Enamines iminoesters

In particular, defluorination brought about by the Mg-TMSCl-DMF system is reliable for the purpose [11]. Difluoroenol silyl ethers [12], enamines [13], phenyl-1,1-difluoroacetate [14], difluorinated Danishefsky diene [15], and trimethylsilyldifluoromethyl benzenes [16] can be prepared in good to excellent yields. The one-pot synthesis of 22 from 21 is of great interest. Defluoro-dechlorinative double silylation occurs under very mild conditions to afford compound 22, a useful synthetic intermediate, in which two different silyl groups can be replaced with various electrophiles in a stepwise manner [17]. Highly functionalized a-iminoesters (27) can be synthesized via 25 and 26 in excellent yields as shown in Scheme 2.15. 

Stork and coworkers [624e] have introduced enamines as a nucleophilic substitute of enols, and a few asymmetric aldol reactions have been performed with enamines. Scolastico and coworkers [1311] have reacted morpholine enamines with chiral oxazolidine 1.84 (EWG = Ts), and in some cases they obtained higher sdectivities than those obtained from enoxysilanes ( 6.9.3) (Figure 6.102). Chiral enamines derived from pyrrolidine 1.64 (R = MeOCI ) react with acyliminoesters of chiral alcohols at -100°C [1313], Double diastereodifferentiation is at work so that from matched reagents, for example the pyrrolidine enamine and iminoester 6.126 shown in Figure 6.102, P-keto-a-aminoesters are obtained with a high diastereo- and enantioselectivity. The esters of either enantiomer of menthol or of achiral alcohols give mediocre asymmetric induction. 

T riethylamine Enamines from iminoesters Cyclic compds. 

Semicorrins have been previously prepared as intermediates in the synthesis of corrinoid and hydroporphinoid compounds [12,13]. The classic route by iminoester-enamine condensation, devisal by Eschenmoser [12], is ideally suited for preparing chiral C2-symmetric semicorrins 1 (Scheme 1). Starting either from L-pyroglutamic acid (-)-3 or from the D-enantiomer, which are both commercially available at moderate prices, the crystalline, enantiomerically pure (S,Sy and (/ ,/ )-diesters (-)-la and (+)-la are readily synthesized in multigram quantities with an overall yield of 30-40 % (Scheme 2) [8,14]. The diesters can be converted to a variety of differently substituted semicorrins by selective trans-... 

The acidic NH proton of the sulfonamide group in chiral organocatalyst 41 turned out to be of key importance for both aspects of selectivity, an anti/syn ratio >20 1 and e.e. >99 %. The mechanism of this reaction comprises an intermediary formation of enamine between the aldehydes and chiral catalyst and protonation of a-iminoester. In the transition state two reactants are oriented in parallel planes affording anri-isomers of TM 4.18a-e in (25,37 )-absolute configuration (Fig. 4.6). 

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