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Nature relies on reading and synthesizing the genetic code with high fidelity. Nucleic acid building blocks that are orthogonal to the canonical A‐T and G‐C base‐pairs are therefore uniquely suitable to facilitate position‐specific labeling of nucleic acids. Here, we employ the orthogonal kappa‐xanthosine‐base‐pair for in vitro transcription of labeled RNA. We devised an improved synthetic route to obtain the phosphoramidite of the deoxy‐version of the kappa nucleoside in solid phase synthesis. From this DNA template, we demonstrate the reliable incorporation of xanthosine during in vitro transcription. Using NMR spectroscopy, we show that xanthosine introduces only minor structural changes in an RNA helix. We furthermore synthesized a clickable 7‐deaza‐xanthosine, which allows to site‐specifically modify transcribed RNA molecules with fluorophores or other labels., Orthogonal nucleic acid building blocks to the canonical A‐T and G‐C base‐pairs are uniquely suitable to facilitate position‐specific labeling. Here, the orthogonal kappa‐xanthosine‐base‐pair for in vitro transcription is employed. Through NMR spectroscopy, it is shown that xanthosine introduces only minor structural changes. A clickable 7‐deaza‐xanthosine, which allows to site‐specifically modify transcribed RNA, is also synthesized.

The assigned structure of the dinoflagellate-derived toxin belizentrin was prepared by total synthesis in form of the corresponding methyl ester for stability reasons. The successful route features an unusual solution for the preparation of a recalcitrant ylide on a C-glycosidic segment; moreover, it involves an asymmetric hetero-Diels-Alder reaction en route to the tertiary hemiacetal substructure, a Negishi cross-coupling of two elaborate building blocks, and a macrocyclization based on an intramolecular aminolysis of a spirolactone. A modified Kocienski olefination ultimately allowed the polyol side chain to be attached to the macrocycle although this transformation faced the exceptional base sensitivity of this polyunsaturated target compound.