Epoxides organometallic cleavage

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Unlike the corresponding phosphonium salts, addition of sulfonium salts to aldehydes results, not in the alkene products, but in the formation of epoxides (see Section 1.1.5.2). However, sulfones can be used to prepare alkenes, by way of the a-metallo derivatives, in what is termed the Julia olefination (alkenylation). Addition of the organometallic species to an aldehyde or ketone gives a p-hydroxy sulfone which, in the form of its 0-acyl or 0-sulfonyl derivative, undergoes reductive cleavage with, for example, sodium amalgam in methanol to form the alkene. The reaction is regioselective and can be used to prepare mono-, di- and trisubstituted alkenes (2.91). 

Table 5.7. We have attempted to activate the epoxide using Lewis acids to accelerate the organometallic epoxide-opening reaction, but to no avail. Analogously, intramolecular cyclizations via an epoxide cleavage process can also be realized. For example, treatment of 6-bromo-l,2-epoxyhexane with thienyl-based copper resulted in intramolecular cyclization subsequent...

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