Umpolung (German: [ˈʔʊmˌpoːlʊŋ]) or polarity inversion in organic chemistry is the chemical modification of a functional group with the aim of the reversal of polarity of that group. This modification allows secondary reactions of this functional group that would otherwise not be possible.

Among the known methods of removal of thioacetal protecting groups2, the one using cerium (IV) ammonium nitrate (CAN) has the advantages of a fast reaction with easy work-up and good to very good yields3. This method has been widely applied for preparative purposes using four equivalents of the cerium salt.

However since sulfur is more readily polarisable than oxygen, it can more efficiently stabilise proximal negative charge by setting up induced dipoles and through pπ-dπ bonding. As a result, the α-C−H in a dithioacetal is much more acidic (pKa(dithiane) ~31) than the corresponding hydrogen in an O,O-acetal (pKa >40).

A combination of a catalytic amount hydrobromic acid (10 mol%) and an excess of hydrogen peroxide is found to be an effective reagent for expeditious regeneration of carbonyl compounds from their 1,3-dithiane as well as 1,3–dithiolane derivatives.

Acyl anion equivalents, among the most common umpolung synthons, can be pro-duced by many strategies. For instance, nitroalkanes can be used as nucleophiles and the nitro function can be cleaved to the carbonyl group. Thus nitronates can be thought of as acyl anion equivalents.

In retrosynthetic analysis, a synthon is a hypothetical unit within a target molecule that represents a potential starting reagent in the retroactive synthesis of that target molecule. The term was coined in 1967 by E. J. Corey.

Cyclic acetals and ketals are the most useful carbonyl (aldehyde or ketone) protecting groups. Common diols used to form ketals are show below in order of their relative rate of formation.

Ketones react with ethylene glycol in the presence of dry HCl gas to give a cyclic product known as ethylene glycol ketals.

Cinnamic acid derivatives The Perkin reaction is an organic reaction developed by English chemist William Henry Perkin that is used to make cinnamic acids. It gives an α,β-unsaturated aromatic acid by the aldol condensation of an aromatic aldehyde and an acid anhydride, in the presence of an alkali salt of the acid.

Q2: What catalysts are used in the Perkin reaction? A2: The alkali salt of the acid anhydride is used as a catalyst. However, other bases can be used for this purpose.

oxidation reaction
The primary alcohol is converted to aldehyde by the oxidation reaction using mild oxidizing reagent.

The HWE reaction of 2-diethylphosphoryl dithiane (413) with aldehydes 412 leading to the ketene ditioacetal 414 has been used for the C C bond formation in the synthesis of carbocyclic peptidomimetics ( Scheme 120 ). 153 Scheme 120. Hanessian, S.; Maji, D. K.; Govindan, S.; Matera, R.; Tintelnot-Blomley, M. J. Org. Chem. 2010, 75, 2861–2876.

Accordingly, dithiane 12 was exploited as a nucleophile to construct the backbone of the target by coupling with the diene 21 as an electrophile (Scheme 16). This planned alkylation is not trivial in view of the hindrance of both 12 and 21, but the conciseness brought to the synthetic plan was appealing.

M. Henrot, M. De Paolis, in Strategies and Tactics in Organic Synthesis, 2016 Accordingly, dithiane 12 was exploited as a nucleophile to construct the backbone of the target by coupling with the diene 21 as an electrophile ( Scheme 16 ).

Deprotonation of the polymer-bound dithianes using n -butyllithium followed by alkylation with alkyl iodides occurs readily in THF. The resulting disubstituted dithianes are cleaved to ketones using mercury (II) perchlorate.