First synthesis of (8-2H3)-(all-rac)-d-tocopherol

Sixth International Electronic Conference on Synthetic Organic Chemistry (ECSOC-6), http://www.mdpi.org/ecsoc-6, 1-30 September 2002

[C001]

First synthesis of (8-²H₃)-(all-rac)-d-tocopherol

Francesco Mazzini*, [a] Emanuele Alpi,[a] Piero Salvadori, [a] Thomas Netscher[b]

[a] Dipartimento di Chimica e Chimica Industriale, Università di Pisa Via Risorgimento 35, 56126 Pisa, Italy E-mail: lcap@server1.dcci.unipi.it

[b] Research and Development, Roche Vitamins Ltd CH-4070 Basel, Switzerland E-mail: thomas.netscher@roche.com

· Introduction

4 Choice of deuteration position

4Synthesis of alfa and beta tocopherol

4Synthetic approach to d-tocopherol: % route A L route B

& References

Introduction

Vitamin E (tocopherols and tocotrienols) is protective against many diseases (cancer, cardiovascular diseases^[1], oxidation of low-density lipoproteins^[2]). Tocopherols are present in oil seeds, leaves and other green parts of higher plants. Since vitamin E is only synthesized by plants, it is a very important dietary nutrient for humans and animals^[3].

For accurate simultaneous quantitative determination of tocopherols in food matrices by LC-MS trideuterated tocopherols were needed. Procedures for trideuterated a- and b-tocopherol have been recently optimized, whilst no method is reported as regards d-tocopherol.

Different synthetic approach are discussed, as well as procedure for the synthesis of (8-²H₃)-(all-rac)-d-tocopherol, (5-²H₃)-(all-rac)-b-tocopherol and (5-²H₃)-(all-rac)-a-tocopherol.

Top Of Page

Choice of the deuteration position

In order to avoid cross talk phenomena and to follow the most abundant ion in quantitative analyses by tandem mass spectrometry, the position of labelling was chosen on the base of tocopherol fragmentation pattern. The fragment resulting from loss of the aliphatic chain and breaking of the pyran ring is the most abundant in this kind of analyses^[4]. Hence we considered to introduce convenient labeling into the aromatic methyl group for our purposes.

characteristic fragment

Top Of Page

Synthesis of alfa and beta tocopherol

Several synthetic procedures for the preparation of poly-deuterated tocopherols are known. Mainly the deuterium is introduced by use of labeled formaldehyde and reducing reagents^[5]. For the synthesis of trideuterated (all-rac)-a- and b-tocopherol we used a recently optimized procedure^[6], by which the corresponding (²H₂)-morpholinomethyl tocopherol is prepared by a Mannich reaction and then reduced with NaCNBD₃

(5-2H3)-(all-rac)-b-tocopherol

(5-2H3)-(all-rac)-a-tocopherol.

i) morpholine (4 equiv.), (CD₂O)_n (4 equiv.), 80 °C, 2 h, 94%; ii) NaCNBD₃, isobutanol, reflux, 91%; iii) morpholine (1.3 equiv.), (CD₂O)_n, (1.3 equiv.), 80 °C, 2 h, 78%; iv) NaCNBD₃, isobutanol, reflux, 90%

Top Of Page

Synthetic approach to d-tocopherol

The synthesis of (all-rac)-d-tocopherol containing labeling in the aromatic methyl group required a totally different approach. We envisaged two routes for the preparation of a suitable trideuterated aromatic building block to be used in subsequent acid-catalyzed condensation reaction with isophytol. Protection of the phenolic group in position 5 is generally required to avoid the formation of 5- and 7-monomethyl regioisomers and possible double-alkylation products in the condensation step.

Top Of Page

Route b

Successful examples for the transformation of the least hindered phenolic group into a benzyl ether (O-benzyl group: stable under the condensation reaction and mild removal conditions) in 2,5-dihydroxybenzoic acid or esters are reported in literature^[7].

On a theorical base, the ester 1 moiety can be reduced to a methyl group through a multi-step reduction sequence with the benzylic alcohol 2 as an intermediate. Unfortunately, we were unable to isolate the corresponding MsO-, TsO-, Br-, or Cl- for the last step because they proved to be very unstable. Neither performing the whole sequence one pot was successful. These results prompted us follow the route a.

Top Of Page

Route a : Synthesis of (8-2H3)-(all-rac)-d-tocopherol

To prepare (a,a,a-²H₃)-2,5-dihydroxytoluene we envisaged and followed two possible ways.

Very good conversion was observed in the metalation of the MOM and methyl hydroquinone derivatives (up to 92%; BuLi 1.1eq., CH₃I 5 eq., rt), but we were not able to separate the products from about 7-10% of the starting reagents.

Conversely synthesis of pure 3 was performed starting from commercially available 2,5-dihydroxybenzoic acid with 50% yield over six steps^[8].

i) MeOH, H₂SO₄, 96%; ii) 1. NaH/THF 2. CH₃I, 92%; iii) LiAlD₄/THF, 92%; iv) PBr₃/CH₂Cl₂, sublimation, 82%; v) LiAlD₄/THF, 91%; vi) BBr₃/CH₂Cl₂, 82%.

3 was then converted to a mixture of monoprotected benzoates 4 and 5 (ca. 65:35). 4 was isolated by fractionating crystallization and careful flash cromatography (45% on a 800-mg scale). Though this result is not exalting, this procedure is the best up to now for the synthesis of 5-benzoyloxy-2-hydroxytoluene.

The ZnCl₂ catalyzed condensation of 4 with isophytol gave pure (8-²H₃)-(all-rac)-d-tocopherol 6 after saponification in 60% yield with a 98.5% deuteration of CD₃.

i) PhCOCl/Py; ii) flash chromatography separation, recrystallization, 45% ; iii) ZnCl₂ (1.5 equiv.), iBuOAc, aq. HCl (cat.) , isophytol, iv) KOH/MeOH, rt, iii+iv 60%.

Top Of Page

Top Of Page

References