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Synthetic Studies Towards Compounds Related to Sterpurene and Protoilludene.-As part of synthetic strategies towards the title sesquiterpenes (I) and (II), resp., the intramolecular aldol condensation of the diastereomeric cyclopentanes (XII) and (XIII) is studied. Mixtures of the cis-and trans-bicyclononenones (XIV) and (XV) with (XVI) are obtained in ratios varying from 1:1 to 12:1 depending on different acidic or basic reaction conditions. The preparation of the trans compound (XV) in ≈70% yield constitutes formally a new synthesis of (I). -(BIRKENES, O. J.; HANSEN, T. V.; M'DACHI, S.; SKATTEBOEL, L.; STENSTROEM, Y.; Acta Chem. Scand. 52 (1998) 6, 806-812; Dep. Chem., Univ. Oslo, N-0315 Oslo, Norway; EN)