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) = 304.3, is monoclinic, space group C2, with a = 23.619(1), b = 8.2168(5), c = 19.093(1) A, beta = 118.72(1) degrees, V = 3249.6(8) A3, Dc = 1.244 g cm-3, mu (MoK alpha) = 0.97 cm-1 and Z = 8. The structure was refined to R = 0.044 and Rw = 0.039 for 1969 observed reflections. There are two independent molecules in the asymmetric 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 have similar conformation except for the orientation of one of the acetoxy groups. Each molecule is a normal 1C4 chair with Cremer-Pople puckering parameters Q = 0.557(6) A, theta = 174.6(2) degrees and psi = 144.6(9) degrees for molecule A and 0.564(4) A, 177.9(1) degree and 30.8(8) degrees for molecule B, respectively. The acetyl 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.