Metathesis catalytic asymmetric

The significant potential of the ruthenium complex 65 was further underlined in the catalytic asymmetric ring-opening/cross metathesis of the cyclic alkene 70 (Scheme 44). This transformation is catalyzed by 5% mol of 65 at room temperature, in air, and with undistilled and nondegassed THF to deliver the corresponding diene 71 in 96% ee and 66% isolated yield. In standard conditions (distilled and degassed THF), the alkene 70 reacts in 75 min to give the diene in 95% ee and 76% yield, with only 0.5 mol % of catalyst. 

Tab. 11.13 Catalytic asymmetric olefin metathesis reactions promoted by supported chiral catalyst (81). >...

For a review of asymmetric Mo-catalyzed metathesis, see Catalytic Asymmetric Olefin Metathesis, A. H. Hoveyda, R. R. ScHROCK, Chem. Eur. J. 2001, 7, 945-950 for reports on chiral Ru-based complexes, see (b) Enantioselective Ruthenium-Catalyzed Ring-Qosing Metathesis, T.J. Sei-DERS, D.W. Ward, R.H. Grubbs, Org. Lett. 2001, 3, 3225-3228 (c) A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening/Cross Metathesis In Air, J. J. Van Veldhuizen, S. B. 

The arena in which catalytic asymmetric olefin metathesis can have the largest impact on organic synthesis is the desymmetrization of readily accessible achiral molecules. Two examples are illustrated in Scheme 4. Treatment of achiral triene 18 with 2 mol % 4a leads to the formation of (R)-19 in 99% ee and 93% yield [ 12]. The reaction is complete within 5 min at 22 °C and, importantly, does not require a solvent. Another example is illustrated in Scheme 4 as well here, BINOL complex 11a is used to promote the formation of optically pure (R)-21 from siloxy triene 20 in nearly quantitative yield. Once again, solvent is not needed [15]. Readily accessible substrates are rapidly transformed to non-ra-cemic optically enriched molecules that are otherwise significantly more difficult to access without generating solvent waste. 

Catalytic Asymmetric Olefin Metathesis Chiral Biphen-Mo Catalysts... 

The arena in which catalytic asymmetric olefin metathesis can have the largest impact on organic synthesis is the desymmetrization of readily accessible achiral molecules. Two examples are illustrated in Scheme 4. Treatment of achiral triene 18 with 5 mol% 4a leads to... 

The molybdenum complex Mo(NAr)(CHCMe2Ph)[(3)-Me2SiBiphen] 43 was used for catalytic asymmetric olefin metathesis reactions such as desymmetrization of trienes, kinetic resolution of allylic ethers, tandem catalytic asymmetric ring-opening metathesis/cross-metathesis. Interestingly, tandem catalytic asymmetric ring-opening... 

Hoveyda, A. H., Schrock, R. R. Catalytic asymmetric olefin metathesis, in Organic Synthesis Highlights V 210-229 (VCH, Weinheim, New York, 2003). 

As well as ring-closing metathesis reactions, the catalytic asymmetric Homer-Wadsworth-Emmons reaction has been achieved using phase-transfer catalysts including ammonium salt (12.77), with rubidium hydroxide as base. The achiral ketone (12.78) is converted into the alkene (12.79) with reasonable enantioselec-tivity. Currently, the long reaction time is a drawback. ... 

So far, enantioselective olefin metathesis has the largest impact on organic synthesis in the desymmetrization of achiral polyenes [81] an illustrative example refers to the total synthesis of endo-brevicomin [82]. The catalytic asymmetric cycli-zation of achiral trienes and meso-tetraenes via ARCM proceeds in excellent enan-tioselection (e.e. > 99 %) as demonstrated for dihydrofuran 163 (Scheme 11.40). 

An area in which catalytic olefin metathesis could have a significant impact on future natural product-directed work would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) or asymmetric ROM... 

A similar strategy served to carry out the last step of an asymmetric synthesis of the alkaloid (—)-cryptopleurine 12. Compound 331, prepared from the known chiral starting material (l )-( )-4-(tributylstannyl)but-3-en-2-ol, underwent cross-metathesis to 332 in the presence of Grubbs second-generation catalyst. Catalytic hydrogenation of the double bond in 332 with simultaneous N-deprotection, followed by acetate saponification and cyclization under Mitsunobu conditions, gave the piperidine derivative 333, which was transformed into (—)-cryptopleurine by reaction with formaldehyde in the presence of acid (Scheme 73) <2004JOC3144>. 

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