Total Synthesis of the Ocular Age Pigment A2-E: A Convergent Pathway

The autofluorescent age pigments (lipofuscin) which accumulate in the human retinal pigment epithelium (RPE) cells with age are considered to be responsible for causing various eye diseases including age-related macular degeneration (AMD), the leading cause of blindness in elderly people for which no remedy exists.1 The prevalence associated with AMD among Americans above 40 years of age was estimated to be 9.2%,2 and a rising tide in AMD is foreseen with the increase of median age. The nature of lipofuscin accumulation in RPE remains a mystery. It is thought that the pigments are accumulative debris resulting from incomplete digestion of phagocytosized outer segment disks in lysosomes, which contain autofluorescent retinoids.3 Among them, the orange fluorophores have attracted wide interest because of their suggested involvement in agerelated decline in photoreceptor cell function. The structure originally assigned4 to the major orange fluorophore isolated from over 250 donor eyes has been revised to 1 (Scheme 1) through a biomimetic reaction of 2 equiv of retinal (vitamin A) and 1 equiv of ethanolamine (thus the name “A2-E”; 0.5% yield after extensive chromatography) and structural studies.5 The unprecedented wedge shape of this amphiphilic compound should be noted (Scheme 1).6 Preliminary assays with human fibroblast lysosomes and red blood cells suggest that A2-E could be a factor leading to the lipofuscin formation.7 Studies also suggest that A2-E induce phototoxicity on human RPE,8 indicating that photodynamic inactivation of photoreceptor cells may lead to AMD. We report herein the total synthesis of A2-E via a convergent double Wittig olefination of bis-aldehyde 2 with Wittig reagent 39 containing the moiety common to both side arms (Scheme 1), followed by alkylation at the pyridine nitrogen. The initial attempted route to the key bis-aldehyde 2 is illustrated in Scheme 2. Thus, 2-bromo-4-methylpyridine (5), prepared from 2-amino4-methylpyridine (4) by diazotization and bromination10 was subjected to Stille coupling reaction11 with tin reagent 6 (prepared by lithiation of 1-bromo-2-methyl-1-propene followed by treatment with tributylstannyl chloride12) to give 7. Oxidation of 7 with SeO2 under various conditions gave, instead of the desired bis-aldehyde 2, mono-aldehyde 8 as the sole product with oxidation occurring on the less-hindered methyl group (Scheme 2).13 The inability of the 4-methyl group to undergo further oxidation to 2 is presumably due to the electronwithdrawing nature of the enal moiety in 8 and coordination of SeO2 to the pyridine nitrogen. In support of this coordination, addition of SeO2 to 7 or 8 in CDCl3 induced substantial downfield shifts of all proton NMR signals by 0.8-1.2 ppm. When 2-bromo-4-methylpyridine (5) was subjected to SeO2 oxidation, no trace of 2-bromo-4-formylpyridine was formed, presumably due to the inductive effect of the Br substituent. We then chose 4-methylpyridone (9) as the starting material.14 The NMR olefinic proton signals (in CDCl3) of 9 appear at 6.5-7.2 ppm, showing that it exists exclusively in the pyridone form.15 Addition of SeO2, however, results in downfield shifts of >1.2 ppm, indicating that 9 tautomerizes to 2-hydroxy-4methylpyridine (10) by coordination of SeO2 to the nitrogen atom with concomitant hydrogen-bond formation, as in 11. * Author to whom correspondence should be addressed. (1) (a) Vision Problems in the United States: Data Analysis; National Society for the Prevention of Blindness: New York, 1980; pp 1-46. (b) Hyman, L. In Age-Related Macular Degeneration: Principles and Practice; Hampton, G. R., Nelson, P. T., Eds.; Raven Press: New York, 1992; pp 1-35. (c) Evans, J. R.; Wormald, R. P. L. InVest. Opthalmol. Visual Sci. 1994, 35, 2003. (2) Third National Health and Nutrition Examination Survey, cf., Klein, R.; Rowland, M. L.; Harris, M. I. Ophthalmology 1995, 102, 371-381. (3) Eldred, G. E. in Retinal Degeneration. Clinical and Laboratory Applications; Hollyfield, J. G., Anderson, R. E., LaVail, M. M., Eds.; Plenum Press: New York, 1993; pp 15-24. (4) (a) Eldred, G. E.; Lasky, M. R. Nature (London) 1993, 361, 724. (b) Eldred, G. E. Nature (London) 1993, 364, 396. (5) Sakai, N.; Decatur, J.; Nakanishi, K.; Eldred, G. E. J. Am. Chem. Soc. 1996, 118, 1559. The numbering of carbon in structure 1 (Scheme 1) refers to those in the retinoid side chains, thus indicating its biogenesis from the retinal (vitamin A). (6) Fuhrhop, J.-H.; Koning, J. Membranes and Molecular Assemblies: The Synkinetic Approach; The Royal Society of Chemistry: Cambridge, 1994; Chapter 3, p 28. (7) (a) Eldred, G. E. Gerontology 1995, 41, 15. (b) Eldred, G. E.; Katz, M. L. Exp. Eye Res. 1988, 47, 71. (8) Organisciak, D. T.; Winkler, B. S. Prog. Retinal Eye Res. 1994, 13, 1. (9) (a) Loeber, D. E.; Russell, S. W.; Toube, T. P.; Weedon, B. C. L.; Diment, J. J. Chem. Soc. (C) 1971, 404. (b) Mayer, H. Pure. Appl. Chem. 1979, 535. (10) Allen, C. F. H.; Thirtle, J. R. Organic Syntheses;Wiley, New York, 1955; Collect. Vol. III, p 136. (11) Echavarren, A. M.; Stille, J. K. J. Am. Chem. Soc. 1987, 109, 5478. (12) Serferth, D.; Vaughan, L. G. J. Organomet. Chem. 1963, I, I38I52. (13) SeO2 undergoes ene-reaction at allylic and arylic positions by coordination to π-systems yielding allylic alcohols which are then oxidized to aldehydes. See: Bulman, P. C.; McCarthy, T. J. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: New York, 1993; Vol. 7, p 83. (14) The methyl groups of 1,4-dimethylcarbostyril,14a 6-methyluracil,14b and triacetic lactone methyl ether14c undergo facile SeO2 oxidation to aldehydes: (a) Cook, D. J.; Stamper, M. J. Am. Chem. Soc. 1947, 69, 1467. (b) Zee-Cheng, K. Y.; Cheng, C. C. J. Heterocycl. Chem. 1967, 163. (c) Suzuki, E.; Hamajima, R.; Inoue, S. Synthesis 1975, 192. (15) Boulton, A. J.; McKillop, A. In ComprehensiVe Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W., Eds.; Pergamon Press: New York, 1993; p 56. Scheme 1