Sigmatropic transformations

The nitrogen analogues of the selenoxides, the selenimides, should in principle be capable of [2,3]sigmatropic transformations similar to those of the selenoxides (Eq. 6) ... 

Berson and Nelson provide an interesting example of a highly selective sigmatropic transformation which neatly demonstrates the predictive power of symmetry rules (7). Symmetry conservation requires that the bicycloheptene I (bicyclo[3.2.0]-2-hepten-endo-6-yl acetate-exo-7-d) undergoes a concerted transformation to II with inversion at carbon-7 (5). 

Similarly the substituted decalin 74 was obtained in good yield by the microwave irradiation of 73. The reaction proceeds via an initial Cope rearrangement to generate the allyl vinyl ether 75 for subsequent [3,3]-sigmatropic transformation to ketone 76. Transannular ene cyclization yielded the product 74. ... 

The propargyl Claisen rearrangement affords functionalized allenes through the [3,3]-sigmatropic transformation of propargyl vinyl ethers (Scheme 35)." ... 

The retrosynthetic simplification of 164 which has just been outlined was initiated by the application of a [3,3] sigmatropic rearrangement transform. Other tactical combinations involve the use of such rearrangement transforms to link a pair of disconnective transforms. 

Preparation of y-pyrone by the reaction of methoxybutenone with formic acid esters (10°C, MeONa, CgHe, 1.5 h) has been reported (73 JPP7229512 80MI2). The intermediate 292 undergoes [3,3]-sigmatropic rearrangement to form methoxydi-hydropyrone 293 which further eliminates methanol, thus transforming to y-pyrone. 

There arc no methods for the preparation of azepines by ring transformation of other seven-membered systems however, interconversion between the four tautomeric azepines, either by [1,5]-H sigmatropic shifts or under base catalysis, is common and almost always results in formation of a 3//-azepine. Such transformations are dealt with in Section 3.1.1.5.7. 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

A neat twofold transformation, obviously a consequence of a sigmatropic [ rearrangement rather than by an ionic pathway, occurs in the case of a propargyl sulfinate20 (equation 3). 

The data presented demonstrate that allylic sulfoxides can provide an easy and highly stereoselective route to allylic alcohols taking advantage of the facility of the allylic sulfoxide-sulfenate [2,3]-sigmatropic rearrangement. This is of considerable synthetic utility, since a number of stereoselective and useful transformations of allylic alcohols and their derivatives have become available in recent years107-109. 

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 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]. 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

Following studies on the rearrangement of allylic arenesulfinates, Braverman and CO workers have investigated a number of natural extensions of this unique transformation, including the predictable [2,3]sigmatropic rearrangements of allylic sulfenates to sulfoxides and of propargylic sulfenates and sulfinates to allenic sulfoxides and sulfones respectively. The last reaction is described below, while the other two are described in Chapter 14. 

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 ... 

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