An Example of Crossed Ship Conformation: The Tetrahydropyran Ring in Trioxa-bis-Spirocital Series

Abstract The three 3JHH coupling constants observed in the 1H NMR spectra (500 MHz) of trioxa-bis-spirocital 1 and 2 serve to calculate the dihedral angles Ψ(8,9) and Ψ(9,10) of these two molecules. The values obtained, 54° for 1, 25° for 2, reflect the change in conformation: chair → crossed ship when passing from 1 to 2. This change in conformation corresponds in 13C NMR to strong shielding (−3.6 to −5.5 ppm) of the lines of the carbons C8, C9 and C10.