Cyclobutenediones synthesis

TRANSITION METAL CATALYZED REACTIONS OF CYCLOBUTENEDIONES SYNTHESIS OF 1202 QUINONES... 

A further example of the utility of the cyclobuta[ >]chroman system in synthesis (see 6.4.1.3) is provided by the conversion of the cyclobutenedione derivatives (19) into substituted xanthones. Compounds (19) are obtained in high yield from salicylaldehydes and squarate esters and their reaction with alkenyl, aryl and heteroaryl li compounds is both facile and high yielding <96JA12473>. 

Strained ring compounds undergo insertion of a low-valence metal complex to give metallacycles and the cycloaddition of metallacycles has a potential in synthesis, as described above. This method is useful in ring transformations of cyclobutenediones and cyclobutenones. 

Conjugate addition of vinyl-, aryl-, heteroarylcuprates 90 to the cyclobutenedione 63, followed by in situ protection with (methoxyethoxy) methyl chloride of the enolates, provides a method for synthesizing substituted catechol derivatives 91 [44], Regiocontrolled synthesis is achieved by using cyclobutenedione monoacetals 92 as starting substrates. (Scheme 32)... 

Quinone synthesis. Two laboratories12 have found that the adducts formed by addition of an aryl-, alkynyl-, or heteroaryllithium to a cyclobutenedione rearrange when heated (138-160°) to hydroquinones, which are usually isolated as the quinone after air or chemical oxidation. The rearrangement involves an interme-... 

All materials containing cyclobutenedione listed in Table I were synthesized in my laboratory and identified by elementary analysis, mass spectrometry, melting point, infrared spectrum, and so on. Full details for the synthesis of cyclobutenediones will be reported separately. UV spectra were obtained with Hitachi spectrophotometer (U-3400). 

Quinone synthesis (13, 97-98 209-210). Liebeskind1 has reviewed the use of organolithiums for preparation of benzoquinones from cyclobutenedione and of the reaction of benzocyclobutenediones with C1Co[P(C6Hj)3]3 to form either benzoquinones or naphthoquinones. These routes are particularly useful for regioselective synthesis of highly substituted quinones. 

Stannylquinones.1 The quinone synthesis based on addition of alkynyllithiums to substituted cyclobutenediones (13, 209-210, 284) can provide stannylquinones. Thus thermolysis of the alkynylcyclobutenol 1 with Bu3SnOCH3 results in rearrangement to the stannylquinone 2. As expected, these stannylquinones undergo palladium-catalyzed cross-coupling with organic halides (Stille reaction, 14, 35), particularly with allylic halides. 

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). 

Annulation of heterocycles with a cyclobutene ring has been achieved by photochemical (2 + 2)-cycloaddition with acetylenes. Both maleic anhydride and N-substituted maleimides yielded 3-oxa- or 3-aza-bicyclo[3.2.0l-hept-6-ene-2,4-diones (167).,89,9° Vinylene carbonates also entered into a cycloaddition reaction with acetylenes to afford 168, which has been employed as starting material for the synthesis of cyclo-butadiene(tricarbonyl)iron or cyclobutenedione.19,-193 3,4-Dihydro-2/f-pyran and 5-methyl-2,3-dihydrofuran reacted with diphenylacetylene to... 

This procedure describes a synthetic route to vinylcyclobutenediones that are important intermediates in the synthesis of other cyclobutenediones (Table) and quinones.6 7 For example vinylcyclobutenedione 2 readily undergoes 1,6-addition of nucleophiles such as thiols and organocuprates to form more highly substituted products as shown in the generalized scheme below. 

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