Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 1 / 0775 867

I. INTRODUCTION

Sections I V of this chapter deal with the syntheses of unsaturated organic compounds playing an essential role in biochemical processes of life. Numerous polyunsaturated compounds have been synthesized in order to elucidate their physiological role, for instance in brain. However, the main impact on permanent searches for new improved methods of synthesis of isotopically labelled dienes and polyenes comes from nuclear medicine and nuclear pharmacy. The deuterium and carbon-13 labelled polyunsaturated compounds are needed as internal standards in mass spectral determinations of very low concentrations of biologically active substances in biological fluids.

The mechanism of protective action of some unsaturated compounds against cancer and the mechanism of reactions of compounds possessing cytoprotective activity, of compounds needed for treatment of cardiovascular diseases, of gastrointestinal ulcers in man, of neonatal hyperbilirubinemia, or of breast carcinoma, unsaturated inducers of colon cancer, receptor interactions in biological membranes, etc, are the frequent topics addressed by the isotopic chemical synthetic papers reviewed in Sections II V of this chapter. Sections III and IV deal with isotopically labelled prostaglandins which are the object of synthetic studies and with the impressive progress which has been made in the synthesis of 11C-labelled compounds of very high specific activity, applied in non-invasive PET methods in diagnosis and treatment.

A large number of papers are published annually on isotopic studies of the mechanisms of chemical reactions of unsaturated compounds. In spite of the large efforts of theoretical chemists and isotope physical chemists the mechanism of two important classes of organic reactions, namely Diels-Alder addition reactions and thermal aliphatic Claisen rearrangements, opening the route to the synthesis of unsaturated carbonyl compounds, has not been clarified satisfactorily. Experimental studies of the elementary acts in these organic reactions by the methodology of carbon and hydrogen isotope effects are difficult, time consuming and expensive and the examples presented in Section VI of kinetic isotope effect (KIE) investigations are very fragmentary. In many cases one can consider them rather as a more or less important introduction, but not a complete solution of the problem. They inform the reader about the contemporary state of fundamental studies in this field.

II. SYNTHESIS OF DIENES AND POLYENES LABELLED WITH STABLE

ISOTOPES

A. Synthesis of Deuterium-labelled Compounds

1. Synthesis of the deuterium-labelled 2-exo-vinylbicyclo[2.2.1]hept-5-en-2-ols

The title compound has been deuterium labelled1 with 2H at C 3 (1), and in the vinyl group (2) by deuterium exchange of the enolizable hydrogen atoms in 3 followed by

The reaction sequences shown in equations 4 and 5 involve the reduction of the appropriate acetylenic tetrahydropyranyl (THP) ethers with (Ph3P)3RhCl and deuterium gas.

2. Silver resin chromatography 1. n-BuLi/ THF, MeOOC(CH2 )6 CHO

Et(CD2 )2 CH2 CH CHCH2 CH CHCH2 CH CH(CH2 )6 COOMe

(10) 25% combined yield

CD2 CD3

(15) 76% step yield, 19% overall yield

COOEt

CD2 CD3

(16)62% yield

5.Synthesis of deuteriated leukotriene A4 methyl ester

14,15,17,17,18,18-[2H6]-Leukotriene A4 methyl ester, 17, has been synthesized14 by Wittig olefination of epoxy dienal 18 with the key reagent 3,4,6,6,7,7-[2H6]-(Z)-(3-nonen- 1-yl)triphenylphosphonium iodide, 19 (equation 9). 17 is employed as stable isotope internal standard for the MS trace analysis of eicosanoids15 17.

(9)

O

D D CH2

(CD2 )2 Et

(17)[2 H6 ]-LTA4 methyl ester, 78%

6.Synthesis of [14,15,17,17,18,18-2H6]-leukotriene-B4

The deuteriated title compound 20, needed for quantitative determination of endogenous LTB4 in various biological fluids by GC/MS18,19, has been obtained20 by enzymatic hydration with human monocytes of D6-LTA4 precursor14,21. Leukotriene D6-LTB4 has been separated from its trans isomers, 6-trans-D6-LTB4 and 12-epi-6-trans-D6-LTB4, in high isotopic purity (99.4%) by reversed-phase HPLC and identified by GC/MS. Leukotrienes B4 and C4 are potent inflammatory mediators22.

OPFB

D D

CD

D D D

(20) pentafluorobenzyl (PFB) - TMS derivative of the enzymatic product used in SIM (selected ion monitoring) GC/MS analysis of the title compound

7. Synthesis of [6,7,14,15-2H4 ]-leukotriene B4 methyl ester

The title compounds LTB4, 21 (Z) and 22 (Z), have been synthesized23 by stereoselective reduction with deuterium gas of a 1:1 mixture of the suitable diacetylenic precursors 23 and 24 using Lindlar catalyst or palladium on barium sulphate catalyst (equation 10). Leukotriene B4, a 5-lipoxygenase metabolite of arachidonic acid, playing

a major role in allergic, inflammatory and immunological states24, had to be deuterium labelled for its quantification in biological samples25,26 and for defining its physiological role.

PhCOO

COOMe

H X Y

(23) (5S, 12R), X = H, Y = OH (24) (5R, 12R), X = OH, Y = H

(10)

D D

(22)

8. Synthesis of 13-cis-retinoic 8,(9,9,9-methyl)-D4 acid

The D4 acid 25a, which according to MS had 94% of D4, has been prepared27 from citral 25b and acetone-D6 as before28 (equation 11).

9. Synthesis of tri-, tetraand penta-deuteriated forms of vitamin A

Four deuteriated retinols, 26 29, with 3 to 5 deuterium atoms have been synthesized29 for metabolism of vitamin A studies in humans30. Deuterium has been introduced into appropriate intermediates, used in the reaction scheme shown in equation 12, by basecatalysed exchange with 2H2O or perdeuterioacetone. The numbering system for retinol (vitamin A alcohol) is shown in equation 12.

In the case of the synthesis of 10,19,19,19-2H4-vitamin A, the most useful for biological studies, three deuterium atoms were incorporated into ˇ-ionone 30, in >98% by deuterium exchange with excess D2O in the presence of NaO2H (and pyridine). The tri-deuteriated 30, utilized in Wittig Horner reaction with dideuterio triethyl phosphonate, provided tetradeuteriated ethyl ˇ-ionilidene acetate 31 with more than 98% 2H4 (by NMR). No deuterium loss in the subsequent synthetic steps was observed as evidenced by MS and NMR analysis.

10. Synthesis of deuteriated 2H8 -ˇ-carotene

Dietary ˇ-carotene, a nutritionally important source of vitamin A, exhibits a protective effect against cancer risk31,32. The deuteriated compound, 10,100,19,19,19,190 ,190,190 - 2H8-ˇ-carotene, 32, has been obtained33 by double condensation of the C-15 Wittig salt 33 with the symmetrical C10 dial 2,7-dimethyl-2,4,6-octatrienedial, 34 (equation 13) for the study of ˇ-carotene metabolism in humans.

11. Synthesis of š -16,17,18,19-[2H4 ]-prostaglandin D2 , 35

Using dimethyl 3,4,5,6-[2H4]-2-oxoheptylphosphonate, 36, prepared in two steps as shown in equation 14, the title prostaglandin D2, 35, has been synthesized34 in thirteen steps (equation 15).