Oxidative cyclization of γ-alkylidene butenolides . Stereoselective preparation of spirolactones

A new route to 1,6-dioxaspiro[4.4]non-3-en-2-ones is established by bromoetherification of dihydroxybutenolides. An asymmetric total synthesis of 8-epi-crassalactone D starting from methyl cinnamate has been accomplished.


Introduction
Spiro-γ-lactones constitute an important class of oxygen-containing heterocyclic compounds, and such groups can be found in many biologically active natural products. 1Among them, the 1,6dioxaspiro [4.4]non-3-en-2-ones constitute a family that have attracted our attention very recently due to their structural originality and biological activity.For example, the styryl-lactone (+)-crassalactone D (1a, Figure 1) has recently been isolated from an extract of the leaves and twigs of Polyalthia crassa Parker (Annonaceae), and its structure was determined on the basis of spectroscopic methods.Single-crystal X-ray analysis and the Mosher ester method were used to confirm its absolute stereochemistry.Spirolactone 1a showed broad cytotoxic activity against murine lymphocytic leukemia, human colon, nasopharyngeal, lung and breast carcinomas. 2 Pyrenolide D 2, however, was isolated from the phytopathogenic fungus Pyrenophora teres (Diedicke) Drechsler (FO 7508), 3 and its absolute configuration was determined by pioneer synthetic work in this field. 4Although 2, like other pyrenolides, was not active against fungi, it was found to be cytotoxic toward HL-60 cells at IC 50 4 µg/ml.
(+)-Massarinolin A 3 is a bioactive sesquiterpenoid isolated by Gloer et al. from liquid cultures of the aquatic fungus Massarina tunicata Shearer & Fallah. 5It shows biological activity against Bacillus subtilis (ATCC 6501) and Staphylococcus aureus (ATCC 29213).It appears to be biosynthesized from a farnesyl-type precursor and even though its relative stereochemistry was deduced by spectroscopic methods its absolute stereochemistry remains a challenge for synthetic chemists.The ring system found in compound 3 has previously been encountered only in expansolides A and B (4, and 5), reported as metabolites of Penicillium expansum. 6Their absolute configuration was established using the modified Mosher method. 7gure 1.Biologically active spiro γ-lactones.
Although pioneer synthetic approaches to spirobutenolides have been reviewed by Knight in 1994, 8 further synthetic efforts in this topic have been reported. 9The first total synthesis of (+)pyrenolide D 2 started from tri-O-acetyl-D-galactal and was reported by Gin et al. in 2001.This pioneer synthetic work led to the absolute stereochemical assignment of 2. 10 The enantiospecific synthesis of four hydroxylated analogues of 2 was reported by Robertson et al. 11 This synthetic approach started from D-glucose and was based on furan oxidative spirocyclization.Shortly afterwards, Vassilikogiannakis et al. reported the photooxygenation of 2-(γ-hydroxyalkyl)furans as a newly developed technology applied to the synthesis of γ-spiroketal γ-lactones such as (+)crassalactone D (1a), and three different epimers of pyrenolide D, 2. 12 The asymmetric total synthesis of (+)-crassalactone D from the commercially available 3-bromo-1-phenyl-1-propene was published by Yang et al. in 2009.Their successful synthesis was elegantly achieved by employing an oxidative spirocyclization of a dihydroxylated 2-substituted furan as the key step.Two close analogues of (+)-crassalactone D 1a, have also been prepared in the course of the synthetic work. 13

Results and Discussion
In order to test whether the bromoetherification of hydroxybutenolides might afford an efficient way towards the synthesis of 1,6-dioxaspiro [4.4]The preparation of both stereoisomers of 7-epi-goniobutenolide 6 was accomplished by following pioneer synthetic work in this field. 14Isolation of the aldehyde 11 afforded the opportunity to assay the vinylogous Mukaiyama aldol reaction using the 2-silyloxyfuran 12a (TMSOF) in the presence of Lewis acids such as TiCl 4 or SnCl 4 . 15The reaction yielded a mixture of stereoisomers 13a and 13b with similar results (72% yield and a moderate stereoselectivity, favouring the threo adduct 13a: 13b = 1.7: 1) (Scheme 1).

Scheme1. Vinylogous aldol reaction on cinnamyl aldehyde derivatives.
A possible explanation for the preference towards 13a may lie in hydrogen bonding formation between the hydroxy function and the lactone oxy group (see Figure 3). 16The stereoselective formation of 13a in the vinylogous Mukaiyama aldol reaction led us to undertake computational studies with a view to determining the the relative stability of both isomers : 13a and 13b.After a conformational search performed with MM2 , the lower energy conformer 13a was found to be 4.63 kcal/mol more stable than 13b.Additionally, the hydrogen bonding formation for 13a, for a O-H distance of d: 2.269 Å, was seen to be more feasible in comparison with the value of d: 3.872 Å obtained for the same O-H distance in 13b.Elimination of the hydroxyl function on C-6 in both isomers 13a and 13b required the transformation into the corresponding acetates (14a and 14b) and further treatment with base (Scheme 2).Treatment of 13b with acetic anhydride in pyridine led to the isolation of acetate 14b in 82% yield.Elimination of the acetate was accomplished by treatment with DBU and allowed us to isolate the butenolide 15a in quantitative yield.In the case of 13a, however, the use of dimethylaminopyridine (DMAP) to obtain the acetate 14a was necessary.The reluctance of the hydroxyl function to undergo the transformation into the acetate in this case may be due to the above-mentioned formation of hydrogen bonding with the lactone oxy function.Chromatographic separation of the crude product on silica gel led to the recovery of the starting material, and the desired product, 14a, with 25% and 68% yields respectively.Treatment of 14a with DBU led to the elimination product 15a, with 65% yield.It is known that this elimination takes place through an E 1 cb mechanism, and we assume that the convergent stereoselectivity obtained in both cases would mostly be due to stereoelectronic factors, which may be explained in terms of electronic repulsion between the lactone oxygen lone pairs and those of acetonide functionality, working on an identical intermediate. 17he deprotection of acetonide 15a took place smoothly without epimerization by treatment with p-toluenesulfonic acid in methanol and we were able to isolate the 7-epi-goniobutenolide (E)-6 in 73% yield.Access to both isomers 13a and 13b was achieved following a modified previously-reported procedure 14c starting from the cinnamyl aldehyde diethyl acetal 16 (Scheme 3).The trienes 17a and 17b were obtained at a 1: 1 ratio in 87% combined yield.The E-and Z-isomers, whose stereochemistry was established by NOE experiments, were readily separated by flash chromatography.The dihydroxylation of 17a under standard conditions afforded (Z)-6 in 85% yield and 98% ee.The other isomer 17b, however, yielded either isomer (Z)-6 or (E)-6 depending on the reaction temperature, in both cases with high yields (82% and 80%, respectively).The formation of (Z)-6 took place when the reaction was performed at room temperature and can be explained in terms of isomerisation occurring under the reaction conditions and concomitant formation of a hydrogen bonding between the hydroxyl function on C-6 and the furanone oxy function which renders this stereoisomer more stable.

Scheme 4. Oxidative cyclization of dihydroxy butenolides.
The reaction mixture proved to be chromatographically unresolvable on a flash column of silica gel.However, the structural assignment was possible by full spectroscopic analysis of the reaction mixture, which included COSY, ROESY, HMQC and HMBC spectra.
Structural assignment of 7a was based on the multiplicity found for the hydrogen atoms on C9 at δ= 4.82ppm (d, J = 4 Hz), C8 at δ = 4.62 ppm (dd, J 1 = J 2 = 4 Hz), and C7 at δ = 5.51 ppm (d, J = 4 Hz), which suggests the cis stereochemistry for the three hydrogen atoms (Scheme 5).Additionally, the absence of correlation between the protons at C9 and C4 suggests the trans stereochemistry between the bromo and the lactone oxy functions.In the case of 7b, however, the hydrogen at C9 appears as a singlet centered at δ = 4.48 ppm, which suggests the trans stereochemistry with respect to the OH function on C8.Again, the absence of correlation between the protons at C9 and C4 suggests the trans relationship between the bromine and the lactone oxygen.
From a mechanistic point of view, the formation of 7a and 7b with the exclusion of 7d and 7c respectively obeys the stereoelectronic effect that is developed at the transition state of the bromoetherification reaction: the electronic repulsion between the electronic lone pairs on the bromine and the lactone oxygen.The formation of bromolactone 7a as the major isomer of the reaction mixture with respect to 7b can mainly be assigned to a stereoelectronic effect (Figure 4). 20Although the cyclization via antiperiplanar attack through the transition state TS-2 should be easier than that with the synclinal orientation (TS-1), the strong steric hindrance developed between the bromine atom and the C4-C5 bond of the furanone nucleus in TS-2 helps to rationalise the formation of the major isomer 7a from TS-1 , which is much less sterically hindered.
Treatment of the bromolactones 7a and 7b with tri-n-butyltin hydride in refluxing toluene afforded the 8-epi-crassalactone D 1b, and 5,8-epi-crassalactone D 1c, in quantitative yields.The hydroxyspirolactones 1b and 1c were obtained at the same (3:1) ratio and were separated by flash chromatography on silica gel.Treatment of 1b under Mitsunobu conditions 21 led to the isolation of the benzoate 18 with 75% yield.Since the transformation of the benzoate 18 into (+)-crassalactone D 1a, has been recently accomplished by Yang et al, 13 our present contribution may also be considered as a formal total synthesis of the biologically active compound.

Conclusions
In summary: we have developed a new route to 1,6-dioxaspiro [4.4]non-3-en-2-ones through the oxidative cyclization of hydroxybutenolides.The bromoetherification reaction of 7-epigoniobutenolides A [(Z)-6] and B [(E)-6] afforded a mixture of bromospirolactones 7a and 7b in a 3:1 ratio, which was reduced by tributyltin hydride to afford the target molecules 1b and 1c.After chromatographic separation of the reaction mixture, the stereochemical outcome of the cyclisation reaction was elucidated by full spectroscopic analysis.

Experimental Section
General.Melting points are uncorrected. 1H-NMR spectra were measured at either 200 or 400 MHz and 13 C-NMR were measured at 50 or 100 MHz in CDCl 3 and referenced to TMS ( 1 H) or solvent ( 13 C), except where indicated otherwise.IR spectra were recorded for samples in CHCl 3 solution on NaCl plates, unless otherwise stated, with an FT-IR instrument.HRMS determinations (EI) were recorded at the Mass Spectrometry Service of the University of Salamanca, Spain.Microanalyses were performed on a Perkin-Elmer 240-B analyzer.All reactions were conducted under a positive pressure of argon, utilizing standard bench-top techniques for handling of air-sensitive materials.Chemicals and solvents were obtained from commercial sources and used as received with the exception of benzene, toluene and dioxane ARKAT-USA, Inc which were distilled from sodium and benzophenone.Yields reported are for chromatographic pure isolated products unless stated otherwise.

Methyl (4S,5R)-2,2-dimethyl-5-phenyl-1,3-dioxolane-4-carboxylate (10).
A catalytic amount of p-toluenesulfonic acid was added to a solution of 9 (3.83 g, 19.5 mmol) and 2,2dimethoxypropane (4.8 mL, 39.04 mmol) in 100 mL of CH 2 Cl 2 .The reaction mixture was stirred for 3 hours at room temperature.Then, 20 mL of a saturated aqueous solution of NaHCO 3 was added and the mixture was stirred for 15 minutes.The mixture was extracted with CH 2 Cl 2 , the combined organic layers were washed with brine, dried over anhydrous Na 2 SO 4 and the organic solvent was evaporated off under reduced pressured to afford 10 (4.14 g, 90%) as a yellow oil.

Scheme 5 .
Scheme 5. Mechanism of formation of the spirobutenolides 7a and 7b.