in tetrahydrofuran T=20°C; 0.5 h;

water; silica gel in dichloromethane N2, water added to suspended silica gel in CH2Cl2, stirred until homogenous, B-compound added at room temp., stirred for 4.5 h; silica gel filtered off and washed (CH2Cl2); solvent removed; distd.; elem. anal.;

1: 2,2-dimethoxy-propane With bis(cyclohexanyl)borane T=0 - 20°C; Cyclization; 1 h; 2: propargyladehyde di-ethyl acetal With trimethylamine Addition; Heating; 3: With 2,3-dimethylbutane-2,3-diol T=20°C; Reduction; 12 h; Further stages.;

Dess-Martin periodane in dichloromethane 1.5 h;

1: 9.2 g / citric acid / methanol / 0.08 h 2: Dess-Martin periodinane / CH2Cl2 / 1.5 h View Scheme