Synthesis of the enantiomers of myo-inositol 1,2,4,5-tetrakisphosphate, a regioisomer of myo-inositol 1,3,4,5-tetrakisphosphate

Routes for the synthesis of racemic myo-inositol 1,2,4,5-tetrakisphosphate DL-Ins(1,2,4,5)P45ab and the chiral antipodes D- and L-myo-inositol 1,2,4,5-tetrakisphosphate 5a and 5b, respectively, are described. For the synthesis of racemate 5ab, 3,6-di-O-benzoyl-1,2:4,5-di-O-isopropylidene-myo-inositol 7ab is prepared in two steps from myo-inositol. The ketals are hydrolysed under acidic conditions to give DL-1,4-di-O-benzoyl-myo-inositol 8ab. Phosphitylation of compounds 8ab using chloro(diethoxy)-phosphine in the presence of base, followed by oxidation and a three-step deprotection strategy, gives DL-Ins(1,2,4,5)P45ab. The chiral tetrakisphosphates 5a and 5b are synthesized using a different route. The 4,5-isopropylidene group of DL-3,6-di-O-benzyl-1,2:4,5-di-O-isopropylidene-myo-inositol 13ab are selectively removed under mild acidic conditions to give diol 14ab. p-Methoxybenzylation at the 4,5-positions followed by acid hydrolysis of the cis-isopropylidene ketal affords cis-diol 16ab. Selective coupling of (S)-(+)-O-acetylmandelic acid with diol 16ab at the equatorial hydroxy group provides two diastereoisomers 18 and 19, which are separated by chromatography. Basic hydrolysis of the individual diastereoisomers provides the enantiomers 16a and 16b. Acidic hydrolysis gives D- and L-3,6-di-O-benzyl-myo-inositol 20a and 20b, respectively. Phosphitylation and oxidation of tetraols 20a and 20b gives the fully blocked derivatives, which are deprotected to give tetrakisphosphates 5a and 5b, respectively. The absolute configuration of compound 20a is established by a chemical method. DL-1,2:4,5-Di-O-isopropylidene-myo-inositol 12ab is coupled to (S)-(+)-O-acetylmandelic acid to give a mixture of bis-esters 26 and 27 and crystallisation of the mixture of diastereoisomers affords pure isomer 27. Basic hydrolysis gives the pure enantiomer 12a (for which the absolute configuration is known) and benzylation followed by acid hydrolysis gives tetraol 20a with the same physical properties as compound 20a prepared by a different route described previously. D-Ins(1,2,4,5)P45a is a potent mobiliser of intracellular Ca2+ions in permeabilised platelets, while L-Ins(1,2,4,5)P45b is inactive.