Versatile synthetic procedure

The Cp2TiCl/H20 combination can also be used for the chemoselective reduction of aromatic ketones. The reaction discriminates between ketones and alkenes, between ketones and esters and, remarkably, between conjugated and non-conjugated ketones [80]. There is strong evidence that this reduction proceeds via ketyl-type radicals, which are finally reduced by H-atom transfer from 42 [81]. Under dry conditions, titanium-promoted ketyl radicals from aromatic ketones can be used for intermolecular and intramolecular cross-coupling of ketones [82], Thus, depending on whether water is added or not, complementary and versatile synthetic procedure protocols are available. 

A versatile synthetic procedure has been developed by Vilsmaier and coworkers. The method involves treatment of an enamine with S, -dimethyl-A succinimidosulphonium fluorosulphonate to give an enaminosulphonium salt (117). The latter gives the cyclopropane derivative under the influence of a nucleophile and base254-259 (Scheme 111). 

The intramolecular cycloaddition of ketenes with alkenes is a versatile synthetic procedure for the preparation of bicyclo[3.2.0]heptan-6-ones and bicyclo[3.3.1]heptan-6-ones19a-b. Three-atom tethers offer the best compromise between product strain, which is prohibitive with two-atom tethers, and entropy of activation, which decreases the rate of reaction with longer tethers. Cycloadditions with four-atom tethers arc quite rarc19a,b. As in intcrmolccular cycloadditions, syn addition to the alkene normally occurs. 

By and large, these early heterocyclic compounds were isolated from natural sources versatile synthetic procedures followed only after many years of research. Some examples of early compounds are as follows ... 

Initially, it was decided that the donor substituent should contain the aromatic diamine functionality necessary for polymerization (27). However, the target monomer M-5 proved impossible to prepare by classical electrophilic substitution reactions. All attempts to couple triaiylamines substituted with peripheral electron donor groups with diazonium salts failed and resulted instead in deaiylation 18). The desired monomer M-5 was finally prepared by the Mills reaction of tris-p-aminophenylamine with p-nitronitrosobenzene 12 as shown in Scheme 2. This is an exceptionally versatile synthetic procedure limited only by the ability to prepare substituted nitrosobenzene partners. 

The versatile synthetic procedures for the preparation of a wide variety of DHBCs, as well as their intriguing solution properties have promoted and suggested the utilization of such polymers in several demanding and smart technological applications. Some of them are discussed in the following seetions. 

Extending the use of zeolites into larger dimensions, say to catalyse enzymatic reactions and for the purification of colloidal precious metals, was the aim of researchers at Mobil Corporation (USA), who in 1992 discovered a viable and versatile synthetic procedure to prepare mesoporous materials, i.e. materials with ordered porosity in the range between 20 and 500 A (2-50 nm Kresge et al. 1992). Their first material was termed MCM-41 (Mobil Composition of Matter) and the mechanisms involved templating of a silica sol-gel synthesis by an amphiphilic surfactant. 

Radicals are versatile synthetic intermediates. One of the efficient procedures for radical generation is based on one-electron oxidation or reduction with transition metal compounds. An important feature is that the redox activity of transition metal compounds can be controlled by appropriate ligands, in order to attain chemoselectivity in the generation of radicals. The application to small ring compounds provides useful methods for organic syntheses. Reductive transformation are first reviewed here. 

The Suzuki reaction (the palladium-catalyzed cross-coupling of aryl halides with boronic acids) is arguably one of the most versatile and at the same time also one of the most often used cross-coupling reactions in modern organic synthesis [32], Carrying out high-speed Suzuki reactions under controlled microwave conditions can today be considered almost a routine synthetic procedure, given the enormous literature precedent for this transformation [7]. 

It was a result of demand from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures that led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic [1, 2]. Additionally, comparisons of the efficiencies of the various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols and comparable with the cryptands [e.g. 3, 4], which have fewer proven applications and require higher... 

The enedione (15), a tetracyclic intermediate on a synthetic route to fusidic acid, has been synthesized from the a-methylene-ketone (16) (Scheme 2). The first step, involving a Diels-Alder reaction with a substituted acrylate (17), provides a new versatile annelation procedure. Further modification of (15) by a route worked out on model systems (see Vol. 4, p. 318) afforded the tetracyclic enone (24) with the desired trans-syn-trans geometry (Scheme 3). This compound (24) has also been prepared by degradation of fusidic acid. Attempts to introduce the C-11 oxygen function necessary for the synthesis of fusidic acid have not been very... 

In this section the synthesis of fluoroalkyl (Section 15.1.4.1.3), a,a-difluoroalkyl (Section 15.1.4.2.3), and trifluoromethyl- and perfluoroalkyl ketones are discussed collectively. The second most widely used method for synthesizing peptide fluoromethyl ketones is the Henry nitro-aldol condensation reaction, which involves the use of (3-nitro alcohols to build the fluoromethyl ketones. As with the modified Dakin-West procedure, the Henry reaction has also been used to synthesize mono-, di-, tri-, and extended fluoromethyl ketones, making it another extremely versatile synthetic method.19 12 19 27 29 33 341 However, similar to the Dakin-West procedure, the products of the Henry reaction are not chiral, since an achiral carbanion is involved in the crucial carbon bond forming step. 

Palladium(0)-catalyzed allylation of nucleophiles (the Tsuji-Trost reaction) is a versatile synthetic method that has gained immense popularity in recent years. Rarely applied to ambident nucleophilic aromatic heterocycles before 1991, the Tsuji-Trost reaction has been extensively used in the chemistry of these compounds since 1991. Two factors have played decisive roles in this increased interest in the Pd(0)-catalyzed allylation of such heterocyclic rings one is that, unlike other alkylation procedures, the Pd(0)-catalyzed allylation can sometimes give the product of thermodynamic control when applied to ambident nucleophiles and the second is that the Tsuji-Trost allylation has become one of the standard methods for synthesizing carbanucleosides, which are important antiviral compounds (93MI1, 93MI2). Of course, the double bond of an allylic system can be modified in different directions, thus adding versatility to the Tsuji-Trost reaction. 

Sml2, which can be prepared conveniently from samarium powder and 1,2-diiodoethane in THF, finds application as a versatile one-electron reducing agent in organic synthesis. Two typical synthetic procedures mediated by Sml2 are the pinacol coupling of aldehydes and the Barbier reaction, as shown in the following schemes ... 

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