Cerium(III) Acetate-Boron Trifluoride Etherate


[537-00-8]  · C6H9CeO6  · Cerium(III) Acetate-Boron Trifluoride Etherate  · (MW 317.27) (BF3.OEt2)

[109-63-7]  · C4H10BF3O  · Cerium(III) Acetate-Boron Trifluoride Etherate  · (MW 141.95)

(two-component Lewis acid catalyst system1,2 for the cyclocondensation of aldehydes with siloxydienes3)

Physical Data: complex generated in situ.

Solubility: slightly sol toluene.

Form Supplied in: generated in situ from anhydrous cerium(III) acetate and Boron Trifluoride Etherate.

Preparative Methods: anhydrous Ce(OAc)3 is required for the preparation of this Lewis acid system and is obtained by drying cerium(III) acetate hydrate at 100 °C under high vacuum for at least 3 h. This material is then slurried into toluene under an argon atmosphere for 30 min, cooled to -78 °C, and treated with freshly distilled BF3.OEt2 to give the title reagent.

Handling, Storage, and Precautions: this binary Lewis acid system is best used as prepared. Strictly anhydrous conditions are required. Cerium reagents are generally of low toxicity. BF3.OEt2 is extremely moisture sensitive. This reagent should be handled in a fume hood.

Cyclocondensation of Aldehydes with Siloxydienes.

Thus far, reported applications of Ce(OAc)3-BF3.OEt2 have been limited to the area of Lewis acid-catalyzed cyclocondensation reactions of activated dienes with aldehydes.1-3 In the context of efforts aimed at developing a fully synthetic route to tunicaminyl uracil, a subunit of the tunicamycins, Danishefsky and co-workers examined a number of Lewis acid promoters for the cyclocondensation of aldehyde (1) with the oxygenated diene (2).1,4 They found that the binary Lewis acid Ce(OAc)3-BF3.OEt2 (toluene, -78 °C, 2 h) afforded a 45% yield of a single diastereomeric product, dihydropyrone (3), along with ca. 20% of recovered aldehyde (1) (eq 1). The facial sense of the reaction can be rationalized by invoking chelation of the b-alkoxy aldehyde by the highly oxophilic5 Ce(OAc)3-BF3.OEt2, followed by a-face attack of the diene (2) on the chelated ensemble.6 Apparently both oxygen atoms of the (benzyloxy)methoxy C-5 appendage play a role in the observed extraordinary diastereofacial selectivity.2 The use of alternative Lewis acids, including those known to favor chelation control (e.g. SnCl4), afforded (3) along with significant and varying amounts of two of the other three possible diastereomeric dihydropyrones.1

One limitation of the Ce(OAc)3-BF3.OEt2 mediated cyclocondensation chemistry was found during attempts to extend the scope of the process to include aldehyde substrates incorporating a nucleoside subunit. For example, the aldehydes (4) failed to react with diene (2) to give the targeted dihydropyrones (5) (eq 2).2 Evidently the two-component catalyst system is modified or destroyed by the uracil ring.

1. Danishefsky, S. J.; Barbachyn, M. JACS 1985, 107, 7761.
2. Danishefsky, S. J.; DeNinno, S. L.; Chen, S.; Boisvert, L.; Barbachyn, M. JACS 1989, 111, 5810.
3. Danishefsky, S. J.; DeNinno, M. P. AG(E) 1987, 26, 15.
4. Danishefsky, S. J.; Maring, C. J. JACS 1985, 107, 1269.
5. For a comprehensive review of lanthanide reagents and their characteristics, see: Molander, G. A. CRV 1992, 92, 29.
6. For an analysis of stereochemical issues associated with Lewis acid promoted addition of nucleophiles to oxygenated aldehydes, see: Reetz, M. T. AG(E) 1984, 23, 556.

Michael R. Barbachyn

The Upjohn Company, Kalamazoo, MI, USA

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