Ethanol effect organic esters

Schniirch and Stanetty have shown that synthesis of boionic add esters could be effectively carried out in ball mill (Scheme 4.49) [37]. Equimolar quantities of pinacol or 2,2-dimethylpropan-l,3-diol milled with boronic adds for Ih and washed up with organic solvent afford boronic esters 174 and 175 in excellent purity and high yields (Table 4.17). This ball-milling method is atom efficient and waste free, requiring only a small amounts of nontoxic solvents (ether, ethanol) for isolation. Reaction is much faster (1 h) than the classical methods (several hours or days) which also need removal of water by azeotropic distillation. The scale-up by factor 20 was performed on a Fritsch Analysette 3 PRO ball mill, with the same efficiency as in small-scale batches. 

Once the acetates have served their purpose their removal is most easily effected by the Zemplen procedure [54] (catalytic NaOMe in MeOH) (Methods 16 and 17) but more selective methods—such as guanidine in ethanol [55], or DBU in benzene (Figure 2.38) [56]—can leave less reactive benzoate esters intact. For deprotection, lipases can again offer exquisite discrimination between acetates which can be particularly useful in sensitive substrates where classical methods fail. An idea of the selectivity that can be attained is given in Figure 2.39 which shows the specific sites of deacetylation in peracetyl sucrose using lipases from different organisms [57]. 

Ester or lipid hydrolysis is most conveniently effected in the laboratory by refluxing with aqueous ethanolic alkali. Acidification of the hydrolysate liberates fatty acids which can be extracted with hexane or other organic solvent. Non-acidic compounds (unsaponi-fiables) such as hydrocarbons, long-chain alcohols, sterols and glycerol ethers will also be present in the organic extract, but glycerol remains in the aqueous phase. The hydrolysis may be effected quantitatively for determination of saponification equivalent or saponification value (Section 6.2.10). 

Dilute solutions ( 10" moldm" ) of Wilkinson s complex in organic solvents such as ethanol/benzene actively catalyse the hydrogenation of alkynes and alkenes at ambient temperature and pressure (1 atm, H2). Traces of oxygen must be excluded. Hydrogenation rates are dependent on steric effects. The following general order has been established 1-alkenes > cis-2-alkenes > trans-2-alkenes > trans-3-alkenes. An exception is ethene itself, which forms a rather stable complex (Fig. 5.19) so that higher temperatures are required. Arenes, esters, ketones, carboxylic acids, amides and nitro compounds are unaffected, but aldehydes are slowly decarbonylated. 

The classical method for removal of the phthaloyl function is treatment with aqueous or ethanolic hydrazine hydrate. The hydrazide by-product separates out by precipitation. Hydrazine is a good nucleophile (so-called a-effect) which will attack the anhydride-like phthaloyl function under conditions that will not affect peptide linkages. However, there is a danger of hydrazinolysis of any ester linkages, if present. Other protecting groups will not be affected by the hydrazinolysis. The use of hydrazine in hydroxylic solvents may be replaced by phenylhydrazine in organic solvents, in which... 

---