Metal-catalyzed enantioconvergent cross-coupling reactions of alkylelectrophiles are emerging as apowerful tool in asymmetric synthesis.T od ate,h igh enantioselectivity has been limited to couplings of electrophiles that bear adirecting group or ap roximalp / p orbital. Herein, we demonstrate for the first time that enantioconvergent cross-couplings can be achieved with electrophiles that lack such features;specifically, we establish that achiral nickel catalyst can accomplish Negishi reactions of racemic a-halosilanes with alkylzinc reagents with good enantioselectivity under simple and mild conditions, therebyproviding access to enantioenriched organosilanes,an important class of target molecules.

As a part of our program aimed at developing new ringexpansion reactions from strained heterocyclic precursors [1] we are exploring vinyl oxetanes as potential candidates. We were particularly eager to learn if our success with the ring expansion of vinyl oxiranes to 2,5-dihydrofurans using [Cu-(hfacac) 2 ] could be translated into an equivalent transformation for vinyl oxetanes (Scheme 1). Given the allylic nature of the system and the favorable release of strain energy, vinyl oxetanes seemed like perfect ring-expansion candidates. Furthermore, we predicted that a broader range of catalysts would be competent for this new ring-expansion reaction because the competing 1,2-hydride shift pathway we needed to overcome for the vinyl oxirane ring expansion is less likely to occur for the vinyl oxetanes.In general, vinyl oxetanes have received limited attention, where the few published studies have focused on nucleophilic ring openings [2] of vinyl oxetanes and insertions of heteroatoms [3] into the oxetane. [4] There is only one report in the literature of a related lactonic vinyl oxetane ring expansion (Scheme 1). In 2000, when trying to expand the scope of their cationic-palladium-mediated b-lactone synthesis to include conjugated aldehydes Hattori et al. realized that in many cases d-lactones were formed. [5] The scope and yield of their cascade reaction, which was developed using classic palladium chemistry and relies on a superior leaving group to that in our ether ring expansion, was reasonable but limited to a few enals; enones were found to be poor substrates.We selected vinyl oxetane 1 as our model substrate. It is readily accessible by a nucleophilic addition to a commercially available b-chloro carbonyl precursor [6] and it has a phenyl group that we postulated would aid oxetane bond breaking and stabilization of a cationic intermediate. When 1 was subjected to the standard catalytic [Cu(hfacac) 2 ] conditions we used for the vinyl oxiranes, [1a] ring expansion to 3,6dihydro-2H-pyran 2 did indeed occur after prolonged heating at 150 8C, albeit in poor yield (Table 1, entry 1). The detrimental hydride shift pathway is presumably a far less likely competing path for vinyl oxetanes than vinyl oxiranes, therefore we decided to evaluate other stable readily available catalysts. Metal(II) triflates were the most effective catalysts and dichloromethane the best solvent. Copper(II) triflate (Table 1, entries 5-7) proved to be a remarkably wellsuited catalyst for the ring expansion, thus affording the desired product (2) both rapidly and quantitatively using only 1 mol % of catalyst at À78 8C. These results prompted us to Scheme 1. Proposed catalytic ring expansion of vinyl oxetanes. Table 1: Catalyst screening for vinyl oxetane ring expansion. Entry Catalyst mol % Solvent T [8C] t [h] % Conv.

A Diels–Alder reaction, a desymmetrizing aldol reaction, and a reductive Heck cyclization are employed in a short synthesis of a tetracycle relevant to exiguaquinol, a potential antibiotic. Ground-state energies of this advanced model system and the natural product rationalize the incorrect hemiaminal configuration experimentally obtained, and point to the importance of the sulfonate in dictating the relative configuration of the natural product.

The halenaquinol family of naphthoquinol natural products includes a few closely related polycyclic compounds that feature an activated, electrophilic furan ring. This motif is likely responsible for the rich biological activity attributed to these secondary metabolites. Their interesting structures—related via their electrophilic furan to the biologically important furanosteroids—and their activities prompted significant efforts by organic chemists that resulted in many strategically compelling laboratory syntheses of these targets. These different strategies are compared and contrasted in this Microreview, and the authors’ recent work on the structurally different but biogenetically related natural product exiguaquinol is put into the context of the previous studies on halenaquinol-type targets.

Metal‐catalyzed enantioconvergent cross‐coupling reactions of alkyl electrophiles are emerging as a powerful tool in asymmetric synthesis. To date, high enantioselectivity has been limited to couplings of electrophiles that bear a directing group or a proximal p/π orbital. Herein, we demonstrate for the first time that enantioconvergent cross‐couplings can be achieved with electrophiles that lack such features; specifically, we establish that a chiral nickel catalyst can accomplish Negishi reactions of racemic α‐halosilanes with alkylzinc reagents with good enantioselectivity under simple and mild conditions, thereby providing access to enantioenriched organosilanes, an important class of target molecules.