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Biosynthetic Origin of the Oxygen Atoms of Tetracenomycin C.

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NMR (75 MHz. CDCI,): 6 =148.18. 144.78, 124.81, 43.92, 16.97, 2.62. HRMS Calcd for C,,H,,Si,: 774.3780; found: 774.3779.5: colorless crystals, m.p.
180-182 C (decomp.); IR (KBr): 3 =1613. 1512, 1495. 1463, 1430, 1339,
1261. 1154. 1094. 1003. 918, 861.802. 751 cm-': MS (70eV): mi: 348 ( M ' ,
11).316(8).261(10).248(9). 176(21). 175(100). 174(11), 156(61). 135(22),
127 (20). 123 (30), 121 (22), 107 (18). 85 (11). 73 (38); ' H N M R (400MHz,
CDCI,): ;i =7.46 (AAm. 6H), 7.38 (BB'm. 6H): I3C{'H) N M R (100 MHz,
CDCI,): iS =143.71. 131.50, 122.64, 76.25.
a ) A. de Meijcre in Cuge Hydrocurbuns (Ed.: G. A. Olah), Wiley. New York,
1990. p. 261. h) H. Prinzbach. D. Stusche. Angew. Chem. 1970,82,836: Angrvv.
Chwn. Inr. Ed. Engl. 1970, 9. 799; c) M. Engelhard. W. Liittke. ihid. 1972.84.
346 and 1972.11,310; d ) H. Prinzbach. R. Schwesinger, ibid. 1972.84.988 and
1972. 11, 940: e) P. Binger, G. Schroth, J. McMeeking. did. 1974,86. 518 and
1974, 13,465: f) A. de Meijere, D. Kaufmann, 0. Schallner, ibid. 1971.83. 404
and 1971. 10,417; TWuhedron Lctr. 1973,553; g) P. Binger, J. McMeeking, U.
Schuchardl. Chcvn. Ber. 1980, 113. 2372; h) D. Kaufmann, H.-H. Fick, 0.
Schallner. W. Spielmann. L:U. Meyer. P. Golitz. A. de Meijere. ibid. 1983, 116,
5x7; W. Sptelmann. H:H. Fick. L.-U. Meyer, A. de Meijere, Terruhrdron L e t / .
1976, 4057; i) C. Rucker, H. Prinzbach. ihid. 1983, 24, 4099: j) W.-D.
Braschwitz. T. Otten, C. Rucker, H. Fritz. H. Prinzbach, Angew. Chem. 1989,
I O l , 1383; Angew. Chew. In/. Ed. Engl. 1989, 28. 1348.
H . - 0 . Kalinowski. S. Berger, S. Braun, ' 3 C N M R Spekrroskopie. Thieme,
Stuttgart. 1984.
Crystal si7e 0.21 x 0.28 x 0.50 mm'. space group P2,/fi, 20 scan range 3-45".
u = 11.6696(20) A, h = 21.345(7) A, c = 20.933 (4) A, a = 90.0. fi =
93.132(13).;=90.0. V = 5206.4(36)A3,2=4,pcaIcd
= 1 . 0 6 g ~ m - ~ ,=1.9
p
cm- I, 6784 unique reflectlons collected at 170 K, of which 4098 were taken as
observed IF2 3u(F2)], R = 0.0514, R , = 0.0580. Hydrogen atoms were assigned idealized positions, adjusted based on residual peaks in the difference
Fourier map during the last cycles of refinement 1231.
C. Kruger. P. J. Roberts. C r w . Strucr. Commun. 1974, 3, 459.
a) C. Kabuto, M. Ydgihara. T. Asdo, Y. Kitahard. Angew. Chem. 1973,85,860;
Angrw. ( % r m Inl. Ed. Engl. 1973. 12, 836; b) W. Littke, U. Driick, ibid. 1974,
86. 557 and 1974, 13, 539; c) E. Vogel, A. Breuer, C.-D. Sornmerfeld, R. E.
Davis, L:K. Liu, ihid. 1977.89, 175 and 1977, 16, 169; d) I. Erden. P. Golitz,
R. Niider. A. de Meijere. ;bid. 1981, 93, 605 and 1981, 20, 583; e) R.
Schwesinger. K. Piontek. W. Littke, 0. Schweikert, H. Prinzbach, C. Kriiger.
Y-H. Tsay. 7i~rruhedronL e t / . 1982. 23, 2427; f) G. McMullen, M. Lutterbeck,
H. Fritz. H. Prinzbdch, C. Kriiger, Isr. J Chem. 1982, 22. 19; g) R.
Schwesinger. K. Piontek, W. Littke, H. Prinzbdch. Angew. Chem. 1985,97,344;
Angew. C%rm. I n f . Ed. Engl. 1985, 24, 318.
R. Boese. D. Bliiser, W. E. Billups, M. M. Haley. A. H. Maulitz, D. L. Mohler,
K. P. C. Vollhdrdt. A n g w . Chem. 1994, 106, 321: Angew. Chem. I n / . Ed. Engl.
1994.33. 313
K. P. Moder. E. N. Duesler, N. J. Leonard, A c / n Cr.vsrallogr. Sect. B 1981.37,
289.
R. Boese. D. Bliser, Angew. Chem. 1988,100.293; Angew. Chem. Inr. Ed. Engl.
1988.27, 304.
J. Spanget-Larsen, R. Gleiter. Angrn. Cliem. 1978. 90. 471; Angew. Chem. I n t .
~ d Engi.
.
1978, 17,441
B. Zipperer. K.-H. Miiller, B. Gallenkamp, R. Hildebrand, M. Fletschinger, D.
Burger. M . Pillat, D. Hunkler, L. Knothe, H. Fritz, H. Prinzbach. Chem. Err.
198n,121,i57.
a ) S. W. Benson, Thermuchemicul Kinetics, 2nd ed., Wiley-Interscience, New
York. 1976; b) J. M. Schulman, R. L. Disch, J Am. Chem. Soc. 1993, l l S ,
11153; c ) 0. V. Dorofeeva. Thermochim. Acru 1992, 194, 9.
R . Jeyaraman. R. W. Murray, J. Am. Chem. Soc. 1984.106,2462; W. Adam, R.
Curci, J. 0. Edwards. Acr. Chem. Rrs. 1989, 22. 205.
a) R . Schwesinger, H. Fritz, H. Prinzbach, Chem. Ber. 1979. 112, 3318; b) E.
Vogel. H.-J. Altenbdch. C.-D. Sommerfeld, Angrw. Chem. 1972, 84. 986;
Angew. Chem. Int. Ed. Engl. 1972, 11, 939; c) M. Stobbe, U. Behrens, G.
Adiwidjaja. P. Golitz. A. de Meijere. ;bid. 1983. 95, 904 and 1983, 22, 867;
A n j i w i . Chmi. Suppl. 1983, 1221.
a) J. Kammerer. G. Rihs. H. Prinzbach, Angekv. Chem. 1990,102.1087: Angrw.
Chem. Inr. Ed. EnjiI. 1990,29, 1038: b) N. R. Easton, Jr.; F. A. L. Anet, P. A.
Burns. C. S. Foote. J Am. Chem. Sor.. 1974. 96, 3945.
Crystal size 0.18 x 0.22 x 0.38 mm3, space group P 2 , h 20 scan range 3-45".
u =7.9135(9) A. h =17.8413(23) A, c =12.4051(16) A, a = 90.0. fi =
102.593(10),~=90.0". V=1709.3(7)A3, Z = 4 . pcaIc,=1.35 g c m - . f i = 0 . 8
urn-'. 2229 unique reflections collected at room temperature, of which 1670
were taken 21s observed [FZ>3u(F2)], R = 0.0292, R, = 0.0374. Hydrogen
atoms were assigned idealized positions, adjusted based on residual peaks in
the difference Fourier map during the last cycles of refinement 1231.
Further details of the crystal structure analysis may be obtained from the
Fachinformationszentrum Karlsruhe. D-76344 Eggenstein-Leopoldshafen
( F R G ) on quoting the depository number CSD-58663.
Biosynthetic Origin of the Oxygen Atoms of
Tetracenomycin C**
Gyorgyi Udvarnoki, Christina Wagner, Reinhard
Machinek, and Jurgen Rohr*
Dedicated to Professor Wolfgang Liittke
on the occasion of his 75th birthday
There is currently great interest in the details of the polyketide
biosynthetic pathway, one of nature's most important routes for
forming a diversity of bioactive cornpounds.['l For the past few
years we have focused our attention on polycyclic, aromatic
polyketide~,['-~]so-called type I1 p01yketides.I~~
Recent investigations of the sources of oxygen in the metabolites have provided novel insights, for example, unexpected deoxygenation steps
before aromatization during the general assembly of these
p~lyketides.[~I
The tetracenomycins (e.g. tetracenomycin C (I))
and elloramycins (e.g. elloramycin A (2)) form a small, distinct
group of tetracyclic, aromatic polyketides whose biosyntheses
have been extensively studied.16- 'I Nevertheless, the oxygenation reactions, the last steps of the biosynthesis of tetracenomycin C (1)19-121 from its immediate precursor tetracenomycin A, (3), are not well understood and are still subjects of
debate. We describe here the biosynthetic sources of all the oxygen atoms of tetracenomycin C (1) and, to our knowledge, the
first experimental proof in biosynthetic studies of an oxygen
atom deriving from H,O.
CH
OCH,
1 R=H
4
R=CH,
3
R=CH,
5
R=H
In a study using an I80,-enriched atmosphere Rickards et
al.['*] showed that only three oxygen atoms in tetracenomycin X
(4), a close relative of 1 produced by Nocardia mediterranea,
derive from molecular oxygen, namely 0-4, 0-5, and 0-12a.
These findings seemed to be in contradiction to the general
biosynthetic pathway for the tetracenomycins outlined by
Hutchinson et al.[91(see also Scheme 1) and to the results of our
studies with blocked mutants," since the oxygen atom attached to C-4a did not derive from atmospheric oxygen. If this
['I Priv.-Doz. Dr. J. Rohr, DipLIng. G. Udvarnoki, DipILChem. R. Machinek
Institut fur Orgdnische Chemie der Universitat
Tammannstrasse 2, D-37077 Gottingen (Germany)
Telefax: Int. code (551)39-9660
+
Dr. C. Wagner
Hans-Knoll-Institut fur Naturstoff-Forschung e. V
I**]
This work was supported by the Deutsche Forschungsgemeinschaft, the Bundesministerium fur Forschung und Technologie, and the Fonds der Chemischen Industrie.
Angeu. Chem. In!. Ed. Engl. 1995, 34, N o . S
0 VCH
Ver/ugsgrsells~hufrmbH, 0-69451 Weinheim, 1Y9S
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565
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0
oxygen atom is part of the
acetate unit incorporated
there, as one could assume
1
from its position as well
1
as from studies on related tetracycles bearing
similar angular hydroxyl
groups,[3*1 3 ] then the biosynthesis of tetracenomycin C (1) does not proceed via aromatic intermediates such as tetraH
3 CH,COH 0O
p
cenomycinD, (5) and A,
(3).["- 141 Unfortunately,
H,CO
OCH,
Rickards et al. did not resolve this ambiguity by
0, H,@
carrying out additional
feeding experiments, for
1
example with '80-labeled
acetate.
In a first series of experiOCH,
ments to find optimal incorporation conditions,
Scheme 1. Biosynthesis of tetracenowe fed [l-13C]acetate to a
mycin C (1).
growing culture of Streptomyces glaucescens (strain
Tii 49) ,L1 the producer of
tetracenomycin C (l), and obtained sufficient incorporation
rates (Table 1). The signal of the critical carbon atom C-4a, a
broad singlet under standard NMR conditions, was not well
enough resolved for possible detection of an l80upfield shift
after labeling of 4a-OH. Systematic NMR studies with 1 finally
yielded conditions under which C-4a gave rise to a sharp singlet
(acetone as the solvent, T = - 30 "C). After we completed the
unambiguous assigment of the complete 13CNMR spectrum of
1 (A in Table
we approached the consequent feeding experiment with [l-'3C,'80z]acetate. The resulting samples of tetracenomycin C (1) showed l80upfield shifts of the expected
magnitude for the signals of C-I, C-3, C-8, 9-C=0, 9-OCH3,
C-11, and C-12, but clearly not for C-4a (C in Table 1 ) . Thus
4a-OH either derives from molecular oxygen, in contradiction
to the earlier experiment,["] or from water. Atmospheric oxygen has been proved to be the source for similar angular oxygen
atoms in tetracyclines (tetracyclic aromatic polyketides structurally related to 1) .["I Water has never been implicated as a
source of oxygen atoms in studies on polycyclic, aromatic
polyketides.
We examined this second, more attractive possibility by carrying out a fermentation of Streptomyces glaucescens (Tii 49) in
500mL of H,180 (10% "0). The resulting sample of 1[18]
showed an l80upfield shift for the signals of 3-OCH3,C-4a,
8-OCH3, and 9-OCH3. The l80enrichments are about tenfold less than that obtained in the experiment with
[l-'3C,180,]acetate because of the tenfold lower l80enrichment of the labeled precursor (D in Table 1). The upfield shifts
observed for the signals of the OCH, groups are surprising on
first glance, but can be explained by partial I8O exchange at the
stage of the biosynthetic building blocks (acetate) and/or intermediates (e.g. tetracenomycin D, (3)). Nevertheless, this I8O
exchange is insignificant: the shifts are observable only for the
signals of OCH, groups, which are much more intensive in the
I3C NMR spectrum than those of quarternary carbons. The
oxygen atom linked at the quarternary C-4a, however, seems to
stem directly from the 180-labeled water, because " 0 exchange
is unlikely at this position. The " 0 enrichment of 4a-OH result10 H,C-bH
I
566
0 VCH
Verlagsgesellschafi mbH. 0-69451 Weinheim. 1995
Table 1. A: "C NMR data for tetracenomycin C (1); 6 values relative to TMS in
(DJacetone, -30 'C, 125.7 MHr, multiplicities from APT (attached proton test)
and C,H-COSY experiments [a]. B: Specific incorporation rates [b] from the feeding
experiment with [I-"Clacetate. C: Upfield shifts (A6 'nC-180)in 1 obtained by
feeding[1-13C,1802]a~etate.
D: Upfield shifts (AS ''C-'"O) in 1 obtained from the
fermentation in H,'*O ('*O enrichments in % [c]).
A
c-1
C-2
c-3
3-OCH3
c-4
C-4a
c-5
C-5a
C-6
C-6a
c-1
C-8
8-0CH3
c-9
9-c=o
9-OCH3
c-10
10-CH,
C-lOa
c - 1I
C-lla
c-12
C-12a
190.8 s
99.1 d
175.3 s
57.2 q
70.3 d
85.2 s
194.1 s
141.2 s
120.7 d
129.0 s
108.4 d
158.3 s
56.6 q
129.6 s
168.0 s
52.8q
137.9 s
21.1 q
121.0s
167.5 s
109.8 s
197.9s
83.6 s
B
C
10.5
0.05 (53)
-
-
8.5
0.02 (54)
-
-
-
-
11.3
-
-
5.5
-
-
-
5.7
-
-
-
7.5
-
11.8
-
5.2
-
D
0.02 (53)
-
0.04 (32)
0.01 (25)
-
6.3
0.01 (47)
-
-
10.1
0.04 (50)
-
-
[a] The
signals were further assigned through long-range C.H couplings
(COLOC, HMBC) [25]. [b] Incorporation rates (+_I %) calculated according to
Townsend, Scott, et al. [26]. [c] '"0 Enrichment = [ ( I , 3 c ~ , ~ , / I , 160
,, +
4x
100%.
ing from the experiment with the labeled water is by far the
highest (D in Table 1).
The findings described here (Scheme 1) are in agreement with
all the results from earlier biosynthetic investigations on the
tetracenomycins.[8- The studies were also aimed at comparing oxygenation steps in the biosynthesis of the tetracenomycins
with those leading to other tetracyclic decaketides such as angucyclines and anthracyclines. Tetracenomycin C (1) is the first
example among the polycyclic, aromatic polyketides requiring a
molecule of H,O to obtain its oxygen pattern. Thus, a novel
enzymatic feature is implied, which is interesting for approaches
to hybrid antibiotics through combination of the biosynthetic
genes of the tetracenomycin producer S . glaucescens with related polyketide synthases of other producers of polycyclic, aromatic polyketides.[' 99 For the oxygenation in the biosynthetic
conversion of 3 into 1, an epoxide intermediate such as 6
(Scheme 2) has to be assumed. It is unclear whether 6 is formed
by two monooxygenases or only one dioxygenase; the latter
possibility is in agreement with the suggested mechanisms found
in oxygenation studies of aromatic
as well as
those proposed for the mode of action of vitamin K.[9c.21b-e1
This epoxide intermediate, however, would require an interesting, enzymatically controlled, but chemically unlikely "cis opening" to generate the stereocenters found in 1.16]Alternatively,
the overall cis opening of the epoxide may result from an initial
attack of an enzyme-bound SH group, which is then replaced by
water in a SN2-typereaction (Scheme 2).["]
Experimental Procedure
Cultivation of the microorganism: Strrptomyces glaucescens (TU 49) was cultivated
on slants (20% agar-agar, 10% malt extract, 4 % glucose, 4 % yeast extract, pH 7.0.
adjusted before autoclaving) at 28 "C until sporulation occurred, then stored at 4 "C.
The culture for the fermentations was innoculated directly from the slants and
OS70-0833195jOSOS-OS66$10.00+ .25j0
Angew. Chem. Int. Ed. Engl. 1995, 34, No. 5
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-
I-
j
V
I
H
H-02
1
f S-Enzvme
HS-Enzyme
6
1
Scheme 2. Details of the oxygenation in the biosynthetic conversion of 3 into 1.
shaken on a rotary shaker (type Multitron, Infors AG, Switzerland) in ten 250 mL.
triply baffled Erlenmeyer flasks, each filled with 100 mL of R2YENG medium 1231
for 96 h at 30 "C and 250 rpm.
Isolation of tetracenomycin C (1): The combined cultures were treated with 300 mL
of MeOH and celite and filtered. The combined culture filtrates were extracted with
1 L of ethyl acetate and the extract concentrated to dryness. The residue was dissolved in MeOH and chromatographed on a Sephadex LH 20column (2.5 x 100 cm.
MeOH). Yield ca. 20 mg of 1.
Feeding experiments with labeled precursors: a) Feeding of sodium [l-l3Cfacetate:
The labeled substrate (750 g L-' culture) was dissolved in 75 mL of sterile water and
neutralized with 0 . 1 HCI.
~
One-sixth (12.5 mL) of this solution was distributed
every eight hours (36 h to 76 h after inoculation) among ten Erlenmeyer flasks each
containing 100 mL of the growing culture of S.glarrcescens Tu 49. b) Feeding of
[l-13C.'s0,]acetate (1 gL- ') : As in a); to avoid "0 exchange with the solvent, the
labeled material was added in portions (6 x 166 mg); each portion was dissolved in
10 mL of sterile water directly before the feeding and neutralized immediately with
5~ NaOH. c) Feeding of H,"O: The cultivation (500 mL culture) was carried out
as above using H,"O (10% " 0 ) for the preparation of the medium and as the
cultivation solvent.
Labeled compounds: [l-'3C]Acetate (99% I3C) was obtained from Cambridge
Isotope Laboratories (Cambridge, MA, USA) and H,"0 (97.9% '*O, 10% "0)
from Isotec Inc. (Miamisburg, OH, USA). [1-13C,'80,]Acetate was synthesized
from [1-"Clacetate and H Z i 8 0(97% " 0 ) 1241.
Received: September 22, 1994 [Z 7342IEl
German version: Angew. Chem. 1995, 107, 643
Keywords: biosynthesis . natural products . tetracenomycin C
[I] D. E. Cane. Science 1994, 263, 338-340, and references therein; D. O'Hagan.
The Polyketide Metabolites, 1st ed. Horwood, Chichester, 1991
[21 J. Rohr, J. Org. Chem. 1992. 57, 5217-5223; J. Rohr. M. Schonewolf, G.
Udvarnoki, K. Eckhardt, G. Schumann, C. Wagner, J. M. Beale, S. D. Sorey,
ihrd. 1993, 58. 2547-2551,
[3] G. Udvarnoki, T. Henkel, R. Machinek, J. Rohr, J. Org. Chem. 1992, 57,
1274-1276.
141 H. Bockholt. G. Udvarnoki, J. Rohr. U. Mocek, J. M. Beale, H. G. Floss, J.
Org. Chem. 1994,59.2064-2069; S. Weber, C. Zolke, J. Rohr, J. M. Beale. ibid.
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[91 a) S. Yue. H. Motamedi, E. Wendt-Pienkowski, C. R. Hutchinson, J. Bacteriol.
1986, 167. 581-586; b) H. Motamedi, E. Wendt-Pienkowski. C. R. Hutchinson, ibid. 1986.167, 575-580; c) H. Decker, H. Motamedi, C. R. Hutchinson,
ihid. 1993, 175. 3876-3886.
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Angen. Chem. Int. Ed. Engl. 1995, 34, No. 5
6
[12] M. G. Anderson, C. L.-Y Khoo, R. W. Rickards, J Antihiot. 1989, 42. 640643.
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Chem. Soc. Chem. Commun. 1982, 902-904.
1141 These studies were based on sometimes ambiguous cosynthesis experiments
with various blocked mutants of the tetracenomycin producer.
[15] We wish to thank Prof. Dr. H. Zahner, University of Thbingen, Germany. for
providing us with the tetracenomycin producer S t w p o m w e s glaucescens
(strain TU 49).
[I61 A combination of homo- and heteronudedr (correlation) experiments
('H NMR; " C NMR; C.H-COSY; COLOC [25a] = correlation spectroscopy
via long-range couplings; HMBC 125 b] = hetero multiple bond connectivity
spectroscopy) on I and its derivatives gave unambiguous results; earlier 13C
NMR assignments [6,7] are in part incorrect.
1171 L. A. Mitscher, J. K. Swayze, T. Hogberg, I. Khanna. G. S. Raghav Rao, J.
Antibiot. 1983.36.1405- 1407; R. Thomas, D. J. Williams.J. Cbem. Soc. Chem.
Commun. 1985, 802-803.
[IS] Since the homogeneity of the magnetic field used in the NMR experiments was
stable for only 1 h at - 30 "C, five 1 h experiments were added to obtain a better
signal-to-noise relationship.
1191 We are currently examining this in cooperation with Dr. H. Decker, University
of Tubingen, Germany.
1201 a) L. Katz, S . Donadio, Annu. Rev. Microhid. 1993, 47, 875-912. and references therein; b) R. McDaniel, s. Ehert-Khosla. D. A. Hopwood, C. Khosla,
Science 1993,262. 1546-1550; e) R. McDaniel, S. Ebert-Khosla, D. A. Hopwood, C. Khosla, J. Am. Chem. Soc. 1993, 115, 11 671 - 11 675; d) H. Fu, s.
Ebert-Khosla, D. A. Hopwood, C. Khosla, ibid. 1994, 116. 4166-4170.
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1992,57, 3514-3516.
I
Am. Chem. Soc. 1981, 103. 5939-5941 ; b) P. C. Preusch,
1221 a) R. B. Silverman, .
J. W. Suttie, J. Org. Chem. 1983, 48, 3301-3305.
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ATh,,N,X,, (A = Li.. .Rb; X = C1, Br):
A New Type of Thorium Cluster with a
ThlzN6 Core
Thomas P. Braun, Arndt Simon,* Fred Bottcher, and
Fumio Ueno
The reduced halides of the rare earth elements exhibit a rich
chemistry, in which discrete or condensed metal clusters play a
major role.[', Primarily the 5d electrons are involved in
metal-metal bonds in the clusters; the 4f electrons are much
lower in energy. The possible participation o f f electrons in the
formation of metal-metal bonds is still in question. One prerequisite for this is significantly higher energies, as found in the 5 f
metals. We therefore started experiments to prepare reduced
thorium halides as a "window towards the actinoids". Like the
related element zirconium, thorium forms cluster comp o u n d ~-[ ~if interstitial heteroatoms are allowed for stabilization. We have already reported on thorium cluster compounds
with an octahedral Th,Br,, unit,[6.'I and we now describe compounds with the new cluster framework Th,,Br,,.
[*I Prof. Dr. A. Simon, Dr. T. P. Braun, Dr. F. Bottcher, Dr. F. Ueno
Max-Planck-Institut fur Festkorperforschung
Heisenbergstrasse 1, D-70569 Stuttgart (Germany)
Telefax: Int. code + (711)689-1642
VCH Verlugsgesellschufi mbH, 0-69451 Weinheim, 1995
0570-0833/95/050S-O5673 10.00 + ,2510
567
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