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Homolytic Reactions of the Alkyl-Metal Bond.

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0 “C. When the temperature of the solution was allowed to
fall slightly below the new solubility limit only a smaI1
precipitate was formed (about 50 mg). After equilibration
under carefully controlled conditions, the precipitate and
solution were separated, the precipitate was dissolved in
water, and its optical activity was measured with respect to
the solution as standard. After many preliminary experiments, ten measurements performed under very strict
conditions all gave the same negative optical rotation r21 of
the precipitate, namely ct N 10-3 ’.
I n o u r search for a n explanation of this remarkable phenomenon we considered the following possible causes:
1 . Systematic errors in the measuring technique; they could
be ruled out by switching the cells, repeated measurements,
etc. - 2. Optically active impurities; this could be discounted on the basis of fractional (chromatographic) purification processes. - 3. Bacterial contamination of the
solution, which would result in preferential degradation of
the dextrorotatory [21 (natural) tartrate. We devoted the
most attention to this last possible source of error and
found: a) Concentrated solutions in open vessels are not
infected a t all within a week. b) If the observed activity
were nevertheless due to an attack by bacteria then any
variation in the time between precipitations would alter
the magnitude of the effect. This was not the case. c) Addition of the bactericide toluene to the solution had n o
influence o n the effect. - 4 . The thought that external fields
(such as the earth’s magnetic field) could lead to D / L discrimination is untenable.
At the present time, the only remaining interpretation of
our findings involves the assumption of different heats of
solution for D and L crystals of sodium ammonium tartrate.
And this means effectively that the lattice energies of the
enantiomorphic crystals must differ, which in turn can only
result from an asymmetric contribution t o the interaction.
Our measurements would indicate a relative difference in
the heats of solution (or lattice energies) of a few 10-5. We
are currently examining the consequences of this phenomenon by other techniques and for other systems.
Received: July 10 1970
[Z 251 IE]
Get-man version: Angeu. Chem. 82. 776 (1970)
[*I Dr. W. Thiemann and Prof. Dr.
K. Wagener
Institut fur Physikalische Chemie der
Kernforschungsanlage Julich GmbH and
Lehrstuhl fur Riophysik der
Technischen Hochschule Aachen
517 Jiilich 1, Postfdch 365 (Germany)
[l] E . L . Niel: Stereochemistry of Carbon Compounds.
McGraw-Hill, New York 1962, p. 44-45.
[ 2 ] Sign and magnitude of the optical rotation of a compound
depend upon the wavelength of the polarized light. We used a
Cary 60 spectropolarimeter, prod wed by Cary Tnstrumentcl
Varian, and worked at h = 280 nm.
Errata
In the communication “Aminoazimines by Addition of
Aminonitrenes to a-Carbonylazo Compounds” by K . H.
Koch and E. Fnhr, Volume 9, August 1970, the following
corrections should be made:
On page 634, right-hand column, line 30 “alcohol-free glacial
acetic acid” should read “alcohol-free ethyl acetate”.
On page 635, left-hand column, line 1 “(la)--(lf)” should
read “(ld)--(lf)”.
C O N F E R E N C E R E PORTS
The overall reaction is that shown in eq. (4).
Homolytic Reactions of the Alkyl-Metal Bond
By Alwyn G . Dnvies[*l
X Y + MRn
Bimolecular homolytic substitution ( S H ~ )reactions are
normally recognized to occur a t peripheral hydrogen atoms
[eq. (l)] or, less commonly, at halogen, oxygen, or sulfur
atoms.
X . + H-R
k
4
H-X+
R.
(1)
Recent stereochemical, kinetic, and ESR studies have shown
that this process will also take place, often much more
rapidly, at a multivalent metallic center [eq. (2); M = e.g.
Mg, Zn, Cd, B, AI, Sn, P, As, Sb, Bi], and provides the key
to t h e interpretation of many organometallic reactions.
Reaction (2) can also occur as a propagation step in a chain
process if a reagent X Y is chosen so that the chain carrier X *
is rapidly regenerated by the reaction (3).
R-+XY
+
R Y + P
Angew. Chem. internat. Edit. 1 Vol. 9 (1970) No. 9
X-MRn-l+
RY
(4)
Chain reactions of this type have been established for the
radicals R’C(CH3)zO. r41, C6H5S‘ [51, (CH3)2N-[6], and
succinimidyl[71 where the reagent XY is R’C(CH3)2OCI,
C ~ H S S H ,(CH&NCl, and bromosuccinimide respectively.
The reaction which has been studied most thoroughly, however, is the autoxidation of a n organometallic compound,
where the overall addition process (5) involves the propagation steps (6) and (7) 18991.
O r + MRn
ROO.
P +0
Reaction (2) can be studied as a step in a non-chain process
if the radical X . (e.g. (CH3)3CO.[ll, (CH3)3CS*r21, or
(CH3)zN- 131) is generated photolytically (from
or
(CH3)3C-O-O-C(CH3)3,
(CH&C-S-S-C(CH&,
(CH~)ZN-N=N-N(CH~)Z, respectively) in the cavity of an
ESR spectrometer, when the spectrum of the displaced
radical R . can be observed.
4
+
ROOMRn-I
+ MRn +
2
+ ROO.
ROOMRn-1
(5)
+ R’
(6)
(7)
The absolute rate constants, k2, for a number of these s H 2
replacements at metal centers have been determined by a
variety of techniques. Some of the values obtained, compared
Table. Rate constants kl and kz (mole-Is-’) for a few S H reactions
~
(Bu = butyl).
X.
(3)
74 1
with the corresponding values, k l , for reaction at hydrogen
centers, are given in the Table [eq. (l)].
Lecture at Dortmund on June 16, 1970 [VB 244 IE]
German version: Angew. Chem. 82, 704 (1970)
[ * ] Prof. Dr. Alwyn C. Davies
Chemistry Department, University College
20 Gordon Street, London, W.C. 1 (England)
111 A . G. Davies and B. P . Roberts, Chem. Commun. 1969,699;
J. organometallic Chem. 19, P 17 (1969); P . J . Krusic and J . K .
Kochi, J. Amer. chem. SOC.91, 3942, 3944 (1969).
[Z] B. P. Roberts, unpublished.
131 A . G. Davies, S . C. W . Hook, and B. P . Roberts, J. organometallic Chem. 22, C 37 (1970).
141 A. G. Davies, D . Griller, B. P. Roberts, and R. Tudor, Chem.
Commun. 1970, 640.
[ 5 ] A . G.Davies and S . C. W . Hook, J. chem. SOC.(London) B
1970, 736.
[6] A. G . Davies, S . C. W. Hook, and B. P . Roberfs, J. organometallic Chem. 23, C 11 (1970).
[7] A . G. Davies, B . P . Roberts, and J . M . Smith, Chem. Commun. 1970, 557.
[8] A . G. Dnvies and B. P . Roberts, J. chem. SOC.(London) B
1967, 17; 1968, 1014; 1969, 311, 317; P . G . Allies and P . B.
Brindley, ibid. 1969, 1126.
[9] K . U . Ingold, Chem. Commun. 1969, 911; A . G . Davies,
K . U . Ingold, B. P . Roberts, and R . Tudor, unpublished.
Further elucidation of the biogenetic pathway has shown
that this route leads from hydroxypregnenone to progesterone, then to a C P Isteroid having a 14P-OH group, and
finally to the cardenolides. Steroids having a butenolide
ring o n C-17 but n o 14F-OH group are not converted into
cardenolides.
Hydroxypregnenone also plays a significant role in the
biogenesis of other plant steroids.
For instance, the formation of hellebrigenin (a cardioactive
bufadienolide from the Christmas rose) and conessin (a
steroidal alkaloid) from radioactive hydroxypregnenone
has been established. As in animals, this precursor can also
be formed from cholesterol in plants. Indeed, cholesterol is
very important in the biogenesis of plant steroids containing 27 carbon atoms. Steroid saponins such as spirostanols,
tigogenin, diosgenin, and gitogenin are formed from
administered radioactive cholesterol. Alkaloids having a
steroid structure, tomatidin and solanidin, are also formed
from cholesterol.
The relatively good yields observed in these incorporations
rule out the possibility of the products being formed in
side reactions. The metabolic utilization of hydroxypregnenone and of cholesterol in plant tissues and the formation of phytosterols from these typical zoosterols shows
that cholesterol and hydroxypregnenone play a n important role in the biogenetic processes of higher plants.
Lecture at Mainz on June 18, 1970
[VB 245 IE]
German version: Angew. Chem. 82, 704 (1970)
Biogenesis of Steroids in Higher Plants
By Herwig Hulpkef”l
A great variety of steroids is encountered as natural
products in the animal and plant kingdoms. For this reason
a n early interest was shown in elucidation of the biogenetic
pathways leading to steroids. Tracer studies with acetic
and mevalonic acids labeled specifically with radiocarbon
revealed a characteristic incorporation of radioactivity
into the steroid formed (labeling pattern). Bloch and Lynen
have used this technique to determine the biogenesis of
animal steroids.
The following simplified account may now be regarded as
valid: Three molecules of acetyl-coenzyme A unite to give
mevalonic acid which is converted into “biological isoprene” by dehydration and decarboxylation. Squalene is
formed via farnesyl phosphate as intermediate and then
undergoes a concerted reaction to give lanosterol, from
which the steroids are formed.
This pathway was first established for the animal cell, but
there is strong evidence suggesting that a similar route is
followed in plants. The first cyclization product formed
from squalene is cycloartenol, a triterpene common t o
many plants.
Among the phytosterols, the cardenolides are known as
highly cardioactive substances. They differ from most
other steroids in a number of characteristic features: Rings
A and B are cis-linked; C-14 bears a @-OHgroup; and a n
a,$unsaturated lactone ring (butenolide ring) is found a t
C-17. In biogenesis studies with specifically labeled mevalonic acid in Digitalis species, it is seen from the degradation of the butenolide ring that it contains more than
one and less than three carbon atoms from the side chain
of a n “original steroid”. Thus the cardenolides are possibly
formed from a steroid containing 21 C atoms. Additional
support for this concept comes from the observation that
steroids containing 21 C atoms - termed digitenols by
R. Tschesche - can be isolated from plants known to
contain particularly large amounts of cardenolides. If
[21-14C]hydroxypregnenone[* *I is administered to Digitalis Zanata in the form of the glucoside, the plant is
found t o produce the cardenolides digitoxigenin, xysmalogenin, digoxigenin, and gitoxigenin.
742
[‘I Dr. H. Hulpke
Farbenfabriken Bayer AG
56 Wuppertal-Elberfeld (Germany)
[**I FormerIy known as pregnenolone.
Carcinogenic Alkylating Substances, Chemical
Constitution and Action[**]
By Hermann Druckrey [*I
The prevention of cancer calls for an elucidation of the
potential causes and their modes of action. At the same time
it is important to produce in experimental animals the
various types of tumor which are required as “models” for
morphogenetic, biochemical, immunological, and chemotherapeutic studies. In order t o achieve specific effects, the
“transport principle”, which has previously proved useful in
cancer chemotherapy, (Honvan, Endoxan) has been used,
i.e. the use of substances from which the actual “active
form” occurs only after metabolic activation. Three groups
of substances have been systematically investigated: Nnitroso compounds; hydrazo-, azo-, and azoxyalkanes; and
l-aryl-3,3-dialkyltriazenes.The first and decisive step in the
activation of these groups of compounds is an enzymatic a-C
hydroxylation of a n alkyl residue, which is then cleaved off
as the corresponding aldehyde. Thus, probabIy a n alkyl
diazohydroxide or a n alkyldiazonium ion occurs as a n
alkylating intermediate. The alkylation of nucleic acids,
particularly a t N-7 in guanine, has been observed and the
resulting change of the genetic code in the cells is looked
upon as the initiation of their carcinogenic transformation.
These investigations revealed highly specific effects. Symmetrical dialkylnitrosamines, for instance, induce predominantly
liver cancer, whereas the asymmetrical compounds produce
carcinoma of the esophagus. Methylbutylnitrosamine proved
to be carcinogenic when inhaled even at concentrations as
low as 0.05 ppm. However, acylalkylnitrosamides, which
undergo easy heterolysis, have a local effect and produce
cancer of the forestomach when administered orally; acetylmethylnitrosourea, o n the other hand, specifically causes
cancer of the glandular stomach. In contrast, the stable and
chemically important N-methyl-N-nitroso-p-toluenesulfonamide proved to be comparatively harmless. Striking differences were observed when injecting the compounds intraAngew. Chem. internat. Edit.
/ Vol. 9 (1970) / NO.9
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