Synthesis and Conformational Analysis of Pyran Inter-Halide Analogues of D-Talose

In this work, we describe the synthesis of halogenated pyran analogues of D-talose using a halo-divergent strategy from known 1,6-anhydro-2,3-dideoxy-2,3-difluoro-β-D-mannopyranose. In solution and in the solid-state, all analogues adopt standard 4 C 1 -like conformations despite 1,3-diaxial repulsion between the F2 and the C4 halogen. Moreover, the solid-state conformational analysis of halogenated pyrans reveals deviation in the intra-annular torsion angles arising from repulsion between the axial fluorine at C2 and the axial halogen at C4, which increases with the size of the halogen at C4 (F < Cl < Br < I). Crystal packing arrangements of pyran inter-halides show hydrogen bond acceptor and nonbonding interactions for the halogen at C4. Finally, density functional theory (DFT) calculations corroborate the preference of talose analogues to adopt a 4 C 1 -like conformation and a natural bonding orbital (NBO) analysis demonstrates the effects of hyperconjugation from C-F antibonding orbitals.