In general, intramolecular reactions that form rings are often disfavored entropically because it makes a flexible starting material more rigid. In the case of ring-closing metathesis, however, the entropy penalty for the closure is not especially large for most systems, such as the one below used as part of a synthesis of an anti-tumor compound known as epothilone A. Why might that be?

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TBSO THESE OTBS Grubbs II (30 mol %) CH,CI,, 45 °C, 48h 89% 1:1 E:Z isomers TBSO w OTBS N