DBU, diazabicycloundecene

Chemoselective E2 eliminations can be carried out with sterically hindered, sufficiently strong bases. Their bulkiness causes them to react with an H atom at the periphery of the molecule rather than at a C atom deep within the molecule. These bases are therefore called nonnucleo-philic bases. The weaker nonnucleophilic bases include the bicyclic amidines DBN (diazabi-cyclononene) and DBU (diazabicycloundecene). These can be used to carry out chemoselective E2 eliminations even starting from primary and secondary alkyl halides and sulfonates (Figure 4.17). 

Quinone methides formed during, for example, alkaline pulping reactions may have other mechanisms for rearomatization. Most commonly, in 8-0-4-, 8-5-, and 8-8-quinone methides QM1-QM3 (Fig. 12.2), retro-aldol elimination of formaldehyde to give styryl aryl ethers or stilbenes is common.40 Retro-aldol reactions using a strong base, for example, diazabicycloundecene (DBU) in CH2CI2 can also provide these compounds conveniently at room temperatures.41 3... 

The key step in the following synthesis of indenes by the azolide method is an intramolecular C-acylation of a fulvene intermediate. The added dimethylaminopyridine (DMAP) was thought to act as an acyltransfer agent, and diazabicycloundecene (DBU) as cyclization initiator via fulvene deprotonation [116]... 

An alternative procedure for effecting the condensation of phophonates is to carry out the reaction in the presence of lithium chloride and an amine such as /V,/V-diisopropyl-V-ethylamine or diazabicycloundecene (DBU). The lithium chelate of the substituted phosphonate is sufficiently acidic to be deprotonated by the amine.156... 

An essential step in the process is the (reversible) formation of the anion RO . Alkylacetylenes, which are less acidic than most other acetylenic derivatives, react less readily. We found that the oxidation of alkylacetylenes such as C2H5C3CH can be carried out at a convenient rate when to the solution in pyridine a small amount of diazabicycloundecene (DBU) is added. This condensed bicyclic amine is a stronger base than other amines and is expected to facilitate the proton removal from the acetylene. 

The basicity of 3,4-dihydro-2//-pyrido[ 1,2-[Pg.106]

Sinou and co-workers also studied the allylation of uracil 190 and thymine 191, but in an aqueous solvent (water/acetonitrile in a ratio 17/2) and in the presence of trisodium triphenylphosphine trisulfonate (tppts, 199) as water-soluble phosphine. Their results are summarized in Table VI. Cinna-myl acetate was used with one equivalent of diazabicycloundecene (DBU) as a base instead of mixed carbonate. Under these conditions, good regiose-lectivities at N-l (to 193 and 185) were observed, as well as lack of diallyla-tion products for uracil and thymine, even with an excess of cinnamyl acetate. It seems that the nonformation of diallylated products is related to the precipitation of the N-l isomers 193 and 185 in the aqueous medium (94TL7085). 

Aliphatic alkynes, e.g. HC CCjHj j and HCfeCCH2SiMe3 react slowly. If a small amount (- 3 ml) of diazabicycloundecene (DBU) is added, a smooth reaction takes place and the diyne CjHj j(CsC)2CjHjj is obtained in an excellent yield. The yield of Me3SiCH2--(C=C)2CH2SiMe3 (b.p. 105 015 mmHg, solid at room temperature) is only about 50%. There is a considerable amount of high-boiling residue. 

Diazabicycloundecene (DBU) and other pyrimidoazepines, chemistry of, 42, 83 Diazepines, chemistry of, 8, 21... 

Bromination of an olefin and double dehydrobromination of the resulting 1,2-dibromide is a classical method for the generation of 1,3-dienes (Table 1). Bromination of a double bond can be done with molecular bromine or, more conveniently, with pyridinium bromide perbromide . A variety of bases has been employed for dehydrobromination. While potassium hydroxide and sodium methoxide have been used for a long time, lithium carbonate-lithium chloride in DMF or hexamethylphosphoric triamide (HMPA) works well in many cases . Double dehydrobromination with hindered bases such as potassium r-butoxide or diazabicyclononene (DBN) and diazabicycloundecene (DBU) give good results. 

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