The synthesis and isolation of the four racemic 2,5,5,8a-tetramethyl-3,4,4a,5,6,7,8,8a-octahydro-2H-1-benzopyran stereoisomers from β β β β β -ionone and α α α α α -ionone is reported. It was based on the cyclization of dihydro-β β β β β -ionone and dihydro-α α α α α -ionone with chlorosulfonic acid at low temperature to yield stereospecifically the trans-fused and cis-fused hexahydrobenzopyran derivatives, respectively. The direct cyclization of dihydro-β β β β β -ionol and dihydro-α α α α α -ionol under identical conditions was also explored. In addition to the complete spectroscopic characterization of every isolated isomer, a description of their olfactory properties is displayed.
ExperimentalReactions were monitored by gas chromatography (GC) on a Varian CP-3800 gas chromatograph fitted with a methyl silicone (CP-Sil 8 CB) capillary column (30 m × 0.25 mm i.d., film thickness 0.25 µm); carrier gas, He; flow rate, 1 mL/min; oven temperature programme, 50-290°C at a rate of 8°C/min; injector temperature, 250°C; flame ionization detector temperature, 300°C; retention times (R t ) are expressed in minutes; retention indices ( RI) referred to n-alkanes, determined on a DB-5 typephase capillary column according to reference 16. The reaction products were purified by conventional column chromatography (Merck silica gel 60, 70 -230 mesh) or by flash chromatography (Scharlau silica gel 60, 230-240 mesh), in both cases using appropriate mixtures of hexane and Et 2 O.