Functionalization of trans-Decalm. IV. A Stereoselective Synthesis of dl-β-Costol, dl-Arctiol, and the Related Eudesmane Type Sesquiterpenes

Abstract

Abstract The efficient synthetic procedures to dl-β-costol (1a), dl-arctiol (2), and the related eudesmane type sesquiterpenes are described. trans-8,8-Ethylenedioxy-4aβ-methyldecalin-2α-ol (6a), prepared from trans-1,1-ethylenedioxy-4aβ-methyl-Δ6,7-octalin by epoxidation and subsequent reduction of the epoxy ring, was converted into 1a as follows: (1) deacetalization of 6a followed with mesylation, giving 2α-methylsulfonyloxy-4aβ-methyldecalin-8-one (7b), (2) condensation of 7b with methyl sodiomalonate and subsequent Wittig reaction with methylenetriphenylphosphorane affording dimethyl (trans-4aβ-methyl-1-methylene-7β-decalinyl)malonate (9), (3) reduction of sodium salt of 9 with NaAl(OCH2CH2OMe)2H2. Oxidation of 1a with PCC gave dl-β-costsd. dl-Arctiol (2), structurally related to 1a, was prepared from trans-5,5-ethylenedioxy-8aβ-methyldecalin-2-one. Introduction of two equatorial substituents, such as hydroxyl and 1-hydroxy-1-methylethyl groups at the C-2 and C-3 carbons of 2, was carried out as follows: (1) methoxycarbonylation followed by methylation of the sodium salt of keto ester with MeLi, (2) subsequent reduction with lithium in liquid NH3, giving trans-5,5-ethylenedioxy-3β-(1-hydroxy-1-methylethyl)-8aβ-methyldecalin-2α-ol (14), and (3) deacetalization of 14 followed by the reaction with methylenetriphenylphosphorane. dl-Eudesma-4(14),7(11)-dien-8-one was also prepared from 2 by oxidation with PCC followed by dehydration and subsequent isomerization of double bond.