Synthesis of blastidic acid and cytosinine, two components of blasticidin S

Article abstract of DOI:10.1055/s-2001-18108

Blastidic acid and cytosinine, two components of antibiotics blasticidin S, have been synthesized.

Full text of DOI:10.1055/s-2001-18108

LETTER
1763
Synthesis of Blastidic Acid and Cytosinine, Two Components of Blasticidin S
S
ynthesis of Blasti
o
dic
A
cid and
s
Cytosinin
h
e
iyasu Ichikawa,* Masayoshi Ohbayashi, Keiko Hirata, Rena Nishizawa, Minoru Isobe
Laboratory of Organic Chemistry, School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan
Fax +81(52)7893222; E-mail: ichikawa@nuagrl.agr.nagoya-u.ac.jp
Received 28 August 2001
enantioselective hydrolysis of meso-diester 5 with pig liv-
Abstract: Blastidic acid and cytosinine, two components of antibi-
er esterase as reported by Ohno (Scheme 2).6 Hence, our
otics blasticidin S, have been synthesized.
initial effort focused on the functional group interconver-
Key words: antibiotics, amines, cyanate, nucleosides, rearrange-
sion of the carboxylic acid of 6 into the N-methyl guani-
ments
dine moiety. Diborane reduction of the carboxylic acid 6
yielded the alcohol 7 in 74% yield, which was further con-
verted into the benzyl amide 8 (54% yield) by reaction
Blasticidin S (1) was isolated from Streptomyces griseo-
chromogenes and at one time used as a fungicide for the
with benzylamine in refluxing benzene.7 Transformation
of the hydroxyl group of 8 into the N-methyl amine moi-
prevention of rice blast in Japan.1 The structural studies by
ety was accomplished according to the protocol of Fuku-
Otake et al. revealed that blasticidin S (1) is a member of
yama.8 Accordingly, N-methyl 2-nitrobenzenesulfon-
the peptidyl-nucleoside family of antibiotics as shown in
amide was alkylated with 8 under Mitsunobu conditions
Figure 1.2
(DEAD, Bu3P, CH2Cl2) to furnish 9 in 95% yield.9 Depro-
tection of the o-nitrobenzenesulfonamide group of 9 was
CO2H
O
N
NH
NH2
O
carried out by treatment with thiophenol and cesium car-
bonate in acetonitrile, giving the N-methylamine 10,
which further reacted with N,N-di-(tert-butoxycarbon-
yl)thiourea in the presence of mercuric chloride.10 The
bis-Boc-protected guanidine 11 was isolated in 75% over-
all yield from 9.
O
N
H2N
N
N
NH2
CH3
H
Blasticidin S (1)
Figure 1
The final stage of the blastidic acid synthesis is the two-
step process of amide activation and hydrolysis of the
benzyl amide 11.11 Accordingly, benzylamide 11 was
subjected to exhaustive acylation with di-tert-butyl dicar-
bonate in THF using DMAP as catalyst to afford the N-
Boc imide 12 in 60% yield. During this acylation, the tert-
butoxycarbonyl groups were also introduced onto the ni-
trogen atoms of both the benzyloxycarbonyl group and the
Boc-protected N-methyl guanidine group. Although sa-
ponification of imide 12 was complicated by base-cata-
lyzed -elimination of the imide group at C-3, this
reaction could be avoided by lowering the leaving group
ability. Thus, imide 12 was transformed into the Boc-car-
bamate 13 by catalytic hydrogenolysis of the Cbz group of
12. Methanolysis of the resulting compound 13 with tet-
ramethyl guanidine in methanol furnished the methyl es-
Careful acid hydrolysis of blasticidin S (1) yielded blas-
tidic acid (2) and cytosinine (3), as shown in Scheme 1.
Structurally, cytosinine (3) is characterized as a peculiar
nucleoside possessing -amino acid functionality, and
this structural feature inspired us to propose a new type of
DNA-analogue 4 based on a peptide backbone (peptide-
nucleoside, PNA).3 With this in mind, we here focused
our attention on the synthesis of blasticidin S (1).
Although the first synthesis of cytosinine (3) was reported
by Kondo and Goto,4 there has been no report on the total
synthesis of blasticidin S. A recent report on the synthesis
of blastidic acid (2) by Nomoto prompted us to present our
synthetic effort in this field.5
The synthesis of blastidic acid began with (3S)-3[(benzy-
loxycarbonyl])amino]-glutarate 6, which was prepared by
Base
O
Base
O
Base
O
CO2H
O
N
NH
NH2
O
H
O
N
Blasticidin S (1)
H2N
N
OH
H2N
NH2
N
N
N
CH3
H
O
H
O
O
H
Cytosinine (3)
Blastidic acid (2)
4
Scheme 1
Synlett 2001, No. 11, 26 10 2001. Article Identifier:
1437-2096,E;2001,0,11,1763,1766,ftx,en;Y17301ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

1764
Y. Ichikawa et al.
LETTER
NO2
H
H
H
pig liver
esterase
CbzN
H
CbzN
CbzN
O
O
H2N
Ph
O
CbzN
O
BH3, THF
74%
O
O
SO2NHMe
benzene
reflux
HO
OMe
HO
OMe
HO
N
H
Bu3P, DEAD
95%
Ph
MeO
OMe
5
6
7
8
54%
S
H
R
O
H
CbzN
O
BocN
3
NBoc
N
CbzN
O
BocN
BocN
Boc2O
DMAP
BocHN
NHBoc
R
N
N
Ph
BocN
H
N
Ph
N
N
Boc
Ph
H
Et3N, HgCl2, DMF
75%
THF
60%
H
Me
Me
Boc Me
9 R = o-Ns
10 R = H
11
PhSH,
12 R = Cbz
13 R = H
H2/Pd-C
EtOH
78%
Cs2CO3, MeCN
NH
Boc
Boc
LiOH,
THF / H2O
NH
NH
HCl•2HN
BocN
O
BocN
O
HCl•HN
H2N
O
1) TFA, CH2Cl2
Me2N
NMe2
92%
BocN
N
OMe
BocN
N
OH
MeOH
87%
N
OH
2) ion exchange
chromatography
Boc Me
Boc Me
Me
16
15
14
Scheme 2
ter 14 in 68% overall yield from 12.12 Finally, the methyl using [3,3]-sigmatropic rearrangement of an allyl cyanate
ester 14 was hydrolyzed with lithium hydroxide in aque- as shown in Scheme 3.17
ous THF to afford the Boc-protected blastidic acid 15,
Ferrier-type glycosylation of 2-acetoxy-tri-O-acetyl-D-
glucal (17) with p-methoxyphenol catalyzed by boron tri-
which is ideal for the synthesis of blasticidin S.13 Confir-
mation of the structure was performed by transforming 15
fluoride diethyl etherate gave 18 in 49% yield after recrys-
into the blastidic acid dihydrochloride 16 by treatment
tallization.18 Treatment of 18 with lithium aluminum
with TFA and purification using ion exchange chromatog-
hydride gave the diol 19,19 which was selectively protect-
1
raphy. The H and 13C NMR spectra of our synthetic 16
ed with tert-butyldimethylsilyl chloride and triethylamine
were found to be identical with those of a sample prepared
in the presence of a catalytic amount of DMAP to afford
from natural blasticidin S.14,15
the silyl ether 20 in 53% overall yield from 18.20 With the
Cytosinine (3) has the structure of 2,3-dideoxy-4-amino- hex-3-enopyranoside 20 in hand, we have undertaken the
D-hex-2-enopyranoside (Scheme 1), and synthesis of such allyl cyanate-to-isocyanate rearrangement of 20. Thus,
an unsaturated amino sugar moiety posed synthetic prob- treatment of 20 with trichloroacetyl isocyanate followed
lems in previous studies, because they relied upon intro- by hydrolysis with potassium carbonate in aqueous meth-
ducing the nitrogen substituent by replacement reaction anol gave carbamate 21. Dehydration of 21 with triphe-
with sodium azide.4,16 To solve this problem, we have de- nylphosphine, carbon tetrabromide and triethylamine at –
veloped a new approach for the synthesis of cytosinine by 40 °C gave the allyl cyanate 22, which underwent [3,3]-
sigmatropic rearrangement at 0 °C for 60 min to afford the
OMe
RO
AcO
AcO
AcO
AcO
TBSO
1) Cl3CCONCO,
CH2Cl2
O
O
O
O
1) LiAlH4, THF
HO
OPMP
OPMP
OPMP
2) K2CO3,
MeOH / H2O
BF3·OEt2
benzene
49%
2) t-BuMe2SiCl, DMAP
Et3N, CH2Cl2
53%
OH
AcO
OAc
OAc
OCONH2
19 R = H
21
18
17
20 R = t-BuMe2Si
PMP = p-methoxyphenyl
1) Swern oxidation
2) NaClO2, NaH2PO4,
t-BuOH / H2O
TBSO
TBSO
TBSO
H
CO2Me
1) PPh3, CBr4
Et3N, CH2Cl2
O
O
O
O
H
N
OPMP
N
OPMP
N
OPMP
OPMP
3) CH2N2, MeOH
70%
2) Cl3CCH2OH
75%
C
Troc
Troc
O
O
23
N
C
24 R = t-BuMe2Si
25 R = H
n-Bu4NF, AcOH
THF
26
22
79%
silated
CO2Me
O
CO2Me
CO2Me
O
O
N
1)(DPAH)2Ag·H2O
CH3CN : H2O
N4-4-t-butylbenzoyl
H
O
N
H
N
O
H
cytosine
1) Zn, AcOH
2) Ac2O, Py
N
N
NHR
N
NHR
N
OR
TMSOTf
CH2Cl2
2) Ac2O, Py
75%
Troc
Ac
Troc
3) separation
29
63% (β : α, 1 : 1 )
30
27 R = H
R = 4-t-butylbenzoyl
28 R = Ac
Scheme 3
Synlett 2001, No. 11, 1763–1766 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Synthesis of Blastidic Acid and Cytosinine
1765
h and then at r.t. overnight. The reaction mixture was washed with
1 N HCl and aq sat. NaHCO3 solution, and dried (Na2SO4). Concen-
tration under reduced pressure gave a crude residue, which was pu-
rified by silica gel chromatography to furnish 24 (10.1 g, 75% for
the two steps).
allyl isocyanate 23. Since isolation of 23 using an aqueous
work-up caused a decrease of yield due to the high reac-
tivity of the isocyanate function, allyl isocyante 23 was
transformed in situ into the trichloroethoxy carbamate 24
by reaction with 2,2,2-trichloroethanol.21 The resulting
carbamate 24 was isolated in 75% overall yield from 20
after chromatographic purification.
Acknowledgement
We would like to thank Prof. T. Kondo, Nagoya University, for his
generous gift of blastidic acid and cytosinine. This research was fi-
nancially supported by a Grant-In-Aid for Scientific Research from
the Ministry of Education, Science, Sports and Culture and by
JSPS-RFTF.
The next stage of the synthesis is the transformation of 24
into the corresponding 2,3-dideoxy-hex-2-enopyranour-
onate and cytosine glycosidation. Accordingly, the tert-
butyldimethylsilyl group of 24 was removed with tetrabu-
tylammonium fluoride in a mixture of acetic acid and tet-
rahydofuran to provide 25 in 79% yield. Two-step
oxidation of 25 involving Swern oxidation followed by
sodium chlorite oxidation and esterification of the result-
ing carboxylic acid with diazomethane in methanol fur-
nished 26 in 70% overall yield for the three steps.
Oxidative hydrolysis of p-methoxyphenyl glycoside 26
with silver (II) bis-(hydrogen dipicolinate) gave the unsta-
ble lactol 27,18 which was immediately subjected to acetic
anhydride and pyridine to provide the acetyl glycoside 28
in 75% yield. Condensation of 28 with silated N4-(4-tert-
butylbenzoyl)cytosine22 in the presence of TMSOTf af-
forded a 1:1 mixture of the fully protected cytosinine de-
rivative 29 and its -isomer in 63% yield.23 Finally, 29
References and Notes
(1) Takeuchi, K.; Hirayama, K.; Ueda, H.; Sakai, H.; Yonehara,
H. J. Antibiot., Ser. A 1958, 11, 1.
(2) (a) Otake, N.; Takeuchi, S.; Endo, T.; Yonehara, H.
Tetrahedron Lett. 1965, 1405. (b) Otake, N.; Takeuchi, S.;
Endo, T.; Yonehara, H. Agric. Biol. Chem. 1966, 30, 126.
(c) Otake, N.; Takeuchi, S.; Endo, T.; Yonehara, H. Agric.
Biol. Chem. 1966, 30, 132.
(3) (a) Nielsen, P. E.; Egholm, M.; Berg, R. H.; Buchardt, O.
Science 1991, 254, 1497. (b) Egholm, M.; Buchardt, O.;
Nielsen, P. E.; Berg, R. H. J. Am. Chem. Soc. 1992, 114,
1895.
(4) (a) Kondo, T.; Nakai, H.; Goto, T. Tetrahedron Lett. 1972,
1881. (b) Kondo, T.; Nakai, H.; Goto, T. Tetrahedron 1973,
29, 1801.
(5) Nomoto, N.; Shimoyama, A. Tetrahedron Lett. 2001, 42,
1753.
(6) (a) Ohno, M.; Kobayashi, S.; Iimori, T.; Wang, V. F.; Izawa,
T. J. Am. Chem. Soc. 1981, 103, 2405. (b) Ohno, M.;
Otsuka, M. Organic Reactions, Vol. 37; John Wiley & Sons:
New York, 1989, 1.
(7) Since N-methylamine 32, which was produced upon
removal of the sulfonyl group of 31, rapidly cyclized to form
lactam 33, benzyl amide was chosen as the carboxylic acid
protecting group (Figure 2).
24
was transformed into 30 which was identical with the
product derived from natural blasticidin S.25
In summary, the synthesis of Boc-protected blastidic acid
15 was achieved in 9 steps starting from chiral carboxylic
acid 6. Allyl cyanate-to-isocyanate rearrangement has
been successfully employed for the construction of an un-
saturated amino sugar moiety of cytosinine, and our syn-
thesis of the fully protected cytosinine derivative 29
required 13 steps starting from 2-acetoxy-tri-O-acetyl-D-
glucal 17. Further studies toward the total synthesis of
blasticidin S are now underway in our laboratory.
NHCbz
NHCbz
PhSH,
Cs2CO3,
H
CbzN
O
Preparation of p-Methoxyphenyl 2,3,4,6-Tetradeoxy-4-
trichloroethoxycarbonylamino- -D-erythro-hex-2-
enopyranoside 26 from 20
R
CO2Me
MeCN
N
OMe
NH
Me
O
N
Me
Me
To a solution of 20 (28.2 g, 77.0 mmol) in CH2Cl2 (300 mL) cooled
to 0 °C was added trichloroacetyl isocyanate (10.5 mL, 92.4 mmol).
After stirring at 0 °C for 1 h, the reaction mixture was concentrated,
and the resulting residue was dissolved in methanol (100 mL). Wa-
ter (100 mL) and potassium carbonate (16.1 g, 231mmol) were add-
ed at 0 °C, and the cooling bath was removed. After stirring at room
temperature for 2.5 h, the reaction mixture was concentrated under
reduced pressure to remove methanol. The resulting aqueous phase
was extracted with CH2Cl2 and the combined organic layer was
dried (Na2SO4) and concentrated to afford the carbamate 21 (25.6
g), which was used for the next reaction without further purifica-
tion.
31 R = o-Ns
33
32
Figure 2
(8) (a) Fukuyama, T.; Jow, C.-K.; Cheung, M. Tetrahedron Lett.
1995, 36, 6373. (b) Mitsunobu reactions of N-methyl p-
toluenesulfonamide in THF afforded sulfonamide in
moderate yield due to the formation of DEAD N-alkylation
compound. In the case of N-methyl 2-nitrobenzenesulfon-
amide, such a by-product has never been observed. See:
Henry, J. R.; Marcin, L. R.; McIntosh, M. C.; Scola, P. M.;
Harris, G. D.; Weinreb, S. M. Tetrahedron Lett. 1989, 30,
5709.
(9) When we employed triphenylphosphine in this Mitsunobu
reaction, we had difficulties in purification of product 9,
which shows a similar chromatographic behavior than
triphenylphosphine oxide.
To a solution of the carbamate 21 (8.27 g, 20.2 mol), triphenylphos-
phine (13.3 g, 50.5 mmol) and triethylamine (7.04 mL, 50.5 mmol)
in CH2Cl2 (170 mL) cooled to –40 °C under nitrogen atmosphere
was added carbon tetrabromide (18.8 g, 56.6 mmol) in CH2Cl2 (30
mL). The reaction mixture was gradually warmed to 0 °C over 30
min and then stirred at 0 °C for 1 h. 2,2,2-Trichloroethanol (11.6
mL, 0.12 mol) was added, and stirring was continued at 0 °C for 2
(10) Kim, K. S.; Qian, L. Tetrahedron Lett. 1993, 34, 7677.
Synlett 2001, No. 11, 1763–1766 ISSN 0936-5214 © Thieme Stuttgart · New York

1766
Y. Ichikawa et al.
LETTER
(11) Flynn, D. L.; Zelle, R. E.; Grieco, D. A. J. Org. Chem. 1983,
48, 2424.
(12) Grehn, L.; Gunnarsson, K.; Ragnarsson, U. Acta Chem.
Scand., Ser. B 1987, 41, 18.
(18) We were extremely frustrated with a serious problem during
transformation of the ethyl glycoside 35 into the
corresponding acetyl glycoside, because acid-catalyzed
hydrolysis of 35 resulted in rapid formation of pyrrole 36
(Figure 4). Taking this preliminary experiment into
consideration, we employed p-methoxyphenyl glycosides,
which can be hydrolyzed under neutral conditions. See:
Noshita, T.; Sugiyama, T.; Kitazumi, Y.; Oritani, T.
Tetrahedron Lett. 1994, 35, 8259.
(13) The synthesis of 15 was first reported in 1999 at the Chubu-
Kansai Branch Joint Meeting and Symposium of the Japan
Society for Bioscience, Biotechnology, and Agrochemistry
at Gifu University. See the abstract on page 39.
(14) The specific rotation of our blastidic acid dihydrochloride
(16) is found to be [ ]20D = +13.3 (c 0.52, H2O), while the
values reported were [ ]15D = +25.0 (c 1.0, H2O; see
reference 2), [ ]18D = +21.0 (c 1.0, H2O; see reference 5),
and [ ]26D = +9.1 (c 0.52, H2O; see reference 15). Due to the
discrepancy between these values, we felt it necessary to
determine the optical purity of our synthetic 16.
Accordingly, compound 15 was transformed into (R)-(+)- -
methylbenzylamide 34, and then the optical purity was
confirmed to be >97%. Similar transformation of 15 using
racemic -methylbenzylamide gave a 1:1 mixture of the
diastereoisomers to confirm the validity of this analysis
(Figure 3).
TBDPS
OTBDPS
O
H
H
N
OEt
N
MeOOC
OH
MeOOC
36
35
Figure 4
(19) (a) Isobe, M.; Ichikawa, Y.; Goto, T. Tetrahedron Lett. 1985,
26, 5199. (b) Ichikawa, Y.; Isobe, M.; Bai, D.-L.; Goto, T.
Tetrahedron 1987, 43, 4737.
Me
(20) Chaudhary, S. K.; Hernandez, O. Tetrahedron Lett. 1979,
14, 99.
(21) Ichikawa, Y.; Osada, M.; Ohtani, I.; Isobe, M. J. Chem. Soc.,
Perkin Trans. 1 1997, 1449.
(22) Will, D.; Breipohl, G.; Langner, D.; Knolle, J.; Uhlmann, E.
Tetrahedron 1995, 51, 12069.
(23) (a) Schmidt, R. R.; Wagner, W. Tetrahedron Lett. 1983, 24,
4661. (b) Ueda, T.; Watanabe, S.-I. Chem. Pharm. Bull.
1985, 33, 3689. (c) Vorbruggen, H.; Ruh-Pohlenz, C.
Organic Reactions, Vol. 55; John Wiley & Sons: New York,
2000, 1.
1)
H2N
Ph
BOP, i-Pr2NEt,
CH2Cl2
HCl•2HN
HCl•HN
H2N
O
Me
15
N
N
Ph
2) TFA, CH2Cl2
3) 3N HCl
H
Me
34
Figure 3
(24) Due to the difficulties associated with separating a mixture
of - and -isomers, we could not estimate the yields of this
transformation.
(15) Cooper, R.; Conover, M.; Patel, M. J. Antibiot. 1988, 123.
(16) Watanabe, K. A.; Wempen, I.; Fox, J. J. Chem. Pharm. Bull.
1970, 18, 2368.
(25) An authentic sample of 30 was prepared from cytosinine (3)
by the following reaction sequence involving i) Ac2O, Py; ii)
CH2N2, MeOH; iii) MeOH, heated at reflux temperature;
and then iv) 4-tert-butylbenzoyl chloride, Py.
(17) Ichikawa, Y.; Kobayashi, C.; Isobe, M. J. Chem. Soc.,
Perkin Trans. 1 1996, 377.
Synlett 2001, No. 11, 1763–1766 ISSN 0936-5214 © Thieme Stuttgart · New York