OTBDMS group

To remove the conformational ambiguity which obscures interpretations of diese results, additions of various dienophiles to 3-substituted 1-vinylcyclopentenes (274) were studied. Preferential cycloaddition from the diene face anti to the Me, OMe and, particularly, the OTBDMS group was observed (Scheme 65, Table 13). Cycloaddition of the hydroxy-substituted [Pg.349]

First, the MeS group was removed with Raney nickel giving the correct stereochemistry at the new centre in 168. The stereochemistry actually comes from the protonation step and presumably arises because EtOH donates an H atom to the less hindered side of the desulfurised enolate opposite the OTBDMS group. Then oxidation with the Dess-Martin periodinane gives the vital aldehyde 169. 

Very little or no diastereoselectivity was observed in the formation of pyrro-lines 35, whereas somewhat better results were obtained with pyroglutamate 36 (R = er/-Bu), which was formed in a 1 2.5 cisjtrans ratio. An efficient stereocontrol of the key cyclization step was however achieved with suitably designed starting materials bearing a bulky OTBDMS group vicinal to the site of radical addition [22b]. Cyclization of the. syn-isonitrile 39 with either ethanethiol or mer-... 

Various a-phenylated cyclohexanones have also been prepared in high yields by reaction of the corresponding lithium enolates with diphenyliodonium triflate (eq 2). The cis/trans ratio depends on the nature of the R group. For example, 4-ferf-butylcyclohexanone affords the phenylated product with a cisitrans ratio >20 1, whereas the ratio is only 5 1 in the case of 4-phenylcyclohexanone and 1 15 if cyclohexanone bears a OTBDMS group at C4. 

Fig. 19. Molecular structure of polytriacetylenes (PTA). The conjugation ends are substituted by donor (dimethylaniline) and acceptor (nitrophenyl) groups to modify the nonlinear optical properties. The lateral OTBDMS ((ferf-butyl)dimethylsilyloxyl) groups serve for improved solubility in the solvent (typically chloroform)...

Other compounds, such as ketols (1 l-keto-3-ol), as well as 11-monoketo, characterized by this p stereochemistry (for C-14), have a similar behavior concerning the formation of these ions. In addition to the interest in distinguishing ring junction geometries, Gaskell and co-workers [206a,b] studied the behavior of a and p stereochemistry of the di-OTBDMS (t-butyldimethylsilyl) groups in derivatives of androstane-3,17-diol (Fig. 56). 

The silyl enol ether 64 is formed from 63 with the Z configuration because of the enormous OTBDMS protecting group and reacts with benzaldehyde to give essentially one diastereoisomer of the vyn-aldol 65. The 1,3-stereochemical control across the carbonyl group is discussed in chapter 21. 

DMT 0—V P2 i OTBDMS first coupling , diester link formation Protecting groups... 

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