Die präparative Chemie der Cyclobutendione; III. Synthese von Quadratsäure, Benzocyclobutendion und deren Derivaten

In Part I of the review^1a, the syntheses of cyclobutenediones from open-chain precursors were described; Part II^1b dealt with the reactivity and transformation of alkyl-, alkenyl-, and arylcyclobutenediones. The following Part III summarizes the preparation of Squaric acid (dihydroxycyclobutenedione), the benzocyclobutenediones, and derivatives thereof. 1. Synthesis of Squaric Acid, its Dichloride (Dichlorocyclobu- tenedione), and its Esters (Dialkoxycyclobutenediones) 1.1. Thermal Cycloaddition of Perhaloalkenes 1.2. Intramolecular Cycloaddition of Vinylketenes 1.2.1. Perchlorocyclobutenone 1.2.2. Cyclobutenediones from Perchlorocyclobutenone 1.3. Squaric Acid from Hexachloro-1,3-butadiene 1.4. Squaric Acid from Carbon Monoxide by Electrochemical Cyclotetramerisation 1.5. Simple Cyclobutenediones from Squaric Acid or its Derivatives 2. Synthesis of Benzocyclobutenediones and Higher Aromatic Analogs 2.1. Synthesis from 1,2-Bis[dibromomethyl]-benzene 2.1.1. via 1,2-Dinitryloxybenzocyclobutene 2.1.2. via 1,1,2,2-Tetrabromobenzocyclobutene 2.1.3. via 2,2-Dibromo-1-benzocyclobutenone 2.2. Synthesis from 1,3-Diphenyl-2-benzofuran and Cyclobutene 2.3. Synthesis from 1,4-Dioxo-1,2,3,4-tetrahydrophthalazine and Cyclopentadiene 2.4. Synthesis from N-Aminophthalimide, 3-Benzoyloxyphthalide, or Ninhydrin 2.5. Synthesis from Cyclohexene-4,5-dicarboxylic Esters 2.6. Synthesis from Tetrachlorobutenyne 3. Final Remarks