Dienes sigmatropic reactions

Alkyl shift is evident in the Cope rearrangement. A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene. This reaction leads to the formation of a six-membered ring transition state. As [3,3] sigmatropic rearrangements involve three pairs of electrons, they take place by a suprafacial pathway under thermal conditions. 

Fig. 10. Structure criteria for the 3,3-diphospha Cope rearrangement as a sigmatropic reaction process of the 1,2-diphospha cycloalkane dienes, of which structures were determined by X-ray investigations.

The l,5-dien-3-ols produced by 2,3-Wittig rearrangement of diallylic ethers can serve as pivotal intermediates for further elaboration through tandem or sequential 3,3-sigmatropic reactions. " The three sequences examined to date are summarized in Scheme 10. 

Pleiadiene (9), a naphtho[a,fi]cycloheptatriene resulting from photoinduced ring opening (unusual two photon process) or thermal isomerization at 250°C (forbidden sigmatropic reaction) -of l,8-naphthobicyclo[3.2.0]hepta-2,6-diene (10), can alternatively be obtained by photolysis of l,8-naphthotricyclo[4.1.0.0 ]heptene (11). All three of these 1,8-naphtho (C4H4) hydrocarbons 9,10 and 11 undergo thermal and photochemical interconversions. - 34... 

The rearrangement of 1,5-dienes by the [3,3]-sigmatropic reaction is referred to as the Cope rearrangement. This reaction was discussed in Part A, Section 10.2,... 

Luckily, both products end up at N-oxide 579 on treatment with methyl iodide and CsF. A subsequent 2,3-sigmatropic reaction led to the oxygen-substituted diene 578 as a useful building block in terpene synthesis. 

Orbital symmetry rules apply to sigmatropic reactions within ionic species also. For this reason, [1, 2]-shift in a carbonium ion is symmetry allowed [1, 4]-shift occurs with inversion of configuration, e.g., in but-2-en-l-yl cation [1, 6]-shift in hexa-2,4-diene-l-yl cation is expected through suprafacial transition state. 

A different type of sigmatropic reaction has been observed for a variety of acyclic alkyl substituted 1, 5-dienes. The reaction occur on direct irradiation and in competition with the [1, 3] sigmatropic shift, discussed earlier. The product of this rearrangement is an allyl cyclopropane and it arise by a [1, 2] sigmatropic shift. 

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 ... 

The Diels-Alder adduct isolated from the reaction with 2,3-dimethylbuta-l, 3-diene at elevated temperatures [200] is in fact the product of a two step reaction a 1,3-cycloaddition followed by a 3,2-sigmatropic rearrangement [199] (equaPon 49)... 

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. 

When 1,5-dienes are heated, they isomerize, in a [3,3] sigmatropic rearrangement known as the Cope rearrangement (not to be confused with the Cope elimination reaction, 17-8)When the diene is symmetrical about the 3,4 bond, we have the unusual situation where a reaction gives a product identical with the starting material ... 

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the Diels-Alder reaction. The 2+2 cycloadditions were classified into the symmetry-allowed 2+2 cycloaddition reactions [6, 7], It was quite momentous when Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme 11) [53, 54], The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products. 

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>. 

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9.100). An example of this is the photochemistry of cis,cis-1,3-cyclooctadiene [Eq. (9.94)].<169) A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the cis,trans-1,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the cis,trans-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the cis, cis-1,4-cyclooctadiene ... 

The first evidence that the radical cation generated by a single-electron transfer (SET) of an unsymmetrical 1,5-diene 408 can undergo a [3,3]-sigmatropic shift (Cope reaction)... 

---