Grignard reagent ketone, stereoselective

Addition of Grignard reagents to ketones and aldehydes was one of the reactions which led to the formulation of Cram s rule. Many ketones and aldehydes have subsequently been examined to determine the degree of stereoselectivity. Cram s rule is obeyed when no special complexing functional groups are present near the reaction site. One series of studies is summarized in Table 8.2. 

For ketones and aldehydes in which adjacent substituents permit the possibility of chelation with a metal ion, the stereochemistry can often be interpreted in terms of the steric requirements of the chelated TS. In the case of a-alkoxyketones, for example, an assumption that both the alkoxy and carbonyl oxygens are coordinated with the metal ion and that addition occurs from the less hindered face of this chelate correctly predicts the stereochemistry of addition. The predicted product dominates by as much as 100 1 for several Grignard reagents.120 Further supporting the importance of chelation is the correlation between rate and stereoselectivity. Groups that facilitate chelation cause an increase in both rate and stereoselectivity.121 This indicates that chelation not only favors a specific TS geometry, but also lowers the reaction barrier by favoring metal ion complexation. 

Chiral 2,2-disubstituted cyclobutanones have been obtained by asymmetric rearrangement of chiral sulfinyl- 177,178 and sulfanylcyclopropanes.179 Using readily available cyclopropyl 4-tolyl (/ )-sulfoxide (l),180 the requisite sulfinylcyclopropanes 3 and 3 were obtained by a sequence of lithiation, reaction with carboxylic acid esters and stereoselective addition of Grignard reagents to the ketones 2 thus formed.178 The corresponding sulfanylcyclopropanes 4 and 4 resulted from a sequence of protection, reduction and deprotection.179... 

Addition to a-aikoxy carbonyl compounds. n-Butylmagncsium bromide reacts stercosclcctivcly with protected, chiral a-hydroxy ketones. For example, reaction of the Grignard reagent with the MEM ether of 3-hydroxy-2-decanone in THF leads to the f/ireo-product [>95% isolated yield (equation I)]. The stereoselectivity is much lower when n-butyllilhium is used in addition the solvent plays an important role. Stereoselectivity is lower in C5H, 2, CH2CI2, and ether. A number of protective... 

A different type of reaction, in which the intermediate formation of a chelate ring is crucial, is the reaction of pyridyl thioates with Grignard reagents, which achieves an overall conversion of an alkyl or aryl halide to a ketone (Scheme 105).378 Grignard reactions are highly susceptible to chelation factors and stereoselectivity is observed in the addition of organomagnesium or... 

The vinyl Grignard reagent was used with Cu(I) catalysis and the reduction of both ester and ketone was achieved with UAIH4. The stereoselectivity was excellent and 75 could easily be separated from the minor equatorial alcohol. In the next chapter we shall revisit both the use of copper in getting regioselectivity and the stereoselectivity of such reactions. 

As part of their studies of the synthesis of hirsutic acid C, Lansbury and co-workers developed a stereoselective synthesis of norhirsutanes (Scheme 52).333 Model experiments involved the addition to ketone 333 of Grignard reagents derived... 

The acid or base elimination of a diastereoisomerically pure p-hydroxysilane, 1, (the Peterson olefination reaction4) provides one of the very best methods for the stereoselective formation of alkenes. Either the E- or Z-isomer may be prepared with excellent geometric selectivity from a single precursor (Scheme 1). The widespread use of the Peterson olefination reaction in synthesis has been limited, however, by the fact that there are few experimentally simple methods available for the formation of diastereoisomerically pure p-hydroxysilanes.56 One reliable route is the Cram controlled addition of nucleophiles to a-silyl ketones,6 but such an approach is complicated by difficulties in the preparation of (a-silylalkyl)lithium species or the corresponding Grignard reagents. These difficulties have been resolved by the development of a simple method for the preparation and reductive acylation of (a-chloroalkyl)silanes.7... 

Chiral acetals can be used as auxiliaries in the diastereoselective reactions of Grignard reagents with acyclic as well as cyclic a-keto acetals. Nucleophilic addition to the monoprotected diketone (69 equation 18) occurs with excellent stereoselectivity to generate the corresponding tertiary alcohol (70) as the major product, usually with greater than 95 5 selectivity. Removal of the ketal yields a-hydroxy ketones of high optical purity. In most examples, enantiomeric excesses of 95% and higher are observed in the resultant keto alcohols. Table 17 represents the results of additions to cyclic and acyclic substrates. 

Sydowic acid (100) was prepared as outlined in Scheme 10. ° The crucial transformation of ketone (101) to cafbinol (102) was accomplished by stereoselective 1,2-addition of trimethylaluminum, which afforded superior facial selectivity for the re-face compared to Grignard reagents. In contrast, methyltri-chlorotitanium addition resulted in a 5i -face stereoselectivity. 

Systematic studies on the chelation-controlled additions were carried out, varying the type of alkoxy group, the carbon nucleophile, the solvent and the temperature. It was found that a-alkoxy ketones react highly stereoselectively with Grignard reagents in THF (equation 24). Alkyllithiums were not effec-tive. -6 The generalization was made use of in the synthesis of the polyether antibiotic monensin. ... 

Thiazolines activated with an equivalent of BF3 readily react with a wide range of organometals, giving tranj-4,5-disubstituted thiazoles stereoselectively. Alkyllithiums, Grignard reagents, lidiium alkynides, nitronates, ester and ketone enolates have been employed as the nucleophile. Stereocontrolled construction of three contiguous asymmetric centers is performed with a lithiated isothiocyanatoacetate, and the product is successfully transformed to (+)-biotin (Scheme 27), ° ... 

Stereoselective addition of a dithiane anion to chiral 2-methyl-3-trimethylsilyl-3-buterud combined with the stereoselective addition of a Grignard reagent to the chi a-alkoxy ketone affords a practical method for the construction of a,y-dimethyl-a,3-dihydroxy compounds, useful intermediates for the synthesis of erythronolides (Scheme 33). -Hydroxy cartmxylic esters were synthesized by the addition of ethyl l,3-dithiolanyl-2-carboxylate enolate to a chiral aldehyde, followed by desulfurization. ... 

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