Arylidene groups

In 2-arylidene-1,3-indandiones 6, which can be considered as ethylene derivatives with two end-carbon atoms attached to 1,3-indandione and to electron-acceptor and electron-donor groups (R), respectively, photoconductivity depends on the substituents of the arylidene group 81>. 

Oxidation of Schiff s bases derived from l-phenyl-3-aminopyrazoline gives l-phenyl-3-aroylaminopyrazoles,469 the hetero ring and the arylidene group being oxidized simultaneously. See also Duffin and Kendall.520... 

In this section, consideration will be given to the actual processes of acetal- or ketal-formation and not to the more indirect methods by which acetals and ketals of the polyhydric alcohols may be synthesized from compounds (e.g. derivatives of the monosaccharides) containing preformed alkylidene or arylidene groupings. The condensation of a carbonyl compound with a glycol is facilitated by acidic catalysts, and, since the reaction is reversible, by dehydration. The catalysts most frequently employed are concentrated sulfuric, hydrochloric and hydro-bromic acids, gaseous hydrogen chloride, zinc chloride and cupric sulfate others are phosphorus pentoxide, sulfosalicylic acid, and anhydrous sodium sulfate. The formation of benzylidene compounds is promoted less efficiently by phosphorus pentoxide than by either concentrated sulfuric acid or concentrated hydrochloric acid 1" the reaction is assisted by chloro- and nitro-substituents on the aromatic nucleus, but hindered by methyl- and methoxy-groups.18... 

The different mechanisms of reaction of benzylidene and isopropylidene acetals, compared with that of methylene acetals, have not yet been elucidated, but, assuming, in both cases, that the acetal ring is ruptured to give an acetyl substituent together with a trifluoroacetoxymethyl substituent, in place of the alkylidene or arylidene group, it is evident that, when either... 

Bis(3-pyrazolin-5-ones) in which the linking chain is not attached to the pyrazolinone nucleus by functional groups are listed in Tables XVI and XVII. Most of these compounds have as the linking chain an arylidene group and are prepared by the reaction of 3-pyrazolin-6-ones with aryl aldehydes,B66 e68,569,807,1001,1133 as shown in eq. 117 (p. 50). Some aliphatic aldehydes have also been used in this reaction.409 3-Pyrazolin-5-ones react with formaldehyde or formamide to form the corresponding bis compounds (eq. 142).910-1192 Under basic conditions... 

This fragmentation mode was minor or absent under the following conditions (a) in the absence of an ortho-nitro group on the S-aryl ring, as in compounds 169b, k, 1 (b) when the AT-arylidene group was replaced by a iV-alkylidene, as in compounds 170 and 171 (c) when the imine functional group was saturated, as in compound 172. 

The diphenylphosphoryl group has been used as an alternative to arylidene groups for the protection of glucosaminyl halides during the synthesis of nucleosides [231] and disaccharides [232]. On completion of the reaction sequence the group is converted into the dibenzylphosphoryl group by transesterification with benzyl alcohol containing ammonia, then removed as described above. 

Subsequent to Hantzsch s communication for the construction of pyridine derivatives, a number of other groups have reported their efforts towards the synthesis of the pyridine heterocyclic framework. Initially, the protocol was modified by Beyer and later by Knoevenagel to allow preparation of unsymmetrical 1,4-dihydropyridines by condensation of an alkylidene or arylidene P-dicarbonyl compound with a P-amino-a,P-unsaturated carbonyl compound. Following these initial reports, additional modifications were communicated and since these other methods fall under the condensation approach, they will be presented as variations, although each of them has attained the status of named reaction . 

In contrast to the saturated azlactones, the Friedel-Crafts reaction of 2-substituted-4-arylidene-5-oxazolones is quite complex and may follow several different courses, often concurrently, depending on both reaction conditions and structural variations in the arylidene ring. This behavior is readily interpreted in terms of the a,)S-unsaturated carbonyl moiety and the cross-conjugated system containing nitrogen, both of which provide potential reaction sites in addition to the lactone carbonyl group. The reaction has been investigated " ... 

Amides can add to aldehydes in the presence of bases (so the nucleophile is actually RCONH ) or acids to give acylated amino alcohols, which often react further to give alkylidene or arylidene bisamides. If the R group contains an a hydrogen, water may split out. 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

Quite recently, a French group reported on the synthesis of 2-substituted 5-arylidene-pyridazinones as represented by the general formula (21, R1 = Ph, 2-ClC6H4 R2 = Ph(CH2)2, PhCO). These compounds were found to exhibit significant dose-dependent analgesic activity (phenylquinone-induced writhing test in mice oral administration) [92]. For a novel type of aminopyridazine-derived inhibitors of prostacyclin biosynthesis, see [93],... 

Reaction of 4-arylidene-2-phenyl-5(4//)-oxazolones 593 with A-phenacylpyri-dinium bromide proceeds by the same sequence to give oxazolo[5,4-/>]pyridines 594 (Scheme 7.188) while the reaction of 4-(aminomethylene)-2-phenyl-5(4//)-oxazolone 418 and phenyl isothiocyanate gives pyrimidin-2-thiones 595 (Scheme 7.189). In this latter case initial attack of the exocylic amino group produces an intermediate thiourea (not shown) that subsequently cyclizes to 595. 

Several examples of oxidative ring closures by attack on an azomethine group by an amine have been reported78 thus arylidene-o-phenylenedi-amines (32) have been oxidized to 2-arylbenzimidazoles (33) [Eq. (35)]. 

Addition of bromine in dichloromethane or of thioacetic acid in ethanol onto the methylene group of 9-arylidene and 9-carboxymethylene moieties of 6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidines 546 stereoselectively gave 9-substituted 6,7,8,9-tetrahydro derivatives 547 and 548, respectively, at ambient temperature (Scheme 34) (90JHC247). Addition was also stereoselective with respect to the C(9) and C(9)—C centers, giving the erythro diastereomers as the primary products, which may then undergo epimerization to the threo isomers. The structure and epimerization of the products were studied by H and l3C NMR spectroscopy and by molecular mechanics calculations. 

Notable progress in the structural analysis of methylene derivatives of the polyhydric alcohols resulted from the investigations of Hann, Hudson and their co-workers26 80,40-4 into the behavior of these compounds during acetolysis. It was found that a mixture of acetic anhydride, acetic acid and 1-2% sulfuric acid ruptures preferentially any methylene bridge which spans a primary and a secondary position, giving the acetate ester of the primary hydroxyl and the acetoxymethyl ether of the secondary hydroxyl subsequent treatment with sodium methoxide removes each of these substituents. Under similar conditions, the acetolysis of a benzylidene compound results in the replacement of the arylidene residue, wherever it is located in the molecule, by two acetyl groups.16 29 47 48... 

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