Aldol boron-mediated

Other reactions adapted from asymmetric aldol reactions suffer in comparison from the fact that (probably due to the strength of the boron-nitrogen bond) boron-mediated processes generally yield the intermediate 2-halo-3-aminoester products rather than aziridine products directly [51]. 

Paterson, I., Delgado, O., Florence, G.J., Lyothier, I., Scott, J.P., Sereinig, N. (2003) 1,6-Asymmetric Induction in Boron-Mediated Aldol Condensations Application to a Practical Total Synthesis of (-F)-Discodermolide. Organic Letters, 5, 35-38. 

Double asymmetric induction (See section 1.5.3) can also be employed in aldol reactions. When chiral aldehyde 15 is treated with achiral boron-mediated enolate 14, a mixture of diastereomers is obtained in a ratio of 1.75 1. However, when the same aldehyde 15 is allowed to react with enolates derived from Evans auxiliary 8, a syn-aldol product 16 is obtained with very high stereo-... 

The same natural product was synthesized by Paterson et al. [45] who assembled the carbon skeleton of the macrolide from three larger subunits as well. Instead of the Evans-Metternich variant they used their boron-mediated antz-selective aldol strategy which relies as the Evans-Metternich aldol on stereo-induction from the a-chiral center and translates the E-enolate geometry, established due to the use of Cy2BCl, to the anti aldol product (Scheme 33). 

ANTI-SELECTIVE BORON-MEDIATED ASYMMETRIC ALDOL REACTION OF CARBOXYLIC ESTERS SYNTHESIS OF (2S, 3R)-2,4-DIMETHYL-1,3-PENTANEDIOL... 

The present procedure is based on the original report by the author and co-workers, and utilizes the characteristic features of the boron-mediated aldol reaction of carboxylic... 

Table Boron-Mediated Asymmetric Aldol Reactions...

Dialkylboron trifluoromethanesulfonates (triflates) are particularly useful reagents for the preparation of boron enolates from carbonyl compounds, including ketones, thioesters and acyloxazolidinones.4 Recently, the combination of dicylohexylboron trifluoromethanesulfonate and triethylamine was found to effect the enolization of carboxylic esters.5 The boron-mediated asymmetric aldol reaction of carboxylic esters is particularly useful for the construction of anti (3-hydroxy-a-methyl carbonyl units.6 The present procedure is a slight modification of that reported by Brown, et al.2... 

ANTI-SELECTIVE BORON-MEDIATED ASYMMETRIC ALDOL REACTION OF 116... 

Anti-selective Boron-mediated Asymmetric Aldol Reaction of Carboxylic Esters. 

Previous work by our group had demonstrated that high levels of 1,4-syn diastereosclection were possible in boron-mediated aldol reactions of... 

Our persistence with the boron-mediated aldol reaction of 4 and 5 was rewarded when the reaction was conducted without recourse to the usual oxidative workup. Other work conducted by our group had shown that oxidative cleavage of certain aldol borinates under standard conditions (H2O2, pH 7 buffer, H20/Me0H) led to poor yields of the aldol products. In the case of 44, the oxidative step was omitted and the reaction mixture was placed directly on silica gel and then eluted to afford aldol adduct 44 in excellent yield (89%) and with improved diastereoselectivity (90 10 ds) relative to the corresponding lithium conditions.17... 

A new thiol auxiliary (45, R = COEt) participates in boron-mediated anti-aldol reactions with aldehydes with high yield and de.124 Reaction of the product with (g> nucleophiles displaces it (in the form of the thiol, 45 R = H), converting the aldol product under mild conditions into esters, thiolates, phosphonates, alcohols, or acids. 

Boron-mediated ketone-ketone aldol reactions have been described, using boron enolates formed with dicyclohexylboron chloride and triethylamine.124 Following addition of the acceptor ketone to form a boron aldolate, oxidation with peroxide yields the aldol product. 

An effective control of the simple diastereoselectivity in boron-mediated aldol reactions of various propionate esters (162) was achieved by Abiko and coworkers (equation 45) °. They could show that under usual enolization conditions (dialkylboron triflate and amine) enol borinates are formed, which allowed the selective synthesis of 5yw-configured aldol products (Table 11). The enolization at low temperature (—78 °C) generated a (Z)-enolate selectively, which afforded mainly the syn diastereomer 164 after reaction with isobu-tyraldehyde (163), following a Zimmerman-Traxler transition-state. The anti diastereomer 164 instead was obtained only in small amounts (5-20%). 

TABLE 11. Diastereoselective onate esters (equation 45) boron-mediated aldol reactions of propi-... 

Although the asymmetric aldol reaction of benzaldehyde and di ketene has been reported with a catalyst generated from di-iso-propyl tartrate and iso-propanol, low induction and low yields were observed for the d-hydroxyl-y5-keto ester 27 [8], Low induction was also observed for aldol reactions mediated by chiral aluminum catalysts generated from a-amino acids [9]. These types of catalyst have been very successful when employing boron as the Lewis acid, as illustrated in the aldol reaction of ketene acetal 10 with the boron catalyst 31 derived from (5)-valine (Sch. 4) [9,10]. Catalysts derived from A-tosyl-(5 )-valine and Et2AlCl and i-BuyAl were relatively ineffective (< 15 % ee) [9]. 

Stoever, M, Luetzen, A, KoeU, P, New glyco-oxazolidin-2-ones as chiral auxiliaries in boron-mediated asymmetric aldol reactions. Tetrahedron Asymmetry, 11, 371-374, 2000. 

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