Ring Expansion from Four-membered Rings

Ring Expansion from Four-membered Rings 

In many reactions, cyclobutane derivatives are intermediates which lead to ring enlarged compounds. Within the scope of this book it will only be possible to discuss reactions which may be of common preparative interest. 

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IV. Ring Expansion from Four-membered Rings or via Four-membered Intermediates... 

From four-membered rings An acid-catalyzed transformation has been observed in the conversion of l-[l-methylsulfinyl-l-(methylthio)alkyl]cyclo-butanol to 3-methyl-2-(methylthio)cyclopentanone [9]. - Rearrangement of a /3-lactone to a y-lactone derivative in the presence of magnesiumdibromide [10]. - A borontrifluoride catalyzed cyclobutene to cyclopentene rearrangement [11]. - Ring expansion of a [2+2] photoadduct to a five-membered ring [12]-... 

Cyclopropanes adjacent to an electron-deficient centre X (carbon or heteroatom) undergo C3 - C4 ring enlargement into four-membered ring derivatives via routes involving cations, radicals or car bene intermediates. Furthermore, when the cyclopropanes also bear on the same carbon an electron-donor substituent Y they undergo a specific C3 - C4 ring expansion, important from the synthetic point of view, into cyclobutanone (or related) derivatives (equation 67). 

To show that ring expansion from three- to four-membered rings and ring contraction the other way are about as easy. 

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