Rearrangements Ramberg-Backlund

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. 

Yoshimoto, N. Ishida, and Y. Kishida, Chem. andPharm. Bull. Japan), 1971,19, 1409. 

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Recent developments in Ramberg-Backlund rearrangement and episulfone chemistry 99CC217. 

Sulfonic acids (e.g. 58) should be sufficiently stable to be isolated and identified, as proved to be the case in the Ramberg Backlund rearrangement of 2-halothiirane dioxide115 (equation 16). 

Thiepin-1,1-dioxide undergoes a number of chromium(0) mediated [6jt + 4jt] cycloaddition reactions with a range of 1,3-dienes. The intermediate adduct undergoes a Ramberg-Backlund rearrangement to form new benzannulated products <96JOC7644>. 

The base-induced Ramberg-Backlund rearrangement later initiated extensive mechanistically - and synthetically -oriented investigations, and played a significant role not only with respect to the study of thiirane dioxides (3b), but also contributed substantially to the present state of the art concerning three-membered rings containing sulfur (equation 2). 

Based on extensive studies associated with the Ramberg-Backlund rearrangement s and its mechanism , including the treatment of thiirane oxides with bases, the following conclusions emerge ... 

One of the most important reactions of sulfones in general is the Ramberg-Backlund rearrangement, which involves the conversion of a-halosulfones to olefins with accompanying loss of hydrogen halide and sulfur dioxide under basic conditions (equation 48) ... 

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... 

Two illustrations that show the power of this reaction for the preparation of strained cycloalkenes are the contractions of 102 to the propellane 103 , an application that has been reviewed , and of 104 to the bicyclo[2.1.1]hexene 105 . The utility of the Ramberg-Backlund rearrangement in the preparation of various natural products such as steroids , terpenoids and pheromones has been demonstrated. In addition to the synthetic applications mentioned in the previous subsection, several selected examples taken from the recent literature are given in equations 66-69. These examples further demonstrate the potential of this method for alkene synthesis in general. 

Halothiirane 1,1-dioxides are known to be intermediates in the Ramberg-Backlund rearrangement of a,a-dihalo sul-fones. These three-membered cyclic suhones are not... 

A new and convenient method of preparation of trichloro- and trifluoro-methyl sulfones has found application in /3-elimination of haloform via an unusually facile Ramberg-Backlund rearrangement under extremely mild and non-aqueous conditions. Thus, 9-fluorenyl trichloromethyl sulfone in CHCI3 affords... 

Table 5 Olefination of 125 by Ramberg-Backlund rearrangement <2004JOC984>...

The desulfurative C-C bond formation process developed by Eschenmoser [217] is somewhat akin to the Ramberg-Backlund rearrangement. An episulfide intermediate is formed, which on being attacked by a phosphine, begets a carbon skeleton with polarity alternating sequence. 

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