Trachylobane, synthesis

In Bettolo and co-workers approach to (+)-methyl trachyloban-18-oate (16), enone 13 was subjected to a photocycloaddition with 1,2-propadiene (1) to afford the [2 + 2]-cycloadduct 14 as a single product in 67% yield (Scheme 19.3) [5]. The addition proceeded exclusively from the /3-face. The resulting exocyclic olefin was eventually converted to a ketone using osmium tetroxide and NaI04 and taken on to 15, constituting a formal total synthesis of 16. 

Scheme 19.3 A formal total synthesis of (+)-methyl trachyloban-18-oate.

Bicycloannelation.1 The a -enolate of an a,j -cyclohexenone reacts with this phosphonium salt to form a tricyclof3.2.1,0 ]octane in low to moderate yield. This reaction was used in a short synthesis of the pcntacyclic dilcrpcnc trachyloban-19-oic acid (4). Reaction of the lithium enolale of 2, prepared from podocajpic acid, with I provided the pcntacyclic ketone 3, which was reduced by the Wolff-Kishncr reaction to 4. 

A novel Wittig reaction of cyclohexenones using vinylphosphonium ylides leads to the construction of the cyclopropane ring and this has found application in the stereoselective synthesis of trachyloban-19-oic acid. A further synthesis of gibberone has been reported.The sesquiterpenoid santonin formed the starting material for syntheses of pachydictyol and dictyolene. ... 

Reductive solvolysis of unsaturated alcohols with double bond participation can be exploited for the synthesis of cycloalkanes. A pleasing example is the synthesis of the pentacyclic diterpene natural product trachylobane (equation 120). ... 

The sjmthesis of (+)-chanoclavin I (582) has been achieved by a multistage route from the acetal (580) by way of the key intermediate (581) whose aliphatic double bond is readily cleaved by ozone. A third, convenient synthesis of the diterpenoid trachylo-bane (586) has been achieved by a reductive homoallylic cyclization. Reaction of the mesylate ester (583) with methylsulphinyl carbanion gives the ketone (584) and the alcohol (585). Oxidation of the crude product mixture with Jones reagent gave ketone (584) which can be reduced to trachylobane (586). ... 

---