Rearrangement summary

Since the foregoing summary was prepared a preliminary account of an ingenious explanation of these results, involving a molecular rearrangement of a new type, has been published by Robinson which cannot be adequately dealt with here and for which the reader must be referred to the original. 

In summary, the evidence described above demonstrates three main mechanistic features of the rearrangement of allylic sulfenates to sulfoxides (1) spontaneous and wholly concerted [2,3]-sigmatropic shift of allyl or a-substituted allyl esters (7 a, b) at one extreme (2) complete stability of the y-aryl and y,y-dialkyl substituted allyl sulfenates as well as... 

A comprehensive summary of the theoretical contributions over the past 40 years towards the understanding of the relationship between two-dimensional aromaticity (hydrocarbons) and the electronic structure of three-dimensional deltahedral boranes. Note in particular that rearrangement processes and the isocloso problem in metallaboranes have also been discussed.11... 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

In summary, zirconocene and half-sandwich zirconium-based catalysis has been developed both in surface and solution systems. In general, the activation of Zr-POSS and silica-supported zirconium system with MAO proved inappropriate (see silsesquioxane displacement and leaching, respectively, vide supra), while the acti-vahon with BArl proved more efficient, albeit structural rearrangement complicates the activation chemistry and simple Zr(IV) cationic alkyl species are seldom the outcome of the activation. 

In summary, it should be pointed out, however, that the exact mechanism of the thermal rearrangement of sulfur rings and chains is still unknown and that further investigations are necessary in this connection. For any further discussion it may also be interesting to take into account the recent results on the molecular rearrangement reactions of cyclic selenium sulfides and of elemental selenium which take place at considerably lower temperatures compared with elemental sulfur and for which — as far as solid selenium is concerned — interesting ionic mechanisms have been proposed... 

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other live triose phosphate molecules (15 carbons) are rearranged in steps to (S) of Figure 20-10 to form three molecules of ribulose 1,5-bisphosphate (15 carbons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic C02 assimilation, six NADPH and nine ATP are required. 

V. 10(5 -> 6pH)Abeostero ds (A-Homo-B-Norsteroids) / 389 Solvolysis of 5a,6a-epoxy-6-methyl steroids / 389 Pinacol rearrangement of 5a-hydroxy-6a-tosylates / 392 Summary / 394... 

Table 1. Summary of various better alkyladamantane rearrangement runs 27) (Reproduced from Ref. 27). 

All aspects of the structure, reactivity and chemistry of fluorine-containing, carbon-based free radicals in solution are presented. The influence of fluorine substituents on the structure, the stability and the electronegativity of free radicals is discussed. The methods of generation of fluorinated radicals are summarized. A critical analysis of the reactivities of perfluoro-n-alkyl, branched chain perfluoroalkyl and partially-fluorinated free radicals towards alkene addition, H-atom abstraction, and towards intramolecular rearrangement reactions is presented. Lastly, a summary of the synthetically-useful chemistry of fluorinated radicals is presented. 

In summary, a stereoselective 10-step total synthetic route to the antimalarial sesquiterpene (+)-artemisinin (1) was developed. Crucial elements of the approach included diastereoselective trimethylsilylanion addition to a,p-unsaturated aldehyde 16, and a tandem Claisen ester-enolate rearrangement-dianion alkylation to afford the diastereomerically pure erythro acid 41. Finally, acid 41 was converted in a one-pot procedure involving sequential treatment with ozone followed by wet acidic silica gel to effect a complex process of dioxetane formation, ketal deprotection, and multiple cyclization to the natural product (+)-artemisinin (1). The route was designed for the late incorporation of a carbon-14 label and the production of a variety of analogues for structure-activity-relationship (SAR) studies. We were successful in preparing two millimoles of l4C-l73 which was used for conversion to I4C-arteether for metabolism75 and mode of action studies.76,77... 

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