From aldol reaction

The reaction conditions needed for aldol dehydration are often only a bit more vigorous (slightly higher temperature, for instance) than the conditions needed for the aldol formation itself. As a result, conjugated enones are usually obtained directly from aldol reactions without isolating the intermediate jS-hydroxy carbonyl compounds. 

Hydroxy-substituted iron-acyl complexes 1, which are derived from aldol reactions of iron-acyl enolates with carbonyl compounds, are readily converted to the corresponding /i-methoxy or /1-acetoxy complexes 2 on deprotonation and reaction of the resulting alkoxide with iodomethane or acetic anhydride (Tabic 1). Further exposure of these materials to base promotes elimination of methoxide or acetate to provide the a,/ -unsaturated complexes (E)-3 and (Z)-3 (Table 2). 

C-H insertion a to oxygen results in the formation of /3-hydroxyester derivatives that are generally prepared from aldol reactions (Figure 3). For the C-H insertion chemistry to be a viable surrogate of the aldol reaction, the reaction would need to be highly diastereoselective and enantioselective. 

The addition prodnct from aldol reactions frequently dehydrates by heating in acid or in base... 

Considerable efforts have been directed towards the development of a system to maximize enantiomerically pure diastereomers from aldol reactions. The most effective systems... 

The (0)-reagent (91) is formed almost exclusively from S-3-(3-ethyl)pentyl propanethioate (92) and borolanyl triflate (90). Despite its apparent steric demand, (91) still retains a high degree of reactivity towards aldehydes (in Scheme 21 the interaction between R and Me is not exceedingly large). Summarized in Table 6 are the results obtained from aldol reactions of representative aldehydes with (91 Scheme 38). All reactions proceed smoothly at -78 °C and the major products have the 2,3-anti stereochemistry (antiisyn > 30 1). With (2 ,55)-(90) the aldehydes examined provide, in most cases, the (2R) aldol products with more than 98% ee. It is important to note that the external chiral moiety can be recovered as its 2,2-dimethylaminoethanol complex, and that the aldol products are equipped with a versatile thioate functionality for further synthetic transformation. 

Beside being used to form a variety of compounds (some of which are listed in the experimental section), the aldol reaction has been used to synthesize the following antibiotic macrolides, especially for the macrolide acutiphycin, which is primarily prepared by five steps of consecutive aldol reactions. Among the listed structures, the bonds arising from aldol reactions are labeled with many lines. 

Scheme 6.76). Heathcock developed a synthetic strategy wherein the 1,2-stereoselection obtainable from aldol reaction is parlayed by a subsequent Claisen rearrangement into 1,4 and 1,5-stereoselection [118]. Claisen-Johnson, Claisen-Eschenmoser and Claisen-Ireland rearrangements were examined in this study. However the orthoester Claisen-Johnson rearrangement was not the most convenient in this case. The rather harsh reaction conditions often led to dehydration and other byproducts. An example of this rearrangement is given in Scheme 6.76. 

A) The option to reductively dehalogenate chiral a-halo-/3-hydroxy imides, resulting from aldol reactions with chiral bromoacetates, offers an altemative route to chiral... 

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