Synthesis of the Tricyclic Core of Solanoeclepin A through Intramolecular [2+2] Photocycloaddition of an Allene Butenolide

Studies are reported towards the synthesis of solanoeclepin A (1), the hatching agent of potato cyst nematodes. Two approaches are investigated to access the tricyclic core including the intricate bicyclo[2.1.1]hexanone moiety. The first approach is based on the intramolecular [2+2] photocycloaddition of dioxenone butenolide 10 and is shown to be less practical due to the limited synthetic utility of the photoproduct 11. The second approach uses as the key step the intramolecular [2+2] photocycloaddition reaction of allene butenolide 39. This latter photosubstrate is prepared through silver-mediated coupling of silyloxyfuran 9 and allenic bromide 34. A five-step sequence starting with the Baylis–Hillman reaction between benzyl butadienolate and paraformaldehyde leads to bromide 34. The crucial photocycloaddition of 39 proceeds with excellent regioselectivity and produces the adduct 40 in good yield. This methylenecyclobutane-containing product 40 is deemed to contain the appropriate functionalities for future studies towards the natural product as is indicated through a model study leading to cyclobutanone 25. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)