SYNTHESIS OF PHENYLACETYL MIDA BORONATES BY OXIDATIVE CLEAVAGE OF VICINAL DIOLS

Abstract

A synthetical approach to acyl(N-methyliminodiacetyl)boronates starting from 1-substituted alkenylboronates has been developed. A comparison of different methods of oxidative cleavage of an α-borylated C-C bound was made. It was found, that the best results can be obtained by sequential osmium tetroxide-catalyzed dihydroxylation of an alkene moiety followed by cleavage of the obtained vicinal diol. The cleavage procedure takes place at 0°C in homogeneous conditions (solution of periodic acid in THF) and it is complete in 10 minutes (more prolonged contact with an oxidant solution results in degradation of the target compound). On the other hand, using of ruthenium tetroxide-based reagents results in overoxidation with simultaneous loss of boron moiety. Potassium permanganate protocols leads to the α-borylated-α-hydroxyketone, which is prone to further oxidation. Although 1-alkyl-vinylboronates react smoothly with 3-chloroperbenzoic acid to give corresponding oxiranes (without cleavage of C-B bound), the latter ones are stable toward action of sodium meta-periodate or periodic acid. The results were shown on the model compound – phenylacetyl MIDA boronate. Precursor of this compound, namely, Z-2-(N-methyliminodiacetylboryl)-1-phenylbut-2-ene was prepared in four steps, starting from common-use reagents with 32% overall yield. Thus the new approach allows acetyl MIDA boronates to be prepared just in 6 linear steps. It is remarkable, that mild and homogeneous conditions of the oxidation step permit to carry out this transformation on gram scale. A preliminary investigation of these substances stability towards common methods of working up and purification procedures was made. It was found, that phenylacetyl MIDA boronate and preceding diol, both are stable to storage at ambient conditions (tightly closed vessel, ambient temperature) at least for one month, showing no changes in its NMR spectra. Also, these compounds are stable to extractive work up with NaHCO3, Na2S2O3 and diluted acids. Stability toward chromatography on silica, prolonged contact with water or alcohols is limited. Structures and purity of compounds in this work was established by 1H, 13C – NMR and HPLC-analyses.