Carbonyl pericyclic reactions

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

C8—Ge—C18 137.7° (3)] (Figure 24). The distortion of the internal O—Ge—C angles leads to an enhanced Lewis acidity, which was illustrated by the promotion of pericyclic reactions involving activation of aldehyde carbonyl groups. 

The triester 70 was needed to study pericyclic reactions with electron-rich (a) and electron poor (b) alkenes.12 The a, 3-unsaturated carbonyl disconnection reveals an enolisable ester 72 (X is some activating group such as CCbR) and a very electrophilic keto-diester 71. The synthesis of the allyl ester 72 is all right but the tricarbonyl compound 71 with two 1,2-diCO relationships, is a challenge. 

Iminium catalysis directly utilizes the higher reactivity of the iminium ion in comparison to the carbonyl species and facilitates Knoevenagel-type condensations, cyclo- and nucleophilic additions, and cleavage of cr-bonds adjacent to the a-carbon. Enamine catalysis on the other hand involves catalytically generated enamine intermediates that are formed via deprotonation of an iminium ion, and react with various electrophiles or undergo pericyclic reactions. ... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

The in situ generation of an iminium ion from a carbonyl compound lowers the LUMO energy of the system. Iminium catalysis is comparable to Brpnsted- or Lewis acid activation of carbonyl compounds. The LUMO energy is lowered, the a-CH acidity increases, and nucleophilic additions including conjugate additions as well as pericyclic reactions are facilitated (Eq. 34). 

Aluminum-Carbonyl Complexation, Activation, and Nucleophilic Reaction Pericyclic Reaction and Asymmetric Reaction... 

The first step 202 is now a pericyclic reaction. It looks like a cycloaddition, though it involves a hydrogen transfer as well. It is in fact a carbonyl (or oxo- ) ene reaction. It is like a Diels-Alder cycloaddition in which a C-H bond has replaced one of the double bonds in the diene and a C=0 group is the dienophile. Many Prins reactions are probably carbonyl ene reactions. In his excellent review30 in Comprehensive Organic Synthesis, B. J. Snider says The (carbonyl) ene reaction and the Prins reaction are not mechanistically distinct . Though this step is pericyclic, it is very polar and the transition state 203 no doubt contains partial charges. It is therefore stabilised and the reaction accelerated by protic acids 205 and Lewis acids 207. 

C/C-connections pericyclic reactions, ene reactions, allylation of carbonyls, Grignard-type additions to carbonyl, aldol-type additions of silyl enol ethers to carbonyl, epoxide opening with cyanide, etc. 

The Diels-Alder reaction occupies a prominent place in organic synthesis, as we have seen, but there are other pericyclic reactions that are also quite useful. One of them is the [2-1-2]-cycloaddition that occurs between two alkenes or between an alkene and a carbonyl or imine.250 Cyclobutanes and oxetanes are... 

In this case, Cucurbituril (53) reveals a number of enzymelike features The reaction exhibits saturation behaviour, it becomes independent of substrate concentration with sufficient amounts of 54 and 55, high concentrations of 54 retard the cycloaddition (substrate inhibition), and release of product 56 from its complex with Cucurbituril (53) is the rate determining step. NMR spectroscopic data suggest that both starting materials of the cycloaddition are hydrogen bonded to the carbonyl groups of 53 with their ammonium moiety and that the reactive substituents extend into the interior of Cucurbituril (53). In this cavity the pericyclic reaction takes place to form the 1,2,3-triazole 56. Kinetic data indicate that the formation of the ternary complex of Cucurbituril (53) with the two starting materials 54 and 55 is not strainless. Since the reaction is still accelerated very much it is assumed that the transition state of the reaction corresponds to the size of the cavity more closely than the substrates. This is a further indication that this case is a useful enzyme model. 

The hetero Diels-Alder reaction has also been carried out efficiently in water. For instance, glyoxylic acid undergoes cycloadditions with various dienes [20] although the carbonyl function is almost exclusively present as its hydrate form. Other pericyclic reactions such as 1,3 dipolar [21] or [4+3] cycloadditions [Eq. (1), Table 1], and Claisen rearrangement [22] gave better results when conducted in aqueous media than in organic solvents. 

In our previous book on domino reactions [4h], we have classified domino reactions according to the mechanism of the different steps. This organizing principal will also be used in this book, and you will find chapters about transition metal catalysis including carbonylation, metathesis and CH-activation, nucleophiHc substitutions, radical reactions, pericyclic reactions, Michael reactions, aldol reactions, oxidations, and reductions. 

Toward the biomimetic synthesis of chloropupukeananin, Kobayashi et al. [22] successfully implemented a domino sequence of two pericyclic reactions. Masked benzoquinone 59 as the diene reactant classifies the Diels-Alder reaction as inverse-electron-demanding one to which the vinylallene 58 can contribute its extra double bond forming a caged skeleton for the subsequent carbonyl-ene reaction. Longer reaction times and higher pressures were able to direct the reaction pathway toward the desired endo product 60 against the backdrop of numerous conceivable side mechanisms. Choosing the pure enantiomer of 59, as depicted in Scheme 6.12, furthermore increased the stereoselectivity and completed the synthesis of the model compound 61 in 70% yield. 

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