Benzal group

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... 

Except in the case of formaldehyde, the electrophilic character of carbon atom of an aliphatic aldehyde is not strong enough to allow its condensation on a CH3 reactive group. However, such a condensation can occur with an aromatic or pseudoaromatic substance such as benzal-dehyde or pyrroloaldehyde, and the of the resulting dimethine dyes have been used in this last case to obtain the basicity scale of various rings (16). 

In 1937, Kondo and Nagasawa confirmed the reactivity of the sole 2-methyl group in the condensation of 2,4-dimethylthiazole with benzal-dehyde (505) and in the cyclizafion to tbiazolopyrrole in the reaction with phenacyl bromide (506) (Scheme 113). 

Substitution Reactions on the Methyl Group. The reactions that give substitution on the methyl group are generally high temperature and free-radical reactions. Thus, chlorination at ca 100°C, or in the presence of ultraviolet light and other free-radical initiators, successively gives benzyl chloride, benzal chloride, and benzotrichloride. 

The methyl groups of 2,4-dimethyl-3//-l, 5-benzodiazepine are CH acidic reaction with benzal-dehyde gives a mixture of mono- and distyryl derivatives 13 and 14.256... 

Reaction of 1,2-glycol systems, such as in the hemiacetal sesquiterpene 1751, with Me3SiCl/NaI in acetonitrile for 5 min at ambient temperature affords more than 80% of the corresponding olefin 1752 [15, 16] (Scheme 12.4). On reacting active methylene groups such as in ethyl acetoacetate or acetylacetone with benzal-dehyde in the presence of TCS14/Nal in acetonitrile, the intermediate unsaturated ketones 1753 and 1755 are reduced to ethyl 2-benzylacetoacetate 1754 [17] or 2-benzylacetylacetone 1756 [18] in 82% and 80% yield, respectively. 

The Co(ni) perchlorate oxidation of substituted and unsubstituted benzal-dehydes has kinetics and a low isotope effect (2.3 at 10 °C) in complete analogy with cyclohexanol and formaldehyde . Ring-substitution by electronegative groups accelerates reaction. 

In the case of benzal chloride, the carboxylation in conventional diaphragm systems fails, leading to poor yields in phenylacetic and mandelic add [180], At an Al anode, carboxylation occurs because the self-esterification of the first carboxylate anion onto the second chloride group is hindered by the formation of Al complex salts [181]. Yields of phenylmalonic and chlorophenylic acetic acids up to 30% each have been obtained [178],... 

As an alternative to radical chlorination, use has been made of carbon tetrachloride and hexachloroethane in the presence of a quaternary ammonium salt, as source of the chloronium ion for reaction with activated alkylbenzenes [38], Benzyl chlorides need the additional activation of a nitro group for their conversion into the corresponding nitrobenzotrichlorides, whereas benzal chlorides do not need the extra activation for a similar conversion. The same synthetic protocol, using hexachloroethane, has been used for the conversion of allylic sulphones into the 1,1-dichloro derivatives [39],... 

When a stream of phosphine is passed into an ether solution of benzal-dehyde saturated with HCl, a colourless, crystalline solid, insoluble in the reaction mixture is formed. The infra-red spectrum of this product shows an absorption band at 1145 cm which is typical for the P=0 group of a tertiary phosphine oxide. This, together with the chemical behaviour, indicates that the product is benzyl-bis(ot-hydroxybenzyl)phosphine oxide, 6,... 

Benzal chloride was tested in two experiments in mice by skin application, the results of which were reported together. In the first experiment, the total dose of benzal chloride was about 289 mg per mouse during a 50-wcek dosing period, after which all mice were killed at week 82. No skin tumours developed in 20 controls, while, in the treated group of 19 (14 of which had died by the end of the experiment), nine mice had squamous cell carcinomas of the skin and two had skin fibrosarcomas. In the other experiment in which the total benzal chloride dose was about 1109 mg per mouse, but which was terminated after just 43 weeks, 2/10 mice developed skin papillomas compared with 0/10 in the controls (lARC, 1982). 

Such reactions have also been carried out on benzofurans carrying benzyl, fluoro, chloro, bromo, methoxy, cyano, and phenyl groups in the benzo ring. Substitution in the stilbene part of the molecule can also be obtained by the use of Schiff s bases derived from differing benzal-dehydes.26,28,29... 

Formation of Benzal- Acetone Structures. Formation of a peroxy group at C-5 (oxidation of structure lid) leads to ring opening between C-4 and C-5 and to formation of benzal acetone structures, which are assumed to be the possible source of the obtained degradation products (acetone and acid V, respectively, w-propyl methyl ketone and acid VI). 

The mechanism of the Perkin condensation involving benzal acetate has been revised.17 It has been demonstrated that the enolate of the gem-diacetate derived from the aromatic aldehyde and acetic anhydride-rather than the enolate of acetic anhydride itself-adds to the aldehyde in the key step. The deprotonation of the diacetate to the enolate appeared to be assisted electrophilically by the neighbouring acetate group. 

The point of addition of the radical to a phenylated compound can be reversed when another ester group is introduced into the substrate molecule. Thus, benzal diethyl malonate reacts with formamide under ultraviolet irradiation in the presence of benzophenone to give derivatives of l-phenyl-l,2,2-tricarboxyethane [61),... 

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