Vinyloxocarbenium ions

Table 4.23 Diels-Alder reactions via vinyloxocarbenium ions...

Diastereoselective Diels-Alder reactions via cyclic vinyloxocarbenium ions [91]... 

A review cataloging intramolecular Diels-Alder reactions as key steps in the total synthesis of natural products has been published.78 A key step in the total synthesis (g) of (+)-dihydrocompactin (66) is the intramolecular ionic Diels-Alder reaction of the trienone (63) to yield the (+)-compactin core compound (65) via the intermediate cyclic vinyloxocarbenium ion (64) (Scheme 17).79 The intramolecular Diels-Alder reaction of the Asp-Thr tethered compound (67) produced the cycloadduct (68) with high regio- and stereo-selectivity (Scheme 18).80 Mixed quantum and molec- (g) ular mechanics (QM/MM) combined with Monte Carlo simulations and free-energy perturbation (FEP) calculations have been used to show that macrophomate synthase... 

Diels-Alder reactions. Enones substituted at the a -position with an acetal group form cyclic vinyloxocarbenium ions. The enhanced dienophilic reactivity of such species is synthetically useful, and diasteroselective processes have been developed in which the ionization is mediated by fluoroboric acid. Scandium(III) triflate provides lower diastereoselectivity, and BFj OEtj is useless, as extensive decomposition of the substrates occurs. 

Cha and coworkers also employed a vinyloxocarbenium ion for the total synthesis of ( )-colchicine 152 (Figure 19.15)... 

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