Cyclopropanols enolate cyclopropanation

When a-halo ketones are treated with diiodomethane and samarium at 0 °C, cyclopropanols can be obtained in reasonable yields. Curiously, under the same conditions 1,2-dibenzoylethane also leads to cyclopropanol products (equations 34 and 35). Several pathways for conversion of a-halo ketones to the observed cyclopropanols can be envisioned. It has been proposed that the mechanism of this reaction involves reduction of the a-halo ketone by Sm (or Smh) to a samarium enolate. Cyclopropanation of this enolate with a samarium-based carbenoid subsequently provides the observed product. 

Another example is the asymmetric synthesis of ( )-pinidine 208 and its isomers. These syntheses are achieved via asymmetric enolization, stereoselective cyclopropanation, and oxidative ring cleavage of the resulting cyclopropanol system (Scheme 5-68).123... 

Vicinally donor-acceptor-substituted cyclopropanol carboxylic esters have been proven to be versatile synthetic building blocks in organic synthesis [11]. They readily undergo a retroaldol reaction, thus creating a stable enolate that at the same time can be considered as a homoenolate in relation to the newly formed carbonyl function. Shimada et al. applied this strategy to the preparation of y-substituted lactones starting from cyclopropane 21 (Scheme 3) [12]. 

Enol esters have been used under Simmons-Smith conditions, and the prodncts are valuable precursors for cyclopropanols. Alternatively, the Uthinm enolate which can be obtained from the corresponding enol ester can be converted into the cyclopropanol derivative (equation 22). The direct cyclopropanation of zinc or lithinm enolates nsing... 

A very useful class of chiral auxiliaries has been developed for alkenes substituted with a heteroatom. These auxiliaries, attached to the heteroatom, allow for the preparation of enantiomerically enriched cyclopropanols, cyclopropylamines and cyclopropylboronic acids. Tai and coworkers have developed a method to efficiently generate substituted cyclopropanol derivatives using the cyclopropanation of a chiral enol ether (equation 78) . The reaction proceeds with very high diastereocontrol with five- to eight-membered ring sizes as well as with acyclic enol ethers. The potential problem with the latter is the control of the double bond geometry upon enol ether formation. A detailed mechanistic study involving two zinc centers in the transition structure has been reported. ... 

Diastereoselective cyclopropanation of enol ethers preparation of cyclopropanol derivatives27 ... 

Vinyl cyclopropanols have been prepared by the addition of alkenyl Grignard reagents to a variety of cyclopropanone equivalents. Upon treatment with acid, the vinyl cyclopropanols rearrange to a-substituted cyclobutanones. Alternatively, a variety of a-heteroatora-substituted cyclopropyllithiura reagents have been developed. These react with aldehydes and ketones to afford cyclopropylcarbinols which also rearrange to cyclobutanones under acid catalysis.Lastly, vinyl cyclopropanols and cyclopropylcarbinols have been prepared by the cyclopropanation of enol silyl... 

Samarium(II) iodide and diiodomethane efficiently cyclopropanated lithium enolates 119 under very mild conditions, providing a convenient route to cyclopropanols 120. 

Enolisation 1 involves the removal of the a-proton from a carbonyl compound to form an enolate ion 2. Homoenolisation involves the removal of a (i-proton 3 to form the homoenolate ion 4 or 5. Both the enolate and the homoenolate can be represented as carbanions, but whereas the enolate version 2b is merely a different way of representing a single delocalised structure, the homoenolate 5 is a different compound from the cyclopropane 4. No literal examples of homoenolates 5 are known so they have the status of synthons which may be represented in real life by reagents derived from cyclopropanols 4 among many other possibilities.1... 

Treatment with triisobutylaluminum converts the saturated acetal 6 into the optically active enol ether 770. Cyclopropanation of 7 with the Simmons-Smith reagent then provides the cyclopropanol derivative 8 with a d.r. of 95 5. Even better side discrimination is observed when the more active diiodomethane/diethylzinc system is applied under optimized conditions70 the cyclopropanol derivative is obtained diastereomerically almost pure. 

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