And pyrolytic eliminations

The configurations, conformations, chiroptic properties, and pyrolytic elimination reactions of steroidal sulphoxides and sulphinates, at C-3, C-6, and C-7, have been studied in detail. In general, though not without exception, the direction of elimination is controlled by the configuration at the sulphur atom, whenever two possibilities exist for the essential syn mechanism. The favoured... 

They established gas chromatographic and mass spectrographic procedures coupled with sensory threshold odor analysis for the determination of off-flavor emanation from packaging materials. They found reaction products from the polymeric materials as well as a combination of oxidation and pyrolytic elimination reactions in polyethylene or paperboard to be responsible for off-odors. 

Extensive discussion of these and many other types of elimination reactions can be found in several reviews.We will focus here on 1,2-eliminations, especially dehydrohalogenation, dehydration, dehalogenation, deamination reactions, and pyrolytic eliminations. 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Pyrolytic elimination from isoindoline A -oxides also affords iso-indoles, but yields were found to be generally lower than those obtained by Kreher and Seubert s procedure. The considerable amount of polymeric material formed in the pyrolytic reaction makes isolation of the isoindole difficult, but a convenient method for separation of the product was found utilizing complex formation with 1,3,5 -trin i tro benzene. 

These topics are discussed in more detail in other chapters of this text. Formally, the pyrolytic elimination of sulphur dioxide from a sulphone, with the concomitant formation of a new carbon-carbon bond, constitutes a reduction at sulphur. These reductions have been valuable in the formation of new molecules, especially macrocycles and cyclophanes, and have been reviewed by Vogtle and Rossa205. Pyrolytic elimination of sulphur dioxide has been used by Julia and co workers in the formation of mixtures of isoprenoids206, and by Takayama and collaborators in the stereoselective synthesis of vitamin D, 19-alkanoic acids207. 

Several types of compound undergo elimination on heating, with no other reagent present. Reactions of this type are often run in the gas phase. The mechanisms are obviously different from those already discussed, since all those require a base (which may be the solvent) in one of the steps, and there is no base or solvent present in pyrolytic elimination. Two mechanisms have been found to operate. One involves a cyclic transition state, which may be four, five, or six membered. Examples of each size are... 

Furthermore, the authors showed that compounds of type 1-78, easily accessible from 1-77, can be used as starting materials for the production of a new cyclo-propa[c]chromane framework 1-80 (Scheme 1.21). Oxidation of 1-77 to the corresponding sulfoxide and subsequent pyrolytic elimination generated the labile cyclobutene 1-78, which was directly epoxidized leading to the desired tricyclic compound 1-80 in 46% yield, probably via the epoxycyclobutane 1-79, again in a domino fashion. 

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