What is the Structure of the Calmodulin Antagonist Konbamide from Theonella sp.1 Synthesis of Two Isomers by Direct Biomimetic Introduction of Bromine in Hydroxytryptophan-Containing Cyclic Peptides

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wiis removed under reduced pressure. The resulting solid material was purifed by
flash chromatography on silica gel ( 5 : 1 hexanes:ethyl acetate) lo yield pure 3a
(183 mg, 82%) as a white solid (90% PP. G C analysis. 145 'C. Chiraldex G-DA
(Alltech and Assoc.. Inc.): [1]i5= - 9.2 (< = 1.0 in MeOH)).
Received: January 29, 1996 [287581E]
German version: Angew. Clrem. 1996. 108. 1439- 1440
Keywords: asymmetric catalysis - copper compounds
propanations diazo compounds macrocycles
-
*
cyclo-
[l] Reviews: C. 1. Roxburgh. Telruhedroon1995.51.9767-9822; H. Stach. M. Hesse.
h i d . 1988. 44, 1573-1590; I. Paterson. M. M. Mansuri, ihid 1985. 41, 35693624; K. C. Nicolaou, ibrd 1977. 33.683-710; M. A. Titus, Chen?. Rev. 1988,
88.719-732: G . R. Newkome. J. D. Sauer, J. M. Roper. D. C. Hager. ihid. 1977,
77, 513- 597
[2] W. Oppolzer. R. N. Radinov. J. A m . Chem. Soc. 1993, 115. 1593-1594.
[3] Reviews: M. P. Doyle in Comprehmsiw Orgunonwrullic Chmrirtri.. Vol. 12 (Ed.:
L. S. Hegedus), Pergamon, New York. 1995. Chapter 5.1; H.-U. Reissig in
Srereoselec~iveSvnrhesis of Organic Compounds/ Methods of Orgunic Chemi.s/ry
(Houben-Weyl). 4fh ed.. 1952.. Vol. Zlc, pp. 3179-3270; T. Ye. M. A. McKervey, Chem. Rev. 1994, 94, 857-1160: A. Padwa. K. E. Krumpe. Tefruhedroit
1992,48. 5385-5453. Recent developments in catalytic asymmetric intramolecular metalcdrbene cyclizations: M. P. Doyle, A. B. Dyatkin. M. N . Protopopo.
Puys-Bus
va, C. I. Yang, C . S. Miertschin, W. R. Winchester, Rec. 7 7 ~ 1 ,Cliim.
1995, lf4.163-170: T. G. Gant, M C . Nor, E. J. Corey. Tetruhedroii L e f t . 1995,
36. 8745-8748; C. Pique. B. FPhndrich. A. Pfaltz. Svdefr 1995.491 -492
[4] M. P. Doyle. M. N. Protopopova, C. D Poulter. D. H . Rogers. J. Am. Clzem.
Soc. 1995, 117. 7281-7282.
[5] G . J. Kubas, Inorg. Syntlt 1979. 19. 90-92.
[6] D. A. Evans, K. A. Woerpel, M. M. Hinman. M. M. F a d . J Ani. Chern. Soc.
1991, 113, 726-728; D. A. Evans, K. A. Woerpel. M. J. Scott, Angew Chem.
1992, 104. 439-441; A n g e x Cliem. I n f . Ed. Engl. 1992, 31. 430-432.
[7] M. P. Doyle, R. E. Austin. A. S. Bailey. M P. Dwyer. A. B. Dyatkin, A. V.
Kalinin, M. M. Y. Kwan, S . Liras. C. J. Oalmann. R. J. Pieters. M. N. Protopopova. C. E. Raab, G. H. P. Roos. Q.-L. Zhou. S. F. Martin. J Am. Chern. Sue.
1995, 11 7, 5163- 5775.
181 A. Pfaltz. Acc. Cltenr. Re.?. 1993, 26, 339-345; P. Muller. C. Baud. D Ene, S.
Motallebi, M. P. Doyle, B. D. Brandes. A. B. Dyatkin, M. M. See, Hell,. Cltim.
A r m 1995. 78, 459-470: H. Nishiyama, Y Itoh. Y Sugawara. H. Matsumoto.
K. Aoki. K. Itoh, Bull. Chem. Soc. Jpn. 1995. 68. 1247-1262.
[9] Product control in [Rh,(SS-mepy),]-catalyzed reactions of 2 and 6 is consistent
with the relative reactivities of olefins and arenes for intramolecular cyclizations
catalyzed by dirhodium(i1) carboxamidates: A. Padwa, D. J. Austin. A. T. Price,
M. A. Semones. M. P. Doyle, M. N. Protopopova. W. R. Winchester, A. Tran.
J. Am. Chem. So?. 1993. tf5.8669-8680.
What is the Structure of the Calmodulin
Antagonist Konbamide from Theonella sp.?
Synthesis of Two Isomers by Direct Biomimetic
Introduction of Bromine in HydroxytryptophanContaining Cyclic Peptides""
Ulrich Schmidt* and Steffen Weinbrenner
Many marine plants and animals store halogens. Their peptidic components often include halogenated tyrosine"] and
tryptophan. For example, the nonribosomal amino acid 6-bromotryptophan is incorporated into clionamide['] and celenamide,f3] which are isolated from algae. Similarly, the cyclic
natural product jaspamide'" contains 2-bromo-N-methyltryptophan as a ring element. 2-Bromo-5-hydroxytryptophan is a
[*] Prof. Dr. U . Schmidt. Dr. S. Weinbrenner
Institut fur Organische Chemie und Isotopenforschung der Universitit
Pfaffenwdldring 55, D-70569 Stuttgart (Germany)
Fax: Int. code +(711)685-4321
[*'I Amino Acids and Peptides, Part 101.This work was supported by the Deutsche
Forschungsgemeinschaft and by the Fonds der Chemischen Industrie.
Part 100: U. Schmidt, C. Braun. H Sutorts. Synfhesrs 1995. 223.
1336
0 VCH
VerlugsgeseNschujtm h H , 0.69451 Wemherm, 1996
component of the cyclic peptides konbamide,['] orbiculamide A,r61and keramamides B-D,[71 which are isolated from
the sea sponge Theonella, and we have synthesized the halogenated compound by direct bromination of hydroxytryptophan derivatives.['] Both the amino acid and its N-acyl derivatives are unstable and therefore cannot be used in natural
product synthesis.
In the biosynthesis of bromine-containing peptides the brornination of the peptide in the last step is a subject of debate; even
electrophilic. nonenzymatic substitution by bromine (formed
from oxidation of Br- ions, perhaps by symbiotic algae) seems
plausible. In the following, we describe our experiments on the
biomimetic synthesis of konbamide by electrophilic bromination of a preformed cyclic peptide. Konbamide has structural
characteristics (Scheme 1) that complicate its synthesis: the sensitive 2-bromo-5-hydroxytryptophan, lysine in which the Eamino group is a ring member, and the urea side chain.
Leu'
MeLeu
(5)
I
Ala
Leu2
Scheme 1. Postulated structure of konbamide. Strategic bonds in retrosynthetic
analysis are marked with arrows.
Intramolecular acylation at the alanine (position 1) or leucine
(position 2) nitrogen atoms were both possibilities for the ring
closure. Both positions seem to be unhindered and both activated alanine and lysine are quite configurationally stable. Based
on our experience in the synthesis of g l i d ~ b a k t i n , ' ring
~ ] closure
at the E-amino group of lysine (position 3) is not promising.
Ring closure at the hydroxytryptophan (position 4) or Nmethylleucine (position 5 ) nitrogen atoms was ruled out because
of the difficulty in acylating secondary nitrogen centers and the
facile racemization of activated N-methylleucine.
The linear pentapeptide 5, which could undergo ring closure
at the alanine nitrogen atom, was obtained by straightforward
reaction of 2 and 4 (Scheme 2). From this point on, all protective groups had to be compatible with 2-bromo-5-tryptophan,
which was to be formed after ring closure. For protection of the
phenolic OH group we chose a pivaloyl ester, which had already
been used successfully in synthesis of 2-brorno-5-hydroxytryptophan. It prevents 4- and 6-substitution of hydroxytryptophan
during bromination, is conserved during hydrolysis of the
methyl ester 6, and can be cleaved from 11 without destruction
of the sensitive tryptophan functionality.
Ring closure of 8 at position 1 was achieved with the pentafluorophenyl (PFP) ester method. which we developed, in a
biphasic system to produce 9 in 63 % yield. The corresponding
ring closure at the leucine nitrogen atom proceeded in 60%
yield. The ring closure could be performed by the same method
without protection of the phenolic group, but this had a yield of
0570-0833j96/3512-1336 $15.00+ 2510
Angew. Chem. Inr. Ed. Engl. 1996, 35. No. 12
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only 30%. Exchange of the Z-protective group for a trifluoroacetyl (TFA) protective group gave substrate 10, which was
brominated with N-bromosuccinimide (NBS) without a radical
starter or light in a straightforward reaction. The TFA and
pivaloyl protective groups were removed from the resultant bromide 11 by careful alkaline hydrolysis to produce 12. The urea
side chain was formed by using allyl (S)-2-isocyanato-4methylpentanoate[ l o ] (from leucine). Finally, the allyl ester 13
was cleaved with Pd'I"] to afford the acid 14.
Minor differences between the synthetic product and the natural compound were evident by HPLC." 1' Similarly the optical
rotation and the N M R spectra of the synthetic product and the
natural substance were not completely identical. In particular,
there were noticeable differences in the cr-H and N H signal regions (Table 1 ) . I n contrast, the mass spectra were the same.['31
1 R-Boc
b)C
2 R=H.HCI
3 R-Bn
4 R-H
d)C
2+4
Tdble 1. Comparison ofcharacteristic N M R data for natural konbamide [5] and 14.
5
R' -Me, R 2 - Boc. R'- H
6 R'-Me,R2-Bo~,R5-P~~
)'
7 R'
h,
8 R'
= C,Fs.
- C.F,.
RZ * Bac, R'
-
R'
Pi"
-
H, R3
-
Piv
YI
Konbamide
Assignment [5]
4.01 -4.08 (m. 1 H)
4.10-4 18 (m, 1 H)
4.28-4.30 (m. 1 H)
4.61 -4.65 (m. 1 H)
4.71 -4.78 (m, 2 H )
3.93 (rn. 1 H)
4 03 (dd. J = 9.0, 5.0 Hz. 1 H )
4.18 (rn, 1 H)
4.61 (ddd, J=11.3.9.0. 3 . 5 H ~ 1. H )
4.72 (m, 2 H )
6.20 (d, J = 7.9 Hz, 1 H)
6.33 (d. J = 8.3 Hz. 1 H)
7 41 (d. br.. 1 H)
8.26 (d, J = 5.6 Hz. 1 H)
8.43 (d, J = 9.0 Hz. 1 H)
8.84 (d, J = 5.9 Hz. 1 H)
6.26 (d, J = 9.0 Hz. 1 H)
6.42 (d, J =7.4 Hz. 1 H)
7.44 (dd, J =7.8, 3.3 Hz. 1 H )
6.99 (d, J = 5.7 Hz. 1 H )
8.29 (d, J = 9.0 Hz, 1 H)
8.74 (d, J = 6.2 Hz. 1 H )
Lys-x-NH
Leu2-a-H
Ala-a-H
BhTrp-a-H
MeLeu-a-H
Leu -a-H
Leu -NH
Lys-a-NH
Lys-F-NH
Ala-NH
BhTrp-NH
Leu'-NH
One possible explanation is that the natural compound and
the synthetic product are diastereomers. During their investigation of the natural substance by 2D-NMR, FAB-MS analysis,
and total hydrolysis, the Japanese authorsrs1determined that
the amino acids are L-configurated after the hydrolysis, by using
GC and HPLC o n chiral phases. Bromohydroxytryptophan decomposes during hydrolysis and thus its configuration was left
open.
The amino acids resulting from hydrolysis of the synthetic
product also proved to be L-configurated. To exclude a different
configuration at bromohydroxytryptophan as the reason for the
difference between the synthetic product and the natural substance, we also synthesized the diastereomer of 14 with D-bromohydroxytryptophan. However, the HPLC retention time of
this product also deviated from that of the natural Substance.
Thus the proposed structure of konbamide cannot be correct.
HOWX,.
I
14
1
Received: February 21. 1996 [28844IE]
German version: Angm.. Chem. 1996, 108. 1432-1433
NH
H
H N A O
A**.Acoon
Keywords: configuration elucidation
products . peptides
.
konbamide
natural
13 R = A I I
1
1
4 R-H
Scheme 2. Synthesis of "konbdmide" 14. a) THF. Z(H, HCl)-(S)-Lys-OMe. BopCI. Et2NH. 0 C. I6h. 80%: b) HCI, dioxane. room temperature (RT); 2h;
c) CHZCI2.Boc-(S)-Ala-OH. TBTU, EtiNH, 0°C. 3 h, 88%. d) MeOH, Pd/C/H,.
RT, 3h. 9 8 % . e ) DMF, 2-(diphenylphosphoryI)azide, - 20°C. 48h, 86%.
f ) CH,CI,. PivOCI. Et,N. RT, 3h. 97%; g) dioxane, H 2 0 . NaOH, RT. 97%.
(EDC), PFP, - 20'C
CH,CI,, I-ethyl-3-(3-dirnethylaminopropyl)carbodiimide
RT. 16h; h) HCI, dioxane, RT, 2 h ; j ) CHCI,. NaHCO,, RT, 5 h ; g)-h): 6 3 % ;
k) MeOH. Pd;C:H,. RT. 16h; THF. TFA anhydride, RT, 16h. 93%; 1) CH,CI,.
NBS, RT. 30 min. 7 5 % . m) MeOH. H,O. NaOH. RT, 24h; n) T H E ally1 (S)-'-isoc y ~ n d t O - 4 - ~ e t h y ~ p e ~ l RT,
~ ~ o24h;m)-n):
dle.
40%:o)THF. [Pd(PPh,)J, RT, 5 h.
75%.
-
A I I ~ ( ' w( '. I ~ c I I I . I n t . E d EnxI. 1996. 35. N u . 12
[I] W. R. Chan, W. F. Tinto, P. S . Mauchaud, L. J. Todaro, J. Org. Chem. 1987,52,
3091 ; N. K. Guldvita. A. E. Wright. P. J. McCarthy. S A. Pornponi. M. KellyBorges. J. N o t . Prod. 1993, 56, 1613.
[2] Isolation and structure determination: R. J. Andersen. R. J. Stonard. Can J.
Cliem. 1979.57.2325: Synthesis: U. Schmidt, A. Lieberknecht, H. Griesser. H.
Bokens, Tetruhedrun Lrrt. 1982, 173. 491 1.
[3] Isolation and structure determination: R. J. Andersen. R. J Stonard, Con. J
C k m 1980. 58. 2121 ; Synthesis: U. Schmidt. J. Wild, Angeii.. CIwn. 1984, 96.
996; Angeu.. Chem. Int. Ed. Engl. 1984, 23, 991.
[4] Isolation and structure determination. T. M. Zabrisky, J. A Klocke. C. M.
Ireland. A. H Marens, T. F. Molinsky, D. J. Faulkner, C. Xu. J C. Clardy. J
A m . Chrm. Suc. 1986. fO8,3123; P Crews, L. V. Manes, M Boehler, Tetruhcdrun Lett. 1986, -77. 2797; Synthesis: P. A. Grieco. Y. S . Hon, A. J. Perez-
c? VCH &dugsgesellschuf!ftmhH. 0.69451
Wernlieinl. 1996
057O-0833/96/3512-1337$ 15 O O i 2 5 0
1337
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Medrano. J Am. Clrum. Soc. 1988. I I O , 1630: K. S. Chu, C. R. Negrete. J. P.
Konopelsky. ihid. 1991, 1 / 3 , 5196; R. Rao, M. K. Gujar, B. R. Nallagandu,
A Bhandari, TL.!rahedron LPI!.1993. 34, 7085.
[S] J. Kobayashi, M. Sato, T. Murayama. M. Ishibashi. M. R. Wilchi. M Kanai.
J. Shoji, Y. Ohizumi, J. CAefn. Soc. C h ~ m Comfnim.
.
1991. 1050
161 N. Fusetani. T. Sugawara. S. Matsunaga. J. Am. C h i r Sot. 1991. / / 3 , 7811.
171 J. Kobayashi, F. Itagaki, H . Shigemori, M. Ishabashi. K. Takahashi. M. Ogura.
S. Nagasdwd, T Nakamura, H. Hirota, T. Ohta, S. Nozoe, J A m . C k m So(..
1991, 113, 7812
[8] U. Schmidt, S. Weinbrenner, SJJ~I~CJ.T;.T
1996, 28.
[9] U Schmidt, A. Kleefeldt, R. Mangold, J Cheni. Soc. Chern Commun. 1992.
1681.
[lo] Synthesis analogous to that of the methyl ester: J. S. Nowick, N. A Powell.
T. M. Nguyen, G. Noronha. J. Org. Clreni 1992,57. 7364.
1111 H. Kunz, H. Waldmann, AII~FII..
Clrem. 1984, Y6. 49: Angtw Clienr. In! E d
€ng/. 1984, 23, 71.
[I21 We thank Prof. J. Kobayashi for providing a sample of the natural substance
[I31 The measurements were kindly performed by Dr. K. Eckart at the MaxPlanck-Institut fur experimentelle Medizin in Gottinpen.
This type of trinuclear complex possessing two highly polar
metal-metal bonds is particularly rare; the only fully characterized and studied example is Casey's ZrRu, complex
[Cp,Zr{Ru(CO),Cp),] (Cp = C5H5).[41More recently, Palyi
et al. have given a preliminary account of the generation of
[Cp,Zr(Co(CO),),] by salt metathesis or alkane elimination.[51
Based on early work by Burger et
we synthesized
the difunctional amidozirconium complexes of the type
[CH,(CH2NSiMe,),ZrC1,(D)2] (D = thf, pyridine),"] which
have proved to be ideally suited for the synthesis of such trinuclear complexes. Reaction of the complex [CH,(CH,NSiMe,),ZrCl,(thf),] 1 with two molar equivalents of the carbonyl metalates K[CpM(CO),] (M = Fe, Ru) and Na[Co(CO),(PPh,)]
yields the corresponding trinuclear ZrFe,, ZrRu,, and ZrCo,
compounds 2-4 (Scheme 2). While both 2 and 3 are stable in
TH F
A Chelate-Amidozirconium Fragment as
Building Block for Unsupported Trinuclear
ZrM, Heterobimetallic Complexes
(M = Fe, Ru, Co)""
Stefan Friedrich, Lutz H. Gade,* Ian J. Scowen, and
Mary McPartlin
With the recent development of halide complexes with tripodal amido ligands, monofunctional building blocks for the generation of directly metal - metal bonded M'- M heterobimetallic
compounds (M' = Ti, Zr, Hf; M = late transition metal) of unprecedented stability have been made available[*,'I and have
enabled a systematic investigation into their chemical reactivit ~ . [We
~ ]have now extended this strategy to the development of
difunctional building blocks (Scheme I), which may be employed in the synthesis of stable trinuclear complexes containing
two equivalent Zr-M bonds, thus opening up the possibility of
extending the cooperative reactivity of early -late heterobimetallic compounds to involve three complex fragments.
2
3
Scheme 2. Synthesis of the ZrM, heterobimetallic complexes 2 4
solution, the ZrCo, complex 4 slowly decomposes generating
[Co(CO),(PPh,)], and an intractable Zr-containing material.
The existence of metal-metal bonds in 2-4 was initially deduced from the IR spectra, in which the v(C0) bands are shifted
to higher wavenumbers relative to those of the alkali metal salts
of the anions (Table I). It is interesting to compare the position
of the asymmetric l2CO stretching vibration of 3 with that of
[ C ~ , Z ~ { R U ( C O ) , C ~ )For
, ] . ~3 ~AV(v(CO),,)
~
was found to be
97 cm- relative to the free carbonyl metalate [V(v(CO,,)) =
1896,1811 cm- '1, while the value for Casey's ZrRu, compound
is only 71 crn-'.['l The greater Lewis acidity of the amidozirconium fragment in comparison to that of the Cp,Zr unit apparently induces a higher degree of charge redistribution from the
late transition metal to the early transition metal through the
metal-metal bond, which therefore assumes a more covalent
character.
This difference in the IR spectroscopic data should be reflected in the molecular structures of the compounds. To establish
such a relationship, and in order to structurally characterize a
ZrFe, species for the first time, single-crystal X-ray structure
analyses of 2 and 3 were carried out (Fig.
The compounds
are isomorphous and their gross features are identical. In each,
the central Zr atom adopts a distorted terahedral coordination geometry (2: N(l)-Zr-N(2) = 97.5(3), Fe(l)-Zr-Fe(2) =
116.2(1)"; 3: N(1)-Zr-N(2) = 97.6(2), Ru(l)-Zr-Ru(2) =
115.84(2)") and links to iron or ruthenium complex fragments through direct unsupported metal-metal bonds.
'
Scheme 1. Relationship between monofunctional and difunctional amidohalide
complexes of the metals of the titanium triad used as building blocks for heteronuclear compounds (*). Formal removal of an anionic "amido-arm" in a tridentate
ligand.
['I Dr. L. H. Gade, S . Friedrich
Institut fur Anorganische Chemie der Universitlt
Am Hubland, D-97074 Wiirzburg (Germany)
Fax: Int. code +(931)888-4605
Dr. 1. J. Scowen, Prof. M. McPartlin
School of Applied Chemistry, University of North London
Holloway Road, London N7 8DB (UK)
['"I This work was supported by the Deutsche Forschungsgemeinschaft (L.H . G.,
S. F.),the Engineering and Physical Science Research Council (M. McP ,
I. J. S), the Deutscher Akademischer Austauschdienst, and the British Council
(ARC grant). We thank Professor Werner for his support and Degussa AG for
a gift of basic chemicals.
1338
;f.
VCH Vrr/ug,sgesellrchafim h H , 0-69451 Wemhrinl, I Y Y 6
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Angmr Cliem I n ! . E d €nnl. 1996, 35. No. 12