Transannular cyclisations ot 6-substituted cyclodecynes and attempted synthesis of samanine

Abstract

Part I The transannular cyclisation reaction of cyclodecyn-6-one (21), cyclodecyn-6-ol (38) and 6-methyl cyclodecyn-6-ol (39) were investigated, with the view of synthesising bicycle[5.3.0] decane system ( 1 ). Compound 21 when treated with several proton acids in a variety of solvents ranging in polarity from hexane to methanol, yielded only octalin, which is a bicyclo[4.4. 0] decane system ( 2 ) . Treatment of 21 with lithium in ammonia-THF followed by oxidation, yielded Δ[sup 1,7]bicycle[5.3.0] decane-2-one (29) in only 4% yield. The major product was Δ[sup 1,6]bicycle[4.4.0] decan-2-one (19). Similarly boron trifluoride etherate in methelene chloride reacted with 21 to yield only 19. Compound 38, on treatment with hydrochloric acid in methanol yielded only decanone-2. Treatment of 39 with hydrochloric acid in methanol yielded a mixture of two compounds having systems 1 and 2. Again the product having system 2 was the major product. Treatment of 38 as well as 39 with boron trifluoride etherate in methylene chloride yielded a fluoroolefin. Through spectral analysis and chemical degradation studies, the compounds were established as having system 2. A mechanism is proposed to rationalise the preponderance of system 2 in all these reactions. Part II A new route for the synthesis of salamander alkaloid samanine ( 1 ) was attempted. Thus testosterone ( 10 ) was converted to 17 β-acetoxy-2-oxo-2,3-seco-5 β-androstane-3-nitrile ( 48 ) through the method of K. Paisley. Reduction of the aldehyde to the alcohol followed by tosylation yielded 17 β-acetoxy-3-cyano-2,3-seco-5 β-androstane-2-p-toluenesulphonate ( 53 ). The nitrile and the acetate moieties were selectively reduced with diborane and the product thus obtained was treated with 10% excess of acetic anhydride in pyridine to yield 17-hydroxy samane-N-acetate ( 57 ), in about 30% yield. A mechanism is proposed for this cyclisation. In a similar way, treatment of the diborane reduction product with benzoic anhydride yielded the corresponding N-benzoate ( 72 ). Compound 57 was oxidised to 17-oxosamanine-N-acetate ( 77 ), which was then converted to the 16-oximino-17-oxcsanianine-N-acetate ( 78 ). Attempts to remove the C-17 oxygen from 78 were unsuccessful. Oxidation of 72, followed by treatment with benzaldehyde and base yielded the corresponding 16-benzylidene-17-oxosamanine-N-benzoate 74. Again attempts to remove the 17-oxo group failed.