A new cyclopentenone annulation method

Concise and Protective Group-Free Syntheses of (±)-Hamigeran B and (±)-4-Bromohamigeran B

“…In contrast, we sought to first construct the B and C rings and envisaged an intermolecular Diels−Alder cyclization as the key step in forming the A ring toward the latter part of the synthesis (Scheme ). Although studies toward a direct route to 4 via cyclopentenone annulation chemistry developed in the Piers laboratory showed limited success, we were able to take advantage of some previously reported work in order to build the desired enone 4 through an enone migration protocol on enone 5 …”

Tetraquinane Diterpenoids: Total Synthesis of (±)-Crinipellin B

“…In 1985, Steglich and co-workers reported 1 the structural elucidation of a small family of tetraquinane diterpenoids, crinipellin A ( 1 ), O -acetylcrinipellin A ( 2 ), tetrahydrocrinipellin A ( 3 ), crinipellin B ( 4 ), and dihydrocrinipellin B ( 5 ). These structurally unprecedented natural products, which share the 12-isopropyl-4,8,11-trimethyltetracyclo[6.6.0.0 1,11 .0 3,7 ]tetradecane skeleton 6 , were isolated from different strains of the fungus Crinipellis stipitaria (Agaricales). 1,2 On the basis of circular dichroism measurements, it was concluded 1 that the absolute configurations of these substances are as shown in the structural formulas 1-5 .…”

“…As it turned out, two of these rings (B, A) were fashioned by use of new five-membered annulation methods that were developed in our laboratories. 10,11 The stereocontrolled conversion of the starting material 7 into the bicyclic enone 11 was accomplished as summarized in Scheme 1. Treatment of 7 , in a mixture of tetrahy-drofuran (THF) and hexamethylphosphoramide (HMPA), with isopropylmagnesium bromide in the presence of a copper(I) catalyst and trimethylsilyl chloride 12 afforded, after workup and product purification, the alkenyl silyl ether 8.…”

“…It was also evident that the actual bond-forming reaction that would be employed to close the required ring had to be an irreversible, kinetically controlled process. The route that was eventually developed 11 is summarized in Scheme 4.…”