Additional Instructive Examples

Another instructive example is found in Fig. 2 The first three systems therein (r + he+ e) illustrate the same features as Fig. 1 with respect to the Kekule structure counts (/iQ. The systems are again obtained by successive additions of hexagons, but four hexagons are added each time in this case. The last system (Fig. 2), obtained by adding four hexagons into the corona hole of the e system, is a concealed non-Kekulean (o). This set of single coronoids illustrate nicely the rheo classification. 

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Another instructive example is provided by a series of a-phenyl-a,P-dibromo-phosphonates 170, 171, 172. While the phosphonate dianion 170 fragments instantaneously at room temperature with formation of the POf ion (see also Sect. 4.1.3), the analogous reaction of the phosphonic monoester anion 171 leading to methyl metaphosphate 151 requires more drastic conditions and is at least 1000 times slower the diester 172 is essentially stable under the reaction conditions described for 171 addition of triethylamine leads to slow demethylation H0). The behavior of 171 contrasts with that of simple (3-haloalkylphosphonic monoesters which merely eliminate HHal on treatment with bases94. Thus it is the possibility of formation of a phenyl-conjugated double bond which supports the fragmentation of 171 to olefin + 151. 

Another instructive example of the relevance of ambient stimuli to the nature of the effect of serotonergic manipulations on locomotor activity is provided by Brody s (27) study of PCPA. He monitored the locomotion of vehicle- and PCPA-treated rats in an open field to which the animals had been previously familiarized. Animals were tested either with or without additional stimulation in the form of flashing lights and 90 dB(A) noise bursts. Without stimulation, PCPA-treated rats were less active than controls with stimulation, PCPA-treated rats were more active. The results of these and other such studies indicate that the central serotonergic systems may not directly modulate the level of locomotor activity per se, but they may profoundly influence locomotor activity by virtue of their effects on the sensory responsivity of the animal to a wide variety of environmental stimuli. 

The oxidative addition of acids is another instructive example. It resembles the reactions with alkyl halides and may result in an "amphoteric" hydride ... 

The same methods work well with six-membered rings so we shall look at a few instructive examples. A dramatic demonstration of the advantages of intramolecular reactions comes in the synthesis of tetramethyl piperidone 72. Removal of the nitrogen with conjugate addition of ammonia to the dienone 73 opens the possibility of a double aldol disconnection to reveal three molecules of acetone. 

Procedures are given for (1) resolution by the use of a simple ester (2) resolution by crystallization of brucine salts of acid phthalates which are readily separated and (3) a similar resolution in which the separation of the diastereoisomeric salts is difficult. In addition to the preparations described in this chapter other instructive examples, showing various further modifications and helpful devices, may be found in references 53, 74, 76, 128, 69, and 125. 

Studies directed toward the synthesis of amaryllidaceae alkaloids provide instructive examples of the combined use of spirocylization and Michael addition pathways in phenolic oxidations (03MI1). For example, treatment of the norbelladine derivative 164 with BTIB leads, by way of C,C-bond formation, to the spiroannulated azepine 165 (Scheme 47) (96JOC5857, 98JOC6625). Hydrolysis of the amide moiety in 165 results in Michael addition of the nitrogen center to the dienone ring and affords ( )-oxomaritidine (166). BTIB-oxidation of the appropriate... 

Another instructive example of electrophilic or H-bonding assistance of protic solvents (or co-solvents) in SnI reactions is the accelerated acetolysis rate of 2-bromo-2-methylpropane upon the addition of phenols to a tetrachloromethane/acetic acid solution of the reactant [582] see reference [582] for further examples. The usefulness of phenol as a solvent for SnI solvolysis reactions, in particular phenolysis of 1-halo-l-phenylethanes, has been stressed by Okamoto [582], In spite of its low relative permittivity (fir = 9.78 at 60 °C), its low dipolarity fi = 4.8 10 Cm = 1.45 D), and its low nucleophihcity, it represents a solvent of high ionizing power due to its electrophilic driving force. 

Gordon and Danishefsky [112] used the reaction of a chromium Fischer car-bene complex 164 with a cycloalkine 163 to build the naphthoquinone core 165 (Dotz reaction, review [113]), a procedure often used for synthesis of the linearly condensed anthracyclinones (e.g., [114]). The quinone ketone 165 has nucleophilic and electrophilic centers correctly positioned to furnish a ben-zo[fl]anthraquinone. However, treatment with NaH or Triton B gave the spiro-compounds 166 as a mixture of two stereoisomers. These products evidently arose from Michael addition of the ketone enolate to the naphthoquinone double bond. But the weaker base DBU induced cyclization at ambient temperature to the benzo[a]anthraquinone 167 in 65% yield (Scheme 42). The primary aldol adduct apparently eliminated water and the resulting dihydrobenzo[a]anthraquinone aromatized under basic conditions in the presence of air. This is an instructive example of the influence of the base on the cyclization mode. 

Additional regio- and stereo-chemical information was obtained from palladium-ene reactions of strained alkenes. An instructive example is the reaction of stoichiometric amounts of the allylpalladium species (52) which undergo rapid, reversible formation of the cr-allylpalladium-norbomadiene complexes (53) to give at 37 °C the cis insertion products (54) (Scheme 12). Carbon-carbon bond formation (at the least substituted allyl terminal) and palladium transfer from the same (exo) face of norbomadiene were ascribed" to a o-allypalladium alkene insertion. 

The formation and reactions of guanine radicals in DNA and their reactions in the presence of carbon-centered radicals derived from lipid molecules provide instructive examples of free radical reactions in solutions involving these biologically relevant species. The reactivities of free radicals derived from biomolecules depend on their structures. The carbon-centered radicals produced by either by hydrogen atom abstraction or the addition of oxyl radicals to double bonds of polyunsaturated fatty acids (PUFAs) are primary intermediates of lipid peroxidation... 

There are few cases in which the stereochemistry of cyclic aldol addition reactions has been carefully examined. The situation is complicated by the problem of retro-aldolization and by the propensity of such aldols to dehydrate to the enone. There is a suggestion that under strictly kinetic conditions, aldols in which the carbonyl and hydroxy groups can chelate a metal cation are favored. However, in protic medium, it does not appear that the kinetic product is generally the more stable one. A particularly instructive example is seen in equation (135). The complex keto aldehyde (101) was treated with dilute KOH in methanol for 15 min at various temperatures. At -20 °C, +25 °C, and +65 °C, the (102) (103) ratios were 93 7,50 50 and 0 100, respectively. 

Biphasing by deliberate addition of a second liquid phase was considered in Chapter 18. We extend the treatment in this section to aspects of biphasing that involve extractive reaction. Thus we focus the discussion on extractive reactions that depend on the dissociation equilibria of the components involved, and we restrict the treatment to a few instructive examples. 

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