3.6- Dihydrophthalic anhydride

The intramolecular variant leads to formation of more than one ring an interesting example is the formation of an intermediate in the synthesis of tetraasterane 16 by Musso and coworkers from 3,6-dihydrophthalic anhydride 15 by two subsequent [2 + 2] cycloaddition reactions, an intermolecular step followed by an intramolecular one ... 

Tetraphenylphthalic anhydride has been prepared by condensation of tetraphenylcyclopentadienone and maleic anhydride in nitrobenzene,1 followed by dehydrogenation of the tetraphenyl-dihydrophthalic anhydride with sulfur.2 3... 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

This double Claisen-Schmidt reaction takes place under the influence of ethano-lic potassium hydroxide (Expt 7.7) and presumably proceeds in the stepwise manner (cf. formulation in Section 6.12.2, p. 1032). The four aryl groups in tetra-cyclopentadienone effectively stabilise the cyclopentadienone system, which otherwise has only a transient existence and readily undergoes dimerisation by way of a diene-dienophile interaction (Diels-Alder reaction, Section 7.6). The use of tetracyclone as a dienophile for the preparation of 3,4,5,6-tetraphenyl-dihydrophthalic anhydride is noted on p. 1121. 

Photochemical synthesis of a 1,4-difluorobenzene-naphthalene biplanemer (50) and its photochemical and physical properties were investigated by Kimura et al. [130] (Scheme 20). The compound 50 was prepared through the electrolytic oxidative decarboxylation of the (4 + 4) photocycloadduct 49 between naphthalene and 3,6-difhioro-l,2-dihydrophthalic anhydride (48). The direct irradiation of 50 gave 1,4-difluorobenzene and the excited singlet and triplet states of naph-... 

Anthracene-benzene biplanemers (119) were prepared through the electrolytic oxidative decarboxylation of (4 + 4) photocycloadducts between substituted anthracenes and 1,2-dihydrophthalic anhydrides [165,166] (Scheme 36). Chemiluminescence was observed for 119 only in the solid state at > 120°C, but not in a liquid phase, whereas 119 (X = F or Cl, Y = Z = H) were not chemiluminescent [167], Efficient chemiluminescence was observed in the photocycloreversion of all biplanemers tested in both phases. 

Dihydro-4-phenylcarbostyri1, 900 Dibydrophosphopyridine nucleotide, 1082 ciis l,2-Dihydrophthalic anhydride, 556 Dihydropyrane, 256-257, 386, 412, 634,... 

The synthesis of unsubstituted Dewar benzene was accomplished by van Tamelen (3) 4). Thus, dihydrophthalic anhydride was photochemically isomerized to a [2.2.0]-ring system which was decarboxylated oxidatively to Dewar benzene. This compound has a higher stability than expected from the high strain of its ring system (t1/2 = 37.2 h at 24.3°). This stability was later explained by the rule of the conservation of the orbital symmetry by Woodward and Hoffmann. 

Pyran 2-ones undergo Diels-Alder reactions with activated alkenes or alkynes. For instance, from 27/-pyran-2-one and maleic anhydride, the e Jo-adduct 15 is formed which undergoes thermal decarboxylation to the 1,2-dihydrophthalic anhydride 16 ... 

The photocycloaddition of cis-1,2-dihydrophthalic anhydride to benzene, naphthalene, or anthracene affords in each case products arising from ( 4j + 4J reaction, a type of addition unknown for cyclohexa-1,3-diene itself. Energy transfer from benzene to the diene anhydride still provided the main reaction pathway, namely electrocyclic closure to give the cis-dihydroDewarbenzene anhydride. The naphthalene photo-adduct (393) on xanthone-photosensitized irradition gave the cage compound (394) similar reaction also occurs with the benzene photo-adduct. 

Fuchs, B. and Scharf, G., Photochemical fragmentation of 7-Oxonorbornen-2,3-dicarboxylic anhydride and 1,2-dihydrophthalic anhydride,/. Chem. Soc., Chem. Commun., 226, 1974. 

Phthalic anhydride -+ 1,2-dihydrophthalic acid Europe 100 Pb cathode in H2S04... 

Similarly, phthalic acid anhydride can be reduced to dihydrophthalic acid, if dioxane is present as cosolvent [18] ... 

The use of cycloaddition reactions for the synthesis of partially reduced heterocyclic systems was shown to be an attractive approach to dihydrobenzimidazoles, dihydroquinazolines, and dihydro-//n-benzopurines (Scheme 14) <86JOC6i6>. The dihydroxylation of the Diels-Alder adduct dimethyl 3,6-dihydrophthalate (128) with 0s04 and NMO followed by protection of the diol as the iso-propylidene derivative afforded compound (129). Saponification, dehydration with ethoxyethyne, and rearrangement with TMS—N3 effected conversion to the substituted tetrahydroisatoic anhydride (130), and subsequent treatment with formamidine acetate yielded compound (131). The substituents at the 6,7-positions of compound (131) were not amenable, however, for annelation of an imidazole. 

Electrocyclic reactions have been used to prepare four-membered rings. Irradiation of 1,2-dihydrophthalic acid anhydride gave the bicyclo[2,2,0]hexene (76) which was oxidatively decarboxylated to Dewar-benzene (77) (Scheme 25). ... 

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