Stereochemical outcome of electrocyclic reactions

During electrocyclic ring-opening as well as electrocyclic ring-closure of a polyene system there are two possibilities due to S5mimetiy of the system. One possibility may be disallowed on account of molecular geometry or steric factor operational during transtional state. Stereochemical outcomes for different membered polyene systems are discussed below  

In compound (I) outward movement of two methyl groups is easy and therefore rate of reaction rate is high but in compound(II) methyl groups move inwardly which is difficult as steric hinderance is there. 

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The choice of whether to use thermal or photochemical conditions also has an impact on the stereochemical outcome of electrocyclic reactions with four Tt electrons ... 

These stereochemical observations puzzled chemists for many years, until Woodward and Hoffmann developed their theory describing conservation of orbital symmetry. This single theory is capable of explaining all of the observations. We will first apply this theory to explain the stereochemical outcome of electrocyclic reactions taking place under thermal conditions, and then we will explore electrocychc reactions taking place under photochemical conditions. 

TABLE 18.1 Stereochemical Outcomes of Electrocyclic Reactions Number of Electrons Conditions Stereochemical Outcome... 

There is a general agreement on the stepwise nature of the [2+2] cycloaddition between ketenes and imines. The first step consists of a nucleophilic attack of the iminic lone pair on the v/ -hybridized atom of the ketene to form a zwitterionic intermediate. The subsequent four-electron conrotatory electrocyclization leads to the corresponding (3-lactam. The final stereochemical outcome of the reaction depends on the combination of the following features (1) endo/exo attack of the imine on the ketene (2) inward outward disposition of the substituents at the conrotatory transition structure and (3) relevance of the isomerization pathways, including those of the starting imines. 

This stereocontrol is observed in many other electrocyclic transformations and is governed by the symmetry properties of the relevant tt molecular orbitals. The Woodward-Hoffmann rules describe these interactions and predict the stereochemical outcome of all electrocyclic reactions as a function of the number of electrons taking part in the process and whether the reaction is carried out photochemically or thermally. A complete treatment of this subject is best left to a more advanced course in organic chemistry. However, the predicted stereochemical course of electrocyclic reactions can be summarized in the simple manner shown in Table 14-2. 

Dienes may be involved in electrocyclization reactions as well. Two well-documented examples are the cyclobutene ring opening244 and the 1,3-cyclohexadiene formation245 reactions. Predictions regarding the stereochemical outcome of these rearrangements can be made applying the orbital symmetry rules. The thermally... 

The predicted photochemical disrotatory closure of protonated divinyl ketones has been documented in several laboratories, most notably by Nozaki, Noyori, and Cerfontain (equation 3). The stereochemical outcome in these reactions was discernible due to secondary processes which preserved the sense of electrocyclization. 

We have just seen why the configuration of the product formed under photochemical conditions is the opposite of the configuration of the product formed under thermal conditions The ground-state HOMO is symmetric—so disrotatory ring closure occurs, whereas the excited-state HOMO is asymmetric—so conrotatory ring closure occurs. Thus, the stereochemical outcome of an electrocyclic reaction depends on the symmetry of the HOMO of the compound undergoing ring closure. 

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

The symmetty of the HOMO of the compound undergoing ring closure controls the stereochemical outcome of an electrocyclic reaction. 

Although 2,8, etc., electron electrocyclic reactions are less common, they do exist, so it is useful to summarize the stereochemical outcomes quite generally (Table 18.1). You don t need to memorize the whole of this table—if you remember just one entry, and the fact that 4h and 4 + 2 are different, and thermal and photochemical are different, then you have it all. Some examples of electrocyclic reactions are given in Figure 18.34. 

Electrocyclic reactions were first described by Woodward and Hoffmann in their classic series of articles. One very interesting aspect of such reactions is, that for a given conjugated polyene photochemical transformation leads to the opposite stereochemical outcome than the thermal one314). 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

A4) of a compound with three tt bonds is asymmetric (Figure 29.3). Therefore, under Lm 3u LergJngring°doVu photochemical conditions, (2 ,4Z,6Z)-octatriene undergoes conrotatory ring closure, controls the stereochemical outcome so both methyl groups are pushed down (or up) and the cis product is formed. of an electrocyclic reaction. 

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