Hetero-Diels-Alder trimerization

In 2012, Liao et al. disclosed total synthesis of ( )-schefflone 97 and tocopherol trimers 100 and 101 by the hetero-Diels-Alder trimerization of o-quinone methides 96 and 99, generated from the silver oxide-mediated oxidation of espintanol 95 and tocopherol 98, respectively (Scheme 28.33) [46]. 

A further hetero-Diels-Alder reaction with inverse electron demand between o-QM 3 as the dienophile and either of the two diastereomers of spiro dimer 9 as the diene provided the spiro trimers 31 and 32 (Fig. 6.25). The absolute configuration was derived from NMR experiments. It was moreover shown that only two of the four possible stereoisomeric trimers were formed in the hetero-Diels-Alder reaction the attack of the o-QM 3 occurred only from the side syn to the spiro ring oxygen.28... 

Marty, M., Clyde-Watson, Z.,Twyman, L.J., Nakash, M. and Sanders, J.K.M. (1998) Acceleration of a hetero-Diels-Alder reaction by cyclic metalloporphyrin trimers. Chem. Commun., 2265-2266. 

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