α-Methoxy-o-xylylene: Formation by LiNR2-Induced 1,4-Elimination of o-Tolualdehyde Dimethyl Acetal

Article abstract of DOI:10.1021/jo00194a005

o-Tolualdehyde dimethyl acetal (3) undergoes 1,4-elimination on treatment with LiNR2, generating α-methoxy-o-xylylene (6), as shown by trapping with various olefinic dienophiles.Norbornene gives two isomeric cycloadducts, one of which (trans) undergoes further elimination; the new o-xylylene formed in this step reacts with a second norbornene to form a symmetrical bis(norbornene)adduct.Similarly, the trans cycloadduct of 6 and norbornadiene undergo further elimination, yielding naphthalene by subsequent retro-Diels-Alder reaction.Cycloadducts of 6 with cyclopentene,isoprene, and 1-hexene are formed in moderate to low yields.The reaction with cyclopentene provides a single isomer (cis) which is resistant to further elimination.Isoprene and 1-hexene give complex mixtures of cycloadducts.Control reactions demonstrate that, in the absence of a reactive dienophile, 6 partitions between dimer formation and electrocyclic closure to α-methoxybenzocyclobutene (7); 7 undergoes rapid elimination with LDA to generate benzocyclobutadiene, as shown by trapping with 1,3-diphenylisobenzofuran.Thermal reaction of 7 with norbornene and cyclopentene give the same cycloadducts as formed in the base-induced reactions of 3, suggesting that these arise from the E isomer of 6