Thermolysis of cyclobutenes

The thermolysis of cyclobutene has received much study, especially after it was discovered that the reactions of cis- and rra s-3,4-dimethylcyclobutene proceed stereospecifically.89... 

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... 

It is not surprising that thermolysis of cyclobutenes leads to ring opening since the strain in the four-membered ring is relieved. What is particularly significant about these reactions is that they are stereospecific cw-3,4-dimethylcyclobutene is... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

The benzo-fused derivative 8 of l,4-dimethyTl,4-dihydro-l,4-diazocine can be prepared by gas phase thermolysis of the cyclobutene derivative 7,31 a reaction that can be reversed photo-chemically. In contrast to the parent system, the heterocyclic ring of the benzannulated compound displays no aromatic character. 

The thermolysis of cycloalkenes is often a more facile process than for the cycloalkenes. Cyclobutene undergoes thermolysis at 175 °C and yields butadiene in an orbital symmetry controlled reaction as shown by stereochemical studies of... 

Similarly, thermolysis of naphthacene photooxide (329) leads to naphtha-cenequinone (330), the bicyclic acetal 331, and the cyclobutene diether 332.178 These products are shown to have been formed by the involvement of the meso-diepoxide 333 by means of trapping experiments. 

Thermolysis of 3-(ort o-anisoyl)-l-(l-piperidinyl)-3-cyclobutenes 807 in the presence of mesitylene affords angular-fused xanthones 809 via formation and ring closure of the intermediate 808 (Scheme 226) <1997TL3663>. Linear-fused xanthones 810 are prepared by nucleophilic addition of aryl and heteroaryl lithiates to dithiane protected benzopyrone-fused cyclobutenediones 811 followed by hydroysis of the dithiane protecting group (Scheme 227) <1996JA12473>. 

An additional example is a two-step approach to the preparation of 1,5-cyclo-decadiene. By a photochemical cycloaddition of a substituted cyclobutene and 2-cyclohexenone, a strained tricyclo[4.4.0.02,5]decane system is generated. Thermolysis of the tricycle gave 1,5-cyclodecadiene [52]. 

Intact cyclobutanes, especially cyclobutanones, can serve as obvious precursors to many cyclobutenes. For example, cyclobutanones have been converted to their silylated enol ethers by standard methodology. Thermolysis of these intermediates gives rise to 1-alkoxysilyl-1,3-butadienes (equation 7). 

The temperatures required for these expansions are typically below 1(X) °C if one or two electron-donating heteroatom substituents is attached to the bridgehead positions. Considerably higher temperatures are required in the absence of such substituents. For example, thermolysis of (97) requires 180-2(X) C, the amide (98) must be heated to over 450 C in order to achieve ring expansion, and the cyclobutene (99) is reported to be thermally stable. ... 

Thermolysis of these yields the diketones (95) which are useful compounds for the synthesis of the perhydroazulenes This specific example (95) was converted into the natural product daucene (96). Others have also described the photocycloaddition of the enone (92) to the cyclobutene (93) to afford the (2+2)-adduct (94) in 24 % yield. The adduct was transformed into the natural product balanitol (97). The triplet state of conjugated enones can be photochemically reduced by sodium borohydride. ... 

Reduction of the photo-adduct (446) derived from (+)-isopiperitone and cyclobutene-1-carboxylic acid with NaCNBH3 gives the lactone (447). Thermolysis of this compound affords the 6a-epimer of isoaristolactone (448) and the elemanolide (449).213 A novel approach to the synthesis of germacranes involves the thermal opening of a bridgehead cyclobutene which, in turn, is derived by an oxy-Cope rearrangement (Scheme 57).214... 

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