4-Methyl-7-hydroxycoumarin (I) was alkylated with 3-chloro-3-methyl-1-butyne (II) in the presence of K2CO3 and KI to afford the corresponding aryl propargyl ether (III). Claisen rearrangement of (III) in refluxing diethylaniline furnished the tricyclic system (IV). The target 3'R,4'R-dihydroxylated derivative (V) was obtained by Sharpless asymmetric dihydroxylation of (IV) with hydroquinine 2,5-diphenyl-4,6-pyrimidinediyl diether ((DHQ)2-PYR) as the chiral catalyst. Diol (V) was finally esterified with (-)-S-camphanoyl chloride (VI) to afford the title diester.
Vilsmeier formylation of orcinol (I) by means of POCl3 and DMF afforded 2,4-dihydroxy-6-methylbenzaldehyde (II). Coumarin (V) was then prepared by Wittig condensation of (II) with carbethoxymethylene triphenylphosphorane (III) followed by intramolecular cyclization in refluxing xylene. Alkylation of (V) with 3-chloro-3-methyl-1-butyne (VI) in the presence of K2CO3 and KI produced the corresponding alpha,alpha-dimethylpropargyl ether (VII), and subsequent thermal cyclization in refluxing N,N-diethylaniline gave rise to the tricyclic system (VIII). Osmium-catalyzed asymmetric dihydroxylation of (VIII) in the presence of the enantioselective ligand hydroquinone 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQ)2-PYR] produced the required (R,R)-(+)-cis-diol (IX). This was finally acylated with (S)-(-)-camphanic chloride (X) in the presence of pyridine to produce the target dicamphanoyl ester.