How does the synthesis of [18]annulene proceed stepwise?

Finding good literature on the synthesis of [18]annulene is challenging, as most sources are well old and quite archaic, so I was hoping some explanation in regard to the mechanism of synthesis.


My biggest question is in regard to the formation trimeric intermediate formed by Glaser coupling of 1,5-hexadiyne. It is not clear to me why this reaction seems to be "templated" towards forming a ring, instead of forming a linear structure formed by the terminal carbon's. In any case, I could not find any reason why this reaction could not be used for forming larger or smaller rings than being 18 carbons long, but I must be missing something.


Here is a good basic synthesis template: [OrganoSynth](https://www.orgsyn.org/demo.aspx?prep=cv6p0068)


