Chiral acetates, aldol stereochemistry

A similar case of enolatc-controlled stereochemistry is found in aldol additions of the chiral acetate 2-hydroxy-2.2-triphenylethyl acetate (HYTRA) when both enantiomers of double deprotonated (R)- and (S)-HYTRA are combined with an enantiomerically pure aldehyde, e.g., (7 )-3-benzyloxybutanal. As in the case of achiral aldehydes, the deprotonated (tf)-HYTRA also attacks (independent of the chirality of the substrate) mainly from the /te-side to give predominantly the t/nii-carboxylic acid after hydrolysis. On the other hand, the (S)-reagcnt attacks the (/ )-aldebyde preferably from the. S7-side to give. s wz-carboxylic acids with comparable selectivity 6... 

Another chiral auxiliary for controlling the absolute stereochemistry in Mukaiyama aldol reactions of chiral silyl ketene acetals has been derived from TV-methyl ephedrine.18 This has been successfully applied to the enantioselec-tive synthesis of various natural products19 such as a-methyl-/ -hydroxy esters (ee 91-94%),18,20 a-methyl-/Miydroxy aldehydes (91% ee),21 a-hydrazino and a-amino acids (78-91% ee),22 a-methyl-d-oxoesters (72-75% ee),20b cis- and trans-l1-lactams (70-96% ee),23 and carbapenem antibiotics.24... 

Under either the catalytic (eq 1) or the stoichiometric conditions (eq 2), the reagent undergoes addition to chiral aldehydes with complete reagent control , i.e. the stereochemistry of the aldol reaction is totally controlled by the chiral catalyst regardless of the inherent diastereofacial preference of the chiral aldehydes (eq 4). Titanium(IV) chloride and tm(TV) chloride mediate the addition of the title reagent to chiral a-alkoxy aldehydes and -alkoxy aldehydes with complete chelation control (eq 5), whereas the corresponding silyl ketene acetal is unselective. 4... 

Although a majority of the catalytic complexes employed in the aldol reaction are bidentate, Carreira and coworkers published the synthesis of a new chiral tridentate chelating ligand for the efficient asymmetric induction of stereochemistry in aldol adducts. The Ti(IV) complex 68, an analog of the BINOL catalyst previously mentioned, was further stabilized by 3,5-di-tert-butylsalicyclic acid as a counterion to increase the yields, selectivity, and efficiency of the asymmetric reaction. This new catalyst is particularly effective in the addition of either O-trimethylsilyl, or O-ethyl, or O-methyl ketene to both aliphatic and aromatic aldehydes enantioselectively to obtain the respective aldol adduct. For example, the reaction of the silylketene acetal 90 with the aromatic aldehyde 89 in the presence of 68 obtains the aldol adduct 91 in high yield (91%) and excellent enantioselectivity (97% ee). 

We decided to investigate the E wis aldol reaction (54,55) of the chiral a-hydroxy acetic anion equivalent 47 with the A -Cbz imine of trifluoropyruvate 33. Screening of several enolates and reaction conditions revealed that titanium enolate of the oxazolidinone 47, prepared with 1 equiv. ofTiCU and l.l equiv. of/-Pr2NEt, reacts with excellent stereocontrol affording the "Evans" syn diastereomer 48, in 88% isolated yield, having the correct stereochemistry to be used as intermediate for the synthesis of the targeted 2-Tfm-sphingolipids. 

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