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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.

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