1,4-Benzoquinones asymmetric cycloaddition

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Certain olefinic substrates undergo thermally-forbidden [2 + 2] type cycloaddition in the presence of Lewis acid catalysts through coordination to the acceptor molecules. Scheme 111 illustrates the enantiose-lective version of this reaction (271). Some thio acetylenes can also be used. Substantial asymmetric induction has been observed in cycloadditions of styrenes and 1,4-benzoquinones using a stoichiometric amount of a chiral Ti(IV) complex (272). The [2 + 2] cycloadducts are readily rearranged to 2-ary 1-2,3-dihydrobenzofirrans. 

By using a stoichiometric amount of the chiral titanium reagent prepared by mixing chiral diol, Titanium IV) Chloride, and titanium tetraisopropoxide, the asymmetric [2 + 2] cycloaddition reaction of 1,4-benzoquinones and styrenes gives the corresponding cyclobutane derivatives with high optical purity. These rearrange to 2,3-dihydrobenzofuran derivatives on mild acid treatment (eq 13). ... 

Following the same strategy, further improvements in similar catalytic asymmetric [4 + 2] cycloaddition reaction have been made by Lectka group. The cyclic 1,4-benzoxazinones 3 (Scheme 10.4) that rely on the highly enantioselective [4 + 2] cycloaddition of o-benzoquinone imides with chiral ketene enolates were efficiently constructed, which can be derivatized in situ to provide a-amino acid derivatives in good to excellent yields and with virtual enantiopurity [9]. 

The chiral titanium catalyst prepared by mixing the diol 1, TiCl4, and Ti(0-i-Pr)4 in a 1 1 1 ratio promotes the asymmetric [2+2] cycloaddition reaction between styrenes 16 and 1,4-benzoquinones 17 to afford cyclobutane derivatives in good optical purity (Scheme 10 and Table 5) [37]. This reaction is not a truly catalytic reaction because excess amounts of the catalyst (5 molar amounts) have to be employed in order to obtain a high enantiomeric excess. The catalyst in this reaction is thought to be different from the one prepared from the diol 1 and TiCl2(0-i-Pr)2 in a 1 1 ratio as described previously. 

The reactions of ketene acetals with Schiff bases derived from (S)-valine esters 1.59 (R = /-Pr) under TiCl4 catalysis are highly selective, but the chiral auxiliary has not been removed [264], Engler and coworkers [1532] performed the asymmetric [2+2]-cycloaddition of 2-methoxy-l,4-benzoquinone with 1-arylpro-penes substituted by electron-donating groups. These reactions occur at -78°C when catalyzed by chiral titanium complexes derived from 2.50 (R = Me, Ar = R = Ph). Cycloadducts are obtained with a good selectivity (Figure 9.6). 

Similar applications of chiral titanium Lewis acid catalysts to asymmetric [2 + 2] cycloadditions, with up to nearly quantitative asymmetric induction, have employed 4-benzoquinones as additions and substituted styrene-type substrates11. In all of these asymmetric [2 + 2]-cvcload-dition reactions, the Lewis acid catalyst presumably is attached to peripheral functional groups and thus, similar to Lewis acid catalyzed Diels Alder reactions (see Section 1.5.8.3.5.4.), is only indirectly involved in the reaction course7. 

The asymmetric [2 + 2] cycloaddition of 1,4-benzoquinones with styrenes can be furnished using 5 equiv of TADDOL-TiCl2 complex to give the corresponding cyclobutane derivative with 90% ee that can be transformed to a dihydrobenzofurans upon warming with some loss of enantioselectivity [185] (Scheme 14.82). 

A review of the cycloaddition reactions of o-benzoquinones as carbodiene, heterodiene, dienophile, or heterodienophile has been published. In the Diels-Alder reaction of furans with masked o-benzoquinones (145), the furans unexpectedly behaved as dienophiles to yield cycloadducts (146) (Scheme 56). Masked benzoquinones behave as dienes which undergo Diels-Alder reactions with electron-rich dienophiles such as enol ethers and thienol ethers.The asymmetric Diels-Alder reactions of 5-substituted and 5,6-disubstituted (S)-2-(p-tolylsulflnyl)-l,4-benzoquinones with cyclopentadiene and fran -piperylene show complete regio- and jr-facial selectivities. The hetero-Diels-Alder reactions of o-benzoquinones with tetracyclone produce cyclopenta[I ][l,4]benzodioxinone derivatives in high yield. 

Scheme 14 Asymmetric double [3+2] cycloaddition of azomethine ylides with 1,4-benzoquinone...

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