Majetich annulation

What is Majetich annulation?

In 1985, Majetich reported a reaction that is now known as the Majetich annulation. This reaction involves a Lewis acid or fluoride promoted intramolecular cyclization of allylsilanes with their inherent olefinic group. The reaction is termed the Majetich cyclobutane annulation or Majetich reaction when a 7-trialkylsilyl-1,5-diene system forms vinyl cyclobutanes with fluoride promotion, while cyclopentanes or cyclohexanes may form with the use of a Lewis acid.

Majetich annulation - general reaction scheme - Majetich cyclobutane annulation - Majetich reaction
Majetich annulation

R = H, alkyl

The fluoride ion confers an anti-orientation of the silyl alkene through kinetic control, leading to vinyl cyclobutanes, while the Lewis acid prefers the synclinal orientation, resulting in cyclopentanes or cyclohexanes.

This reaction is particularly useful in the preparation of bicyclic ring systems, such as 5-5, 6-5, 5-7, and 6-7 ring systems, through the intramolecular addition of allylsilanes to cycloalkenones. The formation of cyclohexane in a bicyclic ring system promoted by a Lewis acid is not sensitive to the size of the existing cycloalkenone, and the regioselectivity is not affected by the silicon ligand either.

However, this reaction prefers an excess amount of Lewis acid and may lead to silyl dienol ethers if no acidic hydrolysis or incomplete hydrolysis is applied during the workup process. When a corresponding annulation is promoted by fluoride, it is better to carry out the reaction in DMF in the presence of three equivalents of HMPA instead of the conventional solvent of THF (see list of acronyms).

References

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