Diazaborolidines, chiral

The possibility of using chiral auxiliaries or chiral catalysts to achieve enantio-selective Claisen rearrangements has been explored.253 One approach is to use chiral boron enolates. For example, enolates prepared with the chiral diazaborolidine bromide O lead to rearranged products of more than 95% enantiomeric excess.254... 

Entry 6 is an example of application of the chiral diazaborolidine enolate method (see p. 572). Entry 7 involves generation of the silyl ketene acetal by silylation after conjugate addition of the enolate of 3-methylbutanoyloxazolidinone to allyl 3,3,3-trifluoroprop-2-enoate. A palladium catalyst improved the yield in the rearrangement... 

Chiral diazaborolidines catalysts in asymmetric reductions have been less described than the corresponding oxaza-borolidines. Although not isolated, the formation of compound 47 has been characterized by nB NMR spectroscopy with the detection of a signal at 24 ppm (from BF3.Et20 as an external standard) <2000TA4329>. 

Asymmetric synthesis of P-amino acid esters. The chiral diazaborolidine 1 (16,155) also effects diastereo- and enantioselective reactions of (S)-r-butyl thiopro-ponoate (2) with N-benzyl or N-allyl aldimines 3 to form j3-amino acid esters 4, precursors to chiral irons-pAactams (5) in 90-99% ee. 

Results of the asymmetric 2-propenylborations of several chiral a- and /i-alkoxy aldehydes are presented in Table 11 74a-82 84. These data show that diisopinocampheyl(2-propenyl)borane A and l,3-bis(4-methylphenylsulfonyl)-4,5-diphenyl-2-propenyl-l,3,2-diazaborolidine C exhibit excellent diastereoselectivity in reactions with chiral aldehydes. These results are in complete agreement with the enantioselectivity of these reagents in reactions with achiral aldehydes (Section 1.3.3.3.3.1.4.). In contrast, however, the enantioselectivity of reactions of the tartrate 2-propenylboronate B (and to a lesser extent the tartrate (/i)-2-butenylhoronate)53b is highly... 

Several chiral heterocyclic borylating agents have been found useful for enantio-selective aldol additions. The diazaborolidine 14 is an example.136... 

Rearrangements. By applying Corey s diazaborolidine catalyst to aromatic Claisen rearrangement, a chiral side chain is introduced into the ortho position of catechols. Highly enantioselective rearrangement of 0-allylimidates 111 containing a methoxybinaphthyl substituent has been witnessed. ... 

Diazaborolidines that are structurally analogous to oxazaborolidine were prepared from a chiral diamine and borane [109]. Their catalytic activity is similar to that of oxazaborolidine. iV-Sulfonyldiazaborolidine derived from 2-amino-methylpiperidine was used in the reduction of acetophenone (>95% 72% ee)... 

Corey and Kania reported an enantioselective Claisen rearrangement reaction of macrocychc lactone for the synthesis of (-l-)-dollabellatrienone (56) [13], Reaction of the lactone 53 with chiral (5,5)-diazaborolidine L2BBr 54 and Barton s base (6) resulted in Claisen rearrangement to give carboxylic acid 55 in 86% yield with >98% ee (diastereos-electivity >98 2) (Scheme 7.10). In this reaction, rapid deprotonation by sterically hindered guanidine base is the key to suppress side reactions. 

General. Chiral 2-bromo-l,3-bis(4-methylphenyl sulfonyl)-4,5-diphenyl- 1,3,2-diazaborolidine (1) is used to control the stereochemistry of enantioselective aromatic Claisen rearrangements, allylations of aldehydes, aldol reactions, and formation of chiral propa-l,2-dienyl and propargyl alcohols. Included is the discussion of both the R,R) and the (5,5) chiral controllers. 

Remarkably, the induced stereoselectivity of enolates 303 and 306 is opposite despite the homochiral diazaborolidine skeleton cis-enolate 303 attacks predominantly from the Re-face to the aldehyde, whereas traws-enolate 306 approaches from the Si-face. The opposite stereochemical outcome was explained by Zimmerman—Traxler-like transition state models 308 and 309, respectively. It was assumed that transition state 308 is favored because it avoids repulsion between the phenylthio and the arylsulfonyl group, whereas 309 prevents steric hindrance between the arylsulfonyl moiety and the aldehyde (Scheme 4.68) [151d]. The chiral controller group 300 was also applied to acetates and thioacetates, but the reactions were found to be plagued by distinctly lower enantioselectivity of 52-80% ee with benzaldehyde - another example of the problematic asymmetric acetate aldol addition. 

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