Crystal boron enolate

Oppolzer s auxiliary opened, in addition, an access to a/iti-configured aldol adducts 272 (Scheme 4.62). For this purpose, silyl ketene N,0-acetal 271 was prepared from propionic sultam 92, obtained as a single diastereomer, according to the NMR spectra of the crude product, and isolated as a crystalline compound it was characterized as a cis-silicon enolate by a crystal structure analysis. For the subsequent Mukaiyama aldol addition, titanium tetrachloride was found to be the optimum Lewis acid to yield the awti-diastereomers 272 in excellent diastereoselectivity. Their formation under attack of the enolate to the Re-face of the aldehyde is consistent with an open transition state 275, wherein the Lewis acid-coordinated aldehyde is located on the face opposite to the sulfonyl group (Scheme 4.62) [136b]. An alternative approach to the a fi-aldol adducts was also elaborated, based upon cA-boron enolates 267 when they are reacted with aldehydes in the presence of titanium tetrachloride, an ti-selective aldol addition occurs leading to the products 272 rather than to sy -aldols 268 that result in the absence of the Lewis acid [136c]. 

Treatment of chromium (III) acetylacetonate with acetic anhydride and boron trifluoride etherate yielded a complex mixture of acetylated chelates but very little starting material. Fractional crystallization and chromatographic purification of this mixture afforded the triacetylated chromium chelate (XVI), which was also prepared from pure triacetylmethane by a nonaqueous chelation reaction (8, 11). The enolic triacetylmethane was prepared by treating acetylacetone with ketene. The sharp contrast between the chemical properties of the coordinated and uncoordinated ligand is illustrated by the fact that chromium acetylacetonate does not react with ketene. 

Oppolzeds sultams 1.133 are also efficient auxiliaries in asymmetric aldol reactions [209,404,407,457,1271], Boron, titanium or Sn (IV) enolates of W-pro-pionoylsultams lead stereoselectively to either enantiomeric syn aldol at -78°C. These products are easily purified by fractional crystallization (Figure 6.83). After treatment with Li0H/H202 and CH2N2, syw-P-hydroxyesters are obtained with an excellent enantiomeric excess. The drawback of this method is the need to use an excess of aldehyde to obtain good chemical yields. As in the case of oxazolidi-... 

Table 4. It is interesting to note that the tin enolate corresponding to (57), upon reaction with aldehydes, also provides syn aldol products (60), diastereomeric to (59), with high diastereoselection. This opposite sense of asymmetric induction is believed to be due to coordination of the sultam oxygen to the metal (Sn) in the transition state, which is absent in the boron counterpart. Notably, the products can be easily purified by flash chromatography and/or crystallized to nearly perfect (>99% de) diastereomeric purity.

In the presence of boron trifluoride etherate, 39 (R=H and Ar==C6H5) reacts smoothly with silyl enol ethers (the achiral anion source) at low temperatures ( — 80 to — 30 °C) to provide a chromatographically separable mixture of diastereomeric acids from which either 40 or 41 can be obtained in good yield. Oxidative decarboxylation of the chiral auxiliary with freshly crystallized lead tetraacetate occurs without racemization of the newly formed chiral center to provide either 42 or 43 with 98% ee. Noteworthy is the fact that if pure cis isomers are used, such as 39, they undergo facile isomerization to a 65 35 cis trans mixture at the low reaction... 

After almost half century of intensive, fundamental, and fruitful investigations of enolate structures, there is now clear evidence indicating that enolates of groups 1, 2, and 13 metals - lithium and boron being the most relevant ones - exist as the O-bound tautomers 1 the same holds in general for silicon, tin, titanium, and zirconium enolates [4]. Numerous crystal structure analyses and spectroscopic data confirmed type metalla tautomer 1 to be the rule for enolates of the alkali metals, magnesium, boron, and silicon [5]. 

The few crystal structures obtained from aluminum enolates that are less important in synthesis than their boron counterparts reveal dimeric aggregates (Scheme 3.5). This maybe illustrated by the enolates 15 [43a], obtained fromiV,Af-dimethyl methyl glycinate through transmetallation of the lithium enolate, and 16 [43b] that was prepared by direct enolization of 2,4,6-trimethylacetophenone and trimethylaluminum. Both dimers feature an AI2O2 core unit and clearly demonstrate the O-bound character of aluminum enolates. 

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