Triquinanes ring opening

An elegant application of this photocycloaddition in the field of triquinane synthesis has been reported by Reddy and Rawal [160]. The initially formed oxetane 163, formed from the Diels-Alder adduct 162, is easily cleaved reductively (by use of LiDBB = lithium di-/er -butyl-biphenylide) to give 164. This approach was also used for the construction of structurally diverse di- and (propellane-type as well as linear or angular anellated) triquinanes [161]. The classic linear triquinane hirsutene is also available via this route [162]. Star-like molecules like the tiene 167 are available via a sequence of intramolecular photocycloaddition (from the 1,3-cyclohexadiene/acylallene adduct 165) and oxetane (166) ring-opening (Sch. 58) [163]. Further examples of intramolecular... 

Cyclopropene esters give aminobicyclo[2.1.0]pentane derivatives, which can undergo ring-opening reactions to cyclopentenes For example, methyl 3,3-dimethylcyclo-propene-l-carboxylate (37), obtained by photolysis of the corresponding pyrazolinic derivative, reacted with the diquinanic enamine 36 to give a mixture of the alkylation product 38 and the [2 + 2] cycloadduct 39 which, on acid treatment, was transformed into the triquinanic cyclopentene alcohol 40 (equation 6). 

Photolysis of enaminones 432 gives tricyclic tetrahydropyridones 433 in good yields. Ring-opening of 433 affords bicyclic dihydropyridones 434, Scheme 119. Both 433 and 434 are useful synthons for the preparation of triquinanes and various sesquiterpenes (92TL7347). 

Whereas the preceding processes involving ring openings of cyclopropanes provide useful [3+2] entries to cyclopentanes, the use of simpler substrates in [3+2] cycloadditions is made possible by the development of reductive cycloaddition pathways. Such reactions were initially developed in the context of stoichiometric processes, where metallacycles were prepared by oxidative cyclizations of enones and alkynes, followed by either protonation or alkylation of the nickel 0-enolate functionality. Catalytic protocols involving various intramolecular combinations of 7t-systems include formal [3+2] reductive cycloadditions of bis-enones to form bicyclooctanols (Scheme 3-33), of enones with unsaturated acyl oxazolidinones to form triquinane derivatives, and of enals with alkynes to form bicyclooctenols. ... 

Fraser-Reid s group have described further examples of the use of serial radical cyclization to prepare di- and triquinane natural products. The previously-reported intermediate (4) (Vol. 23, p. 264) has been ring-opened by a Vasella-Bernet reaction, and hence converted to the triquinane (5) with the silphinene skeleton (Scheme 2). Similar initial steps were involved in the conversion of (6) into silphiperfolene (7), and to transform (8) (Vol. 23, p. 265) into (-)-a-pipitzol (9) (mannose carbons numbered). ... 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

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