Related Eliminations

Since a cycloreversion, the reverse of a cycloaddition, travels the same reaction path as the forward reaction, the considerations of stereochemistry and orbital symmetry that govern concerted cycloadditions are equally applicable to cycloreversions. The number of such reactions that have been studied in detail is not large, but there is sufficient information to establish that orbital-symmetry controls are indeed operating. The principles of orbital-symmetry conservation specify which processes can occur in concerted fashion and the stereochemical restrictions that are imposed by a concerted mechanism. We will first discuss some reactions that do occur by concerted mechanisms, and then turn to some of the elimination processes that involve discrete intermediates. 

A good example of a concerted elimination is the reaction that takes place on treatment of 3-pyrrolines with N-nitrohydroxylamine. The reactive intermediate C is capable of elimination of molecular nitrogen. The reaction has been shown to be 

On treatment with difluoramine, aziridines undergo deamination to afford alkenes in a process that should not be concerted, according to the principles of 

The elimination of carbon monoxide from cyclic ketones is a fairly common process. The reaction occurs photochemically with saturated systems the mechanism of this process was discussed in Part A, Section 11.3. Facile thermal expulsion occurs only in molecules having special structural features. The elimination of carbon monoxide from bicyclo[2.2.1]heptadien-7-ones can occur by a concerted mechanism. In fact, generation of bicyclo[2.2.1]heptadien-7-ones is usually accompanied by 

Exceptionally facile elimination of CO also takes place from structure 8, in which homoaromaticity can stabilize the transition state  

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A related elimination is the formation of 4-pentyn-l-ol from tetrahydrofurfuryl chloride and sodamide [139] ... 

In this chapter, we shall discuss substitution reactions that proceed by ionic or polar mechanisms in which the bonds cleave heterolytically. We also will discuss the mechanistically related elimination reactions that result in the formation of carbon-carbon multiple bonds ... 

It is interesting to note that related elimination studies (Sera et al., 1973 Alunni et al., 1974), using different substrates (phenylsulfonyl acetates and l-phenyl-2-propyl chloride), have revealed contrasting... 

Dehydrosulfonylation. Reaction of a-methoxyalkyl phenyl sulfones with potassium t-butoxide effects 1,2"elimination of benzenesulfinic acid to give enol ethers (equation I). A related elimination (equation II) has been reported.-... 

DehydrohalogettatUm and related eliminations. DMSO is superior to hydroxylic solvents for dehydrohalogenation with potassium t-butoxide.The bis-ethylene-ketal (I) could not be converted satisfactorily into (2) with potassium t-butoxide... 

THETA (168 FIX) 3 - ANTIBODY RESPONSE LAG TIME FROM T=0 ONLY THETA (0,0.4) 4 - VMAX OF ANTIBODY RELATED ELIMINATION THETA (0,5) 5 - KM OF ANTIBODY RELATED ELIMINATION THETA (0.0005,0.0013,0.005) 6 - KE (LINEAR ELIM)... 

A related elimination of sulfur dioxide occurs during the Ramberg-Backlund synthesis of alkenes, which generates an episulfone as a transient intermediate, although episulfones are isolable under controlled... 

The purposeful inhibition of metabolic-related elimination or clearance has already been discussed. By applying these same principles in reverse it becomes possible to accelerate drug metabolism and clearance. For example, in the prior situation conveyed in Figure 19.20 and its accompanying text, why do you suppose that salmeterol, 46, was constructed so as to not contain the gem-dimethyl arrangement depicted as follows, that had worked so well to avoid GYP450-mediated N-dealkylation when placed in albuterol, 45, in the form of a tertiary-butyl group ... 

A related elimination reaction occurs when (3-halocarboxylic acids, especially those that have two substituents on the carbon atom next to the acid group, are treated with base... 

As a related elimination, allyl alkyl carbonates of primary, secondary, allylic, and benzyl alcohols are converted to ketones and aldehydes in MeCN via facile y3-H elimination of jr-allylpalladium alkoxide 537, formed from allyl alkyl carbonate 536 [198], The reaction offers a good method of oxidation of alcohols under neutral conditions without using inorganic oxidants. 

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