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It was found that rhodosaminylanthraclinone-type anthracyclines are readily N-mono-demethylated upon irradiation with visible light, leading to 3′-N-methyl-α-L-daunosarninylanthracyclinones in good yields. The reaction is preferentially observed with rhodosaminylanthracyclinones in which OH-4′ of the sugar (rhodosamine) moiety is not further glycosylated. 3′-N-methyl-α-L-daunosaminyl anthracyclinones provide key compounds that can readily be further derivatized.

It was determined that β-rhodomycin-II (rhodomycin A) (1), β-rhodomycin-I (rhodomycin B, betaclamycin T) (2) and γ-rhodomycin-I (iremycin) (3) may regioselectively be acylated at OH-4′ of the sugar (rhodosamine) moiety(ies), using a two-phase (chloroform/water) solvent system and sodium bicarbonate as a base. This report exemplarily describes the synthesis of the corresponding p-methoxybenzoate esters of 1, 2, and 3, i. e. derivatives 4–7, and highlights the structure elucidation of isomers 5a and 5b.

The syntheses of 7-O-α-L-daunosaminyl-∊-isorhodomycinone (6) and 7-O-α-L-rhodosaminyl-∊-isorhodomycinone (7) are described. The glycosyl donors 1,4-di-O-p-nitrobenzoyl-3-N-trifluoroacetyl-α,β-L-daunosamine and 1,4-di-O-acetyl-α,β-L-rhodosamine have proven to be the most suitable for the glycosylation of ∊-isorhodomycinone (∊-iso RMN) (3), using the TMS triflate method. Deblocking (0.5 N NaOH) of the protected glycoside 4 led to the desired 7-O-glycosyl-∊-isorhodomycinones 5 and 6. The daunosaminyl glycoside 6 was methylated (CH2O, NaCNBH3) to provide the rhodosaminyl derivative 7. The photolytic demethylation of this product selectively provided the 7-O-(3′-N-methyl-α-L-daunosaminyl)-∊-isorhodomycinone (8).