"Slim" nucleosides and nucleotides. I. Synthetic, structural and biophysical investigations of showdomycins.

A new, convenient, and short synthesis of 2'-deoxyshowdomycin, along with an improved procedure for the preparation of showdomycin, have been presented. A single-crystal X-ray structure of 1-benzyl-2'-deoxyshowdomycin (9) has been reported. Conformational studies using C.D. indicated that showdomycin exists predominantly in an anti conformation in aqueous solution. Molecular mechanics calculations using AMBER point to comparable binding energy of showdomycin-adenosine pair with the natural uridine-adenosine pair, but with a significant base-ribose conformational deviation from the natural array in the former. Implications of such a conformational deviation on tumor and viral replications have been discussed. Base-pairing studies employing high resolution NMR spectroscopy indicate that both showdomycin and epishowdomycin base-pair with adenosine-5'-monophosphate (AMP); however, while showdomycin also shows evidence of stacking, that was absent in epishowdomycin. Molecular modeling studies using QUANTA/CHARMm show that showdomycin is capable of forming a homopolymer duplex by base-pairing with poly(A), but with a considerably broader and deeper major groove. A heteropolymer duplex with a single insert of showdomycin exhibits tighter coiling at the point of insertion. A ten-picosecond dynamics simulation of the above heteroduplex revealed relaxation of the helix with disruption of H-bonding for two base pairs on either side of the insertion point, forming a large central cavity.

Synthesis of pseudo-C-nucleosides.

Whereas the reaction of 1,2-O-isopropylidene-alpha-D-xylo-5-hexulofuranuronamide (1) with the Wittig reagent ethoxycarbonylmethylenetriphenylphosphorane gave 3-(1,2-O-isopropylidene-beta-L-threofuranos-4-yl)maleimide (2, 15%) and ethyl 5-carbamoyl-5,6-dideoxy-1,2-O-isopropylidene-alpha-D-xylo-hept-5-enof uranuronate (3, 76%), a similar reaction of 3-O-benzyl-1,2-O-isopropylidene-alpha-D-ribo-5-hexulofuranuronamide++ + (4) gave only 3-(3-O-benzyl-1,2-O-isopropylidene-alpha-D-erythrofuranos-4-yl)mal eimide (5), 80%), and that of 3-O-benzyl-1,2-O-isopropylidene-alpha-D-xylo-5-hexulofuranuronamide++ + (6) gave only ethyl 3-O-benzyl-5-carbamoyl-5,6-dideoxy-1,2-O-isopropylidene- alpha-D-xylo-hept-5-enofuranuronate (7, 85%). The formation of the maleimide ring depended on the orientation and substitution of HO-3'. Compounds 2 and 5 are analogous of showdomycin.

Showdomycin analogues: synthesis and antitumor evaluation.

The synthesis of N-beta-D-ribofuranosyl derivatives of maleimide, 3-methylmaleimide, and 3-chloromaleimide was accomplished in three steps from ribosylamine. The synthetic ribosides can be considered N-nucleoside analogues of showdomycin, which is an antitumor antibiotic of the C-nucleoside type. Although the three analogues were cytotoxic to cultured L1210 cells, no in vivo antitumor activity was found with the murine P388 leukemia test system. Drug transport studies were done in an attempt to trace the biological fate of the analogues.

Degradation and intracellular phosphorylation of showdomycin.

The anti-tumor agent showdomycin is degraded in the presence of 0.05 M NH4HCO3 (pH 8.0) to a compound which carries a negatively charged group. Together with previous data (Darnall et al., 1967) it is suggested that ammonia causes a basic hydrolysis of the maleimide moiety under formation of a maleic acid-like compound. During this modification the C-glycosidic bond is not split. Intracellularly, in mouse lymphoma cells (L5178y), showdomycin is phosphorylated to showdomycintriphosphate, via the mono- and diphosphate stage.