Cyclisation of ethoxylactam 5 in formic acid at room temperature gives rise to stereospecific formation of 9. Similar reaction of the olefin 6 possessing unnatural geometry also proceeds stereospecific and in quantitative yield to the tetracyclic product 10. The acetylene 7 equally affords 11. Upon Me-substitution of the olefinic double bond the cyclisation of 8 has to be carried out in trifluoroacetic acid-dichloromethane in order to obtain a 84% yield of 12 ...
