Toward the Synthesis of Phomoidride D

Cyclopropane Hydrocarbons from the Springtail Vertagopus sarekensis─A New Class of Cuticular Lipids from Arthropods

The epicuticle of insects is usually coated with a complex mixture of hydrocarbons, primarily straight-chain and methyl-branched alkanes and alkenes. We were interested in whether springtails (Collembola), a sister class of the insects, also use such compounds. We focused here on Vertagopus sarekensis, an abundant Isotomidae species in European high alpine regions, exhibiting coordinated group behavior and migration. This coordination, suggesting chemical communication, made the species interesting for our study on epicuticular hydrocarbons in springtails with different degrees of group behavior. We isolated a single hydrocarbon from its surface, which is the major epicuticular lipid. The structure was deduced by NMR analysis and GC/MS including derivatization. Total synthesis confirmed the structure as cis,cis-3,4,13,14-bismethylene-24-methyldotriacontane (4, sarekensane). The GC/MS analyses of some other cyclopropane hydrocarbons also synthesized showed the close similarity of both mass spectra and gas chromatographic retention indices of alkenes and cyclopropanes. Therefore, analyses of cuticular alkenes must be performed with appropriate derivatization to distinguish these two types of cuticular hydrocarbons. Sarekensane (4) is the first nonterpenoid cuticular hydrocarbon from Collembola that is biosynthesized via the fatty acid pathway, as are insect hydrocarbons, and contains unprecedented cyclopropane rings in the chain, not previously reported from arthropods.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

Catalytic Carbonylative Spirolactonization of Hydroxycyclopropanols

A palladium-catalyzed cascade carbonylative spirolactonization of hydroxycyclopropanols has been developed to efficiently synthesize oxaspirolactones common to many complex natural products of important therapeutic value. The mild reaction conditions, high atom economy, broad substrate scope, and scalability of this new method were highlighted in expedient total syntheses of the Turkish tobacco natural products α-levantanolide and α-levantenolide in two and four steps, respectively. The hydroxycyclopropanol substrates are readily available in one step via a Kulinkovich reaction of the corresponding lactones. Mechanistic studies utilizing high-resolution electrospray ionization mass spectrometry (ESI-MS) identified several key intermediates in the catalytic cycle, as well as those related to catalyst decomposition and competitive pathways.

Construction of Optically Active Isotwistanes and Aminocyclitols Using Chiral Cyclohexadiene as a Common Intermediate

We have developed a new method for synthesizing chiral isotwistane and homoisotwistane skeletons as well as aminocyclitols in a highly stereoselective manner. These results were achieved through the use of a common intermediate, which was derived from the ytterbium-catalyzed asymmetric Diels-Alder reaction of Danishefsky diene.

Endeavors to access molecular complexity: strategic use of free radicals in natural product synthesis

Free radicals, which in the past were considered unruly chemical species, have become manageable and indispensable for synthetic organic chemistry. The unique nature of free radicals has allowed practitioners in organic synthesis to design flexible approaches to produce various materials ranging from small molecules to polymers. The present Personal Account describes the author's endeavors to create molecular complexity by the strategic use of free radicals, with an emphasis on the synthesis of bioactive natural products.