Cyclobutanones formation

Table 3. Stereoreversed Cyclobutanone Formation from Oxaspiropentanes...

For example, an oxaspirohexane <52, readily available by condensing cyclobutanone 61 with dimethylsulfonium methylide, rapidly rearranges (isomerizes) to the cyclopentanone 63 upon exposure to a catalytic amount of lithium bromide55). The high diastereoselectivity of the initial cyclobutanone formation translates into a high diastereoselectivity for cyclopentanone annulation as this example of Eq. 74 demonstrates. 

Cyclobutanone formation becomes more bothersome in the acetone-sensitized rearrangements of the homologs (20b-d) to (21b-acetone solutions are irradiated. For these substrates, and generally for all 1-substituted bicyclooctenones of type (20), it is therefore advantageous to employ acetophenone sensitization at >340 nm, where the absorbance of the enone chromophore is negligible. I paratively attractive high substrate concentrations of up to 10% of (20) can be employed under the latter conditions. 

An intriguing combination of two cyclopropyl systems allows cyclobutanone formation according to equation 105 base-induced fragmentation of an intermediate cyclobutanol stereoselectively forms a functionalized olefin . 

The use of aluminium chloride leads to some concomitant cyclobutanone formation, when possible. Lithium iodide and lithium thiocyanate have also been used as catalysts, and in these cases the reaction occurs stereospecifically with net retention of configuration at the migration terminus, presumably via the intermediacy of a lithium 2-iodocyclobu-toxide (equation 170) ... 

Trost also found that treatment of these cyclobutanones with sodium methoxide and diphenyl disulfide leads to in situ bisulfenylation and ring cleavage (secosulfenylation). This was applied to the total synthesis of verrucarol, a complex tetrahydrochromanone substituted at the ring junction by a hydroxymethyl substituent. The synthetic strategy twice used the cyclobutanone formation from 1-lithiocyclo-propyl phenyl sulfide and a ketone, followed by the secosulfenylation process and Baeyer-Villiger type rearrangement of cyclobutanone. Only this part of the synthesis is described in Scheme 28. 

A stereoreversed cyclobutanone formation was realized starting from oxaspiropentanes by using the selenoxide function as a leaving group. Treating oxaspiropentanes 94 with sodium benzeneselenolate in ethanol affords j8-hydroxy selenides 95 which, on oxidation with 3-chloro-peroxybenzoic acid at — 78 to — 30 °C, led directly to the corresponding cyclobutanones 96 (Table 3). The stereochemistry in this reaction is opposite to that normally observed in the acid-catalyzed rearrangement of oxaspiropentanes. 

Cyclobutanone formation from keteniminium derivatives and alkenes... 

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