Long-range stereo-relay: Relative and absolute configuration of 1,n-glycols from circular dichroism of liposomal porphyrin esters

Article abstract of DOI:10.1021/ja047741a

The relative and absolute configurations of long-chain syn- and anti-1,5-, 1,7- and 1,9-glycols were determined from exciton-coupled circular dichroism (ECCD) of the corresponding bis-5-(4′-carboxyl)-5,10,15,20-tetraphenylporphyrin esters measured in uniform (φ = 26 nm), unilamellar liposomes. Long-range transmission of configuration by ECCD is made possible through partial ordering of glycol ester-lipid molecules by liposomal bilayers. Copyright

Full text of DOI:10.1021/ja047741a

Published on Web 07/24/2004
Long-Range Stereo-Relay: Relative and Absolute Configuration of
1,n-Glycols from Circular Dichroism of Liposomal Porphyrin Esters
John B. MacMillan and Tadeusz F. Molinski*
Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616
Received April 19, 2004; E-mail: tfmolinski@ucdavis.edu
Long-chain natural product lipids, bearing multiple hydroxyl
substituents (e.g. caylobolide A, 1, from the cyanobacterium
Lyngbya majuscula)1 are often encountered in natural products
polyketides and glycolipids and exemplify a particularly difficult
problem in stereochemical determinationshow to relate the relative
configurations of two or more isolated stereogenic centers.
Polyketides may contain segments that embody 1,3-, 1,5-, or even
1,7-glycols. Successful approaches to solving 1,2- and 1,3-glycol
configurations2 include 13C NMR analysis of the corresponding 1,3-
glycol acetonides,2a exciton coupling CD (ECCD) of dibenzoates
Figure 1. Ordering of long-chain bis-arylcarboxylate esters of 1,5-glycols
in liposomes. (a) Sign of bisignate CD of 1,5-glycol arylcarboxylate
(or other aryl carboxylates) of 1,2- and 1,3-glycols,2c,d NMR-based
1
1
J-based analysis of H-1H and H-13C coupling constants,2e and
the proposed “universal NMR database”2f-h that addresses stereo-
chemistry through least-difference analysis of 13C NMR chemical
shifts (∆δ). The latter methods have powerful advantages but rely
upon adequate NMR signal dispersion and reliable chemical shift
assignments. Within acyclic chains or macrocylic polyketides, OH
groups separated by four or more C-C bonds are effectively
“insulated” from each other as stereogenic elements and do not
convey configurational information from their NMR or CD spectral
properties. For example, unlike 1,2-dibenzoates,3 the CD spectra
of acyclic 1,5-glycol diarylcarboxylate esters in isotropic solution
show only baseline signal.
esters: positiVe for 1,5-(R,R), (b) negatiVe for 1,5-(S,S).
A solution to the problem resides in pre-alignment of the long
chains by imposing partial ordering within lipid bilayers (Figure
1) to allow nonaveraged orientations of the chromophore charge-
transfer electronic dipole moments. We describe herein long-range
transmission of stereochemical information within 1,n-glycol lipids
(where n ) 5,7, and 9) that gives relative and absolute configura-
tions of glycol molecules from interpretation of ECCD of their
derived porphyrin carboxylate diesters within submicrometer lip-
osomes.
Figure 2. Model glycols, esters, and chromophores for ECCD.
A model study served to demonstrate the proof of principle. C2
symmetric enantiomeric glycols, (R,R)-2 and (S,S)-3 (>99% ee),
were prepared from (R)-1,2-epoxypentane (R)-4 and (S)-4, respec-
tively, through double C2-alkylation of 1,3-dithiane (t-BuLi),
followed by removal of the dithiane (Raney Ni) (see Supporting
Information). The meso-isomer, 5, was separated from the mixture
obtained by double addition of n-propylmagnesium bromide to
pentane-1,5-dinitrile followed by reduction of the resultant 4,8-
diketone.4
Figure 3. TEM image of liposomes containing (R,R)-2c. Scale bar ) 200
nm (mean liposome diameter φ ) 26 ( 5.1 nm).8
Three arylcarboxylate chromophores, commonly used in con-
figurational analysis by ECCD, were chosen for initial examination
(Figure 2, X ) OH) 2-naphthoic acid (a, λmax 234 nm, ꢀ 58000),
and two red-shifted chromophores, 7-(diethylamino)coumarin-3-
carboxylic acid (b, λ 540 nm, ꢀ 80000)5 and 4-(10,15,20-
triphenylporphyrin-5-yl)-benzoic acid (TPP, c, λmax 418 nm, ꢀ
350000).6b Stereoisomers, 2, 3, and 5 were acylated with either the
free carboxylic acid a-c (5 equiv, glycol, DCC, DMAP, CH2Cl2)
or the corresponding N-acylimidazole (glycol, DBU, CH3CN) to
obtain the diesters 2a-c, 3c, and 5c which were purified by HPLC.
Liposomal glycol bis-TPP esters were formulated according to a
procedure developed for the purpose of this work.7 Transmission
electron microscopy (TEM) of the product (Figure 3) revealed
highly uniform, unilamellar liposomes of narrow size distribution
(φ ) 26 ( 5.1 nm).8
Measurement of the CD of (R,R)-2a-c in MeOH or (R,R)-2a-b
in liposomes gave only baseline spectra; however, the CD spectrum
of (R,R)-2c in liposomes (Figure 4a) showed a strong positive
bisignate signal due to exciton coupling [λmax 430 nm (∆ꢀ +27),
9
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J. AM. CHEM. SOC. 2004, 126, 9944-9945
10.1021/ja047741a CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank Grete Adamson (UC Davis,
School of Medicine) for TEM images, and Michio Murata (Osaka
University) and Gideon Berger (UC Davis) for helpful discussions
on liposome preparation. This work was supported by the NIH
(AI39987).
Supporting Information Available: Preparation of model glycols,
1H and 13C NMR and MS of compounds, graphs of ECCD concentration
dependence and distance dependence of A values. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) MacMillan, J. B.; Molinski, T. F. Org. Lett. 2002, 4, 1535-1538
(2) (a) Rychnovsky, S. D.; Rogers, B.; Yang, G. J. Org. Chem. 1993, 58,
3511-3515. (b) Wiesler, W. T.; Nakanishi, K. J. Am. Chem. Soc. 1990,
112, 5574-5583. (c) Wiesler, W. T.; Nakanishi, K. J. Am. Chem. Soc.
1989, 111, 3446-3447. (d) Zhao, P.; Zhao, N.; Rele, D. N.; Berova, N.;
Nakanishi, K. J. Am. Chem. Soc. 1993, 115, 9313-9314. (e) Matsumori,
N.; Kaneno, D.; Murata, M.; Nakamura, H., Tachibana, K. J. Org. Chem.
1999, 64, 866-876 (f) Kobayashi, Y.; Hayashi, N.; Kishi, Y. Org Lett.
2002, 4, 411-414. (g) Kobayashi, Y.; Tan, C.-H.; Kishi, Y. J. Am. Chem.
Soc. 2001, 123, 2076-2078. (h) Kobayashi, Y.; Lee, J.; Tezuka, K.; Kishi,
Y. Org. Lett. 1999, 1, 2177-2180.
(3) (a) Vazquez, J. T.; Wiesler, W. T.; Nakanishi, K. J. Am. Chem. Soc. 1987,
109, 5586-5592. (b) Zhou, P.; Berova, N.; Nakanishi, K.; Knani, M.;
Rohmer, M. J. Am. Chem. Soc. 1991, 113, 4040-4042. (c) Harada, N.;
Nakanishi, K. Circular Dichroic Spectroscopy: Exciton Coupling in
Organic Stereochemistry; University Science Books: Mill Valley, 1983.
Figure 4. CD spectra of TPP glycol esters in DPC liposomes (2 mg/mL,
1,2-distearoyl-sn-glycero-3-phosphocholine). (a) (R,R)-2c, (b) (S,S)-3c, (c)
meso ester 5c (c ) 1 × 10-6 M), (d) (R,R)-6c, (e) (R,R)-7c, and (f) pseudo-
meso (S,R)-8c (c ) 6.5 × 10-7 M).
413, (-25), A value3c ) max - min ) 52]. The enantiomer, (S,S)-
3c, gave a bisignate CD spectrum of opposite sign and magnitude
[λ 413 nm (∆ꢀ -34); 429 nm, ∆ꢀ +31, A ) 65) while the CD
spectrum of meso-5c ester showed only baseline signal.
(4) All new compounds gave satisfactory 1H and 13C NMR and HRMS spectra.
Reduction of 4,8-undecanedione (NaBH4, MeOH) gave diastereomeric
glycols, which were separated by prep. HPLC (5 µ silica, Microsorb, 10
mm × 250 mm, 1:9 i-PrOH/hexane) to give pure (()-2 (Rt ) 12.7 min,
identified by co-injection with authentic (+)-2) and meso-5 (Rt 14.3 min)
(See Supporting Information).
The signs of the bisignate CD spectra of liposomal TPP glycol
diesters (R,R)-5c and (S,S)-5c correlate with the helicity predicted
from consideration of the extended conformation of the lipid chain
(Figure 1a,b)9 and the well-known dependence of the signs of
bisignate CD spectra with the absolute helicity of the electric
transition dipole moments of coupled chromophores.3c The ECCD
of (R,R)-5c showed a nonlinear concentration dependence above c
) 10 µM; optimal concentration appears to be e1 µM. As has
been noted by Nakanishi and co-workers,6a,b TPP diol esters provide
very high extinctions and good ∆ꢀ/ꢀ ratios which, in the present
case, leads to excellent sensitivity (limit of detection ∼40 pmol).
The distance dependence of liposomal ECCD in TPP diesters
of acyclic 1,n-glycols was briefly examined. Diastereomeric glycols
6 (n ) 7) and 7 (n ) 9) have the two OH groups disposed anti-
(“pseudo-C2” symmetric), while in 8 (n ) 9) they are syn- (“pseudo-
meso”). The CD spectra of (R,R)-6c and 7c, (Figure 3b) exhibited
strong positive bisignate curves13 with A values (A ) 51 and 27,
respectively) that diminish roughly linearly with n and the
interatomic distance between the ester oxygens (∼10 Å for n ) 9,
Supporting Information). Again, the pseudo-meso (8c) showed only
baseline signal. Assuming linearity beyond n ) 9, extrapolation of
the A versus n plot suggests that the limiting distance for ECCD
detection should occur at n ) 13 (∼15 Å, Chem3D model).
The present work demonstrates transmission of stereochemical
information across extraordinary atomic distances (8 C-C bonds)
in liposomal acylic diol esters. This method will find use in critical
stereochemical determinations of hydroxylated long-chain natural
product polyketides.
(5) Lo, L.-C.; Yang, C.-T.; Tsai, C.-S. J. Org. Chem. 2002, 67, 1368-1371.
(6) (a) Matile, S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W.
J. Am. Chem. Soc. 1996, 118, 5198-5206. (b) Matile, S.; Berova, N.;
Nakanishi, K.; Novkova, S.; Philipova, I.; Blagoev, B. J. Am. Chem. Soc.
1995, 117, 7021-7022. (c) Huang, X. F.; Nakanishi, K.; Berova, N.
Chirality 2000, 12, 237-255.
(7) A solution of TPP ester and DPC (2 mg/mL) in CHCl3 was “shell-
evaporated” in a 50-mL round-bottom flask. Distilled H2O (2 mL) was
added and the flask sonicated (∼60 s). The crude liposome preparation
was twice heated (60 °C) and cooled to room temperature, then repeatedly
extruded (×21) through a 100-nm membrane filter secured between two
gastight 0.5-mL syringes (Liposofast, Avestin, Canada) to give unilamellar
liposomes (see Figure 3).
(8) Uniform liposomes of a diameter (φ ) 26 ( 5.1 nm) smaller than the
λ
max of TPP excitation (λ 418 nm) greatly reduce loss of CD signal from
non-Rayleigh light scattering. CD of 1,5-glycol TPP esters in micelles
(aq. 1-O-octyl glycoside) or 2D-liquid crystals gave inferior results.
(9) We cannot exclude secondary effects, e.g. π-π stacking of porphyrin
rings.
(10) Methyl carbinols 6-8 were prepared from (R)-1,2-epoxynonane (>99%
ee). Epimeric C2 carbinol mixtures of 6-8 were resolved by lipase-
catalyzed acetylation (Novozym 435,12 vinyl acetate, CH3CN). The
separated product diacetates and unreacted (2S) alcohols were converted
into stereomerically pure glycols by ammoniolysis (NH3, MeOH). (>99%
de by Mosher’s analysis) (see Supporting Information).
(11) CD (6c): λ 426 nm (∆ꢀ +36), 415 (-15). CD (7c): 426 (∆ꢀ +20), 415
(-7).
(12) Ferna´ndez, F.; Garcia´-Mera, X.; Rodr´ıquez-Borges, J. E. Tetrahedron:
Asymmetry 2001, 12, 365-386.
(13) Bis-TPP esters of rigid dimeric sterol diol scaffolds have been shown to
exhibit comparable ECCD at separations of ∼18 Å.6a
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