Precursors acid-catalysed hydrolysis

Levulinic acid (4-oxopentanoic acid) obtained from cellulose and hemicellulose materials by acid-catalysed hydrolysis can be hydrogenated to y-valerolactone (5-methyldihydrofuran-2(3H)-one) which is a useful platform chemical precursor to liquid fuels, polymers and fine chemicals. Carbon-supported tin-ruthenium catalysts were active and selective in the hydrogenation of levulinic acid (Scheme 21.4). ... 

Allenic silylated ethers are efficient precursors of H-a,/S-unsaturated aldehydes and acyl silanes (Scheme 34), " and unsaturated acyl derivatives of silicon, germanium, and tin have been prepared from 1-lithio-derivatives of conjugated vinyl ethers (Scheme 35). " Acid-catalysed hydrolysis of the metallovinyl ethers formed in the latter sequence leads to the acylmetallanes. However, 3-substituted vinyl ethers do not give exclusively the conjugated enones. 

In a comprehensive study of 2-multiprenylphenols which are involved as biosynthetic precursors of ubiquiniones, three approaches were adopted (ref.6). In the first, to avoid 4-substitution found in acid-catalysed reaction of phenol, a blocking 4-methoxycarbonyl group was used. Thus reaction of methyl 4-hydroxybenzoate with phytol in dioxane containing boron trifluoride etherate at 50°C afforded a low yield of methyl 4-hydroxy-2-phytylbenzoate which upon hydrolysis and decarboxylation gave 2-phytylphenol. 

The in situ sol gel silica process is an alternative method of incorporating silica into rubbers. The in situ sol-gel approach, in preparing silica-reinforced rubber, is considered a novel technique. In principle, a rubber is swollen in a silica precursor, e.g. tetraethyl orthosilicate (TEOS) and the TEOS-swollen rubber is then subjected to hydrolysis and condensation reactions which could be acid- or base-catalysed. Subsequently, silica particles are generated in the rubber or rubber network. The chemical reactions involved in the formation of silica particles are shown in Scheme 7.1 where the overall reaction indicates that 1 mole of TEOS requires 2 moles of water to be converted into anhydrous silica. 

In kinetic resolutions (Scheme 3.2-3.5) it is often the case that one of the products is required, while the other is not and must be discarded or recycled (e.g. racemised). Such operations can be wasteful or expensive. On the other hand, the biotransformation of wcso-compounds or prochiral compounds allows for the possibility of preparing an optically pure compound in quantitative yield. In Scheme 3.7, two examples of the use of meso-compounds are described. The diester (11) is made up of a complex dicarboxylic acid unit derivatised as the dimethyl ester. Pig liver esterase catalyses the hydrolysis of one of the ester groups to give the acid (12) (95% e.e.) in 96% yield. This compound is an excellent precursor of the natural product neplanocin. Note that the acid (12) is not a substrate for pie, and thus the reaction stops at the half-way stage. The compound (13), like (11), possesses a plane of symmetry. Hydrolysis catalysed by porcine pancreatic lipase (ppl) affords the alcohol (14) (>98% e.e.) in quantitative yield. The latter compound has been used to make fluorocarbocyclic adenosine (C -adenosine), a stable analogue of the naturally occurring nucleoside adenosine. 

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