Dienes formation from sulfolenes

The main part of this chapter is split into four sections, each of which highlights a particular aspect of the chemistry of sulfolenes which is important to those contemplating the use of sulfolene intermediates in organic synthesis. In the first section we describe some important methods for construction of particular sulfolene nuclei in the second section we review methods for installing substituents selectively into various sulfolene rings in the third section we consider the selective formation of dienes from sulfolenes and finally we use examples to illustrate how dienes generated from sulfolene intermediates have been incorporated in Diels-Alder based synthetic routes. 

Sulfur dioxide extrusion (desulfonation) from sulfolenes provides a useful procedure for generation of the diene in the presence of a dienophile, thus facilitating in situ Diels-Alder cycloadditions as illustrated by Scheme 72. The reaction in Scheme 72 involves the in situ formation of 2-cyanobutadiene (181) which then undergoes [4+2] cycloaddition with the dienophile maleimide to yield the adduct (182). 

Retrosulfolene reaction. The formation of 1,3-dienes by extrusion of SO2 from sulfolenes when heated is a useful reaction (2, 390) and of theoretical interest. 

A variety of methods are available for preparing 2-substituted sulfolenes, as described in Section 6.3. The extrusion of sulfur dioxide is a concerted stereospecific process, and has been shown to follow a disrotatory pathway. Sulfur dioxide extrusion from mono-2-substituted sulfolenes could in principle lead to ( )- or (Z)-diene formation through alternative disrotatory modes of reaction. 

An example of the stereoselective formation of ( )-dienes was the synthesis of a red bullworm moth pheromone (160). Sulfolene (159) was available from 3-sulfolene by selective alkylation, and on heating gave diene (160) as a single isomer [116,117] (Scheme 6.51). 

Cycloadditions Interestingly, sulfur dioxide participated as a dienophile in the [4+2] cycloaddition reaction with 1,3-dienes. In this manner, sulfur dioxide reacts similarly to the related selenium dioxide and the other sulfur dienophiles RN=S=0, RN=S=NR and R2C=S=0 (sulfines). However, the [4+2] cycloadducts derived from 1,3-dienes and sulfur dioxide are only obtained at low temperatures (—80 °C) in a kinetically controlled reaction and the cycloaddition reactions often require the presence of a Lewis acid (CF3COOH or BF3). Above —50 °C the Diels-Alder adducts undergo a cycloreversion and a cheletropic addition of the generated sulfur dioxide to the diene occurs with formation of the corresponding 2,5-dihydrothiophene-1,1-dioxides (sulfolenes). According to ab-initio computations, electrostatic solvent effects are predicted to be of importance in the control of the selec-tivities in this reaction . From linear dienes, the [4+1] cycloadducts are usually obtained. For example, from 1,3-butadiene and SO2 at -20 °C, the cyclic sulfone 25 is obtained in 95% yield. ... 

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