Relay approach

However, if those natural products have been previously synthesised one may start from them, provided that it simplifies the synthesis, since repetitions and waste of time should be avoided. Moreover, once a key intermediate has been synthesised, but may also be obtained easily by degradation from some other product, including the same natural product that is being synthesised, the chemist should resort to such a source and use it to continue the synthesis. This approach to total synthesis is referred to as the relay approach. 

Davies, Renaud, and Sibi independently reported the chiral relay approach to control the enhanced steric extension inside a substrate to achieve increased asymmetric induction. However, as our study proves, the asymmetric activation of a tropos catalyst clearly differs from the chiral relay approach, in which substrate conformational control is utilized, since asymmetric activation controls the chiral environment of a tropos catalyst by the addition of a chiral external source (a chiral activator). 

Percy and co-workers applied a similar strategy in the synthesis of a trisubstituted cyclooctene derivative in an effort to determine the limits of relay ring-closing metathesis.Reaction of diene 315 (R = H) in the presence of 4 (three additions 10 mol %, then 5 mol % then 5 mol % over a 12-day period) and Ti(Oi-Pr)4 in refluxing dichloromethane furnished the corresponding octocycle 316 in only 24% yield. However, when a relay approach was employed, diene 317 (R = (CH2)3CHCH3) gave the... 

Y. Yamamoto, H. Hayashi, T. Saigoku, H. Nishiyama, J. Am. Chem. Soc. 2005, 127, 10804-10805. Domino coupling relay approach to polycyclic pyrrole-2-carboxylates. 

Note The above exercise is merely an approach to the selection of the most appropriate relay and its protective settings for a particular machine. The exact selection of the relay and its setting will depend upon the type of relay, its sensitivity and protective features supplied. 

Y. Cheng-Ching. Autotuning of PID Controllers Relay Feedback Approach (Advances in Industrial Control). Springer-Verlag, London, 1999. 

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