Total synthesis of GEX1Q1, assignment of C-5 stereoconfiguration and evaluation of spliceosome inhibitory activity.

An enantioselective total synthesis of GEX1Q1 has been accomplished in a convergent manner. The C-5 asymmetric center has now been assigned through synthesis. GEX1Q1 displayed slightly better spliceosome inhibitory activity over its C-5 epimer. The salient features of this synthesis include an asymmetric hetero-Diels-Alder reaction to construct the tetrahydropyran ring and a Suzuki cross-coupling to assemble the key segments.

GEX1Q1 (1, Figure 1) is one of the six natural products including GEX1A (herboxidiene) isolated from a culture broth of Streptomyces sp. by Yoshida and co-workers in 2002.[1] It has shown cytotoxicity with an IC50 value of 0.93 μM against human tumor cell lines in vitro.[1] GEX1Q1 contains 10 chiral centers. Nine of them have been assigned by extensive spectroscopic studies. However, the C-5 hydroxy stereochemistry had not been assigned.[1,2] The related natural product, 5-deoxy GEX1Q1, known as herboxidiene (2), displayed important biological properties. It showed induction of cell cycle arrest in G1 and G2/M phase in human normal fibroblast cell lines.[3] It also caused reduction of plasma cholesterol by upregulating gene expression.[4] Furthermore, it displayed potent spliceosome inhibitory activity.[5] As a consequence, there has been immense interest in total synthesis[6,7] and biological studies[8] of herboxidiene. We recently reported a convergent total synthesis[9] and spliceosome inhibitory properties[10] of herboxidiene. In our continuing interest in herboxidiene and its derivatives, we have explored the chemistry and biology of GEX1Q1. Herein, we report a convergent synthesis of GEX1Q1, assignment of the stereochemistry of the C-5 asymmetric center and evaluation of spliceosome inhibitory activity of GEX1Q1 and its C-5 epimer.

Figure 1

Retrosynthesis of GEX1Q1.

Our retrosynthetic analysis of GEX1Q1 is outlined in Figure 1. We planned to carry out Suzuki cross-coupling of vinyl iodide 3 and vinyl boronate 4 to construct GEX1Q1. A similar strategy was employed by Forsyth and co-workers[7a] and by us[9] during the synthesis of herboxidiene. The functionalized tetrahydropyran ring 3 would be synthesized from tetrahydropyranone derivative 5. Vinyl boronate 4 was synthesized by us previously.[9] We planned to carry out an asymmetric synthesis of 5 via a hetero-Diels–Alder reaction with diene 6 and propargylaldehyde derivative 7.

An asymmetric synthesis of tetrahydropyranone derivative 11 is shown in Scheme 1. Silyloxy diene 6 was prepared from aldehyde 8 as described by us previously.[11] An asymmetric hetero-Diels–Alder reaction of diene 6 with commercially available aldehyde 7 in the presence of Jacobsen’s catalyst 9(12) (5 mol %) afforded cycloadduct 10 in 94% yield. Removal of the triethylsilyl group by treatment with tetrabutylammonium fluoride (TBAF) provided pyranone 5 in 85% yield. HPLC analysis of 5 on a chiral OD column showed enantiomeric purity of 94% ee.[13] The synthesis of segment 3 required inversion of configuration of the C-6 methyl center. This was accomplished by treatment of 5 with DBU and EtOH in toluene at 80 °C for 24 h.[14] This condition resulted in the deprotection of the TMS group as well as epimerization of the C-6 methyl center. Pyranone 11 was separated by silica gel chromatography and about 10% of starting pyranone 5 was recovered. This was subjected to further epimerization to provide 11 in 75% yield after one recycle.

Scheme 1

Synthesis of Tetrahydropyranone 11

The syntheses of vinyl iodides are shown in Scheme 2. To obtain the equatorial alcohol, we carried out reduction[15] of 11 with NaBH4 in ethanol at 0 °C to furnish alcohol 12 in 82% isolated yield. Acetylene derivative 12 was treated with a catalytic amount of HgSO4 in 3 M H2SO4 as described by Yates and co-workers[16] to afford the corresponding ketone. The hydroxyl group was protected as the TBS ether to provide bis-TBS derivative 13 in 70% yield over two steps. This was converted to methyl ester 14 in a three step sequence involving: (1) selective deprotection of the primary TBS group by exposure to CSA in MeOH at 0 °C for 2 h; (2) oxidation of the resulting primary alcohol with TEMPO in the presence of PhI(OAc)2 in aqueous CH2Cl2 at 23 °C for 4 h; and (3) esterification of the resulting acid with MeOH using DCC and DMAP in CH2Cl2 at 23 °C for 2 h. Methyl ester 14 was then converted to vinyl iodide 15 using a procedure described by Takai and co-workers[17] by exposure to CrCl2 and CHI3 in THF at 23 °C for 15 h, providing 15 in 50% yield (brsm).

Scheme 2

Stereoslective Synthesis of 5-(R)- and 5-(S)-Tetrahydropyran derivatives 14 and 19

An asymmetric synthesis of tetrahydroprane derivative 19 bearing 5-(S)-configuration is also shown in Scheme 2. To set the 5-(S)-configuration with an axial alcohol, we planned to reduce tetrahydropyranone 11 with L-Selectride.[18] Treatment of 11 with L-Selectride in THF at −78 °C for 1 h afforded alcohol 16 in 81% yield. This was converted to bis-TBS ether 17 in 52% yield over three steps.[19] Compound 17 was converted to methyl ester 18 and then to vinyl iodide 19 as described for compound 15.

Toward the synthesis of GEX1Q1, we first carried out Suzuki cross-coupling of boronate 4(9) and vinyl iodide 15 containing the 5-(R)-configuration. As shown in Scheme 3, reaction of 4 and 15 in the presence of Pd(Ph3P)4 (5 mol %) and Cs2CO3 in THF at 55 °C for 2.5 h provided the coupling product 20 in 82% yield. Treatment of 20 with 1 M HCl in methanol at 23 °C for 45 min resulted in the deprotection of TBS-ethers to furnish the corresponding diol. Exposure of the diol to a catalytic amount of VO(acac)2 in the presence of t-BuOOH at −15 °C for 48 h afforded the epoxide 21 as a single product in 60% yield in two steps. Saponification of methyl ester 21 with K2CO3 in aqueous MeOH at reflux for 2 h afforded synthetic GEX1Q1 (1) with 5-(R)-hydroxy stereochemistry in 92% yield. To our delight, 1H NMR and 13C NMR data of synthetic 1 ([α]23D −13.8, c 0.12, MeOH) are identical to that reported for the natural GEX1Q1 ([α]20D −13.5, c 0.13, MeOH).[1,2]

Scheme 3

Synthesis of (+)-GEX1Q1 and its C-5 Epimer

To further compare the spectral data, as well as to evaluate the effect of the C-5 hydroxyl group stereochemistry of GEX1Q1, we synthesized the C-5-(S) stereoisomer of GEX1Q1. As shown, Suzuki coupling of vinyl iodide 19 with boronate 4 resulted in coupling product 22 in 85% yield. Due to steric hindrance, the deprotection of the TBS group at C-5 was sluggish under methanolic HCl conditions. However, reaction of 22 with Py.HF at 60 °C for 20 h provided the desired product in 72% yield. The resulting diol was then converted to 5-epi-GEX1Q1 23 by epoxidation, followed by saponification of the methyl ester as described for GEX1Q1. Spectroscopic properties (1H NMR and 13C NMR) and optical rotation ([α]23D +16.9, c 0.13, MeOH) of compound 23 are significantly different from that of GEX1Q1.[1,2]

The biological properties of GEX1Q1 1 and compound 23 were evaluated in an in vitro splicing system as previously described.[10] We added the compounds to splicing reactions containing a synthetic pre-mRNA substrate, ATP, and nuclear extract from HeLa cells. As previously characterized for this extract system, spliceosomes assemble on only a portion of the pre-mRNA substrate and catalyze intron removal.[10] We examined splicing activity by denaturing PAGE to separate the substrate and product mRNA, and splicing efficiency was quantified as the percent of pre-mRNA converted to mRNA. In the system, DMSO alone, which has no effect on splicing efficiency, is used as a control.[10] GEX1Q1 inhibits splicing relative to DMSO with an IC50 of 0.3 μM (Figure 2A and C). Compound 23 showed a slight reduction in potency with an IC50 of 0.8 μM. However, the difference is within the variation of splicing efficiency measured by the assay. These values are also comparable to what we have previously observed for herboxidiene.[10]

Figure 2

Impact of analogues on in vitro splicing. (A) Denaturing gel analysis of radiolabeled RNA isolated from splicing reactions. The first five lanes include a time course of splicing reactions in 1% DMSO followed by 30 min time points of splicing reactions incubated with indicated compound concentration. Identities of bands are schematized to the left as (from top to bottom) lariat intermediate, pre-mRNA, mRNA, 5′ exon intermediate. (B) Native gel analysis of spliceosome assembly. Aliquots of the splicing reactions described above were separated under native conditions. The identity of splicing complexes is denoted with assembly occurring in the following order: H/E → A → B → C. (C) Quantification of normalized splicing efficiency vs inhibitor concentration for the splicing reactions shown in A and B, respectively. IC50 refers to the concentration required to reduce in vitro splicing efficiency by half compared to DMSO control.

We also examined the effect of these compounds on spliceosome assembly. Spliceosomes assemble on pre-mRNA substrates via an ordered series of intermediate complexes. A subset of these complexes can be visualized by native gel analysis of the same in vitro splicing reactions described above. H/E and A complexes form as early intermediates that convert to B and then to C complex, at which point the splicing reaction is catalyzed and the complexes immediately disassemble. As with splicing chemistry, DMSO alone has no effect and spliceosomes assemble normally over time (Figure 2B). With increasing concentrations of GEX1Q1 (1) and compound 23, spliceosome assembly halts at an A-like complex. The block in spliceosome assembly appears to be identical to that produced by herboxidiene and two other splicing inhibitors, pladienolide B and FR901464.[10]

In summary, we have accomplished a stereoselective synthesis of GEX1Q1 and assigned the C-5 hydroxyl group stereochemistry of GEX1Q1. Also, we have evaluated spliceosome inhibitory activity of GEX1Q1 and its C-5 epimer and compared their activity with herboxidiene. The synthesis features a hetero-Diels–Alder reaction to construct the tetrahydropyran ring, inversion of the C-5 asymmetric center of the resulting cycloadduct and a Suzuki coupling reaction to assemble the protected GEX1Q1. We have also probed the importance of the C-5 hydroxyl group stereochemistry of GEX1Q1 and its epimer in terms of their effect on spliceosome activity. Interestingly, both GEX1Q1 and its C-5 epimer showed nearly identical potency relative to herboxidiene.[10] Therefore, the C-5 hydroxyl group stereochemistry does not significantly influence spliceosome inhibitory activity. The design and synthesis of novel herboxidiene and GEX1Q1 derivatives are in progress.