Dialkyl squarates

Substituted cyclobutenediones. These useful precursors to quinones (13,209-210) can be prepared from commercially available dialkoxycyclobutenediones (1, dialkyl squarates). Thus a wide variety of organolithium reagents add to 1 at - 78°, and the adducts (a) are hydrolyzed under mild conditions to the cyclobutenediones 2.2 - Protection of a as the /-butyldimethylsilyl ether (b) permits a second addition... 

Benzoquinones (13, 284). Full details of the use of a dialkyl squarate for a regioselective route to highly substituted benzoquinones are now available. The report includes a note of caution These esters show allerginicity comparable to that of the active principal of poison ivy.1... 

The 1,2-addition of vinyllithium to the carbonyl group of dialkyl squarate-derived bicycloheptenones initiates a low-temperature anion-accelerated oxy-Cope rearrangement to afford bicyclo[6.3.0]undecadienone. H.W. Moore and coworkers accomplished the total synthesis of (+)-precapnelladiene using this methodology. ... 

Hayashi, K.-L, Shinada, T., Sakaguchi, K., Horikawa, M., and Ohfune, Y., Olefination of dialkyl squarates by Wittig and Homer-Emmons reactions. A facile synthesis of 3,4-dioxo-l-cyclobutene-l-acetic acid esters. Tetrahedron Lett., 38, 7091, 1997. 

Two other syntheses of dimethyl squarate have been reported.2 One such method involves the formation of the disilver salt of squaric acid followed by treatment with methyl iodide. This process, unfortunately, is laborious as well as costly. The other synthesis involves the reaction of squaric acid with diazomethane. This reaction not only gives a lower yield, but is inherently unsuited for large scale preparations. The synthesis presented here provides a convenient and inexpensive route to the preparation of dimethyl squarate, and is also suitable for large scale synthesis.4 One should take precautions when synthesizing or using dialkyl squarates, particularly dimethyl squarate, since they are known to cause severe contact dermatitis.5 Thus, all procedures should be carried out in the hood and protective clothing and gloves should be worn. 

In an attempt to improve the synthetic yield of high-sensitivity squaraines, e.g., Sq-1 and Sq-8, mixtures of squaraines were synthesized by reacting either squaric acid or dialkyl squarate with a mixture of N,N-dimethylanilines [171-173]. The resulting squaraine mixtures show very similar solution and solid state properties as comparable symmetrical squaraines. More importantly, they also exhibit acceptable xerographic sensitivity. Structural analyses indicated that each of these mixtures consists of two symmetrical squaraines and one unsymmetrical squaraine. The observation of favorable photoconductive properties from these compositions was initially a surprise because it is generally... 

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