1,3-proton shift

High-Resolution Solid-State NMR Study of Reversible 1,5-Proton Shifts in Organic Solids [90MRC(S)29] NMR of Pyrazoles ... 

It is also possible to oxidize allenes 89 and 92 to unsaturated hydroxy ketones 91 and 93 via the spirodioxide intermediate 90 [29]. In this case the terminating step is a 1,5-proton shift. In the examples shown in Scheme 17.26, the formation of the spirodioxide intermediate is diastereoselective, as is the rearrangement to the unsaturated hydroxy ketone. 

Attempts to achieve an asymmetric 1,3-proton shift reaction of (/ )-33, obtained from ethyl 3,3,3-trifhioro-2-oxopropanoate and (f )-l-phenylethanamine in 81 % yield, resulted in conversion into 34 in 89% yield, but without any reliably delectable enantiomeric excess.26 Even at 10% conversion, the Shiff base 34 formed is completely racemic. Imine 34 undergoes isotopic exchange in triethylamine/methanoI-r/4 at a rate 10 times slower than the isomerization of 33 to 34. The authors reason that if a 1.3-proton shift mechanism is operating, some enantiomeric excess would have to be observable in product 34 at low conversion. Since this is not the ease, a 1,5-proton shift to the carbonyl oxygen, via stabilized anion 37, to form achiral intermediate enol 38, was proposed.26... 

Substituted indolizines have been shown to undergo cycloadditions with suitably substituted alkenes to give pyrroloindolizines.202 Thus, 2,6-dimethylindolizine with methyl acrylate gives 136. The reactions presumably involve [8 + 2] cycloadditions, with subsequent 1,5-proton shifts. 

The intramolecular addition of a hydroxy group to a triple bond has been performed successfully in the presence of RuCl2(PPh3)(p-cymene) as catalyst precursor under mild conditions [18, 19]. The Lewis acid property of the ruthenium active species provides the activation of the triple bond and the Markovnikov addition of the hydroxy group to form 2-methylfuran derivatives after 1,5-proton shift and aromatiza-tion (Scheme 8.8). 

Ethylaluminum dichloride has been described as a catalyst for acylation of alkenes by acyl chlorides, giving nonconjugated enones together with the chloroketone. This is interpreted as a result of the deprotonation by EtAlCh rendering the [1,5] proton shift irreversible. This catalyst has not been utilized to any great extent at present, but should offer advantages in that acylium salts do not need to be prepared. 

From the results of deuterium labelling studies, a 1,5-proton shift has been rejected, however, for transformations of the 3,4-epiminocyclohexanes (209 R = phenyl or benzyl) to the corresponding 2,3-dihydro-l// azepines (210) <83CB563>. The preferred mechanism is that of the formation of an azomethine ylide intermediate as described in Scheme 29, a mechanism supported by isolation of cycloaddition products (211) with DMAD. 

Transformations of this type play an important role in the mechanism of quite a few reactions, in particular, it is they which lie at the root of the intramolecular tautomerism [52]. It is therefore not surprising that the theoretical investigation of the intramolecular proton transfers is represented by a wide range of calculations of methodological importance. Particularly thoroughly was studied the reaction of the 1,5-proton shift in 3-oxy-2-propenal (cis-enol of malonal-dehyde) XI which affords one of the most significant examples of the degenerate intrachelate tautomerism. 

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