Fully Intramolecular Cyclotrimerization

Fully intramolecular [2 + 2 + 2] cyclization of triynes affords tricyclic products with complete chemo- and regioselectivity. Despite this advantage, this method is underdeveloped, which is probably due to the diminished accessibility of the triyne substrates. Peters and Blechert reported the first ruthenium-catalyzed fully intramolecular cyclotrimerization using the metathesis cascade strategy with Grubbs catalyst [23]. Later, Yamamoto et al. demonstrated that complex 6 efficiently catalyzes the cyclization of various triynes [14]. Examples of 1,6,11-triyne cyclization are summarized in Table 3.11. As an example, diether 58 (X = = O, R = H) underwent [2 + 2 + 2] 

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The reactions that yield benzene rings can be categorized further into the following types according to the substrates involved (1) intermolecular cycloaddition of three alkynes (cyclotrimerization), (2) partially intramolecular cycloaddition ofdiynes with alkynes, and (3) fully intramolecular cyclotrimerization of triynes. In the next section, the synthetic routes to benzene derivatives using ruthenium-catalyzed cycloaddition are surveyed according to these classifications. Classic examples of [2 + 2 + 2] alkyne cycloadditions using stoichiometric ruthenium mediators are included since they provide useful information on the further development of ruthenium catalysis. 

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