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Mechanism and Stereochemistry of Wittig-Olefin Syntheses.

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equilibrium between the subunits of the glutamate dehydrogenase (molecular weight 270000) and the octamer proceeds
i n a stepwise manner "1. From the dependence of the X-ray
small-angle scattering on the protein concentration (1-33 mg/
ml) it follows that the radius of gyration ( R , = 30.3 A), the
mass per 1 A length (M/l 8, = 2340), and the shape of the
cross-section of glutamate dehydrogenase are independent
of protein concentration. From the comparison of the experimental scattering curves with theoretical cross-section curves
for elliptical cylinders it is concluded that the cross-section is
either circuIar(diameter 86 8 )or ellipticaI with an axial ratio
of 0.8: 1 (long axis 95 A, short axis 76 A). The length of the
associated molecule was found to be 800-900 A. On the basis
I . Under kinetic control salt-free phosphorus ylids link to
give preferentially the thermodynamically unstable erythrobetaines.
2. In the presence of lithium salts the equilibrium between
the two diastereoisomeric betaines is shifted far towards the
threo-eoimer.
The equilibrium between the diastereoisomeric betaines can
be reliably and rapidly established by way of the betaine
ylids. This new type of compound also offers further prepar-
B
8
S-R'
H-C
fc
erythro
+
cis
O=CH-R'
threo
of these results, the shape of the molecule can best be described as cylindrical; furthermore, the results confirm the earlier
conclusion that the dissociation of the prolate molecule into
subunits involves a transfer cleavage. The mass per unit
length measurements and the pronounced side maxima of the
scattering curve suggest that glutamate dehydrogenase is very
loosely built and posseses voids 121.
Lecture at Giessen on December 19, 1967 and at the Max-Plank-Institut
fur Experimentelle Medizin, Gottingen, February 2, 1968 [VB 157 IE]
German version: Angew. Chern. 80, 669 (1968)
[*I Prof. Dr. H. Sund
Universitat Konstanz, Fachbereich Biologie
775 Konstanz, Postfach 733 (Germany)
[l] Experiments with W. Burchard.
[2] Experiments with M . Herbst and I. Pilz.
trans
ative possibilities: addition of an electrophile X* (X = e.g.,
D, CH3, F, CI, Br) leads to a n or-substituted betaine (erythrol
threo mixture or pure rhreo-form) and thence the corresponding unsaturated compound.
ii
Betaine
o
Betaine Ylid
w
(C&)3P-C-CH-0
Q
-+ (C&)sPO
k
+ R,,C=CHR'
X-
a -subst. Betaine
Lecture at Giessen (Germany) on July 9, 1968 [VB 158 IE1
German version: Angew. Chem. 80, 637 (1968)
Mechanism and Stereochemistry of
Wittig-Olefin Syntheses
[*] Dr. M. Schlosser
Organisch-ChemischesInstitut der Universitat
69 Heidelberg, Tiergartenstr. (Germany)
By M. Schlosser[*l
The velocity of, and the yield obtained in, carbonyl olefination
by triphenylphosphonium ylids is strongly influenced by the
reaction conditions. Kinetic studies showed that the slowest
step is usually the decomposition of the zwitterionic adduct,
the betaine, to olefin and triphenylphosphine oxide by way
of the oxaphosphetane ( I ) .
This decomposition is particularly strongly hindered by
lithium salts, which give betaine-LiX adducts. In sum the
following order of increase in reaction rate is observed:
apolar solvent + presence of lithium salts < polar solvent
< salt-free apolar solvent.
On the basis of this mechanistic knowledge procedures have
been developed that direct the carbonyl olefination according
to choice towards cis- or trans-olefins. For this purpose two
stereochemical principles are used:
650
The Biochemistry of Fructose Metabolism
By W. Lamprechf [ * I
Metabolism of fructose in animal tissues differs markedly
from the normal degradation of glucose with respect to organ
specificty and the sequence of the metabolic pathway. In
mammals, fructose can be degraded only in liver, kidney, and
mucosa of the small intestine; only these organs contain the
appropriate enzyme pattern. Ketohexokinase catalyzes phosphorylation of fructose to its 1-phosphate; in liver this particular enzyme is about twice as active with fructose than with
glucose. Fructose 1-phosphate is further degradedto dihydroxyacetone phosphate and D-glyceraldehyde by liver-aldolase
which differs in its specificity from that of muscle. Dihydroxyacetone phosphate enters the normal glycolytic pathway.
However, D-glyceraldehyde may function as a substrate in
four enzymatic reactions competing with each other:
1. D-glyceraldehyde can be phosphorylated to its 3-phosphate by means of ATP and triokinase.
2. and 3. D-glyceraldehyde can also be reduced to glycerol,
either by a NADH- or a NADPH-dependent alcohol-dehydrogenase. Glycerol is phosphorylated to L-glycerol-3-phosphate (an intermediate of normal glycolysis) by means of
glycerokinase. The reduction, however, seams rather unlikely in liver, due to the high K M of the NADH-specific
Angew. Chem. internat. Edit.
1 Vol. 7 (1968) I No. 8
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