Rearrangements initiation

The acid catalyzed hydration of olefins is frequently attended by decomposition or rearrangement of the acid-sensitive substrate. A simple and mild procedure for the Markovnikov hydration of double bonds has recently been devised by Brown and co-workers 13). This reaction, which appears to be remarkably free of rearrangements, initially involves the addition of mercuric acetate to the double bond to give the 1,2-... 

Capable of polymerization, decomposition or rearrangement. Initiation of the reaction can be spontaneous, by energy input such as thermal or mechanical energy, or by catalytic action increasing the reaction rate. 

The reaction sequence to the latter hydrocarbons is the most flexible one and starts from the allenic alcohols 212, which are first converted to the l,3-hexadien-5-ynes 213 by an elimination reaction the allene group is then generated by a pro-pargylic rearrangement initiated by the addition of a Grignard reagent. 

Aliphatic 3-aza-4-oxa-Cope rearrangement of ester-amide dienolates increases the synthetic utility of anionic [3,3]-sigmatropic rearrangement initiated by N—O bond cleavage. Treatment of the enehydroxylamine 94 with KHMDS in the presence of TMSCl at —80°C provided a mixture of iV,0-disilylated 95 and 0,0-disilylated 96 derivatives (equation 30). Both of these would, on [3,3]-sigmatropic rearrangement, provide the corresponding silyl ethers 97 and 98, from which 99 and 100 are obtained on workup. 

Langlois and co-workers ° developed a stereoselective hetero-Claisen rearrangement of camphor-based oxazoline A -oxides 300, available from hydroxylami-noisobomeol 298 and orthoesters 299. The rearrangement, initiated by acylation of... 

A base-induced rearrangement initiated by cyanide is possible with 2-bromobicyclo[3.2.0]heptan-6-ones. The rearrangement proceeds via a cyclopropane.243,244... 

The di-ji-methane reaction results in a 1,2-shift in a 1,4-diene unit, but such shifts sometimes occur in monoalkenes (2.44, and the mechanism must be different. The substrates are usually tetra-substituted ethylenes, and it is suggested that the reactive excited state is a Rydberg singlet state, which rearranges initially to give a carbene. Support for such a mechanism comes from the structures of products obtained from 1,2-dimethy(cyclohexene (2.45), which are... 

In this discussion, however, the term 4 base-catalyzed eiioxi.lr isomerization is given a somewhat broader meaning, to include also certain rearrangements initiated by proton abstraction from c rWi atoms not directly part of the oxide ring. These will be taken to indurU not only e-carbon atoms, but also, with certain alicyclie epoxidrs. transannular carbon atoms as well. Table 11 lists a number of examples. 

From a mechanistic viewpoint this group of enzymes is best described as oxidoreductases where intramolecular hydride transfer is mediated by enzyme bound NAD+ (Table IV). Different enzymatic end products for these enzymes are a consequence of different ways of stabilization of the 4-ulose intermediates or the 4 carbonium ion. The stabilization of the 4-ulose derivative can occur in several ways as shown by the various examples mentioned. During the molecular rearrangement, initiated by the formation of the 4-ulose, the reaction intermediates are held bound to the enzyme until at the final step enzyme-NADH donates the hydrogen to the last intermediate and enzyme-NAD+ releases the end product from the enzyme. 

The mechanisms proposed by Caspar and coworkers in their approach to the problem have a common point, the formation of the intermediate 9. This silylene then undergoes two prototropic rearrangements initial isomerization to a silene, the 2//-silole (11), followed by a second to give the 1H-silole (2)19 (equation 3). 

Heterocyclic rearrangements initiated by nucleophilic addition to carbonyl... 

Protonation at the latter is favored kinetically because it can occur with little electronic rearrangement. Initial protonation at the metal may give a cation in which the hydride is exchanged between the metal and various ligand sites e.g., protonation of h -CjHjRhfFS-cyclohexadiene) gives a hydride cation (which can be isolated as a [PFj] salt) ... 

Rearrangements Initiated by Bond Cleavage in Radical Cations... 

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

Kopsone (54) represents a rearranged monoterpene skeleton and it was suggested that this occurs through the pathway shown in Scheme 51 (57) from 9-hydroxyskytanthine (388). The key reaction is viewed as a [3.3] sigmatropic rearrangement initiated by the hydroxy group on the iminiutn ion 389. An intermediate such as 389 would also be susceptible to attack by acetate (or acetoacetate) and the product 390 cyclized (and decarboxyl-ated) to afford kopsilactone (53), which co-occurs (57). 

Brossi, M., and C. Ganter. 1988. The adamantane rearrangement of syn- and anti-tricyclo[4.2.1.12,5]decane. Part II. Rearrangements initiated by regioselective formation of carbocations at C(3) and C(9). Helv. Chim. Acta 71 848. 

A specialized cation rearrangement first observed with bornyl systems is the acid catalyzed rearrangement of camphene hydrochloride (73) to isobornyl chloride (76).32 The reaction, which proceeds by a 1,2-alkyl shift called the Wagner-Meerwein rearrangement, initially generates bornyl cation 74, but rearrangement to... 

Synthesis of left-hand segment began with 7-benzyloxyindole 197. A Vilsmeier-Haack formylation followed by condensation afforded nitroalkene 198. Reduction, acylation with succinic anhydride, and subsequent Bischler-Napieralski cyclization provided dihydro-p-carboline 199. Noyori asymmetric reduction of 199, further treatment with A-iodosuccinimide, followed by activation with silver triflate in the presence of dimethoxy-N,N-diallylaniline furnished the desired coupling product 200. Subsequent saponification and cyclization via a ketene intermediate gave the rearrangement precursor 201. Oxidative skeletal rearrangement initiated by m-CPBA followed by removal of the Fmoc group and conversion of the aniline to the hydrazine furnished Fischer indole precursor 202 (Scheme 35). 

Figure 2.3.b-4 Ethanol dehydrogenation. Rearranged initial rate data. 

Demjanov described the rearrangement initially in 1901, but the reaction was not recognized until 1903 when he reported the rearrangement of... 

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