Acceptor synthons

The formal carbanions and carbocations used as units in synthesis are called donor synthons and acceptor synthons. They are derived from reagents with functional groups. 

CH methyl acceptor synthon (from the reagent CHjI)... 

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

All odd numbered acceptor synthons (such as a1 and a3) and even numbered donor synthons (such as d2 and d4) have unnatural polarity. 

You ve now met a variety of synthons and it s useful to be able to classify them as donor or acceptor synthons, We call a negatively polarized synthon a donor synthon and give it the symbol d . Positively polarized synthons are called acceptor synthons and are given the symbol a . 

Notice that the acceptor synthons have odd numbers the donor synthon has an even number donor and acceptor properties alternate along the chain as we move away from a carbonyl group. This natural reactivity5 of carbonyl compounds explains why we find it easy to discuss ways of making 1,3- and 1,5-difunctionalized compounds, because they arise from a1 + d2 and from a3 + d2. Reagents corresponding to synthons like d1 or a2 are rarer, and therefore compounds with 1,2- or 1,4- related functional groups require special consideration retrosynthetically,... 

Heterolytic retrosynthetic disconnection of a carbon-carbon bond in a molecule breaks the TM into an acceptor synthon, a carbocation, and a donor synthon, a carbanion. In a formal sense, the reverse reaction — the formation of a C-C bond — then involves the union of an electrophilic acceptor synthon and a nucleophilic donor synthon. Tables 1.1 and 1.2 show some important acceptor and donor synthons and their synthetic equivalents. "... 

The dissonant charge pattern for 2,5-hexanedione exhibits a positive (-1-) polarity at one of the a-carbons, as indicated in the acceptor synthon above. Thus, the a-carbon in this synthon requires an inversion of polarity Umpolung in German) from the negative (-) polarity normally associated with a ketone a-carbon. An appropriate substrate (SE) for the acceptor synthon is the electrophilic a-bromo ketone. It should be noted that an enolate ion might act as a base, resulting in deprotonation of an a-halo ketone, a reaction that could lead to the formation of an epoxy ketone Darzens condensation). To circumvent this problem, a weakly basic enamine is used instead of the enolate. 

In the case of 5-hydroxy-2-hexanone shown below, Umpolung of the polarity in the acceptor synthon is accomplished by using the electrophilic epoxide as the corresponding SE. 

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