Nucleophilic Alkylation of Iminium Ions and other Electrophiles

2 Nucleophilic Alkylation of Iminium Ions and other Electrophiles 9.05.3.2.1 Boron-Mannich reactions 

Multi-component procedures based upon the Mannich reaction and boron reagents were reported in 1993 as a reaction between vinylboronic acids and the adducts of secondary amines and paraformaldehyde (Table 5, entries 1 and 2).558 

The reaction has been extensively used for the synthesis of a-amino acids (entries 3-5),559 //-/ -amino alcohols (entry 6),560 aminophenol (entry 7),561 optically active A-substituted glycines (entries 7 and 8),562 a-hydrazinocarboxylic acids (entry 9),563 A-sulfinyl and A-alkoxy-a-amino acids (entries 10 and ll),564 and propargyl amines (entry 12).565 

The boron-Mannich condensation was assisted by a hydroxy group adjacent to the aldehyde moiety. The products derived from salicylaldehydes and vinylboronic acids underwent cyclization to 2H-chromene derivatives with elimination of the amine upon heating (Equation (119)).566 

Alkenyl, aryl, and heteroaryl boronic acids reacted with 1,2-diamines and glyoxylic acid giving directly piperazi-nones (Equation (123)).571 2-Hydroxymorpholines were synthesized from glyoxal, 2-aminoethanols, and aryl- or 1-alkenylboronic acids (Equation (124)).572 

---