| Popis: |
The lactones 2a and 2b are different only by the configurationof the carbon C5 of the side-chain, but present quite different features in the ring conformation. Inspection of the NMR spectra of 2b , compared to 2a , shows a evident flattening of the butyrolactone ring and an axial orientation of the side-chain, which is a new type of this remote effect, with the 3-methyl trans to the side-chain. These special features arise from the steric interaction which would occur in the regular conformation between the 3-methyl and the substituents at C5. During the synthesis of these lactones, a striking stereochemical behaviour is observed in the course of the addition of a chiral allenic Grignard reagent to a chiral aldehyde or during the hydride reduction of ketones having two asymetric centres in a a′. |
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