Use of Cyclic β-Keto Ester Derivatives in Photoadditions. Synthesis of (±)-Norasteriscanolide

1996-08-09T00:00:00Z (GMT) by Gordon L. Lange Michael G. Organ
The [2 + 2] photoaddition of 2-cyclopentenones with derivatives of the cyclic β-keto ester <b>1</b> was investigated. The resultant adducts then underwent fragmentation to 5/8 fused ring systems present in terpenoid natural products such as asteriscanolide (<b>3</b>). For example, photoaddition of 2-cyclopentenone with the trimethylsilyl derivative <b>13</b> gave the head-to-head <i>cis-anti-cis</i> adduct <b>14</b>. Monomethylation and borohydride reduction of the adduct yielded lactone <b>19</b>. Cleavage of the silyl ether in <b>19</b> with fluoride ion followed by spontaneous fragmentation gave norasteriscanolide (<b>20</b>). Substrate <b>20</b>, which possesses all the stereochemical features of the natural product <b>3</b> but lacks two methyl groups, was synthesized in only four steps using this methodology.