α,α-Dimethoxy-o-xylylene (5-(Dimethoxymethylene)-6-methylene-1,3-cyclohexadiene): Formation by 1,4-Elimination and Electrocyclic Routes and Reactions

Article abstract of DOI:10.1021/jo00225a031

The title reactive intermediate 4, which features an unprecedented (Z)-alkoxy substituent, is generated in two independent ways.The ortho ester 5 undergoes 1,4-elimination on treatment with lithium diisopropylamide, at convenient rates in the temperature range 50-70 deg C.In the absence of added dienophile, 4 generated in this manner forms a spiro <2+4> dimer and a <4+4> dimer, both involving bonding between the two unsubstituted methylene groups.Diels-Alder adducts of 4 with norbornene (NB), norbornadiene, and cyclopentene are described.The ketal 8 formed from 4 and NB undergoes further 1,4-elimination, generating a new o-xylylene which in turn adds a second NB to yield the novel bis-adduct 11.Thermal opening of α,α-dimethoxybenzocyclobutene (6) is also used to generate 4.Rate constants for this reaction were determined over the temperature range 132-168 deg C, through the use of N-phenylmaleimide, which efficiently traps the intermediate, thereby preventing reclosure to 6 and other decomposition reactions.The E_a for this electrocyclic opening is 33.6 kcal/mol; comparative data from the literature are discussed.In the absence of a dienophile, 4 generated in this way recloses to 6 as its major reaction pathway but also undergoes an unusual rearrangement to form methyl o-ethylbenzoate (26).The overall rate of this reaction is ca. one-tenth that of the opening of 6 to 4 and restricts the range of dienophiles which can be used to trap 4 generated from 6.For example, even at the lowest temperature (132 deg C) needed to observe electrocyclic opening, and in the presence of a very large excess of NB, competitive cycloadduct formation and rearrangement to 26 are observed.Flash vacuum pyrolysis of 6 was also briefly examined.