Carvone photochemical cycloadditions

The photochemical cycloaddition of two different alkenes leads to multiply substituted cyclobutanes, and allows for a general access to this class of compounds. More specifically, ever since Ciamician observed the light-induced isomerization of carvone (1 —> 2) (Scheme 6.1) in 1908 [1], the inter- and intramolecular reaction between an a,(3-unsaturated carbonyl compound and an alkene has become the most intensively studied and most widely used class of [2 + 2]-photocycloaddition reactions [2-9]. 

The photochemical [2 + 2] cycloaddition reaction of alkenes, the light-induced cycloaddition of two carbon-carbon ir-bonds to produce a cyclobutane, is a potentially highly useful reaction in organic synthesis since two new carbon-carbon bonds are formed and a maximum of four new stereocenters are introduced in the process. Ciamician reported the first example in 1908 when he observed the formation of carvone camphor (2) on prolonged exposure of carvone (1) to Italian sunlight (equation 1). ... 

Schonberg proposed in his book [4] that the 2+2 cycloaddition first reported by Ciamician for the case of carvone [38] should be named in honor of that scientist. Note that a non photochemical reaction, the Ciamician-Dennstedt ring enlargement of pyrroles (1881), does bear his name... 

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