Efficient Approaches for the Synthesis of Substituted Thiazolo[3,2‐a]‐benzimidazole‐Phosphonates and ‐Phosphinic Diamide Derivatives

Phosphrylation of E‐3‐(dimethylamino)‐1‐(3‐methylbenzo[d]‐thiazolo[3,2‐a]imidazol‐2‐yl)‐prop‐2‐en‐1‐one (1) and 1‐(3‐methylbenzo[d]thiazolo[3,2‐a]imidazol‐2‐yl)ethanone (12) was studied in detail. The enaminone (1) reacted with trialkyl phosphites, dialkylphosphonates, and tris(dialkylamino)‐phosphines to afford the corresponding (E/Z) thiazolobenzimidazol‐3‐oxoprop‐1‐enyl‐phosphonate and phosphonic diamides. The reactions proceeded via phospha‐Michael (P‐Michael) addition reaction, followed by extrusion of HNMe2 moiety and intramolecular migration of the phosphono‐alkyl group. Only the major E‐isomer of the respective oxopropenyl phosphonates was isolated by fractional crystallization (∼ 65% yield) in a pure form whereas the two isomers were chromatography separated and identified in the second case of the respective phosphonic diamide mixtures. The trans‐configuration is also confirmed by nuclear Overhauser effect (NOE) experiments. On the other hand, substituted thiazolobenzimidazole‐α‐hydroxyphosphonate‐ and α‐hydroxyphosphinic diamide derivatives were produced from the reaction of the 2‐acetyl thiazolo[3,2‐a]benzimidazole (12) with the same P(III) reagents via Abramov 1, 2‐addition reaction. Facile and efficient routes for prepareing a variety of new γ‐keto‐ and α‐hydroxy phosphonates as well as phosphonic diamide derivatives.