Racemic
4-O-allyl-myo-inositol-1,3,5-orthoesters
were resolved as the corresponding diastereomeric dicamphanates by
crystallization from alcoholic solvents. Crystals of the two diastereomers
of myo-inositol orthoacetate and one diastereomer
each of myo-inositol orthoformate and myo-inositol orthobenzoate were obtained in >99% purity, on gram
scale.
The configuration of all these diastereomers was established by conversion
to known chiral myo-inositol derivatives as well
as by single crystal structure analysis. It is interesting to note
that the procedures for the separation of diastereomeric myo-inositol orthoesters could be evolved due to the knowledge of crystal
growth and crystal structures of inositol derivatives of comparable
molecular structures. Due to the synthetic versatility of myo-inositol orthoesters, the methods described provide
rapid and convenient access to a variety of chiral inositol derivatives
with high synthetic potential.
The crystal structure database was used to identify inositol derivatives that could be crystallizing as racemic conglomerates. Among the six racemic inositol derivatives identified, racemic 4-O-tosyl-6-O-benzyl-myoinositol-1,3,5-orthoformate (A) was found to be a true conglomerate and was resolved on the multigram scale by the preferential crystallization technique. This resolution procedure does not require the use of any enantiomeric resolving agent. The resolved enantiomers of A are useful for the synthesis of natural and unnatural enantiomeric derivatives of inositol, since they carry orthogonal hydroxy protecting groups. Racemic 4-O-methanesulfonyl-myoinositol-1,3,5-orthoformate (B) on crystallization from common organic solvents generally yielded racemic twin crystals, while in the presence of structural analogs as additives, they yielded true racemic crystals. A comparison of the crystal structures of the true racemate, twinned crystal and crystal of one of the enantiomers of B, revealed the reasons for the formation of polymorphic (twin) crystals. Such instances are relatively rarely encountered but nevertheless shed light on our understanding of polymorphism and twinning of crystals.
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