A practical synthesis of L-threo -dihydrosphingosine (safingol), an antineoplastic and antipsoriatic agent

L S - threo -Dihydrosphingosine, an antineoplastic and antipsoriatic drug, has been synthesized by the Henri reaction of hexadecanal and 2-nitroethanol followed by catalytic hydrogenation of the dl-threo -2-nitro-1,3-octadecanediol and resolution of the racemic dl-threo -dihydrosphingosine. Selection of solvents for recrystallization of mixtures of dl-threo and erythro diastereoisomers proved to be of critical importance for the 100-gram scale preparation of the target compound


Introduction
(2S,3S)-2-Amino-1,3-octadecanediol (L-threo-dihydrosphingosine or safingol, CAS # 15639-50-6) (1), an antineoplastic and antipsoriatic drug, 1 has been extensively studied for its role in cell regulation, signal transduction, 2 and inhibition of protein kinase C. 3 CH 3 (CH 2 ) 14 OH NH 2 OH 1 Safingol has two structurally nonequivalent asymmetrical carbon atoms and, accordingly, three more configurational isomers: the D-threo diastereoisomer (6, Scheme 1) and two erythro forms, 8 and the natural D-erythro-dihydrosphingosine (10).There are two general synthetic routes to safingol described in the literature: (i) diastereoselective or enantioselective methods that lead directly to the desired products; (ii) non-stereoselective methods that result in racemic mixtures, which are subsequently subjected to resolution.All these methods were described at milligram-scale and no attempts were made previously to produce safingol at a larger scale.The two-step enantioselective nitroaldol condensation (Henri reaction) of hexadecanal (2) with nitroethanol (3), followed by hydrogenation to L s -threo-dihydrosphingosine; 4 the total synthesis departing from 2(Z)-2-buten-1,4-diol (eight steps); 5 the eight-step procedure starting from palmytoyl chloride having the asymmetric borane reduction of an α-oxoketoxime trityl ether as a key step; 6 and the diastereoselective synthesis via addition of N-hexyl magnesium bromide to the chiral 2,2-dimethylpropionic acid 4(S)-formyl-2,2-dimethyloxazolidin-3-yl ester, 7 are procedures belonging to the first category.The synthesis of the threo 2-nitro-octadecane-1,3-diol by nitroaldol condensation, reduction of the nitroaldol to racemic dihydrosphingosine and subsequent optical resolution [8][9][10] illustrate the second type of syntheses.The most appealing procedure is the enantioselective nitroaldol condensation/reduction.An optically active BINOLlanthanium complex was the catalyst used to induce stereoselectivity in the condensation of hexadecanal with nitroethanol.However, the preparation of the catalyst was a three-step synthesis, 4,11 which raises a series of technological problems, such as continuous cooling for more than 160 hours at temperatures below -40 °C.Although the amount of catalyst required for the reaction is low (10 mol %), the starting materials required for its preparation are quite expensive.Moreover, the synthesis of the chiral catalyst and its ligand are restricted by a recent patent. 12lthough a previously described diastereoselective synthesis 7 reported an overall yield of the reaction of ca. 30 % and seems appealing and elegant, the starting oxazolidine-type synthetic auxiliary had to be obtained by a multi-step synthesis, which makes it unsuitable for large-scale manufacture. 13,14on-stereoselective syntheses are mostly alternatives to the Henry nitroaldol condensation (Scheme 1).This classic method is performed under diverse catalytic conditions and has been frequently reviewed. 15,16][10] The mixture of nitrodiol diastereomers (4 + 5 and 7 + 9) was recrystallized from various solvents to give one single isomer.Diastereoselective methods described for the nitroaldol condensation, [15][16][17] usually do not apply to long chain aldehydes. 18Methods based on stereoselective syntheses, although successful on small scale, often failed to reproduce the expected purity on larger scale. 19Finally, the reduction of the nitroaldol to the racemic mixture (1+6) followed by resolution has been described earlier.

Results and Discussion
Based on preliminary experiments, involving optimization of the catalyst and solvent, we have developed a simple three-step procedure for the 100-g scale production of safingol starting from commercially available hexadecanal, following the chemistry indicated in Scheme 1.
The most difficult issue to overcome was the selection of different solvent systems in order to separate dl-threo-2-nitro-1,3-octadecanediol from the reaction mixture which contained dlerythro-2-nitro-1,3-octadecanediol.We found that pentane : diethyl ether (7: 1, v/v) was an appropriate solvent system which provided the desired diastereoisomer with >95% de.We discuss below the systems selected and the final results.
Hexadecanal (2) was condensed with 2-nitroethanol (3) in the presence of triethylamine with a reported yield in the range of 40-50%. 21Pure dl-threo-2-nitro-1,3-octadacanediol (4 + 5) was obtained by recrystallization of the crude product from ether-pentane (yield 20%).The second step, the hydrogenation, was performed in ethanol in the presence of Pd-C with a 60-80% yield.The resolution of the resulting racemic mixture (1 + 6) consisted of precipitation of the glutamate of the D-isomer 6, followed by concentration of the mother liquors and subsequent recrystallization of the resulting L s -threo-dihydrosphingosine glutamate.The free L s -threodihydrosphingosine (1) was finally released by treatment of the salt with Na 2 CO 3 in chloroform with a ca.6% overall yield calculated from hexadecanal.dl-threo-2-Nitro-1,3-octadacanediol.Pure dl-threo-2-nitro-1,3-octadecanediol was initially obtained as described in the literature with a de of 50%.Recrystallization of mixtures of dl-threo and erythro diastereoisomers from ether/pentane gave yields of 20%.Several diastereoselective methods that have been described in the literature found application for smaller chain aldehydes.Although most of the methods produce a mixture enriched in the erythro isomer, several procedures claim significant enrichment in the targeted threo diastereomer (de > 70%).These methods are: diastereoselective aldol condensations performed via doubly deprotonated nitroalcohols (α-lithio nitronates); 18,21 in the presence of titanium complexes, 22 trialkylsilyl chlorides; 23,24 or miscellaneous salts, such as tetrabutyl ammonium fluoride, 25 and calcium and magnesium chloride.We investigated these methods for the reaction of hexadecanal and nitroethanol: In our experiments, yields of the desired diastereomer having de > 95% were between 10-20 %.Therefore, the aldol condensation was experimented by employing a series of catalysts and solvents with the aim of enriching one diastereomer in the reaction mixture in a reproducible manner.The following two procedures presented synthetic interest in terms of feasibility, reproducibility and cost effectiveness: (i) By using Al 2 O 3 as catalyst and THF as solvent, 26 pure dl-threo-2-nitro-1,3-octadecanediol was obtained, but the yield after recrystallization was poor (9.6 %).Therefore, this procedure was abandoned.(ii) Employing triethylamine as both catalyst and solvent, 20 the isolated yield was 19.4 %.This step was performed at a larger scale resulting in 100 g of final dl-threo-2-nitro-1,3-octadecanediol.dl-threo-Dihydrosphingosine.The literature mentions two methods of hydrogenation that preserve the configuration.One uses Pd-C as catalyst, 4 the other uses Raney nickel 8,24 as catalyst.6][17] Therefore a few attempts of these reactions were performed in order to determine their applicability to our case.We found that during hydrogenation using Raney nickel as catalyst, for larger scale experiments the threo-nitro derivative epimerized under formation of the two amino diastereoisomers, as shown by NMR spectra of the reaction mixtures.Similarly, a mixture of diastereomers was obtained when using lithium aluminum hydride as reducing agent.The successful method was the hydrogenation with hydrogen at atmospheric pressure in the presence of Pd-C as catalyst, where the conversion to the dl-threo-dihydrosphingosine (mp 98-100 ºC) went smoothly in 77% yield.
In a large scale experiment, 65 g (74% yield) of dl-threo-dihydrosphingosine was produced starting from 100 g of dl-threo-2-nitro-1,3-octadecanediol.L-threo-Dihydrosphingosine.The racemic mixture obtained after hydrogenation was subjected to resolution with L-glutamic acid similar to a method that was reported at milligram-scale. 10 Initially, we developed the method at gram-scale and we obtained the desired product in ca.35%