A concise asymmetric synthesis of (2S,3S,7S)-3,7-dimethylpentadecan-2-yl acetate and propionate, the sex pheromones of pine sawflies

Article abstract of DOI:10.1021/jo0497961

(2S,3S,7S)-3,7-Dimethylpentadecan-2-yl acetate (2) and its propionate analogue (3) are the main sex pheromones of all Neodiprion species and Diprion similes, respectively. Starting from (S)-malic acid and employing a highly chemo-, regio-, and stereoselective tandem ester reduction-epoxide formation-reductive epoxide-opening reaction protocol, an efficient total synthesis of (2S,3S,7S)-2 and -3 is reported herein.

Full text of DOI:10.1021/jo0497961

drawbacks such as necessity of resolution of the chiral
intermediates,6e use of expensive nonnaturally occurring
D-amino acids,6g too many synthetic steps, and/or low
overall yields.
A Con cise Asym m etr ic Syn th esis of
(2S,3S,7S)-3,7-Dim eth ylp en ta d eca n -2-yl
Aceta te a n d P r op ion a te, th e Sex
P h er om on es of P in e Sa w flies
Pei-Qiang Huang,* Hong-Qiao Lan, Xiao Zheng, and
Yuan-Ping Ruan
Department of Chemistry and The Key Laboratory for
Chemical Biology of Fujian Province, Xiamen University,
Xiamen, Fujian 361005, P. R. China
pqhuang@xmu.edu.cn
In connection with our interests in developing cheap
and easily available (S)-malic acid as a chiral template
for the asymmetric syntheses of bioactive molecules,8 we
now report a new approach to (2S,3S,7S)-1 and its esters.
Our approach to (2S,3S,7S)-1 is depicted retrosyntheti-
cally in Scheme 1. The key to this approach was the
synthesis of 6 and 7 starting from (S)-malic acid (8),
which required an inversion of configuration at the chiral
center of (S)-malic acid (8).
Received February 4, 2004
Abstr a ct: (2S,3S,7S)-3,7-Dimethylpentadecan-2-yl acetate
(2) and its propionate analogue (3) are the main sex
pheromones of all Neodiprion species and Diprion similes,
respectively. Starting from (S)-malic acid and employing a
highly chemo-, regio-, and stereoselective tandem ester
reduction-epoxide formation-reductive epoxide-opening
reaction protocol, an efficient total synthesis of (2S,3S,7S)-2
and -3 is reported herein.
Although a stepwise approach could be considered for
the conversion of (S)-malate 9 and (2S,3R)-3-methyl-
malate 17 to 6 and 7, respectively,9 we were interested
in exploring a more efficient transformation based on an
one-pot reaction. To this end, known dimethyl (S)-malate
(9),10 easily available from (S)-malic acid 8 in 98% yield,
was treated with tosyl chloride-pyridine system to afford
(S)-10 in 97% yield (Scheme 2). Heating a suspension of
(S)-10 with 6 molar equiv of lithium aluminum hydride
in THF afforded (+)-1,3-butanediol (11, yield 55%) and
1,4-butanediol (yield 15%). Comparing the optical rota-
Pine sawflies (Hymenoptera: Diprionidae) are common
insects widely distributed in the coniferous forests of
Europe, Asia, and North American continents. They are
considered to be severe pests on conifers. Since the
pioneering work of Coppel, J ewett and co-workers,1,2 it
is known that the sex pheromones of several species of
pine sawflies share a common alcohol moiety, namely 3,7-
dimethylpentadecan-2-ol 1; Neodiprion lecontei and N.
sertifer uses acetate 2 as the major component of their
pheromones, whereas Diprion similis uses propionate 3.2
Later studies revealed that the esters of the (2S,3S,7S)-
3,7-dimethyl-2-pentadecanol (1) were the most active
stereoisomers for all Neodiprion species.3 Up to date, a
number of methods4-6 have been developed for the
syntheses of stereoisomers7 and homologous7 of 1 in view
of developing selective methods for monitoring and
controlling the populations of these insects. However,
only four asymmetric syntheses of (2S,3S,7S)-1 have been
reported.6b-i Most of the reported methods suffer from
tion of the diol (+)-11 [[R]20 +30.0 (c 1.0, EtOH)] with
D
that of known (S)-enantiomer (+)-11 [[R]20D +29.0 (c 1.0,
(6) For the syntheses of optically active 1, see: (a) Mugnusson, G.
Tetrahedron 1978, 34, 1385. (b) Mori, K.; Tamada, S. Tetrahedron Lett.
1978, 10, 901. (c) Mori, K.; Tamada, S. Tetrahedron 1979, 35, 1279.
(d) Bystro¨m, S.; Ho¨gberg, H. E.; Norin, T. Tetrahedron 1981, 37, 2249.
(e) Kikukawa, T.; Imaida, M.; Tai, A. Bull. Chem. Soc. J pn. 1984, 57,
1954. (f) Itoh, T.; Yonekawa, Y.; Sato, T.; Fujisawa, T. Tetrahedron
Lett. 1986, 27, 5405. (g) Larcheveˆque, M.; Sanner, C. Tetrahedron 1988,
44, 6407. (h) ref 3a. (i) Ho¨gberg, H. E.; Hedenstro¨m, E.; Wassgren, A.
B.; Hjalmarsson, M.; Bergstro¨m, G.; Lo¨fqvist, J .; Norin, T. Tetrahedron
1990, 46, 3007.
(7) For some syntheses of homologous and diastereomers of 1, see:
(a) Kikukawa, T.; Imaida, M.; Tai, A. Chem. Lett. 1982, 1799. (b) Tai,
A.; Sugimura, T.; Kikukawa, T.; Naito, C.; Nishimoto, Y.; Morimoto,
N. Biosci., Biotechnol., Biochem. 1992, 56, 1711. (c) Hedenstro¨m, E.;
Hogberg, H. E.; Wassgren, A. B.; Bergstrom, G.; Lofqvist, J .; Hansson,
B.; Anderbrant, O. Tetrahedron 1992, 48, 3139. (d) Tai, A.; Higashiura,
Y.; Kakizaiki, M.; Naito, T.; Tanaka, K.; Fujita, M.; Sugimura, T.; Hara,
H.; Hayshi, N. Biosci., Biotechnol., Biochem. 1998, 62, 607. (e) Moreira,
J . A.; Correˆa, A. G. J . Braz. Chem. Soc. 2000, 11, 614. (f) Ebert, S.;
Krause, N. Eur. J . Org. Chem. 2001, 3831. (g) Hedenstro¨m, E.; Edlund,
H.; Lund, S.; Abersten, M.; Persson, D. J . Chem. Soc., Perkin Trans.
1 2002, 1810. (h) Tai, A.; Tanaka, K.; Fujita, M.; Sugimura, T.;
Higashiura, Y. Kasashi, M.; Hara, H.; Naito, T. Bull. Chem. Soc. J pn.
2002, 75, 111.
(8) (a) Huang, P.-Q.; Wang, S. L.; Zheng, H.; Fei, X. S. Tetrahedron
Lett. 1997, 38, 271. (b) He, B.-Y.; Wu, T.-J .; Yu, X.-Y.; Huang, P.-Q.
Tetrahedron: Asymmetry 2003, 14, 2101. (c) Huang, P.-Q.; Meng, W.-
H. Lett. Org. Chem. 2004 1, 99.
(9) For related transformations, see: (a) Forster, R. C.; Owen, L. N.
J . Chem. Soc., Perkin Trans. 1 1978, 822. (b) Plattner, J . J .; Rapoport,
H. J . Am. Chem. Soc. 1971, 93, 1758.
(1) Coppel, H. C.; Casida, J . E.; Dauterman, W. C. Ann. Entomol.
Soc. Am. 1960, 53, 510.
(2) J ewett, D. M.; Matsumura, F.; Coppel, H. C. Science 1976, 192,
51.
(3) (a) Tai, A.; Morimoto, N.; Yoshikawa, M.; Uehara, K.; Sugimura,
T.; Kikukawa, T. Agric. Biol. Chem. 1990, 54, 1753. (b) Hedenstro¨m,
E.; Ho¨gberg, H. E.; Wassgren, A. B.; Bergstro¨m, G.; Lo¨fqvist, J .;
Hansson, B. S.; Anderbrant, O. Tetrahedron 1992, 48, 3139.
(4) For reviews on the syntheses of 1 and related compounds, see:
(a) Mori, K. In The Total Synthesis of Natural Products; ApSimon, J .,
Ed.; J ohn Wiley: New York, 1981; Vol. 4, pp 123-129. (b) Mori, K. In
The Total Synthesis of Natural Products; ApSimon, J ., Ed.; J ohn
Wiley: New York, 1992; Vol. 9, pp 122-129.
(5) For racemic syntheses of 1, see: (a) Reference 2. (b) Magnusson,
G. Tetrahedron Lett. 1977, 31, 2713. (c) Kocienski, P. J .; Ansell, J . M.
J . Org. Chem. 1977, 42, 1102. (d) Place, P.; Roumestant, M. L.; Gore,
J . J . Org. Chem. 1978, 43, 1001. (e) Baker, R.; Winton, P. M.
Tetrahedron Lett. 1980, 21, 1175. (f) Kallmerten, J .; Balestra, M. J .
Org. Chem. 1986, 51, 2855. (g) Gould, T. J .; Balestra, M.; Wittman,
M. D.; Gary, J . A.; Rossano, L. T.; Kallmerten, J . J . Org. Chem. 1987,
52, 3889. (h) Hedenstro¨m, E.; Ho¨gberg, H. E. Tetrahedron 1994, 50,
5225.
(10) (a) Mori, K.; Ikunaka, M. Tetrahedron 1984, 40, 3471. (b)
Axelsson, B. S.; O’Toole, K. J .; Spencer, P. A.; Young, D. U. J . Chem.
Soc., Perkin Trans. 1 1994, 807.
10.1021/jo0497961 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/27/2004
3964
J . Org. Chem. 2004, 69, 3964-3967

SCHEME 1. Retr osyn th etic An a lysis of 1
SCHEME 2. Syn th esis of th e F r a gm en t B
SCHEME 4. Syn th esis of th e F r a gm en t BC
SCHEME 5. Syn th esis of th e F r a gm en t A
SCHEME 3. P ossible Mech a n ism of th e On e-P ot
Rea ction
tosylate (S)-6 (fragment B) proceeded smoothly to provide
(S)-13 in 79% yield (Scheme 4). (S)-13 was then converted
to bromide (S)-4 [[R]20D +3.3 (c 5.1, hex.); lit.6c [R]20D +2.3
EtOH)]11 allowed its configuration to be assigned as S.
Chiral HPLC analysis of the monotosylate 11a showed
that the ee of 11 was 97.3%. Selective monosilylation of
(S)-11 (TBDMSCl, imid, DMF) gave known (S)-1212 in
89% yield, which was then tosylated to give the fragment
B (6) in 85% yield.
The stereoselective conversion of (S)-10 to (S)-11
implicates that an inversion of configuration occurred.
A plausible mechanism for this transformation is de-
picted in Scheme 3, which involves sequential chemo- and
regioselective ester reduction (or chemoselective ester
reduction), SNi reaction with inversion of configuration
(epoxide formation) and regioselective reductive epoxide-
opening reaction at the terminal carbon.
(neat); lit.6e,7a [R]20 +4.0 (c 4.5, hex.)] via a three-step
D
procedure (HCl, 65%; TsCl, Py., 99%; LiBr, acetone, 84%).
We were delighted to find that a one-pot transformation
of 13 to 4 was possible, simply by treating 13 with PPh3/
Br2.13 In this way, 4 was obtained in 91% yield.
The synthesis of fragment A (5) began with 17, easily
available as an inseparable diastereomeric mixture (anti/
syn ) 12:1) from diethyl (S)-malate 16 (LHMDS, THF,
-78 °C; MeI).14 It was observed that using LHMDS in
place of LDA as a base led to slightly improved diaste-
reoselectivity (anti/syn ) 12:1, combined yield: 86%)
(Scheme 5). Tosylation of (2S,3R)-17 gave 18 in 90%
yield. It is worth noting that the tosylation of 17 not only
provided the precursor for the key sequential reaction
but rendered as well the separation of diastereomers
Coupling of the organocopper species, generated in situ
from n-C8H17MgBr (fragment C) and CuI in THF, with
(11) (a) Gerlach, von H.; Oertle, K.; Thalmann, A. Helv. Chim. Acta
1976, 59, 755. (b) Murakam, S.; Harada, T.; Tai, A. Bull. Chem. Soc.
J pn. 1980, 53, 1356. (c) Hungerbu¨hler, Von E.; Seebach, D.; Wasmuth,
D. Helv. Chim. Acta 1981, 64, 1467.
(13) Ashton, P. R.; Ko¨niger, R.; Stoddart, J . F. J . Org. Chem. 1996,
61, 903.
(12) Pilli, R. A.; Victor, M. M. Tetrahedron Lett. 1998, 39, 4421.
(14) Seebach, D.; Aebi, J .; Wasmuth, D. Org. Synth. 1984, 63, 109.
J . Org. Chem, Vol. 69, No. 11, 2004 3965

SCHEME 6. Syn th esis of (2S,3S,7S)-1
Exp er im en ta l Section
(S)-1,3-Bu ta n ed iol (11). To a suspension of LAH (2.06 g, 54.3
mmol) in anhydrous THF (100 mL) was added dropwise a
solution of 10 (2.91 g, 9.22 mmol) in THF (8.0 mL) The mixture
was stirred at 55 °C for 4 h and then re-cooled to -20 °C. To
the resultant mixture were added successively water (2.8 mL),
15% NaOH (2.8 mL), and water (8.4 mL). The mixture was
filtered through Celite. The filtrate was concentrated in vacuo
and the residue was chromatographed (eluent: EtOAc) to afford
(S)-11 (colorless oil, 456 mg, yield 55%) and 1,4-butanediol (124
20
mg, yield 15%). (S)-11: [R]20 +30.0 (c 1.0, EtOH) [lit.11 [R]
D
D
+29.0 (c 1.0, EtOH) for (S)-11]; IR (film) 3337, 1458, 1376, 1288,
1261, 1075, 1053 cm-1; H NMR (500 MHz, CDCl3) δ 1.24 (d, J
1
SCHEME 7. Syn th esis of 2 a n d 3
) 6.3 Hz, 3H), 1.70 (m, 2H), 3.05 (s, br, 2H), 3.81 (ddd, J ) 5.7,
5.7, 10.8 Hz, 1H), 3.88 (ddd, J ) 5.2, 5.2, 10.8 Hz, 1H), 4.07 (m,
1H); 13C NMR (125 MHz, CDCl3) δ 23.8, 40.0, 61.6, 68.1. MS
(ESI) m/z 91 (M + H+, 100); HRMS m/z calcd for C8H21O4+ (2M
+ H+) 181.1440, found 181.1434. Chiral HPLC analysis of both
racemic 11a and (S)-11a using a CHIRALPAK column (hex./
EtOH ) 6/4, tR ) 9.44 min for S-enantiomer; tR ) 8.81 min. for
R-enantiomer) showed that the ee of (S)-11 was 97.3%.
possible. Thus, treatment of pure anti-18 with LAH (6
molar equiv) in THF afforded the desired diol 715 [color-
(2S,3S)-2-Meth yl-1,3-bu ta n ed iol (7). Following the proce-
dure described for the conversion of 10 to 11, 18 was converted
to 7 in a yield of 79%. (R)-2-Methyl-1,4-butanediol was also
obtained in a yield of 10%. (2S,3S)-7:15 colorless oil; [R]20D +10.6
(c 1.1, CHCl3); IR (film) 3337, 1730, 1653, 1458, 1376, 1288, 1261,
1132, 1075, 1053, 1008 cm-1; 1H NMR (500 MHz, CDCl3) δ 0.88
(d, J ) 6.9 Hz, 3H), 1.12 (d, J ) 6.4 Hz, 3H), 1.80 (m, 1H), 2.52
(s, br, 1H), 2.60 (s, br, 1H), 3.68 (m, 2H), 4.02 (m, 1H); 13C NMR
(125 MHz, CDCl3) δ 10.7, 19.5, 40.1, 66.6, 70.8; MS (ESI) m/z
127 (M + Na+, 100), 105 (M + H+, 32); HRMS m/z calcd for
less oil, [R]20 +10.6 (c 1.1, CHCl3)] in 79% yield and
D
2-methyl-1,4-butanediol in 10% yield. Selective mono-
tosylation of 7 followed by protection of the secondary
hydroxyl group yielded known fragment A (5)6e as a
diastereomeric mixture at the acetal carbon. Chiral
HPLC analysis of (2S,3S)-19 showed that the enantio-
meric excess of 7 was 98.1%.
With the fragments A and BC available, their cou-
pling6e,16 were undertaken. Thus, treatment of organo-
metallic species (fragment BC) generated from bromide
4 (Mg/THF; CuI) with tosylate 5 gave desired 20 in 72%
yield (Scheme 6). Finally, deprotection of 20 under acidic
conditions afforded (2S,3S,7S)-3,7-dimethyl-2-pentade-
+
C15H25O4 (2M + H+) 209.1753, found 209.1744.
(2S,3S)-2-Meth yl-1-(p-tolu en esu lfon yloxy)-3-bu tan ol (19).
To a solution of (2S,3S)-7 (68 mg, 0.65 mmol) in CH2Cl2 (0.5
mL) was added Et3N (0.12 mL, 0.81 mmol) at -20 °C. To the
mixture was added a solution of TsCl (124 mg, 0.65 mmol) in
CH2Cl2 (0.5 mL) over 2 h. After being stirred at -20 °C for 3 h,
the mixture was allowed to warm and stirred for an additional
36 h. The mixture was poured into 1.2 mL of water and extracted
with CH2Cl2. The organic layers were washed successively with
an aqueous solution of 2 N HCl (0.5 mL), a saturated aqueous
solution of NaHCO3 (0.5 mL) and brine (0.5 mL), dried (MgSO4),
and concentrated. The residue was chromatographed (eluent:
EtOAc/PE ) 1: 4) to give 19 (118 mg, yield: 70%) as a colorless
canol (1) [colorless oil, [R]20 -9.9 (c 4.0, hexane) [lit.6b,c
D
[R] 20D -9.8 (neat); lit.6e [R]20D -10.4 (c 3.7, hexane); lit.6d
[R] 20D -12.2 (neat); lit.6f [R]23D -9.6 (c 0.65, hexane); lit.6g
[R]20D -11.5 (c 0.8, hexane); lit.6i [R]20D -11.8 (neat)]], the
common alcohol moiety of the sex pheromones of the pine
sawflies, in 91% yield. Spectral properties of our synthetic
1 were identical with those of the natural compound.6
oil, [R]20 +3.2 (c 1.1, CHCl3). Chiral HPLC analysis under the
D
conditions described for racemic 11a showed that the ee of 19
Finally, acylation of 1 afforded the sex pheromone 2
[colorless oil, [R]20 -5.7 (c 1.3, hex.) [lit.6b,6c [R]20 -5.8
was 98.1%: IR (film) 3552, 3426, 2975, 2929, 1598, 1458, 1356,
1
1177, 1097 cm-1; H NMR (500 MHz, CDCl3) δ 0.82 (d, J ) 7.1
D
D
(neat); lit.6e [R]20D -6.0 (c 4.3, hex.)]] in 97% yield (Scheme
Hz, 3H), 1.09 (d, J ) 6.2 Hz, 3H), 1.67 (s, br, 1H), 1.79 (m, 1H),
2.42 (s, 3H), 3.86 (m, 2H), 4.01 (m, 1H), 7.35 (m, 2H), 7.82 (m,
2H); 13C NMR (125 MHz, CDCl3) δ 9.9, 20.3, 21.6, 39.0, 66.8,
72.6, 127.9, 127.9, 129.9, 129.9, 132.9, 144.8; MS (ESI) m/z 276
7). Similarly, propionation of 1 with propionic anhydride
furnished 3 [colorless oil, [R]20 -5.7 (c 0.6, hexane)
D
[lit.6b,6c [R]20 -5.5 (neat); lit.6e [R]20 -6.9 (c 18, hex.)]]
D
D
+
(M+ + H2O, 100%); HRMS m/z calcd for C12H19SO4 (M + H+)
in 93% yield.
259.1004, found 259.0997.
In summary, starting from (S)-malic acid, an efficient
total synthesis of (2S,3S,7S)-3,7-dimethylpentadecan-2-
yl acetate (2) and propionate (3), the active enantiomers
of the pheromones of pine sawflies, was achieved. The
synthesis features tandem ester reduction-epoxide for-
mation-reductive epoxide-opening reaction [(S)-10 f
(S)-11 and (2S,3R)-18 f (2S,3S)-7] as the key steps. This
one-pot, chemo-, regio- and stereoselective transformation
is not only highly efficient, but also proceeds with
inversion of configuration, allowing the establishment of
all the three chiral centers of (2S,3S,7S)-1 from L-malic
acid.
(2S,3S,7S)-3,7-Dim eth yl-2-p en ta d eca n ol (1). A mixture of
19 (570 mg, 2.21 mmol), dihydropyran (0.30 mL, 2.67 mmol),
and a catalytic amount of p-toluenesulfonic acid in dry CH2Cl2
(9.0 mL) was stirred at rt overnight. The mixture was quenched
with aqueous NaHCO3 and extracted with CH2Cl2. The combined
organic layers were washed brine, dried over Na2SO4, and
concentrated in vacuo. The residue was purified by flash
chromatography (eluent: EtOAc/PE ) 1:10), and known 53a,6e,7b
(718 mg, yield 95%) was obtained as a colorless oil.
Grignard reagent, prepared from magnesium (53 mg, 2.21
mmol) and (S)-4 (548 mg, 2.20 mmol) in anhydrous THF (5.0
mL) under argon atmosphere, was chilled to -78 °C. A solution
of (2S, 3S)-5 (343 mg, 1.0 mmol) in THF (2.0 mL) and CuI (cat.)
were added. The reaction mixture was allowed to warm to 5 °C
and stirred at that temperature for 3 h. After being stirred
overnight at rt, the reaction mixture was cooled with an ice bath
and then poured into ice-cooled aqueous NH4Cl and extracted
with ether. The combined ethereal solution was washed succes-
sively with aqueous NH4Cl, aqueous NaHCO3 and brine, dried
(15) For diastereomers of 18, see: Dolby, L. J .; Meneghini, F. A.;
Koizumi, T. J . Org. Chem. 1968, 33, 3060.
(16) Fouquet, G.; Schlosser, M. Angew. Chem., Int. Ed. Engl. 1974,
13, 82.
3966 J . Org. Chem., Vol. 69, No. 11, 2004

(Na2SO4), and concentrated. After flash chromatographic puri-
fication (eluent: EtOAc/PE ) 1:50), known 203a,6e,7b (245 mg,
yield 72%) was obtained as a colorless liquid.
274 (M+ + H2O, 52), 279 (M + Na+, 100). GC analysis showed
that the purity of 1 was 99.5%.
A methanolic solution (8.0 mL) of 20 (245 mg, 0.72 mmol) was
stirred in the presence of a catalytic amount of p-toluenesulfonic
acid at rt for 24 h. The resultant mixture was concentrated under
reduced pressure. The resulted residue was purified by flash
chromatography (eluent: EtOAc/PE ) 1:10) to afford (2S,3S,7S)-
Ack n ow led gm en t. We are grateful to the NNSFC
(20272048; 203900505), the Ministry of Education (Key
Project 104201), and the Specialized Research Fund for
the Doctoral Program of Higher Education (20020384004)
for financial support.
3,7-dimethyl-2-pentadecanol (1) (168 mg, yield: 91%) as a
20
colorless oil: [R]
-9.9 (c 4.0, hex.) [lit.6e [R]20 -10.4 (c 3.7,
D
D
hex.); lit.6f [R]23D -9.6 (c 0.65, hex.); lit.6g [R]20D -11.5 (c 0.8, hex.);
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for compounds 2-4, 6, 10, 12-15,
lit.6i [R]20D -11.8 (neat)]; IR (film) 3373, 2925, 2855, 1463, 1377,
1
1322, 1089 cm-1; H NMR (500 MHz, CDCl3) δ 0.85 (d, J ) 6.4
1
18; H NMR and 13C NMR spectra of 1-4, 6, 7, 13-15, 18,
Hz, 3H), 0.88 (t, J ) 6.7 Hz, 3H), 0.89 (d, J ) 6.3 Hz, 3H), 1.15
(d, J ) 6.4 Hz, 3H), 1.0-1.7 (m, 23H), 3.71 (m, 1H); 13C NMR
(125 MHz, CDCl3) δ 14.1, 14.2, 19.8, 20.3, 22.7, 24.8, 27.1, 29.4,
29.7, 30.1, 32.0, 32.8, 33.0, 37.0, 37.4, 39.8, 71.4; MS (ESI) m/z
and 19. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O0497961
J . Org. Chem, Vol. 69, No. 11, 2004 3967