Electrocyclic reactions stereoselectivity

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

Not all electrocyclic reactions are stereoselective. It turns out that none of the three of the possible interconversions between triplet cyclopropylidene and allene should show SS, according to an analysis given by Borden (1967). 

Similarities between mass spectral and thermal fragmentations are particularly common in certain reaction types. Electrocyclic reactions, for example, are frequently similar in the two processes. The thermal process has in general a higher stereoselectivity (because of the higher aromaticity in even-electron systems). Retro-Diels-Alder reactions are typical examples for the similarity of the two processes. Internal displacement reactions may also be similar in the two processes, mainly in the case of internal radical displacements. The relationship between mass spectra and thermal fragmentation is complex, and it is useful to discuss it for separate classes of compounds. 

The stereoselective synthesis of (+)-trichodiene was accomplished by K.E. Harding and co-workers. The synthesis of this natural product posed a challenge, since it contains two adjacent quaternary stereocenters. For this reason, they chose a stereospecific electrocyclic reaction, the Nazarov cyclization, as the key ring-forming step to control the stereochemistry. The cyclization precursor was prepared by the Friedel-Crafts acylation of 1,4-dimethyl-1-cyclohexene with the appropriate acid chloride using SnCU as the catalyst. The Nazarov cyclization was not efficient under protic acid catalysis (e.g., TFA), but in the presence of excess boron trifluoride etherate high yield of the cyclized products was obtained. It is important to note that the mildness of the reaction conditions accounts for the fact that both of the products had an intact stereocenter at C2. Under harsher conditions, the formation of the C2-C3 enone was also observed. 

Fig. 8. Stereoselective synthesis of coronafacic acid (31) through retro Diels-Alder and electrocyclic reactions...

An electrocyclic reaction is an intramolecular reaction in which the rearrangement of tt electrons leads to a cyclic product that has one fewer tt bond than the reactant. An electrocyclic reaction is completely stereoselective—it preferentially forms one stereoisomer. For example, when (2 , 4Z,6 )-octatriene undergoes an electrocyclic reaction under thermal conditions, only the cis product is formed when (2 , 4 6Z)-octatriene undergoes an electrocyclic reaction under thermal conditions, only the trans product is formed. Recall that E means the high-priority groups are on opposite sides of the double bond, and Z means the high-priority groups are on the same side of the double bond (Section 3.5). 

Electrocyclic reactions are a class of pericyclic reactions in which a conjugated polyene interconverts with an unsaturated cyclic compound containing one less carbon-carbon double bond than the polyene. " The reactions can be promoted thermally or photochemically and take place with a very high degree of stereoselectivity. 

A gentle introduction to an electrocyclic reaction without stereoselectivity. 

Houk, K. N., Spelhneyer, D. C., Jefford, C. W., Rimbault, C. G., Wang, Y., Miller, R. D. (1988). Electronic Control of the Stereoselectivities of Electrocyclic Reactions of Cyclobutenes against Incredible Steric Odds. Journal of Organic Chemistry, 53, 2125-2127. 

In reaction dynamics the PJT may be responsible for stereoselectivity, because of the selection rules for vibronic coupling matrix elements. Via these relaxation matrix elements the Wigner-Eckart theorem is at the basis of the Woodward-Hoffmann ruies [12]. We shall not discuss these rules in generai, but consider some simple illustrations, related to electrocyclic reactions. Take as a simple example the ring ciosure of cw-butadiene, as iiiustrated in Fig. 6.3. The relevant occupied orbitals are... 

The MOVB theory of stereoselection of "real" radical electrocyclic reactions destroys the impasse which has been reached in the field of radical chemistry with regards to the stereoselectivity of radical reactions, in general. 

A review detailing computational methods for determining transition-state geometry in stereoselective cycloaddition reactions has been presented. Diels-Alder reactions, 1,3-dipolar cycloadditions, sigmatropic rearrangements, electrocyclic reactions, and ene reactions have been extensively reviewed. Pressure-induced cycloadditions to strained arenes have been reviewed. ... 

---