Sigmatropic reactions stereochemistry

Problem 30.7 i Classify the following sigmatropic reaction by order Ja J J, and tell whether it will proceed with suprafacial or antarafacial stereochemistry ... 

Classify as sigmatropic reactions and as cycloadditions each of the following, and predict stereochemistry at the migrating center for the ground-state transformation. 

The experimental results outlined in Eqs. 384 and 385 permit considerations of the stereochemistry of the processes.In every case a cis product is obtained. 4,5-Dihydrooxepine derivatives are formed from cfs-oxiranes in an intramolecular concerted 3,3-sigmatropic reaction, while frans-oxiranes yield not only the dihydro-oxepines but also dihydrofurans, the latter by isomerization via an ylide inte-mediate. 

Sigmatropic reactions Continued) stereochemistry, 166-167 Cope rearrangement, 164, 166 component analysis, 168... 

In the absence of any substituent at the C-6 position, the sulfur stereochemistry controls the stereochemical outcome of the [2,3]-sigmatropic reaction, and hence the imine (60) can be stereo specifically converted to the cw-thiadiazolidine (61) <88JOCl 116>. The epimeric dihydrothiazine... 

Treatment for the Rate of Bimolecular, Gas Phase Reactions , if The symmetry rules allowing some reactions and forbidding others were first proposed by Robert B. Woodward and Roald Hoffmann in two letters to the editor Stereochemistry of Electrocyclic Reactions and Selection Rules for Sigmatropic Reactions , Journal of American Chemical Society, 87 (1965) 395, 2511 as well as by Kenichi Fukui and Hiroshi Fujimoto in an article published... 

The orbital symmetry study of sigmatropic reactions will help us to understand the stereochemistry of these reactions. An FMO analysis of this process indicates the interaction between the frontier orbitals of the n system and orbitals of the migrating atom or atoms. Let us consider the simplest case of 1,3-sigmatropic shift of hydrogen. In this case, n system is an allyl radical and the migrating atom is hydrogen. Their frontier orbitals are allyl i2 and hydrogen Is for thermal reaction... 

A review which covers sigmatropic rearrangements in addition to Diels-Alder reactions, 1,3-dipolar cycloadditions, electrocyclic reactions, and ene reactions has appeared. The stereochemistry of [3,3]-sigmatropic reactions of chiral carbon compounds has been reviewed, as have diastereoselective Claisen rearrangements of substrates bearing chiral auxiliary and enantioselective variants of achiral substrates. Examples of [3,3]-sigmatropic rearrangements used in the synthesis of various types of 3-chromene derivatives have been reviewed. ... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

The reaction between o-quinones and electron rich dienes leads to benzodioxanes. It is proposed that an initial HDA followed by a [3,3] sigmatropic rearrangement account for the stereochemistry of the products <96JCS(P1)443>. 

Some of its special chapters are the Pericyclic Reactions, which includes Cheletropic, Electrocyclic, Sigmatropic and Cycloaddition reactions. The concept of Stereochemistry and Conformation deserve special attention not because they cater to the needs of higher students, but they are immensely useful for candidates trying for UGC and CSIR sponsored competitive examinations, but also those preparing for Union Public Service Commission and State Public Service Commission Exams. The candidates will find the chapters immensely useful and is sure to rouse interest in them in knowing more about mechanistic chemistry. 

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

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