ON THE EXTENT OF CHAIR DISTORTION IN TETRA-AXIAL DERIVATIVES OF 2,3,4-TRI-O-BENZOYL-BETA-D-XYLOPYRANOSE

An X-ray analysis of 2,3,4-tri-O-benzoyl-2-chloro-2-deoxy-β-d-xylopyranosyl chloride (7) revealed a 1C4 chair conformation slightly distorted toward the 5Ho half-chair form. The four substituents deviate from idealized axial orientations by only 3-14°, hence, the conformation approximates to a "tetra-axial" arrangement. 1,2,3,4-Tetra-O-benzoyl-β-d-xylopyranose (5) also crystallizes in a 1C4 → 5Ho form, 2,3,4- tri-O-benzoyl-β-d-xylopyranosyl fluoride (1) is more markedly deformed, and 2,3,4-tri-O-benzoyl-β-d-xylopyranosyl bromide (3) deviates from a 1C4 form so much as to not to be designated "tetra-axial". In chloroform solution, 7 appears to retain the 1C4 conformation adopted in the crystal, whereas, for 1, 3, and 5, 4C1 ⇌ 1C4 equilibria are established. © 1990.