Asymmetric Olefin Metathesis

Asymmetric Olefin Metathesis-26
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).

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Considering the paucity of effective methods for the preparation of nonracemic tertiary alcohols and ethers (22), the present protocol offers a convenient and enantioselective approach to a difficult problem in organic synthesis.

As also depicted in Scheme 3 (12 → 13), meso trienes can be converted to structurally complex polycycles in 80% yield and excellent levels of enantioselectivity (96% to 98% ee) (23).

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).

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).As part of this initiative, we have synthesized, characterized, and examined the catalytic activity of high-oxidation-state Mo-based chiral complexes such as 1–4 (Scheme 1) (1).These investigations have led to the development of a range of methods for catalytic asymmetric ring-closing metathesis (2–6) and asymmetric ring-opening metathesis (AROM) (7–10).Through application of catalytic asymmetric metathesis to multistep synthesis, we aim to demonstrate the unique features of asymmetric olefin metathesis, as well as identify and address any related shortcomings.We report herein key findings in connection with Mo-catalyzed AROM/RCM (11).With an effective catalytic AROM/RCM available, we began to explore the utility of this method through its use in an enantioselective synthesis of africanol.The retrosynthesis, illustrated in Scheme 5, involves access to a through structural modification of b, which in turn may be synthesized enantioselectively through Mo-catalyzed AROM/RCM of diene c.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.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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