Facility access

In most of these examples, the chiral auxiliary is introduced to the allylic reagent at a very late stage in the synthesis of the precursor, thus providing a facile access. It is obvious that in most examples, the central metal atom is kept from becoming stereogenic, and in addition, a C2-symmet-ric cation is desirable, in order to minimize the possible number of competing transition states. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

The X-ray structure of the unsubstituted tris(pyrazolyl)borato zinc nitrate has been solved showing a unidentate coordination mode for nitrate, in contrast with the t-butyl substituted ligand, which shows anisobidentate nitrate coordination due to the steric effects.232 A partial explanation of the reduced activity of cadmium-substituted carbonic anhydrase is offered by Parkin on the basis of the comparison of nitrate coordination to cadmium and zinc trispyrazo-lylborate moieties. A contributing factor may be the bidentate coordination supported by the cadmium that does not allow the facile access to a unidentate bicarbonate intermediate, which could be highly important to carbonic anhydrase activity.233... 

The reaction of fully conjugated 3,5-diaryl-l,2,4-oxadiazoles with butyllithium allows facile access to 5-butyl-3,5-diaryl-4,5-dihydro-l,2,4-oxadiazoles 338 (Equation 70) <2000H(53)191>. 

Of course, the use of tris(trimethylsilyl) phosphite213 214 provides facile access to the free a-hydroxyphosphonic acids. These silyl reagents have been used for the preparation of a wide range of a-substituted phosphonates and -phosphonic acids, starting with ketene,215 a-ketophosphonates,216 ketoesters,217 218 and a,P-unsaturated carbonyl compounds,207/219-221 as well as simple aldehydes and ketones.205 210/219 224 Their use for the preparation of compounds of significant biological interest has been reviewed.125... 

While technically not "organometallics," enamines are reagents that can provide nucleophilic carbon for new bond formation. Two groups of researchers have reported on the use of such reagents for the formation of new carbon-phosphorus bonds through displacement of chloride from phosphorus.72 73 For example, displacement of bromide from phosphorus tribromide has been used for the introduction of a new thiophosphoryl functionality adjacent to an original carbonyl group (Equation 4.28).72 This approach provides a facile access to (3-ketophosphonates. 

Both Raman and infrared spectroscopy provide qualitative and quantitative information about ehemieal species through the interaetion of radiation with molecular vibrations. Raman spectroscopy complements infrared spectroscopy, particularly for the study of non-polar bonds and certain functional groups. It is often used as an additional technique for elueidating the molecular structure and symmetry of a eompound. Raman spectroseopy also provides facile access to the low frequency region (less than 400 cm Raman shift), an area that is more difficult for infrared speetroseopy. 

A multicomponent reaction (MCR) represents a sequence of bimolecular events leading to products that incorporate essentially all atoms of three or more starting materials. MCRs allow for the rapid and facile access to complex target structures... 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

A vast array of piperidine containing cores, both natural and synthetic, are of biological and medicinal interest. These heterocyclic scaffolds have been the subjects of considerable synthetic efforts, especially for the construction of optically active compounds. In this context, Khan et al. reported a catalytic bromodi-methylsulfonium bromide (BDMS) three-component reaction of 1,3-dicarbonyls with aromatic aldehydes and aromatic amines for a facile access to highly functionalized piperidines (Scheme 24) [104]. This strategy is an interesting illustration of... 

Khan AT, Parvin T, Choudhury LH (2008) Effects of Substituents in the beta-Position of 1,3-dicarbonyl compounds in bromodimethylsulfonium bromide-catalyzed multicomponent reactions a facile access to functionalized piperidines. J Org Chem 73 8398-8402... 

It is therefore the right time to give a first comprehensive overview of fullerene chemistry, which is the aim of this book. This summary addresses chemists, material scientists and a broad readership in industry and the scientific community. The number of publications in this field meanwhile gains such dimensions that for nonspecialists it is very difficult to obtain a facile access to the topics of interest. In this book, which contains the complete important literature, the reader will find all aspects of fullerene chemistry as well as the properties of fullerene derivatives. After a short description of the discovery of the fullerenes all methods of the production and isolation of the parent fullerenes and endohedrals are discussed in detail (Chapter 1). In this first chapter the mechanism of the fullerene formation, the physical properties, for example the molecular structure, the thermodynamic, electronic and spectroscopic properties as well as solubilities are also summarized. This knowledge is necessary to understand the chemical behavior of the fullerenes. 

N. Satyamurthy, Synthesis of a new heterobifunctional linker, N-[4-(amino-oxy)butyl] maleimide, for facile access to a thiol-reactive F-labeling agent, Bioconj. Chem. 14 (2003) 1253-1259. 

The present method provides simple and facile access to the variety of furo- 408, benzofuro- 409, thieno- 410, benzothieno- 411, pyrrolo- 412, and indolo- 413 fused [[Pg.45]

It might be expected that transposing the synthetic methods that have worked so well for the preparation of organic polymers would allow similarly facile access to inorganic macromolecules. However, consideration of the main synthetic routes to organic polymers (Scheme 8.1) illustrates the problems associated with this approach. 

The copper mediated cycloisomerisation of alkynyl imines provides a facile access to pyrroles and fused aromatic pyrroloheterocycles.50 In a recent example, shown in 8.39., the pyrrolopyrimidine bearing an acetylene moiety in the appropriate position cyclized smoothly to give the desired tricyclic compound in 89% yield.51... 

S. Hanessian, G. Demailly, Y. Chapleur, and S. Leger, Facile access to chiral 5-hydroxy... 

Whereas preparation of a-amino acid derivatives by asymmetric allylation of an acyclic iminoglycinate gave a modest enantioselectivity (62% ee) in an early investigation [189], the use of conformationally constrained nucleophiles in an analogous alkylation resulted in high selectivities (Scheme 8E.43) [190], With 2-cyclohexenyl acetate, the alkylation of azlactones occurred with good diastereomeric ratios as well as excellent enantioselectivities. This method provides very facile access to a variety of a-alkylamino acids, which are difficult to synthesize by other methods. When a series of azlactones were alkylated with a prochiral gem-diacetate, excellent enantioselectivities were uniformly obtained for both the major and minor diastereom-ers (Eq. 8E.20 and Table 8E.12). 

Liebscher and Anklam [100] have introduced a facile access to novel optically active spiro-(5-lactams 112-113,115-116 through cycloaddition reaction of diazomethane to a-alkylidene-p-lactams (Scheme 29). Diazomethane gave a clean reaction to 7-methylene-(5-lactam 111 affording spiroazetidin-2-ones 112, 113 (d.r. 87 13, 93%) with preference of anti-addition with respect to the CH2OTBS substituent. The 7-ethylene-(5-lactam 114 reacted slowly with diazomethane and... 

One example of the utilization of this approach for the synthesis of natural products is the facile access, Fig. 10, to the southwest tripeptide segment of echinocandin B, 148, a cyclic hexapeptide that is characterized by its antifungal and antiyeast activity. 

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