Cyclobutenediones squaric acid

Thermal electrocyclizations of perhalogenated 1,3-butadienes yield perhalogenated cyclobutenes which can be solvolysed to 3,4-dihydroxy-3-cydobutene-l,2-dione ( squaric acid") and its derivatives (G. Maahs, 1966 H. Knorr, 1978 A.H. Schmidt, 1978). Double CO extrusion from fused cyclobutenediones has been used to produce cycloalkynes, e.g., benzyne from benzocyclobutenedione by irradiation in an argon matrix (O.L. Chapman, 1973) and cyc/o-Ci8, cyclo-Cn, etc. by laser desorption mass spectroscopy of appropriate precursors (see section 4.9.8). 

A related compound, 3,4-dihydroxy-1,2-cyclobutenedione, 5, also has been prepared and is a very strong dibasic acid. It is sometimes called squaric acid ... 

In this paper, I propose a promising new electron acceptor of cyclobutenedione for nonlinear optical materials to prevent centrosymmetric crystal structures by the introduction of chirality and hydrogen bonding property into the acceptor itself. Compared with electron donative groups, electron acceptor is not yet well studied for nonlinear optical materials. The most commonly used electron acceptor is nitro (NO2) group. Therefore, we evaluated the possibility of cyclobutenedione as a new electron acceptor for nonlinear optical materials. One of the most simple cyclobutenediones is squaric acid. Squaric acid is known to be soluble in water and show very strong acidityQ2), as squarylium anion formed in water has a stable 2n delocalized electron system as shown below. 

The most dramatic synthetic achievement with cyclobutenediones is the synthesis of deltic acid derivatives. As illustrated below, photolysis43 of diethyl squarate (16 d) yielded, among other products, diethyl debate (21). The bis-trimethylsilyl ester (16 e) of squaric acid similarly furnished a deltate ester which was successfully hydrolysed to deltic acid itself (22), the lowest member of the series of oxocarbon acids48). 

Over the years, squaric acid and its derivatives have found several uses in medicinal chemistry Squaric acid inhibits glyoxy-lase I (45) semisquaric acid (3-hydroxy-3-cyclobutenedione) is an inhibitor of pyruvate dehydrogenase and transketolase (46) and other derivatives serve as antagonists of the N -methyl-D-... 

Squaric acid Cyclobutenedione, dihydroxy- (8) 3-Cyclobutene-1,2-dione, 3,4-dihydroxy- (9) (2892-51-5)... 

Apart from these methods based on the squaric acid family, direct formation of cyclobutenedione rings by [4 + 2] and [2 -i- 2] cycloaddition reactions is a plausible approach to variably substituted 4-hydroxy-2-cyclobutenone systems [47-54]. 

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