Total synthesis and assignment of the double-bond position and absolute configuration of (-)-pyrinodemin A

Article abstract of DOI:10.1021/ol034700v

(Matrix presented) The first asymmetric total synthesis of a structurally novel cis-cyclopent[c]isoxazolidine alkaloid, (-)-pyrinodemin A (3), which exhibits potent cytotoxicity, has been accomplished through a highly diastereoselective intramolecular nitrone - olefin cycloaddition reaction as the key step. Thus, it has been found that the hitherto unknown absolute configuration of pyrinodemin A is as indicated in the structural formula 3.

Full text of DOI:10.1021/ol034700v

ORGANIC
LETTERS
2003
Vol. 5, No. 15
2611-2614
Total Synthesis and Assignment of the
Double-Bond Position and Absolute
Configuration of (−)-Pyrinodemin A
Yoshiki Morimoto,* Satoru Kitao, Tatsuya Okita, and Takamasa Shoji
Department of Chemistry, Graduate School of Science, Osaka City UniVersity,
Sumiyoshi-ku, Osaka 558-8585, Japan
morimoto@sci.osaka-cu.ac.jp
Received April 28, 2003
ABSTRACT
The first asymmetric total synthesis of a structurally novel cis-cyclopent[c]isoxazolidine alkaloid, (−)-pyrinodemin A (3), which exhibits potent
cytotoxicity, has been accomplished through a highly diastereoselective intramolecular nitrone−olefin cycloaddition reaction as the key step.
Thus, it has been found that the hitherto unknown absolute configuration of pyrinodemin A is as indicated in the structural formula 3.
Recently, an increasing number of structurally and bioac-
tively unique 3-alkylpyridine alkaloids have been isolated
from marine sponges of several genera.1 In 1999, Kobayashi
and co-workers reported the isolation of a structurally novel
cis-cyclopent[c]isoxazolidine alkaloid, pyrinodemin A, from
the marine sponge Amphimedon sp. collected off Nakijin,
Okinawa.2 Pyrinodemin A shows potent cytotoxicity against
murine leukemia L1210 (IC50 ) 0.058 µg/mL) and KB
epidermoid carcinoma cells (IC50 ) 0.5 µg/mL) in vitro and
has relatively weak antifungal activity.3 The plane structure
and relative configuration of pyrinodemin A were primarily
the absolute configuration of pyrinodemins has not yet been
determined.
1
proposed as 1 on the basis of analysis of the H and 13C
NMR and mass spectra. Although Kobayashi et al. also
reported a family of compounds similar to pyrinodemin A
obtained from the same sponge, pyrinodemins B-D, which
possessed prominent cytotoxicity and the cis-cyclopent[c]-
isoxazolidine ring system characteristic of pyrinodemin A,3
(1) (a) Andersen, R. J.; van Soest, R. W. M.; Kong, F. In Alkaloids:
Chemical and Biological PerspectiVes; Pelletier, S. W., Ed.; Pergamon:
Oxford, 1996; Vol. 10, pp 301-355. (b) Faulkner, D. J. Nat. Prod. Rep.
1999, 16, 155-198 and references therein.
(2) Tsuda, M.; Hirano, K.; Kubota, T.; Kobayashi, J. Tetrahedron Lett.
1999, 40, 4819-4820.
(3) Hirano, K.; Kubota, T.; Tsuda, M.; Mikami, Y.; Kobayashi, J. Chem.
Pharm. Bull. 2000, 48, 974-977.
The novel structure of pyrinodemin A coupled with its
bioactive and biogenetic interests prompted synthetic organic
10.1021/ol034700v CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/02/2003

chemists to investigate its synthesis. The groups of Snider4
and Baldwin5 have recently synthesized in a racemic form
the originally proposed structure 1 for pyrinodemin A and
positional isomer 2 of the double bond; further, Baldwin’s
group6 has also reported the racemic synthesis of positional
isomer 3 of the double bond. Both groups concluded that
the position of the double bond in the natural product was
incorrectly assigned between C16′-C17′, mainly on the basis
of critical carbon chemical shift differences (∆δ) observed
between the olefinic carbons, |∆δ| ) 1.0 ppm (Snider), 1.1
ppm (Baldwin) in 1; |∆δ| ) 0 ppm in natural pyrinodemin
A. In light of the difference |∆δ| ) 0.4 ppm in 2, Baldwin’s
group reported that 2 does not correspond to the natural
product as well. In contrast, Snider’s group concluded that
2 is probably the correct structure of pyrinodemin A, albeit
|∆δ| ) 0.4 ppm in 2. On the other hand, Baldwin’s group
suggested the C14′-C15′ double-bond positional isomer 3
as a possible structure of pyrinodemin A because |∆δ| )
0.02 ppm in 3 is nearer to the natural |∆δ| ) 0 ppm. Thus,
there has been inconsistency with the correct structure of
natural pyrinodemin A between both groups. In this paper,
we report the racemic synthesis of possible structures 2-4
for pyrinodemin A and support the C14′-C15′ double-bond
positional isomer 3, which Baldwin’s group proposed, as the
correct structure. Furthermore, we report the first enantiose-
lective total synthesis of (-)-pyrinodemin A (3) through a
highly diastereoselective intramolecular 1,3-dipolar cycload-
dition reaction as the key step and the determination of its
absolute configuration.
Scheme 1a
a Reagents and conditions: (a) 3,4-dihydro-2H-pyran, 10-cam-
phorsulfonic acid, CH2Cl2, 0 °C, 2 h, 100%; (b) LiCtCH‚H2N-
(CH2)2NH2, NaI, DMSO, rt, 4 h, 82%; (c) 9, LDA, THF, -78 °C,
30 min, then Br(CH2)m-1Br (m ) 6, 7, 8), -78 °C to room
temperature, 12 h; (d) lithium acetylide of 8, 1,3-dimethyl-3,4,5,6-
tetrahydro-2(1H)-pyrimidinone/THF (1:1), -15 °C to room tem-
perature, 16 h; (e) H2, 5% Pd-CaCO3, EtOAc, rt, 18 h; (f)
p-TsOH‚H2O, EtOH, rt, 45 min; (g) (COCl)2, DMSO, Et3N, CH2Cl2,
-78 °C to room temperature, 1 h; (h) NH2OH‚HCl, KOH, MeOH,
rt, 30 min, then HCl (pH 3), NaBH3CN, rt, 2 h; (i) 14a, toluene, rt,
40 min, then reflux, 13 h.
In comparison of the synthetic compounds 1-3 with the
natural product, the most critically discriminating point is
∆δ between the olefinic carbons. The farther the position of
the double bond is from the isoxazolidine ring, the nearer
the value of ∆δ is to zero in natural pyrinodemin A.
Therefore, we were interested in the ∆δ of compound 4,
which has the double bond one carbon farther from the
isoxazolidine ring than 3. Thus, we first embarked on the
synthesis of 4 as an alternative possible structure for
pyrinodemin A.
The preparation of hydroxylamine 15c required for the
synthesis of 4 began with THP protection of 6-bromo-1-
hexanol (6) (Scheme 1). Alkylation of the lithium acetylide
ethylenediamine complex with bromide 7 (n ) 6)7 in DMSO
containing NaI4 afforded terminal alkyne 8 (n ) 6) in 82%
yield. Coupling of the lithium acetylide of 8 (n ) 6) with
bromide 10 (m ) 6),8 prepared by alkylating a lithio
derivative of 3-picoline (9)9 with 1,5-dibromopentane, pro-
vided alkyne 11c, hydrogenation of which over Lindlar
catalyst yielded cis-alkene 12c. Deprotection of the THP
ether in 12c and Swern oxidation10 of the resulting alcohol
13c gave aldehyde 14c, which was converted into the desired
hydroxylamine 15c by reductive amination with hydroxyl-
amine and sodium cyanoborohydride in good yields. Dehy-
dration condensation of the hydroxylamine 15c and aldehyde
14a, prepared from 5-hexyn-1-ol (5) by a similar sequence
of reactions, generated a nitrone intermediate, an intramo-
lecular 1,3-dipolar cycloaddition11 of which in situ proceeded
in refluxing toluene to diastereoselectively produce the fourth
double-bond positional isomer 4 in 92% yield.4-6,12 The
synthesis of other possible double-bond positional isomers
(4) Snider, B. B.; Shi, B. Tetrahedron Lett. 2001, 42, 1639-1642.
(5) Baldwin, J. E.; Romeril, S. P.; Lee, V.; Claridge, T. D. W. Org.
Lett. 2001, 3, 1145-1148.
(6) Romeril, S. P.; Lee, V.; Claridge, T. D. W.; Baldwin, J. E.
Tetrahedron Lett. 2002, 43, 327-329.
(7) All new compounds reported here were satisfactorily characterized
on the basis of their 1H and 13C NMR, IR, MS, and HRMS spectra. For
selected characterization data, see Supporting Information.
(8) (a) Torisawa, Y.; Hashimoto, A.; Nakagawa, M.; Seki, H.; Hara, R.;
Hino, T. Tetrahedron 1991, 47, 8067-8078. (b) Morimoto, Y.; Yokoe, C.
Tetrahedron Lett. 1997, 38, 8981-8984. (c) Morimoto, Y.; Yokoe, C.;
Kurihara, H.; Kinoshita, T. Tetrahedron 1998, 54, 12197-12214.
(9) Davies-Coleman, M. T.; Faulkner, D. J.; Dubowchik, G. M.; Roth,
G. P.; Polson, C.; Fairchild, C. J. Org. Chem. 1993, 58, 5925-5930.
(10) (a) Mancuso, A. J.; Huang, S.-L.; Swern, D. J. Org. Chem. 1978,
43, 2480-2482. (b) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651-
1660.
(11) (a) Tufariello, J. J. In 1,3-Dipolar Cycloaddition Chemistry; Padwa,
A., Ed.; Wiley: New York, 1984; Vol. 2, p 87. (b) Confalone, P. N.; Huie,
E. M. Org. React. 1988, 36, 1-173. (c) Wade, P. A. In ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991;
Vol. 4, pp 1111-1168.
2612
Org. Lett., Vol. 5, No. 15, 2003

2 and 3 was also carried out by the same synthetic meth-
odology as that of 4.
phosphine.13 Wittig olefination of the lactol 20 with a
phosphorane generated by treating phosphonium bromide 17
with potassium hexamethyldisilazide (KHMDS) stereose-
lectively provided only (Z)-alkene 21 (J ) 10.9 Hz between
the olefinic protons) in 91% yield. One-carbon homologation
of the alcohol 21 was performed by mesylation and substitu-
tion with KCN in the presence of 18-crown-6 to yield nitrile
23, which was reduced to aldehyde 24 by DIBALH. An
intramolecular asymmetric 1,3-dipolar cycloaddition reac-
tion14 of the alkenylnitrone derived from hydroxylamine 15b
and chiral aldehyde 24 proceeded smoothly to construct the
desired cis-cyclopent[c]isoxazolidine ring system in 95%
yield and excellent diastereoselectivity (>99:1).
The 13C NMR spectral data and |∆δ| ) 0.4 ppm of our
synthetic 27 were identical to those of the authentic sample
reported by Snider and Baldwin.4,5 The |∆δ| ) 0.02 ppm of
our synthetic 3 was also identical to Baldwin’s ∆δ.6
Surprisingly, it has been found that the |∆δ| ) 0.3 ppm of
synthetic 4 becomes larger than that of 3. Moreover, the 13
C
NMR data of 3 were in better agreement with the literature
data of natural product than either 2 or 4.7 Therefore, we
conclude that the structure 3 with a C14′-C15′ double bond
is the correct one of natural pyrinodemin A as proposed by
Baldwin et al.
To solve the problem of the absolute configuration of
pyrinodemin A, we next planned the enantioselective syn-
thesis of the natural product employing chiral aldehyde 24
with an asymmetric center at the allylic C17 position instead
of aldehyde 14a (Scheme 2). Commercially available (R)-
The stereochemistry of the bicyclic ring system in 25 was
unambiguously confirmed by the coupling constant J16-17
) 2.6 Hz and the presence of NOEs observed between the
diagnostic protons14b as shown in Figure 1. The high
Scheme 2a
Figure 1. Diagnostic NOEs observed in 25. Hydrogens on C18
and C19 have been omitted for clarity.
diastereoselectivity might be rationalized as follows. In the
transition states b and c leading to another diastereomer,
sterically encumbered A1,3-strain15 and 1,3-diaxial-like in-
teraction16 occur between respective substituents as shown
in Figure 2. Therefore, because the transition state a without
a Reagents and conditions: (a) 9, LDA, THF, -78 °C, 30 min,
then Br(CH2)8Br, -78 °C to room temperature, 9 h, 65%; (b) HBr,
PPh3, CH3CN, reflux, 29 h, 93%; (c) MOMCl, i-Pr2NEt, CH2Cl2,
0 °C to room temperature, 41 h, 88%; (d) DIBALH, toluene, -78
°C, 1 h, 89%; (e) 17, KHMDS, THF, 0 °C, 1 h, then 20, -78 °C
to room temperature, 30 min, 91%; (f) MsCl, Et3N, CH2Cl2, 0 °C
to room temperature, 1 h, 81%; (g) KCN, 18-crown-6, CH3CN, 50
°C, 20 h, 90%; (h) DIBALH, toluene, -78 °C, 20 min, 81%; (i)
15b, toluene, reflux, 13 h, 95%; (j) HCl, EtOH, 50 °C, 16 h, 95%;
(k) PhOCSCl, DMAP, CH3CN, rt, 41 h; (l) n-Bu3SnH, AIBN,
toluene, 100 °C, 10 min, 59% (two steps).
(+)-R-hydroxy-γ-butyrolactone (18) was chosen as a starting
material. MOM protection of the hydroxy group in lactone
18 and subsequent DIBALH reduction of lactone 19 at -78
°C afforded lactol 20 in good overall yield. Phosphonium
bromide 17, requisite for Wittig reaction with the lactol 20,
was prepared from 3-picoline (9) by alkylation with 1,8-
dibromooctane and subsequent substitution with triphenyl-
Figure 2. Plausible transition states leading to 25. For R and R′,
see Figure 1.
such a repulsive interaction becomes much more stable than
b and c, 25 might exclusively be formed.14
Org. Lett., Vol. 5, No. 15, 2003
2613

Deprotection of the MOM ether in 25 under acidic
conditions gave alcohol 26, wherein deoxygenation of the
secondary hydroxy group was effected by means of the
following stepwise reactions: (1) thionocarbonate formation
with phenyl chlorothionoformate and (2) reduction with
n-Bu3SnH in the presence of AIBN radical initiator.17 The
spectral characteristics (1H and 13C NMR and EI-MS) of
synthetic 3,18 [R]26 -6.42 (c 0.94, CHCl3) (lit.2 [R]25 -9
(c 1.0, CHCl3)), were identical to those reported for the
natural product.2 Thus, it has been found that the hitherto
unknown absolute configuration of pyrinodemin A is as
indicated in the structural formula 3.
In conclusion, we have achieved the racemic synthesis of
possible structures 2-4 for pyrinodemin A and assigned the
C14′-C15′ position to the correct double-bond structure as
had been proposed by Baldwin et al. We have also ac-
complished the first asymmetric total synthesis of (-)-
pyrinodemin A (3), featuring a highly diastereoselective
intramolecular nitrone-olefin cycloaddition, and determined
the absolute configuration of (-)-3. These results may imply
the unknown absolute configuration of pyrinodemins B-D
related to (-)-3 and will be essential for investigating their
structure-activity relationships and biogenesis.
Acknowledgment. We thank Professors J. Kobayashi and
M. Tsuda, Hokkaido University, for copies of the spectral
data of natural pyrinodemin A. This research was partly
supported by a Grant-in-Aid for Scientific Research from
the Japan Society for the Promotion of Science and the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
D
D
Supporting Information Available: Experimental pro-
cedures and characterization data for 4 and (-)-3, charac-
terization data for 2 and the natural product, and copies of
1H and 13C NMR and MS spectra of 2, 4, (-)-3, and the
natural product. This material is available free of charge via
the Internet at http://pubs.acs.org.
(12) For regiochemistry in the intramolecular cycloadditions of
5-alkenylnitrones, see: (a) LeBel, N. A.; Whang, J. J. J. Am. Chem. Soc.
1959, 81, 6334-6335. (b) LeBel, N. A.; Post, M. E.; Whang, J. J. J. Am.
Chem. Soc. 1964, 86, 3759-3767. (c) Baldwin, S. W.; Wilson, J. D.; Aube´,
J. J. Org. Chem. 1985, 50, 4432-4439.
(13) (a) Staab, H. A.; Zipplies, M. F.; Mu¨ller, T.; Storch, M.; Krieger,
C. Chem. Ber. 1994, 127, 1667-1680. (b) Kaiser, A.; Marazano, C.; Maier,
M. J. Org. Chem. 1999, 64, 3778-3782.
OL034700V
(16) Hirsch, J. A. In Topics in Stereochemistry; Allinger, N. L.; Eliel,
E. L., Eds.; Wiley: New York, 1967; Vol. 1, pp 199-222.
(17) (a) Barton, D. H. R.; McCombie, S. W. J. Chem. Soc., Perkin Trans.
1 1975, 1574-1585. (b) Barton, D. H. R.; Jaszberenyi, J. C. Tetrahedron
Lett. 1989, 30, 2619-2622. (c) Nagasawa, K.; Georgieva, A.; Koshino,
H.; Nakata, T.; Kita, T.; Hashimoto, Y. Org. Lett. 2002, 4, 177-180.
(18) Optical purity of synthetic 3 has been determined to be >99% ee
by derivatization of alcohol 26 to a Mosher ester and integration of the
(14) (a) Gothelf, K. V.; Jørgensen, K. A. Chem. ReV. 1998, 98, 863-
909. (b) Tamura, O.; Mita, N.; Okabe, T.; Yamaguchi, T.; Fukushima, C.;
Yamashita, M.; Morita, Y.; Morita, N.; Ishibashi, H.; Sakamoto, M. J. Org.
Chem. 2001, 66, 2602-2610. (c) Chiacchio, U.; Corsaro, A.; Iannazzo,
D.; Piperno, A.; Procopio, A.; Rescifina, A.; Romeo, G.; Romeo, R. J. Org.
Chem. 2002, 67, 4380-4383.
1
signals in the H NMR spectrum (Dale, J. A.; Dull, D. L.; Mosher, H. S.
(15) Hoffmann, R. W. Chem. ReV. 1989, 89, 1841-1860.
J. Org. Chem. 1969, 34, 2543-2549).
2614
Org. Lett., Vol. 5, No. 15, 2003