Cyclization agents/cyclizations mixed anhydrides

Extension of the traditional Yamaguchi/Corey-Nicolaou/Keck macrolactonization protocols (see Scheme 2.1) [5] to catalytic variants can address some of the drawbacks of classical methods. Mixed anhydrides can undergo cyclization with Lewis adds such as Sc,1i, Zr, and Hf [125,126].Trostetol. reported the use of ethoxyacetylene as a stoichiometric activating agent, installed via Ru catalysis [127], later applied to the synthesis of apicularen A (Scheme 2.67) [128]. 

Several successful cyclizations of quite complex structures were achieved using polyphosphoric acid trimethylsilyl ester, a viscous material that contains reactive anhydrides of phosphoric acid.58 Presumably the reactive acylating agent is a mixed phosphoric anhydride of the carboxylic acid. 

Triethyl orthoformate has proved to be a useful cyclizing agent, especially when mixed with acetic anhydride which produces diethoxymethyl acetate (56JA1928,56JOC599, B-78MI40903, p. 995). 

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