Ring-rearrangement metathesis

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

Another form of olefin metathesis widely used for piperidine formation is ring-rearrangement metathesis (RRM), as shown below. The versatility of this reaction can be seen in its ability... 

RING REARRANGEMENT METATHESIS (RRM) - A NEW CONCEPT IN PIPERIDINE AND PYRROLIDINE SYNTHESIS... 

A new concept of diastereoselective ring-rearrangement metathesis has been developed for the synthesis of various carbo- and hetero-cycles (134) and (135) (Scheme 35).107 A strong influence of the substitution pattern of the directing stereocentre on ... 

Cross metathesis (CM) reactions can also be used as the key step in a piperidine synthesis (Scheme 40) <2004TL1167> or in sequence with ring-rearrangement metathesis, for example, in the synthesis of (—)-lasubine (Scheme 41) <2004T9629>. 

ROM-CM Ring-opening cross-metathesis RRM Ring-rearrangement metathesis... 

Multiple cyclo olefinic relays are also suitable substrates to promote cascades of RCM-ROM-RCM reactions [9] (Scheme 16). Thus, W-protected polycyclic amines have been synthesized in moderate to good yields, but an increasing number of cyclopentene relays gave rise to less efficient cascade transformations. This ring rearrangement metathesis (RCM-ROM-RCM) has been extensively reported by Blechert and co-workers for its application in the synthesis of natural products [30-34]. 

Enantioenriched 196 was prepared by ring rearrangement metathesis (RRM) of 197. Through the agency of regio- and stereo-controlled hydroxylation reactions, (+)-castanospermine 198 and both isomers 199 of the pentahydroxylated indolizidine uniflorine-A of putative structure 200 were prepared as shown in Scheme 57 <05T8888>. 

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