Final answer:
The compound blocking access to W15 is an imidazole ring commonly found in the amino acid histidine.
Step-by-step explanation:
The type of heterocycle found on two amino acid residues blocking access to W15 is an imidazole ring. This can be deduced from the synthesis process described where the formation of the imidazole ring is specifically mentioned as a step in the creation of a compound related to isoadenosine.
Within the context of amino acids, histidine is known to possess an imidazole ring which could be the residue implicated in the blockage. In contrast tryptophan contains an indole group, which is not synonymous with imidazole.