Sigmatropic shift vinyl

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

A ring expansion by two carbon atoms was discovered on heating (Z)-l-vinyl-cyclonon-3-en-l-yl-trimethylsilyl ether (V/62), Scheme V/10 [31] [32], Two types of trimethylsiloxy enol ethers were observed, each formed by a [1.3] or a [3.3] sigmatropic shift which, after hydrolysis, gave the ketones V/63, V/64, and V/65. The protection of the tertiary alcohol function is not necessary, if the reaction is done in the presence of potassium hydride [33] [34]. The reaction proceeds when the cyclic 1-vinyl alcohols have either a double bond or a benzo group at the 3-position. Other examples of this reaction type are given in Scheme V/10. 

Scheme 55 Vinyl to allyl isomerization by [3,3]-sigmatropic shift...

Chirahty transmission can also be induced in thio-Claisen reactions by a chiral center adjacent to the allyl vinyl sulfide core. The group of Metz-ner examined the diastereoselectivity of the rearrangement of acyclic S-allyl ketene dithioacetals bearing a chiral center adjacent to carbon 6. hi these substrates, the sigmatropic shift proceeded smoothly and with modest syn anti diastereoselectivity [74]. In contrast, higher selectivities are foimd for thio-Claisen precursors bearing a chiral center adjacent to carbon 1 mainly due to steric effects and allylic strain [75]. In 1991, the group of Beslin examined the effect of a hydroxyl substituted chiral center attached to C-1 of Z and E S-allyl ketene dithioacetals [76,77] (Scheme 7). Under the reaction conditions, these... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

The moderate conditions required for this conversion (E = 25.8 kcalmol" ) suggest that a planar diradical is not an appropriate intermediate ". The interconversions of several vinylmethylenecyclopropanes have been studied and [l,3]sigmatropic shifts can alone account for all the products, although some [3,3]shifts are not excluded For example, the syn and anti isomers of l-allylidene-2-vinylcyclopropane (211) undergo thermal isomerization into 4-vinyl-3-methylenecyclopentene (213) and 3-methylene-1,4-cycloheptadiene (214) on heating in cyclohexane at 42-ATC (equation 142). 

The reaction in the shikimic acid pathway is, of course, the [3,3]-sigmatropic shift in which chorismic acid rearranges to prephenic acid on the way to aromatic rings (p. 1403). The simpler reaction given here is one of the family of reactions from Chapter 36 (pp. 944-6) using an allylic alcohol and an enol derivative of a carbonyl compound. In this case we have the enol ether of a ketone. We must combine these to make an allyl vinyl ether for rearrangement. 

This 2,3-sigmatropic shift has been of substantial utility since Evans and Hoffmann realized that allyl alcohols can be obtained by intercepting the sulfenate with thiophilic agents. One possible way of preparing optically active allylic sulfoxides, which started to racemize even at 0 C was the isomerization of the corresponding vinylic sulfoxide followed by the sulfenate rearrangement in the presence of trimethyl phosphite (Scheme 51). ... 

Recently, the known method for the synthesis of thiolanes by cyclization of diazo ketosulfides (72CC860) has received further development (92TL169 95JOC53). It was found that sulfur-containing a-diazo ketones 437 bearing a tethered alkyne unit decomposed in the presence of Rh2(OAc)4 via the addition of the rhodium-stabilized carbenoid onto the acetylene jr-bond to give a vinyl carbenoid, followed by sulfonium ylide formation and a subsequent [2,3]- or [l,2]-sigmatropic shift. This transfor-... 

Under the same conditions, reaction of 1 with buta-1,3-diene afforded the isomeric 1-chloro-l, 2-divinylcyclopropanes 3. However, trans-3 with its two vinyl groups cis to each other immediately rearranges via a 1,3-sigmatropic shift to produce 2-chloro-l,4-cycloheptadiene (4) (see also Sections 1.2.1.2.2 and l.B.2.4.5.1 for similar examples of the formation of seven-membered rings). 

The mechanism of the thermal and photochemical rearrangement is believed to proceed as shown (1 - 2), with the dilemma of concerted vs. diradical nature not definitely resolved for all cases. A comparison of energy parameters for the thermolysis of the parent 1,2-divinyl-cyclopropane and of l-(hex-l-enyl)-2-vinylcyclopropane ( - and Z-isomers, tram- and cis-cyclopropanes) has been reported. When both alkenes bear a Z-positioned substituent, the rearrangement of the divinylcyclopropane system becomes very slow and other processes, such as the [1,5] sigmatropic shift of alkyl(vinyl)cyclopropanes or the vinylcyclopropane to cyclo-pentene rearrangement, may compete (see also Section 2.4.3.). Both the mechanism and the applications of this rearrangement have been reviewed. 

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