A new approach to the synthesis of prostaglandin and phytoprostanes B(1) is described. The key step is an intermolecular Pauson-Khand reaction between a silyl-protected propargyl acetylene and ethylene. This reaction, promoted by NMO in the presence of 4 A molecular sieves, afforded the 3-tert-butyldimethylsilyloxymethyl-2-substituted-cyclopent-2-en-1-ones (III) in good yield and with complete regioselectivity. Deprotection of the silyl ether, followed by Swern oxidation, gave 3-formyl-2-substituted-cyclopent-2-en-1-ones (II). Julia olefination of the aldehydes II with the suitable chiral sulfone enabled preparation of PPB(1) type I and PGB(1).

1,2,3,4-Tetramethyl-bicyclo[2.2.1]hepta-2,5-diene (TMNBD, for tetramethylnorbornadiene) has been prepared and used successfully as an acetylene equivalent in the synthesis of substituted cyclopentenones. TMNBD is easily accessible on a multigram scale and displays excellent reactivity toward the intermolecular Pauson-Khand reaction. Conjugate additions on the resulting tricyclic compounds proceed with exquisite diastereoselectivity. The retro-Diels-Alder reaction of these TMNBD derivatives occurs under much smoother conditions than those required for its norbornadiene homologues.

Tetramethylnorbornadiene, a Versatile Alkene for Cyclopentenone Synthesis Through Intermolecular Pauson-Khand Reactions. -A number of substituted cyclopentenones can be prepared by this convenient method. The use of chiral cobalt complexes such as (XIII) allows formation of optically active precursors. -(REVES, M.; LLEDO, A.; JI, Y.; BLASI, E.; RIERA*, A.; VERDAGUER, X.; Org. Lett. 14 (2012) 13, 3534-3537, http://dx.