Cyclobutanones Vinylcyclobutanones

Indeed, exposure of the vinylcyclobutanone 67 to acid produces the perhydro-azulene skeleton 68 as summarized in Eq. 78 112). With more ready access to the types of substituted cyclobutanones required for this lateral cyclopentannulation (see substitutive spiroannulation), milder conditions and thus better processes will probably ultimately be developed. 

Vinylcyclobutanones may be converted to cyclohex-2-enones 1 on treatment with Eatons reagent (methanesulfonic acid/phosphorus pentoxide).69 The yields are usually only moderate due to a side reaction leading to cyclopentenones. This side reaction is especially important for 2,2-disubstituted cyclobutanones (R3 4 H).69 In all cases the carbonyl group migrates to the /(-position of the double bond. 

Cyclobutanones are obtained from cycloaddition of ketenes to enamines (see Section IV.A and Chapter 18)266. Such cycloadditions may be concerted or step-wise depending upon the amine moiety and the experimental conditions. Vinylketenes are especially interesting since these may undergo [2 + 2] or [4 + 2] cycloadditions to give after oxidation a vinylcyclobutanone or a cyclohexenone, respectively267,268. 

Analogously, 2-vinylcyclobutanones were obtained on treatment of the corresponding alkylidenecyclopropanes with 3-chloroperoxybenzoic acid. Here, no cyclobutanone was formed for those alkylidenecyclopropanes containing a terminal methylene group. ... 

The oxaspiropentane — cyclobutanone rearrangement has been used to prepare the previously unknown 2-acylcyclobutanones (43) and their mono-acetal derivatives (acetal of side-chain ketone),while a related rearrangement of cyclopropylmethanols (44) provides an entry into 2-vinylcyclobutanones (45). Chiral 2-methylcyclobutanones have been obtained from 1,3-dibromo-propanes by condensation with an optically active TosMIC derivative. ... 

The vinylcyclobutanone (216, R = H) undergoes acid-catalysed rearrangement to the decalin (217) if the C-2 position of the cyclobutanone is blocked however by an alkyl group, then the major products are spiro-cyclopentenones (218) (c/. ref. 146). 

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