Ring enlargement of polyhydroxylated pyrrolidines to piperidines by Mitsunobu reaction: A fortuitous synthesis of 1-deoxy-l-allonojirimycin

Article abstract of DOI:10.1055/s-2004-829566

Chiral 3,4-dibenzyloxy-5-hydroxymethyl-2-thiazolylpyrrolidines under Mitsunobu conditions (R-OH, Ph₃P, DIAD in THF, 80°C) afforded the corresponding R-protected pyrrolidines and 2-deoxy-piperidines in different ratios depending on the stereochemistry of the starting pyrrolidine and the nature of the acid R-OH. A mechanistic scheme is proposed involving the formation of an aziridinium ion as an intermediate. A piperidine derivative obtained in 74% yield was converted in four steps into the title allonojirimycin.

Full text of DOI:10.1055/s-2004-829566

LETTER
1711
Ring Enlargement of Polyhydroxylated Pyrrolidines to Piperidines by
Mitsunobu Reaction: A Fortuitous Synthesis of 1-Deoxy-L-allonojirimycin
R
ing Enlargement
l
of Pyrr
e
olidines ssandro Dondoni,*a,b Barbara Richichi,b Alberto Marra,*a,b Daniela Perronea
a
Laboratorio di Chimica Organica, Dipartimento di Chimica, Università di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
b
Interdisciplinary Center for the Study of Inflammation, Università di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
Fax +39(0532)291167; E-mail: adn@dns.unife.it
Received 30 April 2004
struction of the condensed five-member ring of 4 bearing
the carbonyl and amino groups while the PMP-protected
primary alcohol was transformed into the carboxylic
group.
Abstract: Chiral 3,4-dibenzyloxy-5-hydroxymethyl-2-thiazolyl-
pyrrolidines under Mitsunobu conditions (R-OH, Ph3P, DIAD in
THF, 80 °C) afforded the corresponding R-protected pyrrolidines
and 2-deoxy-piperidines in different ratios depending on the stereo-
chemistry of the starting pyrrolidine and the nature of the acid
R-OH. A mechanistic scheme is proposed involving the formation
of an aziridinium ion as an intermediate. A piperidine derivative
obtained in 74% yield was converted in four steps into the title
allonojirimycin.
Key words: azasugars, 1-deoxynojirimycin, piperidines, pyrrol-
idines, ring expansion
In a recent publication from this laboratory,1 we reported
on an effective synthesis of the 6,7-diacetoxy azabicyc-
lo[3.3.0]octanone amino acid (4, pyrrolizidinone amino
acid, Scheme 1). This hydroxylated rigidified Pro-Ala
dipeptide belongs to the class of fused bicyclic dipeptides2
that simulate the bioactive conformation of the b-turn
sites.3 A rigidified dipeptide of type 4 but lacking the hy-
droxy groups was employed4 as scaffold for the prepara-
tion of a cyclopeptide by insertion of the pharmacophoric
tripeptide Arg-Gly-Asp (RGD) sequence which is impli-
cated in several diseases including angiogenesis and os-
teoporosis.5 The synthesis of 4 represented the initial step
in our program directed toward the preparation of stereo-
diversified libraries of densely hydroxylated rigidified
peptidomimetics constituted of azabicyclo[X.Y.0]al-
kanone amino acids. We have briefly discussed1 the po-
tential of these systems as tools for biological studies and
for improving the delivery and selectivity of drugs in sev-
eral diseases, such as inflammation and cancer, where in-
tegrins are implicated. A simple yet important operation
in the synthetic scheme leading to 4 from the N-benzyl
pyrrolidine 1 (prepared via the nitrone route from D-arabi-
nose),6 entailed the protection of the primary hydroxyl
group as p-methoxyphenyl (PMP) ether via Mitsunobu re-
action with p-methoxyphenol in the presence of PPh3 and
diisopropylazodicarboxylate (DIAD). Then, the product 2
(80% isolated yield) was transformed into the orthogonal-
ly protected polyhydroxylated formyl pyrrolidine 3 via
thiazole-to-formyl conversion. The formyl group in the
latter compound served as the starting point for the con-
Scheme 1
We now report that when the Mitsunobu reaction with p-
methoxyphenol as the nucleophile was applied to the 5-
hydroxymethyl pyrrolidine 57 (prepared via the nitrone
route from D-ribose),6 two products were obtained in 1.3:1
ratio (from NMR analysis), the major one being character-
ized as the piperidine 7a, and the other the expected pyr-
rolidine 6a (Scheme 2).8 The ratio between the piperidine
and pyrrolidine products increased significantly
(7b:6b = 2.3:1) by the use of benzoic acid8 and became
quite substantial (7c:6c = 4.1:1) with diphenyl acetic ac-
id.8 The yields of isolated products 6 and 7 confirmed the
above product distributions. With pivalic acid the reaction
was very sluggish since only 13% of 5 appeared to have
reacted after three hours while affording exclusively the
piperidine 7d. The ring size and the configuration of the
1
5-O-protected piperidines 7 were readily assigned by H
NMR analysis. The two H-6 protons of 7a–c resonate at
much higher field (d = 2.7–3.0 ppm) than those of the
pyrrolidine isomers 6a–c (d = 4.5–5.0 ppm), as expected
SYNLETT 2004, No. 10, pp 1711–1714
Advanced online publication: 15.07.2004
DOI: 10.1055/s-2004-829566; Art ID: G16304ST
© Georg Thieme Verlag Stuttgart · New York
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1712
A. Dondoni et al.
LETTER
for a methylene group linked to a nitrogen rather than an From the above results it appears that the pyrrolidine–pi-
oxygen atom. Moreover, a significant NOE between H-3 peridine ratio in the Mitsunobu reaction of chiral 3,4-
and H-5 of 7a–c indicates that these protons are in a cis- dibenzyloxy-5-hydroxymethyl-2-thiazolyl-pyrrolidines
relationship.
depends on both their stereochemistry and the nature of
the nucleophile R-OH. However in view of the limited set
of data so far collected it is premature to speculate on
those effects. Instead, a mechanistic rationale for the pyr-
rolidine to piperidine ring enlargement can be advanced
on the basis of previous observations of neighboring
group effects in nitrogen, oxygen, and sulfur containing
compounds subjected to Mitsunobu reactions.10 Stereo-
chemical outcomes and/or ring size variations were in fact
explained by invoking aziridinium, oxonium, and thiireni-
um ion intermediates. Hence, we suggest that also in the
present cases after the oxyphosphonium (OPP) salt forma-
tion through the agency of DIAD, the reaction proceeds
through a condensed aziridinium (AZR) ion intermedi-
ate,11 which undergoes the nucleophilic attack by R-O– at
the tertiary carbon atom (route a) to give the enlarged
piperidine (PIP) system (Scheme 4). On the other hand the
pyrrolidine (PYR) should be formed by attack of the nu-
cleophile at the methylene carbon atom of the aziridinium
ring (route b) although it cannot be excluded its direct
formation from the oxyphosphonium salt OPP. A mecha-
nistic scheme involving the initial formation of the pro-
tected pyrrolidine and then its partial transformation into
the piperidine system through the AZR ion intermediate
seems unlikely because we found that the molar ratio be-
tween 6c and 7c did not change during the reaction (NMR
analysis of samples withdrawn from the reaction mixture
after 5, 10, 20, and 120 min). Moreover, pure compounds
6c and 9b were recovered unaltered upon heating for two
hours at 80 °C in dry THF.
Scheme 2
Since diphenyl acetic acid was the most effective reagent
favoring the ring enlargement of 5, this carboxylic acid
was employed to see whether the same process occurred
with the pyrrolidine 1 (see Scheme 1). Quite surprisingly,
no formation of the corresponding piperidine was ob-
served in this case. Hence, we explored the Mitsunobu
reaction of another stereoisomer of 1, namely the 5-hy-
droxymethyl pyrrolidine 89 (Scheme 3) prepared via the
nitrone route from L-xylose.6 The use of p-methoxyphe-
nol-Ph3P-DIAD system furnished a mixture of the pyrro-
lidine 9a and piperidine 10a in 2.5:1 ratio9 by NMR
analysis and, in contrast to our expectation, the substitu-
tion of the phenol with diphenyl acetic acid increased the
above ratio to give products 9b and 10b in 4.8:1 ratio.9
Also in these reactions the yields of isolated products 9
and 10 confirmed the ratios determined by NMR analysis.
Scheme 4
While extensive investigations are needed to gain more
insight on the steric and electronic factors determining the
kinetic of the above concurrent reactions, we envisaged
the implementation of this chemistry for the synthesis of
rare six-member ring 1-deoxy iminosugars, which are
eventually hardly accessible by more common routes.
Scheme 3
Synlett 2004, No. 10, 1711–1714 © Thieme Stuttgart · New York

LETTER
Ring Enlargement of Pyrrolidines
1713
(3) (a) Gillespie, P.; Cicariello, J.; Olson, J. L. Biopolymers
1997, 43, 191. (b) Takeuchi, Y.; Marshall, G. R. J. Am.
Chem. Soc. 1998, 120, 5363. (c) Belvisi, L.; Bernardi, A.;
Manzoni, L.; Potenza, D.; Scolastico, C. Eur. J. Org. Chem.
2000, 2563.
(4) Belvisi, L.; Bernardi, A.; Checchia, A.; Manzoni, L.;
Potenza, D.; Scolastico, C.; Castorina, M.; Cupelli, A.;
Giannini, G.; Carminati, P.; Pisano, C. Org. Lett. 2001, 3,
1001.
This class of polyhydroxylated piperidines has gained
great importance due to their superior activity as specific
glycosidase inhibitors.12 Since such inhibition has signifi-
cance to both viral expression and tumor growth,13 the po-
tential of these compounds as leads for the development
of new drugs has became quite apparent. Thus, given the
good selectivity by which the piperidine 7c was obtained
from 5, we considered the elaboration of this compound as
an example for iminosugar synthesis (Scheme 5). Suc-
cinctly, application of the silver-based thiazole-to-formyl
conversion protocol14 to 7c furnished the aldehyde15 11,
which in turn was readily reduced to the alcohol16 12. Re-
moval of the O- and N-protective groups by standard
methods afforded 1-deoxy-L-allonojirimycin17 13, an imi-
nosugar whose synthesis has been previously reported in
one instance only18 while the antipode was prepared in
various manners by chemical and enzymatic means.19
While the latter product has been proved19a to act as an
inhibitor of rat intestine isomaltase and cellobiase, and
bovine liver cytosolic b-galactosidase, the biological
properties of the enantiomer 13 have not been studied so
far. Application of the same reaction sequence to the
thiazole-substituted piperidine 10b would give the 1-
deoxynojirimycin, a well-known potent glycosidase in-
hibitor.12
(5) (a) Haubner, R.; Finsinger, D.; Kessler, H. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1374. (b) Gottschalk, K.-E.;
Kessler, H. Angew. Chem. Int. Ed. 2002, 41, 3767.
(c) Hynes, R. O. Nat. Med. (N. Y., NY, U. S.) 2002, 8, 918.
(6) (a) Dondoni, A.; Giovannini, P. P.; Perrone, D. J. Org.
Chem. 2002, 62, 7203. (b) This article dealt with the
preparation of O- and N-benzyl-protected 3,4-dihydroxy-5-
hydroxymethyl-2-thiazolyl-pyrrolidines. The selective
debenzylation of the primary hydroxy group to give
compound 1 has been described (ref.1). Under the same
conditions, the 5-hydroxymethyl pyrrolidines 5 (Scheme 2)
and 8 (Scheme 3) were obtained from the corresponding
perbenzylated precursors.
(7) Analytical data for compound 5. [a]D = –4.1 (c 1.0, CHCl3).
1H NMR (400 MHz, CDCl3 + D2O): d = 7.80 (d, 1 H, J = 3.3
Hz, Th), 7.46–7.22 (m, 16 H, 3 Ph, Th), 4.97 and 4.75 (2 d,
2 H, J = 11.6 Hz, PhCH2O), 4.85 (d, 1 H, J2,3 = 1.5 Hz, H-2),
4.53 and 4.40 (2 d, 2 H, J = 11.7 Hz, PhCH2O), 4.31 (dd, 1
H, J3,4 = 6.1, J4,5 = 7.2 Hz, H-4), 4.14 and 3.98 (2 d, 2 H,
J = 13.9 Hz, PhCH2N), 4.06 (dd, 1 H, H-3), 3.82–3.76 (m, 2
H, 2 H-6), 3.40 (ddd, 1 H, J5,6a = J5,6b = 2.7 Hz, H-5).
(8) General Procedure for the Mitsunobu Reaction: To a
refluxing solution of 6-OH thiazolylpyrrolidine (150 mg,
0.30 mmol), PPh3 (120 mg, 0.45 mmol), and phenol or
carboxylic acid derivative (0.60 mmol) in anhyd THF (6
mL) was added DIAD (90 mL, 0.45 mmol). The reaction
mixture was stirred at 80 °C for 2 h and then concentrated.
The residue was eluted from a column of silica gel
(cyclohexane–EtOAc) to give the pyrrolidine 6 and the
piperidine 7. Compound 6a. [a]D = +6.8 (c 0.9, CHCl3).
Compound 7a. [a]D = –32.0 (c 1.4, CHCl3). Compound 6b.
[a]D = –13.7 (c 1.0, CHCl3). Compound 7b. [a]D = –25.5 (c
0.8, CHCl3). Compound 6c. [a]D = –39.2 (c 1.0, CHCl3). 1H
NMR (400 MHz, C6D6): d = 7.57 and 6.55 (2 d, 2 H, J = 3.3
Hz, Th), 7.34–6.90 (m, 25 H, 5 Ph), 5.04 (dd, 1 H, J5,6a = 2.4,
J6a,6b = 12.5 Hz, H-6a), 4.78 (s, 1 H, Ph2CH), 4.65 and 4.60
(2 d, 2 H, J = 12.2 Hz, PhCH2O), 4.58 (dd, 1 H, J5,6b = 7.2
Hz, H-6b), 4.54 (d, 1 H, J2,3 = 1.5 Hz, H-2), 4.18 and 4.08 (2
d, 2 H, J = 11.9 Hz, PhCH2O), 4.10 (dd, 1 H, J3,4 = 5.2,
J4,5 = 6.6 Hz, H-4), 3.86 (dd, 1 H, H-3), 3.78 (ddd, 1 H, H-5),
3.68 (s, 2 H, PhCH2N). Compound 7c. [a]D = –36.0 (c 0.4,
CHCl3). 1H NMR (400 MHz, CDCl3): d = 7.76 (d, 1 H,
J = 3.2 Hz, Th), 7.32–7.13 and 6.93–6.89 (2 m, 26 H, 5 Ph,
Th), 5.06 (ddd, 1 H, J4,5 = 1.8, J5,6a = 4.7, J5,6b = 10.5 Hz, H-
5), 4.97 (s, 1 H, Ph2CH), 4.55 and 4.33 (2 d, 2 H, J = 11.8
Hz, PhCH2O), 4.29 and 4.07 (2 d, 2 H, J = 11.9 Hz,
Scheme 5
Acknowledgment
We thank Mr. P. Formaglio (University of Ferrara) for assistance in
the NMR measurements, MIUR (COFIN 2002) for financial sup-
port, and Interdisciplinary Center for the Study of Inflammation
(Ferrara, Italy) for a fellowship to B. R.
PhCH2O), 4.29 (d, 1 H, J2,3 = 9.5 Hz, H-2), 4.10 (dd, 1 H,
J3,4 = 1.7 Hz, H-4), 3.73 and 3.17 (2 d, 2 H, J = 13.8 Hz,
PhCH2N), 3.71 (dd, 1 H, H-3), 2.78 (dd, 1 H, J6a,6b = 10.3
Hz, H-6a), 2.72 (dd, 1 H, H-6b).
References
(9) Analytical data: compound 8: [a]D = +16.4 (c 0.6, CHCl3).
Compound 9a: [a]D = –4.6 (c 0.6, CHCl3). Compound 10a:
[a]D = +10.0 (c 0.7, CHCl3). Compound 9b: [a]D = +10.9 (c
0.7, CHCl3). Compound 10b: [a]D = +20.0 (c 0.7, CHCl3).
(10) Hughes, D. L. Org. Prep. Proc. Int. 1996, 28, 127.
(1) Dondoni, A.; Marra, A.; Richichi, B. Synlett 2003, 2345.
(2) (a) Hanessian, S.; McNaughton-Smith, G.; Lombart, H.-G.;
Lubell, W. D. Tetrahedron 1997, 53, 12789. (b) Halab, L.;
Gosselin, F.; Lubell, W. D. Biopolymers 2000, 55, 101.
(c) Dietrich, E.; Lubell, W. D. J. Org. Chem. 2003, 68, 6988.
Synlett 2004, No. 10, 1711–1714 © Thieme Stuttgart · New York

1714
A. Dondoni et al.
LETTER
(11) For a recent example of the involvement of an aziridinium
ion intermediate in pyrrolidine to piperidine conversion and
a collection of references on earlier examples, see: Verhelst,
S. H. L.; Paez Martinez, B.; Timmer, M. S. M.; Lodder, G.;
van der Marel, G. A.; Overkleeft, H. S.; van Boom, J. H. J.
Org. Chem. 2003, 68, 9598.
(12) (a) Iminosugars as Glycosidase Inhibitors. Nojirimycin and
Beyond; Stütz, A. E., Ed.; Wiley-VCH: Weinheim, 1999.
(b) Look, G. C.; Fotsch, C. H.; Wong, C.-H. Acc. Chem. Res.
1993, 26, 182. (c) Asano, N. Curr. Top. Med. Chem. 2003,
3, 471. (d) Greimel, P.; Spreitz, J.; Stütz, A. E.; Wrodnigg,
T. M. Curr. Top. Med. Chem. 2003, 3, 513.
(16) To a cooled (0 °C), stirred solution of 11 (30 mg, 0.05 mmol)
in 5:2 Et2O–MeOH (1 mL) was added NaBH4 (5 mg, 0.13
mmol). The mixture was stirred at r.t. for 5 min, diluted with
acetone (0.1 mL) and then 1 M phosphate buffer at pH 7 (5
mL), and partially concentrated to remove the organic
solvents. The suspension was extracted with CH2Cl2 (3 × 20
mL), the combined organic phases were dried (Na2SO4), and
concentrated. The residue was eluted from a column of silica
gel with 3:1 cyclohexane–EtOAc to give 12 (27 mg, 90%) as
a syrup; [a]D = +2.7 (c 0.6, CHCl3). 1H NMR (400 MHz,
C6D6): d = 7.32–6.92 (m, 25 H, 5 Ph), 4.94 (ddd, 1 H,
J1a,2 = 10.6, J1b,2 = 4.2, J2,3 = 2.3 Hz, H-2), 4.92 (s, 1 H,
(13) (a) Tyms, A. S.; Taylor, D. L.; Sunkara, P. S.; Kang, M. S.
L. In Design of Anti-AIDS Drugs; DeClerq, E., Ed.; Elsevier:
New York, 1990, 257–318. (b) Winchester, B.; Fleet, G. W.
J. Glycobiology 1992, 2, 199.
(14) Dondoni, A.; Marra, A.; Scherrmann, M.-C.; Bertolasi, V.
Chem.–Eur. J. 2001, 7, 1371.
Ph2CH), 4.52 and 4.36 (2 d, 2 H, J = 11.6 Hz, PhCH2O),
4.24 and 4.19 (2 d, 2 H, J = 11.5 Hz, PhCH2O), 4.06 (dd, 1
H, J3,4 = 2.4 Hz, H-3), 3.92 (ddd, 1 H, J5,6a = 1.8,
J6a,6b = 11.0, J6a,OH = 9.7 Hz, H-6a), 3.73 and 2.87 (2 d, 2 H,
J = 14.0 Hz, PhCH2N), 3.73 (dd, 1 H, J5,6b = 1.8 Hz, H-6b),
3.54 (dd, 1 H, J4,5 = 9.6 Hz, H-4), 2.83 (dd, 1 H, J1a,1b = 10.4
Hz, H-1a), 2.77 (dd, 1 H, H-1b), 2.76 (ddd, 1 H, H-5), 2.01
(br d, 1 H, OH).
(15) A mixture of 7c (40 mg, 0.06 mmol), activated 4 Å
powdered molecular sieves (140 mg), and anhyd MeCN (0.7
mL) was stirred at r.t. for 10 min, then methyl triflate (13 mL,
0.12 mmol) was added. The suspension was stirred at r.t. for
15 min and then concentrated to dryness without filtering off
the molecular sieves. To a cooled (0 °C), stirred suspension
of the crude N-methylthiazolium salt in MeOH (0.7 mL) was
added NaBH4 (5 mg, 0.13 mmol). The mixture was stirred at
r.t. for additional 10 min, diluted with acetone, filtered
through a pad of Celite, and concentrated. A solution of the
residue in Et2O (30 mL) was washed with H2O, dried
(Na2SO4), and concentrated. To a vigorously stirred solution
of the thiazolidines in MeCN (1 mL) was added H2O (0.1
mL) and then AgNO3 (12 mg, 0.07 mmol). The mixture was
stirred at r.t. for 5 min, then diluted with 1 M phosphate
buffer at pH 7 (10 mL) and partially concentrated to remove
MeCN (bath temperature not exceeding 40 °C). The
suspension was extracted with Et2O (3 × 20 mL), the
combined organic phases were dried (Na2SO4), and
concentrated to give a yellow syrup. A solution of the
residue in Et2O (ca. 30 mL) was filtered through a pad of
Celite and concentrated to afford 11 (30 mg, 82%) as a
colorless syrup ca. 95% pure by 1H NMR analysis. 1H NMR
(400 MHz, CDCl3): d = 9.41 (d, 1 H, J = 4.7 Hz, CHO),
7.35–7.20 and 7.12–7.07 (2 m, 25 H, 5 Ph), 4.96 (s, 1 H,
Ph2CH), 4.91 (ddd, 1 H, J4,5 = 2.4, J5,6a = 4.9, J5,6b = 10.7 Hz,
H-5), 4.50 and 4.30 (2 d, 2 H, J = 11.9 Hz, PhCH2O), 4.44
and 4.32 (2 d, 2 H, J = 11.3 Hz, PhCH2O), 4.12 (dd, 1 H,
J3,4 = 2.0 Hz, H-4), 3.72 and 3.34 (2 d, 2 H, J = 13.8 Hz,
PhCH2N), 3.71 (dd, 1 H, J2,3 = 9.7 Hz, H-3), 3.34 (dd, 1 H,
H-2), 2.71 (dd, 1 H, J6a,6b = 10.6 Hz, H-6a), 2.60 (dd, 1 H, H-
6b).
(17) A solution of 12 (31 mg, 0.05 mmol) in freshly prepared 0.1
M CH3ONa in CH3OH (2 mL) was kept at r.t. for 2 h, then
neutralized with HOAc, and concentrated. A solution of the
residue in CH2Cl2 was washed with 1 M phosphate buffer at
pH 7, dried (Na2SO4), and concentrated. The residue was
eluted from a short column of silica gel with EtOAc
(containing 0.5% of Et3N) to give the 2-OH derivative (17
mg). A vigorously stirred mixture of the alcohol, 20%
Pd(OH)2 on carbon (20 mg), and HOAc (2 mL) was
degassed under vacuum and saturated with hydrogen (by a
H2-filled balloon) five times. The suspension was stirred at
r.t. for 5 h under a slightly positive pressure of hydrogen
(balloon), then filtered through a plug of cotton and
concentrated. A solution of the crude tetrol in H2O was
eluted from a column (0.5 × 3 cm, d × h) of freshly activated
Dowex 1X8-200 (HO– form) to give pure 13 (6 mg, 72%);
[a]D = –28.6 (c 0.2, H2O); lit.18 [a]D = –35.2 (c 0.025,
MeOH). ent-13: lit.19a [a]D = +25.7 (c 0.65, H2O); lit.19b
[a]D = +28.1 (c 0.8, H2O). 1H NMR (400 MHz, D2O): d =
3.92 (dd, 1 H, J2,3 = J3,4 = 2.8 Hz, H-3), 3.62 (dd, 1 H,
J5,6a = 3.0, J6a,6b = 11.6 Hz, H-6a), 3.52 (ddd, 1 H, J1a,2 = 5.1,
J1b,2 = 11.2 Hz, H-2), 3.46 (dd, 1 H, J5,6b = 5.8 Hz, H-6b),
3.30 (dd, 1 H, J4,5 = 10.3 Hz, H-4), 2.67 (dd, 1 H,
J1a,1b = 12.4 Hz, H-1a), 2.56 (ddd, 1 H, H-5), 2.50 (dd, 1 H,
H-1b).
(18) Altenbach, H.-J.; Himmeldirk, K. Tetrahedron: Asymmetry
1995, 6, 1077.
(19) (a) Asano, N.; Oseki, K.; Kizu, H.; Matsui, K. J. Med. Chem.
1994, 37, 3701. (b) Ikota, N.; Hirano, J.; Gamage, R.;
Nakagawa, H.; Hama-Inaba, H. Heterocycles 1997, 46, 637.
(c) Wu, W.-D.; Khim, S.-K.; Zhang, X.; Cederstrom, E. M.;
Mariano, P. S. J. Org. Chem. 1998, 63, 841. (d) Ruiz, M.;
Ojea, V.; Ruanova, T. M.; Quintela, J. M. Tetrahedron:
Asymmetry 2002, 13, 795.
Synlett 2004, No. 10, 1711–1714 © Thieme Stuttgart · New York