Eudesmane alcohol

Successful application of the Mitsonobu epimerization procedure to an eudesmanic alcohol 44 to bring about inversion of configuration at C(l) is the crucial step in the Harapanhalli synthesis of erivanin (50) from santonin (Scheme 7) [16]. Reduction of enone 43, prepared from santonin in 10 steps, with sodium borohydride furnished the )8-alcohol 44 as the sole product. This product results from the approach of the hydride anion from the less hindered Of-face of the molecule. The chemical modification of the C(3)-C(4) double bond to give a 3a-hydroxy-A4-i4 rnoiety was accomplished via the epoxide 46 and its rearrangement in a basic medium. Epoxidation of 44 with MCPA yielded only one product without any directing effect exerted by the homoallylic alcohol. Treatment of 46 with lithium diisopropylamide (EDA) afforded l-e/>/-erivanin (47). For the synthesis of erivanin (50), epimerization at C(l) prior to the A -modification sequence was required. Attempts to epimerize this carbon atom in 44 by acetolysis of the tosyl derivative 45 were unsuccessful as they led to eliminated product 13 (Scheme 3). 

In an attempt to induce a rearrangement to give the pseudoguaianolide (621) the epoxy-alcohol (620) was treated with boron trifluoride etherate. An X-ray analysis, however, has shown that one of the major products is the fluoro-alcohol (622). ° A number of new xanthanolides, grafininacetate (623), desacetyl-xanthanol (624), and xanthanolacetate (625) have been isolated from Pulicaria crispa. These compounds co-occur with the unusual seco-eudesmane sesqui-terpenoid, secocrispiolide (626). 

Hydrogenolysis is not possible for ordinary alcohols, ethers, and esters. Therefore, the phenylmethyl (benzyl) substituent is a valuable protecting group for hydroxy functions. The following scheme shows its use in part of a synthesis of a compound in the eudesmane class of volatile plant oils, which includes substances of importance in both medicine and perfumery. 

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