Ramberg-Backlund rearrangement stereoselectivity

Finally, the most significant mechanistic feature of the Ramberg-Backlund rearrangement is the stereoselective formation of ds-olefin products, as a result of the preferential cis-positioning of the pair of R groups in the episulfone-forming transition state, variously attributed to London forces , to diastereoselectivity in carbanion formation and to steric attraction . However, with the use of stronger bases such as potassium t-butoxide °, the trans-olefin predominates (equation 52), apparently due to prior epimerization of the kinetically favoured cts-episulfone, and subsequent loss of the sulfur dioxide. Similarly, when the episulfone intermediates possess unusually acidic... 

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. 

The reaction of a-halosulphonc with a base to give an olefin is known as Ramberg-Backlund reaction52. A vinylogous version of this rearrangement results in the formation of 1,3-dienes (equation 21)53. Another variation of this reaction is shown in equation 2254. These rearrangements proceed with moderate stereoselectivity. 

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