Types: Doctoral thesis
A study of synthetic routes leading to hydroxyalkaneboronic acids, which were expected to display some useful and interesting biological and chemical properties, has been carried out with reference to reaction sequences involving the hydroboration of alkenes and interaction of Grignard reagents with borate esters. \ud \ud Hydroboration followed by redistribution with trimethylene borage was carried out on the O-tetrahydropyranyl ethers of prop-2-en1-ol, pent-4-en-1-ol, pent-4-en-2-ol, but-3-en-2-ol and pent-1-en-3-ol but in most cases a complex product mixture was obtained. However, 2-(3-[2 –tetrahydropranyloxy]propyl)-1,3,2-dioxaborinane was identified as the major component in the product mixture from 1-(2’-tetrahydropyranyloxy)-prop-2-ene. Evidence was obtained to show that these complex mixtures arose from interaction between trimethylene borate and the tetrahydropyranyloxy functionality. Some mechanistic rationalisation is proposed. \ud \ud Hydroboration followed by equilibration and hydrolysis to a boronic acid was more successful and thus 5-hydroxypentaneboronic acid was prepared from 1-(2'-tetrahydropyranyloxy)-pent-4-ene, 3-hydroxybutaneboronic anhydride from 2-( 2 '-tetrahydropyranyloxy)-but-3-ene, and 3-benzyloxypropaneboronic acid from allyl benzyl ether, although the yields were low. Hydroboration followed by esterification with methanol was carried out on 1-(2'-tetrahydropyranyloxy)-pent-4-ene and dimethy1-1-(5-[2 –tetrahydropyranyloxy]-pentaneboronate was isolated. \ud \ud The use of Grignard reagents with borate esters as a route to hydroxyalkaneboronic acids was less successful and no such compounds could be isolated by this preparative method. 1-Hexaneboronic acid and several of its ester derivatives were successfully prepared to check this synthetic route, and samples of 1-hexaneboronic acid were used to investigate the oxidative and hydrolytic stabilities of a typical alkaneboronic acid. The results showed that it was stable under normal storage conditions.\ud \ud Before proceeding to the study of boronic acid syntheses involving the use of monosaccharide derivatives, a model system, 1,2-0-cyclohexylidene glycerol, was selected for the introduction of an unsaturated site using organocopper reagents. Thus, 1,2-,0-cyclohexylidene-3-\ud -deoxy-3-vinyl glycerol was shown to be formed by the action of divinylcopper lithium on 1,2-0- cyclohexylidene-3-deoxy-3-iode glycerol albeit in low yield and its separation and purification could not be effected.\ud \ud A carbon chain extension process using l,2-0-cyclohexylidene-3-deoxy-3-iode glycerol was considered but an easier route to 1,2-0-cyclohexylidene-4-bromobutane-1,2-diol was devised starting from 4-bromobut-l-cae. Attempted boronic acid synthesis via a Grignard derivative of this compound was unsuccessful.\ud \ud Three typical unsaturated monosaccharides namely, 3,4,6,-tri-0-acetyl-D-glucal, 6-deoxy-1,2:3,4-di-0-isopropylidene-B-L-arabino-bex-5-enopyranose and 1,2:5,6-di-0-cyclohexylidene-3-deoxy-X-D-\ud erythro-hex-3-enofuranose were synthesised and subjected to hydroboration-redistribution processes. Product mixtures were obtained, which were not considered in these cases to arise from interaction of protecting groups with trimethylene borate, and these mixtures were found to contain carbohydrate dioxaborinanes. The major product from the starting material 3,4,6-tri-0-acetyl-D-glucal was separated, purified, and characterised as a boron-containing monosaccharide.
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- A. Finch, J. C. Lockhart, J. Pearn, J.Org.Chem.. 1961, 26, 3250.
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