( S )-Garner aldehyde derived Baylis-Hillman adduct : a substrate for the synthesis of a lactone ceramide analogue via a sequential Heck reaction

The first, general and efficient route has been developed for the synthesis of N-((E,3S,4R)-5benzylidene-tetrahydro-4-hydroxy-6-oxo-2H-pyran-3-yl) palmitamide, a lactone ceramide analogue from (S)-Garner aldehyde-methyl acrylate derived Baylis-Hillman adduct with anti configuration. Reaction conditions have been optimized for the Heck reaction of iodobenzene on Baylis-Hillman adduct with anti configuration to obtain the protected aromatic ceramide analogue intermediate which subsequently on deprotection and N-acylation provided the lactone ceramide analogue. Further, various substituted phenyl iodides have also been employed in the Heck reaction under the optimized conditions for the synthesis of few protected aromatic ceramide analogues intermediates to the test efficacy of the developed methodology.


Introduction
The Baylis-Hillman 1 and Heck reactions 2 are important carbon-carbon bond formation reactions utilized extensively in the synthesis of natural products 3,4 and their analogues, including αalkylidene-β-lactams, α-methylene-γ-butyrolactones, frontalin, sarkaomycin, ilmofosine nuciferol, syributins, terpenoids and alkaloids.In the Baylis-Hillman reaction 5,6 the aldehydes are important substrates and among them the chiral α-amino aldehydes 7 are of special interest as the resulting adducts generate a new stereocenter which can be fine tuned to obtain multifunctional α-methylene-β-hydroxy-γ-amino acid intermediates to be exploited in asymmetric synthesis.The Heck reaction for arylation of unsubstituted alkenes is controlled by the nature of the substrate, solvent, base, nature of catalyst and its concentration used resulting in variable conditions for obtaining the desired intermediates. 8ecently, the work on Pd-catalyzed chemical transformations of Baylis-Hillman adducts has been reviewed in literature. 9In particular the intermolecular Heck type arylation of achiral Baylis-Hillman adducts has led to α-benzyl-substituted β-keto ester as the major product instead of β-aryl substituted Baylis-Hillman adducts. 10Recently Kim et al. 11 have successfully optimized condition for intermolecular Heck reaction of achiral aldehyde derived Baylis-Hillman adducts to yield the β-aryl substituted intermediates as major products.
Sphingolipids, e.g.sphingosine, ceramides, sphingomylein, cerbrosides and gangliosides are important structural and functional components of plasma membrane of essentially all eukaryotic cells and are implicated in the regulation of diverse cellular and physiological processes. 12eramide [N-acylsphingosine 1a (Figure 1)], a long-chain aliphatic 2-amido-1,3-diol with a C(4), C(5)-trans double bond, is known to induce apoptotis, 13 growth arrest 14 and senescence 15 in many human cancers, besides regulating many biochemical and cellular responses to stress, such as radiation, oxidative condition, exposure to heat and chemotherapeutic agents. 16In addition to the natural ceramides, their analogues have also been evaluated for various biological activities. 17In particular, Wascholowski and Giannis 18 have reported sphingolactones as potent inhibitors of the neutral sphingomyelinase (N-SMase).
Due to the wide spectrum of biological and pharmacological significance of ceramide and its analogues considerable effort has been devoted towards developing efficient methods for their synthesis. 19,20Few reports in literature illustrate the use of Baylis-Hillman 21 and Heck reaction 22 for the synthesis of sphingolipids and their analogues.The configurationally stable Garner aldehyde 23,24 has received considerable attention as chiral synthon for the stereoselective synthesis of sphingolipids and their derivatives due to its inherent 2-amino-1,3-diol subunit which is the main backbone of sphingolipids.(S)-Garner aldehyde-methyl acrylate derived Baylis-Hillman adduct with anti configuration 3a was reported by Drewes et al. 21as a potential intermediate for the synthesis of sphingolipid analogue.However, no work has been reported in this direction in literature for further transformations to sphingolipids or their derivatives.
The Baylis-Hillman reaction 7g of (S)-Garner aldehyde 2 with methyl acrylate in the presence of DABCO and ultrasound sonication afforded 6:1 diastereomeric mixture of adducts 3a and 3b as observed by 1 H NMR spectral analysis of crude mixture.7h The anti stereochemistry of the major adduct may also be rationalized on the basis of the Felkin-Anh open-chain model. 26he pure adduct 3a was utilized as a substrate for intermolecular Heck reaction.We first examined the reaction of the adduct 3a with iodobenzene 4a to determine the feasibility and optimum Pd(OAc)2 concentration required for the complete consumption of starting adduct 3a and its maximum conversion to the desired Heck product 6a-E/Z (Table 1).The reaction was screened with different concentration of Pd(OAc)2 starting from 10 mol% and increasing upto 25 mol%.It was observed that in the conversion of adduct 3a to 6a-E/Z, the desired Heck product increases moderately on enhancing the Pd(OAc)2 concentration upto 20 mol% and beyond which there was no significant effect of increased Pd(OAc)2 concentration on the relative composition (Table 1).The best result was obtained (Table 1, entry 3) when a mixture of 3a (1.0 equiv), 4a (2.0 equiv), Pd(OAc)2 (20 mol%), TBAB (1.0 equiv.),KOAc (3.0 equiv) in CH3CN was refluxed for 24 h.The adduct 3a was consumed completely to give 5a (43%), 6a-E (27%) and 6a-Z in 24% relative composition as determined by 1 H NMR spectral analysis of the crude reaction mixture, wherein the characteristic chemical shift of vinylic proton of 6a-E was observed as a singlet at 7.79 ppm and for 6b-Z isomer at 7.01 ppm, while the benzylic -CH2 of α-benzyl-β-keto ester 5a appeared at 3.17 ppm as a multiplet.Purification by column chromatography over silica gel, eluting with hexane:EtOAc/89:11, yielded β-keto ester 5a (36%) as a white solid.Further elution with hexane:EtOAc/86:14 afforded 6a-E (21%) and 6b-Z (18%) as colorless oils.
Under these optimized conditions the Heck reaction was carried out with adduct 3a and various aryl iodides 4b-d to obtain the desired Heck products 6b-d and corresponding β-keto esters 5b-d as shown in Scheme 2 and results are summarized in Table 2. Scheme 2. Heck reaction of adduct 3a with selected aryl halides 4b-d.It has been observed that the adduct 3a was completely consumed except when piodoacetophenone 4d (Table 2, entry 3) was used as the aryl halide, under the optimized conditions.The relative composition of products was again determined by 1 H NMR spectral analysis of respective crude mixtures.It has been observed from 1 H NMR spectra that the isomer E and Z for compounds 6a-d are obtained in approximately 1:1 ratio as explained by Kumareswaran and Vankar. 10a The β-keto esters 5b-d were isolated in 26-43% yield and the desired Heck products were isolated in 21-41% yield.The E and Z isomers of 6b-d were not separated.All compounds were characterized by NMR, IR, and mass spectroscopy.
The N-Boc and oxazolidine groups of pure 6a-E were removed under acidic condition using 1M HCl/MeOH followed by N-acylation 27 using p-nitrophenyl palmitate, Et3N, DMAP (cat.) in dry THF to afford crude lactone ceramide analogue 1c as the major product instead of the expected open chain ceramide analogue.The lactonisation may have occurred due to the intramolecular condensation of primary hydroxyl group of the sphingoid backbone with -COOH group generated during the deprotection step of intermediate 6a-E or by the direct attack of the primary hydroxyl group to the ester group, before its hydrolysis, with subsequent elimination of methanol.The mixture was purified by column chromatography over silica gel to obtain the product 1c in 54% yield (Scheme 3).The title compound 1c was characterized by NMR, IR and mass spectroscopy.The characteristic peaks for 1c were observed for the carbonyl of lactone at 1732 cm -1 in the IR spectrum and at δ 4.30-4.31(d, J = 6.2 Hz, 1H, HCHOH) and 4.40-4.45(m, 2H, HCHOH, -CH-N ) in the 1 H NMR spectrum, which showed downfield shift of these protons as compared to that expected in the corresponding acyclic ceramide analogue.In the 13 C NMR spectrum, the peaks at δ 45.6 and 164.9 were observed for -C _ N and for lactone group respectively, which further confirmed the formation of lactone ceramide analogue 1c.

Experimental Section
General. 1 H and 13 C NMR spectra were recorded on a Bruker Avance II 400 MHz spectrometer at 400 and 100 MHz respectively using TMS as internal standard in CDCl3.IR spectra were recorded using a Perkin-Elmer model RX 1 FT-IR spectrometer.Mass spectra were recorded on Waters Micromass q-Tof Micro spectrometer.Elemental analyses were performed using automatic Perkin Elmer 2400 CHN elemental analyzer.Optical rotations (in degrees) were recorded on Autopol-III polarimeter.The Baylis-Hillman reaction was sonicated in an ultrasonic cleaner (40 ±5 kHz).Column chromatography was conducted on Silica gel 100-200 mesh (E-© ARKAT-USA, Inc. Merck) using ethyl acetate and hexane as eluant.All reactions were carried out using oven dried glassware.All solvents were dried as reported in literature prior to use.

General procedure for the Heck coupling of (S)-Garner aldehyde-methyl acrylate derived Baylis-Hillman adducts 3a with various aryl halides (4a-d)
A stirred solution of (S)-Garner aldehyde-methyl acrylate derived Baylis-Hillman adduct 3a (1.0 equiv), aryl halides 4a-d (2.0 equiv), Pd(OAc)2 (20 mol%), KOAc (3.0 equiv), TBAB (1.0 equiv) in dry CH3CN (8-10 ml) was heated to reflux for 24 h under nitrogen atmosphere.The reaction mixture was allowed to warm to room temperature followed by solvent removal under reduced pressure.The residue was diluted with water and extracted with diethyl ether (5 x 10 ml).The combined organic layers were dried over anhydrous Na2SO4, filtered and the crude product was passed through a small pad of celite.The solvent was evaporated under reduced pressure and 1 H NMR spectra was recorded to determine conversion ratio of the products.The column chromatographic separation over silica gel, eluting with hexane/EtOAc (89:11-72:28) yielded the β-keto esters 5a-d in 26-43% as a white solids and desired Heck products 6a-d (E/

Table 1 .
The reaction a of adduct 3a with Iodobenzene 4a under various Pd(OAc)2 concentration Determined by the 1 HNMR spectral analysis of the crude mixture. b