Stereospecific Synthesis of Enantioenriched Alkylidenecyclobutanones via Formal Vinylidene Insertion into Cyclopropanone Equivalents.

1-Sulfonylcyclopropanols are employed here as efficient cyclopropanone equivalents in a formal vinylidene insertion process, providing the first general synthetic route to enantioenriched alkylidenecyclobutanones. The addition of an alkenyl-Grignard reagent leads to an alkenylcyclopropanol capable of electrophilic activation by N-bromosuccinimide, triggering a regio- and stereospecific 1,2-migration and affording alkylidenecyclobutanones after elimination. Activation of the intermediate with other electrophiles such as HCl or mCPBA leads to the formation of various chiral cyclobutanones and γ-lactones via alternative pathways.

General Synthesis of Cyclopropanols via Organometallic Addition to 1-Sulfonylcyclopropanols as Cyclopropanone Precursors.

The addition of organometallic reagents to ketones constitutes one of the most straightforward synthetic approaches to tertiary alcohols. However, due to the absence of a well-behaved class of cyclopropanone surrogates accessible in enantioenriched form, such a trivial synthetic disconnection has received very little attention in the literature for the formation of tertiary cyclopropanols. In this work, we report a simple and high-yielding synthesis of 1-substituted cyclopropanols via the addition of diverse organometallic reagents to 1-phenylsulfonylcyclopropanols, acting here as in situ precursors of the corresponding cyclopropanones. The transformation is shown to be amenable to sp-, sp2-, or sp3-hybridized organometallic C-nucleophiles under mild conditions, and the use of enantioenriched substrates led to highly diastereoselective additions and the formation of optically active cyclopropanols.

Enantioselective Synthesis of Cyclopropanone Equivalents and Application to the Formation of Chiral ß-Lactams.

Cyclopropanone derivatives have long been considered unsustainable synthetic intermediates because of their extreme strain and kinetic instability. Reported here is the enantioselective synthesis of 1-sulfonylcyclopropanols, as stable yet powerful equivalents of the corresponding cyclopropanone derivatives, by α-hydroxylation of sulfonylcyclopropanes using a bis(silyl) peroxide as the electrophilic oxygen source. This work constitutes the first general approach to enantioenriched cyclopropanone derivatives. Both the electronic and steric nature of the sulfonyl moiety, which serves as a base-labile protecting group and confers crystallinity to these cyclopropanone precursors, were found to have a crucial impact on the rate of equilibration to the corresponding cyclopropanone. The utility of these cyclopropanone surrogates is demonstrated in a mild and stereospecific formal [3+1] cycloaddition with simple hydroxylamines, leading to the efficient formation of chiral ß-lactam derivatives.

Controlled α-mono- and α,α-di-halogenation of alkyl sulfones using reagent-solvent halogen bonding.

The direct and selective α-mono-bromination of alkyl sulfones was achieved through base-mediated electrophilic halogenation. The appropriate combination of solvent and electrophilic bromine source was found to be critical to control the nature of the products formed, where reagent-solvent halogen bonding is proposed to control the selectivity via alteration of the effective size of the electrophilic bromine source. Conversely, the α,α-di-brominated sulfones were selectively obtained in good yields following polyhalogenation followed by selective de-halogenation during workup. Both procedures can be applied on gram scale, and the mono-halogenation was successfully extended to the fully selective α-chlorination, α-iodination and α-fluorination of alkyl sulfones.