Total synthesis of the ocular age pigment A2-E: A convergent pathway

Full text of DOI:10.1021/ja9700414

J. Am. Chem. Soc. 1997, 119, 3619-3620
3619
Communications to the Editor
Scheme 1
Total Synthesis of the Ocular Age Pigment A2-E: A
Convergent Pathway
Rex X.-F. Ren, Naomi Sakai, and Koji Nakanishi*
Department of Chemistry, Columbia UniVersity
New York, New York, 10027
ReceiVed January 6, 1997
The autofluorescent age pigments (lipofuscin) which ac-
cumulate 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.
Scheme 2
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 age-
related 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.
attempted route to the key bis-aldehyde 2 is illustrated in Scheme
2. Thus, 2-bromo-4-methylpyridine (5), prepared from 2-amino-
4-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. Oxida-
tion 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 electron-
withdrawing 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-4-
methylpyridine (10) by coordination of SeO2 to the nitrogen
atom with concomitant hydrogen-bond formation, as in 11.
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
* 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.
(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, I38-
I52.
(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.
(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.
S0002-7863(97)00041-3 CCC: $14.00 © 1997 American Chemical Society

3620 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Communications to the Editor
Scheme 3
Scheme 4
(upon reflux overnight in the dark) followed by silica gel column
purification yielded 1 (70%). The physical properties, i.e., 1H
and 13C NMR, UV, IR, and MS, of this fluorescent isomer were
identical to native A2-E.
This convergent synthesis of ocular age pigment A2-E 1 can
be modified to include preparations of certain isotopically
substituted analogs. A2-E and analogs will be used for critical
mechanistic studies such as the mode of interactions with RPE
cells, RPE enzymes, phototoxicity, and the metabolic changes
of the A2-E molecule itself. If it can be proven that A2-E is
indeed the main cause leading to AMD, the A2-E molecule can
be used to seek for remedies, either by destroying the formed
molecule in RPE cells and/or preventing its formation. An
interdisciplinary approach will be taken for these studies.
Despite this coordination, the electron-donating nature of the
OH group must be sufficient to result in oxidation of the methyl
group to aldehyde. Thus, reaction of 9 with 1.2 equiv of SeO2
gave 2-hydroxy-4-formylpyridine (12) in moderate yield (64%),
which was converted to triflate 13 with Tf2O/pyridine. Stille
coupling with the tin reagent 6 in the presence of a catalytic
amount of CuI afforded 14 (78%).16 Oxidation of 14 with SeO2
was both chemo- and regiospecific, providing bis-aldehyde 2
(83%) despite the presence of the electron-withdrawing 4-formyl
group (Scheme 3).
Double Wittig olefination of bis-aldehyde 2 with 2.2 equiv
of 3 afforded a mixture of four isomers at 11 and 11′ in 4:3:3:2
ratio. Attempts to isomerize this mixture to all-trans isomer
using a catalytic amount of iodine17 and to separate the isomers
on silica gel column were both unsuccessful. However, when
this 4:3:3:2 mixture was heated in nitromethane (bp 100 °C)
under reflux overnight in the dark, the all-trans isomer 15 was
observed as the predominant product (>90% conversion to 15;
stereochemistry determined by NMR), which was purified by
preparative TLC (Scheme 4). Thus, the polyene chains with
cis CdC bonds undergo thermally-induced isomerization.17,18
Alkylation of all-trans 15 with iodoethanol in nitromethane
Acknowledgment. The studies were supported by NIH grant GM
34509. We are grateful to Professor Gilbert Stork for discussions.
Supporting Information Available: Experimental procedures and
spectral data (17 pages). See any current masthead page for ordering
and Internet access instructions.
JA9700414
(18) Alkylation of the 4:3:3:2 mixture with 2-iodoethanol under reflux
in nitromethane in the dark also led to the formation of one predominant
isomer which was identical to native A2-E. In another case, an isomeric
mixture of cis/trans merocyanines was converted into the all-trans species
upon formation of cyanine dye. See: Derguini, F.; Caldwell, C. G.; Motto,
M. G.; Balogh-Nair, V.; Nakanishi, K. J. Am. Chem. Soc. 1983, 105, 646.
(16) (a) Liebeskind, L. S.; Fengl, R. W. J. Org. Chem. 1990, 55, 5359.
(b) Echavarren, A. M.; Gomez-Bengoa, E. J. Org. Chem. 1991, 56, 3497.
(17) Sonnet, P. E. Tetrahedron 1980, 557 and references cited therein.