Carboxyl compounds latent

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

A more generally satisfactory method is the hydrolysis of cyanohydrins. Here a mixture of the carbonyl compound and aqueous sodium cyanide is gradually acidified at room temperature resulting in the sequential formation of the cyanohydrin and its subsequent hydrolysis to the acid (the cyano group is of course a latent carboxyl group). 

The mechanism of the Strecker reaction has received considerable attention over its lifespan.4 The conversion of a carbonyl compound into an a-amino acid, by this method, requires a two-step process. The first step consists of the three-component condensation of cyanide and ammonia with the carbonyl compound 1 to produce an intermediate, a-aminonitrile 3. The second step involves the hydrolysis of the nitrile functional group to reveal the latent carboxylic acid 4. Whereas the second step is fairly straightforward and can be done under basic or acid conditions, the first step is more involved than one may expect. The widely accepted sequence for the first step is the nucleophilic addition of ammonia to the carbonyl carbon to produce the corresponding imine derivative 2. Once formed, this initial species is captured by the cyanide anion to generate the requisite a-aminonitrile 3. 

Further calorimetric studies by Voets and Van Dael [57, 58] have shown that the latent heat for BP j -I decreases as the chiral-ty of the molecule increases and that its magnitude varies considerably as a function of composition in binary mixtures of R- and S-enantiomers. Recently an a.c. and non-adi-abatic scanning calorimetry investigation of the chiral compound S,S-(+)-4"-(2-methyl-butylphenyl)-4 -(2-methylbutyl)-4-biphen-yl-4-carboxylate (S,S-MBBPC), which has very short pitch, yielded the results given in Fig. 12 [55]. By combining these two calorimetric techniques one can show that there are sharp first-order transitions with small latent heats at the N -BP and the BPi-BPm transitions (the BPu phase is absent in this compound). However, no thermodynamic... 

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