Umpoled synthon

For a review of such synthetic equivalents see T. A. Hase, Umpoled Synthons, (J. Wiley, New York, 1987). 

Although extensive lists of umpoled synthons may be found in the chemical literature [3], Table 5.2 gives a limited number of the most useful of them. 

Simple reactivity inversion" implies using an umpoled synthon whose origin has, in principle, nothing in common with the synthon with "unnatural" polarity. An example of this type of reactivity inversion is found in one of the possible synthesis of cw-jasmone (3) in which the nitroethane (4) is used as an equivalent of an "acetyl anion" and reacts with an a,P-unsaturated ketone to give the corresponding 1,4-bifunctional system which can then be transformed by a Nef-type reaction into the dissonant 1,4-dicarbonyl system [5]. An intramolecular aldol condensation finally affords the target molecule (Scheme 5.3). 

TABLE 5.2. Umpoled synthons and their equivalent fragments with unnatural... 

The introduction of umpoled synthons 177 into aldehydes or prochiral ketones leads to the formation of a new stereogenic center. In contrast to the pendant of a-bromo-a-lithio alkenes, an efficient chiral a-lithiated vinyl ether has not been developed so far. Nevertheless, substantial diastereoselectivity is observed in the addition of lithiated vinyl ethers to several chiral carbonyl compounds, in particular cyclic ketones. In these cases, stereocontrol is exhibited by the chirality of the aldehyde or ketone in the sense of substrate-induced stereoselectivity. This is illustrated by the reaction of 1-methoxy-l-lithio ethene 56 with estrone methyl ether, which is attacked by the nucleophilic carbenoid exclusively from the a-face —the typical stereochemical outcome of the nucleophilic addition to H-ketosteroids . Representative examples of various acyclic and cyclic a-lithiated vinyl ethers, generated by deprotonation, and their reactions with electrophiles are given in Table 6. 

For a monograph, see Hase Umpoled Synthons Wiley New York. 1987, For a review see Seebach Angew. Chem. Int. Ed. Engl. 1979,18, 239-258 Angew. Chem. 91, 259-278). 

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