Asymmetric Olefin Metathesis

Similar to diene 7, substrate 15, bearing a trisubstituted olefin, undergoes AROM/CM exclusively to afford optically pure 16 in 77% isolated yield.In reactions of 7 and 15, compounds arising from AROM/RCM (compare 6) or meso products from an additional CM of 14 or 16 with styrene are not detected (net AROM/CM/CM).

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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.

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).

Catalytic asymmetric ring-opening metathesis (AROM) provides an efficient method for the synthesis of a variety of optically enriched small organic molecules that cannot be easily prepared by alternative methods.

The development of Mo-catalyzed AROM transformations that occur in tandem with ring-closing metathesis are described.Efficient AROM/cross metathesis (AROM/CM) of norbornyl substrates with terminal olefins can be promoted by Mo-based chiral catalysts (7).Within this context, we have examined various AROM/RCM reactions carried out in the presence of styrene (two equivalents) to establish whether, under such conditions, AROM/CM effectively competes with AROM/RCM pathways.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).We devised two separate routes for the conversion of b to the target molecule.One possibility (Option 1 in Scheme 5), would involve rearrangement of d to e through catalytic ROM/RCM.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.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).


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