Retro-electrocyclizations

A rather unusual combination of a Ni-catalyzed [2+2+2] cycloaddition of oxaben-zonorbornadiene 6/4-72 with an alkyne 6/4-73 followed by a retro-electrocyclization to give an arene 6/4-75 and benzoisofuran 6/4-76 was described by Cheng and coworkers [298], Under the reaction conditions, 6/4-76 reacted with another molecule of 6/4-72 to give 6/4-77 and 6/4-78 (Scheme 6/4.18). The best yields were obtained employing phenylacetylene with 98% overall yield (58% of 6/4-78 and 40% of 6/4-77). At a lower temperature (18 °C), intermediates of type 6/4-74 could be isolated. On occasion, azabenzonorbornadienes may also be used instead of 6/4-72. 

It is not possible within the restrictions of this review to treat all carbocyclic frameworks (e.g. spirocy-clics) and a conscious decision was also made specifically to exclude the heterocyclic version of this electrocyclization. The number and variety of heteroatoms and the possible permutations along the hexa-triene system that can a priori be considered would justify a separate treatment. The reader is referred to the excellent monograph by Marvell for references to the treatment of electrocyclizations leading to heterocycles. However, in order to provide at least a few leading references, several selected examples of hetero electrocyclizations are included in this chapter together with comments concerning retro electrocyclizations and higher-order electrocyclic processes. 

From a synthetic point of view, the CHT-NCD equilibrium has been exploited in a total synthesis of colchicine and, more recently, in syntheses of tropones (115) and tropolones (119) from 7-halobicy-clo[4,1.0]heptenones (112) or 7-halobicyclo[4.1.0]heptane-3,4 diones (116), respectively. Keto-enol tautomerism of (112) to (113) followed by ring opening to the cycloheptatrienols (114) and loss of HX would explain the formation of the tropone (115). The tropolone (119) probably results from a similar sequence. These examples represent one of few cases that exemplify 6e retro electrocyclizations presented dius far in this review. Additional examples will be presented later in Section 6.2.4.1. 

By contrast, the retro electrocyclization by photochemical irradiation is well known. For example, the photochemical transformation (6e conrotato ring opening) of provitamin D (5) to previtamin D (3) and then thermal isomerization (1,7-H shift) of the latter is a well-established sequence leading to vitamin D (4). It is a sequence involved in vitamin D biosynthesis and in the laboratory synthesis of vitamin D. Moreover, the process is used commercially. 

A5 = — 10.4e.u. Furthermore, protonation of the cyclononatetraenide anion, presumably giving the all cis-cyclic tetraene results in cw-dihydroindene at room temperature consistent with a disrotatory electrocyclization of the all c/ -tetraene. " However, it should be recognized that the cis-6A.O triene cannot give the all cis-tetraene in a concerted retro electrocyclization, so a biradical pathway must be accessible for this process (Scheme 10.12). 

A 1,5-hydrogen shift followed by a retro-electrocyclization is probably responsible for the phenylbutadiene, and a second 1,5-hydrogen shift after the first gives the 1,2-isomer, and naphthalene is always formed from the 1,2-isomer upon heating. 

The reaction appears to be a 1,5-shift of a cycloheptatriene (the retro-electrocyclization product of the norcaradiene) ring carbon over the cyclopentadiene system which is followed by a 1,5-hydrogen shift. 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

It was suggested that the 3,3-shift had occurred, but, the conjugate base of the product underwent a retro-electrocyclic reaction and then incorporated deuterium. Regardless, the approximate free energy of activation for the overall reaction is 33 kcal/mol, which is roughly the expected BDE of the cyclopropane bond undergoing reaction, so if the reaction is a 3,3-shift, it probably involves biradicals. 

The Pt complexes (54), bearing two alltynyl-substituted dimethyldihy-dropyrenes, showed photochromic properties with ring closure (55) and opening. Upon irradiation, the stilbene ether (53) initially underwent 6ti electrocyclization to afford the dihydrophenanthrene (57). The resulting (57) successively underwent an acid catalyzed 1,9-hydrogen shift, 6ti retro-electrocyclization, and hydrolysis to give the ketones (58) and 3-phenylpropanol. ... 

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