Aryl iminoesters

Cossio and Linden et al. studied the reaction of aryl iminoester-derived azomethine ylides 65 with 2- and 1,2-disubstituted nitroalkenes 66 using silver acetate and triethylamine in acetonitrile (Scheme 2.18).35 The product 3-nitropyrrolidines were formed in yields of 24—69% with excellent regioselectivity. These findings mirrored earlier work by Toke et al.36 Unfortunately, diastereoselectivity was generally in the 1 1—4 region. Only in two cases where yields were low were the products formed in a 2 98 ratio. Lithium perchlorate was able to catalyze the reaction as well with similar yields. Remarkably, the diastereoselectivity with lithium perchlorate in some cases resembled that obtained with silver acetate and in others was reversed. Cossio also reported the stereoselective reaction of phenyl isocyanates with azomethine ylides 65 under the same conditions.37 These reactants combined to give the expected products as single stereoisomer in 30-50% yield. 

While virtually all of the research described above has focused on the inter-molecular cycloaddition of azomethine ylides, the intramolecular process holds considerable promise for the synthesis of polycyclic natural products. The Pfaltz group reported an intramolecular catalytic asymmetric cyclization of aryl iminoesters 112 using a complex of silver acetate with PHOX type ligand 100 (Scheme 2.29,... 

Najera reported that silver perchlorate forms a catalytically active complex with (5)-BINAP (87).55 This complex efficiently catalyzes the reaction of aryl iminoesters 108 with /V-methylmaleimide 114 in the presence of the external base triethylamine (Scheme 2.30, Table 2.8). High enantioselectivity is obtained in this reaction. As a consequence of the low solubility of the silver BINAP complex in the reaction medium toluene, it can be quantitatively recovered from the reaction mixture by simple filtration. The recovered catalyst could be recycled at least 4 times with no apparent loss in activity. Carretero reported that /V-phenylmaleimide also reacted with... 

Najera et al. have shown that Feringa s phosphoramidite ligand 102 is superior to other ligands in the cycloaddition of aryl iminoesters 62 with tert-butyl acrylate (106), /V-methylphthalimide 114, and diisopropyl fumarate (118).57 As shown in Scheme 2.31 and Table 2.9, the products 116 and 117 were formed in excellent yields and exquisite endo selectivity. 

While significant progress has been achieved in catalytic asymmetric cycloaddition of aryl iminoesters with dipolarophiles, the field is still in its infancy. Notably,... 

Bashiardes studied the reaction of sugar-derived chiral nonracemic pyran 151 with aryl iminoesters 150 under Grigg s conditions (AgOAc, DBU, MeCN).68 The products 152 were formed in 59-66% yield as single diastereoisomers (Scheme 2.41). 

Thermal reactions of N-aryl cyclopropenone imines 268 are differentiated by the nature of the N-aryl substituent. Imines 268 (Ar = phenyl, p-nitro-phenyl) undergo isomerization to N-aryl-2-phenyl-indenone imines 271 when heated in aprotic solvents202. Since in protic solvents, e.g. ethanol, only the iminoester 272 is isolated, evidence seems to be given for the intermediacy of 269 implying carbene and ketene imine functionality, which may either cause electrophilic ring closure with a phenyl group to form 271 or may add to the hydroxylic solvent (272). 

Conceptually this new three-component reaction resembles the methods applied by Steglich, Enders et al. [8a] and Yamamoto et al. [8b], who treated the preformed iminoesters 26 and 29 at low temperature with the electron-rich olefins 27 and 30, respectively [9]. However, the use of boronic acid derivatives appears to be a versatile method with a great deal of potential. As Petasis and Zavialov note at the end of their paper, unprotected amino acids and peptides, aryl boronic acids, and chiral boronates can also participate in their three component condensation [7a]. Transition metal catalysis also... 

Hoveyda et al. [262] prepared different N-aryhnaleimidobenzoic acids linked to SASRIN resin, whose double bond present in the maleimido moiety could act as a convenient dipolarophile in cycloaddition reactions. Thus, solution-generated a-iminoesters (from different aromatic aldehydes and aminoesters) were reacted vdth the supported maleimides (158) under Tsuge [263] conditions. Formation of the expected syn-endo cycloadduct (160) was observed after only 1 h at room temperature (Scheme 33). From structure-reactivity analysis, the authors concluded that the cycloaddition reaction is more sensitive to steric then to electronic factors on the azomethine yhde counterpart. The advantage of this procedure stems essentially from the fact that the iminoesters (159) are formed in situ. Aldehydes containing a-hydrogens could also be employed. Moreover, the resin in this case also plays the role of a protective group, because, in contrast with N-alkyl and N-aryl (see above) maleimides, N-unsubstituted maleimide is not suitable for 1,3 dipolar cycloadditions. 

The Pd-catalysed cross-coupling ethoxycarbonylation of aryl boronic acids with A-aryl-a-iminoesters has been shown to give aryl carboxylic esters via carbonyl-imino a bond cleavage and involve a 1,2-aryl shift (Scheme 138). ... 

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