Sulfones Ramberg-Backlund reaction

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

The a -halosulfone, required for the Ramberg-Backlund reaction, can for example be prepared from a sulfide by reaction with thionyl chloride (or with N-chlorosuccinimide) to give an a-chlorosulfide, followed by oxidation to the sulfone—e.g. using m-chloroperbenzoic acid. As base for the Ramberg-Backlund reaction have been used alkoxides—e.g. potassium t-butoxide in an etheral solvent, as well as aqueous alkali hydroxide. In the latter case the use of a phase-transfer catalyst may be of advantage. ... 

Cf. three-membered ring sulfones as intermediates of the Ramberg-Backlund reaction L. A. Paquette, Org. React., 25, 1 (1977). 

Block and Aslam441 reported a novel variant of the Ramberg-Backlund reaction in which a,/J-unsaturated a -bromoalkyl sulfones 368 afforded 1,3-dienes upon treatment with base. Since a -bromoalkyl sulfones 368 can be obtained readily by the initial treatment of olefins with bromomethanesulfonyl bromide under photoirradiation and subsequent treatment with triethylamine, this reaction was utilized for preparation of dienes which contain one additional carbon over that of the corresponding olefin. Starting from the cyclic olefins 369-372 the 1,3-dienes 373-376 were obtained442. 

The Ramberg-Backlund reaction has been utilized for the preparation of polyenes. 1,3-Butadienyl allyl sulfones 398 and 399 were transformed into the tri- and tetra-enes 400 and 401 by alkylcuprate addition and the Ramberg-Backlund-type S02 extrusion449. Julia and coworkers450 carried out the Michael addition of various nucleophiles such as ethanol, t-butyl acetoacetate and phenyl thioacetone to allyl dienyl sulfones 402 and then converted them to diallyl sulfones 403. The sulfones were transformed into isoprenoid, 404 by the Ramberg-Backlund reaction. 

One-pot Ramberg-Backlund reaction of sulfones 405 and 406 with KOH in CCl4-t-BuOH afforded the corresponding olefins451,452. 

Finally, an electrochemical reduction of bis-a-bromobenzyl sulfone to stilbene195 and a spectacular, so-called bis-homoconjugative, version of the Ramberg-Backlund reaction, which converts the a-chlorosulfone 100 into the bridged cyclooctatriene derivative 101 (equation 63)196-197, have also been published. 

Most thiirene dioxides (and oxides) have been prepared through a modified Ramberg-Backlund reaction as the last crucial cyclization step, as illustrated in equation 40 for the benzylic series . Synthesis of thiirene dioxides requires two major modifications of the originally employed reaction first, the inorganic base has to be replaced by the less basic and less nucleophilic triethylamine - and second, the aqueous media has to be substituted by an aprotic organic solvent (e.g. methylene chloride). Under these mild reaction conditions the isolation of aryl-substituted thiirene dioxides (and oxides) is feasible . In fact, this is the most convenient way for the preparation of the aryl-disubstituted three-membered ring sulfones and sulfoxides. ... 

The subjects of this section are two reactions that do not actually involve carbo-cation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, an a-halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, an a-halo sulfone gives an alkene. 

The reaction of an a-halo sulfone with a base to give an olefin is called the Ramberg-Backlund reaction.309 The reaction is quite general for a-halo sulfones with an a hydrogen, despite the unreactivity of a-halo sulfones in normal Sn2 reactions (p. 344). Halogen reactivity is in the order I > Br > Cl. Phase transfer catalysis has been used.310 In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone and then elimination of... 

Sulfones have been shown to be versatile intermediates for the introduction of carbon-carbon double bonds [410]. One of the best known routes to do so is the Ramberg-Backlund reaction of an a-halosulfone with an appropriate base. 

Alkene synthesis.1 This sulfone can be subjected to successive alkylation in the order shown in equation (I) to give a tetraalkylated derivative, but the fourth alkylation requires elevated temperatures and an activated reagent such as benzyl bromide. The alkylated derivatives on treatment with base undergo a Ramberg-Backlund reaction to give alkenes. 

Cesium fluoride, 68 Ramberg-Backlund reaction Methyl (trifluoromethylsulfonyl)methyl sulfone, 193 Other eliminations... 

Addition to x,( y,S-dienyI sulfones. Lithium dialkylcuprates react with these diunsaturated sulfones by 1,6-addition, and the new double bond formed has the (Z>-geometry. The adducts on treatment with base extrude S02(Ramberg-Backlund reaction) to give a mixture of polyenes.20... 

The epoxy-Ramberg-Backlund reaction (ERBR) has been used for the conversion of a,/3-epoxy sulfones into a range of mono-, di-, and tri-substituted allylic alcohols.34 Modification of this method has permitted the preparation of enantio-enriched allylic alcohols following the diastereoselective epoxidation of enantio-enriched vinyl sulfones that were accessed efficiently from the chiral pool. 

A review on the Ramberg-Backlund reaction has updated the Paquette review of 1977.45 Benzyl benzyl sulfone systems have been found to give unexpectedly high Z-stereoselectivity (up to E.Z = 1 16) in the Meyers variant of the Ramberg-Backlund (g reaction.46... 

Cyclic sulfones have been prepared by RCM in what appears to be a general process <02OL427> (Scheme 56). These sulfones readily undergo the Ramberg-Backlund reaction to form cyclic dienes in excellent yields. 

In conclusion, the Ramberg- Backlund reaction of glycosyl sulfones is a useful and general route to a variety exo glycals. Simple conversions of the exo-glycals make available a variety of C-glycosides. 

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