Configuration-dependent conformational transmission in trigonal-bipyramidal phosphorus(V) compounds. Enhanced gauche (-) population around the C(5)-C(6) linkage in 6-phosphorus(V) phosphorylated tetramethyl-a-D-galactopyranoside

A 300-MHz 1H NMR variable-temperature study of the 1'-phosphorylated trigonal-bipyramidal (TBP) tetrahydropyran-2-methyl model compound 4 is reported. For this compound, in which both the equatorial and axial sites undergoing phosphorus pseudorotation bear a tetrahydro-pyran-2-methyl group, a precise conformational analysis of the C(1')-C(2) bond is possible. The thermodynamic parameters which result from this analysis allow the assignment of the proton resonating at low field as H(1'). It is shown that, for the group situated axially in the Pv TBP, an enhanced gauche (-) population [O(1') and O(1) trans-situated] occurs around C(1')-C(2) (290 K, 59%; 230 K, 74%) as compared to the corresponding Piv compound 3 (298 K, 10%). The same conformational transmission is observed for 6-phosphorylated pyranosides, but in this case is found to depend upon the configuration at C(4). It appears that 6-phosphorylated methyl 2,3,4-tri-O-methyl--D-galactopyranoside shows a conformational change around C(5)-C(6) towards a higher g- population, going from 6-Piv to 6-Pv TBP, which is absent in the corresponding 6-phosphorylated methyl 2,3,4-tri-O-methyl--D-glucopyranoside. Apparently, the configuration at C(4) is jointly liable for the transmission in the Pv TBP.