Cyclobutenediones

Additional examples of trapping of the bisketene from benzocyclobutenedione (in low yield) by Diels-Alder reactions have also been reported 44). 

In addition to products derived from bis-ketenes, reactions of /7 45) and of diethyl 

One of the reasons for confusion in this area is the fact that cyclobutenediones absorb at much shorter wavelengths than their saturated counterparts, prompting the use of short wavelength light in photolyses this can result in formation of secondary photoproducts. It seems likely that formation of cyclopropenones and of acetylenes is the result of photochemical reaction of bisketenes, the primary products of irradiation. Acetylenes could also be formed from cyclopropenones. 

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). 

Formation of the dianilide of acetylenedicarboxylic acid from still another reaction of 16d, photolysis in ether containing aniline, may involve prior reaction 49) of aniline with 16 d. The bis-anhydride (16f) of squaric acid also afforded an acetylene dicarb- 

---

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 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)... 

Carbene complexes are versatile intermediates in ring construction [47], The insertion of alkoxycarbenechromium complexes 93 into cyclobutenediones 63 leads to the ring-expanded cyclopentenediones 94 as shown in Scheme 33 [48]. 

Liebeskind and associates converted 3,4-diisopropyl squarate (97) to stannylcyclobutenedione 98 via a 1,4-addition-elimination sequence. 98 was then coupled with 2-iodothiophene to afford substituted cyclobutenedione 99 [85]. In another case, 3-lithioquinuclidin-2-ene, generated from the Shapiro reaction of 3-quinuclidinone (100), was quenched with Bu3SnCl to afford a unique enamine stannane 101. The Stille reaction of stannane 101 and 5-bromo-2-formyl-thiophene then furnished 3-thienylquinuclidine 102 [86]. 

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... 

Alkylidenecyclopentene-l,4-diones (cf. 12, 130).2 The cobaltacyclopente-nedione (2) formed on reaction of a cyclobutenedione with 1, when complexed with dimethylglyoxime, reacts with 1-alkynes to form 5-alkylidenecyclopentene-1,4-diones (3). The reaction involves an alkyne-vinylidene rearrangement. Ben-zoquinones are usually formed in minor amounts. 

Two one-pot syntheses of highly substituted pyran-2-ones have been published. One involves the reaction between t-BuNC, dialkyl acetylenedicarboxylates and bromomalonates <99JCR368>. In the other, cyclobutenediones are treated with 0-silylated cyanohydrins to yield a 4-acylcyclobutenone by a 1,4-silyl migration and cyanide displacement which rearranges to the pyranone (Scheme 18) <99JOC2145>. 

Ring enlargcmcnl with isocyanides was also observed with cyclopropenones 133 and triafulve-ncs.134 In these cases, the products of hydrolysis were cyclobutenediones and methylenecy-clobutenones, respectively. 

Reaction with cyclobutenediones. This cobalt complex inserts into di-methylcyclobutenedione (1) to give the cobalt complex 2 in high yield. The complex 2 reacts with alkynes slowly at 120° to give complexes of benzoquinones such as 4. The reaction is facilitated by replacement of the CO ligand of 2 by S(C2H5)2,... 

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-... 

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 ... 

It has been reported that compound 16 with alkali rearranges to phenyl-1,2-cyclobutenedione, 3 (Section 26-2E). This reaction appears to be the first reported reverse benzil-benzilic acid rearrangement (Section 26-4E). Explain how and why this process occurs. 

Cyclobutenones. Cyclobutenones and cyclobutenediones undergo photochemical rearrangement to unsaturated acids in a manner analogous to that of 6,6-disubstituted-2,4-cyclohexadienones. Irradiation of Formula 133 in ether saturated with water gives Formula 134 (49). In... 

Diethyl squarate Cyclobutenedione, diethoxy- (8) 3-Cyclobutene- 1,2-dione, 3,4-diethoxy- (9) (5231-87-8)... 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

---