DABCO synthesis

Gheng, Y., Ye, H.-L., Zhan, Y.-H., and Meth-Gohn, O. 2001. A very simple route to N-methylisatins Rriedel-Grafts acylation of p-substituted N,N-dimethylanilines with oxalyl chloride and DABCO. Synthesis 904-908. 

Finally, we mention the use of the polyquat bromide "Dab-4-Br" - easily prepared from Dabco and 1,4-dibromobutane - in the synthesis (ref. 7) of gmelinite (a 1-D 12-ring zeolite). 

The approach to polyketide synthesis described in Scheme 5.2 requires the relatively nontrivial synthesis of acid-sensitive enol acetals 1. An alternative can be envisioned wherein hemiacetals derived from homoallylic alcohols and aldehydes undergo dia-stereoselective oxymercuration. Transmetallation to rhodium could then intercept the hydroformylation pathway and lead to formylation to produce aldehydes 2. This proposal has been reduced to practice as shown in Scheme 5.6. For example, Yb(OTf)3-cata-lyzed oxymercuration of the illustrated homoallyhc alcohol provided organomercurial 14 [6]. Rhodium(l)-catalyzed hydroformylation of 14 proved successful, giving aldehyde 15, but was highly dependent on the use of exactly 0.5 equiv of DABCO as an additive [7]. Several other amines and diamines were examined with variation of the stoichiometry and none proved nearly as effective in promoting the reaction. This remarkable effect has been ascribed to the facilitation of transmetallation by formation of a 2 1 R-HgCl DABCO complex and the unique properties of DABCO when both amines are complexed/protonated. 

Scheme 61 Three-component synthesis of 2,6-DABCO skeletons...

Mercaptodithiole thiones 93 were treated with sulfur monochloride and DABCO under conditions for the synthesis of tricyclic bis-dithiolothiazines from substituted diisopropylamines (see Scheme 38). Unexpectedly, in all the cases 5-chloro-l,2-dithiole-3-ones 96 were formed in high yields (2006RCB143 Scheme 47). 

Lactams.1 A new synthesis of 0-lactams involves oxidative coupling of dianions of acyclic amides such as 1, prepared as shown from p-anisidine (equation I). The amide 1 is converted into the dianion by BuLi (2 equiv.) and DABCO or by t-... 

Yang Z-Z, He L-N, Dou X-Y et al (2010) Dimethyl carbonate synthesis catalyzed by DABCO-derived basic ionic liquids via transesterification of ethylene carbonate with methanol. Tetrahedron Lett 51(21) 2931-2934... 

Salicylaldehyde reacts with trimethylsilylketene dithioacetal in the presence of a Lewis acid to form the chroman 502, the product of a deoxygenative divinylation (Equation 208) <2001JOC3924>. This reaction can also be applied to salicylaldimines <2003JOC4947>. Treatment of 3,5-dibromosalicylaldehyde with methyl vinyl ketone (MVK) in the presence of DABCO leads to a chroman-4-ol as the major product <2002J(P1)1318>. A stereoselective one-pot synthesis of vy/z-fused chromans from salicylaldehydes, aromatic amines and cyclic enol ethers is carried out in the... 

A novel synthesis of a-unsaturated sulfines has been introduced by Bra-verman et al. [99]. Et3N or DABCO treatment of allylic and benzylic tri-chloromethyl sulfoxides triggered the elimination of chloroform and formation of the sulfines. It must be stressed that these sulfines are thermally relatively stable, and this stands in high contrast to the corresponding thio-carbonyl compounds unsaturated thioaldehydes cannot be monitored under the same experimental conditions and have to be used at very low temperature or trapped in situ. The first synthesis of thioacrolein S-oxide was achieved by flash vacuum thermolysis of an anthracene allyl sulfoxide [100], and both isomers in a (Z E) ratio of 78 22 were characterised by NMR spectroscopy at -60 °C. 

Synthesis. Ammonium sulfite monohydrate, 4,4 -bipyridyl, 1,4-diazabicyclo[2.2.2]octane (DABCO), neodymium(III) nitrate hexahy-drate (Aldrich), 1,10-phenanthroline, ethylenediamine (SD Fine, India), and zinc acetate dihydrate (Qualigens, India) of high purity were used for the synthesis. 

For the synthesis of 18 trichlorophosphine is alkylated to dichloro[bis(tri-methylsilyl)methyl]phosphine by means of the appropriate Grignard reagent. The subsequent elimination of HC1 can be achieved with, for example, diazabicyclo-octane (DABCO), however, the use of triethylamine is advantageous for the work-up.37,38... 

The second in situ technique is NMR. An autoclave fitting with the NMR cavity was designed by Gerardin et al. [59] and allows to follow the evolution of many parameters of the synthesis via the NMR characteristics of the different nuclei versus temperature and reaction time. The first measurement that can be reached now is the absolute value of the pH in hydrothermal conditions and the quantitative evolution of the concentration of protons in the bomb with the parameters of the synthesis [60], They proved that 14N NMR chemical shifts of well chosen amine compounds (imidazole and DABCO which possess complementary pKas) are precise pH indicators in aqueous solutions from room temperature to 475 K. Use of both amines permit to cover a wide range of about 9 pH units, with a precision of 0.1 pH unit. 

The results of in situ 14N NMR study of the reaction vs time at 150°C (Fig. 9(a)) in the presence of imidazole and DABCO show a pH increase from 3.2 to 6 in the first 2 h of the reaction and then an almost constant value of 6 during the rest of the synthesis. This value, which indicates quasi neutral conditions since pKa is 11.5 at 150°C, is precious when considering speciation, protonation of ligands and coordination of A1 atoms. 

In the studies on the synthesis of the antitumor agents esperamicin and calicheamicin by Magnus et al. [93], an aldol reaction was found suitable for macrocyclization after a number of unsuccessful attempts. Thus, as shown in Scheme 54, the diynene core structure (165) of the two antitumor agents was synthesized from the dicobalt hexacarbonyl derivative 163. When 163 was treated with n-BujBOTf/DABCO/EtjN in CHzClj-THF the aldol product 164 was isolated as a single stereoisomer in 45% yield. Although alkynyl aldehydes undergo similar crossed aldol condensation, their dicobalt hexacarbonyl derivatives react with moderate to excellent syn diastereoselectivity [94]. 

The stability of carbocations flanked by n-coordinated organic moieties is dramatically enhanced so that they react easily with nucleophiles [125]. Magnus et al. [126] applied this principle to the synthesis of the core structure (208) of the diynene antibiotics esperamicim and calicheamicins. As shown in Scheme 68, diynene 205 was converted to enol silyl ether which was treated with Co2(CO)g to give dicobalt hexacarbonyl adduct 206. Exposure of 206 to 3 equiv of TiCU and 1 equiv of DABCO at —43 to —35 °C gave macrocycle 207 in 50% yield. 

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