Sigmatropic rearrangement examples

Several other propargylic substrates have successfully undergone [3,3]-sigmatropic rearrangements. Examples are thiocyanates to isothiocyanates [580], the opposite conversion [581] and the equilibrium between such species as exemplified by 173 (Scheme 1.77) [582], cyanates to isocyanates [582] and the formation of isoselenocya-nates [583] and azides [584]. 

There are reports of Diels-Alderase enzymes in the biosynthesis of certain natural products, but their existence is somewhat controversial. For that reason, this discussion has been limited to the sigmatropic rearrangement example cited. 

Sigmatropic rearrangement (Section 24 13) Migration of a a bond from one end of a conjugated tt electron system to the other The Claisen rearrangement is an example... 

The most important sigmatropic rearrangements from the synthetic point of view are the [3,3] processes involving carbon-carbon bonds. The thermal rearrangement of 1,5-dienes by [3,3] sigmatropy is called the Cope rearrangement. The reaction establishes equilibrium between the two 1,5-dienes and proceeds in the thermodynamically favored direction. The conversion of 24 to 25 provides an example ... 

Other examples of [2,3] sigmatropic rearrangements involve amine oxides and diazenes ... 

The following reaction is an example of a [2,3] sigmatropic rearrangement. Would you expect the reaction to be supra facial or antarafacial Explain. 

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. 

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. 

Another version of the double [2,3]-sigmatropic rearrangement, involving the sequence sulfenate - sulfoxide - sulfenate, has also been observed. For example, an effective one-pot epimerization procedure of 17a-vinyl-l 7/i-hydroxysteroids to the rather inaccessible 17-epimers has been achieved by the use of such a rearrangement (equation 35)137. Thus treatment of alcohol 76a with benzenesulfenyl chloride afforded the sulfoxide 77 as a single isomer and E-geometry of the olefinic double bond. Exposure of 77 to trimethyl phosphite in refluxing methanol produced a mixture of 76b and 76a in a 73 27 ratio. 

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

A sigmatropic rearrangement is defined as migration, in an uncatalyzed intramolecular process, of a a bond, adjacent to one or more n systems, to a new position in a molecule, with the n systems becoming reorganized in the process. Examples... 

The order of a sigmatropic rearrangement is expressed by two numbers set in brackets [ij]. These numbers can be determined by counting the atoms over which each end of the s bond has moved. Each of the original termini is given the number 1. Thus in the first example above, each terminus of the s bond has migrated from C-1 to C-3, so the order is [3,3]. In the second example, the carbon terminus has moved from C-1 to C-5, but the hydrogen terminus has not moved at all, so the order is [1,5]. 

A more direct access to the unstable and non isolated sulfonium ylides 58a- c is the reaction of diisopropyl diazomethylphosphonate 57 with allylic sulfides, catalyzed by Cu(II), Rh(II) [39], or ruthenium porphyrins.[40] For example, the a-phosphorylated y,d-unsaturated sulfides 59-61 are obtained through the [2,3] -sigmatropic rearrangement of 58a-c. This method allows the use of a greater variety of starting allylic sulfide substrates, such as 2-vinyl tetrahydrothiophene, or propargylic sulfides (Scheme 15). 

Subsequently, Kametani and coworkers observed a similar allylic sulfoxide-sulfenate-sulfoxide rearrangement. These authors reported the exceptionally facile ringopening reaction of condensed cyclobutenes facilitated by arylsulfinyl carbanion substituents. For example, treatment of sulfoxide 68 with butyllithium in tetrahydrofuran at — 30°C for 10 min, followed by normal workup, results in the formation of product 71, which can be explained by the intervention of a double [2,3]-sigmatropic rearrangement of the initial product 69 via 70 (equation 32). A similar double [2,3]-sigmatropic rearrangement of 1,4-pentadienylic sulfoxides has also been reported by Sammes and 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... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

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