Synthesis and mass spectrometry of phenylboronate esters of polyhydric alcohols

Abstract

1. Phenylboronates of acyclic diols, triols and polyhydric alcohols have been synthesised. 2. Mass spectrometry of phenylboronates of acyclic diols has revealed that the following four processes occur as a result of ionisation by electron impact: (a) Elimination of exocyclic groups by cleavage of C-C bonds. (b) Skeletal rearrangement giving rise to highly unsaturated hydrocarbon ions containing 7~10 carbon atoms. (c) Elimination of oxo molecules. (d) A double elimination exclusive to six-membered phenylboronate rings, which provides a means of detecting this structural unit in compounds of hitherto unknown structure. 3. The mass spectra of phenylboronates of triols, tetritols, pentitols and hexitols were interpreted on the grounds of the processes outlined above, and structures assigned accordingly. 4. The hitherto unsuccessful methylation of hydroxyl groups in phenyl-boronates has been achieved using diazomethane and boron trifluoride etherate as reagents. 5. Methylation of phenylboronates followed by hydrolysis of the ester, acetylation and analysis of the product by gas-liquid chromatography combined with mass spectrometry revealed that the phenylboronates of triols are, in fact, mixtures of different boronate ring modifications. The compositions of such mixtures have been compared and rationalised in terms of the structues.