Concise synthesis of all stereoisomers of β-methoxytyrosine and determination of the absolute configuration of the residue in callipeltin A

Article abstract of DOI:10.1021/ol0513600

(Chemical Equation Presented) All stereoisomers of β-methoxytyrosine (β-OMeTyr), a stereo-undefined component of callipeltin A, were synthesized from L- and D-tyrosine. The stereochemistry of β-OMeTyr in callipeltin A was determined to be 2R,3R by an oxidative procedure and Marfey's analysis.

Full text of DOI:10.1021/ol0513600

ORGANIC
LETTERS
2005
Vol. 7, No. 16
3585-3588
Concise Synthesis of All Stereoisomers
of -Methoxytyrosine and Determination
â
of the Absolute Configuration of the
Residue in Callipeltin A
Angela Zampella,† Rosa D’Orsi,† Valentina Sepe,† Agostino Casapullo,‡
Maria Chiara Monti,‡ and Maria Valeria D’Auria*,†
Dipartimento di Chimica delle Sostanze Naturali, UniVersita` degli Studi di Napoli
“Federico II”, Via D. Montesano 49, 80131, Napoli, Italy, and Dipartimento di
Scienze Farmaceutiche, UniVersita` di Salerno, Via Ponte don Melillo,
Fisciano (SA) 84084, Italy
madauria@unina.it
Received June 10, 2005
ABSTRACT
All stereoisomers of
â
-methoxytyrosine (â-OMeTyr), a stereo-undefined component of callipeltin A, were synthesized from L- and D-tyrosine.
The stereochemistry of
â
-OMeTyr in callipeltin A was determined to be 2R,3R by an oxidative procedure and Marfey’s analysis.
Cyclodepsipeptide callipeltin A1 (1) and its congeners2,3 were
isolated in our laboratories from the marine sponge Callipelta
sp., collected in New Caledonia. Callipeltin A is a decapep-
tide containing three unusual amino acid residues: â-meth-
oxytyrosine (â-OMeTyr), (2R,3R,4S)-4-amino-7-guanidino-
2,3-dihydroxyheptanoic acid (AGDHE), and (3S,4R)-3,
4-dimethyl-L-glutamine (diMeGln). Callipeltin A (1) is
known to exhibit antifungal and anti-HIV activity. The
stereochemistry of callipeltins, however, remains to be
determined because of the uncertainty regarding the â-OMeTyr
unit. Interestingly, the â-OMeTyr residue was also found as
a component of papuamides4 and neamphamide.5 Like
callipeltin A, they show anti-HIV activities. In each instance,
the stereochemistry of the â-methoxytyrosine remains un-
determined because of degradation of this residue under
acidic hydrolysis.
† Universita` degli Studi di Napoli “Federico II”.
‡ Universita` di Salerno.
(1) Zampella, A.; D’Auria, M. V.; Gomez-Paloma, L.; Casapullo, A.;
Minale, L.; Debitus, C.; Henin, Y. J. Am. Chem. Soc. 1996, 118, 6202-
6209.
(2) D’Auria M. V., Zampella, A.; Gomez-Paloma, L.; Minale, L.; Debitus,
C.; Roussakis, C.; Le Bert, V. Tetrahedron 1996, 52, 9589-9596.
(3) Zampella, A.; Randazzo, A.; Borbone, N.; Luciani, S.; Trevisi, L.;
Debitus, C.; D’Auria M. V. Tetrahedron Lett. 2002, 43, 6163-6166.
(4) Ford, P. W.; Gustafson, K. R.; McKee, T. C.; Shigematsu, N.;
Maurizi, L. K.; Pannell, L. K.; Williams, D. E.; de Silva, E. D.; Lassota,
P.; Allen, T. M.; Van Soest, R.; Andersen, R. J.; Boyd, M. R. J. Am. Chem.
Soc. 1999, 121, 5899-5909.
(5) Oku, N.; Gustafson, K. R.; Cartner; Wilson, J. A.; Shigematsu, N.;
Hess, S.; Pannell, L. K.; Boyd, M. R.; McMahon, J. B. J. Nat. Prod. 2004,
67, 1407-1411.
10.1021/ol0513600 CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/13/2005

hydroxyphenylalanine derivatives. The corresponding erythro
diastereomers could be obtained by oxidation of the threo
adducts to the corresponding ketones, followed by reduction
by a suitable hydride, a procedure usually used to access
the not directly obtainable erythro â-adducts.
(2S)-O-Acetyl-N-phthaloyltyrosine methyl ester (2) was
treated with N-bromosuccinimide to afford the corresponding
3-bromo derivatives 3a and 3b (Scheme 1) in 97% yield
(1:1 mixture of diastereoisomers).
Scheme 1. Easton’s Method8
Figure 1. Callipeltin A (1)
The new structure of callipeltin A, together with its
stereochemical ambiguities and its intriguing biological
activity, has prompted considerable interest from the syn-
thetic community.6 To accomplish the total synthesis as well
as the structural determination of callipeltin A, all four
stereoisomers of â-OMeTyr are needed in large quantities.
Recently, two syntheses of all stereoisomers of â-OMeTyr
have appeared in the literature.7 They both feature a strategy
based on the nucleophilic addition of arylmetal reagents to
serine aldehyde equivalents followed by methylation.
In this paper we report a short, inexpensive, efficient
synthesis of all four stereoisomers of â-OMeTyr from
tyrosine methyl ester, using a modification of the method
developed by Easton and Hutton several years ago.8 Besides
providing adequate amounts of the above amino acid unit
for the planned total synthesis, the availability of all four
diastereoisomers of â-OMeTyr enabled us to unambiguously
assign the absolute configuration of the corresponding residue
in callipeltin A, through oxidative procedure and Marfey’s
analysis.
Treatment of bromides with silver nitrate in aqueous
acetone overnight gave the â-hydroxyl derivatives 4a,b
(6:1 mixture of diastereoisomers).
To obtain the corresponding â-methoxytyrosine derivative,
the synthesis then required the methylation of the 3-hydroxyl
group of 4a. We tested several reported methods, but we
only obtained decomposition of the starting material or
unsatisfactory transformation.11
This failure prompted us to explore an alternative access
to â-methoxy tyrosine derivatives 5a,b, which involves the
use of methanol in the place of water as a nucleophile for
the substitution displacement of the bromide atom.
The factors determining the stereoselectivity of the hy-
drolysis of â-bromoarylalanine derivatives were investi-
gated.12 So far, no details on the stereochemical course of
the methanolysis of the bromotyrosine derivatives have been
reported. To gain more insight, we tested several solvolysis
conditions by varying the solvent and the catalyst (Table 1).
In the presence of AgOTf, we observed the concomitant
removal of the O-acetyl protecting group. In all tested
conditions, a modest level of stereoselectivity was observed,
inferring a SN1 character with the absence of neighboring
group participation. Hutton12 reports the formation of threo
and erythro adducts 4a and 4b, respectively, in a 6:1 ratio
when the hydrolysis was performed with silver nitrate in
aqueous acetone. On the same substrate, in comparable
conditions (entry 1), methanolysis afforded a 6:4 ratio of
threo and erythro adducts. Better stereoselectivity was
observed with silver nitrate in MeOH/acetone (entries 2-4),
whereas the use of silver triflate reduced the selectivity of
the solvolysis reactions, giving invariably a 1:1 ratio (entries
Easton’s method, used for the synthesis of the (2S,3R)-
â-hydroxytyrosine residue in vancomycin9 and, recently, for
the synthesis of the (2S,3R)-â-methoxyphenylalanine residue
in cyclomarin,10 provides selective formation of threo-
(6) For the synthesis of the nonproteinogenic units in callipeltin A, see:
(a) Liang, B.; Carroll, P. J.; Joullie, M. M. Org. Lett. 2000, 2, 4157-4160.
(b) Okamoto, N.; Hara, O.; Makino, K.; Hamada, Y. Tetrahedron:
Asymmetry 2001, 12, 1353-1358. (c) Acevedo, C. M.; Kogut, E. F.; Lipton,
M. A. Tetrahedron 2001, 57, 6353-6359. (d) Chandrasekhar, S.; Ram-
achandar, T.; Rao, B. V. Tetrahedron: Asymmetry 2001, 12, 2315-2321.
(e) Guerlavais, V.; Carroll, P. J.; Joullie, M. M Tetrahedron: Asymmetry
2002, 13, 675-680. (f) Thoen, J. C.; Morales-Ramos, A. I.; Lipton, M. A.
Org. Lett. 2002, 4, 4455-4458. (g) Zampella, A.; Sorgente, M.; D’Auria,
M. V. Tetrahedron: Asymmetry 2002, 13, 681-685. (h) Zampella, A.;
D’Auria, M. V. Tetrahedron: Asymmetry 2002, 13, 1237-1239. (i) Ravi,
K. A.; Venkateswara, R. B. Tetrahedron Lett. 2003, 44, 5645-5647.
(j) Turk, J. A.; Visbal, G. S.; Lipton, M. A. J. Org. Chem. 2003, 68, 7841-
7844.
(7) (a) Okamoto, N.; Hara, O.; Makino, K.; Hamada, Y. J. Org. Chem.
2002, 67, 9210-9215. (b) Hansen, D. B.; Wan, X.; Carrol, P. J.; Joullie,
M. M. J. Org. Chem. 2005, 70, 3120-3126.
(8) Easton, C. J.; Hutton, C. A.; Roselt, P. D.; Tiekink, E. R. T.
Tetrahedron 1994, 50, 7327-7340.
(11) During the preparation of this manuscript, a paper (ref 7b) appeared
reporting the same difficulty in methylating a protected 3-hydroxy-tyrosine
derivative.
(9) Rama Rao, A. V.; Chakraborty, T. K.; Laxma Reddy, K.; Srinivasa
Rao, A. Tetrahedron Lett. 1994, 35, 5043-5046.
(10) Wen, S.-J.; Yao, Z.-J. Org. Lett. 2004, 6, 2721-2724.
(12) Hutton, C. A. Tetrahedron Lett. 1997, 38, 5899-5902.
3586
Org. Lett., Vol. 7, No. 16, 2005

Table 1. Ag+-Promoted Methanolysis of Bromotyrosines 3a,b
Scheme 2. Deprotection of â-Methoxytyrosine Derivatives
entry substrate catalyst
solvent
ratiob 5a:5b yieldc
d
1
2
3
4
4
5
6
7
8
3a/3b AgNO3 MeOH
60:40
78:22
65:35
59:41
73:27
60:40
59:41
70:30
45:55
68:32
50:50
75%
70%
70%
77%
75%
87%
85%
70%
72%
70%
73%
3a/3b AgNO3 acetone
3a/3b AgNO3 acetone/MeOH 1:1
3a/3b AgNO3 acetone/MeOH 2:8
3a/3b AgNO3 acetone/MeOH 8:2
3a/3b AgOTf MeOH
3a/3b AgOTf acetone
3a
3a
3b
3b
AgNO3 acetone
AgNO3 MeOH
AgNO3 acetone
AgNO3 MeOH
addition of a 2% hydrazine methanolic solution at 0 °C.
Finally, the removal of the methyl ester was easily obtained
by LiOH hydrolysis. The same deprotection procedure was
applied to the erythro diastereomer 5b to provide (2S,3S)-
3-methoxy-tyrosine 6b with similar results.
The remaining two enantiomers of â-methoxytyrosine
were synthesized using the same procedures starting from
(2R)-O-acetyl-N-phthaloyltyrosine methyl ester to afford the
desired (2R,3S)-â-methoxytyrosine 6c and (2R,3R)-â-meth-
oxytyrosine 6d with similar stereoselectivity and chemical
yield.
Concerning the configuration of the â-methoxytyrosine
residue in callipeltin A (1), recently we applied a new
integrated NMR-quantum mechanical (QM) approach, rely-
ing on the comparison between calculated and experimental
J-values, to the analysis of the relative configuration of four
amino acid units in callipeltin A (1) and proposed an erythro
arrangement (e.g., S*,S*) for the two stereogenic centers in
the â-methoxytyrosine residue.13 With all four stereoisomers
of â-methoxytyrosine in our hands, it was then possible to
confirm the above assignment and to determine the absolute
configuration using a precolumn derivatization method.
We tested several acidic and basic hydrolytic conditions,14
and unfortunately no recovery of â-methoxytyrosine was
invariably observed.15
9
10
a R ) OAc when AgNO3 was used as a catalyst. b 5a:5b ratio was
determined by 1H NMR analysis of the mixture. c Isolated yield by column
chromatography. d Due to the poor solubility of AgNO3 in MeOH/acetone,
10% water was added to the solvent when this catalyst was used.
5 and 6). Results obtained by solvolysis on isolate â-bromo-
arylalanine 3a or 3b (entries 7-10) confirm the findings of
Hutton and indicate some degree of conformational restric-
tion of the cation intermediate with the preferential formation
of the threo adduct. Even if in all tested conditions no
significant selectivity was observed, in light of our need of
a rapid access to both diastereomers of â-methoxytyrosine,
we selected silver triflate/methanol conditions (entry 5). In
these conditions, we obtained easier synthetic elaboration
in terms of yields and number of steps. The two methoxy-
tyrosine derivatives could be easily separated by HPLC (silica
gel, hexane/ethyl acetate 75:25) affording pure 5a and 5b.
To secure the stereochemistry of the diastereoisomeric
â-methoxytyrosine derivatives, a sample of (2S,3R)-â-
hydroxytyrosine derivative 4a previously obtained was
methylated with CH3I and AgOTf. The methylated derivative
was obtained in a modest 10% yield; however, HPLC
separation of the crude reaction mixture followed by
A literature search suggested the opportunity to transform
the phenyl group, responsible of the lability of the â-meth-
1
deacetylation with p-TsOH afforded a product whose H
NMR and optical rotation data were superimposable with
those of derivative 5a.
To complete the synthesis of both diastereomers of
â-methoxytyrosine and to access deprotected derivatives,
useful as standards for the stereochemical studies on cal-
lipeltins, we explored the selective removal of both phthaloyl
and methyl ester protecting groups (Scheme 2).
Scheme 3. Ozonolysis of â-Methoxytyrosines and Their
HPLC Retention Times (tR)
Quantitative removal of the phthaloyl protecting group in
5a was achieved without observable racemization by careful
(13) Bassarello, C.; Zampella, A.; Monti, C.; Gomez-Paloma, L.;
D’Auria, M. V.; Riccio, R.; Bifulco, G. Tetrahedron, submitted.
(14) Davison, I. In Methods in Molecular Biology; Smith, B. J., Ed.;
Totowa, NJ; Vol. 211, pp 111-122.
(15) As determined by LC/MS analysis (ion-selective monitoring for
FDAA-MeOTyr + H+ m/z 464) after Marfey’s derivatization of the crude
hydrolysate.
Org. Lett., Vol. 7, No. 16, 2005
3587

oxytyrosine residue, to a carboxy function, through oxidative
ozonolysis.
was in contrast with that usually observed for hydrophilic
hydroxy and acidic amino acids.16 Noteworthy also is the
deviation from the usual behavior for the threo series, with
the D-isomer eluting before the L-isomer. The selective ion
monitoring at m/z 461 of the hydrolysate of callipeltin A
derivatized with L-FDAA showed only a peak a with
retention time of 17.30 min, corresponding to the L-FDAA
derivative of (2R,3S)-â-methoxy-aspartate 7d. Thus, the
(2R,3R)-configuration for the â-methoxytyrosine residue in
callipeltin A was unambiguously established.
In conclusion, we have reported a concise synthesis of all
four stereoisomers of â-methoxytyrosine. The absolute
configuration of the above residue in callipeltin A (1) was
assigned for the first time. In addition, our method turns out
to be a useful tool for the stereochemical characterization
of â-methoxy-â-aryl amino acids, often found as components
of bioactive natural peptides.
As depicted in Scheme 3, a small sample (1 mg each) of
synthetic diastereomers 6a-d was transformed to the cor-
responding â-methoxy-aspartate derivatives 7a-d by ozo-
nolysis, followed by hydrogen peroxide workup. The com-
pounds 7a-d were derivatized with Marfey’s reagent
1-fluoro-2,4-dinitrophenyl-5-L-alaninamide; L-FDAA).
The same procedure was applied to callipeltin A (1). The
oxidized callipeltin A (1) was hydrolyzed, and the amino
acid residues so obtained were derivatized with Marfey’s
reagent (Scheme 4).
Scheme 4. Configurational Assignment of â-Methoxytyrosine
in Callipeltin A
Acknowledgment. This work was supported by grants
from MIUR (PRIN 2003) “Sostanze Naturali ed Analoghi
Sintetici ad Attivita` Antitumorale”, Rome, Italy. Mass and
NMR spectra were provided by the CRIAS, Faculty of
Pharmacy, Universita` di Napoli Federico II. We thank Dr.
Kirk R. Gustafson (Molecular Targets Development Program,
NCI-Frederick) for helpful discussion.
Supporting Information Available: Experimental pro-
cedures and spectoscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The L-FDAA derivatives of 7a-d were analyzed using
ESI LC/ MS in the positive ion mode. By monitoring for
L-FDAA-OMeAsp at m/z 461, we detected the L-FDAA
derivatives of 7a-d as separate peaks at 10.23, 16.06, 9.86,
and 17.30 min, respectively. The observed good separation
OL0513600
(16) Fujii, K.; Ikai, Y.; Mayumi, T.; Oka, H.; Suzuki, M.; Harada, K.-I.
Anal. Chem. 1997, 69, 3346-3352.
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Org. Lett., Vol. 7, No. 16, 2005