A catalytic allylic cation-induced intermolecular allylation-semipinacol rearrangement

Abstract

A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations has been realized. This tandem reaction is highly efficient under the catalysis of ZnBr, generating a wide range of α-homoallyl substituted ketones which contain all-carbon quaternary centres in good to excellent yields (up to 98%) with moderate to high diastereoselectivities (up to >20 : 1). Synthetic application of this novel methodology in the construction of core structures of natural products is also reported.

Cyclic Hypervalent Iodine Reagents for Azidation: Safer Reagents and Photoredox-Catalyzed Ring Expansion

“…On the basis of these results, we wondered if we could also develop an azidative ring-expansion of alkene-substituted cyclobutanol derivatives via reaction with azido radicals (Scheme C). Such ring expansions are usually initiated by the addition of electrophilic or organometallic intermediates onto the olefin. More recently, the addition of a radical followed by oxidation and an 1,2-shift under photoredox conditions has emerged as a very efficient approach for ring expansion, but it has never been reported for the synthesis of organic azides .…”

Enantioselective Catalytic Aldehyde α‐Alkylation/Semipinacol Rearrangement: Construction of α‐Quaternary‐δ‐Carbonyl Cycloketones and Total Synthesis of (+)‐Cerapicol

“…Over the past few years, there has been considerable research effort from our group in the development of semipinacol rearrangement reactions induced by carbon‐based electrophiles, which include dimethyl acetals, β , γ ‐unsaturated α ‐ketoesters, and propargyl or allyl alcohols . Nevertheless, most of these reactions are racemic and thereby of limited utility in asymmetric synthesis.…”

Palladium-Catalyzed Asymmetric Allylic Alkylation/α-Iminol Rearrangement: A Facile Access to 2-Spirocyclic-Indoline Derivatives

“…Therefore, it is highly desirable to develop efficient methodologies for the construction of chiral 2spirocarbocyclic-indoline skeletons bearing a unique adjacent C3 quaternary stereogenic center. We envisioned that a rationally designed 3-substituted 1H-indole substrate tethered with a built-in cyclobutanol motif could serve as a potential C-nucleophile in Pd-catalyzed asymmetric C3-allylic alkylation [24][25][26] to generate the intermediacy of -hydroxyl imine, which affects the subsequent -iminol rearrangement [27][28][29][30][31][32][33][34] to form biologically important spiro[cyclopentane-1,2'-indolin]-2-one incorporating two adjacent C3 all-carbon quaternary and spiro N-quaternary stereogenic centers (Scheme 1b). However, several formidable challenges are associated with this novel design: (1) the competitive chemoselectivity or site selectivity in the initial allylic alkylation reaction given that three potential nucleophilic sites (C3/N/O) embedded in the designed 3-substituted 1H-indole; (2) the feasibility of racemic branched allylic alcohols serving as the precursors for the key electronic -allylpalladium species, which involves the activation of the inert hydroxyl group and the interaction of racemic allylic alcohol (a pair of enantiomers) with the chiral palladium complex; (3) the enantioselectivity control of the allylic alkylation with prochiral nucleophile to generate a C3 all-carbon quaternary stereogenic center; (4) the highly stereoselective construction of vicinal tetrasubstituted stereogenic centers, one all-carbon and one spiro N-quaternary stereogenic center, is still a great challenge in organic synthesis.…”