Structure of a key intermediate in the asymmetric synthesis of (+)-KDO.

Methyl 3-deoxy-7,8-O-(1-methylethylidene)- 1-O-(phenylmethyl)-alpha-D-manno-2-octalopyranoside cyclic carbonate, (1), C20H26O8, Mr = 394.42, orthorhombic, P2(1)2(1)2(1), a = 10.667 (3), b = 10.7972 (14), c = 16.509 (5) A, V = 1901.4 (8) A3, Z = 4, Dx = 1.38 g cm-3, mu = 0.9960 cm-1, lambda(Mo K alpha) = 0.7107 A, F(000) = 840, T = 298 K, R = 0.0355 for 2248 reflections, Fo greater than or equal to 4[sigma (Fo)]. The X-ray structure was undertaken to confirm the stereochemistry of the substituents on the six-membered pyranose ring. The two rings are cis-fused with ring junction torsion angles of -18.7 (3) degrees for C2-C3-C8-C9 and -18.2 (2) degrees for O4-C3-C8-O7. The six-membered ring assumes a slightly distorted twist conformation while the fused five-membered ring is in the half-chair conformation. The isopropylidene moiety is in the envelope conformation.