Synthesis of Natural Products Containing Macrocycles by Alkene Ring‐Closing Metathesis

Abstract

IntroductionThe impact of metathesis in modern synthetic chemistry is evidenced by the number of publications and reviews that have appeared in the last decade, some of them covering the specific field of macrocyclization in natural product synthesis [1]. It is therefore difficult not to repeat the reports done by other authors or duplicate their critical views on the chemistry. This chapter is exclusively devoted to the total syntheses of natural bioactive macrocycles involving diene and the diene-ene metathesis, and focuses on the reaction conditions used in the macrocyclization key step and on the stereochemical outcome of the reaction. As we shall see, the choice of the catalyst, solvent, temperature, concentration, and reaction time is crucial in these transformations. In addition, we show how subtle variations on the structure of the substrate lead to modifications in the conformational organization of the molecule, which are critical for the success of ring-closing metathesis (RCM). In particular, the substitution pattern, the steric hindrance of the substituents, the ring size to be formed, and the presence of coordinating heteroatoms are particularly important as they have an important influence on the outcome of the reaction. This chapter has been organized around the various families of natural products in order to compare the reaction conditions for compounds bearing structural similarities. We have deliberately decided to reveal only the information regarding the metathesis step and thus not disclose other details of the syntheses.Synthetic efforts toward macrocycles are often laborious and unpractical. The classical strategies involved are macrolactonizations, macrolactamizations, and macroaldolizations. The macrocyclization approach encounters several problems including the competition between the desired intramolecular reaction and intermolecular processes, which result in the formation of polymers. RCM has the advantage of being compatible with a wide variety of functional groups and forming double bonds that can eventually be converted into other functionalities. The main disadvantages are the control of the stereochemistry of the newly formed double bond and the optimization of the yield. On the other hand, the concomitant loss of ethylene makes this reaction highly atom economical and entropically Metathesis in Natural Product Synthesis: Strategies, Substrates and Catalysts. Edited by

At Long Last: Olefin Metathesis Macrocyclization at High Concentration

“…Our goal was to utilize the inherent reversibility of olefin metathesis to produce macrocylic products relevant to the flavor and fragrance (F+F) industry at concentrations much higher than normally used for RCM macrocyclizations (5 mM). We anticipated that under carefully selected conditions ADMET oligomers can be effectively backbited to yield the expected RCM macrocyclic product.…”

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“…Another major consequence of the availability of MAP complexes is the feasibility of carrying out Z -selective macrocyclic RCM. 93 The importance of this set of transformations is reflected in the fact that, despite the absence of catalysts that can ensure high stereoselectivity, it has nonetheless been a commonly used transformation for preparation of large rings. On countless occasions, macrocyclic RCM is relied upon to effect a late-stage transformation in multistep sequence of reactions (cf.…”

Ring-Closing Metathesis in Pharmaceutical Development: Fundamentals, Applications, and Future Directions

“…Cyclizations often proceed without control of the alkene stereochemistry because of the dependence on the substrate thermodynamic preference. Compared with ( Z )-alkenes, ( E )-alkenes sometimes can be obtained by RCM as exclusive or predominant products because of their generally lower energy than the Z isomers . For those that contain ( Z )-alkenes in the macrocycle, the RCM approach often leads to a nearly equal ratio of ( Z )- and ( E )-olefin isomers, except for a few limited examples in which ( Z )-olefins can be obtained selectively .…”

“…123 A large number of chiral Mo-and Ru-based catalysts were evaluated for the challenging ERCM reaction of triene 91 (entry 12) that contains enantiotopic vinyl groups adjacent to a quaternary carbon stereogenic Compared with (Z)-alkenes, (E)-alkenes sometimes can be obtained by RCM as exclusive or predominant products because of their generally lower energy than the Z isomers. 134 For those that contain (Z)-alkenes in the macrocycle, the RCM approach often leads to a nearly equal ratio of (Z)-and (E)-olefin isomers, except for a few limited examples in which (Z)-olefins can be obtained selectively. 135 In the context of macrocyclization, even if the higher-energy product can be kinetically accessible in RCM, it can undergo a ring-opening/ ring-closing process to isomerize to the thermodynamically preferred isomer.…”