Substitution nitrogen heterocydes

Regioselective Substitution in Aromatic Six-Membered Nitrogen Heterocydes, 44, 199... 

A number of Pd-catalyzed carbonylative processes have employed allenes as substrates for the synthesis of nitrogen heterocydes [108], For example, subjection of substituted allene 170 to reaction conditions similar to those employed in related reactions of alkenes led to the formation of pyrrolidine 171 in 68% yield with 2 1 dr (Eq. (1.67)) [108d], Modest asymmetric induction has been achieved in these transformations using simple chiral auxiliaries [108b,d]. This strategy was employed in an asymmetric synthesis of pumiliotoxin 251 D, which involved the aminocarbo-nylation of allene 172 to pyrrolidine 173 as a key step (Eq. (1.68)) [108b]. 

The cross-coupling reactions of hydrazines with aryl halides to give substituted hydrazines is very important from the point of view of nitrogen heterocyde synthesis (for example, indoles, indazoles, pyrazoles, and aryltriazoles, and before this report by Straddiotto s group [28a], there was no literature method for the Pd-catalyzed cross-coupling of hydrazine to aryl electrophiles. Hydrazines are difficult substrates because of their reducing power and the reactivity of the mono-arylated hydrazine product to further arylation. 

The oxygen- or nitrogen-substituted allene adducts as described in the previous section are of major interest because of the possible subsequent cyclization leading to very valuable oxygen- or nitrogen-containing heterocydes (Scheme 8.25). The syn-... 

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