Electrocyclic reactions 6-electron photochemical processes

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

Volume 1 includes nine chapters, the first six of these deal with the main established families of organic photochromes which have a few real and many potential applications. Their photochemical processes are based on pericyclic electrocyclic reactions. The three other chapters concern hydrogen or group rearrangement, and electron transfer. Seven out of the nine main authors, selected from all over the world, have not written chapters for previous books and importantly, three are from companies. Four chapters cover families not reviewed... 

Two electrocyclic reactions, involving three electron pairs each, occur in this isomerization. The thermal reaction is a disrotatory process that yields two cis-fused six-membered rings. The photochemical reaction yields the rrans-fused isomer. The two pairs of n electrons in the eight-membered ring do not take part in the electrocyclic reaction. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

A common type of electrocyclic reaction is the ring-opening of a cyclobutene to a butadiene. The stereochemistry of the new alkene(s) in the diene can be interpreted on the basis of the Woodward-Hoffmann rules. For a four electron component, thermal ring-opening occurs by a conrotatory process (both terminal p-orbitals moving clockwise or anticlockwise), whereas the photochemical reaction... 

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. 

Orbital symmetry considerations dictate that in 4n-electron reactions the thermal process must use a conrotatory motion, whereas the photochemical reaction must be disrotatory.Just the opposite rules apply for reactions involving 4re + 2 electrons. The key to analyzing electrocyclic reactions is to look at the way the p orbitals at the end of the open-chain K system must move in order to generate a bonding interaction in the developing G bond. 

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. 

Vitamin D2 is produced by two pericyclic reactions. One of them is photochemicaUy initiated the second thermally initiated. The first step is a photochemical electrocyclic reaction in which a cyclohexadiene of the B ring is isomerized to a triene. The reaction involves six k electrons and is the reverse of the photochemical cyclization reaction discussed in Section 28.4. Thus, by the principle of microscopic reversibility, this photochemicaUy allowed ring opening involving a 4 +2 71 system must occur by a conrotatory process. 

A common type of six electron electrocycUc reaction occurs in the photochemical reaction of 1,2-diaryl alkenes. ° The parent substrate, stilbene can be converted to phenanthrene, a process that involves conrotatory electrocyclization under photochemical conditions and subsequent oxidation of the product to the polycyclic aromatic structure (3.218). 

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