A new ring expansion for a chiral hexahydroazulene skeleton possessing an angular methyl group

Article abstract of DOI:10.1021/jo201153h

A new synthetic route for a pseudoguaiane ring system is described. The synthesis features an Ireland-Claisen rearrangement for constructing the trans-fused ring system, followed by a new ring expansion to yield a bicyclo[5.3.0]decane ring system possessi

Full text of DOI:10.1021/jo201153h

NOTE
pubs.acs.org/joc
A New Ring Expansion for a Chiral Hexahydroazulene Skeleton
Possessing an Angular Methyl Group
Masaatsu Adachi,* Takema Komada, and Toshio Nishikawa
Laboratory of Organic Chemistry, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan
S Supporting Information
b
ABSTRACT: A new synthetic route for a pseudoguaiane ring
system is described. The synthesis features an IrelandꢀClaisen
rearrangement for constructing the trans-fused ring system,
followed by a new ring expansion to yield a bicyclo-
[5.3.0]decane ring system possessing an angular methyl group.
he naturally occurring guaianes and pseudoguaianes are
members of a large class of azulenic sesquiterpenes and have
Scheme 1. Synthesis of 5-β-Thiophenyl acetate 4
T
been found to exhibit important biological activities.1ꢀ3 These
contain a common bicyclo[5.3.0]decane ring system, the skele-
ton of which differs in the placement of a methyl group. Since the
development of an efficient and easily accessible synthetic meth-
odology has been a major issue for synthesis of the 5,7-membered
fused ring system of these sesquiterpenes, a variety of synthetic
routes have been reported to date.4ꢀ7 We describe herein a new
ring expansion that enables to the synthesis of hexahydroazulene
possessing a methyl group at the angular position.
In the course of our synthetic studies of solanoeclepin A, the
most active hatching agent of potato cyst nematode isolated from
hydroponic potato cultures,8ꢀ10 weplanned to synthesize theC,D-
ring system by an IrelandꢀClaisen rearrangement of 5-β-thiophe-
nyl acetate prepared from an optically active HajosꢀParrish
ketone. The synthesis of 5-β-thiophenyl acetate 4 is illustrated in
Scheme 1. HajosꢀParrish ketone11,12 was treated with TMSCN
in the presence of a catalytic amount of 18-crown-6 and KCN,13
followed by reduction to afford allylic alcohol 1 in 86% yield over
two steps. After transformation of the nitrile to aldehyde, protec-
tion of the allyl alcohol andsubsequentreduction provided alcohol
2. Mesylation of the alcohol 2 and deprotection of two silyl
groups with TBAF gave an intermediate diol, which formed
epoxide 3 in 57% yield over five steps. A Mitsunobu reaction
employing (phenylthio)acetic acid yielded precursor 4 for the
IrelandꢀClaisen rearrangement.
Surprisingly, the product 6 underwent ring expansion in re-
sponse to treatment with methanol at 40 °C, giving the cyclohep-
tadiene 7 in 77% yield (Scheme 3). The structure of compound 7
was determined using one- and two-dimensional NMR analyses,
with the relative configuration being confirmed by the analysis of
NOESY correlations.
The reaction mechanism for the formation of cycloheptadiene
7 isproposed in Scheme4. The IrelandꢀClaisenreaction proceeds
via (Z)-ketene silyl acetal 8 to give a rearrangement product 9,
which would promote cyclization to give the zwitterionic com-
pound 6. We propose that the formation of cycloheptadiene 7
involves 1,2-migration of the vinylic CꢀC bond antiperiplanar to
When the IrelandꢀClaisen rearrangement14ꢀ16 of 4 was
carried out by treatment with an excess amount of KHMDS in
the presence of excess TMSCl and pyridine, followed by elevation
of the reaction temperature to 60 °C, the product was not the
expected 5, but the zwitterionic compound 6 in 57% yield as a
single product (Scheme 2). The structure of 6 was determined by
extensive NMR analysis; the newly formed carbonꢀsulfur bond
was undoubtedly determined by HMBC correlation between
H-3 and the R-carbon of the carboxylate. The stereochemistry of
the ring fusion of 6 was determined by the NOESY correlation of
H-5 with H-1 and H-3.
Received: June 4, 2011
Published: July 01, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/jo201153h J. Org. Chem. 2011, 76, 6942–6945
|

The Journal of Organic Chemistry
NOTE
Scheme 2. IrelandꢀClaisen rearrangement of 4 and Representative NOESY and HMBC Correlations for Zwitterionic
Compound 6
Scheme 3. Ring Expansion of Zwitterionic Compound 6 and
Representative NOESY Correlation for Cycloheptadiene 7
Scheme 4. Proposed Mechanism for the Formation of
Cycloheptadiene 7
the PhS+ group. In this hypothesis, the chairlike six-membered
heterocyclicringofthezwitterionic compound6may beimportant
for the ring expansion as a result of 1,2-migration of the vinylic
CꢀC bond, yielding a zwitterionic intermediate 10 or β-lactone
11. Finally, decarboxylation17,18 of 10 or 11 affords the observed
product, cycloheptadiene 7.
In conclusion, the new ring expansion reaction of the Irelandꢀ-
Claisen rearrangement product was incidentally discovered. Since
the substrate is readily prepared as an optically active form, the
reaction should provide a new entry for synthesis of hydroazulenic
natural products such as presphaerol,19 isoreiswigin,20 and tri-
choaurantianolide A,21 including a 5,7-membered fused ring
system with a methyl group at the angular position.
After being stirred for 2 h, the reaction mixture was poured into an ice-
cooled mixture of water (40 mL) and EtOAc (40 mL). The mixture was
stirred for 2 h, and the aqueous layer was separated and extracted with
EtOAc (30 mL ꢁ 3). The extracts were combined, washed with water
(50 mL) and brine (50 mL), and then dried over Na2SO4. The solution
was concentrated to dryness in vacuo. The residue was purified by
column chromatography (silica gel, hexane/EtOAc 3:2) to give allylic
’ EXPERIMENTAL SECTION
(1R,5R,7aR)-5-Hydroxy-7a-methyl-1-((trimethylsilyl)oxy)-
2,3,5,6,7,7a-hexahydro-1H-indene-1-carbonitrile (1). Hajosꢀ
Parrish ketone (4.00 g, 24.4 mmol) and 18-crown-6 (258 mg,
0.976 mmol) were dissolved in CH2Cl2 (50 mL) and cooled to ꢀ20 °C.
To this cold solution were added KCN (318 mg, 4.88 mmol) and
trimethylsilyl cyanide (3.40 mL, 26.8 mmol) under nitrogen. After being
stirred for 30 min, the reaction mixture was poured into an ice-cooled
saturated aqueous NaHCO3 (50 mL). The aqueous layer was separated
and extracted with CH2Cl2 (30 mL ꢁ 3). The extracts were combined,
washed with water (50 mL) and brine (50 mL), and then dried over
Na2SO4. The solution was concentrated to dryness in vacuo. The residue
was used for the next reaction without further purification. The residue
alcohol 1 (5.66 g, 20.9 mmol, 86% in two steps) as a white solid: [R]23
D
+90.6 (c 1.01, CHCl3); mp 89.5ꢀ90.0 °C; IR (KBr) νmax 3315, 2946,
1
1255, 1156, 845 cmꢀ1; H NMR (400 MHz, CDCl3) δ 0.23 (9H, s,
SiMe3), 1.06 (3H, br d, J = 1.0 Hz, Me), 1.58 (1H, dddd, J = 14.0, 13.0,
9.0, 3.0 Hz, HOCHCHAHB), 1.70 (1H, dt, J = 13.0, 3.0 Hz,
CHAHBCMe), 1.90 (1H, td, J = 13.0, 3.0 Hz, CHAHBCMe), 2.06
(1H, td, J = 11.5, 6.5 Hz, CHAHBCCN), 2.07ꢀ2.16 (1H, m,
HOCHCHAHB), 2.29ꢀ2.40 (2H, m, CHAHBCCN, CHdCCHAHB),
2.56 (1H, tdd, J = 11.5, 5.5, 2.5 Hz, CHdCCHAHB), 4.32ꢀ4.39 (1H, m,
HOCHCHdC), 5.50ꢀ5.53 (1H, m, HOCHCH=C); 13C NMR (100
MHz, CDCl3) δ 1.1, 18.2, 24.9, 29.3, 30.6, 35.6, 48.5, 67.9, 81.6, 121.4,
and CeCl3 7H2O (11.0 g, 29.3 mmol) were dissolved in MeOH
3
(50 mL), and the mixture was cooled to ꢀ78 °C. To this cold solution
was added NaBH4 (1.10 g, 29.3 mmol) in one portion under nitrogen.
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The Journal of Organic Chemistry
NOTE
126.1, 143.8. Anal. Calcd for C14H23NO2Si: C, 63.35; H, 8.73; N, 5.28.
Found: C, 63.26; H, 8.67; N, 5.25.
J = 4.5 Hz, MeCC(O)CHAHB), 4.24ꢀ4.32 (1H, m, HOCHCHdC),
5.42ꢀ5.45 (1H, m, HOCHCHdC); 13C NMR (100 MHz, CDCl3)
δ 21.2, 27.0, 29.1, 29.6, 31.2, 40.4, 53.7, 68.2, 70.0, 122.1, 149.5. Anal.
Calcd for C11H16O2: C, 73.30; H, 8.95. Found: C, 73.13; H, 8.92.
(1R,5S,7aR)-7a-Methyl-2,3,5,6,7,7a-hexahydrospiro[indene-
1,20-oxiran]-5-yl 2-(Phenylthio)acetate (4). Epoxide 3 (450 mg,
2.50 mmol) and triphenylphosphine (1.05 g, 4.00 mmol) were dissolved
in THF (25 mL), and the mixture ws cooled to 0 °C. To the cold solution
were added (phenylthio)acetic acid (0.75 g, 3.75 mmol) and a solution of
diethyl azodicarboxylate (2.2 M solution in toluene; 1.8 mL, 4.00 mmol).
After being stirred at the same temperature for 40 min, the reaction
mixture was concentrated to dryness in vacuo. The residue was purified
by flash column chromatography (silica gel, hexane/EtOAc 9:1) to give
5-β-thiophenyl acetate 4 (662 mg, 2.00 mmol, 80%) as a colorless oil:
(1R,5R,7aR)-7a-Methyl-2,3,5,6,7,7a-hexahydrospiro[indene-
1,20-oxiran]-5-ol (3). Allylic alcohol 1 (2.00 g, 75.4 mmol) was dissolved
in Et2O (20 mL), and cooled to ꢀ78 °C. To this cold solution was added a
solution of diisobutylaluminum hydride (1.02 M solution in hexane;
22.4 mL, 22.0 mmol) dropwise. After being stirred for 15 min, the reaction
mixture was allowed to warm to ꢀ40 °C. After being stirred for 2 h, the
reaction mixture was poured into an ice-cooled mixture of water (40 mL)
and EtOAc (40 mL). To the mixture was added a saturated aqueous
potassium sodium tartrate tetrahydrate (40 mL) solution. The mixture was
stirred for 1 h, and the aqueous layer was separated and extracted with
EtOAc (30 mL ꢁ 2). The extracts were combined, washed with brine
(60 mL), and then filtered through a pad of silica gel (10 g) and Na2SO4
(10 g). The solution was concentrated to dryness in vacuo. The residue was
used for the next reaction without further purification. To a solution of the
residue in DMF (15 mL) were added imidazole (1.28 g, 18.9 mmol) and
tert-butyldimethylsilyl chloride (1.36 g, 9.05 mmol) at room temperature
under nitrogen. After being stirred for 30 min, the reaction mixture was
poured into a mixture of water (60 mL), hexane (40 mL), and EtOAc
(20 mL). The aqueous layer was separated and extracted with a mixture of
hexane and EtOAc (2:1, 30 mL ꢁ 2). The extracts were combined, washed
with water (20 mL) and brine (20 mL), and then dried over Na2SO4. The
solution was concentrated to dryness in vacuo. The residue was used for the
next reaction without further purification. The residue was dissolved in
CH2Cl2 (40 mL) and cooled to ꢀ78 °C. To this cold solution was added a
solution of diisobutylaluminum hydride (1.02 M solution in hexane; 7.4 mL,
7.55 mmol) dropwise. After being stirred for 40 min, the reaction mixture
was poured into an ice-cooled mixture of water (40 mL) and EtOAc
(40 mL). To the mixture was added a saturated aqueous potassium sodium
tartrate tetrahydrate (40 mL). The mixture was stirred for 2 h, and the
aqueous layer was separated and extracted with EtOAc (30 mL ꢁ 3). The
extracts were combined, washed with water (30 mL) and brine (30 mL),
and then dried over Na2SO4. The solution was concentrated to dryness in
vacuo. The residue was used for the next reaction without further
purification. The residue was dissolved in CH2Cl2 (35 mL) and cooled
to 0 °C. To this cold solution were added N,N-dimethyl-4-aminopyridine
(1.50 g, 12.0 mmol), triethylamine (4.9 mL, 35.2 mmol), and methane-
sulfonyl chloride (0.82 mL, 10.6 mmol) under nitrogen. After being stirred
at the same temperature for 30 min, the reaction mixture was poured into a
mixture of water (60 mL), hexane (30 mL), and EtOAc (30 mL). The
aqueous layer was separated and extracted with EtOAc (30 mL ꢁ 2). The
extracts were combined, washed with water (30 mL) and brine (30 mL),
and then filtered through a pad of silica gel (10 g) and Na2SO4 (10 g). The
solution was concentrated to dryness in vacuo. The residue was used for the
next reaction without further purification. To a solution of the residue in
THF (35 mL) was added a solution of tetrabutylammonium fluoride (1.0
M solution in THF; 17.6 mL, 17.6 mmol) at room temperature under
nitrogen. After being stirred for 10 min, the reaction mixture was heated to
50 °C. After being stirred for 12 h, the reaction mixture was poured into a
mixture of water (20 mL) and EtOAc (20 mL). The aqueous layer was
separated and extracted with EtOAc (20 mL ꢁ 3). The extracts were
combined, washed with water (20 mL) and brine (20 mL), and then dried
over Na2SO4. The solution was concentrated to dryness in vacuo. The
residue was purified by column chromatography (silica gel, hexane/EtOAc
1:1) to give epoxide 3 (775 mg, 4.30 mmol, 57% in five steps) as a colorless
oil: [R]26D ꢀ30.8 (c 1.02 CHCl3); IR (KBr) νmax 3368, 2942, 1457, 1032,
843 cmꢀ1;1H NMR (400 MHz, CDCl3) δ1.17 (3H, s, Me),1.34(1H,td,J=
13.0, 3.0 Hz, CHAHBCMe), 1.41 (1H, dt, J = 13.0, 3.5 Hz, CHAHBCMe),
1.59 (1H, tdd, J = 13.0, 9.5, 3.5 Hz, HOCHCHAHB), 1.87 (1H, ddd, J =
14.0, 9.5, 7.5 Hz, MeCCCHAHB), 2.02ꢀ2.10 (1H, m, HOCHCHAHB),
2.10 (1H, ddd, J = 14.0, 11.5, 4.5 Hz, CHdCCHAHB), 2.25 (1H, dddt,
J = 14.0, 9.5, 4.5, 1.0 Hz, MeCCCHAHB), 2.56ꢀ2.65 (1H, m, CHd
CCHAHB), 2.66 (1H, d, J = 4.5 Hz, MeCC(O)CHAHB), 2.92 (1H, d,
[R]27 ꢀ177.0 (c 0.99 CHCl3); IR (KBr) νmax 1727, 1271, 1122,
D
742 cmꢀ1; 1H NMR (400 MHz, CDCl3) δ 1.06 (3H, s, Me), 1.20 (1H,
dt, J = 13.0, 4.0 Hz, CHAHBCMe), 1.27 (1H, td, J = 13.0, 3.5 Hz,
CHAHBCMe), 1.69 (1H, dddd, J = 15.0, 4.5, 3.0, 1.0 Hz, CHAHBCH2CMe),
1.80ꢀ1.87 (1H, m, CHAHBCH2CMe), 1.88 (1H, ddd, J = 14.0, 9.5, 7.0 Hz,
MeCCCHAHB), 2.11 (1H, ddd, J = 14.0, 12.0, 5.0 Hz, MeCCCHAHB), 2.30
(1H, ddd, J = 16.0, 9.5, 5.0 Hz, CHdCCHAHB), 2.63 (1H, dddt, J = 16.0,
12.0, 7.0, 2.5 Hz, CHdCCHAHB), 2.67 (1H, d, J = 5.0 Hz, MeCC-
(O)CHAHB), 2.94 (1H, d, J = 5.0 Hz, MeCC(O)CHAHB), 3.59 (1H, d,
J = 15.0 Hz, (Od)CCHAHBSPh), 3.63 (1H, d, J = 15.0 Hz, (Od)-
CCHAHBSPh), 5.18ꢀ5.24 (1H, m, (Od)COCHCHdC), 5.41ꢀ5.45 (1H,
m, (Od)COCHCHdC), 7.19ꢀ7.24 (1H, m, aromatic), 7.25ꢀ7.31 (2H,
m, aromatic), 7.38ꢀ7.42 (2H, m, aromatic); 13C NMR (100 MHz, CDCl3)
δ 19.7, 24.8, 26.8, 27.2, 29.0, 36.8, 40.2, 53.7, 68.1, 70.0, 115.7, 126.9, 128.9,
130.0, 135.0, 153.2, 169.3. Anal. Calcd for C19H22O3S: C, 69.06; H, 6.71.
Found: C, 68.89; H, 6.60.
(4R,4aR,8aS)-4-Hydroxy-4a-methyl-2-phenyl-2,3,4,4a,5,6-
hexahydro-1H-4,8a-ethanoisothiochromen-2-ium 1-Carbox-
ylate (6). 5-β-Thiophenyl acetate 4 (520 mg, 1.57 mmol) was dissolved
in THF (30 mL), and cooled to ꢀ78 °C. To this cold solution were added
freshly distilled chlorotrimethylsilane (5.90 mL, 46.8 mmol) and pyridine
(3.80 mL, 47.2 mmol) followed by a solution of potassium bis-
(trimethylsilyl)amide (0.5 M solution in toluene; 63.0 mL, 3.14 mmol).
After being stirred for 30 min at the same temperature, the reaction
mixture was allowed to warm to ambient temperature and then heated to
60 °C. After being stirred for 4 h, the reaction mixture was poured into
water (20 mL). The organic layer was separated and extracted with water
(10 mL ꢁ 3). The aqueous layers were combined and concentrated to
dryness in vacuo. The residue was purified by reversed-phase chroma-
tography (Cosmosil 75C18-OPN, 5% CH3CN/H2O) to give zwitter-
ionic compound 6 (298 mg, 0.902 mmol, 57%) as a white powder:
[R]26D +56.4 (c 0.20 H2O); mp 132.0ꢀ133.0 °C; IR (KBr) νmax 3387,
1629, 1339, 749 cmꢀ1; 1H NMR (600 MHz, CD3OD) δ 1.18 (3H, s,
Me), 1.69 (1H, br dd, J = 14.0, 8.0 Hz, H-5), 1.95 (1H, dddd, J = 14.0,
12.0, 6.5, 2.0 Hz, H-9), 2.20 (1H, dt, J = 14.0, 8.0 Hz, H-50), 2.24ꢀ2.36
(3H, m, H-6, H-10, H-100), 2.44ꢀ2.53 (1H, m, H-60), 2.52 (1H, ddd,
J = 13.5, 8.0, 5.5 Hz, H-90), 3.62 (1H, d, J = 12.0 Hz, H-3), 3.87 (1H, d,
J = 12.0 Hz, H-30), 4.32 (1H, s, H-1), 5.73 (1H, dt, J = 9.5, 3.0 Hz, H-7),
5.78 (1H, br d, J = 9.5 Hz, H-8), 7.70 (2H, br t, J = 7.5 Hz, aromatic), 7.78
(1H, br t, J = 7.0 Hz, aromatic), 8.01 (2H, br d, J = 7.0 Hz, aromatic); 13C
NMR (150 MHz, CD3OD) δ 17.5, 23.7, 24.3, 27.1, 36.4, 44.2, 48.3, 51.9,
69.3, 79.4, 125.3, 130.1, 130.6, 131.9, 132.2, 135.4, 168.2. Anal. Calcd for
C19H22O3S: C, 69.06; H, 6.71. Found: C, 69.05; H, 6.68.
(1R,8aR)-8a-Methyl-1-((phenylthio)methyl)-1,2,3,7,8,8a-
hexahydroazulen-1-ol (7). Zwitterionic compound 6 (200 mg,
0.605 mmol) was dissolved in MeOH (6 mL) and heated to 40 °C
under nitrogen. After being stirred for 112 h, the reaction mixture was
concentrated to dryness in vacuo. The residue was purified by column
chromatography (silica gel, hexane/EtOAc 1:9) to give cycloheptadiene
7 (133 mg, 0.464 mmol, 77%) as a colorless oil. [R]25D +101.4 (c 1.03
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NOTE
CHCl3); IR (KBr) νmax 3465, 2926, 1718, 1583, 1481, 1439, 1027 cmꢀ1
;
(17) Mulzer, J.; Zippel, M.; Bruntrup, G. Angew. Chem., Int. Ed. Engl.
1980, 19, 465–466.
(18) Mulzer, J.; Zippel, M. Tetrahedron Lett. 1980, 21, 751–754.
(19) Cafieri, F.; Fattorusso, E.; Blasio, B. D.; Pedone, C. Tetrahedron
Lett. 1981, 22, 4123–4126.
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1995, 36, 1905–1908.
1H NMR (400 MHz, C6D6) δ 1.15 (3H, s, Me), 1.44 (1H, td, J = 12.0,
5.0 Hz, CHAHBCMe), 1.64 (1H, dt, J = 12.0, 3.5 Hz, CHAHBCMe),
1.90ꢀ2.04 (1H, m, HOCCHAHB), 2.11ꢀ2.24 (2H, m, HOCCHAHB,
CHdCCHAHB), 2.29ꢀ2.42 (3H, m, CHdCCHAHB, CH2CHdCH),
3.02 (1H, d, J = 13.0 Hz, CHAHBSPh), 3.16 (1H, dd, J = 13.0, 2.0 Hz,
CHAHBSPh), 5.63 (1H, br d, J = 7.5 Hz, CHdCHCHdC), 5.75 (1H,
ddd, J = 12.0, 5.5, 3.0 Hz, CHdCHCHdC), 5.90 (1H, ddd, J = 12.0, 7.5,
2.5 Hz, CHdCHCHdC), 7.00ꢀ7.06 (1H, m, aromatic), 7.08ꢀ7.14
(2H, m, aromatic), 7.41ꢀ7.46 (2H, m, aromatic); 13C NMR (100 MHz,
C6D6) δ 19.3, 27.5, 28.4, 28.6, 33.9, 42.8, 51.7, 83.5, 118.3, 123.9, 126.3,
129.2, 130.0, 131.4, 138.0, 152.2. Anal. Calcd for C18H22OS: C, 75.48;
H, 7.74. Found: C, 75.48; H, 7.70.
’ ASSOCIATED CONTENT
1
Supporting Information. Copies of H NMR and 13C
S
b
NMR spectra of all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: madachi@agr.nagoya-u.ac.jp.
’ ACKNOWLEDGMENT
This work was financially supported by Grants-in-Aid for
Scientific Research, a SUNBOR GRANT from the Suntory
Institute for Bioorganic Research, and the Global COE program
from MEXT.
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