Disrotatory photochemical ring closure

Figure 14-11 Disrotatory photochemical ring closure of cis,trans-and frans,frans-2,4-hexadiene.

Figure 7.20. Comparison of the AO interactions in the photochemical chain abridgement in a polysilane (top) and in the disrotatory eiectrocyciic ring closure of butadiene (bottom) (by permission from Michl and Balaji, 1991).

Fig. 4.17 Orbital correlation diagrams for the photochemical ring-closure reaction conrotatory (forbidden, left) and disrotatory (allowed, right)...

An example of introducing strain using photochemistry is the synthesis of Dewar benzene. Dewar benzene represents a classic strained ring system that many chemists have studied. One convenient synthesis is shown in Eq. 16.26, the key step being a photochemical disrotatory electrocyclic ring closure. 

In photochemical reactions, we consider the LUMO (the light photon promotes electrons into the LUMO) the symmetry of this is invariably opposite to that of the HOMO, so 4e processes are disrotatory, and 6e processes are conrotatory, exactly the opposite of the thermal reaction. Figure 18.32 shows the LUMO of butadiene, and Figure 18.33 shows the photochemical ring closures of substituted butadienes. 

E,4Z,6Z,8 )-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cycloodati icne. Predict the manner of ring closure—comoiatory or disrotatory— for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case. 

Bridging of a different type has been observed in 3-oxido-l-phenylpyri-dinium (213) which on irradiation in ethyl acetate affords the isomer 214 by way of a photochemically allowed disrotatory ring closure.158 Similarly, stable diaziridines (215) have been isolated on irradiation of 3-oxidopyrida-zinium betaines (216).159 Other related transformations in 2-alkylcin-nolinium-4-olates160 and in 5-oxidopyridazinium betaines161 have been... 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Fig. 20 Energy diagram that illustrates the S0, T, S, and S2 states for the photochemical disrotatory ring closure of butadiene. (Diagram adapted from Salem, 1982)...

Figure 4.41 (a) Orbital overlaps in conrotatory and disrotatory o bond formation in a ring-closure reaction, (b) Stereochemistry of preferred products in photochemical and thermal (dark) processes... 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

In every case, Ri and R2 are trans to each other in the major product. R2 is always cis to the cyclobutene ring in the final product. The initial cycloaddition to the benzene ring occurs syn ring opening proceeds disrotatory and yields a boat-shaped all-cis cyclooctatriene. The photochemical 4-rr ring closure is also disrotatory it occurs in such a manner that the five-membered ring is trans to the... 

When a cyclic polyene is large enough, it can exist in both cis- and iraws-forms. Our approach to polyene cyclization has tacitly assumed an all cis -n chain in the form of a band or ribbon that would slip smoothly on to the surface of a cylinder of appropriate diameter. Should the orbitals of the two polyenes in (36) have a mismatch in their orbital symmetries, a single twist in the tt band of one of them could remedy this (Fig. 10c). Cycloaddition would now be allowed and the reaction would proceed, provided other factors were favorable. Such cases of Mobius (Zimmerman, 1966), anti (Fukui and Fujimoto, 1966b) or axisymmetric (Lemal and McGregor, 1966), as opposed to Hiickel, syn, or sigma-symmetric ring closure are unknown (or, at least, rare). A Mobius form has, however, been proposed as the key intermediate in the photochemical transformations of benzene (Farenhorst, 1966) in (48) in place of the disrotatory cyclization proposed by van Tamelen (1965). 

Irradiation of tropolone alkyl ether 22 (Scheme 14) led to a 4ir-disrotatory ring closure to yield bicyclo[3.2.0]heptadienone 23 with two chiral centers, while prolonged irradiations led to the formation of a secondary product 24 [76-78]. As the same photocyclization was performed in chirally modified zeolites, it is interesting to compare the asymmetric photochemical behavior of 22 in the distinctly different chiral confined media of zeolites and cyclodextrins. Even in the... 

Figure 1 The four stereochemically distinct modes of electrocyclic ring closure wherein the notations inward and outward refer to the axes of rotation as viewed with respect to ( ). The principle of the conservation of orbital symmetry predicts disrotatory (dis) or coniotatory (con) ring closure for the thermally or photochemically induced processes, reqwctively. The same modes (dis or con) are predicted for the reverse ring opening processes...

Electrocyclic closure of butadiene units encased within cycloheptane rings has been used to obtain bicyclo[3.2.0]heptene systems (Scheme 5). For example, irradiation of eucarvone 21 led to the formation of adduct 22 in 52% yield via a disrotatory ring closure . This adduct was used as a key intermediate in the synthesis of the pheromone grandisol, 23, which proceeded in 20% overall yield from 22. in their synthesis of a-lumicolchicine. Chapman and coworkers utilized a photochemically initiated four-electron disrotatory photocyclization of colchicine to produce j6-lumicolchicine 24a and its y-isomer 24b in a 2 l ratio . These adducts were then converted, in a second photochemical step, to the anti head-to-head dimer or-lumicolchicine 25. 

In summary, for butadienes and cyclobutenes four-electron, thermal, conrotatory four-electron, photochemical, disrotatory. The easiest way to visualize the stereochemical result is to make a fist, use your thumbs to designate substituents at the termini of the 77 system, and rotate your fists to determine the stereochemical result upon disrotatory or conrotatory ring closure or opening. 

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