Trienones, intramolecular Diels-Alder reaction

A number of stereospecific intramolecular Diels-Alder reactions of trienones leading to c -fused products have been described. The ketone 34 forms solely compound 35 on treatment with aluminium trichloride at 110°C (equation 25)30. The lower homologue 36 undergoes a spontaneous cyclization to 37 below 20 °C (equation 26)31 and the isomeric ketones 38 and 40 similarly give 3932 and 4133, respectively (equations 27 and 28). 

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... 

More fascinating, however, are intramolecular modes of the Diels-Alder reaction. The vinyl cyclopropanes in discussion open a very short and flexible entry to substrates suitable to undergo this cycloaddition. Cyclopropane 149 — easily obtained by alkylation of the unsubstituted compound 132 (see Table 3, entry 12) — after ring cleavage with fluoride gives a trienone 150, for instance, which has ideal electronic and steric properties for an intramolecular [4 + 2]-cycloaddition. After reaction at... 

Recently, Aube reported the formal synthesis of ( )-stenine through tandem intramolecular Diels-Alder/Schmit-Aube reaction of azido trienone substrate catalyzed by MeAlCb (Scheme 6.135). This reaction proceeds via initial IMDA... 

---