Synthetic equivalent groups reagent

Anions of (3-keto esters are said to be synthetically equivalent to the enolates of ketones. The anion of ethyl acetoacetate is synthetically equivalent to the enolate of acetone, for example. The use of synthetically equivalent groups is a common tactic in synthetic organic chemistry. One of the skills that characterize the most creative practitioners of organic synthesis is an ability to recognize situations in which otherwise difficult transfonnations can be achieved through the use of synthetically equivalent reagents. 

Many other examples of synthetic equivalent groups have been developed. For example, in Chapter 6 we discussed the use of diene and dienophiles with masked functionality in the Diels-Alder reaction. It should be recognized that there is no absolute difference between what is termed a reagent and a synthetic equivalent group. For example, we think of potassium cyanide as a reagent, but the cyanide ion is a nucleophilic equivalent of a carboxy group. This reactivity is evident in the classical preparation of carboxylic acids from alkyl halides via nitrile intermediates. 

The trisannulation reagent 7-acetoxy-l,l l-dodecadien-3-one (134) is derived from the bisannulation reagent 124 in four steps. This reagent is a synthetic equivalent of l-dodecene-3,7,11-trione, and the two ketone groups of the trione are masked as an acetoxy and a terminal alkene. The synthesis of optically active D-homo-19-norandrosta-4-en-3-one (135) by the trisannulation reaction... 

A mole of an organometallic first reacts with the enol proton and a second mole (not necessarily the same reagent) adds exclusively to the carbonyl function, giving (3-hydroxydithioesters in good yields. After protection of the OH group the dithioester function can be used for formation of another carbon-carbon bond, and the starting 0-oxodithioesters can be viewed as a3df or synthetic equivalents [342]. 

When pyrimidinone 3 is treated with ethyl 3-oxobutanoate 19a in the presence of ammonium acetate, a different type of TCRT proceeds, giving ethyl 4-aminopyridine-3-carboxylate 41a (Table 10) [59]. In this reaction, pyrimidinone 3 behaves as the synthetic equivalent of activated diformylamine 5, and the amino group at the 4-position is derived from ammonium acetate. Since 3-ethoxycarbonyl-4-pyridone 14a prepared in Sect. 5.2 is intact under the same conditions, aminopyridine 41a is not formed via pyridone 14a. Furthermore, ammonium ion also causes no change on ethyl 3-oxobutanoate 19a, which indicates enamine is not dinucleophilic reagent in the present reaction. Hence, the keto ester moiety is converted to the enaminone after the addition of 19a to pyrimidinone 3. 

Acetoxy-l,ll-dodecadien-3-one (17) is a synthetic equivalent of 1-dodecene-3,7,11-trione (18), and prepared from l,7-octadien-3-one (16), which is obtained from the butadiene telomer 15 (see Section 5.2). The acetoxy group and the double bond in 17 are precursors of two carbonyl groups. The reagent 17 is used for steroid synthesis. 

The use of dimethyl (R)s-2-(10-isobornylsulfinyl)maleate (87) as a chiral synthetic equivalent of dimethyl acetylenedicarboxylate had several limitations arising from (i) the non-trivial preparation of the chiral auxiliary (10-mer-captoisoborneol) required to produce the dienophile, (ii) the low stereoselectivity observed in the synthesis of the thioether used as precursor of the sulfinyl reagent, and (iii) the lack of differentiation of the two ester groups present in the molecule. In order to avoid symmetrization, selective monodemethylation and further re-esterification were required as previous steps of desulfinylation (see... 

In the process illustrated in Figure 10.1, acetate ion is termed a synthetic equivalent of hydroxide ion because the final product is the same as if hydroxide ion were used directly. But the two-step process results in a higher yield of 2-butanol than could be obtained by a direct substitution reaction of 2-bromobutane with hydroxide ion. The use of a carbonyl group to decrease the basicity or nucleophilicity of a reagent in order to... 

Thus the polarization in the carbonyl group, = secures the opportunity to generate enolates from carbonyl compounds and to employ them as nucleophiles in organic syntheses. This very polarization also accounts for the property of carbonyl compounds r R C = 0 to serve as electrophilic reagents, synthetic equivalents to the cation r r C -OH. 

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