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Base-Free Cationic 14-Electron Titanium and Zirconium Alkyls In situ Generation Solution Structures and Olefin Polymerization Activity.

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- 2.72(q), 133.41(d), 133.45(d), 137.54(s), 141.29(s).MS: m / z ( % ) . 484(4,
Me), 367 (5, M @ - 2 Me,Si-H), 309 (10, Me-3 Me,Si-H), 291 (41,
Mm-2 Me,Si-C,H,), 233 (100, Me-3 Me,Si-C,H,-H).
[6] Addition of crown ether has been reported to increase the reactivity of the
silyl anion by forming a crown ether-sodium ion complex; A. Sekiguchi,
T. Yatabe, C. Kabuto, H. Sakurai, Angew. Chem. f O 1 (1989) 778; Angew.
Chem. Int. Ed. Engl. 28 (1989) 757; M. FuJino, H. Isaka, J. Chem. SOC.,
Chem. Commun. 1989,466.
171 A single crystal with dimensions of 0.5 x 0.2 x 0.2 mm, obtained from
ethanol was used for the X-ray analysis. Lattice constants were determined
by the least-squares method with an angular setting of 25 reflections within
the range 11 < 6 < 13". Intensity data were obtained on a Enraf-Nonius
CAD4 diffractometer equipped with graphite monochromatized Mo,, radiation and using thew-28 scan technique (26 < 50"). Duringdata collection three standards, measured every 120 min, indicated a total loss in
intensity of 18.4%. A h e a r decay correction was applied. Of 3158 independent reflections measured, only 2298 were considered as observed on
the basis of the criterion I > 3.0 o(1). All intensities were corrected for
Lorentz and polarization effects but not for absorption. Crystal data for
1: M , = 483.19; orthorhombic; P,,,; a = 15.037(3), b = 9.029(3), c =
23.231(5) A; V = 3154.3 A3; 2 = 4;
= 1.02 g ~ m - p
~ ;(Mo~.)=
3.0 cm-I. The structure of 1 was solved by direct methods and refined by
full-matrix least-squares methods. The final refinements with anisotropic
temperature factors for the non-hydrogen atoms lowered Rand R, values
to 0.059 and 0.072, respectively.
H. Sakurai, S. Hoshi, A. Kamiya, A. Hosomi, C. Kabuto, Chem. Lett.
f986,1781.
C. Elschenbroich, J. Hurley, W. Mass, B. Baum. Angew. Chem. 100 (1988)
727; Angew. Chem. Int. Ed. Engl. 27 (1988) 684.
a) N. L. Allinger, T. J. Walter, M. G. Newton, J. Am. Chem. SOC.96 (1974)
4588; b) M. G. Newton, T. J. Walter, N. L. Allinger, zbid. 95 (1973) 5652.
Base-Free Cationic 14-Electron Titanium and
Zirconium Alkyls: In situ Generation, Solution
Structures, and Olefin Polymerization Activity **
By Manfred Bochmann,* Andrew J. Jaggar,
and Julian C.Nicholls
Various lines of evidence suggest that cationic group IV
metal alkyl complexes of the type [Cp,M - R]@(M = Ti, Zr,
Hf) are the catalytically active species in homogeneous
metallocene-based Ziegler -Natta polymerization systems."]
We have earlier reported the synthesis of a number of ligandstabilized cationic titanium complexesc2'and, with the aid of
model compounds, have demonstrated the mechanistic pathway which is available to unsaturated substrates capable of
insertion into the Ti< ci bond.t31In independent work it was
shown that cationic alkyl complexes that contain weakly
coordinating ligands such as THFt4]and base-free zwitterionic compounds such as [Cp~Zr-C,H,BPh31cS1polymerize
ethylene in the absence of aluminum alkyl activators, sometimes requiring elevated temperatures and pressures.[51
While these findings are highly suggestive of the catalytic
role played by cationic metal alkyls, no information has as
yet been available about the structure of such species under
the conditions of polymerization catalysis. We now report
the generation in solution of some cationic titanium and
zirconium alkyls and the observation of multiple ethylene
insertion into the Ti-CH, bond.
Treatment of a dichloromethane solution of bis(indeny1)dimethyltitanium (la) with one equivalent of dimeth["I
[**I
780
Dr. M Bochmann, A. J. Jaggar, Dr. J. C. Nicholls
School of Chemical Sciences
University of East Anglia
Norwich NR4 7TJ (UK)
This work was supported by the Science and Engineering Research Council (UK). We thank Dr J. A . Daniels (ICI Wilton) for gel permeation
chromatography measurements.
0 V C H VerlagsgeseNschaftm b H , 0-6940
Weinheim, 1990
ylanilinium tetraphenylborate (2a) gives methane and the
deep brown cation of 3a, [Ind,TiMe]@(Scheme 1). The reaction can be conveniently followed by high-field NMR spectroscopy (see Experimental Procedure). The methyl signal of
the starting material l a (6 = - 0.94) is replaced by a singlet
at 6 = -0.15; the 13CNMR signal for the methyl group of
3a at 6 = 70.8 lies within the range expected for cationic
titanium alkyl
+
[R,MMe,]
la, R
Ib, R
=
Ind, M = Ti
M
lc, R = Cp, M
= Cp,
[*.,Ti
= Ti
= Zr
]t:{
:
[R3NH]BPh,
-CH41-NR3
Za, R3N = PhNMe,
2b, R;N = nBu,N
BPh,
[R,M-MeIBPh,
CH,Ci,
3a/3a', R = Ind, M = Ti
3b, R = Cp, M = TI
3c/3c', R = Cp, M = Zr
t
Et,O
- 3a C,H4
4
[Ind,Ti(CH,CH,)nMe]e
4
Scheme 1. Ind = indenyl, Cp
=
cyclopentadienyl.
By an analogous procedure, deep orange [Cp,TiMe]BPh,
(3b) and pale yeIlow [Cp,ZrMe]BPh, (3c) are synthesized
in dichloromethane solution by allowing l b and lc, respectively, to react with [PhNMe,H]BPh, (2a). The reaction
of l a , or l c with [nBu,NH]BPh, (2b) proceeds equally
facilely to give 3a and 3c, respectively. The addition of diethyl ether to solutions of 3a results in the formation of
[Ind,TiMe(Et,O)]BPh, (4) as poorly soluble black microcrystals, while the addition of acetonitrile to 3b produces the
knownIZbJadduct [Cp,TiMe(NCMe)]BPh, .
Various structural possibilities (A-E; Scheme 2) can be
envisaged for the complex cations. A trigonal-planar geome-
B
A
D
C
E
Scheme 2.
try (A) is ruled out by the observed inequivalence of H-I and
H-3 of the indenyl five-membered ring in 3a; the signal pattern resembles that of isolated pseudo-tetrahedral complexes
of the type [Ind,TiMe(L)]@.[2b1Although not necessarily
conclusive, a comparison of the C-H coupling constants
of the methyl groups of 3a ( J = 128.7 Hz) and of the
nonagostic [Ind,TiMe,] ( J = 125.2 hz) makes an agostic
structure (B) appear unlikely; neutral scandium complexes
isoelectronic to 3b have also failed to provide conclusive
evidence for an agostic methyl Iigand.l6]
0570-0833190/0707-0780$03.SOi .25/0
Angew. Chem. Ini. Ed. Engl. 29 (1990) No. 7
It could be expected that Lewis-acidic cationic metal centers such as those present in 3a-3c would have a tendency to
interact with BPhF. Solutions of 3a-3c, prepared using
[PhNMe,H]BPh, (2a), show a complex ’H NMR pattern in
the aromatic region. In [PhNMe,H]BPh, (2a) the chemical
shifts of the phenyl groups of the anion are strongly influenced by the presence of the anilinium cation, presumably
due to ion pairing, while solvent-separated BPhF ions as in
[nBu,NH]BPh, (2b) gives rise to three well-separated ’H
NMR signals for the para, meta, and ortho proton^.^'] Solutions of 3a-3c at -40°C show the presence of both ionpaired and solvated BPhz ions in ratios varying from 3:l
to 3:2. In addition, the signals of free dimethylaniline
are identified; an adduct of type C is not found. There is
also no evidence that one phenyl ring of a BPhF ion is
coordinated to a metal center or subject to an environment
different from the other three (cf. structure D), even at low
temperature. This finding contrasts with our recent isolation
of an q6-BPhF complex of a trialkylzirconium cation,
[Zr(CH,Ph),(q6-C6H,BPh,)].[81
The observations suggest
that base-free bis(cyclopentadieny1)metal alkyl cations have
a pseudo-tetrahedral solvated structure of type E.[91
By contrast, titanium and zirconium cations prepared
from [nBu,NH]BPh, (2b) and 1 show only solvated BPhF;
there is no indication of an association between anions and
cations. Because of the basicity of nBu,N, the formation of
labile amine complexes as indicated in C is likely in these
cases. In agreement with this assumption, the ethylene polymerization activity of 3a‘ and 3c’, prepared with 2b, is reduced to ca. 20% of the activity of 3a and 3c.
Exposure of a solution of 3a in CD,Cl, at -40 “C to ca.
5 equivalents of ethylene leads to a reduction in intensity of
the Ti-CH, signal at 6 = -0.15 and the appearance of new
signals in the alkyl region at 6 = 0.7, 0.9, and 1,2, indicating
the insertion of ethylene into the titaniumxarbon bond.
These signals grow further on prolonged exposure to an
excess of ethylene, until the precipitation of polyethylene is
seen.1121The Ti-Me signal does not disappear completely
during the initial stages of the chain-growth reaction; this
observation suggests that subsequent CzH4insertions proceed more readily than the first.1131
Solutions of 3a polymerize ethylene at 1 bar over a temperature interval of +10 to -60°C; the activity shows a
maximum at ca. -20°C and decreases rapidly above ca.
15 “C. The polymerization rate is fast for the first 10 to 15
minutes before it decays to a more steady lower level. The
polydispersity of the resulting polyethylene decreases with
the polymerization temperature from R J M , = 2.5 at
f 10 “C to 2.0 at -40 “C; such a narrow distribution is typical of chemically homogeneous active sites. The IR spectra
of hot-pressed films showed the absence of pendent-methylene and terminal un~aturation.[’~]
The catalytic activities of
3b and 3c under identical conditions are lower than that of
3a. Of the three catalysts, the zirconium complex is the
thermally most stable and, although operating at a lower
initial activity than 3a, appears less susceptible to catalyst
deactivation. Propylene is polymerized by 3a at 0°C at a
slower rate. In agreement with earlier observations on nonstereorigid catalysts operating at comparable temperatures,“ 51 the resulting polypropylene is atactic.
+
Experimental Procedure
All reactions were performed under purified argon. Reactions with [PhNMe,H]BPh, (2a): For NMR experiments, 2a (37 mg, 0.084 mmol) was suspended in 0.3 mL of CD2Cl, and cooled to -80°C. A solution of [Ind,TiMe,]
(la) (26.5 mg, 0.086 mmol) in 0.3 mL of CD,CI, was injected, the tube was
sealed, and the sample allowed to warm to 0°C. The solution began to darken
Angew C h m . I n [ . Ed. Engl. 29 (1990) No. 7
at -6O”C, and the reaction was complete after 10-15 min at 0°C to give 3a.
Samples of 3b and 3c were prepared on a similar way. In all cases the formation
of methane was observed (6 = 0.22).
3a: ‘H NMR (400 MHz, CD2CI,, -40°C): 6 = -0.15 (s, 3 H, Ti-Me), 5.98
(dd, poorly resolved, 1 H, indenyl H-I), 5.78 (t, 1 H, indenyl H-2, J = 3.3 Hz),
6.21 (dd, poorly resolved, 1 H, indenyl H-3), 7.2-7.8 (indenyl H-4 to H-7),
unresolved); associated BPhF: 7.63 (“d”, ortho-H), 7.51 (“t”, mera-H), 7.56
(“t”, para-H); solvent-separated BPhF: 7.21 (unresolved, ortho-H), 7.07 (“t”,
meta-H), 6.93 (“t”, para-H); free dimethylaniline: 2.92 (s, N-Me), 6.71 (“d”,
ortho-H), 7.22 (3”.
meta-H), 6.68 (“t”, para-H). 13C NMR: 6 = 70.8 (Ti-Me,
J(C-H) 128.7 Hz). In some samples, a second, as yet unidentified titanium
species is also formed, characterized by ‘H NMR signals at 6 = - 0.19 (Ti-Me),
5.94, 6.01, and 6.35 (indenyl); this species does not participate in polymerization reactions. 3b: ‘H NMR (400 MHz, CD,CI,, -40°C): 6 = 1.26 (s, 3 H,
Ti-Me),6.28(~,10 H , C p ) . k : ‘HNMR(400 MHz,CD,Cl,, -4O”C):J = 0.27
(s, 3 H, Zr-Me), 6.25 (s, 10 H, Cp). The values for solvent-separated and associated BPhF and for dimethylaniline present in solutions of 3b and k closely
correspond to those given for 3a. 4: A solution of [Ind,TiMe,] (la) (0.37 g,
1.2 mmol) in 5 mL of toluene was added to a suspension of 0.441 g of [PhNMe,H]BPh, 2a in 20 mL of dichloromethane-diethyl ether 1:1 at - 70 “C. On
warming to room temperature, black microcrystalline 4 separated and was
filtered off, washed with CH,CI,-Et,O, and dned (0.2 g, 0.29 mmol, 29%).
Satisfactory elemental analysis. ‘H NMR (60 MHz, C,D,N): d = 0.23 (s, 3 H,
Ti-Me), 1.09 (t, 6H, OCHJH,), 3.36 (q, 4 H, OCH,CH,), 6.38 (m. 2 H, indeny]), 6.51 (m, 4H, indenyl), 7.2 (m, 20 H, indenyl, BPhAe, 8.08 (m. broad,
8 H, BPh?).
Reactions with [nBu,NH]BPh, (2b): Procedure as described for
[PhNMe,H]BPh, (2a). 3a’ ‘H NMR (400 MHz, CD,CI,, -40°C): 6 = -0.15
(Ti-Me), 5.99 (dd, poorly resolved, 1 H, indenyl H-1). 5.79 (t, 1 H, indenyl H-2.
J = 3.3 Hz), 6.22 (dd, poorly resolved, 1 H, indenyl H-3), 7.3-7.7 (m, indenyl
H-4 to H-7); BPh,? 7.41 (unresolved, ortho-H), 7.08 (“t”, mera-H), 6.93 (“t”,
para-H); nBu,N: 6 = 0.89, 0.94, 1.18, 2.11, 2.43. k’: ’H NMR (400MHz,
CD,CI,, -40°C): 6 = 0.27 (s, 3 H, Zr-Me), 6.24 (s, 10 H, Cp); the values for
BPh? and nBu,N closely correspond to those given for 3a‘.
General procedure for polymerization reactions: [PhNMe,H]BPh, (2a)
(0.240 g, 0.544 mmol) was suspended in 30 mL of dichloromethane and cooled
to -40°C. 3.05 mL of a 0.18 M solution of [Ind,TiMe,l (la) in toluene
(0.549 mmol) was injected with rapid stirring and purified ethylene was admitted. After two hours the reaction was quenched by the injection of 10 mL of
methanol and polyethylene (3.25 g) was isolated (M.,relative to NBS1475 standard. 16000; M J M ” = 2.0).
Received: March 5 , 1990 [Z 3824 iE]
German version: Angew. Chem. 102 (1990) 830
CAS Registry numbers:
l a , 49596-02-3; lb, 1271-66-5; l c , 12636-72-5; 2a, 118573-45-8; 2b, 6416739-1; 3a, 127686-08-2; 3b, 127686-09-3; 3c, 127686-10-6; 4, 127686-12-8;
[Cp,TiMe(NCMe)]BPh,, 109630-95-7; C,H,, 74-85-1.
a) F. S . Dyachkowski, A. K. Shilova, A. E. Shilov, J Polym. Sci. Part C
16 (1967) 2333; b) J. J. Eisch, A. M. Piotrowski, S . K. Brownstein, E. J.
Gabe, F. L. Lee, J Am. Chem. SOC.107 (1985) 7219; c) P. G . Gassman,
M. R. Callstrom, ibid. 109 (1987) 7875; d) C. A. Jolly, D. S . Marynick,
ibid. 111 (1989) 7968.
a) M. Bochmann, L. M. Wilson, J. Chem. SOC Chem. Commun. 1986,
1610; M. Bochmann, L. M. Wilson, M. B. Hursthouse, R. L. Short,
Organometallics6 (1987) 2556; M. Bochmann, A. J. Jaggar, L. M. Wilson,
M. B. Hursthouse, M. Motevalli, Polyhedron 8 (1989) 1838.
M. Bochmann, L. M. Wilson, M. B. Hursthouse, M. Motevalli,
Organometallics 7 (1988) 1148.
a) R. F. Jordan, C. S . Bajgur, R. Willet, B. Scott, J. Am. Chem. SOC.108
(1986) 7410; b) R. F. Jordan, R. E. LaPointe, C. S . Bajgur, S . F. Echols,
R. Willet, ibid. 109(1987) 4 1 1 1 ; ~ )R. Taube, L. Krukowka, J. Organomel
Chem. 347 (1988) C9.
G. G. Hlatky, H. W. Turner, R. R. Eckman, J. Am. Chem. SOC.111 (1989)
2728.
M. E. Thompson, S . M. Baxter, A. R. Bulls, B. J. Burger, M. C. Nolan,
B. D. Santarsiero, W. P. Schaefer, J. E. Bercaw, J Am. Chem. SOC.109
(1987) 203.
[PhNMe,H]BPh, (2a): ‘H NMR (400 MHz, CD,CI,, 25°C); cation:
6 = 2.92 (N-Me), 6.87 (“d”,orrho-H, J z 8 Hz), 7.26 (“t”, mera-Hi),6.83
(“t”, para-H), anion: 7.61 (“d”, ortho-H, J z 8 Hz), 7.46 (“t”, meta-H),
7.55 (“t”, para-H). [nBuNH]BPh, (26): 6 = 0.89, 1.10, 1.17, 2.13 (N-Bu),
7.44 (unresolved, ortho-H), 7.09 (“t”, meta-H), 6.95 (“t”, para-H,
J z 7.3 Hz).
M. Bochmann, G . Karger, A. J. Jaggar, J. Chem. SOC.Chem. Commun., in
press.
The coordination of CH,CI, to Lewis-acidicmetal centers has precedence,
for example, in [CpM(CO),(CH,CI,)]PF, [lo] and (Pd(p-OTeF,)J
[Ag(q2-CH,C1dJ2 [Ill.
W. Beck, K. Schloter, Z. Naturforsch. 335 (1978) 1214.
0 VCH Verlagsgesellschafr mbH, 0-6940
Weinheim, 1990
0870-0833/90j0707-0781$03.50+.25/0
781
(1 1J T D. Newbound, M. R. Colsman, M. M. Miller, G. P. Wulfsberg, 0. P.
Anderson, S . H. Strauss, J Am. Chem. Suc. 111 (1989) 3762.
[12] No species containing ethylene coordinated prior to insertion into the
Ti-C CI bond (1-11) was detected. i n the absence of d electrons, group IV
metal centers are unable to stabilize x complexes through back-donation,
and species of type I are unlikely to have significant lifetimes. For a theoretical treatment of the ethylene insertion step, see [Id]. Occupancy of d
orbitals in early transition metals leads to energetically highly favorable
back-bonding as a barrier against insertion: 0. Eisenstein, R. Hoffmann,
J Am. Chem. Sue. 102 (1980) 6148; ibid. 103 (1981) 4308.
b]
HO
HO
CHzOH
HO
HO
R e
I
[;.Ti
OH
+ R'R~C-O
3
HO
+
R~OH
4
1
+ [Cp,Ti-CH,CH,RIe
11
[13] This observation is in agreement with earlier results on a Cp,TiMeCI/
AIMeCl, catalytic system where only a fraction of the available Ti-Me
bonds were shown to participate in chain-growth reactions: G. Fink, W
Fenzl, R. Mynott, Z. Narurfursch. 40B (1985) 158.
[I41 J. Haslam, H. A. Willis, D. C. M. Squirrel: IdentifiL.atiun and Analysis of
P[asrics, 2nd ed., Heyden, London 1980.
[lS] W. Kaminsky, Angew. Makrumol. Chem. 1451146 (1986) 149. By contrast,
the thermally unstable Cp,TiPh,/methylaIumoxane system at temperaI
Am.
tures below -4S"C gives isotactic polypropylene: J. A. Ewen, .
Chem. Suc. 106 (1984) 6385.
Development of Tailor-Made Cytostatics
Activable by Acid-Catalyzed Hydrolysis
for Selective Tumor Therapy **
By Lutz E: Tietze,* Matthias Beller, Roland Fischer,
Michael Logers, Eckhard Jahde, KarLHeinz Glusenkamp,
and Manfred E: Rajewsky
The chemotherapy of malignant tumors is extremely problematic because of the narrow therapeutic range of the cytostatics currently available and the resulting side effects."]
The goal of our work['] is to exploit phenotypic differences between malignant and normal cells for the development of tumor-selective cytostatic drugs. Several groups
have independently shown that, owing to an increase in the
blood-sugar level of a tumor-bearing host, the rate of glycolysis in malignant cells is increased relative to that in the
normal cell population.131 The resulting increase in the
amount of lactic acid results in a lowering of the pH value in
the tumor tissue to 6.2 on the average, whereas the pH value
of normal tissue remains nearly constant @H 7.2). We use
this pH difference to release selectively within the tumor
tissue a cytostatic drug generated from a nontoxic precursor
by acid-catalyzed hydrolysis. A crucial problem here is the
development of functional groups that, on the one hand,
guarantee the detoxification of the cytocidal component
and, on the other, ensure sufficient acid lability that, at pH
6.2, the active species is released fast enough. To this end, we
have developed the acetal glycosidest4Iof general formula 1,
which can release a cytotoxic aldehyde or ketone, 3, together
with an alcohol, 4 and a sugar, 2. Although the compounds
we have so far synthesized exhibited increased selectivity in
~ ~ we devitro, their rate of hydrolysis was too I O W . ~Here
scribe the preparation of the glucoside 10c, which satisfies
the above requirements very well.
Reaction of trimethy~silyl-2,3,4,6-tetra-O-acetyl-p-~-glucopyranoside (5) with three equivalents of 4-(tert-butyldiphenylsiloxy)butan-2-one (6)and one equivalent of the
corresponding acetal 7 in the presence of a catalytic amount
of trimethylsilyl trifluoromethanesulfonate (TMSOTQ in
dichloromethane at - 70 "C gave exclusively the acetal-pglucoside 8 a @:a >99: 1) in 39% yield. Owing to the minimal difference between the substituents at the carbonyl
group, however, a nearly 1:l mixture of C-2' epimers was
obtained. The lower yield, compared with the formation of
acetal glucosides from aldehydes, is due to the lower reactivity of
Trehaloses were formed as side products in
the reaction. Cleavage of the trialkylsilyl group with tetrabutylammonium fluoride (TBAF) led to a 94% yield of an
epimeric mixture of alcohols 8b, which, upon treatment with
[(bis(2-chloroethyl)amido]phosphoric acid dichloride (9)@]
in the presence of triethylamine (CH,CI,, 36 h, 20 "C) followed by reaction with NH, (CH,CI,, 1.5 h, 20 "C),afforded
the diamidophosphate 10 bin 74 % yield via 10a. Cleavage of
the acetyl groups to give 10c was accomplished by solvolysis
II
1. TMSOTf, -7OOC
2. TBAF
CH20Ac
Ac
- AcO
0
8a, R = S i ( P h ) , t B u
8b. R=H
1. Cl,P(O)N(CH,CH,Cl),
9 / Et,N
2. NH,
[*I
[**I
782
Prof. Dr. L. F. Tietze, Dr. M. Beller, Dr. R. Fischer,
DiplLChem. M. Logers
Institut fur Organische Chemie der Universitat
Tammannstrasse 2, D-3400 Gottingen (FRG)
Prof. Dr. M. F. Rajewsky, Dr. E. Jahde, Dr. K.-H. Gliisenkamp
Institut fur Zellbiologie (Tumorforschung) der Universitat
Hufelandstrasse 58, D-4300 Essen (FRG)
Glycosidation, Part 16; Anticancer Drugs, Part 12. This work was supported by the Bundesminister fur Forschung und Technologie (Forderkennzeichen 03189-52A9) and the Fonds der Chemischen Industrie. Glycosidation, Part 18, and Anticancer Drugs, Part 11 : L. F. Tietze, M. Beller,
Liebigs Ann. Chem. (1990), in press.
0 VCH
VerlagsgeseIlrchaJi mbH, 0-6940 Weinheim. 1990
3 . K,CO,/MeOH
CH,OR
RO
10a, R=Ac, X=C1
lob, R=Ac, X=NH,
lOc, R = H , X=NH,
0570-0833/9010707-0782 $03.50+ .2510
Angew. Chem. I n t . Ed. Engl. 29 (1990) Nu. 7
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