Sulfones cyclic, pyrolysis

The pyrolytic elimination of sulfur dioxide from cyclic sulfones to give (strained) macrocycles (sulfone pyrolysis) counts among the most important reactions in cyclophane chemistry. There is hardly any research group dealing with cyclophanes which did not make use of the sulfone pyrolysis for the synthesis of such molecules. Since its first application in cyclophane synthesis over twenty years ago [1], the sulfone pyrolysis enabled researchers to synthesize a wealth of strained cyclophanes and developed into a reaction of general importance. 

In 1979, Rossa and Vogtle published a review on sulfone pyrolysis which did not only cover examples from cyclophane chemistry but also discussed the pyrolysis of acyclic sulfones, of small-membered cyclic sulfones as well as questions of mechanisms and practical aspects of the reaction [2]. 

Oxidation of the cyclic sulfides to cyclic sulfones and subsequent thermal extrusion of sulfur dioxide in the gas phase ( sulfone pyrolysis ). 

Vogtle et al. submitted a mixture of cyclic tetrasulfones 169 and 170 (the sulfide products of the cyclization were not separated but directly oxidized) to a sulfone pyrolysis at 600 °C [61], However, the product was shown not to be the desired four-fold-bridged cyclophane but a doubly clamped 1,2-dihydrocyclobutabenzene. Only an increase of the pyrolysis temperature to 750 °C led to successful formation of the desired [2.2.2.2](l,2,4,5)cyclophane 171 in the mixture of products. Manyfold ort/io-clamped phanes prior to this work were not accessible by sulfone pyrolysis. 

Sulfur dioxide reacts with dienes to form a cyclic sulfone in a reaction somewhat reminiscent of the Diels-Alder reaction. In the case of Myrcene (3.1), the adduct produced is (3.2) and its remote double bond is selectively hydrated by mineral acid. Pyrolysis of the product (3.3) under neutral conditions leads to cleavage of the dihydrothiophene ring to give the hydrated diene (3.4) and Diels-Alder reaction of this product then produces Lyral (3.5). 

Benzannulation of the seven-membered ring in 37 and 39 leads to 42-44, as shown in Figure 4.12. Unlike 1,2-benzannulated pleiadiene (42), which is a stable compound [44, 45], 2,3-benzannulated derivatives 43 and 44 were reported by Cava and Schlessinger in 1963 as reactive species that appeared not isolable [46] because they both contain an unstable o-quinodimethane moiety. Pyrolysis of cyclic sulfone 45 at 210 °C in the presence of AT-phenylmaleimide gave the adduct 46 in 70% yield, and resulted in the dimer 47 in 51% yield under similar conditions in the absence of a dienophile, as shown in Scheme 4.10a. These reactions suggest... 

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