Ring-closing metathesis cleavage reaction

As stated above, olefin metathesis is in principle reversible, because all steps of the catalytic cycle are reversible. In preparatively useful transformations, the equilibrium is shifted to one side. This is most commonly achieved by removal of a volatile alkene, mostly ethene, from the reaction mixture. An obvious and well-established way to classify olefin metathesis reactions is depicted in Scheme 2. Depending on the structure of the olefin, metathesis may occur either inter- or intramolecularly. Intermolecular metathesis of two alkenes is called cross metathesis (CM) (if the two alkenes are identical, as in the case of the Phillips triolefin process, the term self metathesis is sometimes used). The intermolecular metathesis of an a,co-diene leads to polymeric structures and ethene this mode of metathesis is called acyclic diene metathesis (ADMET). Intramolecular metathesis of these substrates gives cycloalkenes and ethene (ring-closing metathesis, RCM) the reverse reaction is the cleavage of a cyclo-... 

Until now, the most efficient approach to synthesize Freidinger lactams 147 started from a resin-bound cinnamylamine 144. A Fukuyama-Mitsunobu reaction to 145 followed by sulfonamide cleavage and a consecutive appropriate acylation built up the diene 146, which underwent ring-closing metathesis involving Grubb s catalyst 123 to generate the desired lactams 147 (Scheme 27, Table 5) [35d]. 

For the cleavage of alkenes from a support by metathesis, several strategies can be envisaged. In most of the examples reported to date, ring-closing metathesis of resin-bound dienes has been used to release either a cycloalkene or an acyclic alkene into solution (Figure 3.38, Table 3.44). Further metathesis of the products in solution occurs only to a small extent when the initially released products are internal alkenes, because these normally react more slowly with the catalytically active carbene complex than terminal alkenes. If, however, terminal alkenes are to be prepared, selfmetathesis of the product (to yield ethene and a symmetrically disubstituted ethene) is likely to become a serious side reaction. This side reaction can be suppressed by conducting the metathesis reaction in the presence of ethene [782,783]. 

SCHEME 1. Types of alkene and alkyne metathesis reactions. DBC, double bond cleavage TBC, triple bond cleavage ADMET, acyclic diene metathesis RCM ring-closing metathesis ROMP ring-opening metathesis polymerization... 

Tarchonanthuslactone (27) has been isolated from a compositae, tarchonanthus trilobus (57). All of the known syntheses of optically active 27 reported thus far involve substrate-control and >14 steps (58-61). Our 7-step reagent-controlled asymmetric synthesis begins with the allylboration of acetaldehyde, followed by osmylation, periodate cleavage, a second allylboration, esterification, and ring-closing metathesis reaction sequence (Scheme 9). 

Scheme 6.11. Cyclizative cleavage of the desired product via solid-supported ring-closing metathesis. The undesired byproduct 46 is formed only when the reaction is conducted in solution.

Traditionally, synthetic approaches toward this framework were charactoized by a lack of generality, involving poorly available starting material that require multistep synthetic transformations [22-25]. More recent approaches include tandem cleavage of hydrogenated / -and a-carbolines [26], ring closing metathesis [27], and metal-catalyzed Friedel-Crafts-type reactions of indole derivatives with several electrophiles (such as alkynes, alkenes or epoxides) (see Refs. [118] and [133] in Chap. 1 [28-32]). 

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