2.3- dimethyl-2-butene-l, 4-diyl magnesium

Scheme 1 Reaction of (2,3-dimethyl-2-butene-l,4-diyl)magnesium with a soft electrophile, followed...

When this approach is applied to the magnesium complexes of acyclic 1,3-dienes, it provides an easy route to both spiro y-lactones and y-lactones. Scheme 5 gives an outline for the synthesis of spiro y-lactones from (2,3-dimethyl-2-butene-l,4-diyl)magnesium 18. 

As shown in Scheme 8, reaction of (2,3-dimethyl-2-butene-l,4-diyl)magnesium 18 with an imine afforded the 1,2-addition adduct 27, which after treatment with carbon dioxide, acidic hydrolysis, and subsequent heating afforded a y-lactam 30 accommodating a quaternary carbon center in 62% isolated yield. For other examples of this type of transformation, see Table 7. 

Scheme 4.7 illustrates a route for the synthesis of a y-lactam from the (2,3-dimethyl-2-butene-l,4-diyl)magnesium intermediate (2). Initially, treatment of 2 with A -benzylidenebenzylamine at -78°C resulted in the formation of the... 

In contrast, Rieke magnesium reacts with ( , )-1,4-diphenyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, isoprene, myrcene, or 2-phenyl-1,3-butadiene in tetrahydrofuran (THF) at ambient temperature to afford the corresponding substituted (2-butene-1,4-diyl)magnesium complexes in near quantitative yields. The structures of these complexes have not yet been determined except for (l,4-diphenyl-2-butene-l,4-diyl)magnesium, which has been shown to be a five-membered ring metallocycle [5]. Accordingly, the most likely structures for these complexes are five-membered metallocycles or oligomers. It is also possible that an equilibrium exists between these various forms. 

For example, dichlorodimethylsilane reacted with (l,4-diphenyl-2-butene-I,4-diyl) magnesium at — 78 C to give c/s-1,1-dimethyl-2,5-diphenylsilacyclopent-3-ene in 66% isolated yield (Table 5, entry 1). Asymmetrically substituted silicon-containing heterocycles can also be prepared in this fashion (see Table 5, entries 3-5). 

---