Question

Instead of adding to the 4a position and protonating N-5, the thiolate ion could have added to the 10a position and protonated N-1. (The numbering system is on page 1071.) Why is addition to the 4a position favored? (Hint:Which nitrogen is a stronger base?)

Short answer

N-5 is stronger base than N-1.

Step-by-step solution

Strong base

Strong base have the capability to deprotonate a weak acid. So strong bases are protonated easily.

Reason for no thiolate ion adding to the 10a position and protonating N-1 

From the below given structure it can be seen that N-1 is adjacent to oxygen so here the electron delocatization is not limited to ring instead the delocalized electron can be on oxygen so the protonation of nitrogen will not occur when an nucleophile add to C-10a.

Reason for thiolate ion adding to the 4a position and protonating N-5

As it can be seen here no oxygen is present adjacent to nitrogen so the electron delocalize only to ring so the protonation on nitrogen can occur when nucleophile add to C-4a. So here the stronger base N-5 is protonated.