Reagent cerium

Cerium reagents have also been found to give improved yields in the reaction of organolithium reagents with carboxylate salts to give ketones. 

Alkylations of epoxychalcones 2 has been successfully effected using organo-cerium reagents, displaying excellent yields (80-91 %) and moderate diastereo-selectivities (4 1 - 9 1) of the resultant tertiary alcohols, (Scheme 12). 

Addition of 2,6-dimethoxypyrimidine-4-cerium chloride 371 to the chiral lactone 370 occurred without racemization of the chiral center, and the product 372 was subsequently used in a successful total synthesis of (—)-7-epicylindro-spermopsin <2002JA4950, 2005JOC1963>. The cerium reagent was prepared situ from 4-bromo-2,6-dimeth-oxypyrimidine by sequential addition of butyllithium and cerium trichloride. Addition of the same dimethoxypyrimidine-4-cerium derivative to a chiral lactam has also been reported <1999J(P1)1193>. 

The cerium reagent coordinates to the carbonyl, making only a 1,2 addition possible. 

Bacteria, antimicrobials against, 12, 456 Baeyer-Villiger oxidation, via tin amides, 9, 370 Barbier-Grignard-type reactions, and sonochemical metal insertions, 1, 315 Barbier-type reactions allenyl and propargyl tins, 9, 358 with allylic tins, 9, 357 with antimony(III) compounds, 9, 426 with bismuth(III) compounds, 9, 433 with cerium reagents, 10, 409 with indium compounds, 9, 685... 

Organocerium reagents are prepared in situ by the reaction of organolithium compounds with anhydrous cerium chloride or ceriiun iodide, as shown in equation (1). A variety of organolithium compounds can be employed, including alkyl-, allyl-, alkenyl- and alkynyl-lithiums, which are all converted to the corresponding cerium reagents. 

The chemoselectivity between aldehydes and ketones has been studied by Kauffinann et al Cerium reagents exhibit moderate aldehyde selectivities, although these are much lower than those of the organo-metallic reagents of titanium, zirconium and chrmnium (Scheme 2). ... 

The reactions of ( -PhCH-=CHCOC6H4-p-OMe with cerium reagents provide (Z)-allyl alcohols in excellent yields, suggesting that the addition reactions proceed almost exclusively through a polar pathway. 

The reaction of cerium reagents with imines and nitriles which possess a-hydrogens has been studied by Wada et al ° Available data indicate that the reagents do not effectively d to these substrates, although the results are better than with alkyllithiums themselves. A modification of the procedure improves the yields of addition products. Thus, addition of the organocerium or organolithium reagent to an... 

Corey and Ha have successfully employed a cerium reagent at the crucial step in the total synthesis of venustatriol, as illustrated in Scheme 6. ... 

A notably stereoselective reaction of an arylcerium reagent has been repOTted by Terashiina et al. As shown in Scheme 18, the cerium reagent provides adducts (11) and (12) in a ratio of 16 1 in 95% combined yield. In contrast, the organolithium reagent gives a lower and reversed stereoselectivity. 

Recently, a new method for synthesizing coumarin derivatives has exploited the properties of aryl-cerium reagents, as illustrated in Scheme 19/ Interestingly, the bulky arylcerium reagent (13) adds to the easily enolizable r-butyl acetoacetate in satisfactory yield. 

An example of the reaction of an alkenylcerium reagent with a cyclopentanone derivative has been reported. As shown in Scheme 20, the ( )-cerium reagent adds to the ketone to provide a single adduct in modest yield. However, the (Z)-cerium reagent did not react, presumably due to steric effects. 

The utility of this reagent has been demonstrated by Tamura et al. in its reaction with the readily enol-izable ketone The ethynylation of the ketone proceeds smoothly with the cerium reagent, as... 

Cerium enolates are generated by the reaction of lithium enolates with anhydrous cerium chloride in THF. The cerium enolates react readily with various aldehydes and ketones at -78 °C (Scheme 24). The yields are generally higher than in reactions of lithium enolates. This is presumably due tt> the relative stabilities of the adducts, that of the cerium reagent being greater by virtue of coordination to the more oxo lic cerium atom. The stereochemistry of the products is almost the same as in the case of lithium enolates, as shown in Table 4. The reaction is assiuned to proceed through a six-membeied chair-like transition state, as with lithium enolates. 

RLi-CeCb is an effective all lating reagent for easily enolizable ketones, which give very low yields of the adducts with alkyl-lithiums or -magnesiums. Selective 1,2-addition reactions with a,p-imsatu-rated ketones can be conducted with the cerium reagents (Scheme 7). ... 

The addition of trimethylsilylmethyllithium to enolizable ketones generally gives significant amounts of the enolized carbonyl compound. Johnson and Tait have shown that when the lithium anion is converted to tile cerium reagent it gives high yields of the addition product. For example, a-tetralone (151) reacts with trimethylsilylmethyllithium to give after protonation an 1 1 mixture of the desired exocyclic... 

The union of fragments 78 and 81 was performed by treatment of 81 with t-BuLi to induce a bromine-lithium exchange, followed by transmetal lation to the cerium reagent [49] and finally the addition of aldehyde 78 (Scheme 17). The coupled product was obtained in 85% yield as a diastereomeric mixture of alcohols. Swem oxidation [50] and reaction with MeMgBr at -78 °C provided the tertiary alcohol stereoselectively in 98% yield. A final deprotection with tosic acid gave venustatriol (2). 

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