Substituent effects in tandem ring-closing metathesis reactions of dienynes

Several dienynes bearing different substituents have been synthesized and subjected to a ring-closing metathesis (RCM) reaction using ruthenium carbene complexes. Dienynes containing a pre-existing ring and a quaternary center attached to the alkyne give the expected tandem metathesis products in high yields. For these substrates, a high selectivity for different ring sizes was achieved by modifying the reactivity of one alkene. In one case, an unusual non-metathetic activity of the second-generation Grubbs catalyst was observed and the cycloisomerization product was obtained as the major product. In the absence of favorable factors, especially when the starting substrates contain hindered alkenes, a long tether chain and/or an ester group on the alkyne part, the tandem process is slowed down or completely impeded. In these cases, dienynes behave like simple enynes and afford initial metathesis products whose further reactions give dimeric compounds. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).