Enolsilane, Epoxy

SCHEME 18.40 Reactions of epoxy enolsilane 174 and substituted ftnans. 

This reaction proceeded with a range of substituted epoxy enolsilanes and dienes, but the endo/exo selectivity remained low in most cases. However, for hindered electrophiles and/ or hindered dienes, endo cycloaddition is preferred. For example, the reaction of 182 bearing a more sterically hindered silyl group with the hindered diene 183 afforded the endo cycloadduct 184 exclusively (Scheme 18.41). 

The intramolecular [4+3] cycloaddition of epoxy enolsi-lanes generally proceeded in even higher yield than the intermolecular reactions, and with excellent diastereoselectivity. With enantiomerically enriched epoxy enolsilane 205, cycloadduct 206 was obtained with complete retention of enantiomeric purity, and as a single diastereomer (Scheme 18.46) [40]. Substituted epoxy enolsilane derivatives 207, 209, and 211 tethered to furan moieties generally... 

EtsN SHF, rt, 1 h 40% yield SCHEME 18.45 Cycloadditions of enantiomerically enriched epoxy enolsilane 174. 

FIGURE 18.5 Possible dienophiles in the [4+3] cycloaddition of epoxy enolsilanes. 

The cycloaddition of epoxy enolsilane 214 with one less carbon in the tether, was notdiastereoselective and afforded a mixture of three diastereomeric fused [5,7] carbobicycles 215 bearing the indane carboskeleton, along with alkylated product 216 (Scheme 18.48). 

The reaction of substituted aziridinyl enolsilane 224 and furan also afforded good yields of cycloadducts with good diastereoselectivity (Scheme 18.50). Similar to the epoxy enolsilanes (Section 18.1.3.7), the cycloaddition of (Z)-enol-silane 229 and cyclopentadiene was stereospecific with respect to the enol geometry and yielded the corresponding endo and exo cycloaddition products. 

The species undergoing [4-1-3] cycloaddition in the aziridinyl enolsilane reaction is not the typical siloxyallyl cation, but is probably similar to the intermediate in the analogous reaction of epoxy enolsilanes, because the cycloaddition of enantiomerically enriched aziridinyl enolsilane 217 in nitro-ethane afforded cycloadducts with largely retention of enantiomeric purity (Scheme 18.51). The same reaction mediated by TfOH in dichloromethane gave cycloadducts with higher... 

Chiu and coworkers also reported an intramolecular approach to the cortistatin skeleton using epoxy enolsilanes as the allylic cation precursor (Scheme 19.52) [117-119]. Enantiomerically pure epoxy enolsUane 249 was prepared... 

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