Journal of the Chemical Society. Perkin transactions I p. 3257 - 3266 (1992)
Update date:2022-08-05
Topics:
Fleming, Ian
Lewis, Jeremy J.
Methylation of the enolates 7, 24, 28 and 33 and protonation of the enolates 10, 27, 31 and 36 are diastereoselective in conformity to a general rule, summarised in the drawing 1, governing the stereochemistry of electrophilic attack on a double bond adjacent to a stereogenic centre.The sense of the selectivity is, with one exception, opposite to that of the corresponding nucleophilic attack on a carbonyl group adjacent to a stereogenic centre, which, with the same exception, follows Cram's and the Felkin-Anh rule, summarised in the drawing 2.The exception is probably the reduction 40 -> 38 + 39, with 39 as the major product.This result is inconsistent with Cram's and the Felkin-Anh rule if the isopropyl group is counted as 'larger' than the phenyl group, whereas the Grignard reaction 37 -> 38 + 39, where 39 is again the major product, and the corresponding electrophilic reactions 33 -> 34 + 35, with 34 as the major product, and 36 -> 34 + 35, with 35 as the major product, are all consistent with isopropyl being effectively larger than phenyl.
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