Radical, enantioselective conjugate

Scheme 35 Fluxional template for radical enantioselective conjugate additions...

A recent application of enantioselective conjugate radical additions was seen in the synthesis of (+)-ricciocarpins A and B [95]. The key step in the synthesis was an asymmetric addition of a functionalized radical precursor 141 to afford intermediate 142 (Scheme 37). A chiral catalyst screening revealed that Mgt and bisoxazoline ligand 19 was optimal for achieving... 

Scheme 37 Application of enantioselective conjugate radical addition total synthesis of Ricciocarpin A and B...

Scheme 39 Acetate aldols by enantioselective conjugate radical additions... 

Grignard additions, 9, 66 radical addition of zincs, 2, 401 zinc-containing reagent additions, 2, 398 Nitroolefins, enantioselective conjugate additions, 10, 382 (Nitrophenylthio)osmocenes, preparation, 6, 634 (Nitrophenylthio)ruthenocenes, preparation, 6, 634 Nitropyridines, reductive aminocarbonylation, 11, 543 Nitroso aldol reaction... 

After some modifications to the bisoxazoline ligand, a practical and efficient route was now available for catalytic enantioselective conjugate radical additions... 

Success in diastereoselective Lewis acid-mediated conjugate radical additions using chiral oxazolidinones led Sibi and Porter to evaluate enantioselective variants. Based on previous work from their laboratories as well as information in the literature (control of the s-cis vs s-trans rotamer of the enoate), they surmised that a bidentate Lewis acid in combination with an achiral oxazolidinone template and a chiral ligand would be a good starting point to probe enantioselective conjugate radical additions (Scheme 2). 

Previously, every example of enantioselective conjugate radical additions featured asymmetric induction through the application of bidentate bisoxazoline chiral Lewis acids. A recent report, however, illustrates the utility of a new type of chiral bisoxazoline ligand known as DBFOX-Ph (59), which features a furan-containing bridge that offers tridentate chelation of the Lewis acid for asymmetric conjugate radical additions [25]. 

Lewis acid catalyzed enantioselective conjugate free-radical addition/trapping reactions represent an efficient approach to the formation of adjacent chiral centers. 

Sibi and He have described the formation of 8-lactone from an in situ formed 5-hydroxy ester in the synthesis of ricciocarpins A and B [65] (Scheme 25). Enantioselective conjugate addition of the tertiary radical derived from 133 onto... 

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