Regio- and Diastereoselective Synthesis of Dihydropyridopyrimidines via Cascade Reactions of 2‑Aminopyridines with Morita–Baylis–Hillman Bromides of Nitroalkenes

The Morita–Baylis–Hillman
(MBH) bromides of nitroalkenes
have been employed as bielectrophiles for the first time. The 1,3-bielectrophilic
reactivity of the MBH bromides has been demonstrated in the synthesis
of 3,4-dihydro-2H-pyrido­[1,2-a]­pyrimidines.
The reaction of MBH bromides with 2-aminopyridines takes place in
the absence of any reagent in a cascade SN2′-6-endo-trig
fashion and is completely regioselective and highly stereoselective.
The products, in their hydrobromide salt form, could be conveniently
isolated and purified by crystallization. The high stereoselectivity
has been rationalized in terms of the greater stability of the transition
state in which the Ar and NO2 groups are anti to each other.