Ring‐Opening of N‐Lithio‐2,3(cis)‐diphenylaziridine to 1,3(cis, trans)‐Diphenyl‐2‐azaallyllithium and Its Rearrangement to the trans, trans‐Form

Abstract

According to the above results the "alkene-created part" of the pyrrolidines obtained by [1,3]-anionic cycloaddi-tionr2] always has the configuration of the alkene used in the reaction, thus indicating stereospecific cis-addition of the alkenes (no alternative in the case of acenaphthylene). In the "azaallyl-created part" the phenyl residues of the pyrrolidines always have cis-configuration, thus proving that the stereospecific addition always involves the same 1,3-diphenyl-2-azaallyl anionr5'. The findings are best interpreted in terms of a symmetry allowed one-step [n41 + n2,]-cycloaddition of 1,3-(trans,trans)-( I ) or 1,3-(cis,cis)-diphenyl-2-azaallyl anion ( 5 ) according to the principle of conservation of orbital symmetry [']. The trans,trans configuration of the anion undergoing addition is thus much more probable since the anion of the comparable 1,3-diphenyl-allyllithium in THF at 0°C is always present in this configurationr5. 61. The transpans configuration of the 1,3-diphenyl-2-azaallyl anion at 0°C can also be deduced from the irreversible, and obviously quantitative configurational rearrangement of 1,3-(cis,trans) -diphenyl-2-azaallyl anion (6) on warming from -60°C to O°C[81, since the thermodynamic stability should increase along the series cis& < cis,trans < trans,trans (completely coplanar, strain-free conformation is possible only in the case of transpans). However, confirmation of the trans,trans conformation by direct NMR investigation has not, as yet, been reported.

Cycloadditions of Nonstabilized2-Azaallyllithiums (2-Azaallyl Anions) and Azomethine Ylides withAlkenes: [3+2] Approaches to Pyrrolidinesand Application to Alkaloid Total Synthesis

“…25 We have not observed such products in our tin-derived anions. In Kauffmann's work on 1,3-diphenyl-2-azaallyllithium, similar results were obtained when the anion was generated by deprotonation.…”

“…Kauffmann had found that 2,3-diphenylaziridine, upon deprotonation, afforded 1,3-diphenyl-2-azallyllithium via a conrotatory electrocyclic ring opening, 25 but that the ring-opening of N-lithio-2-phenylaziridine 'proceeds slowly even at 65°C'; 4 no cycloadditions of the resultant anion were reported. Kauffmann had found that 2,3-diphenylaziridine, upon deprotonation, afforded 1,3-diphenyl-2-azallyllithium via a conrotatory electrocyclic ring opening, 25 but that the ring-opening of N-lithio-2-phenylaziridine 'proceeds slowly even at 65°C'; 4 no cycloadditions of the resultant anion were reported.…”

Cycloadditions of Nonstabilized 2‐Azaallyllithiums (2‐Azaallyl Anions) and Azomethine Ylides with Alkenes: [3 + 2] Approaches to Pyrrolidines and Application to Alkaloid Total Synthesis.

“…Interestingly, when the same semistabilized 2-azaallyllithium was formed by the conrotatory ring-opening of N-lithio-2,3diphenylaziridine, the E,Z ('sickle') form of the anion could be captured in a cycloaddition, resulting in a trans-2,5-disubstituted pyrrolidine. 25 We have not observed such products in our tin-derived anions.…”

“…We then sought to reinvestigate Kauffmann's use of Nlithioaziridines as precursors of 2-azaallyllithiums, again with the hope of extending this chemistry to nonstabilized anions. Kauffmann had found that 2,3-diphenylaziridine, upon deprotonation, afforded 1,3-diphenyl-2-azallyllithium via a conrotatory electrocyclic ring opening, 25 but that the ring-opening of N-lithio-2-phenylaziridine 'proceeds slowly even at 65°C'; 4 no cycloadditions of the resultant anion were reported. Hoping to use intramolecular cycloadditions to extend this method to less-stabilized anions, we prepared the aziridine 40 and deprotonated it with n-BuLi (Scheme 10).…”

Stereoselective Conjugate Addition of the Lithium Anion of N-Allyl Imine to Unsaturated Esters: Application to the Enantiospecific Total Synthesis of (−)-Epibatidine

“…Aza-allyl anions, which are broadly classified into nonstabilized, semistabilized, and stabilized anions depending on the substitutions on the imine, are important intermediates in organic synthesis . The seminal observation by Kauffmann that deprotonation of benzylidineimine (obtained from benzaldehyde and benzylamine) forms the semistabilized aza-allyl anion, which reacts with stilbene to form the (3+2) cycloaddition product invigorated considerable interest to explore the reactivity of 2-aza-allyl anions (Figure ). Pearson’s group contributed extensively to the synthesis and reactivity of nonstabilized and semistabilized aza-allyl lithium anions by the tin–lithium exchange reaction of stannyl-substituted imines and widely utilized these intermediates in the total synthesis of alkaloids .…”