Sigmatropic rearrangements typical reactions

The allyl sulphenate-allyl sulphoxide rearrangement is a general reaction and is applicable to structurally diverse allyl alcohols204,205 (Table 13). Mechanistically, it represents a typical example of a [2,3]-sigmatropic rearrangement as shown by the detailed investigations of Mislow and Braverman and their coworkers. 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Labelling experiments using both 15N and 2H indicate that the rearrangement is intramolecular. Reactions are also acid-catalysed and are believed to occur via the Wheland intermediates 74 and 75. The most likely interpretation is that the rearrangement occurs within the Wheland intermediate by a direct 1,3-shift rather than by consecutive 1,2-shifts, and that the process can be regarded as a typical [l,5]-sigmatropic rearrangement. 

Despite the typical use of the 1,5-dienic system in the hetero-Cope rearrangement the participation of triple bonds, C=N and C=C, in the [3,3]-sigmatropic rearrangement was also reported. Synthesis of A-substituted benzimidazolinones 175 by a hetero-Cope rearrangement of the adduct 174 formed by reaction of A-arylhydroxamic acids 172 with cyanogen bromide 173 in the presence of triethylamine and at low temperatures was reported by Almeida, Lobo and Prabhakar (equation 51). 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

Step. In fact, several asymmetric reactions via this process have recently been reported for the preparation of chiral allylic alcohols. In this section, typical results of asymmetric [2,3]sigmatropic rearrangement via chiral allylic selen-oxides to afford the corresponding chiral allylic alcohols are described. 

A typical example of the reaction of N-ethoxycarbonylazepine with a reactive diene is illustrated by the addition of 2,5-dimethoxycarbonyl-3,4-diphenylcyclopentadienone. Heating a mixture of these reactants at 80 C for several hours gave exo [6 -i- 4] adduct (92) and endo [4 + 2] adduct (93) in yields of 18% and 82%, respectively. Alternatively, heating diene (94) in the presence of the same azepine gave only the [2 -i- 4] adduct (95). Low-temperature examination of the cyclopentadienone reaction revealed the intervention of a cycloadduct similar in structure to compound (95), which rapidly led to the [4 + 2] adduct (93) via a facile 3,3-sigmatropic rearrangement. ... 

Intramolecular vinylketene cycloadditions have also found use as an efficient method for the preparation of divinylcyclobutanols suitable for accelerated oxy-Cope rearrangement. A typical reaction sequence is outlined in Scheme 37. In this case the cyclobutanol salt produced by addition of vinyllithium to (270) at -78 C was observed to undergo 3,3-sigmatropic rearrangement simply upon wanning to room temperature the tricyclic enone (272) incorporating a tra/is-cyclooctene ring was thus formed in 62% overall yield from (270). 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

Typical [3,3] sigmatropic rearrangements include the Claisen and Cope rearrangements. The aza-Cope is a [3,3] sigmatropic rearrangement with a nitrogen atom in one of two possible positions 11 or 13. In neither case is a ring of any kind formed in this reaction but in both cases the product is a rather unstable imine and further ionic reactions may lead to heterocycles. 

The aliphatic Claisen (or oxy-Cope) rearrangement is a familiar [3,3] sigmatropic rearrangement 154, typically requiring temperatures of 200 °C or so. The reaction is driven by the formation of... 

Trofimov and co-workers have developed a pyrrole synthesis which involves reactions of ketox-imes with alkynes under strongly basic conditions (Scheme 51) <90AHC178,94H(37)l 193>. The key step in the reaction is a sigmatropic rearrangement of an 0-vinyloxime which is followed by a typical... 

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