrates with InCl3 provided the oxocarbenium ion intermediate,20
which underwent Prins cyclization and formed the tetrahydro-
Figure 1. Prins/Conia-ene cascade cyclization reaction.
pyran-4-one intermediate. Subsequent Conia-ene reaction pro-
vided very good yields of the 1-oxadecalin product with excellent
Prins/Conia-ene cascade cyclization approach, we have decided
diastereoselectivity in one pot. We herein report the develop-
to develop this cascade cyclization into an asymmetric transfor-
ment of a substrate-controlled asymmetric Prins/Conia-ene
mation through substrate control and consider two model
cascade cyclization and the utility of this highly convergent
compounds (1 and 2) that bear the tricyclic furanochroman
skeleton with two all-carbon quaternary centers at C2 and C6
cascade cyclization approach in the synthesis of the tricyclic
furanochroman skeleton of phomactin A.
(Figure 2). The C-ring of the model compounds is anticipated to
Phomactins represented a new class of platelet activating
be established from (()-3 or 4 using an epoxidation/dealkox-
factor (PAF) antagonists isolated from a marine fungus Phoma
ycarbonylation protocol. The A,B-ring subunit could be readily
sp. and exhibited inhibition against PAF-induced platelet
aggregation.21ꢀ26 Phomactin A is structurally the most complex
prepared via a one-pot process by the Prins/Conia-ene cascade
cyclization of β-keto esters 5 or 6 with alkynal 7.
member in the phomactin family and has attracted a tremendous
amount of synthetic effort27ꢀ39 including three elegant total
syntheses reported by Pattenden,40ꢀ44 Halcomb,45ꢀ47 and Hsung.48ꢀ51
To realize an asymmetric total synthesis of phomactin A via the
Received: March 27, 2011
Published: July 08, 2011
r
2011 American Chemical Society
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Table 1. Racemization Study of Prins/Conia-Ene Cascade
Cyclizationa
entry
L.A.
InCl3
solvent
temperature yieldsb (%) % eec (9)
1
2
3
4
5
6
7
8
9
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
40 °C
73
80
n.d.d
InCl3
0ꢀ40 °C
0ꢀ40 °C
0ꢀ40 °C
0ꢀ40 °C
0ꢀ40 °C
0ꢀ70 °C
0ꢀ110 °C
0ꢀ70 °C
trace
62
InBr3
88
87
In(OTf)3
83
Zn(OTf)2 CH2Cl2
trace
42
n.d.d
SnCl4
CH2Cl2
THF
89
84
86
89
In(OTf)3
In(OTf)3
In(OTf)3
34
Figure 2. Construction of the tricyclic furanochroman skeleton of
phomactin A via the Prins/Conia-ene cascade cyclization approach.
toluene
60
CH3CN
85
a The general procedures were followed with the indicated Lewis acid
’ RESULTS AND DISCUSSION
and reaction conditions. b Isolated yields (%) after silica gel column
c
chromatography. The ee values were determined by Daicel Chiracel
Development of Asymmetric Prins/Conia-ene Cascade
Cyclization via Substrate Control. Since the Prins/Conia-ene
cascade cyclization can provide the 1-oxadecalin product with
excellent diastereocontrol, we have decided to develop an
asymmetric cascade cyclization via substrate control which could
become a useful tool for synthesis of 1-oxadecalin containing
natural products. However, side reactions associated with this
type of process including partial racemization and the exchange
of aldehyde and alcohol side chains leading to mixtures of products
have been reported recently.52ꢀ63 Thus, racemization of the
1-oxadecalin product was studied using the reaction between
optically enriched (R)-864 and hex-5-ynal.65 As shown in Table 1,
InCl3 in refluxing CH2Cl211 provided 1-oxadecalin (+)-9 in good
yields, but more than 10% loss of the enantiomeric excess (ee)
value (entry 1) resulted. According to the literature, racemization
of Prins reactions can be suppressed by lowering the reaction
temperature with an appropriate Lewis acid.66 Thus, a variety of
Lewis acids in CH2Cl2 at low reaction temperature were exam-
ined. Prins reaction with InCl3 proceeded very slowly at 0 °C, and
the substrates decomposed slowly under the reaction condition
(entry 2). Switching to a stronger Lewis acid (InBr3) at 0 °C led
to a complete reaction in one hour. Subsequent Conia-ene
reaction was achieved at 40 °C and gave 62% of the cascade
cyclization product ((+)-9) in one pot with only 3% ee loss
(entry 3). The yield of (+)-9 was improved to 83% by using
In(OTf)3 with a similar level of racemization (entry 4). Zn-
(OTf)2 gave a result similar to that of InCl3 (entry 5). Racemiza-
tion was almost completely suppressed by using SnCl4 (89% ee),
but a very poor yield of the product resulted (entry 6). After a
brief survey of the solvent effect (entries 7ꢀ9), the results were
optimized by using In(OTf)3 in CH3CN, which afforded 85%
yield of 9 with almost no ee loss (entry 9).
OD-H column (hexanes: i-propanol = 98:2, 0.3 mL/min: t1 = 38.61 min,
t2 = 44.12 min). d n.d. = not determined.
the 2,6-trans-substituted tetrahydropyran was not observed.
These results suggested that racemization could occur only
before the tetrahydropyran ring formation, and once the 1-ox-
adecalin has been established, ring opening and closing of the
tetrahydropyran could not be effective even at high reaction
temperature. The effects of the Lewis acid on the β-keto ester
substrate alone were investigated. Upon treatment of InCl3 in
CH2Cl2, a 20% decrease in the ee value of (R)-8 was observed
after 1 h at 40 °C. On the other hand, the racemization was
completely suppressed under In(OTf)3 in CH3CN at 0 °C.
These results indicated that the racemization of the cascade
reaction could occur via a stabilized carbocation intermediate.
Thus, a racemization mechanism base on Willis’s work58 was
proposed. As shown in Figure 3, the stabilized carbocation (10)
could be generated upon treatment of a Lewis acid at high
reaction temperature. The resulting carbocation (10) could be
either attacked by the carbonyl moiety of the aldehyde to form
(()-11 or converted to (()-8 followed by condensation with
the aldehyde and formed the same oxocarbenium ion intermedi-
ate. Since the racemization of (R)-8 can be effectively suppressed
under our optimal conditions (In(OTf)3/CH3CN, 0 °C), the
1-oxadecalin product ((+)-9) can be obtained with almost no ee
loss. According to Rychnovsky’s mechanistic studies,67 another
racemization mechanism could also be effective at high reaction
temperature. As shown in Figure 4, condensation between (R)-8
and hex-5-ynal provided oxocarbenium ion 12, which could under-
go either a Prins reaction (cyclization followed by loss of a proton)
and form the expected tetrahydropyran product (14) or an
oxocarbenium ion isomerization followed by a 3,3-sigmatropic
rearrangement and give 18. After a series of CꢀC bond rotations,
oxocarbenium ion 19 could undergo either a Prins reaction and
give 22 (the enantiomer of 15) or a 3,3-sigmatropicrearrangement
and afford 20, which could also lead to 22 via a Prins reaction.
In contrast to the modified MaitlandꢀJapp reaction (Figure 5)13ꢀ18
between β-keto esters and aldehydes that afford the trans-tetrahydro-
pyran ring as the minor product with no racemization being
To determine the racemization-involving steps in the cascade
cyclization reaction, the effects of extending the reaction time at
different reaction temperatures with In(OTf)3/CH3CN were
studied. Interestingly, no significant change in yields or ee values
was observed by extending the reaction time from 1 to 5 h for the
Prins cyclization step at 0 °C. Moreover, similar results were
obtained by extending the reaction time from 5 to 12 h for the
Conia-ene reaction at 70 °C, and the 1-oxadecalin product bearing
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Figure 3. Proposed racemization mechanism for the Prins/Conia-ene cascade cyclization based on a stabilized carbocation intermediate.
Figure 4. Proposed racemization mechanism for the Prins/Conia-ene cascade cyclization based on an oxocarbenium ion isomerization and a 3,3-
sigmatropic rearrangement.
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acetone. Lactone ring opening of 30 with 2-trimethylsilyl
ethanol provided β-keto ester 6 in good yields. However,
Prins/Conia-ene cascade cyclization of 6 with alkynal 7 using
In(OTf)3 in CH3CN did not give the expected cyclized product
((()-4). The dealkoxycarbonylation product (32) was obtained
as the major side product, which may result from hydrolysis of
the 2-(trimethylsilyl)ethyl ester. This side reaction was sup-
pressed by removing water from the reaction mixture by addition
of 4 Å molecular sieves, and the cascade cyclization afforded (()-
4 in 66% yield. Epoxidation of (()-4 using mCPBA afforded the
dealkoxycarbonylation precursor ((()-31), which was treated
with TBAF and afforded the expected crude dealkoxycarbonyla-
tion product ((()-1 or 28). The crude product ((()-1) was
found to be unstable, and it underwent dehydration rapidly in the
purification process to give the furan derivative (33). This
observation is consistent with that reported by Pattenden.40
The crude product was then treated with a catalytic amount of
TsOH in methanol and afforded the stable tricyclic model
compound ((()-2) in decent yields, which has been character-
ized unambiguously by 1H, 13C NMR, and HRMS. The relative
configurations of (()-2 were determined by NOESY (Scheme 2).
Figure 5. Study on the modified MaitlandꢀJapp mechanism.
observed,16 the trans-tetrahydropyran isomers were not observed
in our system. This result indicated that reopening and closing of
the tetrahydropyran ring could be unfavorable under our reaction
conditions. Moreover, Knoevenagel condensation between
β-keto ester 2468 (the dehydroxy derivative of 8) and hex-5-ynal
did not provide the condensation product (25) under our
optimal condition and led to decomposition of the aldehyde
substrate. This result suggested that the modified Mai-
tlandꢀJapp mechanism (Knoevenagel/oxa-Michael) is unlikely
under our reaction conditions.
Synthesis of the Tricyclic Core of Phomactin A. In our
previous study, the Prins/Conia-ene cascade cyclization of
sterically hindered substrate (Scheme 1) provided only modest
yields even in high reaction temperature.11 Thus, the Prins/
Conia-ene cascade cyclization between two sterically demanding
substrates, β-keto ester 569 and alkynal 7,70 was studied. For-
tunately, our optimal condition (In(OTf)3/CH3CN) provided
80% yield of 1-oxadecalin 3 as a single diastereomer.71 The
β-keto ester moiety of (()-3 could be converted to the β-hydroxyl
ketone for construction of the tricyclic furanochroman skeleton of
phomactin A via Totah’s epoxidation/retro-Aldol protocol.31 How-
ever, this approach required a number of functional and protecting
group manipulations. Alternatively, direct dealkoxycarbonylation of
(()-26 should lead to intermediate 27, which could undergo
epoxide ring opening and gave the tricyclic core of phomactin
A (28 or (()-1) in one pot. To study this direct dealkoxy-
carbonylation approach, (()-3 was first converted to (()-26
(a 4:1 diastereomeric mixture) using mCPBA. Unfortunately,
(()-26 was found to be inert with a variety of dealkoxycar-
bonylation conditions72ꢀ77 and decomposed upon prolonged
heating.
’ CONCLUSION
In summary, we have successfully developed an asymmetric
Prins/Conia-ene cascade cyclization via substrate control. The
cyclization has been optimized by using In(OTf)3 in CH3CN
from 0 to 70 °C. These conditions afforded the 1-oxadecalin
product in good yields and effectively suppressed the racemiza-
tion of the 1-oxadecalin product with almost no ee loss. The
racemization-involving steps have been investigated by varying
the reaction conditions. The optimal reaction condition also
provided good yields for (()-4 (66%, single diastereomer) in
one pot by addition of 4 Å molecular sieves for removing water
from the reaction. The 2-trimethylsilyl ethyl moiety of (()-4 was
successfully converted to the tricyclic furanochroman skeleton of
phomactin A in 52% yield (for three steps) via the epoxidation/
dealkoxycarbonylation protocol under a very mild condition. We
are currently employing this cascade cyclization strategy for the
total synthesis of phomactin A.
’ EXPERIMENTAL SECTION
By replacing the ethyl ester with a 2-(trimethylsilyl)ethyl ester,
(()-31 was anticipated to undergo dealkoxycarbonylation under
a much milder condition.78,79 As shown in Scheme 2, synthesis of
(()-31 began with Mukaiyama aldol reaction between 2980 and
General Remarks. All air- and water-sensitive reactions were carried
out under a nitrogen atmosphere with dry solvents under anhydrous
conditions, unless otherwise noted. Reactions were monitored by thin-layer
Scheme 1. Prins/Conia-Ene Cascade Cyclization of Sterically Hindered Substrates
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Scheme 2. Synthesis of the Tricyclic Furanochroman Skeleton of Phomactin A and the Important NOESY Correlations of (()-2
ARTICLE
chromatography (TLC) carried out on 0.25 mm silica gel plates (60F-254)
that were analyzed by fluorescence upon 254 nm irradiation or by staining
with anisaldehyde (450 mL of 95% EtOH, 25 mL of conc. H2SO4, 15 mL of
acetic acid, and 25 mL of anisaldehyde). Silica gel (60, particle size
0.040ꢀ0.063 mm) was used for flash column chromatography. All the
chemicals were purchased commercially and used without further purifica-
tion. Anhydrous THF was distilled from sodium-benzophenone. Toluene
was distilled over Na. CH3CN and CH2Cl2 were distilled from calcium
hydride. Molecular sieves were activated by heating at 200 °C for 12 h at
∼1.0 Torr. Yields refer to chromatographically, unless otherwise stated.
NMR spectra were recorded on either a 300 (1H: 300 MHz, 13C: 75.5
MHz) or 500 MHz (1H: 500 MHz, 13C: 125.8 MHz). The NOESY
experiments were performed on a 600 MHz spectrometer. The following
abbreviations were used to explain the multiplicities: s = singlet, d = doublet,
t = triplet, q = quartet, m = multiplet, br = broad. High-resolution mass
spectra were obtained from a MALDI-TOF Mass Spectrometer. High-
performance liquid chromatography analysis was performed using an OD-
H column (250 ꢁ 4.6 mm) or AD column (250 ꢁ 4.6 mm) with iPrOH/
hexanes as the eluent. Optical rotations were measured on a digital
polarimeter in CHCl3.
0.12 mmol) in CH3CN (2 mL) at 0 °C was added a solution of hex-5-
ynal (0.015 g, 0.16 mmol) and β-keto ester 8 (0.025 g, 0.11 mmol, 91%
ee) in CH3CN (1 mL) via a cannula. The resulting mixture was stirred at
0 °C for 1 h and 70 °C for 5 h. The reaction mixture was then cooled to
room temperature and diluted with a saturated aqueous solution of
NaHCO3 (1 mL). The aqueous layer was extracted with ethyl acetate
(5 mL ꢁ 3), and combined extracts were washed with brine, dried over
sodium sulfate, filtered, and concentrated. Silica gel flash column
chromatography (hexanes:ethyl acetate = 30:1) of the residue afforded
a colorless oil (0.029 g, 0.092 mmol, 85%) as the product (9). 1H NMR
(300 MHz, CDCl3): δ 7.39ꢀ7.32 (m, 5H), 5.23 (d, J = 2 Hz, 1H), 4.85
(dd, J = 12, 3 Hz, 1H), 4.58 (d, J = 2 Hz, 1H), 4.56 (m, 1H), 4.34ꢀ4.26
(m, 2H), 2.93 (dd, J = 15, 12 Hz, 1H), 2.63 (dd, J = 15, 3 Hz, 1H),
2.52ꢀ2.46 (m, 1H), 2.31ꢀ2.14 (m, 2H), 2.09ꢀ1.93 (m, 1H),
1.67ꢀ1.53 (m, 2H), 1.30 (t, J = 7 Hz, 3H). 13C NMR (125 MHz,
CDCl3): δ 204.5, 169.2, 143.0, 140.9, 128.6, 128.1, 125.6, 115.6, 79.1,
78.0, 69.0, 61.6, 47.1, 33.5, 28.6, 20.2, 14.1. IR (neat, cmꢀ1): 3065, 1731,
1710, 1641, 1496, 1455, 1228. 8: [R]2D0 = +32.3° (91% ee (R), c = 0.1,
CHCl3). 9: [R]2D0 = +188.1° (89% ee, c = 0.1, CHCl3), HRMS (+ESI)
m/z calcd. for C19H22O4Na+ (M + Na+) 337.1416, found 337.1406.
Synthesis of Racemic Ethyl 2,2,6,6-Tetramethyl-5-methylene-4-ox-
ooctahydro-2H-chromene-4a-carboxylate (3). The general procedures
General Procedures for Asymmetric Prins/Conia-Ene Cas-
cade Cyclization. To a stirred suspension of In(OTf)3 (0.066 g,
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were followed with In(OTf)3 (0.093 g, 0.16 mmol), alkynal 7 (0.031 g,
0.25 mmol), and β-keto-ester 5 (0.031 g, 0.16 mmol). A colorless oil
(0.038 g, 0.13 mmol, 80%) was obtained as the product. 1H NMR
(300 MHz, CDCl3): δ 5.36 (s, 1H), 4.64 (s, 1H), 4.62 (m, 1H), 4.30ꢀ
4.13 (m, 2H), 2.91 (d, J = 13 Hz, 1H), 2.16 (d, J = 13 Hz, 1H), 1.95ꢀ1.92
(m, 2H), 1.88ꢀ1.76 (m, 1H), 1.31 (s, 3H), 1.29ꢀ1.25 (t, J = 7 Hz, 3H),
1.25ꢀ1.20 (m, 1H), 1.20 (s, 6H), 0.96 (s, 3H). 13C NMR (CDCl3, 75.5
MHz): δ206.9, 170.1, 149.8, 116.6, 75.5, 71.3, 67.6, 61.3, 50.3, 35.9, 33.1, 30.9,
30.5, 29.2, 25.0, 23.7, 13.9. IR (neat, cmꢀ1): 1722, 1710, 1631, 1224. HRMS
(+ESI) m/z calcd for C17H26O4Na (M + Na)+ 317.1723, found 317.1725.
Synthesis of Racemic Ethyl 2,2,6,6-Tetramethyl-4-oxooctahydrospiro-
[chromene-5,20-oxirane]-4a-carboxylate (26). To a stirred solution of
3 (0.035 g, 0.12 mmol) in CH2Cl2 (4 mL) was added m-CPBA (0.088 g,
0.36 mmol, 70%). The resulting mixture was stirred at 40 °C for 24 h.
The mixture was then cooled to room temperature and treated with a
saturated aqueous solution of Na2S2O3 (3 mL) and NaHCO3 (1 mL).
After stirring for 1 h, the aqueous layer was extracted with ethyl acetate
(10 mL ꢁ 3), and the combined organic extracts were washed with
brine, dried over sodium sulfate, filtered, and concentrated. Silica gel
flash column chromatography of the residue (hexanes:ethyl acetate =
30:1) to afford a colorless oil as the product (0.033 g, 0.11 mmol, 92%, a
HRMS (+ESI) m/z calcd for C12H24O4NaSi (M + Na)+ 283.1336,
found 283.1339.
Synthesis of Racemic 2-(Trimethylsilyl)ethyl 2,2,6,6-Tetramethyl-5-
methylene-4-oxooctahydro-2H-chromene-4a-carboxylate (4). The
general procedures were followed with In(OTf)3 (0.078 g, 0.14 mmol),
4 Å molecular sieves (0.050 g), alkynal 7 (0.026 g, 0.21 mmol), and
β-keto-ester 6 (0.036 g, 0.14 mmol). A colorless oil (0.034 g, 0.093
mmol, 66%) was isolated as the product. 1H NMR (300 MHz, CDCl3):
δ 5.36 (s, 1H), 4.63 (s, 1H), 4.62 (m, 1H), 4.29ꢀ4.19 (m, 2H), 2.91 (d,
J = 13 Hz, 1H), 2.16 (d, J = 13 Hz, 1H), 1.95ꢀ1.92 (m, 2H), 1.88ꢀ1.77
(m, 1H), 1.31 (s, 3H), 1.25ꢀ1.20 (m, 1H), 1.20 (s, 6H), 1.07ꢀ0.98 (m,
2H), 0.97 (s, 3H). 13C NMR (CDCl3, 75.5 MHz): δ 200.0, 170.3, 149.7,
116.6, 75.5, 71.4, 67.7, 63.7, 50.3, 36.0, 33.2, 30.9, 30.5, 29.3, 25.0, 23.7,
17.1, ꢀ1.6. IR (neat, cmꢀ1): 1724, 1708, 1630, 1226, 1205, 862, 837.
HRMS (+ESI) m/z calcd for C20H34O4NaSi (M + Na)+ 389.2119,
found 389.2115.
Synthesis of Racemic 2-(Trimethylsilyl)ethyl 2,2,6,6-Tetramethyl-4-
oxooctahydrospiro[chromene-5,20-oxirane]-4a-carboxylate (31).
The procedures for the synthesis of 26 were followed with (()-4
(0.033 g, 0.090 mmol) and m-CPBA (0.060 g, 0.27 mmol, 70%). A
colorless oil (0.033 g, 0.086 mmol, 95%, a 4:1 mixture of diastereomers)
1
1
was obtained as the product. Major product: H NMR (300 MHz,
4:1 mixture of diastereomers). Major isomer: H NMR (300 MHz,
CDCl3): δ 4.68 (t, J = 3 Hz, 1H), 4.33ꢀ4.19 (m, 2H), 2.92 (d, J = 14 Hz,
1H), 2.76 (d, J = 4 Hz, 1H), 2.54 (d, J = 4 Hz, 1H), 2.23 (d, J = 14 Hz,
1H), 2.15 (dt, J = 13, 5 Hz, 1H), 1.87ꢀ1.84 (m, 1H), 1.38 (s, 3H), 1.26
(s, 3H), 1.23ꢀ1.18 (m, 1H), 1.08ꢀ1.02 (m, 2H), 0.99 (s, 3H), 0.81
(s, 3H), 0.06 (s, 9H). 13C NMR (CDCl3, 75.5 MHz): δ 204.0, 169.6,
73.9, 69.3, 63.9, 63.6, 58.3, 52.2, 46.1, 34.0, 31.1, 30.8, 25.9, 24.99, 24.96,
24.9, 17.3, ꢀ1.6. IR (neat, cmꢀ1): 1721, 1250, 863, 836. HRMS (+ESI)
m/z calcd for C20H34O5NaSi (M + Na)+ 405.2068, found 405.2069.
Minor product: 1H NMR (300 MHz, CDCl3): δ 4.93 (q, J = 3 Hz, 1H),
4.31ꢀ4.16 (m, 2H), 2.83(d, J = 4 Hz, 1H), 2.70 (d, J = 4 Hz, 1H), 2.52
(m, 2H), 2.02ꢀ1.96 (m, 1H), 1.92ꢀ1.84 (m, 1H), 1.78ꢀ1.68 (m, 1H),
1.51ꢀ1.43 (m, 1H), 1.29 (s, 6H), 1.07 (m, 2H), 0.93 (s, 3H), 0.88 (s,
3H), 0.04 (s, 9H). 13C NMR (CDCl3, 75.5 MHz): δ 201.8, 167.8, 74.9,
73.9, 65.3, 64.4, 59.5, 50.8, 47.8, 34.3, 34.0, 30.4, 28.9, 27.2, 25.7, 24.9,
17.2, ꢀ1.6. IR (neat, cmꢀ1): 1721, 1250, 860, 838. HRMS m/z calcd for
C20H34O5NaSi (M + Na)+ 405.2068, found 405.2068.
CDCl3): δ 4.68 (t, J = 3 Hz, 1H), 4.30ꢀ4.16 (m, 2H), 2.92 (d, J = 14 Hz,
1H), 2.76 (d, J = 4 Hz, 1H), 2.53 (d, J = 4 Hz, 1H), 2.23 (d, J = 14 Hz,
1H), 2.21ꢀ2.09 (m, 1H), 2.15 (dt, J = 13, 5 Hz, 1H), 1.38 (s, 3H), 1.30
(t, J = 7 Hz, 3H), 1.25 (s, 3H), 1.23ꢀ1.18 (m, 1H), 0.98 (s, 3H), 0.81 (s,
3H). 13C NMR (CDCl3, 75.5 MHz): δ 204.0, 169.5, 73.9, 69.3, 63.6,
61.4, 58.2, 52.2, 46.0, 34.0, 31.1, 30.8, 25.8, 24.9, 24.8, 13.9. IR
(neat, cmꢀ1): 1744, 1720, 1257. HRMS (+ESI) m/z calcd for
C17H26O5Na (M + Na)+ 333.1672, found 333.1673. Minor isomer: δ
4.93 (q, J = 3 Hz, 1H), 4.29ꢀ4.17 (m, 2H), 2.84 (d, J = 4 Hz, 1H), 2.68
(d, J = 4 Hz, 1H), 2.53 (m, 2H), 2.07ꢀ1.96 (m, 1H), 1.92ꢀ1.82 (m,
1H), 1.79ꢀ1.69 (m, 1H), 1.51ꢀ1.43 (m, 1H), 1.33ꢀ1.24 (m, 9H), 0.93
(s, 3H), 0.88 (s, 3H). 13C NMR (CDCl3, 75.5 MHz): δ 201.8, 167.8,
74.8, 73.8, 65.1, 61.9, 59.4, 50.8, 47.9, 34.4, 34.0, 30.4, 28.8, 27.1, 25.7,
24.7, 13.9. IR (neat, cmꢀ1): 1748, 1729, 1259. HRMS (+ESI) m/z calcd
for C17H26O5Na (M + Na)+ 333.1672, found 333.1673.
Synthesis of 6-(2-Hydroxy-2-methylpropyl)-2,2-dimethyl-4H-1,3-di-
oxin-4-one (30). To a stirred solution of acetone (0.093 g, 1.6 mmol) in
Synthesis of Racemic 3a-Methoxy-2,2,6,6-tetramethyl-2,3,3a,5,6,
7,8,8a-octahydrofuro[2,3,4-de]chromene (2). To a stirred solution of
(()-31 (0.028 g, 0.073 mmol) in THF (2 mL) was added TBAF
(0.11 mL of a 1 M solution in THF, 0.11 mmol). The mixture was stirred
at room temperature for 1 h. After removal of volatiles, the residue was
dissolved in MeOH (2 mL) and treated with pTsOH (0.0014 g, 0.0073
mmol). The mixture was stirred at room temperature until TLC analysis
showed consumption of the crude product (1 or 28). The solution was
then treated with 4ꢀ5 drops of triethylamine and concentrated. The
residue was purified by flash column chromatography with basic
aluminum oxide (hexanes:dichloromethane = 5:1) to afford a colorless
CH2Cl2 (5 mL) at ꢀ78 °C was added BF3 Et2O (0.14 mL, 1.1 mmol).
3
The resulting mixture was stirred at ꢀ78 °C for 0.5 h. Then a solution of
silyl dienolate 29 (0.23 g, 1.1 mmol) in CH2Cl2 was added slowly via a
cannula at ꢀ78 °C. The mixture was stirred at room temperature for 1 h.
Then a saturated aqueous solution of NaHCO3 (2 mL) was added. The
aqueous layer was extracted with ethyl acetate (10 mL ꢁ 3), and the
combined organic extracts were washed with brine, dried over sodium
sulfate, filtered, and concentrated. Silica gel flash column chromatogra-
phy of the residue (hexanes:ethyl acetate = 5:1) afforded a colorless oil
(0.14 g, 0.70 mmol, 65%) as the product. 1H NMR (300 MHz,
CD3OD): δ 5.35 (s, 1H), 2.43 (s, 2H), 1.71 (s, 6H), 1.29 (s, 6H).
13C NMR (CDCl3, 75.5 MHz): δ 168.8, 160.9, 106.6, 96.0, 70.5, 47.2,
29.7, 25.2. IR (neat, cmꢀ1): 3434, 1718, 1628, 1203. HRMS (+ESI) m/z
calcd for C10H17O4 (M + H)+ 201.1121, found 201.1125.
1
oil (0.010 g, 0.040 mmol, 55%) as the product. H NMR (300 MHz,
DMSO-d6): δ 4.61 (dd, J = 13, 3 Hz, 1H), 4.38 (dd, J = 13, 1 Hz, 1H),
4.15ꢀ4.13 (m, 1H), 3.01 (s, 3H), 1.98ꢀ1.94 (m, 2H), 1.52ꢀ1.41 (m,
4H), 1.22 (s, 3H), 1.09 (s, 3H), 1.04 (s, 3H), 1.03 (s, 3H). 13C NMR
(CDCl3, 75.5 MHz): δ 146.4, 128.4, 110.1, 73.2, 73.0, 64.7, 49.4, 48.9,
37.1, 32.7, 32.6, 29.4, 27.6, 26.4, 24.3. IR (neat, cmꢀ1): 1708, 1466, 1201,
1157. HRMS (+ESI) m/z calcd for C15H24O3Na (M + Na)+ 275.1618,
found 275.1629.
Synthesis of 2-(Trimethylsilyl)ethyl 5-Hydroxy-5-methyl-3-oxohex-
anoate (6). A solution of 30 (1.1 g, 5.5 mmol) and 2-(trimethylsilyl)-
ethanol (0.97 g, 8.2 mmol) in toluene (40 mL) was stirred at 110 °C in a
sealed tube for 16 h. After removal of the volatiles, silica gel flash column
chromatography (hexanes:ethyl acetate = 1:5) of the residue afforded a
colorless oil (1.2 g, 4.7 mmol, 86%) as the product. 1H NMR (300 MHz,
CDCl3): δ 12.38 (s, 0.1H), 5.00 (s, 0.1H), 4.26ꢀ4.20 (m, 2H), 3.43 (s,
1.8H), 3.39 (s, 1H), 2.74 (s, 1.8H), 2.37 (s, 0.2H), 1.28 (s, 6H),
1.04ꢀ0.98 (m, 2H), 0.04 (s, 9H). 13C NMR (CDCl3, 75.5 MHz): δ
204.1, 175.1, 172.6, 166.9, 92.1, 70.1, 69.6, 63.7, 62.3, 53.7, 50.7, 48.2,
29.2, 17.2, ꢀ1.7. IR (neat, cmꢀ1): 3525, 1739, 1708, 1252, 862, 839.
2,2,5,6,6-Pentamethyl-6,7,8,8a-tetrahydro-2H-chromen-4(3H)-one
1
(32). H NMR (300 MHz, CDCl3): δ 4.42ꢀ4.36 (m, 1H), 2.48 (m,
2H), 1.96 (s, 3H), 1.95 (m, 1H), 1.71ꢀ1.49 (m, 3H), 1.29 (s, 6H), 1.13
(s, 3H), 1.07 (s, 3H). 13C NMR (CDCl3, 75.5 MHz): δ 200.6, 153.1,
131.2, 73.5, 70.2, 53.9, 37.0, 35.9, 30.5, 28.6, 26.5, 26.4, 25.6, 16.2. IR
(neat, cmꢀ1): 1687, 1604, 1220. HRMS (+ESI) m/z calcd for C14H23O2
(M + H)+ 223.1693, found 223.1698.
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The Journal of Organic Chemistry
ARTICLE
2,2,6,6-Tetramethyl-2,3,6,7,8,8a-hexahydrofuro[2,3,4-de]chromene
(33). 1H NMR (500 MHz, DMSO-d6): δ 7.35 (s, 1H), 4.39ꢀ4.36 (m,
1H), 2.56ꢀ2.46 (m, 2H), 1.88ꢀ1.85 (m, 1H), 1.59ꢀ1.57 (m, 2H), 1.33
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221.1538.
’ ASSOCIATED CONTENT
S
Supporting Information. HPLC analysis of (()- and
b
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(+)-9, 1H, 13C NMR, and IR spectra of (+)-9, (()-2ꢀ4, 26, 31,
and 6, 30, 32ꢀ33, and NOESY spectrum of (()-2. This material
is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: lizc@szpku.edu.cn.
’ ACKNOWLEDGMENT
Financial support for this research project from the National
Natural Science Foundation of China (Grant No.: 20972004)
and Peking University Shenzhen Graduate School is gratefully
acknowledged. A special acknowledgment is made to the Hong
Kong Polytechnic University, which is funded by the University
Grants Committee of Hong Kong Special Equipment Grant, for
the 500 and 600 MHz NMR spectrometers.
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