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

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.