Chirality Transfer in Azetidinium
Ylides: An Enantioselective Route to α-Quaternary
Azetidines
作者:François Couty、Bruno Drouillat、Jérome Marrot
DOI:10.1055/s-0028-1087939
日期:——
Enantiomerically pure N-allyl azetidinium ions undergo a stereoselective [2,3]-sigmatropic shift to give azetidines with an α-quaternary center. These compounds are direct precursors of 2-alkyl-2-carboxy-azetidines, a new class of constrained α-amino acids.
Upon treatment with DAST (diethylaminosulfur trifluoride) enantiopure 2-hydroxyalkylazetidines rearrange into 3-fluoropyrrolidines. The reaction is stereospecific and involves a bicyclic 1-azoniabicyclo[2.1.0]pentane intermediate which is regioselectively opened by a fluoride anion.
Meisenheimer rearrangement. This ring expansion leading to isoxazolidines occurs readily, without trapping of the transient N-oxide. Starting with azetidines bearing a nitrile or an ester group at C-2, the rearrangement is regioselective. However, a varying amount of epimerization on the migrating radical is observed, which can also be observed with the related [1,2] Stevens rearrangement.
Regioselective Nucleophilic Opening of Azetidinium Ions
作者:François Couty、François Durrat、Gwilherm Evano
DOI:10.1055/s-2005-871537
日期:——
Azetidinium ions bearing different substitution patterns were reacted with nitrogen nucleophiles (sodium azide and benzylamine) and oxygenated nucleophiles (sodium acetate and alkoxides). High regioselectivity of nucleophilic opening was observed in both cases: the nucleophile reacting on the unsubstituted carbon in the case of α-substituted azetidiniumsalts and on the carbon bearing an ester or cyano
The reaction of an azetidine with a chloroformate can give either the dealkylated heterocycle or the ring-opened product (gamma-chloroamine), which can further cyclize to the oxazinanone. A general study of this underrated reaction was conducted and revealed that azetidines can undergo smooth nucleophilic ring-opening reactions to highly functionalized gamma-chloroamines in the presence of a variety of alkyl chloroformates under mild reaction conditions. Yields are usually good, and parameters governing this reaction were evaluated.