Umpolung synthesis

Scheidt reports using an amended Rokita method in conjunction with an umpolung derivative of several aryl aldehydes for the synthesis of a-aryl ketones.7 The procedure is indeed useful for the synthesis of 2-arylated benzofurans as shown by the synthesis of demethylmoracin I (Fig. 4.8). 

Organic electrochemistry fulfills favorably the requirements defined for a good synthesis (see Sect. 3.2). The Umpolung of reactivity by FT saves steps because it allows a one-step coupling of two donors or two acceptors. In comparable chemical conversions, two or more steps are necessary for that purpose. The selectivity... 

Umpolung The reversal of polarity of the carbonyl carbon atom is termed umpolung (German for polarity reversal). Normally the carbonyl carbon atom of an aldehyde (or a ketone) is partially positive i.e., electrophilic and therefore it reacts with nucleophiles. When the aldehyde is converted to a dithiane by reaction with 1,3-propanedithiol and reacted with butyl lithium the same carbon now becomes negatively charged to react with electrophiles. This reversed polarity of the carbonyl carbon is termed umpolung which increases the versatility of the carbonyl group in synthesis. The sulphur atoms stabilize... 

Reactions of the Nitroso Group. Up until recently, the chemistry of N-nitrosamines was restricted to the reaction involving the nitroso group. It was not until the usefulness of N-nitros-amines in synthesis [concept of "Umpolung (3.)] was demonstrated, that the long-known denitrosation of N-nitrosamines became impor-... 

Ultra-violet absorption, 13, 132, 218 Umpolung, 211 Unsaturated acids afi-, from Perkin reaction, 227 j8y-, synthesis, 234... 

This is another example of umpolung (see 0-95) 1478 the normally electrophilic carbon of the aldehyde is made to behave as a nucleophile. The reaction can be applied to the unsubstituted dithiane (R = H) and one or two alkyl groups can be introduced, so a wide variety of aldehydes and ketones can be made starting with formaldehyde.1504 R may be primary or secondary alkyl or benzylic. Iodides give the best results. The reaction has been used to close rings.1505 A similar synthesis of aldehydes can be performed starting with ethyl ethyl-thiomethyl sulfoxide EtSOCH2SEt.1506... 

The synthesis of C-glycosyl compounds, commonly known as C-glycosides, in ionic reactions relies on the electrophilicity of the anomeric center and, therefore, involves the attack of an appropriate C-nucleophile. An umpolung method has been developed, and is described in the previous chapter. But instead of going from a carbocation to a carbanion, one can also consider homolytic or radical reactions to reverse the philicity (Scheme 1). 

An enantioselective approach to cytotoxic nor-calamenenes via electron-transfer-driven benzylic umpolung of an arene tricarbonyl chromium complex. Synthesis 2003, 1851-1855. 

R ac.tL jity umpoiung and 6ynthztic e.quivaJte.ntA are very fruitful concepts in modern organic synthesis, in particular for the multistep preparation of natural compounds. In this area, organo-phosphorus compounds can be useful tools, particularly for reversible umpolung by heteroatom exchange (.1.-4). 

AFP. See Antifreeze proteins (AFPs) Aglycones and aglycone reactions. See Natural C-glycosides Ramberg-Backlund reaction Reverse polarity (umpolung) approach to aryl C-glycoside synthesis Sialyl Lewis X (sLex)... 

In the oxidative or reductive versions of the reaction types listed above the [nucleophile, electrophile] combination is not allowed instead we use [nucleophile, nucleophile] combinations oxidatively or [electrophile, electrophile] combinations reductively [R—E, Nu-], [C=C( J-), Nu-], [R—Nu, E+] and [C=C(<5+), E+]. This principle (Umpolung see Section 1) vastly increases the number of choices available for synthetic schemes, and particularly allows for short cuts in conventional ones. Thus, the synthesis of ArOH or ArCN from ArH normally requires four steps (18), whereas the... 

You have in fact met one example of each of the unnatural synthons with a2 and d1 reactivity. Such synthons are given the German name Umpolung, meaning inverse polarity because their natural reactivity is reversed, and umpolung reagents are the key to the synthesis of 1,2- and 1,4-difunc-tionalized compounds. 

---