Disrotatory ring

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

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... 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

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... 

Electrocyclic closure of butadiene units encased within cycloheptane rings has been used to obtain bicyclo[3.2.0]heptene systems (Scheme 5)12. For example, irradiation of eucarvone 21 led to the formation of adduct 22 in 52% yield via a disrotatory ring closure123. 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 /Murnicolchicine 24a and its /-isomer 24b in a 2 1 ratio12b. These adducts were then converted, in a second photochemical step, to the anti head-to-head dimer a-lumicolchicine 25. 

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. 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

Figured. RHF, SC and CASSCF(6,6) energy profiles for the disrotatory ring-opening of cyclohexadiene alongthe CASSCF(6,6) IRC (all calculated with a 4-ilG basis).

Figure 5. Symmetry-unique SC orbitals for the disrotatory ring-opening of cyclohexadiene along the CASSCF(6,6) IRC at IRC = -1.2 amul 2 bohr (leftmost column), TS (IRC = 0) and IRC +1.2 bohr (rightmost column). The plot details are as for Fig. 1, except that the isovalue surfaces...

Figure 6. Composition of the active space spin-coupling pattern [ % in Eq. (2)]from the SC wavefunction for the disrotatory ring-opening of cyclohexadiene along the CASSCF(6,6) IRC, expressed in terms of Chirgwin-Coulson weights Pok [seeEq. (4)] in the Rumer basis [see Eq. (3)].

Unsaturation is also important in the metal-catalysed disrotatory ring opening of XXXII ( hexamethyl-Dewar-benzene , HMDB) to hexamethylbenzene. This formally forbidden process is catalysed by monomeric HMDBRhCl (the reaction being of order 1/2 in [HMDBRhCl]2 and order 1 in substrate) Closely related is the conversion of XXXIII(a) to XXXIV, presumably by way of an unsaturated intermediate XXXIII ( ), since free CO or added alkene ligands inhibit the process . ... 

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