Natural product synthesis ring structures

Chelidonine, a representative benzo[c]phenanthridine alkaloid (1,2), was synthesized as the first application of this reaction in natural product synthesis (145). Initial thermal opening of the four-membered ring in 299 led to the formation of a transient -o-quinodimethane (301), which has a dienamide structure in the diene part and is then trapped by the suitably positioned multiple bond in the same molecule. They applied this intramolecular reaction to the acetylenic cyclobutene 300 for the synthesis of ( )-chelidonine (Scheme 110). 

A.B. Smith and co-workers have devised an efficient strategy for the synthesis of the cylindrocyclophane family of natural products. Olefin ring-closing metathesis was used for the assembly of the [7,7]-paracyclophane skeleton. During their investigations they discovered a remarkably efficient CM dimerization process, that culminated in the total synthesis of both (-)-cylindrocyclophane A and (-)-cylindrocyclophane F. They established that the cross metathesis dimerization process selectively led to the thermodynamically most stable member of a set of structurally related isomers. Out of three commonly used ROM catalysts, Schrock s catalyst proved to be the most efficient for this transformation. 

In this beautiful synthesis of periplanone B, Still demonstrated a classical aspect and use of total synthesis - the unambiguous establishment of the structure of a natural product. More impressively, he demonstrated the usefulness of the anionic oxy-Cope rearrangement in the construction of ten-membered rings and the feasibility of exploiting conformational preferences of these medium-sized rings to direct the stereochemical course of chemical reactions on such templates. 

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