Relationships
between the
Cycloadduct, it's
isomer, the
epimeric
Dihydroxytriones
and the
Ethynylcarbinols
The
schemes below
illustrate the
relationships of
the epimeric
ketones and
ethynlycarbinols
formed in the
sequences. The end
result, expectedly
is that the same
anthracycline
(15) will be
formed as the
epimeric hydrogens
at C-10a (as well
as C-6a) are
removed by lead
tetra-acetate in
the oxidation. The
isomerised
cycloadduct
(17) is
presumably formed
by interaction with
a cationic
silica gel -
isomerisation of
(12) to
(17) is also
known to occur
slowly in
deuteriochloroform
(traces of
H+). The
hydrolysis created
only the
dihydroxytrione
(13), as
revealed by NMR
spectroscopy of the
crude material. For
the ethynylation
reaction, however,
a 2:1 mixture of
dihydroxytriones
(13) and
(18) was
used.
The
second scheme,
illustrates a
further
sequence:
Note this time,
hydrolysis of the
isomerised
cycloadduct in the
presence of the
red-coloured
aromatic material
(formed by
decomposition of
the cycloadduct
(12) during
the Diels-Alder
reaction described
earlier) formed the
C-10a epimer
(18).
In the 3D
model below of the dihydroxytrione (18), notice
the chair-like
structure of the
Ring-A: the 10a-H
hydrogen atom
points downwards.
Legend:
Hydrogen,
blue,
oxygen, red
and carbon, dark
grey.
