The title concept involves the use of structurally modified RCM substrates that contain extender arms, terminating in a remote reactive alkene. Initiation of an RCM sequence at that reactive alkene is followed by rapid intramolecular relay of the metal center to an initially less reactive alkene in the parent substrate. This permits one to control the relative timing (or direction) of a metathesis sequence. For example, one can reverse the inherent tendency of an unsymmetrical alpha,omega-diene substrate to close, say, left-to-right, to that of right-to-left. Four distinct types of application of the RRCM concept are demonstrated. Among other things, they show the preparation of tetrasubstituted electron-deficient alkenes using G1 [(Cy3P)2(Cl2)Ru=CHPh], complementary control of directionality (endedness), auxiliary benefits (enzyme specificity) from the incorporation of additional steric bulk, the activation of otherwise ineffective substrates for RCM closure, the use of unorthodox alkenes as initiation sites for ring closure, and control of product olefin geometry.

[reaction--see text] Each of four diastereomers of structure 2, corresponding to the lipophilic side chain of scyphostatin (1), were prepared. Careful analysis of their NMR spectral data and comparison with those of the natural product corroborates the recently reported (Org. Lett. 2000, 2, 505) stereochemical assignment. A strategy for the stereoselective synthesis of 2 has been achieved.

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