2-Buten-l,4-diyl

Interactions polarize bonds. Trimethylenemethane (TMM) and 2-buten-l,4-diyl (BD) dianions (Scheme 6a, b) are chosen as models for hnear and cross-conjngated dianions. The bond polarization (Scheme 7) is shown to contain cyclic orbital interaction (Scheme 6c) even in non-cyclic conjugation [15]. The orbital phase continnity-discon-tinnity properties (Scheme 6d, e) control the relative thermodynamic stabihties. 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

Z)-(l,4-Dioxo-2-butene-l,4-diyl) -COCH=CHCO- the E-form is fumaroyl (Z)-(3-Carboxy-l-oxo-2-propenyl) HOOCCH=CHCO-(l,3-Dioxo-l,3-propanediyl) -COCH2CO-(2-Hydroxy-l,4-dioxo-l,4-butanediyl)-COCH2CH(OH)CO-(Hydroxyphenylacetyl) PhCH(OH)CO--SH... 

Methyl-l,4-dioxo-2-butene-l,4-diyl] -COCH=C(CH3)CO- the Z-formis citraconyl... 

Scheme 9 Reaction of(2-butene-l,4-diyl)magnesium 17 with two different electrophiles, El and EE...

Table 12 Representative Reactions of (2-Butene-l,4-diyl)magnesiums with Electrophiles...

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. 

Diphenyl-1,3-butadiene has been one of the most reactive conjugated dienes towards metallic magnesium. Reaction of this diene with Rieke magnesium affords (l,4-diphenyl-2-butene-l,4-diyl)magnesium 2, which can be treated with various electrophiles, as shown in Scheme 3. 

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

Treatment of (l,4-diphenyl-2-butene-l,4-diyl)magnesium with l,n-dihaloalkanes provides a convenient method for the generation of substituted three-, five-, and six-membered carbocycles. Significantly, the cyclizations are always stereospecific and completely regioselective. 

It is particularly difficult to prepare asymmetric (2-butene-l,4-diyl)magnesium complexes from ordinary magnesium. The reaction is usually complicated by extensive... 

Table 2 Cyclization of (l,4-Diphenyl-2-butene-l,4-diyl)magnesium with l,/ -Dihaloalkanes...

Remarkably, (2-phenyl-2-butene-l,4-diyl)magnesium reacted exclusively at the 4-position with either organosilicon electrophile, as shown in Scheme 4. The resulting allylic organomagnesium intermediate added to cyclohexanone exclusively in the 3-position. 

Reaction of asymmetric (2-butene-l,4-diyl)magnesium with a,to-alkylene dihalides usually gives two isomeric products. For example, treatment of (2-phenyl-2-butene-l,4-diyl)magnesium with 1,3-dibromopropane at — 78°C, followed by warming to room temperature, resulted in the generation of two five-membered carbocycles, 1-phenyl-1-ethenylcyclopentane and a-cyclopentylstyrene, with a ratio of 77 23, in a 76% isolated yield. 

One of the useful applications of substituted (2-butene-l,4-diyl)magnesium complexes is the facile synthesis of silicon-containing five-membered cyclic compounds. This reaction has been reported earlier for the magnesium complex of 1,3-butadiene [7]. 

In contrast with the general 1,2-cyclizations of symmetric (2-butene-l,4-diyl)magnesium complexes with a,a>-alkylene dihalides, reactions of both symmetric and asymmetric (2-butene-l,4-diyl)magnesium complexes with diorganosilyl dichlorides yield exclusively overall 1,4-additions, generating silicon-containing five-membered rings. Some of these results are summarized in Table 5. 

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. 

Reductive alkylation. In a synthesis of mew-chimonanthine and me o-ealycanthine, two acetaldehyde chains in the masked form are introduced as a ( 2I-2-buten-l,4-diyl unit by reductive alkylation of A. lV -dibenzylisoindigo with fZ)-l,4-dichloro-2-butene. The reaction is mediated by Sml -LiCl. 

Anhydro-3-bromo-3,4-(2-butene-l, 4-diyl) -3,4-dideoxy-axab no-hexopyranos-2-ulose [79849-66-4]... 

D-a o-form. 1,6-Anhydro-3,4-dideoxy-3,4-(l-oxo-2-butene-l, 4-diyl)-p-D-allopyra-... 

Anhydro-3,4-(2-butene-l,4-diyl)-3,4-dideoxy-p-D-rj6o -hexopyranos-2-ulose (5aR,9aS)-form, A-514... 

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