2- benzaldehydes, ring

A recent study has examined the antimicrobial activity of various aldehydes against M. tuberculosis [107]. Pentenal, benzaldehyde and phthalalde-hyde showed a kill greater than 10 cfu ml" in 5 minutes, though benzaldehyde was only active in the presence of 2% glutraldehyde. Substituents (nitro, chloro, methyl and methoxy) on the benzaldehyde ring reduced this synergism, but still showed increased activity over 2% glutaraldehyde alone [107]. 

Many aromatic aldehydes (having the -CHO group joined directly to the benzene ring) undergo polymerisation when heated with a solution of potassium cyanide in aqueous ethanol. Thus benzaldehyde gives benzoin, a compound of double function, since it contains both a secondary alcoholic and a ketonic... 

To understand the effect of a carbonyl group attached directly to the ring consider Its polarization The electrons m the carbon-oxygen double bond are drawn toward oxy gen and away from carbon leaving the carbon attached to the nng with a partial posi tive charge Using benzaldehyde as an example... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Reactions of the Aromatic Ring. The aromatic ring of hydroxybenzaldehydes participates in several typical aromatic electrophilic reactions. Ha.logena.tlon, Chlorination and bromination yield mono- and dihalo derivatives, depending on reaction conditions. Bromination of / -hydroxy-benzaldehyde in chloroform yields 65—75% of the product shown (39). 

Benzaldehyde is a versatile iatermediate because of its reactive aldehyde hydrogen, its carbonyl group, and the benzene ring. 

When tetrahydrothiophene is treated in a similar manner, ring opening occurs to give 4-methylthiobutyl fluoride. In the benzaldehyde dithioacetal, gem-di-fluorination takes place to form benzylidene fluoride, though in low yield [6] (equations 9 and 10). 

Ring closure of 2-chloro-l-phenethylpyridinium ion (247) (prepared in situ) to l,2-dihydro-3,4-benzoquinolizium ion involves intramolecular nucleophilic displacement of the chloro group by the phenyl 77-electrons. A related intermolecular reaction involving a more activated pyridine ring and more nucleophilic 7r-electrons is the formation of 4-( -dimethylaminophenyl)pyridine (and benzaldehyde) from dimethylaniline and 1-benzoylpyridinium chloride (cf. Section III,B,4,c). 

The second group of reactions is called vicinal difunctionalization. They embrace the C2 and C3 positions of the furan ring simultaneously. Thus, complex 3 (X = O, R = R = R = H) reacts with benzaldehyde dimethyl acetal to give 4H-furanium cation (the product of electrophile addition at C4), which experiences further attack by the methoxide group with formation of the acetal 8 (950M2861). This reaction is possible in the presence of the Lewis acid (BF3—OEt2). Reaction with methyl vinyl ketone in methanol, when run in identical conditions. 

A methyl group in the 2-position of the selenazole ring shows the same reactivity as the analogous thiazoles toward carbonyl compounds. By reaction of 2,4-dimethylselenazole with benzaldehyde in the presence of anhydrous zinc chloride catalyst, 4-methyl-2-styryl-selcnazole (9), mp 74-75°C, could be prepared. ... 

The mechanism of the cycloaddition reaction of benzaldehyde 2a with Danishefsky s diene 3a catalyzed by aluminum complexes has been investigated theoretically using semi-empirical calculations [14]. It was found that the reaction proceeds as a step-wise cycloaddition reaction with the first step being a nucleophilic-like attack of Danishefsky s diene 2a on the coordinated carbonyl compound leading to an aldol-like intermediate which is stabilized by interaction of the cation with the oxygen atom of the Lewis acid. The next step is the ring-closure step, giving the cycloaddition product. 

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