Ramberg-Backlund sulfur extrusion

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Internal alkylation takes place on reaction of a-haloalkyl alkyl sulfones with bases and produces epi-sulfones as intermediaries. These intermediates are usually unstable and provide alkenes by thermal extrusion of sulfur dioxide. This reaction, known as the Ramberg-Backlund rearrangement, has been widely used in synthesis and has been thoroughly reviewed (Scheme 107, entry e).36-3. 43<.473... 

Halosulfones are important intermediates in the Ramberg-Backlund reaction (1940) which converts dialkyl sulfones into alkenes (87) with extrusion of sulfur dioxide. 4 The reaction requires a leaving group (X) at the a-position and an a -hydrogen atom it is generally carried out with a-chloro sulfones (91), although other halogens or sulfonates are effective (Scheme 38)... 

A very different sulfur dioxide extrusion protocol has been applied to the total synthesis of (+)-estradiol (30) by Rigby and coworkers. In the event, the trimethylsilyl-substituted chromium thiepin complex 27 participated in a [6 r+4 r] cycloaddition reaction to provide a 70% yield of the structurally complex dihy-drothepin 28. Subsequent sulfur dioxide extrusion via a Ramberg-Backlund-type ring contraction gave tetracycle 29. The total synthesis of (+)-estradiol (30) was completed using a four-step sequence of standard reactions (eq 12). ... 

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