Synthesis of Aryllead Triacetates

The second method for the formation of aryllead triacetates is via transmetallation either from the corresponding aryltrialkylstannane [80] or arylboronoic acid [81] catalysed by Hg(OAc)2. This approach is the more general approach and greatly extends the scope of the aryl group to include different substitution patterns which 

This approach has also been utilised in an enantioselective approach to generate axially chiral diaryl phenols using bmcine as the enatiomerically pure amine base at low temperatures of -20 to -40 °C [87]. The phenols could be obtained in up to 83 % ee. Aryllead triacetates can also be used to arylate [)-diketones. Both linear and cyclic P-diketones with an a-substituent already present could be successfully arylated with a variety of aryllead triacetates. The P-diketone 2-methylcyclohexane-l,3-dione 

Other C-arylation reactions have been accomphshed with vinylogues of P dicarbonyls [93], malonic acid derivatives [94], a-cyano esters and malonitriles [95]. The a-arylation of ketone enolates/enamines and nitroalkanes has also been achieved [96-98]. 

73 Synthesis of AryUead Triacetates for the C-Arylation of P-Keto Allyl Esters 

A number of aryUead triacetates were synthesised in this section of the PhD project to be utilised in the C-arylation of p-keto aUyl esters. In total, 11 aryUead triacetates were prepared according to the synthetic methods described in Sect. 4.7.1. AU of the aryllead reagents were prepared using Pb(OAc)4, either by direct plumbalion or by transmetallation from the corresponding aryl stannane or aryl boronic acid. The synthesis of all of the aryllead triacetates used in this project has been previously reported, many for the first time by this research group [53, 80, 81, 99]. 

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The procedure described here serves to illustrate a new, general method for effecting the < -arylation of g-dicarbonyl compounds by means of an aryllead triacetate under very mild conditions. Although the first synthesis of an aryllead triacetate was reported relatively recently, a wide range of these compounds can now be readily prepared. The most direct route to these compounds is plumbation of an aromatic compound with lead tetraacetate, and in the procedure reported here p-methoxyphenyllead triacetate has been prepared in this way. It may also be obtained by reaction of the diarylmercury with lead tetraacetate, a longer, but more general method of synthesis of aryllead triacetates. 

The synthesis of aryllead triacetates has been well developed over the past 40 years [79]. There are two principle methods for the synthesis of aryllead triacetates direct plumbation and transmetallation. The simplest method is direct plumbation where an arene can be reacted directly with Pb(OAc>4 to form an aryllead triacetate by a highly selective electrophilic aromatic substitution reaction. Pb(OAc>4 can be reacted with 1,3,5-trimethoxybenzene (138) in CHCI3 at room temperature to generate 2,4,6-trimethoxyphenyllead triacetate (139) in 76 % yield (Scheme 4.35). Although this method is limited to electron rich arenes it is a remarkably selective reaction. 

Over the past 40 years aryllead triacetates have been applied as both N- and C-arylating reagents and cross-coupling partners in organic synthesis. The synthesis and application of aryllead triacetate reagents is a key aspect of this section of this PhD project and an overview of this key class of arylaling reagents will now be discussed. 

Donnelly, D.M.X., Fitzpatrick, B.M., and Finet, J-P., Aryllead triacetates as synthons for the synthesis of biflavonoids. Part 1. Synthesis and reactivity of a flavonyllead triacetate, J. Chem. 

Chromenes 1079 react with aryllead(rv) triacetates 1080 to afford chroman-3-ones 1081 which upon hydrogenation furnish 4-arylchroman-3-ones, key intermediates for the synthesis of neoflavenes (Scheme 272) <2001T413>. 

Mixed arylation of bifunctional substrates, such as those containing two /3-ketoester groups in their structure can be made. This possibility was used to synthesize unsymmetrically substituted compounds containing two different aryl groups (Scheme 2). One-pot treatment of the dibenzyl 3,7-dioxobicyclo [3.3.0]-octane-2,6-dicarboxylate 6 with a mixture of two aryllead triacetates 7 and 8 afforded a mixture of the unsymmetrical diaryl derivative 10 with the two symmetrical diaryl compounds 9 and 11. When the arylation was performed with 1.1 equiv. of each aryllead reagents 7 and 8, a mixture of 9-11 in a ratio 0.46 1 0.72 was obtained. This ratio became 0.43 1 0.49, when 0.86 equiv. of 7 and 1.33 equiv. of 8 were used. This compound 10, isolated in a 33% yield, was eventually elaborated to complete a total synthesis of ( ) methyl piperitol.25... 

The procedure described here serves to illustrate a new, general method for effecting the -arylation of g-dicarbonyl compounds by means of an aryllead triacetate under very mild conditions. Although the first synthesis of an aryllead triacetate was reported relatively recently, a wide range of... 

The value of 4-hydroxycoumarin in synthesis has been discussed <07THC(11)283> and its use in the synthesis of 3,4-disubstituted coumarins has been reported <07JOC7279>. The 3-arylation of 4-hydroxycoumarins, achieved using aryllead triacetates, sets up a substrate for a cascade of reactions which leads to the [2]benzopyrano[4,3-c][l]benzopyran system <07JOC3293> and the EDDA-catalysed reaction of 4-hydroxycoumarin with a,[3-unsaturated arylaldehydes produces pyrano[3,2-c]benzopyran-5-ones <07T10025>. Propargylation and... 

Aryllead triacetates react with a mixture of isomeric vinylogous keto esters re-giospecifically at C-1. In the synthesis of alkaloids such as ( )-mesembrine [39] and ( )-lycoramine [38], the central quaternary carbon center was installed by this electrophilic arylation reaction (Scheme 13.11). 

Vinylsilanes 132 have been reacted with bromomalononitrile to yield the intermediate 133, which was used for the synthesis of cyclopropane derivatives Addition reactions of dichloromalononitrile with substituted alkenes and alkadienes can also be used for the preparation of intermediates in carbo- and heterocyclic synthesis. 2-Arylmalononitriles 135 have been produced by coupling malononitriles with aryllead(IV) triacetates like 134163. 

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