Short syntheses of gabosine I and gabosine G from δ-D-gluconolactone

Article abstract of DOI:10.1021/jo0709697

(Chemical Equation Presented) A short synthesis of gabosine I (1) from δ-D-gluconolactone (3) in four steps, involving a one-pot TPAP oxidation-K₂-CO₃-mediated intramolecular Horner-Wadsworth-Emmons (HWE) olefination as the key step,

Full text of DOI:10.1021/jo0709697

Short Syntheses of Gabosine I and Gabosine G
from δ-D-Gluconolactone
Tony K. M. Shing* and Hau M. Cheng
Department of Chemistry and Center of NoVel Functional
Molecules, The Chinese UniVersity of Hong Kong, Shatin,
Hong Kong, China
tonyshing@cuhk.edu.hk
ReceiVed May 9, 2007
FIGURE 1. The gabosine family.
acid as the chiral starting material.8,9 The remaining five
constructions of gabosines involved a racemic norbornyl route,10
a chemoenzymatic synthesis from iodobenzene,11 an asymmetric
Diels-Alder reaction of chiral sulfinylacrylate with 2-methoxy-
furan,12 enantioselective acetylation of hydroxyketals,13 and an
enantioselective synthesis from [(p-tolylsulfinyl)methyl]-p-
quinols14 as the key strategies. In 1988, we reported a synthesis
of (6R,7S)-asperlin from D-glucose using an intramolecular
Horner-Wadsworth-Emmons (HWE) olefination as the key
step.15 Now, we extend the HWE strategy to a rapid entry to
hydroxylated carbocycles from δ-D-gluconolactone, affording
short syntheses of gabosines I (1) and G (2) in four and five
steps, respectively. The fabrication of gabosine G (2) has not
been addressed and this paper documents its first synthesis.
The first synthesis of garbosine I (1) was achieved by
Lubineau and Billault in nine steps from tetra-O-benzyl-D-
glucose.5 Since tetra-O-benzyl-D-glucose was prepared from
D-glucose in three steps,16 the total number of synthetic steps
to 1 from D-glucose is therefore 12. Protection of hydroxyls as
benzyl ethers is common in syntheses involving carbohydrates.5
However, in the present synthesis, the starting material is δ-D-
gluconolactone (3) and direct benzylation of all the hydroxyl
groups is not a trivial process. Furthermore, our experience has
shown that deprotection of the four benzyl ethers in the presence
A short synthesis of gabosine I (1) from δ-D-gluconolactone
(3) in four steps, involving a one-pot TPAP oxidation-K2-
CO3-mediated intramolecular Horner-Wadsworth-Emmons
(HWE) olefination as the key step, is described. Regiose-
lective acetylation of the primary alcohol in gabosine I (1)
then furnished gabosine G (2).
Gabosines belong to a family of unusual, hydroxylated
cyclohexenones and hexanones that may be classified as pseudo-
or carbasugars (Figure 1).1 They have been shown to display
interesting bioactivities such as antibiotic,1b anticancer,2 and
DNA binding properties.1c Since the first isolation of gabosines
from Streptomyces strains in 1974,1a twelve total syntheses have
appeared. Four enantiospecific syntheses constructed the car-
bocyclic framework from carbohydrates by using either an
intramolecular nitrile oxide cycloaddition,3 a SnCl4-promoted
aldol-type cyclization of phenylsulfonyl enol silyl ether,4 an
intramolecular Nozaki-Kishi reaction,5 or a ring-closing alkene
metathesis6 as the key step. Three other enantiospecific syn-
theses, including our earlier endeavor,7 employed (-)-quinic
(1) (a) Tatsuta, K.; Tsuchiya, T.; Mikami, N.; Umezawa, S.; Umezawa,
H.; Naganawa, H. J. Antibiot. 1974, 27, 579-586. (b) Bach, G.; Breiding,
M. S.; Grabley, S.; Hammann, P.; Hu¨tter, K.; Thiericke, R.; Uhr, H.; Wink,
J.; Zeeck, A. Liebig. Ann. Chem. 1993, 3, 241-250. (c) Tang, Y. Q.; Maul,
C.; Ho¨fs, R.; Sattler, I.; Gabley, S.; Feng, X.-Z.; Zeeck, A.; Thiericke, R.
Eur. J. Org. Chem. 2000, 1, 149-153.
(2) (a) Huntley, C. F. M.; Hamilton, D. S.; Creighton, D. J.; Ganem, B.
Org. Lett 2000, 2, 3143-3144. (b) Kamiya, D.; Uchihata, Y.; Ichikawa,
E.; Kato, K.; Umezawa, K. Bioorg. Med. Chem. Lett. 2005, 15, 1111-
1114.
(3) Gabosines ent-C and E: Lygo, B.; Swiatyj, M.; Trabsa, H.; Voyle,
M. Tetrahedron Lett. 1994, 35, 4197-4200.
(4) Gabosine C and COTC: Tatsuta, K.; Yasuda, S.; Araki, N.;
Takahashi, M.; Kamiya, Y. Tetrahedron Lett. 1998, 39, 401-402.
(5) Gabosine I: Lubineau, A.; Billault, I. J. Org. Chem. 1998, 63, 5668-
5671.
(8) Gabosine C and COTC: Huntley, C. F. M.; Wood, H. B.; Ganem,
B. Tetrahedron Lett. 2000, 41, 2031-2034.
(9) Gabosines A, B, ent-D, and ent-E: Shinada, T.; Fuji, T.; Ohtani, Y.;
Yoshida, Y.; Ohfune, Y. Synlett 2002, 8, 1341-1343.
(10) (()-Gabosine B and the putative structure of (()-gabosine K:
Mehta, G.; Lakshminath, S. Tetrahedron Lett. 2000, 41, 3509-3512.
(11) Gabosine A: Banwell, M. G.; Bray, A. M.; Wong, D. J. New J.
Chem. 2001, 25, 1351-1354.
(12) Gabosine C and COTC: Takahashi, T.; Yamakoshi, Y.; Okayama,
K.; Yamada, J.; Ge, W.-Y.; Koizumi, T. Heterocycles 2002, 56, 209-220.
(13) Gabosines N and O: Alibes, R.; Bayon, P.; de March, P.; Figueredo,
M.; Font, J.; Marjanet, G. Org. Lett. 2006, 8, 1617-1620.
(14) Gabosine, O: Carren˜o, M. C.; Merino, E.; Ribagorda, M.; Somoza,
AÄ .; Urbano, A. Chem. Eur. J. 2007, 13, 1064-1077.
(15) (a) Shing, T. K. M.; Aloui, M. J. Chem. Soc., Chem. Commun. 1988,
23, 1525-1526. (b) Shing, T. K. M.; Aloui, M. Can. J. Chem. 1990, 68,
1035-1037.
(16) Galudemans, C. P. J.; Fletcher, H. G., Jr. Methods Carbohydr. Chem.
1972, 6, 373-376.
(6) Gabosine C and COTC: Ramana, G. V.; Rao, B. V. Tetrahedron
Lett. 2005, 46, 3049-3051.
(7) COTC: (a) Shing, T. K. M.; Tang, Y. J. Chem. Soc., Chem. Commun.
1990, 312-312. (b) Shing, T. K. M.; Tang, Y. Tetrahedron 1990, 6575-
6584.
10.1021/jo0709697 CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/19/2007
6610
J. Org. Chem. 2007, 72, 6610-6613

SCHEME 1. Synthesis of Enone 7
SCHEME 2. Syntheses of Gabosines I (1) and G (2)
of an alkene functionality is not a high-yielding reaction. On
the other hand, isopropylidene protecting groups could readily
be installed and then removed easily via acid hydrolysis at the
end of a synthesis.17 Thus, acetalization of δ-D-gluconolactone
(3) with an excess of 2-methoxypropene afforded an acid-
sensitive, mixed acetal 4 in good yield (Scheme 1). Nucleophilic
addition of lithiated diethyl methylphosphonate to the lactone
carbonyl in 4 gave a â-ketophosphonate 5, which then was
subjected to an oxidation and intramolecular HWE cyclization
sequence.18,19 Phosphonate 5 exists as an acyclic hydroxy ketone
instead of a cyclized pyranose ring as shown by 13C NMR
spectroscopy. Because of the acid labile mixed acetal protecting
groups, the oxidation conditions for â-ketophosphonate 5 had
to be mildly basic.
Various oxidation and cyclization conditions were attempted
to yield enone 7 from â-ketophosphonate 5 and the results are
shown in Table 1. Oxidation of 5 was initially carried out with
PDC. Since the resultant 1,5-diketone 6 was too unstable for
isolation, it was manipulated to the next step without purifica-
tion. Hence, oxidation of 5 with PDC in the presence of pyridine
followed by typical HWE reaction conditions,20 entries 1a and
1b, furnished enone 7 in 20% and 32% yield, respectively.
Potassium carbonate as base also caused cyclization to give
enone 7 in 28% yield (entry 1c). Other literature conditions
were studied, but no significant improvement was observed
(entries 1d-f).21-23
bases were disappointing (entries 2d-f) and only very low yields
of 7 were obtained. Among the inorganic bases used (entries
2g-i), potassium carbonate was found to be the best candidate
to give enone 7 in 43% yield (entry 2i). (An unidentified mixture
of compounds was isolated and the mixture contains phospho-
nate and isopropylidene groups as shown by 1H and 13C NMR
spectroscopy. This mixture could not be induced to undergo
alkenation under various conditions.) To the best of our
knowledge, this is the first example of TPAP oxidation assisted
by K2CO3 to induce HWE olefination in one pot. When a
stoichiometric amount of TPAP was used, the oxidation-
olefination sequence also occurred and gave 7 in 38% yield
(entry 3). The reduced ruthenium(IV) species24 was speculated
to act as a base to induce cyclization. However, the high cost
of TPAP prohibited its use in stoichiometric amount.
With the blocked enone 7 in hand, we completed a new
synthesis of (-)-gabosine I (1) by a facile acid hydrolysis in
1
95% yield (Scheme 2). The H, 13C NMR spectral data and
Interestingly, oxidation of 5 with a catalytic amount of tetra-
n-propylammonium perruthenate (TPAP) and N-methylmor-
pholine N-oxide (NMO) as oxidant not only provided 1,5-
diketone 6, but also induced 6 to undergo HWE olefination to
give a 4% yield of enone 7 (entry 2a). Addition of lithium salt
inhibited cyclization (entries 2b,c). Different bases were added
in the same pot to promote the intramolecular alkenation. Amine
TABLE 1. Oxidation and Cyclization of Phosphonate 5 to Enone 7
under Various Conditions
oxidation
yield
conditions
cyclization conditions
(a) LiCl, DBU, CH3CN
of 7
ref
1. PDC, 3 Å MS,
pyridine, CH2Cl2 (b) LiCl, DIPEA, CH3CN
(c) K2CO3
20%
20
20
32%
28%
21%
(d) LiCl, DBU, THF, -78 °C
(e) K2CO3, 18-crown-6, toluene 5%
21
22
(17) Shing, T. K. M.; Kwong, C. S. K.; Cheung, A. W. C.; Kok, S.
H.-L.; Yu, Z.; Li, J.; Cheng, C. H. K. J. Am. Chem. Soc. 2004, 126, 15990-
15992.
(18) Fukase, H.; Horii, S. J. Org. Chem. 1992, 57, 3651-3658.
(19) (a) Lim, M.-I.; Marquez, V. E. Tetrahedron Lett. 1983, 24, 5559-
5562. (b) Altenbach, H.-J.; Holzapfel, W.; Smerat, G.; Finkler, S. H.
Tetrahedron Lett. 1985, 26, 6329-6332.
(20) Blanchette, M. A.; Choy, W.; Davis, J. T.; Essenfeld, A. P.;
Masamune, S.; Roush, W. R.; Sakai, T. Tetrahedron Lett. 1984, 25, 2183-
2186.
(f) NaH, THF, -40 °C
(a) without base
(b) DBU, LiCl
decomp 23
4%
2. TPAP
(0.05 equiv),
NMO,
NRa
(c) K2CO3, LiCl
NRa
3 Å MS, CH3CN (d) Et3N
(e) DIPEA
4%
5%
(f) DBU
4%
(g) KOtBu
NRa
(21) (a) Gassama, A.; d’Angelo, J.; Cave´, C.; Mahuteau, J.; Riche, C.
Eur. J. Org. Chem. 2000, 18, 3165-3169. (b) Gonza´lez, M. A.; Ghosh, S.;
Rivas, F.; Fischer, D.; Theodorakis, E. A. Tetrahedron Lett. 2004, 45, 5039-
5041.
(22) Aristoff, P. A. Synth. Commun. 1983, 13, 145-150.
(23) Wada, E.; Funakoshi, J.; Kanemasa, S. Bull. Chem. Soc. Jpn. 1992,
65, 2456-2464.
(h) NaOH
(i) K2CO3
8%
43%
38%
3. TPAP (1 equiv),
3 Å MS, CH3CN
a NR ) no reaction.
J. Org. Chem, Vol. 72, No. 17, 2007 6611

crystallized on standing to afford ulose 5 (1.19 g, 78%) as colorless
specific optical rotation of 1 are in good agreement with the
literature values.1b
crystals: mp 89-90 °C; [R]20 -74.8 (c 0.75, CHCl3); Rf 0.5
D
Regioselective acetylation25 of the primary alcohol in 1 then
gave gabosine G (2) in 65% yield. Now, we document the first
synthesis of gabosine G (2) with 1H and 13C NMR spectral data
in accord with those reported in the literature; however, the
specific optical rotation was not recorded.1b The absolute
configuration of (-)-gabosine G ([R]20D -41.8 (c 1.34, MeOH))
is established as 2R,3S,4R.
(EtOAc:acetone, 3:1); IR (thin film) 3380, 2991, 1721, 1382, 1205,
1
1061, 1026, 964, 891 cm-1; H NMR (300 MHz, CDCl3) δ 1.25
(s, 3H), 1.27 (s, 3H), 1.29-1.35 (m, 9H), 1.41 (s, 3H), 1.50 (s,
3H), 1.62 (s, 3H), 1.83 (s, 1H), 2.84 (dd, J ) 22.8, 13.8 Hz, 1H),
3.24 (s, 3H), 3.35 (s, 3H), 3.58-3.65 (m, 1H), 3.73-3.78 (m, 2H),
3.85-3.9 (m, 1H), 4.03-4.16 (m, 4H), 4.17-4.19 (m, 1H), 4.64
(d, J ) 5.1 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 16.7 (CH3),
16.8 (CH3), 18.6 (CH3), 23.2 (CH3), 24.8 (CH3), 24.9 (CH3), 25.3
(CH3), 28.6 (CH3), 41.2 (CH2) (d, JC-P ) 124 Hz), 50.1 (CH3),
51.8 (CH3), 62.4 (CH2), 62.6 (CH2), 62.7 (CH), 64.7 (CH2), 71.2
(CH), 72.6 (CH), 75.6 (CH), 99.4 (C), 102.4 (C), 103.6 (C), 199.7
(C) (d, JC-P ) 21.9 Hz); MS (ESI) m/z (rel intensity) 537 ([M +
Na]+, 100); HRMS (ESI) calcd for C22H43O11P1 [M + Na]+
537.2435, found 537.2438.
In conclusion, the synthesis of gabosine I (1) was ac-
complished in four steps with 20.3% overall yield and that of
gabosine G (2) was achieved in five steps with 13.2% overall
yield from δ-D-gluconolactone (3) via a key one-pot TPAP
oxidation-K2CO3-mediated HWE olefination. This construction,
the shortest synthetic route reported for gabosines, not only
underscores its potential to access other gabosines for biological
evaluation, but also provides opportunities for the syntheses of
polyhydroxylated cyclohexenoid natural products such as va-
lienaminevalienamine26 and the compounds containing it.17,27
Research along this line is underway.
Enone 7. To a mixture of ulose 5 (151 mg, 0.29 mmol),
N-methylmorpholine N-oxide (NMO) (137 mg, 1.17 mmol), and 3
Å molecular sieves (MS) (251 mg) in dry CH3CN (10 mL) were
added tetra-n-propylammonium perruthenate (TPAP) (5 mg, 0.014
mmol) and K2CO3 (334 mg, 2.41 mmol) under N2. After being
stirred for 24 h at room temperature, the mixture was diluted with
EtOAc (10 mL) then filtered through a pad of Celite and the residue
was eluted with EtOAc:Et3N (50:1, 50 mL). Concentration of the
filtrate followed by flash chromatography (n-hexane:Et2O:Et3N, 2:1:
0.01) gave enone 7 (44.1 mg, 43%) as a colorless oil: [R]20D -148
(c 1.02, CHCl3); Rf 0.17 (n-hexane:Et2O, 1:1); IR (thin film) 2921,
1684, 1383, 1221, 1119, 871 cm-1; 1H NMR (300 MHz, CDCl3) δ
1.41-1.43 (m, 15H), 1.52 (s, 3H), 3.29 (s, 3H), 3.39 (s, 3H), 3.99
(dd, J ) 10.95, 8.1 Hz, 1H), 4.24 (d, J ) 10.8 Hz, 1H), 4.37-4.49
(m, 2H), 4.55 (dd, J ) 5.35, 1.5 Hz, 1H), 5.81 (d, J ) 1.8 Hz,
1H); 13C NMR (75 MHz, CDCl3) δ 22.8 (CH3), 25.8 (CH3), 25.9
(CH3), 26.0 (CH3), 26.0 (CH3), 26.5 (CH3), 50.6 (CH3), 50.7 (CH3),
62.0 (CH2), 73.0 (CH), 74.8 (CH), 76.2 (CH), 101.0 (C), 102.6
(C), 102.9 (C), 121.5 (CH), 157.7 (C), 197.2 (C); MS (FAB)
m/z (rel intensity) 327 ([M - OCH3]+, 10), 73 (100); HRMS
(FAB) calcd for C18H30O7 [M - OCH3]+ 327.1802, found
327.1812.
Gabosine I (1). To a solution of the enone 7 (44.1 mg, 0.12
mmol) in CH2Cl2 (3 mL) were added TFA (0.1 mL) and water
(0.02 mL) at room temperature. The resulting solution was stirred
for 2 h at room temperature. Concentration of the solution followed
by flash chromatography (CHCl3:MeOH, 8:1) yielded gabosine I
(1) (20.4 mg, 95%) as a brownish oil: [R]20D -58.6 (c 0.79, MeOH)
{lit.1b [R]20D -61.4 (c 1, MeOH)}; Rf 0.36 (tBuOH:AcOH:H2O 4:1:
5, upper phase);1b 1H NMR (300 MHz, CD3OD) δ 3.59 (dd, J )
10.8, 8.4 Hz, 1H), 4.03 (d, J ) 10.8 Hz, 1H), 4.29-4.37 (m, 2H),
4.51 (d, J ) 17.7 Hz, 1H), 6.15 (d, J ) 2.1 Hz, 1H); 13C NMR (75
MHz, CD3OD) δ 62.1 (CH2), 73.7 (CH), 78.1 (CH), 79.5 (CH),
121.3 (CH), 168.1 (C), 199.5 (C); MS (ESI) m/z (rel intensity) 197
([M + Na]+, 100), 149 (10); HRMS (ESI) calcd for C7H10O5 [M
+ Na]+ 197.0420, found 197.0427.
Experimental Section
Acetal 4. To a solution of δ-D-gluconolactone (3) (1.01 g, 5.69
mmol) in dry DMF (10 mL) at 0 °C were added 2-methoxypropene
(2.5 mL, 26 mmol) and (()-10-camphorsulfonic acid (36 mg, 0.15
mmol). The solution was stirred for 5 h at 0 °C, and was then
quenched with saturated aq NaHCO3 (10 mL). The resulting mixture
was extracted with Et2O (3 × 30 mL). The combined organic layers
were dried (MgSO4) and filtered. Concentration of the filtrate
followed by flash chromatography (n-hexane:Et2O 5:1) yielded
mixed acetal 4 (1.48 g, 72%) as a colorless oil: [R]20 +22.6 (c
D
1.33, CHCl3); Rf 0.5 (n-hexane:Et2O, 1:1); IR (thin film) 2993, 1770,
1375, 1211, 1076, 1039, 832 cm-1; 1H NMR (300 MHz, CDCl3) δ
1.35 (s, 3H), 1.37 (s, 3H), 1.40 (s, 3H), 1.43 (s, 3H), 1.44 (s, 3H),
1.48 (s, 3H), 3.22 (s, 6H), 3.76 (dd, J ) 10.8, 5.4 Hz, 1H), 3.78 (t,
J ) 10.5 Hz, 1H), 4.01 (dd, J ) 5.4, 1.2 Hz, 1H), 4.06 (dd, J )
10.8, 5.4 Hz, 1H), 4.26 (d, J ) 0.9 Hz, 1H), 4.70 (dt, J ) 10.35,
5.4 Hz, 1H); 13C NMR (75 MHz, CDCl3) δ 19.0 (CH3), 25.1 (CH3),
25.5 (CH3), 25.5 (CH3), 25.8 (CH3), 29.3 (CH3), 49.5 (CH3), 50.2
(CH3), 62.6 (CH2), 67.6 (CH), 73.8 (CH), 74.2 (CH), 75.9 (CH),
100.3 (C), 101.7 (C), 103.2 (C), 169.8 (C); MS (FAB) m/z (rel
intensity) 347 ([M - CH3]+, 41), 331 (36), 299 (100); HRMS
(FAB) calcd for C17H30O8 [M - CH3]+ 347.1700, found
347.1703.
Ulose 5. To a solution of diisopropylamine (1.88 mL, 13.4 mmol)
in dry THF (10 mL) was added dropwise n-butyllithium in n-hexane
(1.6 M solution, 8.4 mL, 13.4 mmol) at -78 °C under N2. The
reaction mixture was stirred for 15 min at -78 °C under N2 and
diethyl methylphosphonate (0.48 mL, 3.32 mmol) was then added.
The reaction mixture was stirred for a further 30 min at -78 °C
and was added slowly to a solution of lactone 4 (1.2 g, 3.31 mmol)
in dry THF (10 mL) at -78 °C. Stirring was continued for an
additional 1 h at the same temperature. The reaction was quenched
with saturated aq NH4Cl (10 mL) at -78 °C and was warmed to
room temperature. The mixture was extracted with EtOAc (4 ×
100 mL). The combined organic extracts were dried (MgSO4) and
filtered. Concentration of the filtrate gave an oily residue that
Gabosine G (2). To a solution of gabosine I (1) (31.2 mg, 0.179
mmol) in collidine (3 mL) was added dropwise acetyl chloride
(0.015 mL, 0.215 mmol) at -78 °C. The resulting white suspension
was stirred at that temperature for 3 h, and then at room tem-
perature for 12 h. Methanol (0.1 mL) was added to quench the
reaction and the mixture was concentrated under high vacuum at
35 °C. The residue was flash chromatographed (CHCl3:MeOH 20:
1) to give gabosine G (2) (25.4 mg, 65%) as a brownish oil: [R]20
D
(24) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 639-666.
-41.8 (c 1.34, MeOH) {lit.1b no optical rotation reported};
Rf 0.6 (tBuOH:AcOH:H2O 4:1:5, upper phase);1b 1H NMR
(300 MHz, CD3OD) δ 2.11 (s, 3H), 3.60 (dd, J ) 11.1, 8.4 Hz,
1H), 4.03 (d, J ) 10.8 Hz, 1H), 4.39 (d, J ) 10.8 Hz, 1H), 4.89
(dt, J ) 17.1, 1.2 Hz, 1H) (part of the peak was obscured
by the solvent peak), 4.97 (dt, J ) 17.1, 1.2 Hz, 1H), 5.99
(q, J ) 2.1 Hz, 1H); 13C NMR (75 MHz, CD3OD) δ 20.6 (CH3),
(25) Ishihara, K.; Kurihara, H.; Yamamoto, H. J. Org. Chem. 1993, 58,
3791-3793.
(26) (a) Shing, T. K. M.; Li, T. Y.; Kok, S. H. L. J. Org. Chem. 1999,
64, 1941-1946. (b) Kok, S. H.-L.; Lee, C. C.; Shing, T. K. M. J. Org.
Chem. 2001, 66, 7184-7190.
(27) Shing, T. K. M.; Kok, S. H. L. Tetrahedron Lett. 2000, 41, 6865-
6868.
6612 J. Org. Chem., Vol. 72, No. 17, 2007

63.7 (CH2), 73.5 (CH), 78.0 (CH), 79.3 (CH), 122.5 (CH), 161.5
(C), 172.0 (C), 199.1 (C); MS (ESI)m/z (rel intensity) 239 ([M +
Na]+, 100); HRMS (ESI) calcd for C9H12O6 [M + Na]+ 239.0526,
found 239.0530.
Supporting Information Available: General procedures and
1
copies of H and 13C NMR spectra of all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was supported by a Strategic
Investment Scheme administered by the Center of Novel
Functional Molecules, The Chinese University of Hong Kong.
JO0709697
J. Org. Chem, Vol. 72, No. 17, 2007 6613