Benzyltrimethylammonium Elimination

Now that the allylic oxidation problem has been solved adequately, the next task includes the introduction of the epoxide at C-l and C-2. When a solution of 31 and pyridinium para-tolu-enesulfonate in chlorobenzene is heated to 135°C, the anomeric methoxy group at C-l 1 is eliminated to give intermediate 9 in 80% yield. After some careful experimentation, it was found that epoxy ketone 7 forms smoothly when enone 9 is treated with triphenyl-methyl hydroperoxide and benzyltrimethylammonium isopropoxide (see Scheme 4). In this reaction, the bulky oxidant adds across the more accessible convex face of the carbon framework defined by rings A, E, and F, and leads to the formation of 7 as the only stereoisomer in a yield of 72%. 

It would appear that when fluoride ion is to be used in stoichiometric amounts, benzyltrimethylammonium fluoride is the preferred source on the other hand, tetra-n-butylammonium fluoride, commercially available as its trihydrate, is more convenient in catalytic situations. However, there are difficulties. (19) in successfully dehydrating the latter source without inducing Hofmann elimination. 

Although 1, -elimination of o xylene polyhalides with sodium iodide, zinc, copper, or iron metal is a fundamental method for the formation of o-xylylene intermediates, it is difficult to carry out the reaction under mild conditions such as at room temperature. o Trimethylsilylmethyl)-benzyltrimethylammonium halides were devised for this purpose and were shown to generate the o-xylylene at room temperature. However, we have successfully generated o-xylylene at room temperature by the reaction of a, 0,-dibromo o-xylene with metallic nickel(51). The Diels-Alder reaction of... 

Elimination of p-[(trimethylsilyl)methyl]benzyltrimethylammonium iodide with fluoride leads via the intermediate p-quinodimethene to [2.2]paracyclophane (7). This very versatile method can also be applied for the syntheses of [2.2](2,5)furano-and thiophenophane 48a). 

Preparation. The reagent is readily prepared by the condensation of benzil with dibenzyl ketone in the presence of a basic catalyst. Dilthey s original procedure, adopted by Organic Syntheses specifies ethanol as solvent and a solution of potassium hydroxide in ethanol as the base. This procedure suffers from the low boiling point of the alcohol and the limited solubility of both potassium hydroxide and the reaction product in ethanol. In an improved procedure - use of the better solvent triethylene glycol permits operation at a higher temperature, and use of the readily soluble benzyltrimethylammonium hydroxide as base eliminates the step of dissolving potassium hydroxide in ethanol, A mixture of 0.2 mole each of benzil and... 

Hauser et were able to isolate the intermediate dimeric halide when 9-chlorofluorene was allowed to react with only one equivalent of sodium amide in liquid ammonia. In t-butyl alcohol containing its potassium salt or a dilute aqueous solution of benzyltrimethylammonium hydroxide, the rate of formation of bifluorenylidene is second-order in 9-bromofluorene and first-order in the basicity of the reaction medium as measured by the ionisation of nitroaniline indicators . Under the same reaction conditions, protium exchange of 9-deutero-9-bromofluorene and elimination from 9-bromo-9,9 -bifluorenyl are much faster reactions than the conversion of 9-bromofluorene into bifluorenylidene. These facts are consistent with the displacement mechanism. 

The one-pot reaction between di(2-bromobenzyl)methylamine (141), n-butyllithium, and antimony trichloride, followed by p-tolyllithium, afforded (142) in 23% yield <88TL5401>. Halogenation of (142) yielded the corresponding dihalides (143 X = Cl, Br or I) and the reaction of (143 X = Cl or Br) with SbCls or AgBp4 resulted in the formation of compounds with Sb—N bonds (144 X = Cl or Br) (Equation (16)). Treatment of (143 X = Cl) with KF in DMF gave the difluoro compound (143 X = F). The structure was assigned to the compounds (143) on the basis of H NMR spectroscopy. The most reasonable structure for (143 X = I) was determined to be the ionic formulation (144 X = I). The addition of benzyltrimethylammonium chloride to (144 X = Cl) converted it back into the neutral compound (143 X = Cl). Methyl iodide oxidatively adds to (142) subsequent reductive elimination of methyl chloride from the intermediate (145) yields the iodostibine (146) in quantitative yield (Equation (17)) <89TL4841>. 

Racemization via a reversible addition/elimination process under mild conditions can be used, with for example cyanohydrins, hemiacetals, hemiaminals and hemithioacetals. SiUca-supported benzyltrimethylammonium hydroxide (BTAH) was used to racemize cyanohydrins and effected an efficient DKR process in tandem with porous ceramic-immobilized lipase (lipase PS-C II) (Scheme 4.22) [57]. 

Desilylation of some compounds can generate very reactive species such as benzynes, pyridynes, xylylenes, and benzofuran-2,3-xylylenes. 1,4-Elimination of o-(a-trimethylsilylalkyl)benzyltrimethylammonium halides with TBAF in acetonitrile generates o-xylylenes, which undergo intermolecular and intramolecular cycloadditions (eq 15). Treatment of a-silyl disulfldes with Cesium Fluoride or TBAF... 

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