Atom-economical synthesis of the N(10)-C(17) fragment of cyclotheonamides via a novel Passerini reaction-deprotection-acyl migration strategy1

Article abstract of DOI:10.1021/ol0165239

Equation presented A novel variant of the atom-economical Passerini reaction between suitably protected argininal, dipeptide isonitrile, and proline components afforded adduct 13. Orthogonal N-deprotection of 13 led, via a smooth O-to N-acyl migration, to 14, which constitutes the N(10)-C(17) fragment of the cyclotheonamide family of serine protease inhibitors. Each reaction in this three-step protocol proceeds in good yield and under very mild conditions.

Full text of DOI:10.1021/ol0165239

ORGANIC
LETTERS
2001
Vol. 3, No. 21
3301-3304
Atom-Economical Synthesis of the
N(10)−C(17) Fragment of
Cyclotheonamides via a Novel Passerini
Reaction−Deprotection−Acyl Migration
Strategy1
Timothy D. Owens and J. Edward Semple*
Department of Medicinal Chemistry, CorVas International, Inc.,
3030 Science Park Road, San Diego, California 92121
ed_semple@corVas.com
Received August 1, 2001
ABSTRACT
A novel variant of the atom-economical Passerini reaction between suitably protected argininal, dipeptide isonitrile, and proline components
afforded adduct 13. Orthogonal N-deprotection of 13 led, via a smooth O- to N-acyl migration, to 14, which constitutes the N(10)−C(17) fragment
of the cyclotheonamide family of serine protease inhibitors. Each reaction in this three-step protocol proceeds in good yield and under very
mild conditions.
Peptidyl and peptidomimetic R-ketoamide scaffolds are
useful in small molecule drug discovery programs as
transition-state analogue (TSA) protease inhibitors.2 Such
covalent inhibitors generally exhibit potent in vitro enzyme
inhibitory activity, with sub-nanomolar equilibrium inhibitor
constants (Ki) being typical of representative members.3
Accordingly, they are finding increasing applications as
potential therapeutics for important disease indications.4-7
The cyclotheonamides CtA-CtE3 (1a-g, Figure 1) con-
stitute a growing family of 19-membered macrocyclic
pentapeptide derivatives isolated from the marine sponge
Theonella swinhoei.8 All contain the unusual vinylogous
L-tyrosine, L-R-ketohomoarginine, and â-linked L-diamino-
propanoic acid moieties in addition to more common
L-proline and hydrophobic D-phenylalanine, D-leucine, or
(4) NS3-4A Hepatitis C protease and inhibitors thereof: (a) Bennett, J.
M. et al. Bioorg. Med. Chem. Lett. 2001, 11, 355. (b) Hoofnagle, J. H.; Di
Bisceglie, A. M. N. Engl. J. Med. 1997, 336, 347.
(5) HIV protease inhibitors: Sheha, M. M.; Mahfouz, N. M.; Hassan,
H. Y.; Youssef, A. F.; Mimoto, T.; Kiso, Y. Eur. J. Med. Chem. 2000, 35,
887.
(6) Calpain inhibitors: (a) De Biasi, R. L.; Edelstein, C. L.; Sherry, B.;
Tyler, K. L. J. Virol. 2001, 75, 351. (b) Harbeson, S. L.; Abelleira, S. M.;
Akiyama, A.; Barrett, R.; Carroll, R. M.; Straub, J. A.; Tkacz, J. N.; Wu,
C.; Musso, G. J. Med. Chem. 1994, 37, 2918.
(7) HCMV protease inhibitors: LaPlante, S. R.; Bonneau, P. R.; Aubry,
N.; Cameron, D. R.; Deziel, R.; Grande-Maitre, C.; Plouffe, C.; Tong, L.;
Kawai, S. H. J. Am. Chem. Soc. 1999, 121, 2974.
(1) Dedicated to Ruth F. Nutt, respected and admired mentor, on the
occasion of her 60th birthday.
(2) Semple, J. E. Abstracts of Papers, 219th National Meeting of the
American Chemical Society, San Francisco, CA, March 26-30, 2000;
American Chemical Society: Washington, DC, 2000; ORGN 667.
(3) (a) R-Ketoamide serine protease inhibitors: Semple, J. E.; Levy, O.
E.; Minami, N. K.; Owens, T. D.; Siev, D. V. Bioorg. Med. Chem. Lett.
2000, 10, 2305. (b) Semple, J. E.; Rowley, D. C.; Brunck, T. K.; Ripka,
W. C. Bioorg. Med. Chem. Lett. 1997, 7, 315. (c) R-Ketoamide cysteine
protease inhibitors: Nakamura, M.; Inoue, J.; Yamada, T. Bioorg. Med.
Chem. Lett. 2000, 10, 2807. (d) Otto, H. H.; Schirmeister, T. Chem. ReV.,
1997, 97, 133.
10.1021/ol0165239 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/27/2001

component reaction-based (MCR) approaches to R-hydroxy-
â-aminocarboxylic acid and amide derivatives. These ver-
satile, stable intermediates readily undergo oxidation to
ketoamide targets.
Utilizing the appropriate N-R-protected amino aldehyde
precursors 2 (Figure 2), we recently disclosed novel varia-
Figure 1. Representative examples of biologically active R-keto-
amides: members of the cyclotheonamide (Ct) class of marine
natural products.
D-isoleucine subunits. They are potent, slow-binding inhibi-
tors of several important trypsin-like serine proteases includ-
ing thrombin (factor IIa), factor Xa, trypsin, plasmin, and
tissue plasminogen activator.9 From this group, CtA (1a) and
CtE (1e) express the most potent thrombin inhibitory activity
with Ki values of e1 and 2.9 nM, respectively. Such potent
biological activity is derived from the key pharmacophore,
an electrophilic R-ketoargininamide group, which docks into
the S1 pocket of serine proteases and engages the catalytic
triad serine hydroxyl group to form a hemiketal (TSA)
intermediate, which effectively but reversibly inhibits the
enzyme.
Previous routes10 to the cyclotheonamides proceeded via
construction of individual protected amino acid subunits,
peptide couplings, macrocyclization, and late stage oxidation
to install the reactive ketoamide residue. In all cases,
protected R-hydroxy-â-homoarginine derivatives served as
the key L-R-ketohomoarginine precursors. They were pre-
pared by multistep homologation-hydrolysis approaches
from arginine or ornithine precursors via cyanohydrin10c,d (6-
11 steps), orthothioformate,10a,e (7-8 steps) or furyllithium
addition-oxidation10b (5 steps) protocols. Our interest in the
design and synthesis of novel classes of R-ketoamides as
small molecule inhibitors of serine proteases, including prolyl
endopeptidase,11 thrombin,12 factor Xa,3a,13 urokinase,14 and
NS3A hepatitis C protease,4 led us to investigate multiple-
Figure 2. Passerini MCR strategy for the construction of R-hy-
droxy-â-amino amides 4 and 6 and their potential elaboration into
the R-ketoamide subunit 7. PG denotes N-protecting group.
tions of the atom-economical Passerini reaction15 for the
direct production of either R-acyloxy-â-amino amides 32,16
or R-hydroxy-â-amino amide derivatives 4.2,16,17 In our hands,
(11) Total synthesis of the PEP inhibitor Eurystatin A: Owens, T. D.;
Araldi, G. L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271.
(12) Thrombin inhibitors: (a) Reiner, J. E.; Lim-Wilby, M. S.; Brunck,
T. K.; Uong, T. H.; Goldman, E. A.; Abelman, M. A.; Nutt, R. F.; Semple.
J. E.; Tamura S. Y. Bioorg. Med. Chem. Lett. 1999, 9, 895. (b) Semple, J.
E. Tetrahedron Lett. 1998, 39, 6645. (c) Semple, J. E.; Rowley, D. C.;
Brunck, T. K.; Uong, T. H.; Minami, N. K.; Owens, T. D.; Tamura, S. Y.;
Goldman, E. A.; Siev, D. V.; Ardecky, R. J.; Carpenter, S. H.; Ge, Y.;
Richard, B. M.; Hakanson, K.; Tulinsky, A.; Nutt, R. F.; Ripka, W. C. J.
Med. Chem. 1996, 39, 4531.
(13) FXa inhibitors: (a) Tamura, S. Y.; Levy, O. E.; Reiner, J. E.; Uong,
T. H.; Goldman, E. A.; Brunck, T. K.; Semple, J. E. Bioorg. Med. Chem.
Lett. 2000, 10, 745. (b) Ho, J. Z.; Levy, O. E.; Gibson, T. S.; Nguyen, K.;
Semple, J. E. Bioorg. Med. Chem. Lett. 1999, 9, 3459. (c) Tamura, S. Y.;
Goldman, E. A.; Bergum, P. W.; Semple, J. E. Bioorg. Med. Chem. Lett.
1999, 9, 2573.
(8) (a) CtA and CtB: Fusetani, N.; Matsunaga, S.; Matsumoto, H.;
Takebayashi, Y. J. Am. Chem. Soc. 1990, 112, 7053. (b) CtC, CtD, and
CtE: Nakao, Y.; Matsunaga, S.; Fusetani, N. Bioorg. Med. Chem. 1995, 3,
1115. (c) CtE2 and CtE3: Nakao, Y.; Oku, N.; Matsunaga, S.; Fusetani,
N. J. Nat. Prod. 1998, 61, 667.
(9) (a) Maryanoff, B. E.; Qui, X.; Padmanabhan, K. P.; Tulinsky, A.;
Almond, H. R.; Andrade-Gordon, P.; Greco, M. N.; Kauffman, J. A.;
Nicolaou, K. C.; Liu, A.; Brungs, P. H.; Fusetani, N. Proc. Natl. Acad. Sci.
U.S.A. 1993, 90, 8048. (b) Lewis, S. D.; Ng, A. S.; Baldwin, J. J.; Fusetani,
N.; Naylor, A. M.; Shafer, J. A. Thrombosis Res. 1993, 70, 173.
(10) Cyclotheonamides, total synthesis: (a) Bastiaans, H. M. M.; van
der Baan, J. L.; Ottenheijm, H. C. J. J. Org. Chem. 1997, 62, 3880. (b)
Deng, J.; Hamada, Y.; Shioiri, T. Tetrahedron Lett. 1996, 37, 2261. (c)
Maryanoff, B. E.; Greco, M. N.; Zhang, H. C.; Andrade-Gordon, P.;
Kauffman, J. A.; Nicolaou, K. C.; Liu, A.; Brungs, P. H. J. Am. Chem.
Soc. 1995, 117, 1225. (d) Wipf, P.; Kim, H. J. Org. Chem. 1993, 58, 5592.
(e) Hagihara, M.; Schreiber, S. L. J. Am. Chem. Soc. 1992, 114, 6570.
(14) uPA inhibitors: (a) Tamura, S. Y.; Weinhouse, M. I.; Roberts, C.
A.; Goldman, E. A.; Masukawa, K.; Anderson, S. M.; Cohen, C. R.;
Bradbury, A. E.; Bernadino, V. T.; Dixon, S. A.; Ma, M. G.; Nolan, T. G.;
Brunck, T. K. Bioorg. Med. Chem. Lett. 2000, 10, 983.
(15) Passerini reaction: (a) Passerini, M. Gazz. Chim. Ital. 1921, 51,
126. (b) Passerini, M.; Ragni, G. Gazz. Chim. Ital. 1931, 61, 964. (c)
Authoritative review: Ugi, I. In Isonitrile Chemistry; Ugi, I., Ed.; Academic
Press: New York, 1971; Chapter 7. (d) TiCl4 activation: Carofiglio, T. et
al. Organometallics 1993, 12, 2726. (e) Seebach, D.; Adam, G.; Gees, T.;
Schiess, M.; Weigand, W. Chem. Ber. 1988, 121, 507. (f) Seebach, D.;
Schiess, M. HelV. Chim. Acta 1983, 66, 1618.
(16) Semple, J. E.; Levy, O. E. PCT Int. Appl. WO 0035868 A2, June,
2000.
3302
Org. Lett., Vol. 3, No. 21, 2001

N-deprotection of 3 under mild conditions generated the
R-acyloxy-â-amino intermediate 5, which underwent a
smooth O- to N-acyl shift and provided adduct 6 in high
yield. Intermediates 4 and 6 routinely serve as useful
advanced precursors to R-ketoamide derivatives 7 by oxida-
tion, preferably at a very late synthetic stage. The recent
report by Banfi et al.18 on a related process prompts us
to disclose our application of the Passerini reaction-
deprotection-acyl migration strategy toward a concise
synthesis of the N(10)-C(17) fragment of CtA (1a) and CtB
(1b).
Scheme 1a
As outlined in Scheme 1,19 commercially available N-R-
Boc-Ng-nitroarginine 8 was converted in two steps to the
corresponding Weinreb amide, which was reduced with
lithium aluminum hydride to afford N-R-Boc-Ng-nitro-
argininal 9 in 65% overall yield (100 g scale).20An alternate
route to 9 proceeded via borane reduction of 8 to the
corresponding argininol (MeOH quench, 300 g scale) fol-
lowed by efficient oxidation in DMSO employing the
Parikh-von Doering protocol21 to deliver 9 in 71-76%
overall yield (10-20 g scale).
Concurrent acid-catalyzed esterification and N-deprotection
of the commercially available tyrosine derivative 10 followed
by EDC/HOBt-mediated peptide coupling with Fmoc-D-
phenylalanine afforded the corresponding dipeptide 11 in
nearly quantitative overall yield. Sequential N-deprotection,
N-formylation, and mild diphosgene-mediated formamide
dehydration according to the Ugi protocol22 afforded the
(17) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy. O. E. Org. Lett.
2000, 2, 2769.
(18) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985.
(19) All new compounds were characterized by full spectroscopic (NMR,
low/high resolution MS) analysis. Yields refer to spectroscopically and
chromatographically homogeneous (g95% by HPLC, TLC) materials.
(20) Tamura, S. Y.; Semple, J. E.; Ardecky, R. J.; Leon, P.; Carpenter,
S. H.; Ge, Y.; Shamblin, B. M.; Weinhouse, M. I.; Ripka, W. C.; Nutt, R.
F. Tetrahedron Lett. 1996, 37, 4109.
(21) (a) Parikh, J. R.; Doering, W. V. E. J. Am. Chem. Soc. 1967, 89,
5505. (b) Reviews: Tidwell, T. T. Synthesis 1990, 857. (c) Tidwell, T. T.
Org. React. 1990, 39, 297. (d) Mancuso, A. J.; Swern, D. Synthesis 1981,
165.
(22) (a) Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977, 16, 259.
(b) Giesemann, G.; von Hinrichs, E.; Ugi, I. J. Chem Res. (S) 1982, 79. (c)
Periasamy, M. P.; Walborsky, H. M. Org. Prep. Proced. Int. 1979, 11,
293.
(23) Dipeptide isonitriles prepared by this method retain their chiral
integrity: Urban, R.; Marquarding, D.; Seidel, P.; Ugi, I. Chem. Ber. 1977,
110, 2012.
a Reagents and conditions: (a) i-BuOCOCl, NMP, THF, -5 °C;
(b) Me(OMe)NH, -5 °C to rt, 88%; (c) LiAlH4, THF, -78 °C;
KHSO4, H2O, -70 to -30 °C, 74%; (d) BH3, THF, -78 to -20
°C, 12 h; MeOH, -78 °C to rt, 95%; (e) SO3‚Pyr, DMSO, Et3N,
5 °C to rt, 1 h, 75-80%; (f) MeOH, HCl, 0 °C to rt, 98%; (g)
Fmoc-D-Phe-OH, EDC, HOBt, NMM, CH3CN, rt, quant; (h) Et2NH,
CH2Cl2, 0 °C to rt, 99%; (i) HCO2H, Ac2O, CH2Cl2, rt to reflux,
79%; (j) CCl3OCOCl, NMM, -40 to 0 °C, CH2Cl2, 75%; (k) Alloc-
Pro-OH, Boc-Arg(NO2)-H (9), CH2Cl2, 0 °C to rt, 2 days, 59%; (l)
HCl, MeOH, 0 °C to rt, quant.; (m) Et3N, pH 9, MeOH, 0 °C to rt,
15 h, 98%.
(24) Experimental Details for 13. A solution of 9 (382.2 mg, 1.26
mmol), N-Alloc-proline (298.0 mg, 1.50 mmol), and isonitrile 12 (590.0
mg, 1.15 mmol) in anhydrous dichloromethane (4.6 mL) was stirred at
ambient temperature for 16 h. Solvent was slowly removed in vacuo, and
the resultant thick residue was stirred for 1 day. Standard extractive workup
in ethyl acetate gave a crude product, which was purified by silica gel flash
chromatography using dichloromethane/isopropanol 98:2 as eluent to afford
690 mg (59.3% yield) of product 13 as an amorphous, colorless solid. RP-
HPLC analysis (Vydac 5 m C18, 5-75% CH3CN, H2O with 0.1% TFA):
tR ) 21.1 and 21.3 min, 1.6:1 mixture at diastereomeric R-acyloxy center.
TLC: (silica gel; dichloromethane/isopropanol 9:1) Rf ) 0.6; (ethyl acetate)
Rf ) 0.4, 0.3; UV, PMA visualization. MS: [MH]+ 1013.9, [MNa]+ 1035.9.
1H NMR (400 MHz, CD3OD): δ 1.40 + 1.45 (2s, 9H), 1.32-1.47 (obscured
m, 2H), 1.49-1.70 (m, 2H), 1.90-2.12 (m, 3H), 2.30 (m, 1H), 2.70-2.89
(complex br m, 1H), 2.95 (m, 1H), 3.02-3.25 (complex m, 4H), 3.39-
3.59 (m, 2H), 3.68 + 3.72 (major + minor s, 3H, ratio ∼1.4-1.6:1), 3.88
+ 4.01 (2m, 1H), 4.40 (m, 1H), 4.50 (m, 2H), 4.57-4.76 (complex m,
2H), 5.03 + 5.11 (minor + major m, 1H R-CHO-acyl methine, ratio 1:2),
5.16-5.37 (m, 2H), 5.25 + 5.27 (2s, 2H), 5.85 + 5.97 (minor + major m,
1H), 6.95 (m, 2H), 7.03-7.14 (m, 4H), 7.16-7.29 (m, 3H), 7.35 (m, 1H),
7.44 (m, 2H).
configurationally stable23 dipeptide isonitrile 12 in satisfac-
tory overall yield.
The key Passerini reaction between argininal 9, isonitrile
12, and N-Alloc-proline fragments proceeded under very
mild, essentially neutral conditions over 2 days and delivered
Org. Lett., Vol. 3, No. 21, 2001
3303

the complex adduct 13 in a gratifying, albeit unoptimized
59% yield.24 Analysis of 13 by RP-HPLC or chiral HPLC
on Chiracel AD revealed two peaks (ratio ≈ 1.6:1), indicating
formation of one diastereomeric pair at the newly created
R-acyloxy center. Therefore, the integrity of all four original
chiral centers was preserved during the course of the reaction.
Acid-catalyzed N-Boc cleavage of 13 afforded the corre-
sponding stable vicinal O-acyloxyamine hydrochloride salt
quantitatively. Dissolution of this salt in methanol and
adjustment of the solution pH to ∼9 with triethylamine led,
via facile O- to N-acyl migration, to the desired advanced
intermediate 14 in practically quantitative overall yield (cf.
5 to 6 in Figure 2).25 Model compound 14 constitutes the
N(10)-C(17) region of CtA (1a) and CtB (1b) and may be
deemed a suitable precursor for their total synthesis.26 We
envision that application of this three-step sequence to the
appropriately protected isonitrile derivative of D-phe-
tyr(vinyl), argininal, and diaminopropionyl-proline compo-
nents would lead directly to the entire acyclic skeleton of
1a and 1b.
Our approach to the advanced cyclotheonamide intermedi-
ate 14 via the key Passerini reaction-deprotection-acyl
migration strategy is convergent and highly efficient. Pro-
ceeding in just eight steps (longest linear sequence) and in
33% overall yield from a commercially available tyrosine
derivative, it is complementary to other current multistep
routes to similar intermediates.9a,10
In conclusion, the utility of this technology for the rapid
construction of relatively complex R-ketoamide natural
product precursors is highlighted by the concise synthesis
of 14, which serves as a model for the N(10)-C(17) segment
of CtA (1a) and CtB (1b). The key Passerini reaction,
deprotection, and O- to N-acyl migration steps proceed in
moderate to high yields and under mild conditions. Other
noteworthy features include operational simplicity and high
atom-economy. Interest in efficient synthetic approaches to
linear3,17 and cyclic R-ketoamide protease inhibitors2,11 leads
us to speculate on alternate intramolecular variants that might
directly provide advanced macrocyclic intermediates, thus
further expanding the utility of this atom-economical proc-
ess.27 Numerous applications of this new technology to the
synthesis of R-ketoamide natural products and protease
inhibitors are envisioned and will form the basis of forth-
coming publications from our laboratories.
(25) Experimental Details for 14. To a solution of 13 (675 mg, 0.666
mmol) in anhydrous methanol (5 mL) at 0 °C was added freshly prepared
5 N HCl in anhydrous methanol (10 mL, 50 mmol). After 40 min at 0 °C
the solvent was removed in vacuo. The residue was dissolved in fresh 10
mL portions of anhydrous acetonitrile, re-evaporated (repeated twice), and
then evaporated once from a 10 mL portion of dichloromethane. High
vacuum pumping for several hours afforded 645 mg (102% of theory,
∼quantitative yield) of the corresponding O-acyloxyamine hydrochloride
as a pale tan foam, which was used in the following reaction. To a solution
of amine salt (625 mg, 0.658 mmol) in anhydrous methanol (3.3 mL) was
added Et3N (134.6 mg, 1.33 mmol, 185 mL). The solution was stirred at
ambient temperature for 2 h, during which time a thick yellow slurry formed.
The solvent was removed, and the residue was dissolved in 200 mL of
ethyl acetate, extracted successively with 20 mL portions of 1 N HCl,
saturated NaHCO3 solution (2×), water, and brine, and then dried over
anhydrous MgSO4. Filtration, solvent removal, and filtration through a short
silica gel column using dichloromethane/2-propanol 9:1 as eluent gave 588
mg (97.9% yield) of product 14 as a nearly colorless, amorphous solid.
RP-HPLC analysis (Vydac 5 m C18, 5-75% CH3CN, H2O with 0.1%
TFA): tR ) 19.1 and 19.5 min, 1.6:1 ratio at diastereomeric R-hydroxy
center. TLC: (silica gel; dichloromethane/2-propanol 9:1) Rf ) 0.5; (ethyl
acetate) Rf ) 0.2, 0.25; UV, PMA visualization. MS: [MH]+ 914.8, [MNa]+
936.8. 1H NMR (400 MHz, CD3OD): δ 1.30-1.45 (br m, 1H), 1.48-1.75
(br m, 3H), 1.94 (m, 2H), 2.07 (m, 1H), 2.32 (br m, 1H), 2.70-3.04
(complex m, 3H), 3.07-3.28 (complex m, 3H), 3.48-3.62 (m, 2H), 3.67
(m, 1H), 3.70 + 3.72 (2s, 3H), 4.05-4.13 (m, 1H), 4.37-4.57 (m, 2H),
4.60-4.78 (complex m, 3H), 5.18-5.40 (complex m, 2H), 5.23+ 5.25 (2s,
2H), 5.86 + 5.96 (minor + major m, 1H), 6.96 (m, 2H), 7.05-7.30 (m,
7H), 7.35 (m, 1H), 7.43 (m, 2H).
Acknowledgment. Stimulating discussions with R. F.
Nutt, T. K. Brunck, O. E. Levy, S. Y. Tamura, and K. E.
Pryor are gratefully acknowledged.
(26) Elaboration of 14 to the corresponding protected vinylogous tyrosine
(v-Tyr) intermediate could proceed via the following sequence of reac-
tions: (a) TBDMSCl, imidazole, DMF, room temperature; (b) LiBH4, THF,
EtOH, 0 °C to room temperature; (c) SO3‚Pyr, DMSO, Et3N, 5 °C to room
temperature; (d) Ph3PCHCO2t-Bu, CH2Cl2, 0 °C to room temperature.
Alternatively, construction of the corresponding v-Tyr analogue of 12
followed by application of the Passerini reaction, deprotection, and O- to
N-acyl migration sequence may also be a viable approach to this intermedi-
ate.
OL0165239
(27) For application of MCR to synthesis of macrocyclic peptides and
natural products, see: Doemling, A. Comb. Chem. High Throughput Screen.
1998, 1, 1 and references therein.
3304
Org. Lett., Vol. 3, No. 21, 2001