General Synthetic Procedure

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

A simple, high-yield procedure for the conversion of ArTlXj into ArjTlX compounds has recently been described 90). This symmetrization reaction, the mechanism of which is not known, can be effected by treatment of the ArTlX2 compound either with triethyl phosphite or with hot aqueous acetone. As a wide variety of ArTlXj compounds can now be easily prepared by electrophilic thallation of aromatic substrates with thallium(III) trifluoroacetate (q. v.), symmetrization represents the method of choice for the preparation of the majority of ArjTlX compounds. Only about twenty mixed compounds, RR TIX, have been prepared so far, and the only general synthetic procedure available consists of a disproportionation reaction between an RTIX2 species and another organometallic reagent [e.g., Eqs. (5)-(7)]. 

Coordination compounds have been produced by a variety of techniques for at least two centuries. Zeise s salt, K[Pt(C2H4)Cl3], dates from the early 1800s, and Werner s classic syntheses of cobalt complexes were described over a century ago. Synthetic techniques used to prepare coordination compounds range from simply mixing the reactants to employing nonaqueous solvent chemistry. In this section, a brief overview of some types of general synthetic procedures will be presented. In Chapter 21, a survey of the organometallic chemistry of transition metals will be presented, and additional preparative methods for complexes of that type will be described there. 

It is consequently not possible to give one general synthetic procedure (for a detailed discussion see refs. 29 and 45). It also became evident that the order of addition of the reagents can play a crucial role, which is nicely illustrated by the following examples. 

This synthetic strategy proved to be unsuitable for [M(CO)5] (M = Cr, W) metal fragments due to the thermal instability of the corresponding non-donor-substituted allenylidenes [M(=C=C=CR R )(C0)5] (R, R = usually alkyl or aryl groups). An alternative general synthetic procedure using deprotonated functionalized acetylenes has been successfully applied (for example tris-amino or alkoxo prop-1-ynes see Equation 2.2) [4a]. 

A more general synthetic procedure to prepare aryltrifluoro-l -sulfanes. e.g. 4, is the fluorination of an aryl disulfide, e.g. 3, with silver(ll) fluoride, although the cost of the silver(II) fluoride precludes large-scale synthesis.80... 

General Synthetic Procedures for Encoded Peptidomimetic and Small Molecule Libraries... 

Compounds such as R2SnCl2 and R2SiCl2 have several uses because two reactive Sn-Cl bonds remain that can undergo hydrolysis, reactions with alcohols, and so on. The reactions of Grignard reagents constitute one of the most general synthetic procedures, and they can be used in a wide variety of cases. 

The synthesis of chloro(2,2 6, 2"-terpyridine)platinum(ll) was first reported by Morgan and Burstall in 1934.4 Better yields for this compound can be obtained by the method of Intille,s which is given below. The (2,2 6, 2"-terpyri-dine)thiolato compounds were reported in 1974,1 and a general synthetic procedure for these complexes was recently described.3 In this method silver ions are introduced during an intermediate step. Since silver is known to bind... 

Polymers containing benzoin terminal groups can act as photochemical macroinitiators and are effective in photogenerating polymeric electron donor radicals. The initiation of polymerization by means of azo-benzoin initiators yields polymers with one or two benzoin end-groups according to the termination mode of the particular monomer involved [72-74], The general synthetic procedure is depicted below as illustrated for the case styrene polymerization (Scheme 18). 

The great power of the general synthetic procedure proposed by Zemva, Bartlett et al. lies in the ability to prepare marginally thermally stable and even thermodynamically unstable fluorides at low temperatures. Originally [67], they... 

Metal ion template mediation in macrocyclic synthesis has been a part of the field since its inception, its importance having been realized early in the development of this area. Two specific roles for the metal ion in template reactions have been proposed. These are, in turn, kinetic and thermodynamic in origin." In the kinetic template effect, the arrangement of ligands already coordinated to the metal ion provides control in a subsequent condensation during which the macrocycle is formed. The thermodynamic effect serves to promote stabilization of a structure which would not be favored in the absence of a metal ion. Schiff base condensations tend to be dependent on this latter type of template effect. Some of the more routine and general synthetic procedures will be described here. A more in-depth treatment can be found in a review by Curtis, with particular emphasis on general methods as well as modifications of preformed macrocycles." ... 

The failure of trifluoroacetate decarboxylation as a general synthetic procedure is perhaps not surprising since such reactions are usually of limited utility. Furthermore, trifluoroacetate groups are quite resistant... 

Subsequently, a significant number of other catenanes have been obtained using the same general synthetic procedure - but derived from different crown ethers and the same or related tetracationic macrocycles. - The extension of the initial success to the production of these new assemblies clearly owes much to the basic understanding gained in the prior studies. 

The recent and continuing surge of interest in transition-metal complexes of o ,/3-unsaturated /3-keto amines (/3-amino ketones), especially with respect to their stereochemical properties, has resulted in increased efforts to secure general synthetic procedures for these complexes. Methods of preparation of these complexes have been reviewed recently. Although most of... 

It is reported that (R3Sn)20 compounds react with acids (HX) that have ionization costants between 10 and 10" ° to produce SnXR3 species. Thus, the preparation of Sn(SPh)(/i-C4H9)3 is an example of a more general synthetic procedure. 

While the aforementioned methodologies constitute the most useful and general synthetic procedures for the preparation of 1,6-anhydropyranoses, a number of alternative routes have been described [27]. As mentioned previously, solutions of a number of sugars in aqueous acid at equilibrium contain considerable amounts of 1,6-anhydro sugars. A simple synthesis of tri-0-acetyl-l,6-anhydro-/J-L-idopyranose has been reported by Stoffyn and Jeanloz that utilizes the acid-catalyzed dehydration of L-idose to form the anhydro ring [28]. 

The mechanism of the reaction is illustrated in Scheme 1 for the conversion of r-butanol, using sulfuric acid and acetonitrile, into A -r-butylacetamide. Three intermediates are involved. Firstly, a carbenium ion (1) is produced under strongly acidic conditions. This reacts with the nitrile to produce a resonance-stabilized nitrilium ion (2), which in turn is converted into the corresponding imidate (3). Finally, the latter is hydrolyzed to the amide. Since all three events occur in a one-pot process, frequently in high yield, the Ritter reaction is a simple, efficient and general synthetic procedure. 

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