New cyanomorpholinyl byproduct of doxorubicin reductive alkylation

Previously we reported that reductive alkylation of doxorubicin with 2,2'-oxybis[acetaldehyde] and NaBH3CN to form the 4''-morpholinyl derivative also gave the intensely potent 3''-cyano-4''-morpholinyl as a byproduct, by addition of CN- to an iminium intermediate in place of hydride. We now find that sugar 4'-OH is a third nucleophile that can add to the iminium intermediate in this reaction. Bridging of the 4'-OH to the morpholine ring at C.5'' formed a novel byproduct with an oxazolidino ring fused to the sugar and morpholine. The new product was minor at neutral pH but predominant at an acidic pH. When tested against tumors in mice it was 4-6 times more potent than doxorubicin. Hence, in comparison with the 3''-cyano-4''-morpholinyl, potency was reduced up to 100-fold by the O bridge. Analytical HPLC showed the presence of three of the four possible diastereoisomers, and two were isolated. The diastereoisomers appeared to differ in stability. In vitro tests suggested that biological potency varied inversely with stability.