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(8a to 10a), the intermediate ketene acetals were isolated (ca. 35 and
1
30%, H NMR of OMe singlets at d 3.63 and 3.73), but these were
accompanied by about 20% of starting malonate (formed by ready
hydrolysis) and rearranged malonates (15 and 2 ± 3%, respectively).
[7] LDA/ZnCl2, effective in certain chelate ± enolate Claisen rearrange-
ments (see U. Kazmaier, Liebigs Ann. Chem. 1997, 285), was tested for
the rearrangement of (R)-5 derived malonate, but gave rise to a slow
reaction with formation of side products.
[8] Alternatively, desilylation can be performed in a separate step (5%
aqueous HCl, THF).
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92
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[33] Alternatively, pure syn-epoxide 17a could also be obtained in 79%
yield by permaleic acid epoxidation (maleic anhydride, H2O2, CH2Cl2,
108C), as concomitant acid-catalyzed lactonization of the anti-epoxide
18a to 20a (10% formed) is much faster than that of 17a to 19a (1%
formed).
[12] Reviews: E. J. Corey, C. J. Helal, Angew. Chem. 1998, 110, 2092;
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with LiAlH4 in Et2O, or more economically with polymethylhydro-
siloxane (PMHS) and catalytic amounts of zinc 2-ethylhexanoate and
NaBH4: H. Mimoun, J. Org. Chem. 1999, 64, 2582.
[17] Under these conditions, the liberated MeOH, a known inhibitor of
lipases, is removed by evaporation. Without KHCO3, the pH of the
initial reaction mixture is about 3, and the reaction does not reach
completion; in some experiments, partial racemization was noted.
[18] For Johnson ± Claisen rearrangements performed on enantiopure
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resolution, see K. Mori, Liebigs Ann. Chem. 1993, 77; E. Brenna, N.
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[34] The reaction was followed by quenching samples with NaHCO3/Et2O,
followed by silylation of the unconverted alcohol with N,O-bis(trime-
thylsilyl)acetamide (BSA) in toluene at 1008C for 5 min and chiral
GC analysis (Megadex 5). Normal GC analysis did not always give
reproducible results (thermal instability of 11a).
[35] The product can be further purified by flash column chromatography
(SiO2 F60, 35 ± 70 m), using CH2Cl2 as solvent.
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Angew. Chem. Int. Ed. 2000, 39, No. 3
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