[4+2] Cycloaddition Reactions Between 1,8-Disubstituted Cyclooctatetraenes and Diazo Dienophiles: Stereoelectronic Effects, Anticancer Properties and Application to the Synthesis of 7,8-Substituted Bicyclo[4.2.0]octa-2,4-dienes

Donor-Acceptor Bicyclopropyls as 1,6-Zwitterionic Intermediates: Synthesis and Reactions with 4-Phenyl-1,2,4-triazoline-3,5-dione and Terminal Acetylenes

The bicyclopropyl system activated by incorporation of donor and acceptor groups in the presence of Lewis acids was used as a synthetic equivalent of 1,6-zwitterions. Opening of both cyclopropane rings in 2'-aryl-1,1'-bicyclopropyl-2,2-dicarboxylates (D-A bicyclopropyl, ABCDs) in the presence of GaI₃ + Bu₄N⁺GaI₄^- results in 5-iodo-5-arylpent-2-enylmalonates as products of HI formal 1,6-addition to the bicyclopropyl system. The use of GaCl₃ or GaBr₃ as a Lewis acid and terminal aryl or alkyl acetylenes as 1,6-zwitterion interceptors allows the alkyl substituent to be grown to give the corresponding acyclic 7-chloro(bromo)-hepta-2,6-dienylmalonates. The reaction of ABCDs with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) catalyzed by Yb(OTf)₃ also results in the opening of both cyclopropane rings. The reaction products are tetrahydropyridazine derivatives - (7,9-dioxo-1,6,8-triazabicyclo[4.3.0]non-3-en-2-ylmethyl)malonates - containing one more PTAD moiety in the malonyl group.

The cubane paradigm in bioactive molecule discovery: further scope, limitations and the cyclooctatetraene complement

The cubane phenyl ring bioisostere paradigm was further explored in an extensive study covering a wide range of pharmaceutical and agrochemical templates, which included antibiotics (cefaclor, penicillin G) and antihistamine (diphenhydramine), a smooth muscle relaxant (alverine), an anaesthetic (ketamine), an agrochemical instecticide (triflumuron), an antiparasitic (benznidazole) and an anticancer agent (tamibarotene). This investigation highlights the scope and limitations of incorporating cubane into bioactive molecule discovery, both in terms of synthetic compatibility and physical property matching. Cubane maintained bioisosterism in the case of the Chagas disease antiparasitic benznidazole, although it was less active in the case of the anticancer agent (tamibarotenne). Application of the cyclooctatetraene (COT) (bio)motif complement was found to optimize benznidazole relative to the benzene parent, and augmented anticancer activity relative to the cubane analogue in the case of tamibarotene. Like all bioisosteres, scaffolds and biomotifs, however, there are limitations (e.g. synthetic implementation), and these have been specifically highlighted herein using failed examples. A summary of all templates prepared to date by our group that were biologically evaluated strongly supports the concept that cubane is a valuable tool in bioactive molecule discovery and COT is a viable complement.

Cyclooctatetraene: A Bioactive Cubane Paradigm Complement

Cubane was recently validated as a phenyl ring (bio)isostere, but highly strained caged carbocyclic systems lack π character, which is often critical for mediating key biological interactions. This electronic property restriction associated with cubane has been addressed herein with cyclooctatetraene (COT), using known pharmaceutical and agrochemical compounds as templates. COT either outperformed or matched cubane in multiple cases suggesting that versatile complementarity exists between the two systems for enhanced bioactive molecule discovery.