Thermal electrocyclic ring opening

A similar result is obtained for the thermal electrocyclic ring-opening of 3,4-dimethylcyclobutene. The trans isomer yields only (2 ,4 )-2,4-hexadiene when heated, and the cis isomer yields only (2 ,4Z)-2,4-hexadiene. On UV irradiation, however, the results are opposite. Cyclization of the 2E,4 isomer under photochemical conditions yields cis product (Figure 30.4). 

Intermediates (6) can yield to the corresponding [3-lactams via conrotatory electrocyclic closures (Fig. 3). The main features of these transition structures closely resemble those found for the thermal electrocyclic ring opening of cyclobutenes [34]. 

Spiro-cyclobutene derivatives prepared from the intramolecular Wittig reaction of DMAD with ethyl oxo-(2-oxocycloalkyl)ethanoates yield cycloalka[6]pyrans via a thermal electrocyclic ring-opening - ring-closing sequence (Scheme 1) <03X2001 >. 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

Thermal electrocyclic ring opening of 2-oxopyran-5-carbaldehyde followed by E/Z isomerisation and electrocyclisation is proposed to account for their conversion to 3- and 5-acetylpyran-2-ones <97CC1293>. 

Thermal electrocyclic ring-opening of tetraphenylcyclobutenone is followed by intramolecular recyclization of the resulting vinylketen (580) to give the naphthol (581) in high yield. The reaction is retarded by the presence of AlClj owing to formation of complex (582). 

The thermal electrocyclic ring-opening of the cyclobut-2-enol (594 R = H) affords tiglic aldehyde (596 R = H) quantitatively, showing that only the -dienol (595) is formed. This was confirmed by rearrangement of (594 R = D) to (596 R = D) and of (594 R = SiMej) to (595 R = SiMe3), providing a useful synthesis of the aldehydic isoprene synthon. ... 

H-Pyrans show the chemical properties ofoxacyclohexadienes for instance, the O/C-2 bond can take part in thermal electrocyclic ring-opening with formation of dienones, for example, 4 5 ... 

Primary products of this photoreaction are bicyclic diaziridines 13, which yield the diazepines 14 in a thermal electrocyclic ring-opening of the three-membered ring. The diaziridines 13 can be thermally reconverted into the pyridine N-ylides 12. 

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