Protection of the five hydroxyls with a suitable group, would allow cleavage of the dibenzyl phosphate to reveal the free alcohol (167). Ideally, the cleavage of the dibenzyl phosphate would be conducted in one hydrolysis step. The group used to protect the hydroxyl groups should therefore be stable to acid or base hydrolysis. Initially, a benzyl group was chosen as an ideal candidate.
The benzylation of the five hydroxyl groups proved to be extremely difficult to achieve. One of the main issues associated with the attempted benzylation was the poor solubility of the starting material. The 1-bis(phenyloxy)phosphoryl-L-myo-inositol (116) could only be dissolved in water and methanol. Whilst methods of benzylation do exist for reaction in protic polar solvents, they are not ideal due to the potential quenching of reactive intermediates.
Initial attempts at the perbenzylation were made using pyridine as a potential solvent. The starting material was not soluble in pyridine, but it was hypothesised that if some reaction was to occur then the partially benzylated product would be taken up by this solvent and the reaction could then proceed more rapidly. Using an excess of benzyl bromide (50 equivalents) as a benzylation reagent did not furnish any reaction at all. Addition of tetrabutylammonium iodide (TBAI) did nothing to help the situation, and again no reaction was observed.
Previously, phenyldiazomethane (115) had been used as a reagent for the selective benzylation of the phosphate group. It was hypothesised that with a tweak to the reaction conditions, this reagent could be used to achieve perbenzylation on either the L-myo- inositol 1-phosphate (19) or 1-bis(phenyloxy)phosphoryl-L-myo-inositol (116). Attempts at the perbenzylation of both of these substrates were conducted by treatment with an excess of the reagent in methanol at room temperature overnight. 1- Bis(phenyloxy)phosphoryl-L-myo-inositol (116) failed to react with phenyldiazomethane and L-myo-inositol 1-phosphate (19) was converted efficiently into 1- bis(phenyloxy)phosphoryl-L-myo-inositol (116), but the reaction did not progress past this point.
Chittenden162 was able to effect the selective benzylation of the secondary hydroxyl group within glycerol (185) using tin (II) chloride as a Lewis acid (Scheme 4.2.10).
Scheme 4.2.10 – Selective secondary alcohol benzylation using phenyldiazomethane and tin (II) chloride.162 Reagents and conditions: i. Tin (II) chloride, MeOH, DCM, 48 hr, 28 %.
Unfortunately, efforts to conduct the Lewis base promoted benzylation of 1- bis(phenyloxy)phosphoryl-L-myo-inositol (116) using phenyldiazomethane proved unsuccessful. Tin (II) chloride and cerium (III) chloride were trialled for this purpose as they were readily accessible, but catalytic addition of neither of these Lewis bases afforded any benzylation of the substrate.
An example was found in the literature of fluoroboric acid being used in conjunction with phenyldiazomethane to efficiently benzylate alcohols.163 Addition of this reagent to the reaction, however, proved unsuccessful with 1-bis(phenyloxy)phosphoryl-L-myo-inositol (116).
Formation of a tin acetal with dibutyl tin oxide (nBu2SnO) and subsequent treatment with
benzyl bromide is known as a method of selectively benzylating diols.48 This method can be used to obtain selectivity in which of the two hydroxyl groups are being protected, but also requires a diol for reaction due to the formation of the tin acetal. This method should therefore lend itself to reaction within methanol. 1-bis(phenyloxy)phosphoryl-L-myo- inositol (116) was refluxed with nBu2SnO in a mixure of methanol and toluene for 6 hours
to form the tin acetal. The reaction was concentrated before dissolution in toluene and addition of benzyl bromide and tetrabutylammonium bromide. The mixture was refluxed overnight, however, no benzylated product was observed, and it appears as though the tin acetal was not formed in the first instance. The reaction was also attempted in acetonitrile with the same result.
With the benzylation of 1-bis(phenyloxy)phosphoryl-L-myo-inositol (116) proving elusive, a different protecting group was sought to effect the protection of the five inositol hydroxyls. Ideally, a protecting group would be chosen that is stable at pH extremes whilst being able to be introduced with water or methanol as the solvent. There was no ideal candidate with this chemistry that could be removed at the end of the epi-mycothiol (164) synthesis without major effects on the rest of the molecule.
With reconsideration of the phosphate cleavage, acetylation of the hydroxyl groups looked like a viable option. Treatment of 1-bis(phenyloxy)phosphoryl-L-myo-inositol (116) with acetic anhydride, pyridine and DMAP furnished the pentaacetate 187 (Scheme 4.2.11) in a 22 % yield. Repetition of the reaction with freshly distilled pyridine and a new bottle of acetic anhydride pushed the yield up to only 27 %. Unfortunately, it did not seem possible to achieve a higher yield for this step. As the reaction progressed, a side product was being formed in the reaction, which meant that formation of the product seemed to be capped at 27 %. The side product was not observed by thin-layer chromatography (TLC) until roughly 5 hours after beginning, but after this point it appeared that no more of the desired product was being produced. Exclusion of DMAP from the reaction mixture had little to no effect on the reaction outcome. Conducting the reaction overnight, with the DMAP absence still produced the side product and a similarly low yield of the desired pentaacetate 187. It is hypothesised that this side product is the result of some type of rearrangement occurring with the phosphate moiety, although the identity of this compound is not clear from crude 1H NMR data. If time had allowed, further investigation into this side product would have been investigated.