Late-Stage Carboranylation of Nucleos(t)ides Enabled by Minisci and Pd-Catalyzed B-N Cross-Coupling Reactions: Toward the Synthesis of Carboranyl-Modified Oligonucleotides.

Carboranyl-modified nucleosides are promising boron-10 (10B) delivery agents for boron neutron capture therapy (BNCT), yet methods for directly forming B-C/B-N bonds at unprotected nucleosides are scarce. We report herein two operationally simple, orthogonal strategies for the late-stage installation of carboranyl units at heteroaromatic C(sp2) centers and exocyclic amino groups of native nucleosides, nucleotides, and analogues. The first method uses a visible-light-mediated Minisci-type radical reaction, where 9-meta-carboranyltrifluoroborate serves as a nucleophilic B(9)-vertex radical precursor to selectively forge C(sp2)-B bonds at the C8-position of guanine, C2-position of adenine, and C5-position of uracil. Theoretical calculations revealed that the selectivity for adenosine and uridine is substrate-dependent: in adenosine, C2-selectivity arises from steric repulsion that disfavors C8-addition, whereas in uridine, C5-selectivity is dictated by stabilizing interactions. The second strategy employs Pd-catalyzed Buchwald-Hartwig-type amination to form B-N bonds at the N6-position of adenosine, the N2-position of guanosine, and the N4-position of cytidine. Furthermore, carboranyl groups were successfully incorporated into DNA oligonucleotides at various positions via solid-phase synthesis. To assess their suitability for BNCT applications, 10B-enriched carboranyl-modified oligonucleotides, carboranyl-modified DNA aptamers (Sgc8c) targeting protein tyrosine kinase 7 (PTK7), along with 10B-enriched carboranyl-modified aptamers, were prepared. Cellular uptake studies showed that the aptamer-carborane conjugates effectively recognize and internalize into PTK7-overexpressing HCT116 cells, thereby providing a basis for future exploration of BNCT applications. These methods expand the chemical space of boron-rich nucleos(t)ides and represent a step toward the development of next-generation BNCT agents.