Dication equivalents

Tanaka and Ohno developed the palladium(0)-catalyzed cyclization of bromoallene 222 bearing a sulfonamide for the synthesis of medium-sized heterocycle 223 (Scheme 37).48b In this reaction, bromoallene acts as an allyl dication equivalent 224, and two different nucleophiles can be introduced regioselectively. The intramolecular nucleophilic... 

A bromoallene was demonstrated to act as an allyl dication equivalent. When treated with Pd(0) in an alcoholic solvent, an ei-hydroxybromoallene provides a mediumsized heterocycle (Scheme 16.101) [106]. The oxidative addition of a bromoallene to Pd(0) generates an allenylpalladium species, which is successively transformed into a Jt-allylpalladium complex through the attack of the hydroxyl group on the sp carbon followed by the protonation of the resulting Pd-carbene complex. Finally, the products are provided as a mixture of regioisomers by the nucleophilic attack of the external methanol. 

A highly regio- and stereoselective synthesis of oxepine derivatives via cyclization of bromoallenes bound with hydroxy group by a four-carbon atom tether in the presence of a palladium(O) catalyst and alcohol is developed (Equation 16). In this reaction, the bromoallenes act as an allyl dication equivalent, and the intramolecular nucleophilic attack takes place exclusively at the central carbon atom of the allene moiety <2004JA8744>. 

Bromoallene 139 was used as an allyl dication equivalent, and palladium-catalyzed cyclization leads to a mixture of regioisomers 140 (major) and 141 (minor) (Scheme 19) <2004JA8744>. 

In this reaction, acetals are used as dication equivalent of one-carbon unit 72, whereas 71 provides a dianion 73 <95TL5581>. A new stereocontrolled synthesis of substituted tetrahydrofiirans starts with dioxalones 74. A titanium-mediated methylenation using dimethyltitanocene (THF, 60 C) with subsequent treatment of 75 with trialkylaluminium reagents results in the formation of tetrahydrofiirans 76 <95JA6394>. 

Are the above DreM tactics feasible in tandem A preliminary but affirmative answer is possible. Thus, the biaryl O-carbamate migration-amide cydization sequence 166 —> 167 —> 168 (Scheme 43), which conceptually constitutes a reaction of a biaryl 2,2 -dianion with a carbonyl dication equivalent (169), has found application in natural product synthesis [65, 72]. 

A different sequential DreM pathway involving O-carbamate ring switch-vinylogous tolyl amide cydization, 170 —> 171 —> 172 (Scheme 44), a formal bridging of a 2,2 -methyl diaryl dicarbanion with a carbonyl dication equivalent (173) has been affirmed in a model study (174 —> 175, Scheme 45) [73] and applied in natural product total synthesis [73, 74]. 

A novel, general route to 2,3,6,7-tetrahydroazepines via RCM was reported by Pearson and Aponick <01OL1327> (Scheme 24). Double-allylation of the amine a,a -dication equivalent 39, followed by in situ trapping with ClCC Ph gives rise to diene 40 which is efficiently converted to the 7-membered ring heterocycle 41 by treatment with the GMC in DCM. This process was effectively applied to a variety of substituted (2-azaallyl)stannanes. 

The a-(diphenylmethylsilyl) esters have been shown to be vinyl dication equivalents 3, and as such are precursors to terminal olefins and deuterated olefins, 1,1-dlsubstituted olefins and tri- and tetrasubstituted olefins. ... 

The Au(I) catalyst system transforms 2,3-benzo-7-oxabicyclo[2.2.1]hept-2-ene into a tetralin-1,4-dication equivalent, as shown by its reaction with allylsilanes. ... 

The a-(diphenylmethylsilyl) esters have been shown to be vinyl dication equivalents 3, and as such are precursors to terminal olefins and deuterated olefins, 1,1-disubstituted olefins and tri- and tetrasubstituted olefins. They are precursors to B-ketosilanes and ketones, wherein the overall transformation results in an ester to ketone conversion. They can also be deprotonated and the enolate anion condensed with aldehydes and ketones to give a,6-unsaturated esters, in particular a-alkylated-a,B-unsaturated esters. Their y-lactone counterparts, o-(diphenylmethy1snyl )-y-butyrolactone 4a and a-(diphenylmethylsilyl)-y-valerolactone 4b, are precursors to 4-oxo acids,1,4-diketones and a-ylidene-y-lactones. ... 

Ohno H, Hamaguchi H, Ohata M, Kosaka S, Tanaka T (2004) Palladium(0)-catalyzed Synthesis of Medium-Sized Heterocycles by Using Bromoallenes as an Allyl Dication Equivalent. J Am Chem Soc 126 8744... 

A -Octalin ring 44, 973 Olefin dication equivalent 43, 879 Oligonucleotides (s.a. Phosphorami-date intemucleotide linkages) Oligonucleotide synthesis (s.a. Gene synthesis) 43,401 44, 25... 

Ph3Si ethylene oxide catalytically adds PhSH to give a- and -adducts in 6 1 ratio, and serves as a vinyl dication equivalent with RCu to give alkenes stereoselectively, while epoxides rearrange in the presence of silylated nucleophiles Diphenylmethylsilylation of cyclopropane carboxylates gives the C-silylated ester which then form C-silylated cyclopropyl ketones, P-silyl cyclopropyl carbinols, and alkylidene cyclopropancs, which ring expand to the... 

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