Synthesis and structural revision of (±)-laurentristich-4-ol

Article abstract of DOI:10.1021/jo7021247

(Chemical Equation Presented) The structure of tetracyclic natural sesquiterpene laurentristich-4-ol was revised based on its synthetic studies. The proposed and the revised structures of laurentristich-4-ol were both synthesized with SmI₂-mediated ketyl radical cyclization as the key step to construct the spirocyclic ether skeleton.

Full text of DOI:10.1021/jo7021247

catalyzed intramolecular C-H insertion of aryldiazoacetates,5
and the intramolecular addition of a vinyl radical to benzofuran
in a low efficiency.6 Driven by our interest in carbon-centered
radical cyclization reactions,7 we envisioned that the SmI2-
mediated ketyl radical cyclization8 onto benzofuran might be
an efficient and convenient way for the construction of the
spirocyclic ether. Thus, we prepared the model substrate 4a to
explore this possibility (vide infra). Treatment of ketone 4a with
SmI2/HMPA in THF at rt for 16 h led to the clean formation of
5-exo cyclization product 5a in 66% isolated yield with excellent
regioselectivity and stereoselectivity, whose configuration was
established by its NOESY experiments (eq 1). No other
stereoisomers could be detected.
Synthesis and Structural Revision of
(()-Laurentristich-4-ol
Pinhong Chen, Junhua Wang,* Kun Liu, and Chaozhong Li*
Joint Laboratory of Green Synthetic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, People’s Republic of China
clig@mail.sioc.ac.cn; chemwoods@yahoo.com.cn
ReceiVed September 30, 2007
The structure of tetracyclic natural sesquiterpene laurentris-
tich-4-ol was revised based on its synthetic studies. The
proposed and the revised structures of laurentristich-4-ol were
both synthesized with SmI2-mediated ketyl radical cyclization
as the key step to construct the spirocyclic ether skeleton.
We then extended this methodology to the cyclization in a
6-exo mode with ketone 4b as the substrate. The reaction of 4b
with SmI2 under the above conditions also showed high
regioselectivity and the expected 6-exo cyclization product 5b
was achieved in 72% yield as the mixture of two stereoisomers
in a 2:1 ratio (eq 2). As a comparison, the intermolecular ketyl
radical addition to benzofuran 6 gave the desired product 79 in
a low yield (eq 3). This observation should be attributed to the
rich electron density of the CdC double bond in benzofuran
while ketyl radicals are nucleophilic.10
Laurentristich-4-ol was one of the 14 sesquiterpenes isolated
from the red alga Laurencia tristicha in the South Sea of China.1
It possesses an unusual novel carbon skeleton not satisfying
the isoprene rule. The proposed structure is shown as compound
1, which has a novel tetracyclic skeleton with a spirocyclic ether
structure. It was proposed that 1 might be biogenetically
synthesized from the possible precursor cyclolauren-2-ol (2),
which was another sesquiterpene also isolated from Laurencia
tristicha, by an oxidative rearrangement procedure. Laurentris-
tich-4-ol was tested against a few human cancer cell lines and
no cytotoxicity was found.1 Other biological activities of
laurentristich-4-ol remain unknown.
On the basis of the above results, the synthesis of 1 was then
carried out as illustrated in Scheme 1. 2-Iodo-5-methylbenzene-
1,4-diol (8) was used as the starting material, which was readily
The unique structure of laurentristich-4-ol prompted us to
study its total synthesis. The stereoselective generation of the
1-oxaspiro[4.4]nonane skeleton (3) was obviously the key to
the synthesis of 1. Surprisingly, few methods are available in
the literature for its formation. These included the carbocation-
based reaction of diols with acids,2 the acid-catalyzed addition
of phenols to CdC bonds,3 the based-catalyzed intramolecular
Diels-Alder reaction of furfuryl propargyl ethers,4 the rhodium-
(3) Van der Mey, M.; Hatzelmann, A.; Van, Klink, G. P. M.; Van der
Lann, I. J.; Sterk, G. J.; Thibaut, U.; Ulrich, W. R.; Timmerman, H. J.
Med. Chem. 2001, 44, 2523.
(4) Wu, H.-J.; Ying, F.-H.; Shao, W.-D. J. Org. Chem. 1995, 60, 6168.
(5) Davies, H. M. L.; Grazini, M. V. A.; Aouad, E. Org. Lett. 2001, 3,
1475.
(6) Parsons, P. J.; Penverne, M.; Pinto, I. L. Synlett 1994, 721.
(7) (a) Lin, H.; Chen, Q.; Cao, L.; Yang, L.; Wu, Y.-D.; Li, C. J. Org.
Chem. 2006, 71, 3328. (b) Liu, L.; Chen, Q.; Wu, Y.-D.; Li, C. J. Org.
Chem. 2005, 70, 1539. (c) Yu, H.; Li, C. J. Org. Chem. 2004, 69, 142. (d)
Liu, L.; Wang, X.; Li, C. Org. Lett. 2003, 5, 361. (e) Fang, X.; Xia, H.;
Yu, H.; Dong, X.; Chen, M.; Wang, Q.; Tao, F.; Li, C. J. Org. Chem.
2002, 67, 8481. (f) Yu, H.; Wu, T.; Li, C. J. Am. Chem. Soc. 2002, 124,
10302. (g) Wang, J.; Li, C. J. Org. Chem. 2002, 67, 1271.
(8) For reviews, see: (a) Molander, G. A. In Radicals in Organic
Synthesis; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: New York, 2001;
Vol. 1, p 153. (b) Molander, G. A.; Harris, C. R. Chem. ReV. 1996, 96, 307.
(9) Hosokawa, T.; Imada, Y.; Murahashi, S.-I. Bull. Chem. Soc. Jpn.
1985, 55, 3282.
(1) Sun, J.; Shi, D.; Ma, M.; Li, S.; Wang, S.; Han, L.; Yang, Y.; Fan,
X.; Shi, J.; He, L. J. Nat. Prod. 2005, 68, 915.
(2) (a) Nishizawa, M.; Yadav, A.; Iwamoto, Y.; Imagawa, H. Tetrahedron
2004, 60, 9223. (b) Tamura, K.; Kato, Y.; Ishikawa, A.; Kato, Y.; Himori,
M.; Yoshida, M.; Takashima, Y.; Suzuki, T.; Kawabe, Y.; Cynshi, O.;
Kodama, T.; Niki, E.; Shimizu, M. J. Med. Chem. 2003, 46, 3083. (c) Mukai,
C.; Yamashita, H.; Sassa, M.; Hanaoka, M. Tetrahedron 2002, 58, 2755.
(d) Nishizawa, M.; Iwamoto, Y.; Takao, H.; Imagawa, H.; Sugihara, T.
Org. Lett. 2000, 2, 1685. (e) Canonne, P.; Foscolos, G.; Belanger, D. J.
Org. Chem. 1980, 45, 1828.
(10) (a) Blot, V.; Ressig, H.-U. Synlett 2006, 2763. (b) Gross, S.; Ressig,
H.-U. Org. Lett. 2003, 5, 4305.
10.1021/jo7021247 CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/11/2007
J. Org. Chem. 2008, 73, 339-341
339

SCHEME 1. Synthesis of the Proposed Structure of
(()-Laurentristich-4-ol
SCHEME 2. Structural Revision of (()-Laurentristich-4-ol
It was clear that the methylene group of the cyclopropane in 1
flipped over to furnish 13. Thus, the correct structure of (()-
laurentristich-4-ol should be 13 rather than 1.
The stereochemical assignment of laurentristich-4-ol in the
literature1 was based on the strong NOE between one of the
benzylic protons and the methyl protons attached to the
cyclopropane ring. However, this was not conclusive as the two
types of protons are at the 1,4-position. In fact, the NOE signal
was strong both in 1 and in 13. The crystal structures of 12 and
14 also revealed that these protons are very close in space in
both cases. Our theoretical calculations at the HF/6-31G* level
indicated that compound 13 is about 2.7 kcal/mol lower in
energy than its stereoisomer 1 (see the Supporting Information).
It can also be seen from the computed structures that the distance
between the abovementioned two types of protons is shorter
than 3 Å in both molecules (2.162 Å in 1 and 2.473 Å in 13).
Although the mechanism of the rearrangement of 1 remains
unclear, it might be possible that the oxidation of the phenoxyl
group to phenoxy radical initiates the cleavage of the spiro-
C-O bond to give the 1,4-benzoquinone intermediate and the
reverse of this process leads to the formation of the more stable
isomer 13. This was evidenced by the fact that both phenols 1
and 13 were very unstable in air and underwent partial
decomposition during their purification by column chromatog-
raphy on silica gel while the O-acylated compounds 12 and 14
were remarkably stable. The role of molecular sieves in
accelerating the isomerization of 1 might be attributed to the
introduction of a catalytic amount of air absorbed in the
molecular sieves into the reaction mixture.
prepared from the commercially available methylhydroquinone
in three steps (methylation with dimethyl sulfate11 followed by
iodination with I212 and subsequent demethylation with BBr3)13
according to the literature procedures.11-13 The palladium-
catalyzed coupling14 of 8 with hept-1-yn-6-one led to the
formation of benzofuran 9 in 60% isolated yield. The hydroxyl
in 9 was then acylated by reaction with acetic anhydride and
potassium carbonate. The treatment of the acylated compound
10 with SmI2/HMPA in THF at 0 °C furnished the ketyl radical
cyclization product 11 as a single stereoisomer in 65% yield.
The dehydration of 11 with methanesulfonyl chloride and
DABCO gave the olefin intermediate that underwent partial
decomposition during its purification by flash chromatography
on silica gel. Therefore, the crude dehydration intermediate was
directly subjected to the treatment with diethylzinc15 and
diiodomethane in CH2Cl2 at room temperature. The expected
cyclopropanation product 12 was thus obtained in the above
two-stage, one-pot procedure in an overall 68% yield as the
only stereoisomer whose structure was unambiguously estab-
lished by its X-ray crystal analysis (see the Supporting Informa-
tion). The stereoselective cyclopropanation could be attributed
to the presence of the benzofuranyl oxygen, which might
coordinate to diethylznic and thus direct the cyclopropanation
at the same side. Finally, the deacylation of 12 with NaOH
furnished the target molecule 1 in a quantitative yield.
In summary, the proposed structure of tetracyclic natural
sesquiterpene laurentristich-4-ol was synthesized in six steps
from compound 8 in an overall 23% yield. The structure of
laurentristich-4-ol was then revised to be 13, which was
synthesized for the first time in seven steps from 8 in an overall
21% yield. Moreover, the SmI2-mediated ketyl radical cycliza-
tion in the construction of sterically congested spirocyclic ether
skeletons described above should find further application in
natural product synthesis.
1
Surprisingly, the H NMR spectrum of 1 was dramatically
different from that of laurentristich-4-ol reported in the literature,
indicating that the structure of laurentristich-4-ol cannot be 1.
Fortunately and to our delight, the CDCl3 solution of 1 in an
NMR tube underwent slow isomerization and after 1 month,
compound 1 was all isomerized and the resulting NMR spectrum
was essentially identical with that of laurentristich-4-ol (see the
Supporting Information for details). Our efforts showed that this
process could be accelerated by the presence of molecular sieves
(4 Å). To characterize the rearrangement product 13, it was
acylated with acetic anhydride to give compound 14 (Scheme
2), whose structure was then unambiguously established by its
X-ray diffraction experiments (see the Supporting Information).
Experimental Section
5-Hydroxy-6-mentyl-2-(4-oxopentanyl)benzofuran (9). Hept-
1-yn-6-one (340 mg, 3.1 mmol) was added to the mixture of 2-iodo-
5-methylbenzene-1,4-diol (8) (308 mg, 1.23 mmol), PdCl2(Ph3P)2
(64 mg, 0.06 mmol), CuI (30 mg, 0.16 mmol), and triethylamine
(1 mL) in DMF (3 mL) at rt under N2 atmosphere. The mixture
was stirred at rt for 1 h and then at 80 °C for another 16 h. The
resulting mixture was then cooled to rt and poured into water (50
mL). The two layers were separated and the aqueous phase was
extracted with ether (3 × 30 mL). The combined organic phase
was washed with aqueous NaOH solution (10 mL) and water (20
mL) and then dried over anhydrous Na2SO4. After the removal of
the solvent under reduced pressure, the residue was purified by
column chromatography on silica gel with petroleum-ethyl acetate
(4:1, v:v) as the eluent to give compound 9 as a white solid. Yield:
170 mg (60%). Mp 144-146 °C. IR (KBr): ν (cm-1) 3380, 2947,
(11) Fuganti, C.; Serra, S. J. Chem. Soc., Perkin Trans. 1 2000, 3758.
(12) Lucht, B. L.; Mao, S. S. H.; Tilley, T. D. J. Am. Chem. Soc. 1998,
120, 4354.
(13) Sontag, B.; Ruth, M.; Spiteller, P.; Arnold, N.; Steglich, W.;
Reichert, M.; Bringmann, G. Eur. J. Org. Chem. 2006, 1023.
(14) Kundu, N. G.; Pal, M.; Mahanty, J. S.; De, M. J. Chem. Soc., Perkin
Trans. 1 1997, 2815.
1
2889, 1703, 1424, 1381, 1182, 953, 854. H NMR (300 MHz,
CDCl3): δ 1.95-2.05 (m, 2H), 2.13 (s, 3 H), 2.32 (s, 3 H), 2.48
(t, J ) 7.2 Hz, 2 H), 2.71 (t, J ) 7.2 Hz, 2 H), 4.52 (br, 1 H), 6.25
(15) Morikawa, T.; Sasaki, H.; Hanai, R.; Shibuya, A.; Taguchi, T. J.
Org. Chem. 1994, 59, 97.
340 J. Org. Chem., Vol. 73, No. 1, 2008

(s, 1 H), 6.85 (s, 1 H), 7.15 (s, 1 H). 13C NMR (75 MHz, CD3-
COCD3): δ 16.8, 22.6, 28.2, 42.7, 102.7, 105.2, 105.4, 112.5, 121.8,
128.1, 150.1, 152.1, 159.2, 207.7. EIMS: m/z (rel intensity) 232
(M+, 14), 174 (100), 173 (20), 161 (22), 105 (8), 77 (7), 43 (21).
Anal. Calcd for C14H16O3: C, 72.39; H, 6.94. Found: C, 72.29; H,
6.96.
the crude product was purified by column chromatography on silica
gel with petroleum-ethyl acetate (20:1, v:v) as the eluent to give
12 as a white solid. Yield: 17.2 mg (68%). Mp 80-82 °C. IR
(KBr): ν (cm-1) 3015, 2956, 2928, 2876, 1756, 1492, 1368, 1203,
1177, 1159, 1010, 911. 1H NMR (300 MHz, CDCl3): δ 0.39 (dd,
J ) 7.8, 5.1 Hz, 1 H), 1.00 (dd, J ) 5.1, 3.9 Hz, 1 H), 1.14 (s, 3
H), 1.16-1.21 (m, 1 H), 1.65-1.80 (m, 4 H), 2.09 (s, 3 H), 2.29
(s, 3 H), 2.74 (d, J ) 15.6 Hz, 1 H), 3.45 (d, J ) 15.6 Hz, 1 H),
6.61 (s, 1 H), 6.79 (s, 1 H). 13C NMR (75 MHz, CDCl3) δ 14.5,
16.3, 16.4, 20.8, 23.4, 24.2, 27.9, 34.2, 38.6, 98.4, 110.7, 118.3,
125.4, 129.1, 142.3, 157.6, 169.9. EIMS: m/z (rel intensity) 272
(M+, 35), 258 (13), 230 (100), 215 (49), 201 (19), 161 (10), 137
(92), 93 (20), 77 (12). HRMS calcd for C17H20O3 272.1412, found
272.1410. The structure was further confirmed by its X-ray
diffractional analysis.
5-Acetoxy-6-methyl-2-(4-oxopentanyl)benzofuran (10). Potas-
sium carbonate (380 mg, 2.75 mmol) was added into the CH2Cl2
(20 mL) solution of 5-hydroxy-6-mentyl-2-(4-pentanon-1-yl)ben-
zofuran (9) (318 mg, 1.37 mmol) at rt. Acetic anhydride (5.2 mmol,
0.30 mL) was then added and the mixture was stirred at rt overnight.
The resulting mixture was concentrated under reduced pressure and
the residue was poured into water (20 mL). The aqueous mixture
was then extracted with ether (3 × 20 mL). The combined extracts
were washed with 1 N hydrochloric acid solution and then dried
over anhydrous MgSO4. After the removal of the solvent under
reduced pressure, the crude product was purified by column
chromatography on silica gel with petroleum-ethyl acetate (5:1,
v:v) as the eluent to give 5-acetoxy-6-methyl-2-(4-pentanon-1-yl)-
benzofuran (10) as a white solid. Yield: 329 mg (88%). Mp 67-
68 °C. IR (KBr): ν (cm-1) 3111, 2935, 1759, 1717, 1602, 1472,
1368, 1157, 913, 873. 1H NMR (300 MHz, CDCl3): δ 1.97-2.03
(m, 2 H), 2.13 (s, 3 H), 2.25 (s, 3 H), 2.33 (s, 3 H), 2.47 (t, J ) 7.2
(1′S*,2′S*)-5-Hydroxy-1′,6-dimethyl-3H-spiro[benzofuran-
2,2′-bicyclo[3.1.0]hexane] (1). NaOH (20 mg, 0.5 mmol) was added
into the solution of compound 12 (13.0 mg, 0.048 mmol) in MeOH
(0.5 mL) and THF (1.5 mL) at rt. After the mixture was stirred for
30 min, 2 N aqueous HCl was added until the pH was close to 7.
The resulting aqueous solution was extracted with ether (3 × 20
mL). The combined organic phase was washed with brine and then
dried over anhydrous Na2SO4. After the removal of the solvent
under reduced pressure, the crude product was purified by column
chromatography on silica gel with petroleum-ethyl acetate (10:1,
v:v) as the eluent to give the pure 1 as a colorless liquid. Yield:
Hz, 2 H), 2.74 (t, J ) 7.2 Hz, 3 H), 6.32 (s, 1 H), 6.09 (s, 1 H). 13
C
NMR (75 MHz, CDCl3): δ 16.7, 20.8, 21.6, 27.5, 29.9, 42.4, 102.5,
112.3, 112.7, 125.6, 127.4, 145.1, 152.7, 159.0, 169.8, 208.2.
EIMS: m/z (rel intensity) 274 (M+, 13), 232 (8), 216 (8), 174 (100),
175 (12), 161 (11), 115 (3), 77 (2), 43 (6). Anal. Calcd for
C16H18O4: C, 70.06; H, 6.61. Found: C, 69.89; H, 6.63.
1
11.5 mg (100%). H NMR (300 MHz, CDCl3): δ 0.39 (dd, J )
7.8, 5.1 Hz, 1 H), 1.00 (dd, J ) 5.1, 3.9 Hz, 1 H), 1.11 (s, 3 H),
1.15-1.20 (m, 1 H), 1.60-1.77 (m, 4 H), 2.19 (s, 3 H), 2.73 (d, J
) 15.6 Hz, 1 H), 3.43 (d, J ) 15.6 Hz, 1 H), 6.55 (s, 1 H), 6.63
(s, 1 H). 13C NMR (75 MHz, CDCl3): δ 14.5, 16.1, 16.3, 24.2,
26.9, 27.9, 34.1, 38.5, 97.6, 110.6, 112.0, 122.7, 125.2, 147.2, 153.9.
EIMS: m/z (rel intensity) 230 (M+, 35), 216 (30), 201 (34), 188
(13), 161 (8), 137 (100), 93 (25), 84 (49). HRMS calcd for C15H18O2
230.1307, found 230.1315.
(1′S*,2′S*)-5-Acetoxy-1′-hydroxy-1′,6-dimethyl-3H-spiro[ben-
zofuran-2,2′-cyclopentane] (11). A solution of 10 (158 mg, 0.58
mmol) and tBuOH (87 µL, 1.15 mmol) in THF (20 mL) was added
to the mixture of SmI2 (0.1 M, 28.8 mL, 2.88 mmol) and HMPA
(2 mL, 11.5 mmol) in THF (5 mL) at 0 °C. The solution was stirred
at 0 °C for 16 h and then quenched with saturated aqueous NaHCO3
(30 mL). The two layers were separated and the aqueous phase
was extracted with ether (3 × 20 mL). The combined organic phase
was washed with brine and then dried over anhydrous MgSO4. After
the removal of the solvent under reduced pressure, the crude product
was purified by column chromatography on silica gel with
petroleum-ethyl acetate (3:1, v:v) as the eluent to give 11 as a
colorless liquid. Yield: 103 mg (65%). IR (neat): ν (cm-1) 3054,
(1′S*,2′R*)-5-Hydroxy-1′,6-dimethyl-3H-spiro[benzofuran-
2,2′-bicyclo[3.1.0]hexane] (13). Molecular sieves (4 Å, 0.5 g) was
added into the solution of 1 (20 mg, 0.087 mmol) in CHCl3 (2
mL) at rt and the mixture was stirred for 24 h. The resulting mixture
was filtered and the filtrate was concentrated in vacuo to give 13
1
as a colorless liquid. Yield: 18 mg (90%). The H and 13C NMR
spectra were identical with those reported in the literature.1
(1′S*,2′R*)-5-Acetoxy-1′,6-dimethyl-3H-spiro[benzofuran-
2,2′-bicyclo[3.1.0]hexane] (14). Compound 14 was prepared by
the acylation of 13 in 85% yield according to the procedure outlined
for the synthesis of 10. White solid. Mp 86-88 °C. IR (KBr): ν
(cm-1) 3025, 2930, 2867, 1759, 1628, 1492, 1369, 1204, 1157,
1
2961, 2933, 1758, 1493, 1370, 1213, 1159, 917. H NMR (300
MHz, CDCl3): δ 1.21 (s, 3 H), 1.76-1.86 (m, 3 H), 1.99-2.11
(m, 6 H), 2.28 (s, 3 H), 2.95 (d, J ) 16.2 Hz, 1 H), 3.35 (d, J )
16.2 Hz, 1 H), 6.55 (s, 1 H), 6.79 (s, 1 H). 13C NMR (75 MHz,
CDCl3): δ 16.4, 19.0, 20.8, 20.9, 33.4, 37.3, 38.5, 81.5, 98.9, 110.7,
118.1, 125.6, 129.2, 142.5, 156.8, 169.9. EIMS: m/z (rel intensity)
276 (M+, 7), 234 (24), 216 (10), 201 (9), 161 (8), 137 (36), 97
(20), 55 (19), 43 (100). HRMS calcd for C16H20O4 276.1362, found
276.1357.
(1′S*,2′S*)-5-Acetoxy-1′,6-dimethyl-3H-spiro[benzofuran-2,2′-
bicyclo[3.1.0]hexane] (12). A solution of 11 (25 mg, 0.09 mmol)
in dry dichloromethane (5 mL) was allowed to cool to 0 °C.
DABCO (103 mg, 0.9 mmol) and methanesulfonyl chloride (69
µL, 0.9 mmol) were added successively. The mixture was stirred
at 0 °C for 4 h and then at rt overnight. The resulting mixture was
diluted with ether (10 mL), washed successively with water,
saturated aqueous NH4Cl, and 10% aqueous Na2CO3, and then dried
over anhydrous MgSO4. After the removal of the solvent under
reduced pressure, the crude product was dissolved in dichlo-
romethane (2 mL) under N2 atmosphere and the solution was cooled
down to -23 °C. Diethyl zinc (0.5 mL, 0.5 mmol, 1.0 M solution
in hexane) and diiodomethane (80.5 µL, 1 mmol) were added. The
reaction mixture was stirred vigorously at -23 to 0 °C for 10 h.
The resulting mixture was then treated with aqueous NH4Cl (10
mL) and extracted with ether (3 × 20 mL). The combined ether
solution was washed with saturated NaHCO3 and then dried over
Na2SO4. After the removal of the solvent under reduced pressure,
1
1007, 914. H NMR (300 MHz, CDCl3): δ 0.31-0.35 (m, 1 H),
0.40-0.43 (m, 1 H), 1.11 (s, 3 H), 1.33-1.36 (m, 1 H), 1.59-
1.73 (m, 2 H), 1.94-2.06 (m, 2 H), 2.09 (s, 3 H), 2.29 (s, 3 H),
2.91 (d, J ) 15.9 Hz, 1 H), 3.34 (d, J ) 15.9 Hz, 1 H), 6.58 (s, 1
H), 6.77 (s, 1 H). 13C NMR (75 MHz, CDCl3): δ 14.3, 14.9, 16.5,
20.8, 24.9, 25.6, 30.9, 35.8, 36.2, 99.3, 110.7, 117.9, 125.6, 129.2,
142.2, 157.0, 170.0. EIMS: m/e (rel intensity) 272 (M+, 34), 258
(2), 230 (100), 215 (48), 201 (12), 161 (10), 137 (66), 93 (21), 77
(12). HRMS calcd for C17H20O3 272.1412, found 272.1407. The
structure was further confirmed by its X-ray diffractional analysis.
Acknowledgment. This project was supported by the
National NSF of China (Grant Nos. 20325207, 20672136,
20702060, and 20772142) and by the Shanghai Municipal
Committee of Science and Technology (Grant No. 07XD14038).
Supporting Information Available: Characterizations of com-
pounds 4-8, 1H and/or 13C NMR spectra of 9-14, and theoretical
calculations on 1 and 13, as well as crystal structures of 12 and 14
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
JO7021247
J. Org. Chem, Vol. 73, No. 1, 2008 341