Disconnection pericyclic

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

Pericyclic disconnections and sigmatropic and other rearrangements give rise to synthons which are themselves reagents. Such disconnections greatly simplify the target molecule (e.g. the refro-Diels-Alder reaction). These disconnections are most commonly applied in alicyclic and heterocyclic systems. 

For each of the following molecules, perform a pericyclic disconnection ... 

So we shall be using discoimections corresponding to ionic and pericyclic reactions, and we shah be looking all the time for a good mechanism to guide us. You should now see what a disconnection means and be ready for the next stage. In the next few chapters we... 

The most important pericyclic reaction in synthesis, indeed one of the most important of all synthetic methods, is the Diels-Alder reaction. We have seen this many times before. What are the clues for a Diels-Alder disconnection ... 

In a pericyclic reaction, the electron density is spread among the bonds involved in the rearrangement (the reason for aromatic TSs). On the other hand, pseudopericyclic reactions are characterized by electron accumulations and depletions on different atoms. Hence, the electron distributions in the TSs are not uniform for the bonds involved in the rearrangement. Recently some of us [121,122] showed that since the electron localization function (ELF), which measures the excess of kinetic energy density due to the Pauli repulsion, accounts for the electron distribution, we could expect connected (delocalized) pictures of bonds in pericyclic reactions, while pseudopericyclic reactions would give rise to disconnected (localized) pictures. Thus, ELF proves to be a valuable tool to differentiate between both reaction mechanisms. 

In the present chapter, however, because the problem is considered from a retrosynthetic point of view, we will distinguish only between heterolytic and homolytic disconnections -to which we will refer to as "retro-annulations"- and concerted or "pericyclic (or cheletropic) cycloreversions". In the same way that Woodward-Hoffmann rules [2] apply to pericyclic reactions, the Baldwin rules [3] may be said to apply to heterolytic as well as to homolytic "monotopic" annulations (see Table 6.1). Although in the preceding Chapter (see 5.5) we have already described some radical "monotopic" annulations, later on in this Chapter (see 6.1.3) and mainly in Chapter 7 we will refer to some new methods, syntheses and strategies which have been developed recently. 

Cycloreversions pericyclic and cheletropic disconnections. The Woodward-Hojfmann rules... 

In addition to being treated in a similar manner to acyclic systems, cyclic systems can also be disconnected according to concerted pericyclic or cheletropic cycloreversions. In this context, the Woodward-Hoffmann rules [2] are of... 

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. 

In the disconnection approach, bonds are usually disconnected by either homolytic or heterolytic fission (Figure 10.11(a) and (b)). However, some bonds may be disconnected by a reverse pericyclic mechanism (Figure 10.11(c)). 

Figure 10.11 (a) Homolytic, (b) heterolytic and (c) pericyclic bond disconnections. Homolytic... 

Another class of reaction where you can see at once that the disconnect ion is the reverse of the reaction is Pericyclic Reactions. An example would be the Diels-Alder reaction between butadiene and maleic anhydride. Draw the mechanism and the product. 

Pericyclic reactions are the ones where the electrons rearrange through a closed loop of interacting orbitals, snch as in the electrocyclization of 1,3,5-hexatriene (88). Lemal pointed ont that a concerted reaction could also take place within a cyclic array, bnt where the orbitals involved do not form a closed loop. Rather, a disconnection occnrs at one or more atoms. At this disconnection, nonbonding and bonding orbitals exchange roles. Such a reaction has been termedpseudopericyclic. 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

The key steps of a synthesis usually involve formation of carbon-carbon bonds. In Chapters 8 and 9, nucleophilic methods involving the disconnect product were discussed in detail. In the chapters that follow, methods of forming carbon-carbon bonds by pericyclic reactions (disconnect products that arise from breaking more than one bond), and by reactions that have cationic intermediates (C disconnect products) or radical intermediates (C disconnect products-carbenes are included in this chapter) will be presented. In each chapter, additional examples of synthetic strategy using those particular reactions will be presented. 

PERICYCLIC CARBON-CARBON BOND FORMING REACTIONS MULTIPLE BOND DISCONNECTIONS... 

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