Divinyl ketones, rearrangement

Symmetrical ketones can be prepared in good yields by the reaction of organo-mercuric halides with dicobalt octacarbonyl in THF, or with nickel carbonyl in DMF or certain other solvents. The R group may be aryl or alkyl. However, when R is alkyl, rearrangements may intervene in the C02(CO)g reaction, though the Ni(CO)4 reaction seems to be free from such rearrangements. Divinylic ketones... 

A variety of different precursors have been shown to rearrange to divinyl ketones or equivalent species under highly ionizing acidic conditions. Equivalents of both cross-conjugated and linear dienones have been generated and cyclize to cyclopentenones in situ. The structural diversity of substrates in this class is remarkable and testifies to the utility of these ring-forming reactions which are unified by the propensity for cationic electrocyclization. 

Kuroda, C.. Sumiya, II.. Murase, A., and Koito, A., Iron(ni)-induced tandem Nazarov cyclization-rearrangement of a-(triincLhylsilylmethyl)divinyl ketone. Synthesis of the bicyclo[4.3.0]nonane ring system via spiro[4.4]nonanc, J. Chem. Soc., Chem. Commun.. 1177. 1997. 

The rearrangement of divinyl ketones to cyclopentenones (the Nazarov rearrangement) can be promoted with BF3-Et20. This transformation has recently been applied to good effect in the construction of the hydroazulene core found in guanacastepene. Other variants of the Nazarov reaction have been developed and are usually coupled with a second event, such as the reductive... 

The conversion of a pentadienyl to a cyclopentenyl cation is an electrocyclic reaction of a system containing loury-electrons delocalized over five atomic orbitals, and should therefore be a csorotatory process. In the example just described, the stereochemistry of the cyclization reaction is masked by subsequent rearrangement of (alkj l groups, which ultimately affords the most stable carbonium ion. The stereochemistry of the cyclization of divinyl ketones under acidic conditions, however, is conrotatory, as predicted ... 

Denmark and co-workers have published extensively on the use of (3-silyl substituted divinyl ketones (see 82) in the Nazarov cyclization. Such silyl groups control the collapse of the intermediate cyclopentenylic cations 84, and thus aid the regioselectivity of elimination, as well as the minimization of side reactions (secondary cationic rearrangements). Such stabilization derives from the known P-cation stabilizing effect of silicon, which through stabilization of 84, ensures maximum efficiency of the cyclization, with controlled formation of the final double bond. An important consequence of the final elimination step is that the double bond is placed in the thermodynamically less stable position (see 85). The most common Lewis acid used in the silicon-directed Nazarov cyclization is anhydrous iron(III) chloride, at temperatures below ambient. Alternatively, in cases where the... 

The reduction of some substituted divinyl ketones under Luche conditions (sodium borohydride and cerium trichloride) using different alcohols as solvents affords polyenyl alkyl ethers in good yields via rearrangement of allylic cations and nucleophilic attack of the alcohol solvent. ... 

The mechanism of chemoselective copper(II)-mediated NazarovAVagner-Meerwein rearrangement sequence of divinyl ketones has been reported to involve an initial 4 r electrocyclization and proceed via two different sequential [l,2]-shifts, with selectivity that depends on either migratory ability or the steric bulkiness of the substituents at C( 1) and C(5) (Scheme 168). ... 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

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