A Conformationally Restricted Cyclic Phosphate Analogue of Inositol Trisphosphate: Synthesis and Physicochemical Properties

The design and total synthesis of DL-6-deoxy-6-(hydroxymethyl)-scyllo-inositol 1:7-cyclic 2,4-trisphosphate (4), a conformationally restricted cyclic phosphate analogue of the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], is described. The protected inosose 2,4,6/3,5-pentahydroxy-3,5-bis-O-(p-methoxybenzyl)-2,4,6-O- methylidynecyclohexanone (7) was obtained from myoinositol orthoformate in two steps, and Wittig methylenation and then hydroboration-oxidation using 9-BBN-H/OH-/H2O2gave the axial hydroxymethyl derivative 9. A series of protection/ deprotection steps provided the diol 13, which was converted into two cyclic phosphate esters 14a and 14b, epimeric at phosphorus, by reaction with (benzyloxy)bis(N,N-diisopropylamino)phosphine/ 1H-tetrazole followed by m-CPBA. Two other hydroxyl groups were then exposed and phosphorylated, and total deprotection gave racemic 4. NMR studies confirmed that in 4 the phosphate group equivalent to the 4-phosphate of Ins(1,4,5)P3is held in the positive gauche orientation and that the inositol ring maintains a chair conformation from pH 2 to 12. Investigation of the acid-base properties of 4 using potentiometric and31P NMR techniques showed that, over the physiological pH range, 4 behaves as a diprotic acid and that the ionization of the phosphate group equivalent to the 5-phosphate of Ins(1,4,5)P3is enhanced. In biological assays, 4 appears to behave as a weak full agonist at the platelet Ins(1,4,5)P3receptor, and the possible interpretation of this result is discussed.