Beckwith studies

No single factor can be identified as determining the outcome of radical addition. Nonetheless, there is a requirement for a set of simple guidelines to allow qualitative prediction. This need was recognized by Tedder and Walton,2 17 Beckwith ei ah, Giese. and, most recently, Fischer and Radom.4 With the current state of knowledge, any such rules must be partly empirical and, therefore, it is to be expected that they may have to be revised from time to time as more results become available and further theoretical studies are carried out. 1 lowever, this does not diminish their usefulness. 

Before discussing Beckwith s data on that basis, a brief mention must be made of investigations by Lown s group (Naghipur et al., 1989, 1990, and other papers cited there) who claim to have observed the formation of benzoxathiete (10.63) and its valence isomer monothio-l,2-benzoquinone (10.64) in the aprotic diazotization of 2-[(2,-acetoxyethyl)sulfinyl]aniline (10.62). Scheme 10-82 is an abbreviated form of the mechanism proposed by the authors. A more detailed experimental study is clearly required. 

Beckwith, R.S. Reeve, R. (1963) Studies on soluble silica in soils. I. The sorption of silicic acid by soils and minerals. Aust. J. Soil Res. 

Determining the underlying factors that control the cyclization reactions of hexenyl and related radicals has been the object of much study. The recent theoretical treatments of Beckwith and Schiesser,6 and Houk and Spellmeyer,7 relate the current state of understanding with many important references to the extensive body of prior work. 

Brocks JJ, Beckhaus H-D, Beckwith ALJ, Ruchardt C (1998) Estimation of bond dissociation energies and radical stabilization energies by ESR spectroscopy. J Org Chem 63 1935-1943 Buxton GV, Langan JR, Lindsay Smith JR (1986) Aromatic hydroxylation. 8. A radiation chemical study of the oxidation of hydroxycyclohexadienyl radicals. J Phys Chem 90 6309-6313 Buxton GV, GreenstockCL, Helman WP, Ross AB (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/O -) in aqueous solution. J Phys Chem Ref Data 17 513-886... 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

In another use of the radical clock principle with the clock reaction incorporated within the substrate to be studied, Beckwith and Storey determined the rate constant for 5-exo-cyclisation of an aryl radical onto an acryloyl double bond, as depicted in Scheme 10.16 [ 14]. Since the rate constant for cyclisation onto the O-allyl double bond to give 9 is known (ko-aiiyi = 5 x 108 s 1 at 25°C, see Scheme 10.3), the unknown rate constant, kacryioyi> can be determined by Equation 10.13 simply by determining the ratio of the products 9 and 10 ... 

Beckwith-Hall BM, Holmes E, Lindon JC, Gounarides J, Vickers A, Shapiro M, Nicholson JK. NMR-based metabonomic studies on the biochemical effects of commonly used drug carrier vehicles in the rat. Chem Res Toxicol 2002 15 1136-1141. 

Beckwith, A. L. S., Norman, R. O. C. Electron Spin Resonance Studies. Part XX. The Generation of Organic Radicals by the One Electron Reduction of Aliphatic Halogeno-Compounds in Aqueous Solution. J. Chem. Soc. A 1969, 400. 

A comprehensive review of experimental as well as theoretical studies has been compiled by Beckwith et al. in 1997 for those systems with acyloxy or phosphatoxy substituents.3 This account will therefore concentrate on the theoretical studies published since then together with the relevant experimental results. Particular... 

More recently, elegant mechanistic studies on intramolecular Meer-wein reactions by Beckwith et al. considerably extended the utility of diazonium salts. They showed that many electron donors could convert an arenediazonium cation into an aryl radical which cyclized in good yield to form dihydrobenzofurans and indolines. The final radical was functionalized as a halide, sulfide, or ferrocene. Thus, the credentials of diazonium salts as electron acceptors were well established, and the stage was set to investigate the interaction between diazonium salts and TTF. 

Information on components of the prokaryotic protein-secretion apparatus has come primarily from genetic studies in E. coli. The chief technique has been to obtain a strain that is defective in some aspect of protein secretion and then to screen for strains in which the defect is suppressed by a compensating second-site mutation (Emr et al., 1981 Bankaitis and Bassford, 1985). Another approach has been to select for mutations that exhibit a pleiotropic export-defective phenotype (Ito et al., 1983 Oliver and Beckwith, 1981). These procedures have identified a host of genetic loci that specify proteins that affect secretion. Two of... 

Beckwith JB, Palmer NF. Histopathology and prognosis of Wilms tumor Results from the first national Wilms tumor study. Cancer. 1978 55 2850-2853. 

Parham DM, Weeks DA, Beckwith JB. The clinicopathologic spectrum of putative extrarenal rhabdoid tumors. An analysis of 42 cases studied with immunohistochemistry or electron microscopy. Am J Surg Pathol. 1994 18 1010-1029. 

Simple model studies show that substitution at C-l or C-3 of the 5-hexenyl radical gives mainly c/.v-disubstituted cyclopentanes, whereas substitution at C-2 or C-4 leads mainly to tran.v-disub-stituted cyclopcntanes. A variety of theoretical treatments L 2 and experimental results now aid in the planning of highly stereoselective reactions, and allow predictions according to Beckwith s guidelines. 

A. L. J. Beckwith and C. H. Schiesser, Tetrahedron, 41, 3925 (1985). Regio-and Stereo-Selectivity of Alkenyl Radical Ring Closure A Theoretical Study. 

Studies on prostaglandin biosynthesis in the early 1970s have shown that molecular oxygen is incorporated into polyunsaturated lipids. It was shown that autoxidation of polyunsaturated species leads to peroxyl radical intermediates that can undergo yff-scission, H-atom abstraction, and allylic rearrangement or/and cyclization. Beckwith looked into the oxygenation of dienes initiated by phenylthiyl radicals... 

O-Stannyl ketyls have been proposed as intermediates for almost 30 years. Much of the early work came from the laboratories of Davies [9a], Pereyre [3, 9b, 9c] and Beckwith [6a, 10] and provided a framework for a modern understanding of O-stannyl ketyls. These seminal studies were often focussed on mechanistic aspects of tin ketyls in a-cyclopropyl- and a-epoxy-ketone ring openings. In one of the first carbonyl-alkene cyclizations, it was determined that the tributyltin radical added to a ketone in a somewhat sluggish manner and that excess amounts of tin hydride were needed to drive the reaction to completion [6a]. It was also understood that a ketyl radical anion is more stable than a simple carbon-centered radical, likely attributed to more effective delocalization. Ketyl reactive intermediates are now being utilized in new strained-ring cleavage-recyclization sequences (see below) and are... 

The free-radical cyclization of enynes as a synthetic strategy was introduced by Stork and coworkers using BusSnH/AIBN-mediated ethynyl cyclization (equation 34) ". From a mechanistic point of view, the first step is the selective addition of the stannyl radical to the triple bond of the enyne generating a vinyl radical, which can cyclize onto the double bond. In the absence of the methyl substituents on the olefinic moiety, the groups of Beckwith and Stork have both postulated, and demonstrated with the aid of kinetic studies, that the 6-endo radical 30 arises from a ring expansion of the kinetically favored... 

---