CONFORMATIONAL FEATURES OF RHAMNOPYRANOSE DERIVATIVES - THE MOLECULAR-STRUCTURE OF METHYL 2,3,4-TRI-O-ACETYL-ALPHA-L-RHAMNOPYRANOSIDE

Methyl 2,3,4-tri-O-acetyl-alpha-L-rhamnopyranoside C13H20O8, M(r) = 30 4.3, is monoclinic, space group C-2, with a = 23.619(1), b = 8.2168(5) , c = 19.093(1) Angstrom, beta = 118.72(1)degrees, V = 3249.6(8) Angst rom(3), D-c = 1.244 g cm(-3), mu (MoK(a)lpha) = 0.97 cm(-1) and Z = 8. The structure was refined to R = 0.044 and R(w) = 0.039 for 1969 obse rved reflections. There are two independent molecules in the asymmetri c unit. The bond lengths and bond angles of the pyranose rings of the two are in good agreement within the limits of error. The molecules ha ve similar conformation except for the orientation of one of the aceto xy groups. Each molecule is a normal C-1(4) chair with Cremer-Pople pu ckering parameters Q = 0.557(6) Angstrom, theta = 174.6(2)degrees and psi = 144.6(9)degrees for molecule Angstrom and 0.564(4) Angstrom, 177 .9(1)degrees and 30.8(8)degrees for molecule B, respectively. The acet yl groups have the planar, (S)-cis conformation most commonly observed . They are oriented with the acetyl planes within +/-35 degrees of the C-H bond at the ring carbon atom to which they are attached.