Pyridine annelation

Whereas in all previously mentioned inverse cycloaddition reactions [h]-fused pyrido annelated systems are formed, some reactions are described which lead to [c]-pyridine annelated bicyclic systems. 5-(Butynylthio)pyrimidines (R = Ph, NHCOCH3) give on heating at 180°C in nitrobenzene 5-R-2,3-dihydrothieno[2,3-c]pyridines (89T803). 5-Propynyloxymethylpyrimidines also readily undergo cycloaddition into l,3-dihydrofuro[3,4-c]pyridines (89T5151) (Scheme 39). Considerable rate enhancements were observed with quaternized pyrimidinium salts. Whereas... 

When the reaction of 20 with the isocyanate is conducted in the presence of pyridine, annelation to give 22 occurs. ... 

Pyridine annelation.1 1,2,4-Triazine can function as a reactive azadiene in Diels-Alder reactions with enamines, particularly pyrrolidine enamines, to form substituted pyridines. 

Pyridines annelated with carbocycles 84HC(14,5)253. Tetrahydroisoquinolines, biochemistry of 83MI10. 

These products can be used as synthetic equivalents of a,/3-unsaturated aldehydes in Michael reactions. Thus C6HsSCH=CHCH2Cl can be used as the equivalent of acrolein, CH2 =CHCHO, which normally undergoes 1,2- rather than 1,4-addition with organometallic reagents. An interesting example is the preparation of an intermediate (1) to the pyridine annelation reagent of Danishefsky and Cain. ... 

Scheme 55 Synthesis of benzo- and pyridine-annelated 3-azabicyclo[3.3.1]nonanes...

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