Enol ethers restricted rotation

In the monoanionic enolates from the ethers and other 2-substituted 1-acylpyrrolidines, chelation should be much weaker or absent, leading to less restricted rotation of the enolate moiety. Furthermore, rotation of the pyrrolidine substituent and conformational mobility of the pyrrolidine ring can lead to inversion of the nitrogen lone pair. These factors may explain the reversed and lower diastereoselectivity observed with these substrates. 

The structure and chiral properties of the enolate intermediate were then investigated. Treatment of 40 with KHMDS (1.1 equiv) in toluene-THF (4 1) at —78°C for 30 minutes followed by t-butyldimethylsilyl (TBS) triflate gave Z-enol silyl ether 54 and its -isomer 55 in respective isolated yields of 57% and 27%.30 In the lH NMR spectra of both 54 and 55, methylene protons of the MOM groups appeared as AB quartets, which indicates restricted rotation of the C(l)-N bonds. The rotational barrier of the C(l)-N bond of the major Z-isomer 54 was determined to be 16.8 kcal/mol at 92°C by variable-temperature NMR measurements in toluene- (400 MHz 1 H... 

The other systems, esters 2.71, enamines 2.72, and enol ethers 2.73, similarly have restricted rotation about the bond drawn as a single bond, but the barrier is... 

The conveyor of chiral memory in the reaction turned out to be restricted rotation in the enolate intermediate, and Fuji was able to show that the enol ether 3, formed as a by-product in the reaction, was chiral because of restriction to rotation about the arrowed bond, and could be recovered enantiomerically enriched. The enantiomeric excess of 3 decayed with a half-life of about 1 h at ambient temperature, and the enantiomers of 3 are atropisomers [2]. 

Reactions like these, in which stereoselectivity is the consequence of steric hindrance to bond rotation, are most well known among the biaryls, and derivatives of binaphthyl have provided chemists with a valuable range of chiral ligands [4-6]. But the biaryls are only a small subset of axially chiral compounds containing two trigonal centres linked by a rotationally restricted single bond. Many others are known, some with much greater barriers to rotation than Fuji s enol ether [7]. Yet until quite recently there were no reports of reactions in which nonbiaryl atropisomers were the source, conveyor, or product of asymmetric induction. 

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