Esters 5-silyl, conformation

The addition of the lithium enolates of methyl acetate and methyl (trimelhylsilyl)acetate to ( + )-(S)-2-(4-methylphenylsulfinyl)-2-cycloalkenones gives, after desulfurization, (/ -substituted cycloalkenones. A higher level of selectivity is observed with the a-silyl ester enolate and in the cyclohexenone series13. The stereochemical outcome is rationalized by assuming attack on a ground-state conformation analogous to that in Section 1.5.3.2.1. 

The combinations of chlorotrimethylsilane-hexamethylphosphoramide (HMPA) or chlorotrimethylsi-lane-4-(dimethylamino)pyridine (DMAP) are also powerful accelerants for copper(I)-catalyzed Grignard conjugate additions,33 and stoichiometric organocopper and homocuprate additions (Scheme 21 ).36 However, these reactions must be performed in tetrahydrofuran instead of ether.37 These procedures are noted for their high yields with stoichiometric quantities of Grignard reagents, excellent chemoselectivity and efficiency with a,3-unsaturated amides and esters and enals.58 Typically, additions to enals proceed via the S-trans conformers to afford stereo-defined silyl enol ethers for example, enals (122) and (124) give the ( )-silyl enol ether (123) and (Z)-silyl enol ether (125), respectively. 

With a more powerful donor than an H—C bond, cyclohexyl esters carrying a [3 silyl group demonstrate the preference for the donor and the acceptor bonds to be anti. The equilibrium proportion of the alcohol 2.89 (R = H) is in favour of the diequatorial isomer, but with esters (R = acyl) the equilibrium shifts to favour the diaxial conformation 2.90. Furthermore, the equilibrium constant correlates... 

This is true also of allylic esters 182 so that the geometry of both alkenes involved in the Claisen-Cope rearrangement can be controlled if the lithium enolate is trapped by silylation to give 183. Drawing the rearrangement in the chair conformation 183a — 184 gives a predictable diastereoisomer of the product 185. 

The diastereomers should have conformational rigidity to maximize their physical differences and so enhance the separation. During the separation of the diastereomeric (-l-)-neomethyl thioureas of amino acid esters, Nambara et al. [9] showed that the separation factors of the t-butyldimethyl-silyl esters were greater than those of the methyl esters. This was believed to be an effect of the increased structural rigidity imparted by the t-butyldimethylsilyl group. 

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