The asymmetric synthesis of fused cyclopentenone ring systems: a formal asymmetric total synthesis of (−)-isocomene

Optically active tricyclic oxazolidine lactams 10 have been prepared using two different routes (Scheme 1). They can be obtained by acid-mediated intramolecular cyclization of bicyclic lactams 13via their acyliminium intermediates producing appended five-, six-, and seven-membered tricyclic systems. Alternatively, 10 can be prepared by cyclocondensation of chiral amino alcohols with cyclopentane-1,2-dicarboxylic acids 12 to give the imide which is reduced or alkylated to the amino alcohols and cyclized to a diastereoisomer mixture of 10. Alkylation of 10 (R″ = H) via its enolate gives stereospecifically α-quaternary products 10 (R″ = alkyl). Degradation of the latter with MeLi or Red-Al® followed by mild acid hydrolysis and aldol cyclization produces the bicyclic ketones 14 and 15 as 1:1 mixtures, readily separated and isolated in > 99% ee. This sequence produced a known non-racemic intermediate 69 for the synthesis of (−)-isocomene