Allenylmetal chiral

The configurational stability of chiral allenylmetal reagents depends to a large extent on the nature of the metal substituent. The mechanism of the racemization process has not been studied in detail, but two reasonable pathways can be proposed, based on known reactivity characteristics of these compounds. The first entails reversible intermolecular SE- rearrangement to the propargylic isomer. This process could proceed by a pure syn or anti pathway, in which case no racemization would take place. However, the occurrence of both pathways would result in racemization (Scheme 9.5). 

Scheme 9.5 Possible racemization pathways for chiral allenylmetal compounds.

Chiral allenylmetal compounds provide convenient access to enantioenriched homopropargylic alcohols through Se2 additions to aldehydes. The syn adducts can be obtained through addition of allenyl tributylstannanes in the presence of stoichiometric boron trifluoride etherate (BF3-OEt2). The use of allenylmetal halides derivatives of Sn, Zn, and In lead to the anti diastereomers. The former additions proceed through an acyclic transition slate whereas the latter are thought to involve a cyclic transition state, thus accounting for the difference in diastereoselectivity. 

Simple diastereoselectivity comes into play when allenylmetal compounds are added to aldehydes, since adducts such as 1 a/b contain both an axis and a center of asymmetry. Hence, diastereomeric mixtures are produced. When chiral aldehydes are used in such reactions, the diastereoselectivity also depends on the relative rate by which the enantiomers of the racemic allenylmetallic species interconvert, i.e., relative to the rate of addition to the chiral aldehyde. Apart from reactions of allenyllithium and -titanium reagents with aldehydes90-94, few such intermolecular, simple diastereoselective reactions yielding allenes have been reported. 

Using (/ ,/ )-Diop as the chiral auxiliary, the amount of chiral induction is not dependent on the temperature, but depends on the type of chiral ligand in the palladium complex and on the allenylmetal reagent. When magnesium or copper is present, a reversal of the configuration... 

The high synthetic utility of alcohols 38 stems from the fact that terminal alkynes are among the most versatile functional groups for the further elaboration of a carbon skeleton. Asymmetric synthesis of alcohols 38 from aldehydes with the concurrent formation of the two stereogenic C atoms has been accomplished mainly by two methods. The first features synthesis of chiral nonracemic allenylmetal compounds from the corresponding chiral nonracemic propargyl alcohols and addition of the former to aldehydes [26] and the second method in-... 

In the following Sections we review the reactions of chiral allylmetal and allenylmetal reagents and their application to the synthesis of complex natural products. These reagents are useful for the enantioselective allylation of achiral aldehydes... 

Marshall s chiral allenylmetal reagents have been utilized in double asymmetric reactions with chiral aldehydes for the synthesis of polypropionate natural products. All four dipropionate diastereomers are accessible from the reactions of chiral allenylmetal reagents with a-chiraI-y5-alkoxy aldehydes 97 (153, 158, 276, 277]. The BF3-OEt2-catalyzed addition of allylstannane (l )-218a to aldehyde 97a occurs in high yield and diastereoselectivity to give the xyn.syn-dipropionate 395, presumably through either the synclinal or antiperiplanar Felkin transition states 396 and 397 (Eq. (11.31)). 

Marshall has applied the chiral allenylmetal reagents in a number of natural product syntheses [154, 278-280]. The synthetic utility of these reagents is maximized when the alkyne functionality of Marshall s products is used to further elaborate the carbon skeleton of the ultimate synthetic target. This methodology is il-... 

Catalytic enantioselective allylations of aldehydes already published can be classified into two methods carried out under the influence of chiral Lewis acid catalysts and chiral Lewis base catalysts. The process by chiral Lewis acid catalysts generally uses allyltrimethylsilane or allyltrialkylstannane as an allylating agent, both of which show low reactivity toward aldehydes without these catalysts. The process by chiral Lewis base catalysts employs allyltrichlorosilane or allylmetals possessing relatively higher reactivity. Both processes can be successfully applied to various substituted allylmetal compounds or allenylmetal compounds. 

This synthesis illustrates the potential of chiral allenylmetal reagents for the synthesis of this polyketide natural product. 

The additions of chiral nonracemic allenylmetal reagents to chiral a-methyl propanal derivatives have been proven useful for the assembly of polypropionate fragments. These reagents rely on allene chirality to favor one of the two possible diastereomeric transition states in the addition and, thus, differ in a fundamental way from the aforementioned methods in which a chiral auxiliary or catalyst provides the control element. For example, a chiral allenylstannane 246 is added to a chiral aldehyde (S)-230, derived from the Roche ester, in the presence of various Lewis acid promoters to afford any of the four diastereo-mers with excellent diastereo- and enantioselectivity, depending on the reaction conditions. Representative results are depicted in Scheme 10.48. From the stereocontrol point of view, these transformations follow Cram-fike open transition state models without or with chelation, respectively. If InBr3, SnCLi, BuaSnCl, or other additives... 

Marshall JA, Grant CMJ. Preparation of chiral allenylmetal reagents from enantioenriched allenyl iodides and propar-gyUc mesylates. A comparison of indium, bismuth, and tin derivatives. Org. Chem 1999 64 8214-8219. 

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