Tertiary‐Amine Controlled (3+3) or (4+2) Annulations of β′‐Acetoxy Allenoates with N‐Sulfonyl Ketimines: An Entry to m‐Teraryl and Fused Dihydropyridines

Abstract

Lewis base dependent (3 + 3) and (4 + 2) annulations of β′‐acetoxy allenoates with N‐sulfonyl ketimines offer m‐teraryl and fused dihydropyridines with varying substituents depending on the tertiary amine as well as subtle changes in the reaction conditions. The triazabicyclodecene (TBD)‐catalyzed (3 + 3) annulation involves 1,2‐elimination followed by 6‐endo‐dig cyclization as key steps in delivering fused 1,4‐hydropyridines. The same reactants under DMAP catalysis offer m‐teraryl via Mannich coupling, rather than 1,2‐elimination followed by C‐N bond cleavage/ aromatization. Key intermediates have been identified by HRMS studies.

Phosphine vs DBU-Catalyzed Annulation Reactions of β′-Acetoxy Allenoates with Acyl-Tethered Benzothiazole Bisnucleophiles: (4 + 3) or (4 + 1) vs (3 + 3) Annulation

“…Subsequent to the seminal discovery of Lu’s PPh 3 -catalyzed (3 + 2) cycloaddition involving allenoates, this class of allenes has been explored as versatile synthons under Lewis base catalysis via nucleophilic zwitterionic intermediates. , By contrast, the δ- or β′-acetoxy allenoates, by virtue of the readily detachable –OAc group, under phosphine/amine catalysis generate electrophilic diene-ammonium/phosphonium intermediates that are amenable to numerous annulation reactions. , Overall, these precursors lead to cyclization processes that may be quite different from the normal substituted allenes or acetoxy allenes (allenyl acetates) . Thus, the Lewis base-catalyzed annulation of β′-acetoxy allenoates has been investigated by several research groups, including ours (Scheme a–d). ,,,, As can be seen readily, the second substrate in all of these cases acts as a bisnucleophile, and the annulation products possess newly generated 5/6-membered rings.…”