1- Cyclobutene-1,2-dicarboxylic acid

Pure crystalline 2,3-dicyanobutadiene has been prepared in high yield by gas-phase thermolysis of cyclobutene (2).2,8 Analogous thermolysis of derivatives of cyclobutene-1,2-dicarboxylic acid appears to represent general procedures for the synthesis of derivatives of butadiene-2,3-dicarboxylic acid of high purity.2,12 These... 

Cyclobutene-1,2-dicarboxylic acid l-Cyclobutene-l,2-dicarbox-ylic acid (8,9) (16508-05-7)... 

Since 3-methylenecyclobutane-l,2-dicarboxylic anhydride is easily converted to 3-methyl-2-cydobutene-l,2-dicarboxylic acid, it is an intermediate to a variety of cyclobutenes. The dimethyl ester of 3-methylenecyclobutane-l,2-dicarboxylic acid is also a versatile compound on pyrolysis it gives the substituted allene, methyl butadienoate, and on treatment with amines it gives a cyclobutene, dimethyl 3-methyl-2-cyclobutene-l,2-di-carboxylate. ... 

One such example is a sequence of solution reactions starting from the dimer 91, which has a cis-1,2-dicarboxylic acid arrangement and is obtained by irradiation of monomethyl fumarate, 90, in the crystal (137). This isomerically pure dimer may be oxidatively decaiboxylated, by a variety of methods, to the thermally unstable cis-disubstituted cyclobutene 92, whose smooth disrotatory ringopening leads to the isomerically pure dimethyl c/s, frans-muconate, 93 (167). 

Formula 366) gives a bicyclic enol ether (Formula 367) which is hydrolyzed to Formula 368 (161,162). The sequence of Formulas 366-368 represents a potentially general synthesis of bicyclic cyclobutanone derivatives. A clever synthesis of m-3-cyclobutene-l,2-dicarboxylic acid anhydride (Formula 369) has been accomplished by irradiation of muconic anhydride (Formula 370) in ether (163). 

As far back as 1894 an attempt was made to prepare cyclobutadiene--1,2-dicarboxylic acid, by the action of alkali on 1,2-dib romocyclo-butane-l,2-dicaTboxylIc acid only cyclobutene derivatives were obtained [2]. 

Whereas the cyclobutane rings in (8 X = O or S) are planar, that in (9) is puckered, and two short Br Br distances of 3.55=A are observed in the crystals. Planarity is also seen in ds-cyclobutene-3,4-dicarboxylic acid and in substituted cyclopentadi-enyl cobalt complexes of tetraphenylcyclobutadiene, in which each metal atom is sandwiched between parallel, planar fo ir- and five-membered rings. Crystal structures have been obtained for a variety of polycyclic compounds containing cyclobutane rings. ... 

Thummel has prepared benzo[l,2 3,4]dicylobutene (137). Diels-Alder addition of vinylcyclobutene to dimethyl cyclobutene-1,2-dicarboxylate gave (138). Hydrolysis and decarboxylation of the dicarboxylic acid with two equivalents of lead tetra-acetate gave (137) directly. The u.v. spectrum of (137) showed only a small hypsochromic shift relative to the unstrained model, 1,2,3,4-tetramethylbenzene. The isomeric benzo-l,2 4,5-dicyclobutene showed a substantial bathochromic shift relative to 1,2,4,5-tetramethylbenzene. Both benzodicyclobutenes showed increased intensity of absorption. 

Dimethyl tricyclo[4.2.2.02 -5]deca-3,7-diene-9,10-dicarboxylate adds bromine and iodine only to the less hindered double bond to give the syn 1,2-addition product of the cyclobutene moiety79. The product composition from this compound depends on the temperature and the solvent. At high temperatures, the 1,2-addition predominates over the transannular reaction, but this predominance is small in a solvent like chloroform and is lost in a protic solvent such as acetic acid (equation 47). 

Pyrolytic scission of cyclobutenes leads to a diene which promises to be useful in a Diels Alder approach to catechols. Heating of the sllyl derivative of the acyloin product from dimethyl glutarate in the presence of dlenophiles affords adducts of the transient diene. The adduct obtained with dimethyl acetylene dicarboxylate was converted in several steps to phthalic acid derivative. ... 

The cycloaddition of 1-pyrrolidylcyclopentene with activated alkynes, such as dimethyl acctylcne-dicarboxylate, is reported to occur at room temperature. In contrast, the less reactive diethyl 1-alkynylphosphonates require reaction temperatures of at least 85°C to permit cycloaddition. Under such conditions, spontaneous ring opening of the thermally unstable cyclobutene intermediate yields the ring-enlarged product. Acid hydrolysis of the product enamine gives the unsaturated P-keto-phosphonate (Scheme 1.22). The best results for the cycloaddition are obtained with rigorous exclusion of moisture and with temperatures held below 100°C. Reaction times varied from 24 h (Rl = H) to 8 days (Ri = MOM). -... 

Vogel was the first to recognize that these reactions were stereospecific. He found that the dimethyl ester of c/5 -cyclobutene-3,4-dicarboxylate gives only the dimethyl ester of cis,trans-m xcomc acid at 120°C (Scheme 5.9). Similar stereochemical observations were made by Criegee who found that cw-1,2,3,4-tetramethylcyclobutene gives on c/5, ra/25 -3,4-dimethyl-2,4-hexadiene while the trans compound gives the trans,trans-6itnt at 200°C (Scheme 5.9). ... 

Dimethyl heptalene-l,2-dicarboxylate (105) is the major product of the reaction between azulene and dimethyl acetylenedicarboxylate in the absence of acid. No heptalene is obtained if acid is present. Cyclopent[c,d]azulene reacts with dimethyl acetylenedicarboxylate to give [2 + 2] adducts which undergo cyclobutene ring... 

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