Diphenylsilyl tether

In the cases of dimethylsilyl IMDA precursors, the exolendo selectivity was poor. However, this ratio could be readily and quite dramatically influenced by varying the alkyl substituents on the silicon template [12]. Thus with dienol 24, tether formation with dimethylvinylsilyl chloride and subsequent IMDA reaction afforded a 4 1 mixture of exolendo products (Scheme 10-7). The ratio could be further improved to 10 1 by using a diphenylsilyl tether, and when bulky Bu groups were used, a single stereoisomer, resulting from exo addition, was observed. This example once more illustrates the potential for tuning the stereoselectivity of the reaction by varying the steric interactions with the tether. 

A modified tether, incorporating (-)-hydrobenzoin instead of glycol, was readily prepared from ketone 78 as before, anticipating that steric interactions between substituents at these new stereogenic centers and the diphenylsilyl group would rectify the problem of r-facial selectivity in the cycloaddition. Thermolysis of triene 84 at 200 °C in toluene for 18 h afforded a 3 2 mixture of cyclized products 85 and 86 respectively, in which the major diastereoisomer resulted from the desired a-approach of the dienophile (Scheme 10-28). Subsequent manipulation generated (-(-)-adrenosterone in a total of only seven steps from enone 78. 

Evans and Murthy used RCM of enantiomerically enriched allylic alcohols tethered through a silicon atom to prepare the reduced carbohydrate D-altritol 390 [164]. Treatment of diphenylsilyl acetal 391 with Grubbs catalyst 385 in CH2CI2, heating at reflux, provided the C2-symmetrical silacycle 392 in excellent yield. Dihydroxylation of the cis-olefin and simple manipulations led to a facile, expeditious route to D-altritol (Scheme 10-126). 

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