Aziridines, electrocyclic ring opening

Synthetic work commenced with evaluation of an azomethine ylide dipole for the proposed intramolecular dipolar cycloaddition. A number of methods exist for the preparation of azomethine ylides, including, inter alia, transformations based on fluoride-mediated desilylation of a-silyliminium species, electrocyclic ring opening of aziridines, and tautomerization of a-amino acid ester imines [37]. In particular, the fluoride-mediated desilylation of a-silyliminium species, first reported by Vedejs in 1979 [38], is among the most widely used methods for the generation of non-stabilized azomethine ylides (Scheme 1.6). 

Triazole derivatives also result from the cycloaddition of DEAZD to azomethine ylids derived from electrocyclic ring opening of aziridines.117 121 For example, the tetrahydro-1,2,4-triazole 73 was prepared by thermolysis of the cts-aziridine in the presence of DEAZD in 96% yield (Eq. 8), and... 

Electrocyclic ring opening of the aziridine in (3.38) affords aza[17]annu-lene 357>. 

One last type of three-membered ring whose electrocyclic ring opening does tell us about the stereochemistry of the process is the aziridine. 

Many aziridines are stable compounds, but those Me02 bearing electron-withdrawing groups are unstable with respect to electrocyclic ring opening. 

The product is clearly derived from a [3 + 2] cycloaddition. Disconnection of the appropriate bonds leads back to an azomethine ylide intermediate, which can be formed by electrocyclic ring opening of the aziridine. 

While the electrocyclic ring-opening of aziridines to azomethine ylides is well known, the reverse reaction has also been studied as a route to aziridines . This process is implicit in the cis-trans equilibration of substituted aziridines <67JA1753, 7UA1779), and may be involved in the reaction of diazoalkanes with imines <84T2569>. Convincing evidence for the electrocyclization of... 

Azomethine ylides are another class of powerful nitrogen-based 1,3-dipoles, which provide access to pyrrolidines. An illustration of their use in stereoselective synthesis [53] was reported by Takano, starting from aziridine 28 (Scheme 18.7) [54]. Thermally induced electrocyclic ring-opening generated dipole 29 in situ, which then participated in a stereospecific cycloaddition process to give 30 in >95 5 dr. This key intermediate was subsequently transformed into acromelic acid A (31), an amino acid with important neurochemical properties. 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

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