2 research outputs found
Structure-reactivity study of O-tosyl Cinchona alkaloids in their new synthesis and in hydrolysis to 9-epibases : unexpected formation of cinchonicine enol tosylate accelerated by microwave activation
New methods for
O
-
tosylation of
the
natural
Cinchona
alkaloids
have been
discovered
as
a
biphasic
processes with Bu
3
N as a
catalyst.
The
optimized
excess of
tosy
l chloride
,
necessary for
transformation
of each
of the
four alkaloid
s
into
O
-
tosy
l derivative
, decreases
in the following
order
:
quinine, quinidine, cinchonidine and cinchonine
. The same
decreasing order has been
noticed
for
the
hy
drolysis
rate
of
the
appropriate
tosylates
to 9
-
epibases
.
D
iffic
ult conversion
of
O
-
tosy
lcinchonine
in the
hydrolytic medium
of
aq
ueous tarta
ric
acid
gives
9
-
epicinchonine
together with
parallel
formation
of
cinchonicine enol
tosylate. The latter product
is
obtained
as
the main
when
both cinchonine and cinchonidine
tosylates
react
in the presence of salicylic acid
under
controlled microwave heating
.
On the basis of X
-
ray
structure of the new alkene product,
the
stereoselective
syn
-
E2
quinuclidine
ring opening process
,
competing to
the
S
N
2 hydrolysis
is
postulated
for this transformation
Monazite trumps zircon: applying SHRIMP U–Pb geochronology to systematically evaluate emplacement ages of leucocratic, low-temperature granites in a complex Precambrian orogen
Although zircon is the most widely used geochronometer to determine the crystallisation ages of granites, it can be unreliable for low-temperature melts because they may not crystallise new zircon. For leucocratic granites U–Pb zircon dates, therefore, may reflect the ages of the source rocks rather than the igneous crystallisation age. In the Proterozoic Capricorn Orogen of Western Australia, leucocratic granites are associated with several pulses of intracontinental magmatism spanning ~800 million years. In several instances, SHRIMP U–Pb zircon dating of these leucocratic granites either yielded ages that were inconclusive (e.g., multiple concordant ages) or incompatible with other geochronological data. To overcome this we used SHRIMP U–Th–Pb monazite geochronology to obtain igneous crystallisation ages that are consistent with the geological and geochronological framework of the orogen. The U–Th–Pb monazite geochronology has resolved the time interval over which two granitic supersuites were emplaced; a Paleoproterozoic supersuite thought to span ~80 million years was emplaced in less than half that time (1688–1659 Ma) and a small Meso- to Neoproterozoic supersuite considered to have been intruded over ~70 million years was instead assembled over ~130 million years and outlasted associated regional metamorphism by ~100 million years. Both findings have consequences for the duration of associated orogenic events and any estimates for magma generation rates. The monazite geochronology has contributed to a more reliable tectonic history for a complex, long-lived orogen. Our results emphasise the benefit of monazite as a geochronometer for leucocratic granites derived by low-temperature crustal melting and are relevant to other orogens worldwide