Enantioselective addition to aldehydes

Chiral reagents for enantioselective addition to aldehydes. Seebach s group2 has prepared a series of chiral reagents of the general type RTi(OR )3 and ArTKOR ), in which R is derived from (-)-menthol, quinine, cinchonine, and (S)-1,T-... 

In other diethylzinc studies, a neural network modelling approach has been used to predict the utility of new enantioselective catalysts,222 norephedrine-derived ligands with three stereogenic centres catalyse enantioselective addition to aldehydes and to chalcones,223 and a chiral sulfonamide ligand based on tartaric acid gives good ees in addition to both aldehydes and ketones.224... 

Enantioselective Addition of Dialkylmagnesium to Aldehydes. Amino alcohol (1) is a chiral ligand for dialkylmagnesium reagents in the enantioselective addition to aldehydes. Reaction... 

The use of these boryl complexes in catalytic, enantioselective additions to aldehydes by silyl ketene acetals has also been the subject of intense investigation by Yamamoto (Eq. 30) [108]. Although ethyl and benzyl acetate-derived enol silanes furnished racemic products, the phenyl acetate-derived trimethylsilyl ketene acetals proved optimal, giving adducts in up to 84% ee. Additionally, Yamamoto has documented the use of 184 in aldol addition reactions of propionate- and isobutyrate-derived enol silanes (Eqs. 31 and 32). Thus, the addition of the phenyl acetate derived (E)-enol silane afforded adducts as diastereomeric mixtures with the syn stereoisomer displaying up to 97% ee (Eq. 32). 

Other approaches to trisubstituted aikenes via organoboranes involve alkynyltrialkyl borates, alkenyltrialkyl borates or the cross-coupling reaction of alkenylboronic acids with alkyl halides. Both ( )- and (2)-l,2-disubstituted alkenyl bromides can also be prepared by the methodology shown in eq 7. The boron trifiuoride etherate-mediated 1,4-addition of 1,2-disubstituted alkenylboronates affords y,5-unsaturated ketones (eq 9). The boronates can also be converted into chiral enoiborates for the enantioselective addition to aldehydes. ... 

Trialkylaluminum compounds are available in bulk quantities and would therefore be valuable alkylating reagents for the enantioselective addition to aldehydes. Several successful attempts along these lines have been reported [52]. In most cases, an excessive amount of Ti(O Pr)4 and use of tetrahydrofuran (THF) as the solvent are necessary in order to get satisfactory results of the catalysis. Similar... 

Other enantiomerically pure B-allylboranes also show excellent stereoselectivity in these reactions." Allyl and 2-butenyl derivatives of the cyclic boronate ester 3, derived from tartaric acid, also give enantioselective additions to aldehydes. ... 

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