Evaluation of Lipases in the Desymmetrization of meso-exo-3 , 5-Dihydroxymethylenetricyclo [ 5 . 2 . 1 . 0 2 , 6 ] decane and the Synthesis of Chiral Derivatives

A dessimetrização mais eficiente do composto meso-exo-3,5-dihidroximetilenotriciclo[5.2.1.0]decano foi com a lipase da Pseudomonas cepacia (PS–C ‘Amano’ II) em acetato de vinila, quando obteve-se alto rendimento químico (93%) e excelente excesso enantiomérico (ee ≥ 99%), determinado por cromatografia gasosa em coluna quiral. Desse sistema quiral, foram preparados derivados enantiopuros com potenciais aplicações como intermediários sintéticos.


Introduction
7][8] Optically active amino alcohols are constituents of many biologically and pharmacologically important compounds such as adrenaline, β-adrenergic receptor blockers and local anaesthetics. 9The abundance and crystallinity of (+)-camphor have attracted considerable interest to the synthesis of enantiomerically pure derivatives including 3-endo-amino-2-endobornanol, 10 exo,exo-amino alcohol, 11 anti-(+)camphorquinone-3-oxime 12 and bridgehead-substituted 2norbornanones and 2-norbonanoximes. 13he desymmetrization of an meso compound by reaction with a suitable enantiomerically pure reagent provides a versatile approach to the preparation chiral synthons for asymmetric synthesis. 14Desymmetrization of meso-compounds or prochiral diols and diacetates in the presence of lipase has become a practical approach for the preparation of chiral compounds due to its high specificity and reproducibility. 9tically active aminooxy alcohols would appear to be very interesting building blocks in the search for novel biologically active compounds and the synthesis of such optically active derivatives by using a lipase-catalyzed acetylation has been described. 9he goal of our research group over the last few years has been the study and application of constrained polycyclic compounds, from both a conformational and stereochemical viewpoint, 15 as well as in terms of their reactivity and enantiomeric resolution. 16n this work we report the evaluation of various lipases for the desymmetrization of meso-exo-3,5-dihydroxymethylenetricyclo[5.2.1.0 2,6]decane in vinyl acetate and the preparation of derivatives as chiral building blocks, carrying functional groups in flexible chains on appropriate positions of the tricyclic framework.
The lipase-catalysed transesterification reaction with vinyl acetate at 23-26 o C was used to afford the kinetic desymmetrization of meso-diol 3 (Scheme 2).
Catalytic desymmetrization reaction of meso-diol 3 mediated by lipase from Pseudomonas cepacia supplied in three different preparations were evaluated: powder (Lipase PS "Amano"), immobilized on ceramic particles chemically modified with methacrylic groups (Lipase PS-C "Amano" II) and immobilized on diatomaceous earth (Lipase PS-D "Amano" I).The same reaction mediated by lipase from Candida rugosa (AY Amano 30) and lipase from Porcine pancreatic (PPL) supplied as a powder were also evaluated.
The AY Amano 30 lipase (from Candida rugosa supplied as a powder) gave only the racemic monoacetate (±)-4 while all the other lipases produced exclusively monoacetate (+)-4 with high enantiomeric excesses (ee ≥ 99%).However, the Amano PS -C II lipase was found to be the best catalyst, giving the highest yields (93%) and the most favorable reaction time (Table 1).
After reaction, the acetoxy alcohol (+)-4 was purified by filtration over Celite with ethyl acetate.The enantiomeric excess (ee%) of the separated acetate was determined by gas chromatography on a Beta-Dex TM 120 chiral column.
The oxidation of the acetoxy alcohol (+)-4 with pyridinium chlorotrioxochromate (PCC) gave the respective enantiomerically pure acetoxy aldehyde (-)-5 in 88% yield with an [α] D 20 = -16 (c =2, CH 2 Cl 2 ); as this aldehyde is unstable we prepared directly its oxime derivative and, consequently, the elemental analysis was made of its oxime.The acetoxy oxime derivative (+)-6 was obtained from the reaction of the aldehyde (-)-5 with hydroxylamine chloride/sodium acetate in a 65-85% yield.The acetoxy oxime (+)-6 is a conformer mixture determined by 1 H NMR (60:40) and it had a specific rotation of [α] D 20 = + 24 (2, CH 2 Cl 2 ).In order to obtain the protected amino alcohol 7 the oxime (+)-6 was reduced with nickel chloride hexahydrate and sodium borohydride in methanol.However, direct extraction and purification of the chiral amino-alcohol afforded poor yields, and to overcome this, in situ acetylation of the reaction product was carried out, producing the protected amino alcohol (-)-7 in a 77% yield

Conclusion
The Amano PS -C II lipase was found to be the best catalyst, giving highest yields and the best reaction rate in the desymmetrization reaction of meso-exo-3,5dihydroxymethylenetricyclo[5.2.1.0 2,6]decane, (3) in vinyl acetate.Chiral derivatives, in non-racemic form, have been prepared, carrying functional groups on flexible chains in appropriate positions of this tricyclic framework.This constitutes an effective and convenient synthetic approach for the preparation of new chiral auxiliaries, synthons and building blocks.

Experimental
General Melting points were determined on an Electrothermal IA9000 apparatus and are presented without correction.Infrared spectra were recorded using a Mattson 3020 FTIR spectrometer.Mass spectra were acquired on an HP 5988A spectrometer.NMR spectra were recorded on a Varian VXR-200 spectrometer at a magnetic field of 4.7 T at 22 o C. Chemical shifts are expressed as δ (ppm) relative to TMS as internal standard and the J values are given in Hz.Elemental analyses were recorded on a Perkin -Elmer 2400 CHN elemental analyzer.The products were analyzed by GC on a Shimadzu GC -17A Gas Chromatograph equipped with an FID detector.Optical rotations were recorded on a Perkin -Elmer 341 polarimeter using the sodium D line or mercury 365 nm line with a 0.1 dm cell at a temperature of 20 o C. Lipase PS "Amano" (Lot.LPSAX10508), PS -C "Amano" I (Lot.IPSAX08531K), PS -C "Amano" II (Lot.ILPSAX01520K), PS -D "Amano" I (Lot.ILPSAX02520K), AK "Amano" 20 (Lot.LAKX09510) from Pseudomonas and lipase AY Amano 30 (Lot.LAYY0450102S), were kindly provided by Amano Enzyme U.S.A. Co. Vinyl acetate was distilled from hydroquinone just before use.All enzymatic resolutions were carried out at 20 o C under anhydrous conditions on a Mistral Multi-Mixer apparatus.GC parameters for achiral analysis.injector 250 ºC; detector 300 ºC; oven 100 ºC for 5 min then 10 ºC min -1 until 300 ºC; column pressure 15 kPa; column flow 9.5 mL min -1 ; linear velocity 84.7 cm s -1 ; total flow 200 mL; split ratio 1:20; column DB1 15 m x 0.53 mm (internal diameter).