Synthesis of novel heteroaromatics structurally related to ellipticine alkaloids via thermolysis of pyridannulated enyne-carbodiimides

Article abstract of DOI:10.1021/jo0202031

New synthetic pathways to the 5H-pyrido[3',4':4,5]pyrrolo[2,3-b]quinolines 6, the 6H-pyrido[3',4':4,5]pyrrolo[2,3-b] [1,6]naphthyridines 7, and the 11H-pyrido[43,3': 4,5]pyrrolo[2,3-b]quinolines 8 via thermolysis of the pyridannulated enyne-carbodiimides 14, 19, and 23 were established. These novel heteroaromatic systems are structurally related to ellipticine alkaloids and could serve as DNA- intercalating agents.

Full text of DOI:10.1021/jo0202031

pyrrolo[2,3-b]quinolines 6, the 6H-pyrido[3′,4′:4,5]pyrrolo-
[2,3-b][1,6]naphthyridines 7, and the 11H-pyrido[4′,3′:
4,5]pyrrolo[2,3-b]quinolines 8.
Syn th esis of Novel Heter oa r om a tics
Str u ctu r a lly Rela ted to Ellip ticin e
Alk a loid s via Th er m olysis of
P yr id a n n u la ted En yn e-Ca r bod iim id es†
Xiaoling Lu, J effrey L. Petersen,‡ and Kung K. Wang*
Department of Chemistry, West Virginia University,
Morgantown, West Virginia 26506-6045
kwang@wvu.edu
Received March 22, 2002
Abstr a ct: New synthetic pathways to the 5H-pyrido[3′,4′:
4,5]pyrrolo[2,3-b]quinolines 6, the 6H-pyrido[3′,4′:4,5]pyr-
rolo[2,3-b][1,6]naphthyridines 7, and the 11H-pyrido[4′,3′:
4,5]pyrrolo[2,3-b]quinolines 8 via thermolysis of the pyrid-
annulated enyne-carbodiimides 14, 19, and 23 were estab-
lished. These novel heteroaromatic systems are structurally
related to ellipticine alkaloids and could serve as DNA-
intercalating agents.
Resu lts a n d Discu ssion
The synthetic sequence outlined in Scheme 1 involves
the use of the Pd-catalyzed cross-coupling reaction be-
tween 4-amino-3-iodopyridine (9)4 with terminal alkynes
10 to form 11. Treatment of 11 with dibromotriphenyl-
phosphorane gave the iminophosphoranes 125 for the
subsequent aza-Wittig reaction with phenyl isocyanate
(13) to produce in situ the pyridannulated enyne-
carbodiimides 14. Thermolysis of 14 under refluxing
p-xylene at 138 °C then afforded the 5H-pyrido[3′,4′:4,5]-
pyrrolo[2,3-b]quinolines 6. In the case of 6b, a small
amount (16%) of a urea derivative, derived from the
reaction between the nitrogen of the pyrrole ring of 6b
and phenyl isocyanate (13), was also isolated. The
structure of 6c was established by the X-ray structure
analysis.
Thermolysis of the benzannulated enyne-carbodi-
imides provides efficient synthetic pathways to novel
heterocyclic aromatic compounds. With a phenyl sub-
stituent at the carbodiimide terminus in 1a , a variety of
the 6H-indolo[2,3-b]quinolines 2a were readily obtained
(eq 1).1 Similarly, the presence of a 4-pyridyl substituent
in 1b led to the formation of the 6H-indolo[2,3-b][1,6]-
naphthyridines 2b,2 which could be regarded as the 5-aza
analogues of ellipticine (3), a naturally occurring alkaloid.3a
The transformation from 14 to 6 could proceed either
through a two-step biradical pathway as in the enyne-
allene system6 or through a concerted intramolecular
Ellipticine and many of its derivatives were found to
exhibit potent antitumor activities.3b-m The azaellipti-
cines 4 and 5 are active on topoisomerase II and initiate
the cleavage of DNA,3c,l,m and 5 has undergone clinical
trials.3c,d We envisioned that by replacing the central
benzene ring in 1a and 1b with a pyridine ring, a simple
synthetic pathway to a variety of new heteroaromatics
having structures related to ellipticine alkaloids could
thus be established. We now report our findings of using
this strategy for the synthesis of the 5H-pyrido[3′,4′:4,5]-
(3) (a) Goodwin S.; Smith, A. F.; Horning, E. C. J . Am. Chem. Soc.
1959, 81, 1903-1908. (b) Dalton, L. K.; Demerac, S.; Elmes, B. C.;
Loder, J . W.; Swan, J . M.; Teitei, T. Aust. J . Chem. 1967, 20, 2715-
2727. (c) Gribble, G. W. In The Alkaloids; Brossi, A., Ed.; Academic
Press: New York, 1990; Vol. 39, pp 239-351. (d) Suffness, M.; Cordell,
G. A. In The Alkaloids; Brossi, A., Ed.; Academic Press: New York,
1985; Vol. 25, pp 89-142. (e) Kansal, V. K.; Potier, P. Tetrahedron
1986, 42, 2389-2408. (f) Gribble, G. W.; Saulnier, M. G.; Obaza-
Nutaitis, J . A.; Ketcha, D. M. J . Org. Chem. 1992, 57, 5891-5899. (g)
Ishikura, M.; Yaginuma, T.; Agata, I.; Miwa, Y.; Yanada, R.; Taga, T.
Synlett 1997, 214-216. (h) D´ıaz, M. T.; Cobas, A.; Guitia´n, E.; Castedo,
L. Synlett 1998, 157-158. (i) Ergu¨n, Y.; Patir, S.; Okay, G. J .
Heterocycl. Chem. 1998, 35, 1445-1447. (j) Ishikura, M.; Hino, A.;
Yaginuma, T.; Agata, I.; Katagiri, N. Tetrahedron 2000, 56, 193-207.
(k) Anderson, W. K.; Gopalsamy, A.; Reddy, P. S. J . Med. Chem. 1994,
37, 1955-1963. (l) Pierson, V.; Pierre, A.; Pommier, Y.; Gros, P. Cancer
Res. 1988, 48, 1404-1409. (m) Vilarem, M. J .; Riou, J . F.; Multon, E.;
Gros, M. P.; Larsen, C. J . Biochem. Pharmacol. 1986, 35, 2087-2095.
(4) Estel, L.; Marsais, F.; Que´guiner, G. J . Org. Chem. 1988, 53,
2740-2744.
* To whom correspondence should be addressed. Phone: (304) 293-
3068 ext 6441. Fax: (304) 293-4904.
† Dedicated to Professor Herbert C. Brown on the occasion of his
90th birthday.
‡ To whom correspondence concerning the X-ray structure should
be addressed. Phone: (304) 293-3435 ext 6423. Fax: (304) 293-4904.
E-mail: jpeterse@wvu.edu.
(1) (a) Shi, C.; Zhang, Q.; Wang, K. K. J . Org. Chem. 1999, 64, 925-
932. (b) Schmittel, M.; Steffen, J .-P.; Engels, B.; Lennartz, C.; Hanrath,
M. Angew. Chem., Int. Ed. 1998, 37, 2371-2373. (c) For a photochemi-
cally induced reaction, see: Schmittel, M.; Rodr´ıguez, D.; Steffen, J .-
P. Angew. Chem., Int. Ed. 2000, 39, 2152-2155.
(2) Zhang, Q.; Shi, C.; Zhang, H.-R.; Wang, K. K. J . Org. Chem. 2000,
65, 7977-7983.
(5) (a) Molina, P.; Alajar´ın, M.; Vidal, A. J . Chem. Soc., Chem.
Commun. 1990, 1277-1279. (b) Horner, L.; Oediger, H. J ustus Liebigs
Ann. Chem. 1959, 627, 142-162. (c) Bo¨deker, J .; Ko¨ckritz, A.; Ko¨ckritz,
P.; Radeglia, R. J . Prakt. Chem. 1985, 327, 723-730. (d) Kulpe, S.;
Seidel, I.; Bo¨deker, J .; Ko¨ckritz, P. Cryst. Res. Technol. 1984, 19, 649-
654.
10.1021/jo0202031 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/25/2002
5412
J . Org. Chem. 2002, 67, 5412-5415

SCHEME 1
SCHEME 3
SCHEME 2
SCHEME 4
Diels-Alder reaction. It is worth noting that the core
structure of 6 represents a new heterocyclic aromatic
system. A sample of 6a was submitted to the National
Cancer Institute for evaluation against human tumor cell
lines, and it was found to exhibit activity against MCF7
(breast) in the primary anticancer assay.
formation of the 11H-pyrido[4′,3′:4,5]pyrrolo[2,3-b]quino-
lines 8. The structures of 8b and 8c were established by
the X-ray structure analysis. Again the core structure of
8 represents a new heterocyclic aromatic system. Inter-
estingly, in addition to 8, minor amounts of 24 were also
isolated with the structure of 24b being established by
the X-ray structure analysis. Presumably, cycloaddition
between 23 and phenyl isocyanate (13) produced 25,
which then eliminated a molecule of diphenylcarbodi-
imide to form the 4-pyridyl isocyanates 26 (Scheme 4).9
A subsequent aza-Wittig reaction with 22 then produced
the pyridannulated enyne-carbodiimides 27 as the pre-
cursors of 24.
Treatment of 11 with triphosgene produced in situ the
4-pyridyl isocyanates 17 (Scheme 2). It was reported that
the parent 4-pyridyl isocyanate is very reactive and
presumably undergoes spontaneous trimerization to form
an isocyanurate.7 Fortunately, with an adjacent alkynyl
substituent in the cases of 17, the 4-pyridyl isocyanates
17 were relatively stable to allow the subsequent aza-
Wittig reaction with the iminophosphorane 182 to furnish
in situ the pyridannulated enyne-carbodiimides 19.
Thermolysis of 19 under refluxing p-xylene at 138 °C
then afforded the 6H-pyrido[3′,4′:4,5]pyrrolo[2,3-b][1,6]-
naphthyridines 7 having a nitrogen in each of the four
rings of the novel heterocyclic system. A sample of 7a
was also submitted to the National Cancer Institute for
screening against human tumor cell lines, and it was
found to exhibit activities against the three-cell line panel
in the primary anticancer assay.
Exp er im en ta l Section
All reactions were conducted in oven-dried (120 °C) glassware
under
a nitrogen atmosphere. Tetrahydrofuran (THF) was
distilled from benzophenone ketyl, and triethylamine was dis-
tilled from calcium hydride prior to use. 1-Alkynes were obtained
from chemical suppliers and were used without further purifica-
tion. Dibromotriphenylphosphorane (Ph3PBr2), Pd(PPh3)2Cl2,
copper(I) iodide, p-xylene (anhydrous), N,N-dimethylformamide
(DMF), N,N-diisopropylethylamine, phenyl isocyanate (13), and
triphosgene were used as received. 4-Amino-3-iodopyridine (9)
was prepared in 98% yield by treatment of the corresponding
trimethylacetamide4 with 4.5 M sulfuric acidic under refluxing
water for 2 h. 3-Amino-4-iodopyridine (20)8 and the iminophos-
phorane 182 were prepared according to the reported procedures.
By starting from 3-amino-4-iodopyridine (20),8 the
synthetic sequence outlined in Scheme 3 led to the
(6) (a) Schmittel, M.; Strittmatter, M.; Kiau, S. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1843-1845. (b) Schmittel, M.; Maywald, M.;
Strittmatter, M. Synlett 1997, 165-166. (c) Musch, P. W.; Engels, B.
J . Am. Chem. Soc. 2001, 123, 5557-5562.
(7) (a) Chambers, J .; Reese, C. B. Tetrahedron Lett. 1975, 2783-
2786. (b) L’abbe´, G. Synthesis 1987, 525-531.
(8) Malm, J .; Rehn, B.; Ho¨rnfeldt, A.-B.; Gronowitz, S. J . Heterocycl.
Chem. 1994, 31, 11-15.
(9) Bo¨deker, J .; Courault, K.; Ko¨ckritz, A.; Ko¨ckritz, P. J . Prakt.
Chem. 1983, 325, 463-474.
J . Org. Chem, Vol. 67, No. 15, 2002 5413

Melting points were uncorrected. 1H (270 MHz) and 13C (67.9
(1 H, s), 9.30 (1 H, s), 8.58 (1 H, d, J ) 5.3 Hz), 8.50 (1 H, d, J
) 5.0 Hz), 7.77 (1 H, d, J ) 6.1 Hz), 7.42 (1 H, d, J ) 5.5 Hz),
3.24 (1 H, s); 13C (6% CD3OD in CDCl3) δ 154.3, 149.3, 149.0,
146.7, 145.4, 143.8, 142.1, 120.4, 120.1, 118.7, 116.7, 106.9, 15.2.
MHz) NMR spectra were recorded in CDCl3 using CHCl3 (1H δ
7.26) and CDCl3 (
13C δ 77.00) as internal standards unless
otherwise indicated.
4-Am in o-3-(2-p h en yleth yn yl)p yr id in e (11c). The following
procedure for the preparation of 11c is representative. The flask
containing 0.051 g (0.072 mmol) of Pd(PPh3)2Cl2 and 0.023 g
(0.120 mmol) of copper(I) iodide was evacuated and then filled
with nitrogen. To the flask were added in sequence via cannula
5 mL of degassed DMF, a degassed solution of 1.34 mL (7.65
mmol) of N,N-diisopropylethylamine and 0.525 g (2.39 mmol)
of 4-amino-3-iodopyridine (9) in 5 mL of DMF, and a degassed
solution of 0.510 g (5.00 mmol) of phenylacetylene (10c) in 10
mL of DMF. After 12 h at rt, the reaction mixture was poured
into a flask containing 20 mL of dichloromethane and 20 mL of
a saturated NH4Cl solution. The organic layer was separated,
washed with water, dried over Na2SO4, and concentrated. The
residue was purified by column chromatography (silica gel,
hexanes/ethyl acetate/95% ethanol ) 5:5:1) to afford 11c (0.412
g, 2.12 mmol, 89%) as a white solid: mp 97-98 °C; IR (KBr)
3454, 3298, 1643, 757, 689 cm-1; 1H δ 8.45 (1 H, s), 8.16 (1 H, d,
J ) 3.4 Hz), 7.55-7.51 (2 H, m), 7.38-7.35 (3 H, m), 6.57 (1 H,
d, J ) 5.8 Hz), 4.79 (2 H, br); 13C δ 153.0, 152.6, 149.2, 131.5,
128.6, 128.4, 122.6, 108.2, 105.0, 97.1, 82.5; MS m/z 194 (M+),
166, 139.
3-Am in o-4-(1-p en tyn yl)p yr id in e (21b). The following pro-
cedure for the preparation of 21b is representative. The flask
containing 0.031 g (0.044 mmol) of Pd(PPh3)2Cl2 and 0.014 g
(0.074 mmol) of copper(I) iodide was evacuated and then filled
with nitrogen. To the flask were added in sequence via cannula
15 mL of degassed triethylamine, a degassed solution of 0.323
g (1.47 mmol) of 3-amino-4-iodopyridine (20) in
5 mL of
anhydrous THF, and a degassed solution of 0.250 g (3.68 mmol)
of 1-pentyne (10b) in 5 mL of triethylamine. After 12 h at rt,
the reaction mixture was poured into a flask containing 20 mL
of dichloromethane and 20 mL of a saturated NH4Cl solution.
The organic layer was separated, washed with water, dried over
Na2SO4, and concentrated. The residue was purified by column
chromatography (neutral alumina/50% ethyl acetate in hexanes)
to afford 21b (0.190 g, 1.19 mmol, 81%) as a brown yellow
liquid: IR (neat) 3456, 3314, 2218, 821 cm-1; 1H δ 7.99 (1 H, s),
7.77 (1 H, d, J ) 5.0 Hz), 6.96 (1 H, d, J ) 5.0 Hz), 4.41 (2 H,
br), 2.31 (2 H, t, J ) 7.2 Hz), 1.51 (2 H, sextet, J ) 7.2 Hz), 0.92
(3 H, t, J ) 7.2 Hz); 13C δ 143.3, 138.1, 136.2, 124.9, 115.1, 99.6,
74.8, 21.7, 21.2, 13.2; MS m/z 160 (M+), 145, 131.
5-P r opyl-11H-pyr ido[4′,3′:4,5]pyr r olo[2,3-b]qu in olin e (8b)
a n d 4-(1-P en tyn yl)-11-p r op yl-6H-p yr id o[4′,3′:4,5]p yr r olo-
[2,3-b][1,5]n a p h th yr id in e (24b). The following procedure for
the preparation of 8b is representative. To a solution of 0.464 g
(1.100 mmol) of Ph3PBr2 and 1.39 mL of anhydrous triethyl-
amine (10.00 mmol) in 10 mL of anhydrous p-xylene was
introduced via cannula a solution of 0.080 g (0.500 mmol) of
3-amino-4-(1-pentynyl)pyridine (21b) in 10 mL of anhydrous
p-xylene. The reaction solution was kept at 80-90 °C for 12 h
before it was allowed to cool to rt. The triethylammonium
bromide precipitate was removed by filtration under a nitrogen
atmosphere and was washed twice with 10 mL of anhydrous
p-xylene. To the combined yellow-brown filtrate and the p-xylene
solutions was then added via cannula a solution of 0.054 g (0.450
mmol) of phenyl isocyanate (13) in 10 mL of anhydrous p-xylene.
After 5 h of stirring at rt followed by heating under reflux at
138 °C for 12 h, the reaction mixture was allowed to cool to rt.
The solution was concentrated, and the residue was purified by
column chromatography (neutral alumina/chloroform) to afford
0.063 g (0.241 mmol, 54%) of 8b as a pale yellow powder and
0.016 g (0.049 mmol, 11%) of 24b as a light yellow solid.
Compound 8b: mp 253-254 °C (sample recrystallized from 5%
11-P h en yl-5H-pyr ido[3′,4′:4,5]pyr r olo[2,3-b]qu in olin e (6c).
The following procedure for the preparation of 6c is representa-
tive. To a solution of 1.020 g (2.420 mmol) of Ph3PBr2 and 2.79
mL (20.00 mmol) of anhydrous triethylamine in 20 mL of
anhydrous p-xylene was introduced via cannula a solution of
0.215 g (1.11 mmol) of 4-amino-3-(2-phenylethynyl)pyridine (11c)
in 20 mL of anhydrous p-xylene. The reaction mixture was kept
at 80-90 °C for 12 h before it was allowed to cool to rt. The
triethylammonium bromide precipitate was removed by filtra-
tion under a nitrogen atmosphere and was washed twice with
10 mL of anhydrous p-xylene. To the combined yellow-brown
filtrate and the p-xylene solutions was added via cannula a
solution of 0.118 g (0.990 mmol) of phenyl isocyanate (13) in 10
mL of anhydrous p-xylene. After 5 h at rt, the reaction mixture
was heated under reflux at 138 °C for 12 h and then was allowed
to cool to rt and concentrated. The residue was purified by
column chromatography (silica gel/10% absolute ethanol in
chloroform) to afford 0.188 g (0.064 mmol, 64%) of 6c as a yellow
powder. Recrystallization from chloroform and absolute ethanol
(100:5) afforded yellow crystals: mp 297-298 °C; IR (KBr) 3426,
1
1605, 759, 699 cm-1; H δ 12.35 (1 H, br), 8.45 (1 H, d, J ) 5.3
absolute ethanol in chloroform); IR (KBr) 3448, 1625, 757 cm-1
;
Hz), 8.30 (1 H, s), 8.20 (1 H, d, J ) 8.4 Hz), 7.86-7.74 (2 H, m),
7.72-7.64 (3 H, m), 7.56-7.42 (3 H, m), 7.35 (1 H, d, J ) 5.3
Hz); 13C δ 152.6, 147.2, 146.6, 146.2, 144.5, 144.0, 135.8, 129.7,
129.3, 129.1, 126.8, 126.4, 124.1, 123.9, 118.1, 115.1, 106.1. Anal.
Calcd for C20H13N3: C, 81.34; H, 4.44; N, 14.23. Found: C, 81.42;
H, 4.45; N, 14.37. The structure of 6c was established by the
X-ray analysis.
1H δ 11.34 (1 H, br), 9.06 (1 H, s), 8.60 (1 H, d, J ) 5.3 Hz), 8.32
(1 H, d, J ) 8.7 Hz), 8.21 (1 H, d, J ) 7.9 Hz), 8.04 (1 H, d, J )
5.3 Hz), 7.83 (1 H, ddd, J ) 8.2, 6.9, 1.3 Hz), 7.57 (1 H, ddd, J
) 8.2, 6.9, 1.1 Hz), 3.68 (2 H, d, J ) 8.0 Hz), 1.99 (2 H, sextet,
J ) 7.6 Hz), 1.23 (3 H, t, J ) 7.4 Hz); 13C δ 153.0, 147.9, 147.8,
140.8, 136.9, 133.7, 130.2, 127.5, 127.4, 124.5, 123.51, 123.47,
117.2, 114.9, 31.2, 23.3, 14.7; Anal. Calcd for C17H15N3: C, 78.13;
H, 5.79; N, 16.08. Found: C, 78.41; H, 5.71; N, 16.00. The
structure of 8b was established by the X-ray analysis. Compound
24b: compound turns dark at 185 °C and becomes black at 215
°C (sample recrystallized from chloroform); IR (KBr) 1626, 1394,
1257, 822 cm-1; 1H δ 10.63 (1 H, br), 9.01 (1 H, s), 8.91 (1 H, d,
J ) 4.2 Hz), 8.61 (1 H, d, J ) 5.0 Hz), 8.08 (1 H, d, J ) 5.3 Hz),
7.80 (1 H, d, J ) 4.2 Hz), 3.90 (2 H, t, J ) 7.9 Hz), 2.38 (2 H, t,
J ) 7.1 Hz), 1.96 (2 H, sextet, J ) 7.7 Hz), 1.62-1.54 (2 H, m),
1.19 (3 H, t, J ) 7.4 Hz), 1.00 (3 H, t, J ) 7.4 Hz); 13C δ 153.0,
150.5, 146.4, 142.9, 140.8, 139.1, 137.3, 135.0, 129.6, 127.6, 126.9,
117.7, 117.3, 102.0, 78.0, 30.1, 23.2, 21.9, 21.5, 14.7, 13.5. The
structure of 24b was established by the X-ray analysis.
11-Met h yl-6H -p yr id o[3′,4′:4,5]p yr r olo[2,3-b][1,6]n a p h -
th yr id in e (7a ). The following procedure for the preparation of
7a is representative. To a solution of 0.0594 g (0.200 mmol) of
triphosgene in 20 mL of anhydrous p-xylene was added dropwise
using a pressure-equalizing addition funnel over 2 h a solution
of 0.28 mL (2.00 mmol) of anhydrous triethylamine and 0.066 g
(0.500 mmol) of 4-amino-3-(1-propynyl)pyridine (11a ) in 10 mL
of anhydrous p-xylene. The reaction mixture was heated to 70
°C for 12 h before it was allowed to cool to rt. The triethyl-
ammonium chloride precipitate was removed by filtration under
a nitrogen atmosphere and was washed twice with 10 mL of
anhydrous p-xylene. To the combined pale yellow filtrate and
the p-xylene solutions was added via cannula a solution of 0.177
g (0.500 mmol) of the iminophosphorane 18 in 20 mL of
anhydrous p-xylene. After the reaction mixture was kept at 50
°C for 6 h, it was heated under reflux at 138 °C for 12 h before
it was allowed to cool to rt. The solution was concentrated, and
the residue was purified by column chromatography (neutral
alumina/5% absolute ethanol in dichloromethane) to afford 0.075
g (0.321 mmol, 64%) of 7a as a pale yellow solid: mp >360 °C;
IR (KBr) 3447, 1605, 736 cm-1; 1H (6% CD3OD in CDCl3) δ 9.54
Ack n ow led gm en t. The financial support of the
National Science Foundation (CHE-9618676) to K.K.W.
is gratefully acknowledged. J .L.P. acknowledges the
support (CHE-9120098) provided by the Chemical In-
strumentation Program of the National Science Foun-
5414 J . Org. Chem., Vol. 67, No. 15, 2002

spectra for 6a -c, a urea derivative of 6b, 7a -c, 8a -c, 11a -
c, 21a -c, and 24a -c; and ORTEP drawings and tables of
crystallographic data for the X-ray diffraction analyses of 6c,
8b,c, and 24b. This material is available free of charge via
the Internet at http://pubs.acs.org.
dation for the acquisition of a Siemens P4 X-ray
diffractometer in the Department of Chemistry at West
Virginia University.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectroscopic data for 6a ,b, a urea derivative of
6b, 7b,c, 8a ,c, 11a ,b, 21a ,c, and 24a ,c; 1H and 13C NMR
J O0202031
J . Org. Chem, Vol. 67, No. 15, 2002 5415