Diastereoselectivity bridged bicyclic

Subsequently, Posner published the completely regioselective and highly stereoselective cyclo additions of racemic 3-(p-tolylsulfinyl)-2-pyrone (141) (Scheme 70) with 1,1-dimethoxyethylene [133],vinylether,and vinylthioethers [134]. With the first dienophile, the best diastereoselectivity (an 88 12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h). A 10 1 endo/exo mixture of cycloadducts was obtained with vinyl-ether in the presence of ZnBr2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures. The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives. Although enantio-merically pure samples of compound 141 could be obtained [134] it was not used as a starting material for asymmetric Diels-Alder reactions (the low yield of (S)-141 precluded this). 

In general, high levels of diastereoselectivity are observed for 5-evo-cyclisa-tions leading to both fused and bridged bicyclic ring systems. For example,... 

In a beautifully choreographed sequence of events, Judd and co-workers installed the C3 to C4 bridged bicyclic lactam moiety of 129 with diastereoselective control via an Ugi 4-component coupling, ring-closing metathesis, and Heck reaction <07OL5119>. 

When the eyclopentadiene ring is fused at C-2, C-3 to bicyclo[2.2.1] or unsymmetrical bicy-clo[2.2.2] frameworks, the diastereoselectivity of the cycloaddition of triazolediones is affected mainly by the presence of unsaturation in one bridge, but also by the presence of substituents at either the eyclopentadiene moiety or the bicyclic framework. 

Martin has made ample use of this strategy for the synthesis of various polyketide natural products. Furan 164, prepared by addition of lithiofuran to a lactaldehyde derivative, gave the bridged ketal 165 upon oxidation and acid catalyzed dehydration <99T3561>. The bicyclic architecture allowed highly diastereoselective reactions which led to 166, a key intermediate for... 

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