Subject 3,2-sigmatropic rearrangement

This section will focus on recent examples of asymmetric [3,3]-sigmatropic rearrangements involving dienes and polyenes. Attention will be given to Cope and Claisen rearrangements, as well as to several of their variants. For more exhaustive reviews of the subject, the reader is referred elsewhere69,70. 

The mixture of 5 and 6 can be converted to 9 by reduction, separation and then epimerization/reduction of one isomer. Alcohol 9 is then further subjected to similar procedure as for 1 to give tricyclic ether 12, through the same Cu(tfacac)2-catalyzed ylide formation/[2,3]-sigmatropic rearrangement of diazo compound 10 (Scheme 2). 

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. 

Cycloadditions, the subject of the last chapter, are just one of the three main classes of peri-cyclic rearrangement. In this chapter, we consider the other two classes—sigmatropic rearrangements and electrocyclic reactions. We will analyse them in a way that is similar to our dealings with cycloadditions. 

Oxopentanals may be synthesized from allylic alcohols by 3,3 sigmatropic rearrangement of their vinyl ethers, and subs uent oxidation of the terminal double bond. Cinnamyl alcohol (31) was converted to the allyl vinyl ether (32), which was subjected to Qaisen reairangement to give 3-phenyl-4-pentenal (33) in 50% yield. Oxidation of the terminal double bond of (33) gave 3-phenyl-4-oxopentanal... 

Cope and Claisen rearrangements are the [3,3]-sigmatropic rearrangements and are among the most commonly used sigmatropic reactions. They have been the subject of numerous... 

This review aims to siunmarize and update the various roles exerted by sulfur-containing fimctionaUties in [3,3]-sigmatropic rearrangements. Some of the subject matter discussed herein has been reviewed before [1-5], therefore, we have made an effort to discuss in depth just the more recent results, as well as mechanistically related transformations that were not included in previous reviews on Claisen and thio-Claisen rearrangements. 

The reduction of organic halides in the presence of aromatic hydrocarbons has been the subject of detailed kinetic studies, which provide rate constants for the homogeneous ET [130] and the follow-up reaction [131]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers in 1978-1980 [132-134]. Homogeneous ET also plays an important role in the protonation of anion radicals [135], and it is also an essential step in the sigmatropic rearrangement in 1-arylindenes [136]. Both reactions are discussed here. 

The ability of charged substituents to accelerate the 3,3-sigmatropic rearrangement of allyl vinyl ethers (the Claisen rearrangement) has also been documented. The effect of oxyanion substituents on the rate and course of aliphatic Claisen rearrangements has been the subject of particular attention. - In 1972, Ireland and Mueller reported that the lithium enolate derivatives of allyl esters undergo rapid and effi-... 

A derivative of (S)-prolinol 10.23, quartemized by CICH2CN and then transformed into the ammonium ylide by KO-fert-Bu in DMSO, has been subjected to a [2,3]-sigmatropic rearrangement at -90°C. After hydrolysis of the aminoni-trile formed in this way, an a-chiral P,y-unsaturated aldehyde is formed with an excellent selectivity [261,290, 1008, 1062] (Figure 10.9). 

The formation of C-O and C-N bonds by sigmatropic rearrangements is referred to in this section only for the sake of completeness. For more detailed discussion on these subjects see Section D.4.11. (C-O bond formation) and Section D.7.6. (C N bond formation). 

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