Enolate chelate

Asymmetric aikyiation of imide etiolates.1 The sodium enolates of 3 and 7 are alkylated with marked but opposite diastereoselectivity by alkyl halides. The selectivity is improved by an increase in the size of the electrophile, with methylation being the least stereoselective process. The asymmetric induction results from formation of (Z)-enolates (chelation) with the diastereoselectivity determined by the chirality of the C4-substituent on the oxazolidone ring (equations I and II). The products can be hydrolyzed to the free carboxylic acids or reduced by LiAlH4 to the corresponding primary alcohols and the unreduced oxazolidone (1 or 2). 

Again Shapet ko s interpretation is that electron redistribution in the enol chelate is the sole influence in operation and that there is no tautomeric equilibrium between the two cis enol species. Since this conflicts with other evidence that the OHO hydrogen bonding is non-centred, all that can be said is that Shapet ko s substituent correlations show a type of hydrogen bond that behaves for NMR purposes as if it were centred. Shapet ko uses the term quasisymmetrical when he uses another method of investigating these /3-diketones. 

TCs are well documented to bind various metal ions, including alkaline earth metals, Al(ni) and transition metals VO(II), Cr(III), Mn(II), Fe(II/III), Co(II), Ni(II), Cu(II) and Zn(II) . TC can form 2 1 TC-metal complexes with transition metal ions in non-aqueous solution, in which the metal is bound at the 2-amido and 3-enolate chelating sites . TCs are present in plasma mainly in the Ca(II)-bound form or Mg(n)-bound form to a lesser extent, when they are not bound to proteins such as serum albumin. Thus, the bioavailability of TCs should be dependent upon the physical and biochemical properties of their metal complexes instead of their metal-free form. 

The chiral P-amino thiol ester (198) gives a lithium enolate that shows excellent diastereofacial preference in its reactions with a,p-unsaturated and aryl aldehydes (equation 119 Table 22). The stereochemistry of the major isomer (199) is consistent with attack of the aldehyde on a relatively rigid enolate chelate. 

Curcumin (31, the active principle in turmeric, is used both for its colour and flavour. It exists as an intramolecular keto-enol chelate containing a six-membered ring. 

---