Cyclobutene 3,4-dihydroxy-

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). 

Ohno M, Eguchi S (2006) Directed Synthesis of Biologically Interesting Heterocycles with Squaric Acid (3,4-Dihydroxy-3-cyclobutene-l,2-dione) Based Technology. 6 1-37 Okino T (2006) Heterocycles from Cyanobacteria. 5 1-19... 

The four-, five-, and six-membered analogs (178,180, and 182) were also obtained from the diprotonation of squaric acid (3,4-dihydroxy-3-cyclobutene-l,2-dione, 177), tri-O-protonation of croconic acid (4,5-dihydroxy-4-cyclopentene-l,2,3-trione, 179), and tetra-O-protonated rhodizonic acid (5,6-dihydroxy-5-cyclohexene-l,2,3,4-tetraone, 181), respectively. These ions were prepared in either Magic Acid (1 1 FSOsH-SbFs) or fluorosulfuric acid at low temperature and characterized by NMR. Ab initio/IGLO calculations showed that di-O-protonated squaric acid (178) is planar and aromatic, whereas the polyprotonated croconic and rhodizonic acids (180 and 182) have more carboxonium ion character, and no indication was obtained for any significant contributing homoaromatic structures. 

X-Ray crystal structure analyses of both squaric acid158 (3,4-dihydroxy-3-cyclobutene-l,2-dione) and the dianion159 are the usual benchmarks used in consideration of structural data of metal complexes. The relevant dimensions are shown in Table 9, from which it can be seen that there is substantial conjugation and C2 symmetry in the acid molecule while the dianion has DAh symmetry... 

Directed Synthesis of Biologically Interesting Heterocycles with Squaric Acid (3,4-Dihydroxy-3-cyclobutene-l,2-dione)... 

Squaric acid 3-Cyclobutene-1,2-dione, 3,4-dihydroxy- (2892-51-5), 76, 190 Stannanes, chiral a-alkoxyallylic, 77, 103 Stereochemical purity, determination of, 76, 52... 

Oxocarbon anions, C 0 (3 < n < 6), are derived from the cyclic dihydroxy compounds depicted in Figure 3. Deltic and squaric acids are made by hydrolysis of suitable cyclopropene and cyclobutene derivatives (Figure 4). Much milder conditions (e.g. n-butanol in diethyl ether) are... 

Chroman 5,7-Dihydroxy-2,2-dimethyl-4-oxo- VI/4, 166 Cyclobuten 3,4-Dioxo-2-ethoxy-l-(2-hydroxy-l-cyclopentenyl)-E15/2, 1426 (NR2 OH) Cycloheptatrien 1,6-Dimethoxycar-bonyl- V/ld, 346... 

Biphenyl 2,5-Dihydroxy- VI/lc, 46 lH- Cycloprop[b]naphthalin) 2,7-Dioxo-1 a-methyl-1 a,2,7,7a-tetrahydro- VII/3a, 95 Cyclobutene, 3,4-Dioxo-2-ethyl-l-phenyl- E17f, 955/960 (Cl/Cl -Oxo)... 

C4H204 3,4-dihydroxy-3-cyclobutene-1,2-dione 2892-51-5 25.00 1.4720 2 2631 C4H5Br04 DL-bromosuccinic acid 923-06-8 25.00 1.7770 2... 

Cyclobutene-l, 2-dione, 3,4-dichloro-, 339 —, 3,4-dihydroxy- (squaric acid), 78 Cyclobutenes perhalogenated synthesis, 78... 

Crystalline [Cr(77 -C5H5)2] [HC404] has been obtained by reaction of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4) with [Ctirf iti THF. The flat shape and small dimensions of the squarate anion... 

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