Disrotatory ring closur

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

Active Figure 30.5 Thermal cyciizaltons of 2,4,6-octatrienes occur by disrotatory ring closures. Sign in atwww.thomsonedu.com... 

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

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 HOMO of the pentadienyl cation is j/, which is antisymmetric, so a conrotatory ring closure occurs, consistent with the four electrons involved in this reaction. The HOMO of the pentadienyl anion is /2, which is symmetric, so a disrotatory ring closure occurs, consistent with the six electrons involved in this reaction. 

Other aliphatic acyclic dienes such as isoprene (2)102,111, 2-isopropyl-1,3-butadiene (24)102, and , -2,4-hexadiene (5)78 also yield the corresponding cyclobutene, all via excitation of the s-cis conformer. The latter yields the disrotatory ring closure product, c -3,4-dimethylcyclobutene (54), stereospecilically78. 

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 14.2. a) Conrotatory electrocyclic reaction showing preservation of a C2 axis of symmetry. (b) Disrotatory ring closure showing preservation of a mirror plane of symmetry. 

The decatetraene cyclizes at — 10°C. by a conrotatory motion, to produce the cyclooc-tatriene with the two methyl groups in a trans orientation. When the cyclooctatriene is warmed to 20°C, it undergoes a thermally allowed disrotatory ring closure to give the final product. 

Irradiation of the substituted cyclohexadiene resulted in the formation of the Dewar benzene skeleton by a disrotatory ring closure. Reaction with lead tetraacetate (a reaction that is not covered in this book) was used to remove the anhydride group and introduce the final double bond of Dewar benzene. Again, because of the forbidden nature of the conrotatory opening to benzene, Dewar benzene has an appreciable lifetime. At 25°C the half-life for its conversion to benzene is 2 days, and at 90°C its half-life is 30 min. 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

The rules based on the Hiickel-Mobius concept have their counterpart among the Woodward-Hoffmann selection rules. There was a marked difference between the suprafacial and antarafacial arrangements in the application of the Woodward-Hoffmann treatment of cycloadditions. The disrotatory and conrotatory processes in elec-trocyclic reactions presented similar differences. The suprafacial arrangement in both of the reacting molecules in the cycloaddition as well as the disrotatory ring closure in Figure 7-25 correspond to... 

Figure 7-25. Comparison of the disrotatory ring closure and the V2S + V2S reaction with the Hiickel ring.

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... 

The inclusion complex 26, shown in Scheme 11, has been used as a host by Toda and coworkers to carry out a number of enantioselective reactions [231]. For example, irradiation of a 1 1 host-guest assembly of a-tropolone methyl ether 27 and (S,S)-( — )-26, in the solid state gave (lS,5R)-( — )-28 of 100% ee. The authors state that the high stereoselectivity is a result of the steric hindrance to disrotatory ring closure from one direction due to the structure of the host. This leads to the formation of only one enantiomer of the product. More details on this topic are available in Chap. 13. 

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