Total Syntheses of (±)-Melicolones A and B

Sc(OTf)3-Catalyzed [3+2]-Cycloaddition of Diazoacetoacetate Enones and N-Aryl Nitrones: Diastereoselective Synthesis of Functionalized Isoxazolidines with Three Contiguous Stereogenic Centers

A catalytic [3+2]-cycloaddition using Sc(OTf)3 as a Lewis acid catalyst is developed. This catalytic 1,3-dipolar cycloaddition dia­stereoselectively transforms diazoacetoacetate enones and N-aryl nitrones into highly functionalized isoxazolidines bearing three contiguous chiral centers. The feasibility of the uncatalyzed 1,3-dipolar cycloaddition is postulated by DFT calculations and substantiated experimentally.

Reflections on the intriguing occurrence of some recently isolated natural products as racemates and scalemic mixtures

This review continues our interest in the intriguing reports of a variety of new racemic natural products (at least 11 in the past 4 years). These include the polyphenolic racemate galewone, the polycyclic prenylated acylphloroglucinol garcinielliptone; variecolortide, a combination of an anthraquinone and a isochinulin-type alkaloid; the isoindoline alkaloid irpexine, the new hybrid phenylproanoid asarone; colletopyandione an indolydenepyradione; the enantiomerically enriched (scalemic) neolignans, gardenifolins; and meroterpenoid pabmaragramin in addition to some marine lipids. We also present a recent biomimetic synthesis of the polyketide preuisolactone A; synthesis of the polyketide spiromamakone A, which also corrected the proposed structure of another metabolite as identical to spiromamakone A; and the melicolones A and B. The continuing reports of natural racemates provoke speculation as to their role in the producing organism.

Enabling Cyclization Strategies through Carbonyl-Ylide-Mediated Synthesis of Malonate Enol Ethers

Malonate enol ethers are afforded in one step by condensation of cyclic ketones with α-diazomalonates under [CpRu(CH₃CN)₃][BAr_F] catalysis. The dual reactivity of these 2-vinyloxymalonates can be used to expand the classical range of cyclizations derived from carbonyl ylide intermediates.

Catalytic asymmetric [3+2] cycloaddition of isomünchnones with methyleneindolinones

An efficient enantioselective [3+2] cycloaddition of isomünchnones with methyleneindolinones that are generated by an in situ intramolecular addition of the carbonyl group to rhodium carbenes is realized with a chiral N,N'-dioxide/Zn(II) complex as a Lewis acid. A series of chiral oxa-bridged 3-spiropiperidines are obtained in high yields with excellent dr and excellent ee values.

Electrochemical ODI-[5+2] Cascade for the Syntheses of Diversely Functionalized Bicyclo[3.2.1]octane Frameworks

A metal- and hypervalent iodine reagent-free electrochemical oxidative dearomatization-induced [5+2] cycloaddition/pinacol rearrangement cascade reaction was described. The electrosynthetic method showed strong tolerance for vinylphenols, ethynylphenols, and allenylphenols, which thus enabled the rapid assembly of diversely functionalized bicyclo[3.2.1]octanes in 41-95% yields and up to >20:1 dr. This protocol could be scaled up to gram amounts and should find wide application in complex natural product synthesis.