DISTAMYCIN-NA - A DNA ANALOG WITH AN AROMATIC HETEROCYCLIC POLYAMIDE BACKBONE - PART 2 - SOLID-PHASE SYNTHESIS OF DISTAMYCIN-NAS CONTAININGTHE NUCLEOBASE URACIL - UNEXPECTED SOLVENT PARTICIPATION IN THE COUPLING STEP

The synthesis of the Fmoc-protected amino acid 2 is presented. First a ttempts of amide-bond formation to the homodimer ? in solution showed only poor coupling yields indicative for the low reactivity of the ami no and carboxy groups in the building blocks 1 and 2, respectively (Sc heme 1). Best coupling yields were found using dicyclohexylcarbodiimid e (DCC) without any additive. The oligomerization of building block 2 adopting the Fmoc ((9H-fluoren-9-ylmethoxy)carbonyl) solid-phase synth esis yielded a mixture of N-terminal-modified distamycin-NA derivative s. By combined HPLC and MALDI-TOF-MS analysis, the N-terminal function al groups could be identified as acetamide and N,N-dimethylformamidine functions, arising from coupling of the N-terminus of the growing cha in with residual AcOH or DCC-activated solvent DME An improved prepara tion of building block 2 and coupling protocol led to the prevention o f the N-terminal acetylation. However, 'amidination' could not be circ umvented. A thus isolated tetramer of 2, containing a lysine unit at t he C-terminus and a N,N-dimethylformamidine-modified N-terminus, not u nexpectedly, showed no complementary base pairing to DNA and RNA, as d etermined by standard UV-melting-curve analysis.