Versatility in the Brook Rearrangement for the Selective Ring‐Opening of Three‐Membered Rings

Abstract

From a single α‐silylated carbinol intermediate, easily accessible by carbometallation of cyclopropenes, various scaffolds featuring a quaternary carbon stereocenter could be obtained selectively. The selectivity towards these different products was achieved by either changing the experimental conditions or the nature of the organometallic species involved.

Unexpected Vinylcyclopropane Rearrangement: New Strategies toward Skipped Dienes Using Sulfonium Ylides

“…We have identified a new rearrangement of vinylcyclopropane into skipped dienes with a stereogenic centre at the central position. Conversely to previously reported rearrangements, this rearrangement was found to be very selective and proceed at room temperature (15 min, r.t., TiCl 4 catalysis). Based on deuteration experiments we proposed a mechanism for this transformation which involves a regioselective ring‐opening of the cyclopropane followed by a 1,2‐migration of an aryl group.…”

“…This latter, unlike 3 , was found to be unreactive upon MgI 2 catalysis. We now have attempted other reaction conditions to rearrange 5aa and, interestingly, found that it leads quantitatively to skipped diene 6aa in the presence of TiCl 4 or FeCl 3 …”

Creating Stereocenters within Acyclic Systems by C–C Bond Cleavage of Cyclopropanes

“…However, when the same intermediates 39 were added to a magnesiate reagent, Bu 3 MgLi, the intermediate ate complexes underwent the Brook rearrangement followed by ring fragmentation. This sequence provides silyl enol ethers 41 in good yields as single geometrical Z isomers, as illustrated in Scheme , path b . Even more fascinating is the observation that the same intermediates 39 treated with a dialkylmagnesium species Bu 2 Mg, provide now skipped dienes possessing the quaternary stereocenter were formed (Scheme , path c) …”

“…This sequence provides silyl enol ethers 41 in good yields as single geometrical Z isomers, as illustrated in Scheme , path b . Even more fascinating is the observation that the same intermediates 39 treated with a dialkylmagnesium species Bu 2 Mg, provide now skipped dienes possessing the quaternary stereocenter were formed (Scheme , path c) …”

C–C Bond Cleavages of Cyclopropenes: Operating for Selective Ring-Opening Reactions

“…The last piece of work at the moment described the synthesis of γ,δ-unsaturated ketones 276 decorated with a quaternary carbon at β-position (Scheme 40C). 193 Once α-silylcarbinolate intermediates were generated from cyclopropenes 275, treatment with two equivalents of n-BuLi, subsequent increase of the temperature and final hydrolysis afforded ketones 276 in moderate overall yields. Again, transfer of chirality was viable as illustrated with the preparation of 276d from the enantioenriched cyclopropene.…”