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Org Biomol Chem. 2008 Dec 21;6(24):4627-33. doi: 10.1039/b813870b. Epub 2008 Oct 27.

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

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Department of Bioorganic Chemistry, Box 581, Biomedical Center, Uppsala University, SE-751 23, Uppsala, Sweden.


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 to the olefinic double bond, yielding about 9% of the 6-endo cyclization product. Possible pathways to give 6-endo cyclization product 9 compared to the intermediates responsible to give the 5-exo cyclization product 5 has been discussed. Based on this unique 6-endo cyclization 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.

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