SYNTHESIS: To a solution of 49.2 g 5,6,7,8-tetrahydronaphthol (5-hydroxytetralin) in 100 mL MeOH, there was added 56 g methyl iodide followed by a solution of 24.8 g KOH pellets (85% purity) in 100 mL boiling MeOH. The mixture was heated in a 55 °C bath for 3 h (the first white solids of potassium iodide appeared in about 10 min). The solvent was stripped under vacuum, and the residues dissolved in 2 L H2O. This was acidified with HCl, and extracted with 4×75 mL CH2Cl2. After washing the organic phase with 3×75 mL 5% NaOH, the solvent was removed under vacuum to give 48.2 g of a black residue. This was distilled at 80-100 °C at 0.25 mm/Hg to provide 33.9 g 5-methoxy-1,2,3,4-tetrahydronaphthalene as a white oil. The NaOH washes, upon acidification and extraction with CH2Cl2 gave, after removal of the solvent under vacuum and distillation of the residue at 0.35 mm/Hg, 11.4 g of recovered starting phenol.
A mixture of 61.7 g POCl3 and 54.3 g N-methylformanilide was heated on the steam bath for 15 min which produced a deep red color. This was added to 54.3 g of 5-methoxy-1,2,3,4-tetrahydronaphthalene, and the mixture was heated on the steam bath for 2 h. The reaction mixture was quenched in 1.2 L H2O with very good stirring. The oils generated quickly turned to brown granular solids, which were removed by filtration. The 79 g of wet product was finely triturated under an equal weight of MeOH, filtered, washed with 20 mL ice-cold MeOH, and air dried to yield 32.0 g of 4-methoxy-5,6,7,8-tetrahydronaphthaldehyde as an ivory-colored solid. The filtrate, on standing, deposited another 4.5 g of product which was added to the above first crop. An analytical sample was obtained by recrystallization from EtOH, and had a mp of 57-58 °C. Anal. (C12H14O2) C,H.
To a solution of 25.1 g 4-methoxy-5,6,7,8-tetrahydronaphthaldehyde in 300 mL CH2Cl2 there was added 25 g 85% m-chloroperoxybenzoic acid at a rate that was commensurate with the exothermic reaction. Solids were apparent within a few min. The stirred reaction mixture was heated at reflux for 8 h. After cooling to room temperature, the solids were removed by filtration and washed lightly with CH2Cl2. The pooled filtrate and washes were stripped of solvent under vacuum and the residue dissolved in 100 mL MeOH and treated with 40 mL 25% NaOH. This was heated on the steam bath for an hour, added to 1 L H2O, and acidified with HCl, producing a heavy crystalline mass. This was removed by filtration, air dried, and distilled at up to 170 °C at 0.2 mm/Hg. There was thus obtained 21.4 g of 4-methoxy-5,6,7,8-tetrahydronaphthol as an off-white solid with a mp of 107-114 °C. An analytical sample was obtained by recrystallization from 70% EtOH, and melted at 119-120 °C. Hexane is also an excellent recrystallization solvent. Anal. (C11H14O2) C,H. As an alternate method, the oxidation of the naphthaldehyde to the naphthol can be achieved through heating the aldehyde in acetic acid solution containing hydrogen peroxide. The yields using this route are consistently less than 40% of theory.
A solution of 21.0 g of 4-methoxy-5,6,7,8-tetrahydronaphthol in 100 mL acetone in a 1 L round-bottomed flask, was treated with 25 g finely ground anhydrous K2CO3 and 26 g methyl iodide. The mixture was held at reflux on the steam bath for 2 h, cooled, and quenched in 1 L H2O. Trial extraction evaluations have shown that the starting phenol, as well as the product ether, are extractable into CH2Cl2 from aqueous base. The aqueous reaction mixture was extracted with 3×60 mL CH2Cl2, the solvent removed under vacuum, and the residue (19.6 g) was distilled at 90-130 °C at 0.3 mm/Hg to give 14.1 g of an oily white solid mixture of starting material and product. This was finely ground under an equal weight of hexane, and the residual crystalline solids removed by filtration. These proved to be quite rich in the desired ether. This was dissolved in a hexane/CH2Cl2 mixture (3:1 by volume) and chromatographed on a silica gel preparative column, with the eluent continuously monitored by TLC (with this solvent system, the Rf of the ether product was 0.5, of the starting phenol 0.1). The fractions containing the desired ether were pooled, the solvent removed under vacuum and the residue, which weighed 3.86 g, was dissolved in 1.0 mL hexane and cooled with dry ice. Glistening white crystals were obtained by filtration at low temperature. The weight of 5,8-dimethoxytetralin isolated was 2.40 g and the mp was 44-45 °C. GCMS analysis showed it to be largely one product (m/s 192 parent peak and major peak), but the underivitized starting phenol has abysmal GC properties and TLC remains the best measure of chemical purity.
A well-stirred solution of 3.69 g 5,8-dimethoxytetralin in 35 mL CH2Cl2 was placed in an inert atmosphere and cooled to 0 °C with an external ice bath. There was then added, at a slow rate, 4.5 mL anhydrous stannic chloride, which produced a transient color that quickly faded to a residual yellow. There was then added 2.0 mL dichloromethyl methyl ether, which caused immediate darkening. After a few min stirring, the reaction mixture was allowed to come to room temperature, and finally to a gentle reflux on the steam bath. The evolution of HCl was continuous. The reaction was then poured into 200 mL H2O, the phases separated, and the aqueous phase extracted with 2×50 mL CH2Cl2. The organic phase and extracts were pooled, washed with 3×50 mL 5% NaOH, and the solvent removed under vacuum. The residue was distilled at 120-140 °C at 0.3 mm/Hg to give 3.19 g of a white oil that spontaneously crystallized. The crude mp of 1,4-dimethoxy-5,6,7,8-tetrahydro-2-naphthaldehyde was 70-72 °C. An analytical sample from hexane had the mp 74-75 °C. The GCMS analysis showed only a single material (m/s 220, 100%) with no apparent starting dimethoxytetralin present. Attempts to synthesize this aldehyde by the Vilsmeier procedure (POCl3 and N-methylformanilide) gave complex mixtures of products. Synthetic efforts employing butyllithium and DMF gave only recovered starting material.
To a solution of 1.5 g 1,4-dimethoxy-5,6,7,8-tetrahydro-2-naphthaldehyde in 20 g nitromethane there was added 0.14 g anhydrous ammonium acetate and the mixture heated on the steam bath for 50 min. The rate of the reaction was determined by TLC monitoring, on silica gel with CH2Cl2 as the moving solvent; the Rf of the aldehyde was 0.70, and of the product nitrostyrene, 0.95. Removal of the volatiles under vacuum gave a residue that spontaneously crystallized. The fine yellow crystals that were obtained were suspended in 1.0 mL of MeOH, filtered, and air dried to yield 1.67 g 2,5-dimethoxy-beta-nitro-3,4-(tetramethylene)styrene with a mp of 151.5-152.5 °C. Anal. (C14H17NO4) C,H.
EXTENSIONS AND COMMENTARY: The road getting to this final product reminded me of the reasons why, during the first few billion years of the universe following the big bang, there was only hydrogen and helium. Nothing heavier. When everything had expanded enough to cool things sufficiently for the first actual matter to form, all was simply very energetic protons and neutrons. These were banging into one-another, making deuterium nuclei, and some of these got banged up even all the way to helium, but every time a helium nucleus collided with a particle of mass one, to try for something with mass five, the products simply couldn’t exist. Both Lithium-5 and Helium-5 have the impossible half-lives of 10 to the minus 21 seconds. Hence, in the primordial soup, the only way to get into something heavier than helium was to have a collision between a couple of the relatively scarcer heavy nuclei, or to have a three body collision. Both of these would be extremely rare events, statistically. And if a few got through, there was another forbidden barrier at mass 8, since Beryllium-8 has a half life of 10 to the minus 16 seconds. So everything had to wait for a few suns to burn down so that they could process enough helium into heavy atoms, to achieve some nuclear chemistry that was not allowed in the early history of the universe.
And in the same way, there were two nearly insurmountable barriers encountered in getting to 2C-G-4 and G-4. The simple act of methylating an aromatic hydroxyl group provided mixtures that could only be resolved into components by some pretty intricate maneuvers. And when that product was indeed gotten, the conversion of it into a simple aromatic aldehyde resisted the classic procedures completely, either giving complex messes, or nothing. And even now, with these two hurdles successfully passed, the presumed simple last step has not yet been done. The product 2C-G-4 lies just one synthetic step (the LAH reduction) away from completion, and the equally fascinating G-4 also that one last reduction step from being completed. Having gotten through the worst of the swamp, let’s get into the lab and finish up this challenge. They will both be active compounds.