2-Vinyl Threoninol Derivatives via Acid-Catalyzed Allylic Substitution of Bisimidates

The synthesis of unnatural α-alkyl- and α-aryl-substituted serine derivatives

The synthesis of α-aryl- and α-alkyl-substituted serine derivatives via [3,3]-sigmatropic rearrangement of allyl carbamates as a key step is reported. Allyl carbamates were obtained from the corresponding allyl alcohols. The former were prepared through three approaches. Aryl-substituted ones were synthesized via the Stille coupling reaction of aryl iodides with enantiomerically enriched vinyl stannanes. Conversely, alkyl-substituted allyl alcohols were prepared by an analogous strategy involving the Negishi coupling reaction of enantiomerically enriched vinyl iodides or by enzymatic kineric resolution of the corresponding racemic alcohols.

Chiral Brønsted acid-catalyzed intramolecular SN2' reaction for enantioselective construction of a quaternary stereogenic center

Construction of a quaternary stereogenic center was accomplished through the enantioselective intramolecular allylic substitution reaction of bis-trichloroacetimidate catalyzed by a chiral phosphoramide derivative.

An enantioselective intramolecular anti-S_N2′ cyclization reaction for the construction of a quaternary stereogenic center was accomplished through the activation of the leaving group using a binaphthol-derived phosphoramide as the chiral Brønsted acid catalyst. The present allylic substitution reaction is beneficial not only for the regioselective nucleophilic substitution at the multi-substituted site of the double bond but also for controlling the stereochemical outcome because of using a geometrically defined double bond. Indeed, the reaction afforded synthetically useful amino alcohol derivatives having a tetra-substituted carbon center in a highly enantioselective manner in most cases, in which the modification of the sulfonamide unit of the phosphoramide catalyst was demonstrated to improve the enantioselectivity. Experimental and theoretical elucidation of the reaction mechanism suggested that the reaction proceeds through a synchronous anti-S_N2′ pathway, although NMR monitoring of the reaction indicated the formation of the phosphorimidate ester via the S_N2 reaction of the catalyst with the substrate, which results in catalyst deactivation. Further theoretical studies of the origin of the stereochemical outcome at the generated quaternary stereogenic center were performed. Structural analysis of the transition states at the enantio-determining step revealed that the distinct discrimination of the substituents attached to the geometrically defined double bond is achieved by the anthryl and sulfonamide substituents of the catalyst through the three-point hydrogen bonding interactions and the T-shaped C–H···π interactions.

Intramolecular cyclopropylmethylation via non-classical carbocations

Cyclopropyl-cyclopropyl rearrangement can be achieved selectively by intramolecular trapping of cyclopropylmethyl carbenium ions with an internal nucleophile. This can be exploited as a useful method for the introduction of a cyclopropyl group into complex molecules using readily accessible disubstituted cyclopropane intermediates.

A Useful Allene for the Stereoselective Synthesis of Protected Quaternary 2-Amino-2-vinyl-1,3-diols

Treatment of readily available allene 1 with Cy₂BH followed by addition of an aldehyde led to quaternary protected 2-amino-2-vinyl-1,3-diols in high yield and excellent stereochemical purity. The choice of benzoyl as N-protecting group is critical since the observed N- to O-Bz transfer during the process prevents later undesired isomerizations in the adducts and keeps all heteroatoms protected.