MM3(92) ANALYSIS OF INOSITOL RING PUCKERING

The molecular mechanics method MM3(92) was used to study the puckering of the nine inositoi isomers. Energy surfaces were generated based on the Cremer-Pople puckering definitions and the geometric plate carree representations of Pickett and Strauss. Chair forms are favoured for all the inositol isomers in agreement with experiment. The lowest ener gy non-chair form, more than 10 kJ/mol in free energy above the chair, is a skew of muco-inositol, in which all the hydroxyls are oriented e ither equatorially or pseudo-equatorially Hydrogen-hydrogen coupling c onstants are in good agreement with experiment. The predicted order of stability for the isomers is scyllo- > myo- > chiro- > neo- > epi > m uco- approximate to allo- > cis-inositol. This order generally agrees with Angyal's proposed stability order based on the number and type of gauche: and 1,3-diaxial ring substituents. In contrast with recent ab initio calculations and values calculated with Angyal's method, the M M3 gas-phase free energy of scyllo-inositol was lower than that of myo -inositol by 5.5 kJ/mol. The calculated transition energy for the chai r-to-chair conversion of cis-inositol differs by 12.5-17.2 kJ/mol from the experimental value, depending on the strength of the electrostati c field.