Carbocyclic C-C Bond Formation: Intramolecular Radical Ring Closure to Yield Diastereomerically Pure (7′S-Me- or 7′R-Me-) Carba-LNA Nucleotide Analogs

In light of the impressive gene-silencing properties of carba-LNA modified oligo DNA and RNA, both in antisense RNA and siRNA approaches, which have been confirmed as proof-of-concept for biochemical applications in post-transcriptional gene silencing, we envision the true potential of carba-LNA modifications to be revealed soon. Herein we provide detailed protocols for synthesis of carba-LNA-A, -G, -^5-MeC, and -T nucleosides on a medium/large scale (gram scale), as well as important guidelines for incorporation of these modified carba-LNAs into DNA or RNA oligonucleotides. Creation of a stereoselective C-C bond during the 5-exo radical intramolecular cyclization involves trapping of a C2′ radical intermediate intramolecularly by the vicinal double bond of a C4′-tethered ─CH₂-CH═CH₂ group. All diastereomers of substituted carba-LNAs are now available in pure form. The present procedure allows carba-LNA to be commercialized for medicinal or biotechnological purposes. © 2017 by John Wiley & Sons, Inc.