1.4- Benzoquinones Lewis acid promoted

The Lewis acid promoted reactions of styrene with 7V-phenylsulfonyl-l,4-benzoquinone monoimine to the 2-aryIbenzofuranoid ring system have been reported previously. This offers a new route to henzofinanoid neoUgnans (( )-hcarin B, eupomatenoid-1, eupomatenoid-12) (Scheme 25, <96TL6969>). 

A general approach to 2-aryl-7-alkoxy-benzofuranoid neolignans is based on a Lewis acid-promoted reaction of styrenes with A -phenylsulfonyl-l,4-benzoquinone monoimines. Regioselective formation of the 2-arylbenzofuranoid ring system is followed by conversion of the aromatic A -phenylsulfonyl moiety into a propenyl substituent (Equation 147) <1996TL6969>. 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

The reaction of chromenes with mono- and bis- imides of 1,4-benzoquinones (6, X = O or NCOPh) is promoted by Lewis acids and affords pterocarpans and azapterocarpans, respectively (95TL2713). A total synthesis of the pterocarpan, neorautenane, involves a chemoselective coupling of a benzodipyran with o-chloro-mercuriophenol (95JCS(P1)949). 

Engler has noticed an interesting dichotomy in his indole synthesis involving benzoquinone monoimides <97TL6135>. For instance, while reaction of the quinone monoimide 96 with N-phenylsulfonyl enol ethers in the presence of Ti(IV) Lewis acid catalysts affords the expected tetrahydrocarbolines 97, reactions promoted by BF3 produce substituted benzofurans. 

Cycloadditions. The regioselectivity in the cycloaddition reactions of 2-alkoxy-5-allyl-l,4-benzoquinones with styrenes is controlled by the choice of Ti" or SnCU Lewis acids (eq 33). The use of an excess of TiCU or mixtures of TiCU and Ti(0-i-Pr)4 produces cyclobutane (13) as the major or exclusive product, while SnCU promotion with one equivalent of Lewis acid results in the formation of (14) only. These reactions represent a classic example of the mechanistic variability often associated with seemingly modest changes in Lewis acid. 

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