Electrophilic cyclizations of vinylcyclopropanols to tethered aldehydes

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.

Alder-ene reactions driven by high steric strain and bond angle distortion to form benzocyclobutenes

A unique aryne-based Alder-ene reaction to form benzocyclobutene is described.

A unique aryne-based Alder-ene reaction to form benzocyclobutene is described. In this process, the thermodynamic barrier to form a four-membered ring is compensated by the relief of the strain energy of an aryne intermediate. On the other hand, the driving force to overcome the high kinetic barrier is provided by the gearing effect of the bulky substituent at the ortho-position of the ene-donor alkene. To maximize the steric strain by the ortho-substituent, a structural element for internal hydrogen bonding is installed, which plays a crucial role for both the hexadehydro Diels–Alder and the Alder-ene reactions. DFT calculations show that the bulky hydrogen bonding element lowers the activation barrier for the Alder-ene reaction by destabilizing the intermediate, which is due to the severe bond angle distortion. The preferred formation of cis-isomers can also be explained by the extent of bond angle distortion.

Strategic innovation in the total synthesis of complex natural products using gold catalysis

This review has been organized from the perspective of synthetic target families, with emphasis on the use of gold-catalyzed transformations and cascade reactions that significantly increase molecular complexity.

Catalysis of diazoalkane-carbonyl homologation. How new developments in hydrazone oxidation enable the carbon insertion strategy for synthesis

Diazo compounds continue both to challenge and to fascinate practitioners of chemical synthesis. The most strategically powerful and unique type of reactivity observed with these reagents is a formal insertion of the donor-acceptor carbon into C-C or C-H bonds alpha to carbonyl groups. Although the reaction does not involve discrete carbon-metal bonds, it can be catalyzed by metal-based Lewis acids. This chapter investigates both classical and modern developments in diazoalkyl carbon insertion with a special emphasis on nonstabilized nucleophiles.

Au(I)-catalyzed ring expanding cycloisomerizations: total synthesis of ventricosene

The gold(I)-catalyzed cycloisomerization of enynes containing an embedded cyclopropane unit leads selectively to the formation of ring systems containing the cyclopropylmethyl cation. A subsequent Wagner-Merwein shift provides diastereomerically pure fused cyclobutanes. The utility of this methodology for the rapid assembly of polycyclic ring systems is illustrated by the total synthesis of the angular triquinane ventricosene.

1,2-Alkyl migration as a key element in the invention of cascade reactions catalyzed by pi-acids

This brief overview highlights recent progress in the field of cascade reactions that are initiated by the activation of a pi-system using platinum and gold catalysts and that are coupled with a 1,2-alkyl migration step. While the reactions discussed aim to rapidly evolve molecular complexity, they are experimentally straightforward and easy to perform. Primarily guided by the type of 1,2-alkyl migration, methods are categorized as shifts to metal carbenoid centers and pinacol-type rearrangements.

Diastereoselective Prins-type reaction of cycloalkenylcyclopropanol silyl ethers and alpha,beta-unsaturated aldehyde acetals

Electrophilic addition of 1-(1-cyclohexenyl)-1-cyclopropanol trimethylsilyl ether to alpha,beta-unsaturated aldehyde acetals under Lewis acidic conditions proceeds with good to excellent diastereoselectivity to afford spirocyclobutanones containing three contiguous stereocenters. A convenient entry to enantioselective syntheses is available by use of a nonracemic C2-symmetric acetal. Elaboration of the resulting adducts provides ready access to medium-sized carbocycles.

Cleavage of Activated Cyclic Amines: Unprecedented Approach toward 2-Substituted Cyclobutanones

We report, for the first time, ring opening of activated four- to six-membered cyclic amines followed by an intramolecular expansion of cyclopropanol to cyclobutanone via carbocation intermediate. In the case of a N-tosylaziridine ester, a cyclobutanol was formed in a stereospecific manner during the Kulinkovich reaction step.

Radical fragmentation of omega-bromoalkyl cyclobutanones. A modular approach to eight-membered carbocycles

[reaction: see text] An eight-membered ring was conveniently appended onto a cycloalkene carboxylate by employing a facile radical cyclization-fragmentation reaction of an omega-bromoalkyl spirocyclobutanone, which was readily accessible by the Kulinkovich cyclopropanation and subsequent electrophilic addition to a 3-bromoalkyl acetal.

Synthesis of new heterocycles through a cation-driven tandem ring-enlargement-annulation reaction

[reaction: see text] The thionium ion, generated through a cyclopropylcarbinyl-cyclobutyl ring expansion, is, for the first time, intramolecularly intercepted by activated aromatic rings to generate new versatile 2a-methyl-8b-(phenylsulfanyl-1,2a,3,8b-tetrahydro-2H-cyclobuta[c]chromenes.