Unusual radical 6-endocyclization to carbocyclic-ENA and elucidation of its solution conformation by 600 MHz NMR and ab initio calculations

Abstract

In our previous paper (J. Am. Chem. Soc., 2007, 129, 8362), we reported the synthesis of 7′-Me-Carba-LNA and 8′-Me-Carba-ENA thymidine through 5-hexenyl or 6-heptenyl radical cyclization. Both 5-hexenyl and 6-heptenyl radical cyclized exclusively in the exo form, giving unwanted exocyclic C7′-methyl group. In the present study, we showed that the regioselectivity of the 5-hexenyl radical cyclization could be favorably tuned by introduction of a hydroxyl group β to the olefinic double bond, yielding about 9% of the 6-endocyclization product. Possible pathways to give 6-endocyclization product 9 compared to the intermediates responsible to give the 5-exo cyclization product 5 has been discussed. Based on this unique 6-endocyclization strategy, a carbocyclic ENA modified thymidine (carba-ENA) has been successfully synthesized, which also enabled us to perform its full solution conformation analysis by using NMR (¹H at 600 MHz) observables for the first time.