Regioisomeric Preference in Ring-Opening Polymerization of 3',5'-Cyclic Phosphoesters of Functional Thymidine DNA...

Regioregularity is a crucial property in the synthesis of DNA analogues, as natural DNA is synthesized exclusively in the 5' to 3' direction. We have focused our attention on the determination of the regioisomeric distribution of poly(3',5'-cyclic 3-(3-butenyl) thymidine ethylphosphate)s obtained from the ring-opening polymerization of (R)-3',5'-cyclic 3-(3-butenyl) thymidine ethylphosphate. The regioisomeric preference was investigated by comparison to synthesized model compounds of 3',3'-, 3',5'-, and 5',5'-linkages, where the model 3'-phosphoester linkages were to the secondary alcohol of 3-hydroxytetrahydrofuran and the model 5'-linkages derived from coupling to the primary alcohol of tetrahydrofurfuryl alcohol. From the ³¹P resonance frequency assignments of those small molecule model compounds, ³¹P NMR spectra revealed the major connectivity in the polymer backbone to be 3',5'-linkages, with ≤30% of other isomeric forms. Model reactions employing a series of alcohol initiators imparting various degrees of steric hindrance, to mimic the increased steric hindrance of the propagating alcohol relative to the initiator, were then conducted to afford the corresponding ring-opened unimer adducts and to gain understanding of the regioselectivity during the ring-opening polymerization. ¹H-³¹P heteronuclear multiple-bond correlation spectroscopy showed ethanol and 4-methoxybenzyl alcohol initiation to yield only the P-O5' bond cleavage product, whereas attack by isopropyl alcohol upon (R)-3',5'-cyclic 3-(3-butenyl) thymidine ethylphosphate afforded both P-O3' and P-O5' bond cleavage products, supporting our hypothesis that the increased steric hindrance of the propagating species dictates the regioselectivity of the P-O bond cleavage. Further model reactions suggested that the P-O5' bond cleavage products can be detected upon the formation of dimers during the ring-opening polymerization. Overall, this work provides a fundamental understanding of the polymerization behavior of six-membered cyclic phosphoesters and broadens the scope of DNA analogues from the ring-opening polymerization of 3',5'-cyclic phosphoesters.