Studies Related to Tandem Reactivity of 1-Carbomethoxy-5-dicyanomethyl-1,3-cyclohexadiene
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A set of studies centered around the reactions of the active methine compound 1-carbomethoxy-5-dicyanomethyl-1,3-cyclohexadiene (the "ring-opened adduct"), obtained by base-induced ring-opening of the Diels-Alder adduct of 5,5-dicyanocyclopentadiene and methyl acrylate, has been carried out. A plan was devised for the anionic (at the dicyanomethyl carbon) ring-opened adduct whereby its reaction with electrophiles, for example Michael reactions with double-bond acceptors, would generate reactive intermediates that would undergo cyclization by tandem conjugate addition to the a,ß,?,d-unsaturated ring p-system to generate bicyclic compounds. In practice, reaction with di-<i>tert</i>-butyl methylidenemalonate, methyl vinyl ketone, and cyclopentenone generated intermediates that exhibited greater tandem reactivity than was anticipated: the bicyclic enolates were found to cyclize further by Thorpe-Ziegler-like reaction with the proximal nitrile to generate, after facile acid hydrolysis, substituted known tricyclic skeleta termed homobrendanes, specifically, tricyclo[5. 2. 1. 0<sup>4,8</sup>]decenes. An attempt was made to generalize the reaction to other substrates, among them singly-activated Michael acceptors and 1,2-heteroatom electrophiles, but the generalization of the homobrendane forming reaction did not meet with success. Attempted functional group manipulations to probe the conversion of the homobrendane derived from di-<i>tert</i>-butyl methylidenemalonate to the homobrendane natural product 2-isocyanoallopupukeanane revealed the unreactivity of the skeletal double-bond toward electrophiles and the high reactivity of the ring ketone toward nucleophiles, among them mCPBA which brought about Baeyer-Villiger reaction, and chloride and hydroxide, which brought about addition/elimination reactions to cleave the last-formed homobrendane ring. <br /> The ring-opened adduct was also envisaged as a potential substrate in intramolecular Heck reactions. To this end, Heck substrates were generated from the ring-opened adduct anion and iodo- and bromo-benzyl halides. A key observation at this stage pertained to the unexpected acidity of the ring-opened adduct C5 proton, which could be deprotonated by DBU to bring about allylic isomerization, a finding that would provide a key insight to the pattern of reactivity later evidenced with alkyl propiolates. Optimization of the Heck substrate-generating reaction was followed by Heck reactions under Jeffery's conditions, which generated angular tricycles as intended, accompanied by aromatic compounds generated by base-induced HCN elimination/rearrangement and dehydrogenation. The Jeffery's conditions were optimized to limit the production of aromatics. <br /> The possibility of ring-opened adduct-derived vinyl silane intermediates undergoing cationic cyclizations led to a minor study based upon the bromination of allylsilanes and the elimination of TMSBr from 1,2-dibromo-3-trimethylsilyl compounds, accessible compounds unaccounted for in the review literature. It was determined that the combination of HBr and Br<sub>2</sub> (perhaps as HBr<sub>3</sub>) was required to eliminate TMSBr, in contravention of the textbook account of electrophilic substitutions being the inherent reactions of allylsilanes and Br<sub>2</sub>. <br /> Unexpected tandem reactivity was observed in the reactions of the anionic ring-opened adduct and alkyl propiolates under catalytic DBU conditions. Rather than tandem cyclization or simple adduct formation, the allenolate intermediates were determined to undergo extremely facile formal allenolate Cope rearrangements involving the ?,d-double-bond of the parent ring. Excess base intercepted the allenolate by deprotonating ring C5 and effecting 1,2-vinyl transfer by 3-<i>exo</i>-trig addition-elimination. The chemistry of the highly delocalized side-chain carbanion in the Cope product was studied in detail.