Acid promoted transformations of fluorescent luminarosine and its 2'-modified analogues

The susceptibility of highly fluorescent luminarine nucleosides to acid promoted anomerization reactions has been studied in order to select a derivative with suitable properties for chemical synthesis of luminarine-labeled oligo(deoxy)ribonucleotides. Both O-acetylated derivatives Ia-c and parent luminarosine IIa, as well as 2'-O-methylluminarosine IIb, and 2'-deoxyluminarosine IIc undergo anomerization at pH = 4 however, at considerably different velocities. In the case of O-protected nucleosides (Ia-c), the anomerization leads to an equilibrium mixture of respective beta and alpha furanosides, the rate and extent of anomerization decreasing in the following order: Ic >> Ia > Ib. Parent nucleosides (IIa-c) bearing free hydroxyls are generally more susceptible to anomerization than the O-acetylated derivatives but a similar order of reactivity (IIc >> IIa > IIb) is observed. In each case, a complex mixture containing both beta and alpha ribopyranosyl and -furanosyl forms is formed. Their structure and anomeric configuration have been proved by 1H and 13C NMR spectroscopy. The results point to 2'-O-methylluminarosine as the fluorophore of choice for further derivatization and chemical introduction into oligo(deoxy)ribonucleotides.