Cyclobutanones rearrangements

The reaction gave only the rearrangement products 333 and 334, and the side product 335, as expected from the reactivity of alkylidenecyclopropane derivatives (Scheme 49). Compound 333 might arise from the 0-0 bond cleavage followed by the rearrangement of a cyclopropyloxy cation to an oxoethyl cation (Scheme 49, path a). Spiro-hexanone 334 could arise from a different fragmentation of ozonide C-O bond and further cyclopropyloxy-cyclobutanone rearrangement (Scheme 49, path b). Oxirane 335 can eventually derive from the same path b or from other side processes [13b]. 

This type of cyclobutanone annelation is feasible with various dibromocyclopropanes. When diaryl ketones are used as electrophiles, the oxaspiropentane-cyclobutanone rearrangement occurs spontaneously, so that the cyclobutanone is obtained directly (equation 63)"° . When 1-bromo-l-lithiocyclopropanes are allowed to react with aldehydes, the formation of cyclopropyl ketones results" . 

Bromoallyl)cyclobutanones rearranged to give bicyclo[4.1.0]heptan-3-ones 7 and 8 on reduction with tributyltin hydride.119... 

The first step is straightforward—-just like the one you have just met. The pinacol dimer from cyclobutanone rearranges with the expansion of one of the rings to give a cyclopentanone fused spiro to the remaining fou(-membered ring. Reduction of the ketone then gives an... 

The methylenecyclopropane oxide —> cyclobutanone rearrangement has been coupled with asymmetric epoxidation to give optically active cyclobutanones, a versatile synthetic intermediate, without loss of the original enantiopurity of the epoxides (Eq. 282) [646]. Both processes occurring in this transformation are promoted by the same titanium species. Representative results are shown in Eqs 283 [646] and 284 [647]. The versatility of these transformations obviously relies on the facile accessibility of optically active epoxides. 

If the hydroxyalkyl function and the oxy substituent are linked to the same cyclopropane carbon, the pattern is perfect for a cyclopropylcarbinyl-cyclobutanone rearrangement. This allows a very convenient access to cyclobutanones (equation The method was used in syntheses of grandisol poitediol and... 

Electronically excited cyclobutanones rearrange in alcoholic media to yield products of ring expansion (2-alkoxytetrahydrofurans) (74AG200 75 ACR209). Spiro[2.3]cyclohexan-3-one 335 is, a priori, more strained than cyclobutanone and hence favors decomposition by the way of ring expansion 336 (73JA3947). 

Treatment of 3a-methoxy-2a,4a,5,7j8-tetramethyltricyclo[5.3.0.0 ]dec-5-en-4-ol with 30% aqueous sulfuric acid in hexane afforded 2a,4a,5,7 8-tetramethyltricyclo[5.3.0.0 ]dec-5-en-3-one (71) in 89% yield. This a-oxycyclopropyl alcohol to cyclobutanone rearrangement is a crucial step in the total synthesis of isocomene. 

Note that a-alkalidene- (also a-cycloalkylidene-) cyclobutanones rearrange on thermolysis to the corresponding cyclohexenone derivatives. Thermolysis of 2-methylene-3,3-disubstituted cyclobutanones results in the formation of the same series of compounds (Scheme 75). " ... 

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. ... 

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