3.4- dichlorocyclobutene-1,2-dione

This addition-elimination reaction can be applied to the preparation of squaric acid derivatives. Thus, the treatment of 3,4-dichlorocyclobutene-l,2-dione 27 with two different zinc-copper reagents furnishes polyfunctional squaric acid derivatives like 28, provided the first zinc-copper reagent bears a secondary or tertiary alkyl group (Scheme 9-26) [56]. 

Diels-Alder dienes Benzene, see Dicyanoacetylene. Cyclooctatetraene, see 4-Phenyl-l,2,4-triazoline-3,5-dione. Cyclopentadiene ketals. cis-7,8-Dichlorobicyclo[4.2.0]octadiene-2,4, see cis-3,4-Dichlorocyclobutene. 5,5-Dimethoxy-l,2,3,4-tetrachlorocyclopentadiene. Hexa-chlorocyclopentadiene. Methyl tra .rl2,4-pentadienoate. A 4 l6-Steroid dienes, see 4-Phenyl-l,2,4-triazoline-3,5-dione. Tetrachlorocyclopentadienone ethylene ketal ... 

Disubstituted cyclobutene-1,2-diones." These products can be prepared by reaction of 3,4-dichlorocyclobutene-l,2-dioncs with functionalized organocopper/zinc reagents. 

---