Dimedone allyl transfer

The CH-acid dimedone (pKa = 5.2) was chosen as the allyl transfer reagent in order to facilitate separation from the Liberated amine. When used in 7- to 8-fold excess, it also protonates the free amino group (pKa = 8) to such an extent that the Latter cannot function as an allyl acceptor. However, if the reaction times are long, an enamine may form with the dimedone in which case dimethylbarbituric acid (pKfl = 4.7) can be used instead.184185 It is also noteworthy that the thioether function of methionine is not detrimental to the activity of the Pd catalyst. A mechanism for the Pd 0)-cata lysed deprotection of Aloe derivatives using dimedone or NtN --dimethylbarbituric acid as scavengers is given in Scheme 8.75. 

The mild and selective nature of the palladium-catalysed allylation reaction has made it suitable for protection. Allyl ethers, esters, amines and carbamates can usually be easily formed by standard procedures. The allyl group can be removed later by exposme to a palladium catalyst and a competitive nucleophile, Y (Scheme 9.68). This is an allyl transfer reaction. Commonly used competitive nucleophiles for allyl esters of carboxylic acids include sodium 2-methylhexanoate, secondary amines such as pyrrolidine (Scheme 9.69) and morpholine, and dimedone. ... 

The Pd(0)-catalyzed transfer of the allyl moiety to dimedone [23], morpholine [22], or N,N -dimethyl barbituric acid [27] resulted in the completely selective cleavage of the Aloe group, whereas the numerous other protecting groups and the glycosidic bonds,... 

Another method for deallylation is the Pd-catalyzed transfer of the allyl group to reactive nucleophiles. Sodium 2-ethylhexanoate [141], morpholine [142], dimedone [143] and AA-dimethylbarbituric acid [144] are used as allyl scavengers. It is claimed that benzenesulfinic acid 331 or sodium toluenesulfinate are the best allyl scavengers [145]. 

The enolate formed in this process then condenses irreversibly with the r -allylpalladium species. At the outset, dimedone and A,A-dimethylbarbituric acid had been more specifically devisedP for the deprotection of allyl carbamates in solution or in solid-phaseP chemistry in which case the enolate species also forms by proton transfer from the (3-dicarbonyl allyl scavenger to the liberated amine but since then it has also been used for the deprotection of allyl carboxylic esters (see Section 2.2.1.1.3).P 1... 

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