Titanium tetrachloride diastereofacial selectivity

Brimble and coworkers172 reported the asymmetric Diels-Alder reactions between quinones 265 bearing a menthol chiral auxiliary and cyclopentadiene (equation 73). When zinc dichloride or zinc dibromide was employed as the Lewis acid catalyst, the reaction proceeded with complete endo selectivity, but with only moderate diastereofacial selectivity affording 3 1 and 2 1 mixtures of 266 and 267 (dominant diastereomer unknown), respectively. The use of stronger Lewis acids, such as titanium tetrachloride, led to the formation of fragmentation products. Due to the inseparability of the two diastereomeric adducts, it proved impossible to determine which one had been formed in excess. 

Hansen and colleagues177 used (+)-pantolactone as a chiral auxiliary to achieve asymmetric induction in the first step toward their synthesis of d.v-perhydroisoq uinol inc 278. The titanium tetrachloride catalyzed reaction between 1,3-cyclohexadiene (275) and chiral acrylate 276 proceeded with high diastereofacial selectivity to give 277 (94% de) in 75% yield (equation 77). 

An interesting variation of the above process involved the addition of optically active (/ )-di-(—)-menthyl allene-1,3-dicarboxylate (28) onto furan in an asymmetric Diels-Alder reaction [27]. Three equivalents of titanium tetrachloride at 40 °C gave the highest diastereofacial selectivity and afforded a 53% isolated yield of the optically active adduct 29, which represents a very useful chiral intermediate for synthetic applications (Scheme 13.11). 

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