Cascade reactions are highly effective, atom- and step-economical processes in organic
synthesis, which are widely used for the streamlined construction of various molecular
architectures. Although there are several types of cascade reactions reported in the
literature, one involving annulation as the key step is more captivating as it gives
quick access to various heterocyclic compounds in a short and effective manner. In
this account, our groups efforts towards cascade addition/functionalization followed
by annulation of simple substrates for accessing various bio-relevant heterocycles
and drug derivatives will be presented. We have used photocatalysis for cascade addition-annulation
type reactions, transition metal catalysis for cascade C-H acyl methylation-annulation
and finally dual transition metal- photoredox catalysis for cascade C-H olefination-annulation
of carboxylic acids. Using these strategies, a broad range of structurally complex
heterocyclic scaffolds, including dihydroquinazolinones, chromanes, benzoxazines,
julolidine, etc. were synthesized.
