Asymmetric Olefin Metathesis

Asymmetric Olefin Metathesis-83
We devised two separate routes for the conversion of b to the target molecule.

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The different levels of asymmetric induction observed in the formation of 6 versus 8 (69% and 92% ee) may be partly the result of such mechanistic intricacies.

The terminal olefin in 5 may be converted to ii ( generation of Mo-methylidene i, which is expected to be a more effective initiator (less sterically hindered) compared with the neophylidene precatalyst (see Scheme 1).

As shown in Scheme 4, reaction of diene 5, bearing a terminal olefin, proceeds to afford 6 in nearly the same yield and enantioselectivity as observed in the absence of the olefin additive (see Scheme 2 for comparison); none of the corresponding AROM/CM product (e.g., 14) is observed.

In contrast, diene 7 affords only triene 14, the product of a catalytic AROM/CM, in 98% ee and 85% isolated yield (see Scheme 2 for reaction outcome in the absence of styrene).

It merits mention that in this class of transformations reaction efficiency can be significantly affected depending on the stereochemical identity of the substrate, as exemplified by rapid polymerization of 9 (Scheme 3) (7).

As the data in Scheme 3 illustrate (10 → 11), Mo-catalyzed tandem ROM/RCM can be effected to afford heterocyclic structures that bear a tertiary ether site (21).It is plausible that in the case of 5, the rapidly formed (7) benzylidene iv (Scheme 4) reacts with the substrate to regenerate styrene and affords terminal alkylidene v, which undergoes facile RCM to deliver 6.With substrates 7 and 15, which bear the more substituted and less reactive olefinic side chains, the catalytic cycle might commence with an AROM through reaction of benzylidene iv with the substrate to afford vi.Accordingly, use of catalytic metathesis as a stereochemistry-determining step in a total synthesis must be accompanied with effective strategies that allow for differentiation of the resulting olefinic sites.In the case of intermediate b (Scheme 5), differentiation of the terminal alkene from the cyclic disubstituted olefin was not expected to be particularly problematic, but in certain cases (e.g., 14 or 16 in Scheme 4), differential functionalization of various olefins may require highly regio- and site-selective methods.Through such protocols, optically enriched organic molecules can be prepared that cannot be easily accessed by alternative approaches.With the availability of various chiral catalysts and the enantioselective transformations that can be promoted by such complexes, we have recently begun to explore the utility of catalytic asymmetric metathesis in target-oriented synthesis.The resulting Mo-alkylidene may then react more readily with another molecule of styrene to afford 14 or 16 rather than participate in an RCM with the neighboring di- or trisubstituted alkene.Whether the five-membered chelate structure vi shown in Scheme 4, which is expected to cause lowering of reactivity of the Mo-alkylidene, plays a role in reducing the facility of ring closure in reactions involving the more substituted olefinic chains (i.e., 7 and 15) is unclear at the present time (internal chelation in v involves a less favorable four-membered structure).Similarly, products corresponding to CM of 6 with styrene are not observed.The difference in the outcome of reactions depicted in Scheme 4 may be attributed to a number of mechanistic factors.


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