Alkyl shift, rearrangements with

Alginates 170,178 Alkali metal ions 206 Alkaline phosphatases 645 active site of 645 occurrence 645 Alkane(s) 382 Alkenes, hydration of 683 Alkyl shift, rearrangements with 527 Alkyl cobalt 867 Alkyl ethers 382 Alkyl cobalamms... 

Reversible, random carbonium ion formation is not required to explain the rearrangements of both 2 and 37 to adamantane in the highly acidic media. Sequential 1,2 alkyl shifts coupled with the well documented 5316,2- and 3,2-hydride shifts of the norbomyl system permit a rearrangement pathway analogous to that discussed earlier as the most likely route for the Lewis acid catalyzed rearrangement of 2 to adamantane. 

The propensity for epoxides to undergo various rearrangements under Lewis acidic conditions is also well-known. Typically, Lewis acid "catalysts", such as BF3-OEt2, must be used in quantities approaching stoichiometric amounts due to their instability under the reaction conditions. However, Yamanoto has recently demonstrated that tris(pentafluoro phenyl)boron, an air-stable and powerful Lewis acid, was not only active in catalytic quantities, but also effective in mediating the alkyl-shift rearrangement of epoxide 70 with... 

Rearrangements that involve carbanions are found to be very much less common than formally similar rearrangements that involve car bo cations (p. 109). This becomes more understandable if we compare the T.S. for a 1,2-alkyl shift in a carbocation with that for the same shift in a carbanion ... 

We have already seen that in [l, 5] alkyl shifts with symmetry allowed will lead to retention of configuration. Another such example with retention of configuration in a [1, 5] alkyl shift is the rearrangement of spirodienes. 

In the course of dolastane synthesis (the dolastanes are a group of marine diterpenes) interesting rearrangements catalyzed by Lewis acids were found. Treatment of the trienone 293 with excess (1.5 eq) ethylaluminum dichloride at low temperatures (—5°C, 48 h) gave the tetracyclic enone 295 in 53% yield while the tricyclic dienone 296 (50%) was formed at room temperature (equation 102)156. It was assumed that both products can be derived from the common zwitterion 294 which undergoes intramolecular alkylation at low temperatures (path a) whereas an alkyl shift takes place at elevated temperatures (path b), followed by a 1,2-hydride shift (equation 102). 

Primary amines are oxidized to the corresponding oximes. The sequence of reactions closely parallels the sequence observed with other mono oxygen donors, i.e., oxidation to alkyl hydroxylamines (V) followed by oxidation to alkylnitroso compounds (VI) which, via a prototropic shift, rearrange to the oximes (VII) ... 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

This reaction occurs with a variety of fully substituted 1,2-diols, and can be understood to involve the formation of a carbenium ion intermediate that subsequently undergoes a rearrangement. The first generated intermediate, an a-hydroxycarbenium ion, rearranges through a 1,2-alkyl shift to produce the carbonyl compound. If two... 

Fig. 14.26. Mechanisms of the photochemicatty initiated and Ag(I)-catatyzed Wotff rearrangements with formation of the ketocarbene E and/or the ketocarbenoid F by dediazota-tion of the diazoketene D in the presence of catalytic amounts ofAg(I). E and F are converted into G via a [1,2]-shift of the alkyl group R1. N2 and a carbene C are formed in the photochemically initiated reaction. The carbene E continues to react to give G. The ketocarbene C may on occasion isomerize to B via an oxacyclo-propene A. The [l,2-]-shift of B also leads to the ketene G.

El with rearrangement by an alkyl shift. The Zaitsev product violates Bredt s rule. 

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