Regioselectivity site selectivity

Before describing the regioselective, site-selective and stereo-selective preparation of oxetanes via the PB reaction, the mechanism of the photochemical reaction will be briefly summarized. The reason for this is that an understanding of the reaction... 

The following sections describe the regioselective, site-selective, and stereoselective synthesis of oxetanes. 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

Regioselective (site-selective) functionalization of unsaturated haloge-nated nitrogen, oxygen, and sulfur heterocycles by Pd-catalyzed cross-couplings and direct arylation processes 07CSR1036. 

To date (ca 1996) many potentially usefiil sucrose derivatives have been synthesized. However, the economics and complexities of sucrochemical syntheses and the avadabiLity of cheaper substitutes have limited their acceptance hence, only a few of them are in commercial use. A change in the price and availability of petroleum feedstocks could reverse this trend. Additional impetus may come from regioselective, site-specific modifications of sucrose to produce derivatives to facilitate and improve the economics of sucrochemical syntheses. For example, the microbe yigwbacterium tumifaciens selectively oxidizes sucrose to a three-keto derivative, a precursor of alkylated sucroses for detergent use (50). Similarly, enzymes have been used for selective synthesis of specific sucrose derivatives (21). 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

The most useful and important application of Fukui function and local softness resides in the interpretation and thereby, prediction of reaction mechanism, especially in the site selectivity or regioselectivity. Since long FMO theory has generally been used to probe the regioselective nature of a reaction, in particular of organic compounds, but the DFT-based local reactivity parameters have emerged as... 

The palladium-catalyzed reaction of iodobenzene and an allenyl malonate provided vinylcyclopropane in a highly regioselective manner (Scheme 16.7) [11, 12]. A jT-allylpalladium complex, generated by the addition of PhPdl to a 2-allenyl malonate, can be trapped by an internal malonate anion to afford a vinylcyclopropyl derivative. The site selectivity in this cyclization is dependent on the nature of the entering RX groups, catalytic systems involving phosphine ligands, solvents and bases. 

The regiochemistry of nickel mediated cycloadditions of substituted norbomadienes has been investigated in detail. The regioselectivity, exo/endo selectivity and site selectivity seem to depend strongly on the substituents on both diene and dienophile. Tetracya-noethene, for example, reacted with 2-acetyloxymethyl substituted norbomadiene on the distal side331. 

Miller also explored the ASD of glycerol derivatives through an enantioselective acylation process which relies on the use of a pentapeptide-catalyst which incorporates an A-terminal nucleophilic 3-(l-imidazolyl)-(5)-alanine residue [171], Most recently, Miller has probed in detail the role of dihedral angle restriction within a peptide-based catalyst for ferf-alcohol KR [172], site selective acylation of erythromycin A [173], and site selective catalysis of phenyl thionoformate transfer in polyols to allow regioselective Barton-McCombie deoxygenation [174],... 

II. REGIOSELECTIVE FORMATION OF ALLYLIC HYDROPEROXIDES A. Site Selectivity... 

As a synonym of site-selectivity, locoselectivity was used, apparently only by a single author, for designating that site out of several possibilities, where chemical reactions occurs 8. Hassner s regioselectivity was preceded by Viehe s proposal directiospecific9 that did not prevail. 

The expedient and regioselective metalation of unprotected biphenyl-2-, -3-, and -4-carboxylic acids has been reported.59 Unprotected biphenyl-2-carboxylic acid has been cleanly metalated with. sex-butyllithium at the position adjacent to the carboxylate and can then be subjected to site-selective electrophilic substitution (Scheme 8). The remote C(2 )-position has been attacked by the superbasic mixture of n-BuLi and t-BuOK (LICKOR) in THF or benzene. The resulting dianion cyclizes to give the fluorenone skeleton. The mechanism of the metalation of homologous compounds, 2-(pyridin-3-yl)benzoic acid derivatives, with strong bases has also been discussed.60... 

The usual site selectivity was observed in a nonpolar solvent, whereas in polar solvents, such as methanol, where a hydrogen-bonding with the hydroxyl is efficient, the regioselectivity reverses (Table 2) [67,68], Photo-oxygenation of... 

Kost, D. M., Gerdt, J. P., Pradeepkumar, P. I., and Silverman, S. K. (2008). Controlling regioselectivity and site-selectivity in RNA ligation by Zn2+-dependent deoxyribozymes that use 2,3,-cyclic phosphate RNA substrates. Org. Biomol. Chem. 6, 4391—4398. 

Adam and coworkers reported the regioselective and diastereoselective formation of oxetanes during the PB reaction of allylic alcohols (Scheme 7.27) [43, 44]. This group proposed that hydrogen-bond interactions in the exciplex played an important role in controlling the selectivity. D Auria and coworkers also observed a site-selective and diastereoselective formation of oxetanes in the PB reaction of 2-furylmethanol derivatives (Scheme 7.27) [45]. 

The Site Selectivity of Diels-Alder Reactions. Site selectivity is another kind of regioselectivity, in which a reagent reacts at one site (or more) of a polyfunctional molecule when several sites are, in principle, available. Thus butadiene reacts faster with the quinone 6.209 at C-2 and C-3 than at C-5 and C-6. The cyano groups will lower the coefficients at C-2 and C-3 more than those at C-5 and C-6. The dimer of hexatriene is 6.210 and not 6.211, which we can similarly explain by looking at the coefficients of the frontier orbitals, essentially narrowing the problem down to assessing the Zc2 term in Equation 3.4. 

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