Transition-metal-free Synthesis of tetra-substituted Vinyl Iodides by Cascade Sequential Reaction of α-Keto Acids, 1-Iodoalkynes, and Alkyl Halides

The cascade sequential reaction of α-keto acids, 1-iodoalkynes, and alkyl halides are reported herein to synthesize tetra-substituted vinyl iodides. It represents an efficient protocol to access a diverse range of tetra-substituted vinyl iodides starting from simple materials in a one-pot fashion, featuring mild reaction conditions, ease of operation, and broad substrate scope.

Enantioselective Cyclopropanation/[1,5]-Hydrogen Shift to Access Rauhut-Currier Product

A Michael addition initiated cyclopropanation/[1,5]-hydrogen shift has been developed for the enantioselective synthesis of Rauhut-Currier products. The reaction of α-alkyl diazoesters and in situ generated o-quinone methides proceeds in the presence of chiral oxazaborolidinium ion, providing Z-stereocontrolled Rauhut-Currier products in high yields (up to 96%) with excellent Z/E selectivities (>20:1) and enantioselectivities (up to >99% ee). The synthetic utility was illustrated by conversion of the product to 3,4-dihydrocoumarins with two adjacent chiral stereocenters.

Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yield (up to 98%) with high to excellent enantioselectivity (up to 99% ee). A rational mechanistic model for the complex of COBI and aldimine is provided to account for these enantioselective reactions.

Asymmetric Synthesis of Enantioenriched 2-Aryl-2,3-Dihydrobenzofurans by a Lewis Acid Catalyzed Cyclopropanation/Intramolecular Rearrangement Sequence

A cyclopropanation/intramolecular rearrangement initiated by the Michael addition of in situ generated ortho-quinone methides (o-QMs) has been developed for the enantioselective synthesis of 2-aryl-2,3-dihydrobenzofurans containing two consecutive stereogenic centers, including a quaternary carbon atom. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction proceeded in excellent yields (up to 95 %) with excellent stereoselectivity (up to >99 ee, up to >20:1 d.r.).

Asymmetric Synthesis of Cyclobutanone via Lewis Acid Catalyzed Tandem Cyclopropanation/Semipinacol Rearrangement

Chiral Lewis acid catalyzed asymmetric formation of cyclobutanones from α-silyloxyacroleins and α-alkyl or α-aryl diazoesters has been developed. In the presence of a chiral oxazaborolidinium ion catalyst, various α-silyloxycyclobutanones possessing a chiral β-quaternary center were synthesized in high yield (up to 91%) with excellent enantio- and diastereoselectivity (up to 98% ee and up to >20:1 dr) through tandem cyclopropanation/semipinacol rearrangement. The synthetic potential of this method was illustrated by conversion of the product to various cyclic compounds such as γ-lactone, cyclobutanol, and cyclopentanone.

Cascade cyclizations of acyclic and macrocyclic alkynones: studies toward the synthesis of phomactin A

A study of the reactivity and diastereoselectivity of the Lewis acid promoted cascade cyclizations of both acyclic and macrocyclic alkynones is described. In these reactions, a β-iodoallenolate intermediate is generated via conjugate addition of iodide to an alkynone followed by an intramolecular aldol reaction with a tethered aldehyde to afford a cyclohexenyl alcohol. The Lewis acid magnesium iodide (MgI2) was found to promote irreversible ring closure, while cyclizations using BF3·OEt2 as promoter occurred reversibly. For both acyclic and macrocyclic alkynones, high diastereoselectivity was observed in the intramolecular aldol reaction. The MgI2 protocol for cyclization was applied to the synthesis of advanced intermediates relevant to the synthesis of phomactin natural products, during which a novel transannular cation-olefin cyclization was observed. DFT calculations were conducted to analyze the mechanism of this unusual MgI2-promoted process.