Structural Variations in 3,4-Dihydro-2\u3ci\u3eH\u3c/i\u3e-pyran Ketals: Acyl and Aryl Warfarin Derivatives

The crystal structures of ( ± )-cis-2-methyl-5-oxo-4- phenyl-3,4-dihydro-2H,5H-pyrano[3,2-c][1]benzopyran-2-yl acetate [C21H18O5, Mr = 350·37, monoclinic, P21/n, a = 12·091 (4), b = 8·288 (3), c = 17·840 (5) Å, β = 106·34 (2)°, V = l715(2) Å3, Z = 4, Dx = 1·356 g cm-3, λ(Mo Kᾱ) = 0·7107 Å, μ = 0·904 cm-1, F(000) = 736, T = 295 K, R = 0·050 for 2767 observations with I ≥ 3σ(I)] and (6R,12S)-( — )-6,8-dimethyl-6, 12-methano-6H,l2H,l3H-[1] benzopyran[4,3-d][1,3]benzodioxocin-13-one [C20H16O4, Mr = 320·36, tetragonal, P43, a = 10·788 (4), c = 13·587 (9) Å, V = 1581 (2) Å3 , Z = 4, Dx = 1·345 g cm-3, λ(Mo Kᾱ) = 0·7107 Å, μ = 0·873 cm-1, F(000) = 672, T = 295 K, R = 0·049 for 1425 observations with I ≥ 2·5σ(I)] are described. They are acyl and aryl ketals of warfarin, respectively, and contain an embedded dihydropyran ring. The molecules were studied as part of a series of axial 2-O-substituted-2-methyl-3,4-dihydro-2H-pyran structures which show (hemi)ketal C—O bond-length variations indentified through factor analysis with the systematic geometrical changes associated with a spontaneous elimination (El-like) reaction from the ketal leading to 2-methyl-4H-pyran. As in α-tetrahydropyranyl acetals the C—0 lengths in dihydropyranyl ketals can be expressed as a function of the electron-withdrawing ability of the substituent conjugate base, and the slopes of the relat1onsh1ps for the two systems are similar. Corresponding endocyclic C—0 lengths are about 0·052 Å longer in these model dihydropyranyl ketals