Bases Collidine

The transient intermediate 13-5 is not isolated as it dehydrates spontaneously to give the intermediate 14-1 under the strongly acidic reaction conditions (Scheme 6.14). Treatment of that product with the hindered base collidine leads to formation of an anion on C2. This displaces the side chain chlorine to re-form the cyclopropyl ring, yielding the progestin cyproterone (14-2). 

Until recently, pyridine-type bases have been commonly used to produce conjugated enones from 2-halo ketones yields are usually poor °° and these reactions are frequently accompanied by rearrangement, reduction and salt formation. Thus, Warnhoff found that dehydrobromination of (28) with 2,4-lutidine gave a mixture of (29), (30) and (31) in the ratio 55 25 20. Collidine gave a ratio of 38 25 37, whereas pyridine gave mainly the salt (32). 

Ester eliminations are normally one of two types, base catalyzed or pyrolytic. The usual choice for base catalyzed j5-elimination is a sulfonate ester, generally the tosylate or mesylate. The traditional conditions for elimination are treatment with refluxing collidine or other pyridine base, and rearrangement may occur. Alternative conditions include treatment with variously prepared aluminas, amide-metal halide-carbonate combinations, and recently, the use of DMSO either alone or in the presence of potassium -butoxide. 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

For the complex-catalyzed acetylation of l-phenyl-3-propanol, an equimolar amount of a base such as collidine or triethylamine was necessary. 

Cyclobutanones (11, 560-561). Ketenimium salts are more reactive than ke-tenes in [2 + 2] cycloadditions with alkenes to prepare cyclobutanones. The salts are readily available by in situ reaction of tertiary amides with triflic anhydride and a base, generally 2,4,6-collidine. The cycloaddition proceeds satisfactorily with alkyl-substituted alkenes and alkynes, but not with enol ethers or enamines.1... 

The diazoketone first formed is decomposed in a high-boiling solvent (benzyl alcohol or octanol 2) at 160°-180°. Although no catalyst is required, but the addition of a collidine base improves the yield. The second step is the rearrangement of the diazoketone which is also called Wolff rearrangement. 

Scheme 5.7 (a) Mechanism of the w-pentenyl glycoside-based glycosylation method. Electrophiles commonly employed include iodonium di-sym-collidine perchlorate (IDCP) and A-iodosuccinimide/triethylsilyl triflate (NIS/Et3SiOTf) (b) Mechanism for the activation of NIS by EtsSiOTf.55... 

The electrooxidation of substituted pyrazolines in the presence of a base (pyridine, collidine), which facilitates deprotonation from the pyrazoline ring, gives the corresponding pyrazoles (Scheme 90). In the absence of a base, an Ar-Ar coupling product is obtained [135-138]. 

Acid sites were shown to be located in the three-pore system of protonated samples (HMWW), and methods were recently proposed for determining the distribution of these sites as well as their respective role in o-, m-, and p-xylene transformations. While xylene transformation was shown to occur in the three locations, benzene alkylation with ethylene was catalyzed by the acidic sites of the external hemicups only. Indeed, the activity for this reaction is completely suppressed by adding a base molecule (collidine) to the feed that is too bulky to enter the inner micropores. Moreover, adsorption experiments show that collidine does not influence the rate of ethylbenzene adsorption, so that the suppression of alkylation activity was not caused by pore mouth blocking. ... 

An unusual osmium(Vl) monooxo species stabilized by amido ligands, [Os (0)(tmen-2H)(tmen-H)]" ", has been prepared by treatment of [0s (0)2(tmen)2] " (97b) with a base such as collidine in acetonitrile." The crystal structure of the perchlorate salt has been determined the osmium atom is displaced 0.67 A from the plane of the four N atoms. The Os=0 distance is 1.72 A, and the Os—N distances of 1.85 A and 1.99 A indicate double bond character. 

Dehydrobromination with a base such as collidine then establishes the 1,4-diene function to yield fluoroprednisolone acetate (19-3) [13]. 

In the recent patent literature a procedure is given for coupling a diazonium salt with ethyl a-methylacetoacetate to afford an azo compound which could be used as an acetylating agent because its acetyl group was readily cleaved under mild conditions [26], In the coupling reaction, relatively weak bases such as pyridine, picolines, quinoline, and collidine are said to be effective. The example is given here for illustration only. It is to be noted that this is a composition of matter patent. 

Carboxylic esters where R is methyl or ethyl can be cleaved by heating with lithium iodide in refluxing pyridine or a higher-boiling amine.1039 The reaction is useful where a molecule is sensitive to acid and base (so that 0-10 cannot be used) or where it is desired to cleave selectively only one ester group in a molecule containing two or more. For example, refluxing O-acetyloleanolic acid methyl ester with Lil in s-collidine cleaved only the 17-carbomethoxy... 

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