1.4- Silyl migrations diastereoselectivity

Silyl migrations from O to C were also found during the synthesis of a-halomethylsilanes415 and diastereoselective preparations of (Z)-vinyl-, epoxy- and cyclopropylsilanes389,416. 

The tin(IV) chloride-promoted [3 + 2] cycloaddition of allysilanes and a-ketoesters provides a versatile diastereoselective route to silyl-substituted tetrahydrofurans via 1,2-silyl migration (Equation (106)) <94CL627, 94TL8401>. 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

There is a dichotomy in the sense of syn-anti diastereofacial preference, dictated by the bulkiness of the migrating group [94]. The sterically demanding silyl group results in syn diastereofacial preference but the less demanding proton leads to anti preference (Sch. 35). The anti diastereoselectivity in carbonyl-ene reactions can be explained by the Felkin-Anh-like cyclic transition-state model (Ti) (Sch. 36). In the aldol reaction, by contrast, the now inside-crowded transition state (Ti ) is less favorable than Tg, because of steric repulsion between the trimethylsilyl group and the inside methyl group of aldehyde (Ti ). The syn-diastereofacial selectivity is, therefore, visualized in terms of the anti-Felkin-like cyclic transition-state model (T2 )-... 

There is a dichotomy in the sense of syn- vs antz-diastereofacial preference, dictated by the bulkiness of the migrating group [80]. The sterically demanding silyl group shows syn-diastereofacial preference but the less demanding proton leads to anti-preference (Scheme 33). The anfi-diastereoselectivity in carbonyl-... 

When 2-substituted (methyl or trimethylsiloxy) norborn-2-enes and l,7,7-trimethylnorborn-2-enes are treated with Oj, ene products are formed efficiently. In case of the trimethylsiloxy-substituted derivative 23, migration of the trimethylsilyl group occurs, and O-silylated a-hydroperoxy ketones 24, which are readily reduced to O-silylated a-hydroxy ketones 25 (Scheme 14) are isolated. High exo selectivity is observed for the 7,7-unsubstituted substrates and endo selectivity for the 7,7-dimethyl ones. The steric demand seems to be higher for the silyl-ene reaction, for which endo/exo ratios from 0 100 to 94 6 were observed, whereas 2-methyhiorborn-2-ene shows a 98.5 1.5 exo diastereoselectivity. ... 

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