Chloro-sulfone, Ramberg-Backlund reaction

Variations of the Ramberg-Backlund reaction have been described in which the usual halide ion is replaced by p-toluenesulfonate or p-toluene-, alkane- or trifluoromethane-sulfinate leaving groups. For example, Meyers has described a single transformation of the Ramberg-Backlund type of an a-tosyloxy sulfone (Scheme 9). The reaction is surprisingly slow, nearly 1000 times slower than that of the corresponding a-chloro sulfone under the same conditions, and this, of course, limits its value in synthesis. Another problem is the fact that the precursors are not easy to prepare. 

The Ramberg-Backlund reaction has also been applied to the synthesis of cyclopentadienes. For example, the a-chloro sulfone (12), a single unidentified diastereoisomer, was converted regiospecifi-cally into the cyclopentadiene (13). In principle, the acidity of the product requires the use of at least 2 equiv. of base for optimum yields, but, in practice, more than 1 equiv. of Bu"Li causes appreciable polymerization. 

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)... 

Early in his academic career in 1964, Paquette carried out the Ramberg-Backlund reaction of chloro-sulfone 65 to make phenanthrene 66.44 He discovered that conformational requirements of a-sulfonyl carbanion prolonged the reaction time (5 days) in some cases. In 1986, applying the Meyers modification, Staab et al. converted sulfone 67 to phenanthrene 68.45 The two chlorine atoms on 68 were the result of overchlorination where tetrachloro-sulfone was generated in situ. 

In a formal total synthesis of steroid brassindole, sulfide 102 was used as the substrate of the Ramberg-Backlund reaction.57 Chlorination of 102 followed by oxidation with m-CPBA, and treatment of the resulting chloro-sulfone with potassium t-butoxide, provided symmetrical olefin 103. 

The Ramberg-Backlund rearrangement was the key step in the total synthesis of the marine alkaloid manzamine C by D.l. MaGee and E.J. Beck. The azacycloundecene ring was stereoselectively formed by exposing the a-chloro sulfone to a strong base. The use of weaker bases either resulted in no reaction or gave rise to mixtures of ( )- and (Z)-alkenes. 

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