Harnessing the Reactivity of Cyclopropenes in the Synthesis of Spiroketals via Putative Generation of Donor-Acceptor Cyclopropanes

TMSOTf-mediated 5/6-exo-trig hydroalkoxylation followed by the (3 + 2) cycloaddition cascade reaction of hydroxy cyclopropenes with aldehydes gave an expedient, stereoselective synthesis of [5.5]-and [6.5]-spiroketal derivatives. The developed protocol provides a new approach toward the synthesis of spiroketals from hydroxy cyclopropenes via a transiently generated donor-acceptor cyclopropane intermediate. These spirocyclic derivatives could be transformed to facilitate the access of intricate polycyclic heterocycles via a metal halogen exchange reaction and a copper-catalyzed C-O cross coupling reaction. The decarboxylation of the diester diastereoselectively sets the fourth chiral center of the spiroketal product.

Highly Enantioselective O-H Bond Insertion Reaction of α-Alkyl- and α-Alkenyl-α-diazoacetates with Water

Catalytic asymmetric reactions in which water is a substrate are rare. Enantioselective transition-metal-catalyzed insertion of carbenes into the O-H bond of water can be used to incorporate water into the stereogenic center, but the reported chiral catalysts give good results only when α-aryl-α-diazoesters are used as the carbene precursors. Herein we report the first highly enantioselective O-H bond insertion reactions between water and α-alkyl- and α-alkenyl-α-diazoesters as carbene precursors, with catalysis by a combination of achiral dirhodium complexes and chiral phosphoric acids or chiral phosphoramides. Participation of the phosphoric acids or phosphoramides in the carbene transfer reaction markedly suppressed competing side reactions, such as β-H migration, carbene dimerization, and olefin isomerization, and thus ensured good yields of the desired products. Fine-tuning of the ester moiety facilitated enantiocontrol of the proton transfer reactions of the enol intermediates and resulted in excellent enantioselectivity. This protocol represents an efficient new method for preparation of multifunctionalized chiral α-alkyl and α-alkenyl hydroxyl esters, which readily undergo various transformations and can thus be used for the synthesis of bioactive compounds. Mechanistic studies revealed that the phosphoric acids and phosphoramides promoted highly enantioselective [1,2]- and [1,3]-proton transfer reactions of the enol intermediates. Maximization of molecular orbital overlap in the transition states of the proton transfer reactions was the original driving force to involve the proton shuttle catalysts in this process.

Visible-light mediated oxidative ring expansion of anellated cyclopropanes to fused endoperoxides with antimalarial activity

Hetero- and carbocyclic anellated cyclopropanes were converted in one step by a visible light induced photooxidation to their corresponding polycyclic endoperoxides, which show promising antimalarial activity.

Cascade Radical Cyclization on Alkynyl Vinylogous Carbonates for the Divergent Synthesis of Tetrasubstituted Furans and Dihydrofurans

A single alkynyl vinylogous carbonate was elaborated to tetrasubstituted furan or dihydrofuran via a cascade inter-intramolecular radical reaction by changing the radical being added. The strategy could be used in the synthesis of polycyclic heterocycles as well as bis-furan exhibiting atropisomerism. Installation of a new furan motif on the existing one was feasible by iteration. Stannyl dihydrofuran derivative was used in Stille coupling, whereas intramolecular Friedel-Crafts acylation on the furan gave furanonaphthol.

Asymmetric Protonation of Cumulenolates: Synthesis of Allenyl Aldehydes Facilitated by an Organomanganese Auxiliary

Chiral ammonium salts were used to catalyze the isomerization of organomanganese-complexed alkynyl aldehydes to chiral allenal building blocks in moderate to good enantiomeric excesses. Normally, conjugated alkynyl aldehydes do not isomerize to their thermodynamically less stable allene isomers. However, with a manganese auxiliary in place to promote allene formation, asymmetric protonation of cumulenolate intermediates was realized using a variety of cinchonidinium salts in a weakly basic biphasic reaction system. Optimal results were realized using a novel cinchonidinium geranyl derivative with its C-9 hydroxyl group playing a crucial role in enantioselectivity.

Organo-manganese η2-auxiliary directed reactions: a diastereoselective approach to 2,3-allenols

Propargyl aldehydes underwent isomerization to allenyl aldehydes under mildly basic conditions when complexed to an organo-manganese auxiliary using methylcyclopentadienyl manganese tricarbonyl (MMT). This traceless auxiliary magnifies the axial chirality of the allene moiety, allowing for highly diastereoselective additions to the aldehyde carbonyl and subsequent access to an array of 2,3-allenols. Using this strategy, a nitrile-substituted 2,3-allenol was prepared and efficiently converted to Hagen's gland lactone.

A stereocontrolled synthesis of Hagen's gland lactones via iterative proline catalyzed α-aminoxylation and oxa-Michael addition reactions

A simple and efficient synthesis of Hagen's gland lactones was achieved using a sequential α-aminoxylation/oxa-Michael approach in a highly diastereoselective manner with assignment of relative configurations.

Donor-acceptor substituted cyclopropane to butanolide and butenolide natural products: enantiospecific first total synthesis of (+)-hydroxyancepsenolide

An oxygen substituted donor-acceptor cyclopropane (DAC) is used as a common intermediate in the enantiospecific collective total synthesis of butanolide- and butenolide-based natural products like (+)-juruenolide C and D, (+)-blastmycinone, (+)-antimycinone, and (+)-ancepsenolide. Enantiospecific first total syntheses of (+)-hydroxyancepsenolide and its acetate are achieved confirming their absolute stereochemistry.