Reaction conditions: (a) 70 °C; (b) 2-lithiofuran; (c) benzene reflux; (d) Ag20; (e) hv (visible light)

Scheme 18 Tandem 8n-6n electrocyclic reactions of 2-(vinylphenyl)-3-isobutenylcyclo-butenone [99-101]

The cascade rearrangements following double addition of alkenyl anions to the squarate ester was initiated by Paquette from the clue of Asensio's finding that twofold addition of organolithium (MeLi, PhLi, etc.) leads to the facile electrocyclic ring opening to 1,4-diketones [102]. This expedient method for construction of complex polycycles is achieved by a simple one-pot process amenable to regioselective operation, stereochemical control, self-immolative chirality transfer, 1,5-asymmetric induction, and chemical modulation [72]. In this regard, synthesis of oxa- and aza-heterotricycles was exemplified in the case where 2-lithiated dihydrofuran and dihydropyrrole were used as the alkenyl anions. The stereochemical course was completely dissected on the basis of cascades as shown in Scheme 19 (see also Fig. 5) [103,104]. Total synthesis of hypnophilin (triquinane epoxide) has proved the method to be a valuable synthetic tool [105,106].

MeO yO
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