Expedient synthesis of spiro[3.3]heptan-1-ones via strain-relocating semipinacol rearrangements

Formal Carbene Insertion into Cyclopropanones: Access to 2-Aroyl Cyclobutanones via Sulfonium Ylides

This report presents a method for the synthesis of 2-aroyl cyclobutanones via the reaction of in situ-generated cyclopropanones with acyl sulfonium ylides representing a formal carbene insertion into cyclopropanones. The reaction is highly stereoselective in the case of 2-substituted cyclopropanones, and the cyclobutanones thus obtained are well suited to α-alkylation, offering versatile synthetic applications.

Concise Synthesis of Optically Active Cyclopropane β-Amino Acid Derivatives via Olefination of Cyclopropanone Surrogates

An expedient synthesis of cyclopropane β-amino acid derivatives is reported from readily accessible cyclopropanone surrogates. The addition of stabilized phosphorus ylides to 1-sulfonylcyclopropanols leads to the formation of highly electrophilic alkylidenecyclopropanes shown to be reactive in a telescopic aza-Michael reaction, in mild conditions. The transformation proceeds with complete diastereocontrol in favor of the trans products and is amenable to the rapid production of highly enantioenriched β-amino acid derivatives, peptidomimetics and spirocyclic analogues.

Shifting Lithium Amide Reactivity to the Radical Domain: Regioselective Radical C-H Functionalization of 3-Iodooxetane for the Synthesis of 1,5-Dioxaspiro[2.3]hexanes

Strained spiro-heterocycles (SSHs) have gained significant attention within the medicinal chemistry community as promising sp3-rich bioisosteres for their aromatic and non-spirocyclic counterparts. We herein report access to an unprecedented spiro-heterocycle-1,5-dioxaspiro[2.3]hexane. Our synthetic approach leverages a lithium-amide induced single-electron transfer to benzophenones generating an N-centered radical and a ketyl radical anion-reminiscent of a frustrated radical pair. This pair works synergistically to selectively abstract the β-hydrogen from 3-iodooxetane, initiating an exergonic radical-radical coupling reaction. This process enables the formation of the desired bond between the oxetane core and benzophenone derivatives, ultimately yielding the novel 1,5-dioxaspiro[2.3]hexane core. The stability and synthetic utility of the novel 1,5-dioxaspiro[2.3]hexane motif are showcased. An in-depth mechanistic investigation is presented, including cyclic voltammetry studies, as well as computational calculations and experiments to support the mechanism of this new single electron synthetic tactic.

Recent advances in asymmetric synthesis via cyclopropanol intermediates

Cyclopropanols have attracted significant attention in organic synthesis as versatile three-carbon synthons, as this readily available class of donor-activated cyclopropanes undergoes miscellaneous transformations, either via ring-opening or with retention of the cyclopropane ring. This review summarizes stereoselective and stereoretentive transformations suitable for asymmetric synthesis. The utility of cyclopropanols is discussed for two main strategies: (i) substrate-controlled transformations using enantiomerically enriched cyclopropanol intermediates through a traditional approach, and (ii) the use of nonchiral or racemic cyclopropanols, where asymmetric induction is achieved through a chiral catalyst, representing a direction that has recently emerged.

Ring-Enlargement of in Situ Generated Cyclopropanones by the Reaction with Sulfonium Ylides: One-Pot Synthesis of Cyclobutanones

In this report, we describe a simple method for the synthesis of 2-aryl-2-vinyl-cyclobutanones through the reaction of in situ generated cyclopropanones and cinnamylsulfonium ylides, representing an example of a formal carbene insertion into these three-membered rings. The cyclobutanones thus obtained are ideal substrates for palladium-catalyzed coupling reactions upon enol triflate formation, thereby providing access to densely functionalized cyclobutenes. A mechanistic proposal for the ring-enlargement is presented based on experimental evidence.

Brønsted acid-catalyzed synthesis of spirocyclobutanes via heteroannulation of vinyloxyphenylbicyclobutanes with water

We report a perchloric acid-catalyzed heteroannulation for the synthesis of spirocyclobutanes using vinyloxyphenylbicyclobutanes with water. This metal-free reaction yields high product outputs and is consistent with the formation of a cyclobutene intermediate originating from an isomerization of a bicyclobutane.

Iron-Catalyzed Oxidative Rearrangement of Cyclopropanone Hemiaminals: General Access to Pyrroloindolones from Indoles

A concise synthetic approach to medicinally relevant pyrroloindolones and related fused heterocycles is reported via the diastereoselective N-addition of unprotected indoles to readily accessible cyclopropanone equivalents. The resulting stable hemiaminals are shown to smoothly rearrange to pyrroloindolones in mild conditions using Fe(III) catalysis in the presence of inexpensive ammonium persulfate as a stoichiometric oxidant. Experimental evidence points toward the formation of a β-carboxylic radical intermediate prone to cyclization and oxidative rearomatization as the operative mechanistic pathway.

Spiro[3.3]heptane as a Saturated Benzene Bioisostere

The spiro[3.3]heptane core, with the non-coplanar exit vectors, was shown to be a saturated benzene bioisostere. This scaffold was incorporated into the anticancer drug sonidegib (instead of the meta-benzene), the anticancer drug vorinostat (instead of the phenyl ring), and the anesthetic drug benzocaine (instead of the para-benzene). The patent-free saturated analogs obtained showed a high potency in the corresponding biological assays.

Divergent Synthesis of β-Fluoroamides via Silver-Catalyzed Oxidative Deconstruction of Cyclopropanone Hemiaminals

An expedient approach for the synthesis of challenging β-fluoroamides from readily accessible cyclopropanone equivalents is reported. Following the addition of pyrazole used here as a transient leaving group, silver-catalyzed regiospecific ring-opening fluorination of the resulting hemiaminal leads to a β-fluorinated N-acylpyrazole intermediate reactive to substitution with amines, ultimately affording β-fluoroamides. The process could also be extended to the synthesis of β-fluoroesters and γ-fluoroalcohols via the addition of alcohols or hydrides as terminal nucleophiles, respectively.