Mysore S. Shashidhar

Convenient synthesis of 4,6-di-O-benzyl-myo-inositol and myo-inositol 1,3,5-orthoesters

Convenient high yielding methods for the preparation of 4,6-di-O-benzyl-myo-inositol, myo-inositol 1,3,5-orthoformate and myo-inositol 1,3,5-orthoacetate, without involving chromatography are described. Myo-inositol was converted to racemic 2,4-di-O-benzoyl-myo-inositol 1,3,5-orthoformate by successive treatment with triethyl orthoformate and benzoyl chloride. The dibenzoate obtained on benzylation with benzyl bromide and silver(I) oxide gave 2-O-benzoyl-4,6-di-O-benzyl-myo-inositol 1,3,5-orthoformate. Deprotection of the benzoate and the orthoformate with isobutylamine and aqueous trifluoroacetic acid, respectively gave 4,6-di-O-benzyl-myo-inositol in an overall yield of 67%. Myo-inositol orthoformate and orthoacetate were prepared and isolated as their tribenzoates. The free orthoesters were regenerated by deprotection of the benzoates by aminolysis with isobutylamine.

Sulfonate protecting groups. Regioselective sulfonylation of myo-inositol orthoesters-improved synthesis of precursors of D- and L-myo-inositol 1,3,4,5-tetrakisphosphate, myo-inositol 1,3,4,5,6-pentakisphosphate and related derivatives.

The regioselectivity of sulfonylation of myo-inositol orthoesters was controlled by the use of different bases to obtain the desired sulfonate. Monosulfonylation of myo-inositol orthoesters in the presence of one equivalent of sodium hydride or triethylamine resulted in the sulfonylation of the 4-hydroxyl group. The use of pyridine as a base for the same reaction resulted in sulfonylation of the 2-hydroxyl group. Disulfonylation of these orthoesters in the presence of excess sodium hydride yielded the 4,6-di-O-sulfonylated orthoesters. However, the use of triethylamine or pyridine instead of sodium hydride yielded the 2,4-di-O-sulfonylated orthoester. Sulfonylated derivatives of myo-inositol orthoesters were stable to conditions of O-alkylation but were cleaved using magnesium/methanol or sodium methoxide in methanol to regenerate the corresponding myo-inositol orthoester derivative. These new methods of protection-deprotection have been used: (i) for the efficient synthesis of enantiomers of 2,4-di-O-benzyl-myo-inositol, which are precursors for the synthesis of D- and L-myo-inositol 1,3,4,5-tetrakisphosphate; (ii) for the preparation of 2-O-benzyl-myo-inositol which is a precursor for the preparation of myo-inositol 1,3,4,5,6-pentakisphosphate.

Crystal-to-crystal transformation amongst dimorphs of racemic 2,6-di-O-(p-halo benzoyl)-myo-inositol 1,3,5-orthoformates that achieves halogen bonding contacts

Racemic 2,6-di-O-(p-halobenzoyl)-myo-inositol 1,3,5-orthoformates (bromo (1) and chloro (2)) produced two polymorphs each, thin needle type crystals (Form I) were obtained from methanol, whereas larger rectangular crystals (Form II) were produced from ethyl acetate. Both forms could be produced concomitantly on crystallization (of 1 or 2) from ethyl acetate–light petroleum ether mixture; the yield of Form II crystal was always much more compared to Form I crystals. Although, a one-dimensional isostucturality linking molecules via O–H⋯O hydrogen bonding is seen in both forms, the difference arises in linking these chains. In larger Form II crystals (of 1 and 2), the adhesions are viahalogen bonding (C–X⋯OC, X = Cl, Br) contacts, whereas in smaller Form I crystals C–H⋯X contacts join them. Interestingly, DSC and X-ray crystallographic studies confirmed the thermal crystal-to-crystal transition of Form I to Form II crystals. Transformation of a minor to major polymorph containing ‘C–Br⋯OC’ contacts, similar to the phase transition previously reported by us in the case of 2,4,6-tri-O-(p-bromobenzoyl)-myo-inositol 1,3,5-orthoformate, suggests the role of X⋯O short contacts in preferential nucleation and crystal growth.

Enhancing Intermolecular Benzoyl‐Transfer Reactivity in Crystals by Growing a “Reactive” Metastable Polymorph by Using a Chiral Additive

Racemic 2,4-di-O-benzoyl-myo-inositol-1,3,5-orthoacetate, which normally crystallizes in a monoclinic form (form I, space group P2(1)/n) could be persuaded to crystallize out as a metastable polymorph (form II, space group C2/c) by using a small amount of either D- or L- 2,4-di-O-benzoyl-myo-inositol-1,3,5-orthoformate as an additive in the crystallization medium. The structurally similar enantiomeric additive was chosen by the scrutiny of previous experimental results on the crystallization of racemic 2,4-di-O-benzoyl-myo-inositol-1,3,5-orthoacetate. Form II crystals can be thermally transformed to form I crystals at about 145 degrees C. The relative organization of the molecules in these dimorphs vary slightly in terms of the helical assembly of molecules, that is, electrophile (El)...nucleophile (Nu) and C-H...pi interactions, but these minor variations have a profound effect on the facility and specificity of benzoyl-group-transfer reactivity in the two crystal forms. While form II crystals undergo a clean intermolecular benzoyl-group-transfer reaction, form I crystals are less reactive and undergo non-specific benzoyl-group transfer leading to a mixture of products. The role played by the additive in fine-tuning small changes that are required in the molecular packing opens up the possibility of creating new polymorphs that show varied physical and chemical properties. Crystals of D-2,6-di-O-benzoyl-myo-inositol-1,3,5-orthoformate (additive) did not show facile benzoyl-group-transfer reactivity (in contrast to the corresponding racemic compound) due to the lack of proper juxtaposition and assembly of molecules.

Sulfonate protecting groups. Improved synthesis of scyllo-inositol and its orthoformate from myo-inositol.

A convenient high yielding method for the preparation of scyllo-inositol and its orthoformate from myo-inositol, without involving chromatography is described. myo-Inositol 1,3,5-orthoformate was benzoylated to obtain 2-O-benzoyl-myo-inositol 1,3,5-orthoformate. This diol was tosylated and the benzoyl group removed by aminolysis in a one-pot procedure to obtain 4,6-di-O-tosyl-myo-inositol 1,3,5-orthoformate. Swern oxidation of the ditosylate, followed by sodium borohydride reduction and methanolysis of tosylates gave scyllo-inositol 1,3,5-orthoformate (isolated as the triacetate). Aminolysis of the acetates followed by acid hydrolysis of the orthoformate moiety with trifluoroacetic acid gave scyllo-inositol in an overall yield of 64%.

Regioselective O-acylation of myo-inositol 1,3,5-orthoesters: the role of acyl migration

Abstract An efficient and general method for the preparation of 2-O-acylated derivatives of myo-inositol 1,3,5-orthoesters via isomerization of the corresponding 4(6)-O-acylated derivatives has been described. The isomerization involves a novel 1(axial)→3(equatorial) intramolecular acyl migration assisted by a metal ion.

Synthesis of the aminocyclitol units of (-)-hygromycin A and methoxyhygromycin from myo-inositol.

Concise and efficient syntheses of the aminocyclitol cores of hygromycin A (HMA) and methoxyhygromycin (MHM) have been achieved starting from readily available myo-inositol. Reductive cleavage of myo-inositol orthoformate to the corresponding 1,3-acetal, stereospecific introduction of the amino group via the azide, and resolution of a racemic cyclitol derivative as its diastereomeric mandelate esters are the key steps in the synthesis. Synthesis of the aminocyclitol core of hygromycin A involved chromatography in half of the total number of steps, and the aminocyclitol core of methoxyhygromycin involved only one chromatography.

Capturing a metastable chiral polymorph of an achiral molecule--hexa-O-benzoyl-myo-inositol.

myo-Inositol hexabenzoate having meso configuration produces chiral polymorph (form I) when crystallized rapidly but yields achiral polymorph (form II) when allowed to crystallize slowly; in the mother liquor form I slowly but completely disappears to give form II.

Sulfonate protecting groups. Regioselective O-sulfonylation of myo-inositol orthoesters

Abstract Sulfonylation of myo -inositol 1,3,5-orthoesters with alkyl or aryl sulfonyl chlorides in the presence of sodium hydride gives the corresponding 4,6-di- O -sulfonates in good yields. These sulfonates can be cleaved with magnesium in methanol to generate the free myo -inositol derivative. This methodology was used for the preparation of racemic 2,4-di- O -benzyl- myo -inositol and 2- O -benzyl- myo -inositol, which are precursors for some phosphoinositols.

Crystal-to-crystal thermal phase transition amongst dimorphs of hexa-O-p-toluoyl-myo-inositol conserving two-dimensional isostructurality

Triclinic (P-1) crystals of hexa-O-p-toluoyl-myo-inositol obtained from common organic solvents exhibited single crystal-to-single crystal irreversible phase transition centered at ∼250 °C. The transformation of these crystals to monoclinic P21/n form was revealed using DSC and X-ray diffraction studies. The latter crystals could also be produced by melt crystallization. Crystal structure analysis revealed that the molecules in both forms are linked via bifurcated C–H⋯O interactions to make almost identical centrosymmetric dimers. The neighbouring dimers are bridged via C–H⋯O and aromatic π⋯π stacking interactions to create two-dimensional isostructural assemblies. The difference in the two crystal forms arises from linking of the centrosymmetric dimers along the third dimension; the dimers are centrosymmetrically bridged in the triclinic form, while they have n-glide relationship in the monoclinic form. Comparison of the dimorph structures further revealed that they are actually an excellent case of morphotropism since dimorphs are related by non crystallographic rotation and translation of their basic motif (centrosymmetric dimers) that transforms the triclinic (P-1) phase to a monoclinic (P21/n) phase.