NMR Spectral Assignment of Per-substituted Key-Intermediates of β-Cyclodextrin and Implications in the Structures of the Derivatives

Abstract

The compounds, 6-per-O-(t-butyldimethylsilyl)-β-cyclodextrin(1), 2,3-per-O-benzyl-6-per-O-(t-butyldimethylsilyl)-β-cyclodextrin(2), 2,3-per-O-benzyl-β-cyclodextrin (3),2,3,6-per-O-benzyl-β-cyclodextrin (4),2,3,6-per-O-benzoyl-β-cyclodextrin (5), are used as keyintermediates in the synthesis of selectively substituted β-CD derivatives. Simple and assignable ¹H and ¹³C NMR spectra (chemical shifts and coupling constants) were obtained for compounds1–4 indicating C₇ symmetry, ⁴C₁ glucose conformation and major ≫ arrangement of H6, H6' atoms at the primary side. The derivative 5, however, gave very broad peaksat room temperature. The peaks could partially be assigned at 270 K, but the broadening was still present at 220 K. This implies that there exist several conformers of similar energyand C₁ symmetry that continuously interchange, since there is not a single type of stabilizing interaction thatpredominates. We attributed this phenomenon to the presence of the carbonyl group, which probablydisfavors π-π stacking and induces random arrangements of the aromatic rings.