Phytosphingosines — a facile synthesis and spectroscopic protocol for configurational assignment

Halisphingosines A and B, modified sphingoid bases from Haliclona tubifera. Assignment of configuration by circular dichroism and van't Hoff's principle of optical superposition

Halisphingosines A (1) and B (2), modified long-chain sphingoid bases, from the marine sponge Haliclona tubifera collected in Brazil, were characterized after conversion to their N-Boc derivatives. The 2R,3R,6R configuration of halisphingosine A, a compound first reported from Haliclona sp. from South Korea, was confirmed using a novel CD approach: deconvolution of exciton coupling from mono- and trinaphthoyl derivatives obtained by derivatization of the natural product. The sensitive CD deconvolution method, applicable to submilligram samples, simultaneously predicted the relative and absolute configuration of three stereocenters in halisphingosine A with precision and accuracy. Halisphingosine B was assigned by correlation to halisphingosine A.

Bioactive lipids from the sponge Spirastrella abata

Three sphingosine 4-sulfates (1-3) and a lysophosphatidylglycerol (4) were isolated from the Korean sponge Spirastrella abata. The structures of these compounds were determined based on the combined results of spectroscopic analyses. Based on the results of combined synthesis and comparison of specific rotation and circular dichroism, the absolute configurations of 1-3 were found to be enantiomeric to the previously isolated metabolites. The configurations of 4 were also partially determined by similar chemical and spectroscopic methods. The compounds exhibited significant cytotoxicity and weak antimicrobial activity (1), as well as weak-to-moderate inhibitory activity against isocitrate lyase and Na(+)/K(+)-ATPase. A structure-activity relationship was found for the sphingosine 4-sulfates.

Asymmetric synthesis of D-ribo-phytosphingosine from 1-tetradecyne and (4-methoxyphenoxy)acetaldehyde

An asymmetric synthesis of d-ribo-phytosphingosine (1) was achieved by utilizing the ProPhenol (12)-catalyzed alkynylation of unsaturated aldehyde 8 to afford allylic propargylic alcohol (S)-6 followed by asymmetric epoxidation and opening of propargylic epoxy alcohol anti-5 with NaN(3)/NH(4)Cl. Deprotection and reduction of the resulting acyclic azide 3 then gave 1. Alkyne-azide 3 was subjected to an intramolecular click reaction, generating a bicyclic triazole, which was found to have unexpected vicinal coupling constants. Application of the advanced Mosher method verified the configurations of the three contiguous stereogenic centers of 1. An alkynyl azide analogue of 1, which may be useful as a glycosyl acceptor in the synthesis of alpha-galactosylceramide derivatives, was also readily prepared by this route.

Enhancement of exciton coupled circular dichroism with sterically encumbered bis-porphyrin tweezers

Porphyrin tweezers have been successfully used as hosts for the absolute stereochemical determination of a variety of chiral compounds. A set of new porphyrin tweezers with substituted aryl groups on the meso position of the porphyrin rings have been synthesized. The modified tweezers are used as hosts for the stereochemical determination of chiral diamines and carrier-derivatized alpha-chiral carboxylic acids in order to monitor the influence of the various substitutions of the aryl group on the amplitude and sign of the ECCD couplet. t-Butyl substitution at the meta positions of the porphyrin's meso phenyl substituents leads to enhanced ECCD amplitudes.

Divergent syntheses of all stereoisomers of phytosphingosine and their use in the construction of a ceramide library

Sphingolipids such as ceramide and sphingosine-1-phosphate have recently attracted intense research interests because of their functional roles as signaling molecules in many important physiological processes, such as growth arrest, apoptosis, and inflammatory responses, and cell proliferation, vascular maturation and trafficking of lymphocytes. The well-defined modular structures of ceramides and related glycosylceramides are ideally amenable to library formation for medicinal chemistry investigation. We have developed divergent synthetic routes to all eight phytosphingosine stereoisomers and then proceeded to prepare phytosphingosine-based ceramide library composed of more than 500 compounds.

Dicliptercerebroside, a novel cerebroside from Dicliptera chinensis

A novel cerebroside named dicliptercerebroside (1) was isolated from the whole herb of Dicliptera chinensis, together with two known compounds asperglaucide (2) and amidoalcohol (3) for the first time from the title plant. Their structures were elucidated using spectroscopic methods. The structure of dicliptercerebroside was determined as 1-O-beta-D-glucopyranosyl-2N-[(2'R)-2'-hydroxy-(9Z)-palmitoleoyl]-2S,3S,4R-C(20)-phytosphingosine.

Dihydroxylation of 2-vinylaziridine: efficient synthesis ofd-ribo-phytosphingosine

An efficient and highly stereoselective synthesis of D-ribo-(2S,3S,4R)-phytosphingosine was accomplished in 62% overall yield starting from commercially available (2S)-hydroxymethylaziridine via osmium-catalyzed asymmetric dihydroxylation as a key step.

Alpha-c-galactosylceramides: synthesis and immunology

The immunostimulant activity of alpha-galactosylceramides provided the impetus for the research described here. The activity was first discovered via screening of extracts of a marine sponge. The active materials purified from the extracts were alpha-O-galactosylceramides. The work described herein focuses on syntheses of alpha-C-galactosylceramides. Crucial methodologies for the syntheses were (i) Ramberg-Bäcklund reaction, (ii) modified Julia olefination, (iii) olefin cross-metathesis, and (iv) Sharpless asymmetric epoxidation in four independent routes. The immunostimulant activity of the synthetic alpha-C-galactosylceramide far surpasses that of the O-galactosyl material. A discussion of the reasons for the difference in activity is presented.

Synthesis and biological evaluation of α-galactosylceramide (KRN7000) and isoglobotrihexosylceramide (iGb3)

Glycoceramides can activate NKT cells by binding with CD1d to produce IFN-gamma, IL-4, and other cytokines. An efficient synthetic pathway for alpha-galactosylceramide (KRN7000) was established by coupling a protected galactose donor to a properly protected ceramide. During the investigation, it was discovered that when the ceramide was protected with benzyl groups, only beta-galactosylceramide was produced from the glycosylation reaction. In contrast, the ceramide with benzoyl protecting groups produced alpha-galactosylceramide. Isoglobotrihexosylceramide (iGb3) was prepared by glycosylation of Galalpha1-3Galbeta1-4Glc donor with 2-azido-sphingosine in high yield. Biological assays on the synthetic KRN7000 and iGb3 were performed using human and murine iNKT cell clones or hybridomas.

A direct organocatalytic entry to sphingoids: asymmetric synthesis of d-arabino- and l-ribo-phytosphingosine

The organocatalytic asymmetric synthesis of D-arabino- and L-ribo-phytosphingosine is described employing a diastereo- and enantioselective (S)-proline-catalyzed aldol reaction of 2,2-dimethyl-1,3-dioxan-5-one and pentadecanal as the key step.

A general, efficient and stereospecific route to sphingosine, sphinganines, phytosphingosines and their analogs

Sphingosine, sphinganines and phytosphingosines and their analogs were synthesized by an aldol condensation between an iminoglycinate bearing a (+)-(1R,2R,5R)-2-hydroxy-3-pinanone group as chiral auxiliary and an appropriate aldehyde. All condensations proceeded with excellent enantioselectivity to generate the (2S,3R)-D-erythro structures in good yields.

Effects of lipid chain lengths in alpha-galactosylceramides on cytokine release by natural killer T cells

Glycolipid presentation by CD1 proteins has emerged as an important aspect of antigen recognition, and presentation of alpha-glycosylceramides by CD1d to natural killer T cells has become a central focus in understanding how glycolipid presentation can influence immune responses. An alpha-galactosylceramide containing relatively long lipid chains has been the subject of intense study because, when presented by CD1d to natural killer T cells, it stimulates the release of both proinflammatory and immunomodulatory cytokines. Using an efficient synthesis of alpha-galactosylceramides, we have prepared a series of glycolipids in which the lipid chain lengths have been incrementally varied. The responses of natural killer T cells to these glycolipids have been determined, and we have found that truncation of the phytosphingosine lipid chain increases the relative amounts of immunomodulatory cytokines released. In similar fashion, the length of the acyl chain in alpha-galactosylceramides influences cytokine release profiles.

Novel, short, stereospecific synthesis of lyxo-(2R,3R,4R)-phytosphingosine and erythro-(2R,3S)-sphingosine

Lyxo-phytosphingosine and erythro-sphingosine have been elaborated from a common intermediate. The key step in the reaction sequence involves stereo- and regiospecific functionalization of an olefin by intramolecular nucleophilic sulfinyl group participation.

A facile synthesis of phytosphingosine from diisopropylidene-D-mannofuranose

In the present study, an efficient method with a high overall yield for preparing phytosphingosine and an analogue was developed. Starting with commercially available 2,3;5,6-di-O-isopropylidene-d-mannofuranose, a variety of lipid moieties were incorporated to obtain phytosphingosine and an analogue. Through an eight-step manipulation, phytosphingosine was obtained with an overall yield of 57%.

Absolute stereochemical determination of chiral carboxylic acids

Monoamidation of 1,4-diaminobenzene with alpha-chiral carboxylic acids leads to a carrier strategy for absolute stereochemical determination with bis-zinc porphyrin tweezers by exciton-coupled circular dichroism (ECCD). The helicity induced in the porphyrin tweezers upon complexation with the derivatized carrier originates from the preferred conformation of the C(carbonyl)-C(chiral) bond. Correct ECCD signs can be predicted by the rotamer that places the large group perpendicular to the carbonyl group with the small group facing the porphyrin. [reaction: see text]

Synthesis of D-erythro-dihydrosphingosine and D-xylo-phytosphingosine from a serine-derived 1,5-dioxaspiro[3.2]hexane template

[reaction: see text] A serine-derived 1,5-dioxaspiro[3.2]hexane template is shown to be a useful precursor for both aminodiol and aminotriol sphingoid bases by its conversion to D-erythro-dihydrosphingosine and D-xylo-phytoshingosine.

CD exciton chirality method for determination of the absolute configuration of beta-hydroxy-alpha-amino acid derivatives

The absolute configuration of beta-hydroxy-alpha-amino acids was studied by CD exciton chirality method using 7-diethylaminocoumarin-3-carboxylate as a red-shifted chromophore. The CD spectra of bischromophoric derivatives of (S)-serine and (2S,3R)-threonine methyl esters (2 and 7) were compared with those of acyclic vic-aminoalcohols and diols (3--6 and 8--9). This study indicates that the polar carboxylate group of beta-hydroxy-alpha-amino acids makes them a unique subclass of vic-aminoalcohols. By combining the data of CD and NMR coupling constants, we are able to correlate their preferred conformer B and positive CD to the corresponding absolute configuration.

Synthetic phytoceramides induce apoptosis with higher potency than ceramides

Ceramides are naturally occurring compounds recognized to mediate apoptosis. N-acylsphingosines, containing a double bond at carbons 4 and 5 of their sphingoid backbone, are thought to be the active form, because N-acylsphinganines with completely saturated sphingoid are inactive. In the present study, we synthesized a series of N-acyl-4D-ribo-phytosphingosines (phytoceramides) that contain a hydroxyl group at carbon 4 and investigated structure-cytotoxicity relationship of the presumed functional groups in ceramides. N-Acetylphytoceramide (PCer2) and N-hexanoylphytoceramide (PCer6) were found to be more cytotoxic than ceramides as determined by released lactate dehydrogenase activity and morphological criteria. This was not caused by intracellular conversion of phytoceramides to ceramides, because no N-hexanoylsphingosine was formed after incubation of cell lysate with PCer6. Among phytoceramides having acyl chains two to eight carbons long, the cytotoxicity was highest with five or six carbons. The carbonyl group of the amide bond did not seem to be critical, because substitution of the oxygen with sulfur did not influence the cytotoxicity. The phytoceramide-induced cell death was observed to be apoptotic in nature with the use of terminal deoxynucleotidyl transferase dUTP nick-end labeling and propidium iodide staining. Because phytoceramides can be readily synthesized from yeast sources, they may present a potential and economical alternative to ceramide in future studies and therapies.

Functional characterization of sphingolipid C4-hydroxylase genes from Arabidopsis thaliana

In the genome of Arabidopsis thaliana, two genes were identified encoding isoenzymes for C4-hydroxylation of long chain bases (LCB) in plant sphingolipids. Both predicted proteins consist of 258 amino acid residues (77% identity) which show sequence similarity to di-iron-binding enzymes, such as Sur2p and Erg3p from yeast, involved in oxygen-dependent lipid modifications. Heterologous expression of these genes in a yeast sur2Delta-null mutant lacking C4-LCB hydroxylation resulted in the formation of D-ribo-C(18)- and -C(20)-phytosphinganine. The identity and stereochemical configuration of the isolated trihydroxybases was confirmed by electrospray ionization-mass spectroscopy, gas-liquid chromatography-mass spectrometry and 1H-nuclear magnetic resonance spectroscopy. These results represent the first functional identification of SUR2 genes from plants as well as from any organism other than yeast.