Stereochemistry of [2,3]Sigmatropic Rearrangements

Abstract

Stereoselective synthesis is attracting more and more attention. Successful stereoselective syntheses require the availability of reactions of known stereochemistry which give maximum yields of the desired products. This is particularly so with reactions proceeding via cyclic transition states. Of these, the stereochemistry of [3,3]sigmatropic processes has long been known. This article now summarizes the factors determining the stereoselectivity of [2,3]sigmatropic rearrangements.Volume 18 . Number 8 August 1979Pages 563 -640 I . Introduction [3,3]Sigmatropic rearrangements (a) (Scheme 1; Claisen, Cope, oxy-Cope, hetero-Cope rearrangement)" 41 have been thoroughly studied with regard to their stereochemical course[5 and belong to the standard repertoire of stereoselective syntheses of ~lefins[~-'l or centers of chirality[* ''I. Recent outstanding applications can be seen in a juvenile hormone synthesis ['21, transformation of sugars into prostagland i n~ [ '~' , or the synthesis of tocopherol['4'. I IAmong the [2,3]sigmatropic rearrangements there is a great variety of reactions of types I and I1 [Scheme 1, eq. (b) and (c)]''~ 2'1 which frequently occur at lower temperatures than the [3,3]sigmatropic rearrangements. The five-membered cyclic transition state of [2,3]sigmatropic rearrangements shows greater conformational "flexibility" than the six-membered transition state of [3,3]sigmatropic rearrangements and should therefore be far more susceptible to the effects of stereochemical control by substituents122'. In view of this synthetic potential it was most appropriate that studies on [2,3]sigmatropic rearrangements should have been accompanied by a thorough examination of their stereochemistry which began almost simultaneo~sly[~~l. It is thus all the more surprising that the starting materials for the preparative investigation of [2,3]sigmatropic rearrangements were (purposely?) chosen in such a way that no stereochemical problems occurred. However, this meant forgoing stereochemical information.The following compilation of results should provide an insight into the stereochemistry of [2,3]sigmatropic rearrangements.Scheme I . Survey of 13.31-and [2,3jsigmatropic rearrangements. The suggested valences may be bonds to other atoms or lone pairs. Formal charges follow from the nature and the bonding situation of atoms X, Y, Z. Stereochemkal Observables['I Prof. Dr The rearrangements (b) and (c) lead to formation of a new double bond between C-1 and C-2. Given two different substituents on C-I this double bond may result in an E or 2 configuration. Hence the factors determining the configuration of the new double bond are of interest. 563The centers X and C-I of the reactants of rearrangements (b) and (c) are chiral if they each bear two different substituents. A lone pair will also be regarded as a substituent in these considerations. C-3 [rearrangements (b) [rearrangements (b)] are prochiral if the substituents on these centers are different. In the course of rearrangement, the chirality of X and c -I di...

Asymmetric Catalytic [2,3] Stevens and Sommelet–Hauser Rearrangements of α‐Diazo Pyrazoleamides with Sulfides

“…According to our previous work, [10a] the absolute configuration of the major products, [12,16] control experiments,a nd deuterium-labelling experiments,a sw ell as the general mechanism of [2,3] sigmatropic rearrangements, [18] the catalytic models for the enantiocontrol in the [2,3] Stevens and Sommelet-Hauser rearrangements are proposed ( Figure 1). When 1a is mixed with the chiral nickel(II) complex catalyst (Figure 1b,t op), decomposition by loss of nitrogen proceeds readily to form chiral nickel carbene intermediate A1.T he (phenylthio)acetate prefers to attack from the Re-face of nickel carbene because the Si-face is blocked by the left amide unit of the ligand (Figure 1a).…”

Phosphite‐Catalyzed C−H Allylation of Azaarenes via an Enantioselective [2,3]‐Aza‐Wittig Rearrangement

“…[17] Interestingly,f or all three classes of azaarenes,b oth cis and trans substrates furnished the same major enantiomer of the allylation product, which suggests that the chiral phosphite dictates the stereochemical outcome of the rearrangement of intermediate 4 to 5. Ther eaction of cis-1a with catalyst 2k produced the same enantiomer of 6a,a lbeit with slightly reduced enantioselectivity (81 vs.9 2% ee).…”

Asymmetric Enzymatic Synthesis of Allylic Amines: A Sigmatropic Rearrangement Strategy

“…It is therefore likely that in the sulfimidation/ rearrangement reactions,t he imidation event also proceeds with high enantioselectivity,w hile the rearrangement proceeds with imperfect stereofidelity,v ia competing endo and exo rearrangement transition states. [14] Notably,h owever, Figure 2. Evolution of acatalyst for sulfimidation/[2,3]-rearrangement of Z-1.Reaction conditionsa sgiven in Figure 1, with aDTT work-up employed for reactions giving > 30 %y ield.…”

“…Since sigmatropic rearrangements often proceed with high stereospecificity,t he stereochemistry set in the P411-catalyzed sulfimidation step would be transferred to the allylic amine product. [14] This strategy would establish ab iocatalytic route to chiral allylic amines, compounds that are biologically active [15] as well as valuable synthetic intermediates. [16,17] Furthermore,e xpanding the breadth of enzymatic amination reactions will facilitate the construction of non-natural in vivo pathways for chiral amine production.…”