T enolate

The rx bromination of carbonyl compounds by Br2 in acetic acid is limited tc aldehydes and ketones because acids, esters, and amides don t enolize to a suffi cient extent. Carboxylic acids, however, can be a brominaled by a mixture of Br and PBr3 in the HeJI-Volhard-Zelinskii (HVZ) reaction. 

The synthesis in Scheme 13.44 is also based on a carbohydrate-derived starting material. It controlled the stereochemistry at C(2) by means of the stereoselectivity of the Ireland-Claisen rearrangement in Step A (see Section 6.4.2.3). The ester enolate was formed under conditions in which the T -enolate is expected to predominate. Heating the resulting silyl enol ether gave a 9 1 preference for the expected stereoisomer. The... 

The mercury-induced cyclization of the (T )-enol ether (E)-5 is less stereoselective and a 70 30 (a// ) diastereomeric mixture, as determined by HPLC, is obtained. This result has been explained as being due to both the structure of the mercurinium ion and the anomeric effect94. 

Notice that in the transition state (1) R chooses to go equatorial (2) the methyl group is for axial because it is cis to OLi (3) the f-butyl group forces the aldehyde to add to the back face of t enolate as drawn. 

Molybdenum and tungsten C-enolates can be generated by reaction of complexes with a-chlorocarbo-nyls (Scheme 67).212-213 These 2-oxaally 1(t) 1 -C-enolate) complexes react with aldehydes in a photoreaction to produce aldol products, by way of the T -enolate. 

The dearest empirical evidence for the productive involvement of an 1] , O-bound nickel enolate comes from the intermolecular reductive coupling of alkynes and enals (Scheme 8.9) [27]. The extremely high levels of Z-isomer stereoselectivity (>98 2) can best be rationalized via the metallacycle intermediate 5 which undergoes o-bond metathesis to afford nickel hydride 6, followed by reductive eUmina-tion to yield the Z-selective enol silane product 7. A mechanism consisting of a nickel Jt-allyl species would not be expected to lead to high selectivities of Z-enol silanes, and has been imphcated in reactions leading to the selective production of T-enol silanes [28],... 

A reverse aldol (formaldehyde T) enol. Arthrobacter simplex Nocardia corallina 260-262 260-262 M-597 M-597... 

NOTE For source ATCC 19-hydroxy-4-androstene-3,17-dione 1 A reverse aldoi (Formaldehyde T enol. M-597... 

Use of an chloromethyloxirane excess for epoxy novolac synthesis increases the conversion of t enolic hydros groups to epo groups and minimizes the reaction of phenolic hydroxy groups with epoxy groups that are attached to the novolac molecules. The chloromethyloxirane excess limits bianehing reactions. Thus, equation 46 is usually driven as close to completion as possible since hydrolyzable and nonhydrolyzaHe, oiganically bounded ehlorine reduces the... 

Edwards, G.G., A.G. Reynolds, A.V. Rodriguez, MJ. Semon, and J.M. Mills. 1999a. Implication of acetic acid in the induction of slow/stuck grape juice fermentations and inhibition of yeast by Lactobacillus sp. Am. T. Enol. Vitic. 50 ... 

The question here is all about control—can we direct the process to give us our target Although we will develop this theme further in Chapter 20, there are some simple paradigms that work. If we condense two species where one cannot enolize, then this principle reduces our number of primary products to two. If we make sure that the species that cannot enolize is the more electrophilic component, or we use it in large excess, so that the enol is likely to meet this rather than the keto form of itself, then we can get successful reactions. An example is provided by the reaction of 4-chlorobenzaldehyde (can t enolize) with acetophenone... 

With simpler, less stabilized, enols and enolates, the same selectivity issues arise in this addition process as we saw for aldol and related reactions—these reactions are related to the aldols, but a double bond simply intervenes as an electron relay. Consider the example shown in Figure 17.68. We have chosen a nucleophile that can enolize readily and an a,p-unsaturated aldehyde that can t enolize. The enal is more electrophilic than the ketone, so self-condensation of the ketone is not... 

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