164070-83-1Relevant articles and documents
Diastereoselectivity in the SE2″ reaction of chiral pentadienylsilanes: A test for the relative importance of steric and electronic effects
Fleming, Ian,Jones, Graeme R.,Kindon, Nicholas D.,Landais, Yannick,Leslie, Colin P.,Morgan, Ian T.,Peukert, Stefan,Sarkar, Achintya K.
, p. 1171 - 1196 (2007/10/03)
The homochiral pentadienylsilanes (3Z,5E)-(hepta-3,5-dien-2-yl)dimethyl(phenyl)silane 9, (3Z,5E)-(hepta-3,5-dien-2-yl)trimethylsilane 13, (4Z,6E)-(2-methylocta-4,6-dien-3-yl)dimethyl(phenyl)silane 14 and (4Z,6E)-(2-methylocta-4,6-dien-3-yl)trimethylsilane 17 undergo Lewis acid catalysed reactions with isobutyraldehyde and its dimethyl acetal stereospecifically anti with surprisingly high levels of stereoselectivity, ca. 90:10. The pentadienylsilanes (3Z)-hexa-3,5-dien-2-yldimethyl(phenyl)silane 20aa, (4Z)-(2-methylhepta-4,6-dien-3-yl)dimethyl(phenyl)silane 20ab, (3Z)-(hexa-3,5-dien-2-yl)-trimethylsilane 20ba and (2Z)-(1-phenylpenta-2,4-dienyl)trimethylsilane 20bc undergo dipolar cycloadditions to 2,2-dimethylpropanenitrile oxide regioselectively at the terminal double bond and stereoselectively anti to the silyl group to a somewhat lower extent, ca. 70:30. The pentadienylsilane (3Z,5E)-(6-cyclohexylhexa-3,5-dien-2-yl)trimethylsilane 25 undergoes deuteriodesilylation stereospecifically anti to a lower extent still, ca. 55:45. The pentadienylsilane (3Z,5E)-(8-methyl-8-methoxyethoxymethoxynona-3,5-dien-2-yl)trimethylsilane 34 undergoes an intramolecular reaction stereospecifically anti again to the extent of about 60:40, whereas the reaction of the corresponding allylsilane (3Z)-(6-methyl-6-methoxyethoxymethoxyhept-3-en-2-yl)trimethylsilane 32 is essentially completely anti. These results show that SE2″ reactions can be highly stereoselective in the anti sense, that the high level is probably best accounted for by the steric effect of the silyl group, and that when the steric effect is minimised, the stereospecificity is low, but still measurable. The pentadienylsilanes were prepared by aldol reactions between β-silyl esters and the appropriate α,β-unsaturated aldehyde, followed by decarboxylative elimination. The products of the SE2″ reactions were identified and their stereochemistry determined by comparison with authentic materials or by degradation and synthesis, using chiral auxiliaries to determine the enantiomeric purity. The products of two of the dipolar cycloadditions were identified by degradation and stereospecific vinylogous Peterson elimination, but the vinylogous Peterson elimination taking place with (1RS,2Z,4SR,6SA)-(4,6-dihydroxy-7,7-dimethyl-1-phenyloct-2-enyl)trimethylsilane 67bc and its (1SR) diastereoisomer 68bc was not stereospecific, giving (5E,7E)-2,2-dimethyl-8-phenylocta-5,7-dien-3-ol 73 from both isomers. The stereochemistries of all the reactions are summarised.
Diastereoselectivity in the Preparation of β-Silyl Esters from αβ-Unsaturated Esters and Amides Attachhed to Chiral Auxiliaries
Fleming, Ian,Kindon, Nicolas D.
, p. 303 - 316 (2007/10/02)
The conjugate addition of the phenyldimethylsilyl-cuprate reagent to cinnamate and crotonate esters and amides 1 of various known chiral auxiliaries, a-e, is diastereoselective.The sense of diastereoselectivity of silyl-cuprate addition to the esters 1b-d, 8 and 9 is different from established precedent based on carbon-cuprates, but is normal for silyl-cuprate addition to the amide 1a, the imines 1e and 21, and the oxazolidine 6.The chiral auxiliary e gives the best results of those tested, and the silicon-containing group can be removed from the chiral auxiliary using alkoxide ion in aprotic media, making available β-silyl esters 27-29 of high enantiomeric excess, with recovery of the chiral auxiliary 30.
