2-Azaallyl anions from imines

In general, metalated 2-azaallyl anions derived from imines of a-amino esters serve both as Michael donors and as 1,3-dipolar reagents the course of the reaction, as well as the stereochemical outcome depends upon the base and the reaction conditions82,83. 

Ethyl (bornylideneamino)acetate (2) and the imines of (-)-(lf ,2, 5 )-2-hydroxy-3-pinanone and glycine, alanine and norvaline methyl esters were particularly successful as Michael donors. The chiral azaallyl anions, derived from these imines by deprotonation with lithium diisopropylamide in THF at — 80 C, add to various a,/i-unsaturated esters with modest to high diastereoselectivity (see Section 1.5.2.4.2.2.5.). Thus, starting with the imine 2, (R1 = CH,) and ethyl ( )-2-butcnoate, the a,/i-dialkylated glutamate derivative 3 is obtained as a single diastercomer in 90% yield91-92. 

While the abstraction of protons adjacent to the carbon-nitrogen double bond of imines/imine derivatives has been utilized for tiie regioselective generation of azaallyl anions (which are useful in asymmetric ketone synthesis), it competes with and often prevents the addition of nucleophiles to imines. For this reason, imine additions often involve azomethines (e.g. benzylidineanilines) which are not capable of enolization. Many potentially useful additions, however, involve substrates capable of proton abstraction. By avoiding in certain instances some of the structural features of imines/imine derivatives and the reaction conditions responsible for proton abstraction, products resulting from this serious side reaction can be minimized. 

Diaryl-2-thiazolines (331) are synthesized by reaction of l,3-diaryl-2-azaallyl anions (330) with 0-ethyl thiocarboxylates (Scheme 83). The anions (330) are generated in situ from imines <95SL809>. 

Among the cycloaddition reactions which lead to imidazole products is the [3 + 2] cycloaddition of imines to 2-azaallenyl radical cations (derived from azirines by photolysis) which yields 1-substituted imidazoles <91AG(E)1336,93CB543). Similar addition of 2-azallyl anions (made by deprotonation of A-alkylated Schilf bases) to aromatic nitriles can give rise to either 3-imidazolines or imidazoles depending on the substitution pattern in the azaallyl species (229) <83CB492>. These reactions have been reviewed (Scheme 164) <90CHE1>. 

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