| Popis: |
X-Ray and 1H N.M.R. studies on pyranoid rings 1,2- cis -fused to dioxolane rings in acetylated D -gluco- and D --galactopyranose derivatives demonstrate that the configuration of the dioxolane ring influences the conformation of the pyranoid ring in the D -gluco but not in the D -galactopyranose series. The crystal structure of 3,4,6-tri- O -acetyl-1,2- O -(R)--(l-cyano-ethylidene)-α- D -glucopyranose ( 1 ) and 3,4,6-tri- O -acetyl-1,2- O -( R )-(1-cyano-ethylidene)-α- D -galactopyranose ( 2 )have been determined by X-ray analysis. Lattice parameters for 1 are a=20.6021 (11), b=8.0438 (2), c=5.5541 (1) A and β= 95.588 (3)° for a cell with P21 symmetry. These parameters for 2 are a=20.3361 (7), b=10.0907 (2), c=18.9115 (5) A, β =112.399 (2)°, C2, with two crystallographycally independent molecules. The conformation of the pyranoid ring in both compounds can be described as flattened 4C1 and that of the dioxolane ring as distorted E1. The importance of the torsion angles for describing problems of configuration is remarked and the use of relative configurational angles is stressed. The 1H N.M.R. spectra of 1 and 2 and 3,4,6-tri- O -acetyl-1,2-O-(S)- and (R)-ethylidene-α- D -glucopyranose ( 5 and 7 ), 3,4,6-tri-O-acetyl--1,2-O-(S)- and ( R )-ethylidene-α- D -galactopyranose ( 6 and 8 ), and 3,4,6-tri- O -acetyl-1,2- O -( S )-and ( R )-benzylidene-α- D -glucopyranose ( 9 and 10 ) have been analyzed by using iterative computer methods and N.O.E. measurements. The results indicate that the major solution conformation of the pyranoid ring of the derivatives in the D -gluco series 1 , 5 and 9 may be described as flattened 4C1 and that of 7 and 10 as 2 S 5. The major solution conformation of the pyranoid ring in all compounds in the D -galacto series ( 2 , 4 , 6 , 8 ) may be described as flattened 4 C 1. |
