Total syntheses of tanshinone-II (II) and cryptotanshinone (III) from 3-methylbenzofuran-4, 7-quinone (X) and 6,6-dimethyl-l-vinylcyclohexene (IX), and of tanshinone-I (I) from quinone X and o-methylstyrene (XVIII) are described. Preliminary accounts for the Diels-Alder reaction of methoxybenzoquinone revealed that the effect of substituents for orientation phenomena can be considered to be of electronic nature.
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Inouye et al. (1969) studied this question.
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