Oxabicyclo heptenes, formation

Shortly after Trost s works, two investigations demonstrated the high reactivity of platinum halide salts for this type of reactions. Blum reported a PtCU-catalyzed rearrangement of allyl propargyl ethers to 3-oxabicyclo[4.1.0]-heptenes (Scheme 79).294 This series of reactions also represented the premiere entry into the versatile formation of cyclopropyl products based on skeletal rearrangements of enynes.295 This intriguing aspect is discussed further. 

CAB 3c derived from N-tosyl (a5 / )-/3-methyltryptophan catalyzes the Diels-Alder reaction of 2-bromoacrolein and furan with 96 4 enantioselectivity, enabling efficient synthesis of numerous chiral 7-oxabicyclo[2.2.1]heptene derivatives (Eq. 17) [18]. Interestingly, the analog 3b of catalyst 3c which lacks the y8-methyl group (derived from M-tosyl-(5)-tryptophan) is not as effective at catalyzing the formation of Diels-Alder product, and the reaction rate and yield are considerably lower. 

Paquette s [27] synthetic studies on sesquiterpenes furanoheliangolides such as zexbrevin possessing in their framework a 6,9-epoxycyclodeca[b]furan involve as the key synthetic steps high-pressure-promoted Diels-Alder reaction of 4-methyl-2-[(phenylmethoxy)methyl] furan 58 with 1-cyanovinyl acetate (Scheme 14). To suppress side reactions, hydroquinone was added. Near-quantitative formation of single diastereomeric adduct 59 was achieved at 12 kbar. Its conversion to tricyclic core 61 of furanoheliangolides was achieved after elaboration of the 7-oxabicyclo[2.2.1] heptene moiety and final [3,3] sigmatropic rearrangement of 60. 

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