A Stereoselective Synthesis of (−)-Viridiofungin A Utilizing a TiCl₄-Promoted Asymmetric Multicomponent Reaction

Abstract

A stereoselective synthesis of (−)-Viridiofungin A is described. The convergent synthesis utilized a unique highly diastereoselective multicomponent reaction between optically active phenyldihydrofuran and an α-ketoester to provide two chiral centers including a quarternary carbon center in a single step. Other key steps include an acyloxycarbonum ion-mediated tetrahydrofuran ring-opening reaction and a Julia-Kocienski olefination.

Viridiofungins were first isolated by Harris and co-workers in 1993 from the fungus, Trichoderma viride.¹ This family of alkyl citrates exhibited broad spectrum of anti-fungal properties with minimum fungicidal concentrations in the range of 1–20 μg/mL against a number of species. Furthermore, viridiofungins inhibited rat and yeast squalene synthesis with IC₅₀ values of 0.4–15 μM.² This antifungal activity is unrelated to the inhibition of ergosterol biosynthesis. Instead, viridiofungins showed very potent nanomolar inhibitory activity against serine palmitoyltransferase, the first enzyme in the sphingolipid biosynthesis which is responsible for viridiofungin’s antifungal properties.³ Sphingolipids are abundant membrane lipids important for cell recognition and signal transduction. Due to the ability of sphingolipids to form so called lipid rafts, inhibition of serine palmitoyltransferase has been recognized as a possible target for the treatment of hepatitis C.⁴ There are some indications that viridiofungin analogs could inhibit farnesyltransferase, which might lead to viridiofungin-based compounds with anti-cancer activity.⁵

The initial structural assignment of the viridiofungins was carried out by chemical degradation and spectroscopic studies.¹ The relative and absolute stereochemistry of viridiofungin A (1) was assigned by Hatakeyama and co-workers through their first total synthesis in 27 steps.⁶ Several other synthetic studies led to the synthesis of ester derivatives of viridiofungins. Hiersemann and co-workers⁷ and Barrett and co-workers⁸ have reported the synthesis of viridiofungin ester derivatives. Hatakeyama and co-workers have reported a second generation synthesis of viridiofungin A in 22 steps.⁹ Herein we report an asymmetric synthesis of viridiofungin A which can be utilized for analog preparation.

Our synthetic strategy to viridiofungin A is shown in Figure 1. Strategic bond disconnections provided funtionalized aldehyde 2, sulfone 3, and tyrosine derivative 4. A Julia-Kocienski olefination of 2 with 3 is planned to provide trans-olefin in 1. This sidechain attachment was previously explored by Hiersemann and co-workers.⁷ The highly functionalized aldehyde could be derived from the oxidative cleavage of styrene derivative 5, which in turn could be obtained from a ring-opening reaction of the corresponding functionalized tetrahydrofuran derivative of 6. The key intermediate would be synthesized by an asymmetric multicomponent reaction of optically active phenyldihydrofuran 7 and an appropriately functionalized ketoester. Such asymmetric multicomponent reactions and acyloxycarbonium ion mediated opening of tetrahydrofuran ring were developed in our laboratory.^{10, 11}

Figure 1.

Retrosynthetic Analysis

The synthesis begins with the titanium mediated multicomponent reaction between optically active phenyldihydrofuran 7,¹¹ α-ketoester 8¹², and triethyl silane. The reaction involved TiCl₄-promoted activation of α-ketoester followed by attack of phenyldihydrofuran presumably anti to the phenyl group formed an oxocarbenium ion. Subsequent reaction of this presumed oxocarbenium ion with a hydride from triethylsilane formed two adjacent chiral centers including a quarternary carbon center diastereoselectively. This reaction proceeded with good yield and an excellent diastereomeric ratio (dr> 20:1). The stereochemical outcome can be rationalized based upon product-like transition-state models 9a and 9b. Presumably, the model 9b is preferred as the developing non-bonded interactions are less for exo-oriented bulky 5-membered Ti-chelate compared to endo-oriented Ti-chelate in 9a. Similar transition-state models were proposed by us previously.^10d

The acetylene functionality in 6 was converted to the corresponding carboxylic acid via hydroboration with borane-THF complex followed by oxidation with alkaline hydrogen peroxide.¹³ The ethyl ester was subsequently hydrolyzed with aqueous lithium hydroxide to provide the corresponding diacid. This diacid was converted to the di-tert-butyl ester 10 by treatment with N,N-diisopropyl-O-2-tert-butylisourea in CH₂Cl₂. The tert-butyl ester is necessary due to incompatibility with base at the later stages of the synthesis.⁷

With the synthesis of requisite tetrahydrofuran 10, we then explored the ring opening reaction. Previously, we carried out similar ring opening reactions using a catalytic amount of ZnCl₂ in the presence of acetic anhydride.¹¹ However, subjecting 10 to these conditions led to unwanted side reactions. After surveying a number of Lewis acids, we found that, exposure of 10 to a catalytic amount of Cu(OTf)₂ (20 mol%) and acetic anhydride in refluxing toluene gave rise to smooth ring opened product through a presumed acyloxycarbonium ion intermediate followed by an unexpected collapse of the di-tert-butyl esters to provide anhydride 11 in near quantitative yield.

Basic hydrolysis conditions only provided the elimination product. However, treatment with aqueous acetic acid in THF resulted in anhydride opening to the corresponding diacid smoothly. The resultant diacid was esterified by treatment with N,N-diisopropyl-O-2-tert-butylisourea in CH₂Cl₂ to give di-tert-butyl ester 5. For the subsequent Julia-Kocienski olefination,¹⁴ protecting group manipulations were necessary. Standard conditions for acetate hydrolysis such as K₂CO₃ in methanol resulted in elimination rather than deprotection. To circumvent this problem, diacetate 5 was treated with allyl magnesium bromide at −78 °C to give the diol. The primary alcohol was protected as its tert-butyldimethylsilyl ether with TBSOTf and 2,6-lutidine at −78 °C. Subsequent protection of the tertiary alcohol as its triethylsilyl ether provided styrene derivative 12.

Oxidative cleavage of styrene 12 with ozone followed by reductive workup afforded the corresponding aldehyde. This aldehyde was subjected to Julia-Kocienski olefination with known sulfone 3.⁷ Thus, treatment of sulfone 3 with KHMDS in THF at −78 °C followed by addition of the aldehyde via cannula afforded the alkene 13 as a single trans-stereoisomer. Removal of the silyl protecting groups with HF-pyridine followed by oxidation of the primary alcohol using Jones’ reagent afforded the acid 14 with concomitant deprotection of dioxolane protecting group.

To complete the synthesis of viridiofungin A, coupling of acid 14 with tyrosine tert-butyl ester 4 was carried out by utilizing EDCI hydrochloride, N-methylmorpholine and hydroxybenzotriazole in DMF to provide tri-tert-butyl ester 15. While deprotection of tri-tert- butyl ester 15 is known from the second generation synthesis by Hatakeyama⁹, commercial grade (88%) formic acid gave incomplete conversion to (−)- viridiofungin A and instead gave a mixture of mono- and di-tert-butyl esters. However, when 96% formic acid was used removal of all three tert-butyl esters was accomplished in one hour. As such, deprotection of the tert-butyl esters by treatment with neat 96% formic acid at 23°C for one hour furnished synthetic (−)-viridiofungin A (1, [α]²³ _D -11.0 (c 0.39, MeOH). The spectral data (¹H-and ¹³C- NMR) of synthetic (−)-viridiofungin A are identical with those reported for the natural (−)- viridiofungin.¹

In summary, we have achieved a stereoselective synthesis of (−)-viridiofungin A (1). The convergent synthesis features a highly diastereoselective multicomponent reaction to form two key stereocenters including a quaternary stereocenter in high yield. Both stereogenic centers are derived from (S)-2-phenyl-2,3-dihydrofuran. The synthesis will provide a convenient access to a variety of viridiofungin derivatives.

Scheme 1.

Multicomponent Reaction

Scheme 2.

Tetrahydrofuran Ring Opening

Scheme 3.

Completion of Viridiofungin A