Acetalization of benzaldehyde

It is important to mix the mandelonitrile with hydrochloric acid immediately it has been separated from the water. Standing results in rapid conversion to the acetal of benzaldehyde and mandelonitrile C(H,CH[OCH(CN)C H,] and/or the iso-nitrile the yield of mandelic acid will, in consequence, be reduced. 

A mixed acetal of benzaldehyde, methanol, and salicylic acid has also been studied. It, too, shows a marked rate enhancement attributable to intramolecular general acid catalysis ... 

The PhSiH3/RhCl(PPh3)3 combination reduces trimethyl orthobenzoate to the dimethyl acetal of benzaldehyde with small amounts of methyl benzyl ether product (Eq. 315) 493... 

The simple cleavage of lactones 1 or 2 with alcohol and acid has not been reported. However, when 1 is treated with benzaldehyde diethyl acetal and hydrochloric acid, ethyl 3,5 4,6-di-0-benzylidene-L-gulonate (47) is formed in >90% yield.77,78 No other isomers were observed, and other acetals of benzaldehyde, as well as aliphatic aldehydes, afford similar products in good yield.77 D Addieco prepared36 similarly protected derivatives of L-gulonic acid by oxidation of l,3 2,4-di-0-ethylidene-D-glucitol (15), followed by esterification of the resulting acid with diazomethane. 

Acetals of benzaldehydes may undergo EGA-catalyzed aldol reactions also with alkyl enol ethers, (22) (R = alkyl), as nucleophiles [31] but in contrast to the reaction with enol silyl ethers the threo isomer is favored in this case. 

An acetal of benzaldehyde may become a protective group of polyhydroxy compounds. The hydroxyl groups in the 1 and 3 positions of glycerol may form a cyclic acetal with benzaldehyde. The secondary hydroxyl group left intact is available for esterification. Hydrogenolytic removal of the benzal group allows the synthesis of the P-monoglyceride (eq. 13.21).46... 

Hydroboration is faster than cleavage with borane, but H2BCI etherate cleaves unsaturated acetals without giving hydroboration (equation 10). Lower yields are obtained with the less reactive Me2S complex, but that reagent gives good yields of dihydroxy diethers from chiral bisacetals (equation 11). The diethyl acetal of benzaldehyde can be reduced to ethyl benzyl ether in moderate yield by decaborane in toluene at 120-130 C (equation 12). °... 

Dimethyldioxirane, acetone, CH2CI2, 0°C, 24h, >95% yield." Although ketone dioxolanes are cleaved to ketones, aldehyde dimethyl acetals will gives the ester, but the generahty of the later process has not been established beyond the acetal of benzaldehyde. Ethers are also oxidized under these conditions. 

Brown and Kirby [46] found relatively efficient intramolecular general acid catalysis for the hydrolysis of the three acetals 3.10, 3.11 and 3.12 (Scheme 2.21). Acetals of benzaldehyde were used to ensure measurable rates of reaction for the much less reactive aliphatic systems. They offered the further advantage that EMs could be estimated, since intermolecular general acid catalysis can be observed for dialkyl acetals of benzaldehyde. 

A mixed acetal of benzaldehyde, tetrahydropyranol, and salicylic acid has also been studied. It, too, shows a marked rate enhancement attributable to intramolecular general acid catalysis. In this case the pH-rate profile (Figure 7.11) shows a plateau in the region Ph 2-5. As the carboxy group becomes protonated below pH 6, it provides an increment owing to intramolecular general acid catalysis. 

Polystyrene-Aluminum chloride (4, 13 5, 13). ( )—AICI3 catalyzes formation of acetals of benzaldehydes. Some catalysis is observed by the polymer alone, probably because it can entrap the water formed on acetalization. ... 

Diastereoselective deprotonation has been observed with enantiopure complexes (e.g.,42) prepared by acetalization of benzaldehyde with asymmetric 1,2-diols (Fig. 3) [32,34,36,50,85,116,122,125]. A series of experiments with S-bu-tane-l,2,4-triol as a source of chiral acetal auxiliary failed to demonstrate synthetic utility [122]. 

The acetalization of benzaldehyde with 1,3-propanediol has been examined in the presence of 1 mol% of a fluorous super-Br0nsted acid, 13 or 8, at azeotropic reflux in cyclohexane with the removal of water for 3 h [Eq. (6)]. Both solid acids are soluble in cyclohexane under reflux conditions, and promote the reaction well to give the desired acetal in good yields. Post-reaction, 13 has been recovered in 96% yield by precipitation at room temperature. However, 13 can not be recovered in the same manner. Besides this acetalization, 8 is also effective as a fluorous catalyst for the acylation of l-menthol with benzoic anhydride [Eq. (7)] and esterification of 3-phenylpropionic acid in methanol [Eq. (8)] [11]. 

The two following examples incorporate the fluorous super Brpnsted acid catalyst 13 for the acetalization of benzaldehyde with 1,3-propanediol and for the... 

The pH-rate profile shown in Fig. 8.2 exhibits a sharp maximum just above pH 6. The observed maximum rate is 2.7 x 10 faster than the hydrolysis of the dimethyl acetal of benzaldehyde under comparable conditions. The bell-shaped nature of the pH-rate profile indicates that of the species available, the monoanion of the acetal is most reactive. It is the species the concentration of which maximizes in the inter-... 

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