Diels-Alder reaction magnesium compound

A different synthetic approach to benzoannelated [2.2]paracyclophanes was realized by the Diels-Alder reaction of bis (methylene) [2.2] paracyclophane (13) and tetrakismethylene[2.2]paracyclophane (15) with para-benzoquinone [20]. The bis(exomethylene)paracyclophane derivatives were obtained from 6 and 7 via a copper-mediated coupling with methyl magnesium bromide, bromination and debromination. The dienes 13 and 15 react readily to the bis- and tris-phanes 16 and 17 when heated with para-benzoquinone in dichlorobenzene. However, both compounds are extremely poorly soluble in organic solvents and... 

In a much more complicated piece of work recently disclosed, two stereospecific aldehyde Diels-Alder reactions have been used in preparation of hikosamine derivative 29, a component of the antibacterial compound hizikimycin (Scheme 4-XIII).45k Diene 25 reacted with furfural to give cw-y-pyrone 26, which was transformed in several steps to aldehyde 27. Condensation of 27 with 25 using magnesium bromide as catalyst afforded only adduct 28, presumably via chelated intermediate 27A. Compound 28 was converted in a series of steps to acetylhikosamine 29. The methodlogy described here allowed total synthesis of this unusual sugar having 10 contiguous chiral centers with complete stereocontrol. 

Benzyne can be produced by the reaction of magnesium with o-bromofluor-obenzene in THE solution. The reaction of benzyne produced in this way with 2,5-dimethyl-2,4-hexadiene did not lead to a Diels-Alder adduct. Instead, the product isolated from the reaction mixture was 2,5-dimethyl-3-phenyl-l,4-hexadiene. Propose a mechanism for the formation of this compound. 

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