Diastereo- and enantioselective preparation of cyclopropanol derivatives

Regio- and Diastereoselective Carbometalation Reaction of Cyclopropenes

The various facets of the chemistry of cyclopropane derivatives, the smallest carbocycle, are amazingly diverse and continue to fascinate theoreticians, synthetic or structural chemists having interest in fundamental physical, medicinal chemistry, and natural product synthesis. The challenges generated by this intriguing cyclic arrangement of only three tetravalent carbons represent a wide area of the chemical spectrum. From fundamental aspects of bonding through the synthesis of highly strained molecules, the understanding of the mode of action in biological systems to the selective cleavage into acyclic substrates makes the chemistry of these small rings fascinating. Therefore, efficient routes to prepare differently polysubstituted cyclopropanes have always been of a primordial importance. In the past decade, we and others have expanded the scope of the carbometalation reaction of cyclopropenes as a broad and general method to the formation of stereodefined cyclopropane derivatives. Although cyclopropenes, with their even higher strain energy, easily undergo addition reactions of organometallic reagents, their carbometalation reactions generate new regio-, diastereo-, and enantioselectivity issues that needed to be addressed. These various stereochemical aspects accompanied our research from its origins to today, and we are proposing in this Account, a didactic overview of the different ways by which cyclopropenes can lead to the formation of polysubstituted cyclopropanes or open-products possessing several stereogenic centers as a single regio- and diastereomer.We initially launched our research campaign on the chemistry of these strained three-membered rings by the regio- and diastereoselective copper-catalyzed carbomagnesiation of enantiomerically enriched cyclopropenyl carbinols. The directing alcohol governed both the regio- and diastereoselectivity of the addition and also served as a good leaving group as it undergoes a selective 1,2-elimination reaction to provide enantioenriched alkylidenecyclopropanes in excellent yields and enantiomeric excesses. Then, we turned our attention to the regio- and stereoselective synthesis of stereodefined tri- and tetrasubstituted cyclopropanes through the diastereoselective addition to sp²- monosubstituted cyclopropenyl ester derivatives. With the aim to further expand this concept to the formation of penta- and hexa-substituted cyclopropanes as single isomer, we had first to design the preparation of the required 1,2-disubstituted cyclopropenes that would control the regioselective addition of the organometallic derivatives. The synthesis of penta- and hexa-substituted cyclopropanes was then reported for the first time as a single regio- and diastereomer. It should be noted that the in situ formed cyclopropyl-metal intermediate is configurationally stable and can be subsequently functionalized with pure retention of the configuration by addition of electrophiles. Then, the enantioselective-catalyzed carbometalation reaction of achiral cyclopropenes allowed the synthesis of several new classes of cyclopropane derivatives in high enantiomeric ratios. Finally, by combining the regio- and diastereoselective carbometalation reaction of a cyclopropene with a subsequent reaction of the resulting cyclopropylmetal species, a selective carbon-carbon bond cleavage was observed to lead to the preparation of acyclic substrates possessing several stereocenters including a quaternary carbon stereogenic center. Our original vision of using strain within an embedded double bond in a three-membered ring has provided new routes to the stereoselective synthesis of polysubstituted cyclopropanes and has been extremely successful, as it represents a current new tool for the synthesis of persubstituted cyclopropanes as a single diastereomer.

Enantioselective Hydrothiolation: Diverging Cyclopropenes through Ligand Control

In this article, we advance Rh-catalyzed hydrothiolation through the divergent reactivity of cyclopropenes. Cyclopropenes undergo hydrothiolation to provide cyclopropyl sulfides or allylic sulfides. The choice of bisphosphine ligand dictates whether the pathway involves ring-retention or ring-opening. Mechanistic studies reveal the origin for this switchable selectivity. Our results suggest the two pathways share a common cyclopropyl-Rh(III) intermediate. Electron-rich Josiphos ligands promote direct reductive elimination from this intermediate to afford cyclopropyl sulfides in high enantio- and diastereoselectivities. Alternatively, atropisomeric ligands (such as DTBM-BINAP) enable ring-opening from the cyclopropyl-Rh(III) intermediate to generate allylic sulfides with high enantio- and regiocontrol.

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.

Developments in the construction of cyclopropanols

The ring-opening of cyclopropanols is one of the most active areas of research and it has been well documented in recent years owing to subsequent coupling with various partners, thus providing the facile syntheses of a large number of multifunctional compounds that may otherwise be difficult to access. Evidently, the useful cascade reaction requires easy access to diversely functionalized cyclopropanol substrates. However, developments in the construction of cyclopropanols have not received adequate attention. Herein, recent reports on the formation of cyclopropanols are summarized, and the highly stereoselective production of new promising substrates for the cyclopropanol ring-opening/cross-coupling reactions are introduced and improved syntheses of known cyclopropanols are depicted. This review may facilitate more interesting applications of the cyclopropanol ring-opening/coupling reaction in the synthesis of pharmaceutical compounds, natural products, and structurally more diversified organic synthetic intermediates.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.