Azetidiniums: Ring‐Expansion to Pyrrolidines, Piperidines, Azepanes, and Azocanes

Chiral N,N'-Dioxide Ligands Tune Diastereoselectivity in Mg(II)-Catalyzed Asymmetric Ring-Opening Desymmetrization of Azetidiniums

A diastereodivergent asymmetric desymmetrization of azetidinium salts with benzothiazoleamides as carbon nucleophiles through a chiral N,N'-dioxide/Mg(II) complex-promoted ring-opening reaction is realized by tuning ligands. Both syn- and anti-chiral δ-amino acid derivatives bearing benzothiazole structure were obtained in moderate to good yields and dr and ee values. DFT calculations indicated that the diastereodivergency stems from the different size of the chiral pocket formed by variable substructures of the ligands, leading to the opposite attack direction of the nucleophiles.

Forging Medium Rings via I(I)/I(III)-Catalyzed Diene Carbofunctionalization

A main-group catalysis-based strategy to access 8-membered carbocycles via the direct carbofunctionalization of 2-phenethyl-substituted 1,3-dienes is disclosed. Through the intervention of an I(I)/I(III) catalysis cycle, the synthesis of densely functionalized, fluorinated benzocyclooctenes can be achieved in an operationally simple manner. Modulating the oxidation/activation regime, and the external nucleophile, the process has been extended to unify the challenging cyclization with formation of allylic C-O, C-N, and C-C bonds (>30 examples). Derivatization of the product benzocyclooctenes is demonstrated together with X-ray conformational analysis, preliminary validation of enantioselective catalysis and a scalable resolution protocol.

Robust Synthesis of Terpenoid Scaffolds under Mn(I)-Catalysis

The 6/6/5-fused tricyclic scaffold is a central feature of structurally complex terpenoid natural products. A step-economical cascade transformation that leads to a complex molecular skeleton is regarded as a sustainable methodology. Therefore, we report the first Mn(I)-catalyzed C(sp²)-H chemoselective in situ dienylation and diastereoselective intramolecular Diels-Alder reaction using iso-pentadienyl carbonate to access 6/6/5-fused tricyclic scaffolds. To the best of our knowledge, there is no such report thus far to utilize iso-pentadienyl carbonate as a substrate in C-H activation catalysis. Extensive mechanistic studies, such as the isolation of catalytically active organo-manganese(I) complexes, 1,3-dienyl-intermediates, and isotopic labeling experiments have supported the proposed mechanism of this cascade reaction.

Synthesis of Tetrahydroazepines through Silyl Aza-Prins Cyclization Mediated by Iron(III) Salts

A new methodology for the synthesis of seven-membered unsaturated azacycles (tetrahydroazepines) was developed. It is based on the powerful aza-Prins cyclization in combination with the Peterson-type elimination reaction. In a single reaction step, a C–N, C–C bond and an endocyclic double bond are formed. Under mild reaction conditions and using iron(III) salts as sustainable catalysts, tetrahydroazepines with different degrees of substitution are obtained directly and efficiently. DFT calculations supported the proposed mechanism.

Modulating chitin synthesis in marine algae with iminosugars obtained by SmI2 and FeCl3-mediated diastereoselective carbonyl ene reaction

Strategies for synthesizing polyhydroxylated piperidines such as iminosugars have received broad attention. These substances are known to interact with carbohydrate related enzymes glycosidases and glycosyltransferases, to which also the large...

Synthesis of Azocanes from Piperidines via an Azetidinium Intermediate

α-Trifluoromethyl azocanes are accessible from 2-(trifluoropropan-2-ol) piperidines by metal-free ring-expansion involving a bicyclic azetidinium intermediate. The opening of the azetidinium intermediate was achieved by various nucleophiles (amines, alcoholates, carboxylates, phosphonates, halides and pseudo-halides) with an excellent regio- diastereo- and enantioselectivity and in good yields. The relative configuration of the piperidines and azocanes were assigned and the deprotected azocanes offer opportunities for further derivatization.

Divergent, Strain-Release Reactions of Azabicyclo[1.1.0]butyl Carbinols: Semipinacol or Spiroepoxy Azetidine Formation

The azetidine moiety is a privileged motif in medicinal chemistry and new methods that access them efficiently are highly sought after. Towards this goal, we have found that azabicyclo[1.1.0]butyl carbinols, readily obtained from the highly strained azabicyclo[1.1.0]butane (ABB), can undergo divergent strain-release reactions upon N-activation. Treatment with trifluoroacetic anhydride or triflic anhydride triggered a semipinacol rearrangement to give keto 1,3,3-substituted azetidines. More than 20 examples were explored, enabling us to evaluate selectivity and the migratory aptitude of different groups. Alternatively, treatment of the same alcohols with benzyl chloroformate in the presence of NaI led to iodohydrin intermediates which gave spiroepoxy azetidines upon treatment with base. The electronic nature of the activating agent dictates which pathway operates.