Electrocyclic cleavage

The allyl cation generated by the electrocyclic cleavage of dibromocyclopropanes (cf. Section 4.7.3.7.1) has been trapped by a carboxyl group in a highly efficient synthesis of furanones (equation 111) and pyranones.236 Acid-catalyzed openings of dihalocyclopropanes also give similar results.237... 

CH, CyCl° " 2 addition h3co 1 < < > I K electrocyclic cleavage h3co... 

X = CH or CPh) electrocyclic cleavage is more commonly observed. Of such examples the domino-Diels-Alder-like synthesis of semibullvalenes deserves... 

Table 3. Formation of 5-Lactones and Tetrahydropyrans by Electrocyclic Cleavage of Bromocyclopropanes ...

The mesoionic 3-oxido-A-alkylpyridinium betaine 13 undergoes 1,3-dipolar cycloadditions as the dipoles a and b. Thus, acetylenedicarboxylate addition via O and C-2 is followed by electrocyclic cleavage of the pyridine N/C-2 bond in the intermediate 16, resulting in the formation of fiiran derivatives 17, In contrast, phenylacetylene and acrylic ester add to the betaine 13 via C-2 and C-6 producing 8-azabicyclo[3.2.1]octane derivatives 14 and 15 [85] ... 

Biosynthesis IV-Methyltetrahydroprotoberberinium salts such as I are the precursors of S. a. Dehydrogenation of the tetrahydroberberinium ions to ions II and subsequent electrocyclic cleavage of the N-C-8 bond furnish Intermediates III which can cyclize to give the spirobenzylisoquinolines, ochotensimine or ochotensine in the example shown above. ... 

Bridgehead olefins (834 n = 3 or 4) are formed in the solvolysis of the dibromo-propellanes (833 n = 3 or 4) by loss of halide anion, electrocyclic cleavage of the cyclopropane ring, and solvent capture/ The products isolated depend upon the precise reaction conditions in the case of (833 n = 3) acetolysis affords (835 53%) and (836 18 %) provided that water is not present. The details of the mechanism of the Ag" "-assisted solvolysis of (833 n = 4) have been elucidated with the aid of labelling. " ... 

This corresponds to the electrocyclic ring-opening of the cyclopropyl cation the preferred reaction pathways should involve + 2 ] or [ jOa + 2a] interactions, (Equation 6.1). In simple unfused ring systems the evidence for disrotatory scission comes from kinetic measurements (see below). The stereochemical test is not available because of the interception of the allyl cation by a counter-ion, (Equation 6.2). However, when the cyclopropyl cation is part of a bicyclic system, for example (1), it is found that electrocyclic cleavage occurs readily even when the number n has the small value of 3 or 4. In these circumstances the conrotatory mode, which would yield the unstable trans-... 

Direct observation of the electrocyclic cleavage of the cyclopropyl anion has been reported for one case, (Equation 6.8) the observed conrotatory mode is the predicted thermochemical pathway. An isoelectronic system has provided... 

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