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Total synthesis of certain hydrochrysene analogues

University of British Columbia

Two sequences leading to the total synthesis of several octahydrochrysene derivatives are described; in particular, the synthesis of B-nor-cis-2-methoxy-8α-acetoxy-5-keto-10a-methyl-5,6,7,8,9,10,10a-octahydrochrysene (XL) is achieved. In one sequence an olefinic bond at the 5,6 position of ring B (XXXII) was introduced by reduction of cis-2-methoxy-8α-acetoxy-5-keto-10a-methyl-5,6,6a, 7,8,9,10,lOa-octahydrochrysene (XIV) with sodium borohydride, followed by dehydration with phosphorus pentoxide.Subsequent reaction at the olefinic linkage by osmium tetroxide provided the α-diol (XXX), which upon treatment with periodic acid yielded a hemiacetal (XXXIV). Cyclization on alumina then provided the B-nor internal aldol condensation product (XXXIX). The latter readily underwent / oxidation with Jones' reagent to result in the B-nor-5-keto acetate (XL). In an alternate sequence the cis-2-methoxy-8α-acetoxy-5-keto-5,6,6a,7,8, 9,10, 10a-octahydrochrysene (XIV) is oximinated at the 6-position with isoamyl nitrite and the oxime is converted by acid hydrolysis to a 5,6-diketo-8-hydroxy compound (XLII). An attempt was made to accomplish a benzilic acid rearrangement on the α-diketo compound and the resulting product (XLVII) was subjected to lead tetra-acetate oxidation in the hope that the B-nor-5-keto system (XLVIII) could be obtained. The structures from the last two reactions were only tentatively established. A critical discussion of the n.m.r. data is reserved to a separate section of the thesis. Tentative assignments of all the aromatic protons were made. Of special note is the chemical shift dependence of the proton at C₄ on the substitution at C₅. Configurational and conformational assignments of the ring A/B system were made on the basis of angular methyl and 8β-proton frequencies.

Text

2011-09-15

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http://hdl.handle.net/2429/37396

Chemistry

University of British Columbia

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