Ramberg—Backlund reaction systems

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... 

Without doubt, the most important modification of the Ramberg-Backlund reaction is the result of the work of Meyers. In 1969, he reported that a suspension of powdered KOH in a mixture of CCU and Bu OH is a powerful chlorinating system for a variety of compounds with acidic methylene groups. ... 

If the fragmentation of the two atoms from the group, to which they were bonded, takes place in a concerted process resulting in the formation of a conjugated system, this is a cheletropic extrusion. Thus, the two possibilities can be found in the case of the loss of a SO2 group SO2 extrusion from an episulfone, as in the Ramberg-Backlund reaction, is an extrusion reaction, but SO2 extrusion from a cyclic sulfolene is a cheletropic reaction. ... 

The Ramberg-Backlund reaction is now widely utilized in organic synthesis. It has been applied in synthesizing cyclobutenes, cyclopentenes, cyclohexenes, phenanthrenes, large ring systems, dienes, enynes, polyenes, alkynes, and many natural products. 

Review articles have featured advances in the cycloisomerization of l,n-allenynes and Rn-allenenes, " reactions for synthesis of quaternary centres bearing a nitrogen substituent, " the Ramberg-Backlund reaction transforming a-halosulfones into alkenes, " and ring expansion of l,2-benzisoxazol-3-ones with a Vilsmeier reagent system, POCI3 and dimethylformamide (Scheme 75). " ... 

Methods used for opening of bicyclic oxazines by the formation of a new carbon-carbon bond to give highly substituted ring systems containing multiple stereocenters have been reviewed. A review of olefination reactions used in the synthesis of cyclophanes has been presented. This covers the Ramberg-Backlund reaction, the Wittig and related... 

Particularly useful in cyclic dialkyne chemistry is sulfide as a nucleophile. This is due to a discovery made by Regen and co-workers, who found that the yield of dialkyl sulfides is strongly enhanced when the sulfide reagent is first adsorbed onto AI2O3 thus, the substitution reaction takes place on the surface of the aluminum oxide. As the preparation of cyclic sulfides from dibromides is a stepwise process, these conditions were expected to increase the yield of cyclic sulfides dramatically as one end of the chain is immobilized after displacement of the first bromine [14]. Nicolaou first used this principle in the preparation of a series of cyclic thiadiynes [Eq. (6)] [15]. The cyclic thiadiynes prepared in this way are very useful as they can be converted to cyclic enediyne systems via the Ramberg-Backlund ring-contraction reaction (see below). 

In an approach to the general synthesis of bicyclo[2,l,l]hexenes in which the double bond is introduced simultaneously with the construction of the basic ring system, the Ramberg-Backlund rearrangement of 2-chloro-3-thiabicyclo[3,l,l]heptane-3,3-oxides has been investigated. Thus (573), which is available in six steps from the cis + irons mixture of cyclobutane-1,3-dicarboxylic acids, is converted into the parent alkene (574) by reaction with sodium t-pentyloxide in dephenyl ether. 

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