Synthesis and stereochemical assignment of (+)-chamuvarinin.

A stereocontrolled total synthesis of (+)-chamuvarinin, isolated from the root extract of Uvaria Chamae, utilizes a convergent modular strategy to construct the adjacently linked C15-C28 ether array, followed by a late-stage Julia-Kocienski olefination to append the butenolide motif. This constitutes the first total synthesis of (+)-chamuvarinin, defining the relative and absolute configuration of this unique annonaceous acetogenin.

Isolated in 2004 by Laurens and co-workers from root extracts of the West African plant, Uvaria chamae, chamuvarinin (1, Scheme 1)[1,2] is a unique member of the annonaceous acetogenin family of natural products.(3) The crude extracts of Uvaria chamae are widely used in traditional medicinal practices for the treatment of a range of ailments including parasitic-borne West African sleeping sickness and has proven to be a rich source of novel acetogenins.(4) An initial biological screening of chamuvarinin showed significant cytotoxicity toward KB 3-1 cervix cancer cell lines (IC50 = 0.8 nM). Structurally, chamuvarinin is the first acetogenin to contain a tetrahydropyran (THP) ring linked adjacently to a bis-tetrahydrofuran (THF) ring system, spanning the C15−28 region of the carbon backbone. In common with the majority of acetogenins,(3)1 bears the 36S-configuration, but the assignment of the relative and absolute configuration within the C15−C28 region proved to be a more significant challenge. This quandary was partially resolved by Poupon and co-workers in 2007,(2) who showed that 1 is not derived directly from squamocin leading to the proposed relative configuration as shown,[3,5] with two possible diastereomeric structures for the structure of 1. Herein, we report the total synthesis of chamuvarinin and stereochemical assignment of 1, utilizing a highly convergent strategy for the construction of the central C15−C28 region. We chose to pursue the 15R-diastereomeric series based on a comparison of acetogenins bearing a C15-carbinol linked to a bis-THF motif and the biosynthetic likelihood that 1 would share the common 15R-configuration.[5,6] Our synthetic strategy relied on a late-stage olefination of the C1−C8 aldehyde 2 and a suitable coupling partner derived from the C9−C34 intermediate 3, as outlined in Scheme 1. In turn, assembly of the central C15−C28 polyether array found in 3 would arise from the coupling of aldehyde (4, C9−C20) and alkyne (5, C21−C34), followed by reduction and cyclization to install the C20−C23 THF motif.

Scheme 1

Synthetic Strategy for Chamuvarinin 1

As outlined in Scheme 2, the synthesis of the C1−C8 aldehyde 2 started with the alkylation of iodide 6(7) with the lithium enolate of lactone 7 to provide 8 in 75% yield.[8,9] Oxidation to the sulfoxide (mCPBA) and 1,2-syn elimination, followed by cleavage of the C8-benzyl ether with BCl3·SMe2, provided alcohol 9 (81% over 2 steps).(10) Finally, Dess−Martin oxidation of 9 provided the C1−C8 aldehyde 2 in excellent yield, as required for the final C8−C9 bond coupling.

Scheme 2

Synthesis of C1−C8 Aldehyde 2

As shown in Scheme 3, the synthesis of the C9−C20 subunit 4 began with the Cu(I)-promoted opening of (S)-TBS-glycidol ether 10 with allylmagnesium bromide to provide 11 (82%). TBS ether formation (TBSCl, ImH) gave 12 (98%). Ozonolysis and reductive PPh3 workup of 12 provided aldehyde 13 which was used directly in the Julia−Kocienski olefination with sulfone 14(11,12) (readily prepared from alcohol 15(13) via Mitsunobu with DIAD, PPh3, 1H-mercaptophenyltetrazole, and Mo(VI)/H2O2 oxidation). Treatment of sulfone 14 with NaHMDS in DME at −78 °C, followed by addition of aldehyde 13, provided olefin 16 in 86% yield from 12 (E/Z = 97:3). The C16−C19 THF ring was conveniently installed by sequential application of protocols developed by Forsyth and co-workers.[14,15] Cleavage of the TBS ethers in 16 with TBAF gave diol 17, which was cleanly converted to epoxide 18 by treatment with NaH and TrisIm (98%).(14) Subsequent asymmetric dihydroxylation of 18 (AD-mix-β),(16) followed by base-mediated 5-exo cyclization (K2CO3, MeOH), provided the 2,5-anti-THF diol 19 in 89% yield with >97:3 dr.(15) A three-step sequence was then required to complete the C9−C20 aldehyde 4, involving TBS ether formation (TBSOTf, 2,6-lutidine), selective primary silyl cleavage, and Dess−Martin oxidation (41% over three steps). Using this approach the C9−C20 subunit was completed in ten steps and 21% overall yield from 10.

Scheme 3

Synthesis of C9−C20 Subunit 4

PTSH = 1H-mercaptophenyltetrazole; Mo(VI) = (NH4)6Mo7O24·4H2O.

As outlined in Scheme 4, the synthesis of the C21−C34 subunit 5 began with the opening of (S)-1-epoxyoctane 20(17) with homoallylmagnesium bromide(18) and catalytic CuI to afford 21. In light of the initial uncertainty around the substitution of the C24−C28 THP ring system in 1,1 we adopted a divergent approach to access both syn-22 and anti-23 from 21. Thus, epoxidation of 21 with mCPBA, followed by addition of catalytic CSA (20 mol %) cleanly promoted the in situ 6-exo cyclization to provide readily separable THP alcohols 22 (41%) and 23 (32%). Oxidation of 22 and addition of the lithium anion of trimethylsilylacetylene to the intermediate aldehyde provided 24 with the undesired 23R configuration in 88% yield (8:1 dr).(19) The minor 23S diastereomer proved readily separable by column chromatography, and Mitsunobu inversion of the C23-hydroxyl in 24,(20) followed by base-mediated methanolysis/desilylation (K2CO3, MeOH), gave 25 (64%) with the requisite 23S configuration. TBS protection (TBSCl/ImH, 91%) then completed the C21−C34 subunit 5 in six steps and 11% overall yield.

Scheme 4

Synthesis of C21−C34 Subunit 5

Ar = 4-BrC6H4.

With our key subunits in hand our attention turned to the C20−C21 bond construction and the installation of the central C20−C23 syn-substituted THF motif. In practice, treatment of 5 with n-BuLi, followed by addition of the resulting lithium anion to 4, gave the expected Felkin−Anh adduct 26 in 71% yield with good levels of stereocontrol at C20 (3:1 dr). Diimide reduction of 26 with TsNHNH2 and NaOAc in DME at reflux proved the most efficient reduction protocol to afford 27 in 79% yield.(21) At this point the C20 diastereomers were readily separated by column chromatography.(19) The stage was now set for the construction of the central C20−C23 THF ring. In the event the C20-hydroxyl in 27 was readily activated as its mesylate (MsCl, Et3N), which upon treatment with TBAF promoted silyl ether cleavage and concomitant 5-exo ring closure to provide 3 in excellent yield. At this point, comparison of the 1H and 13C NMR spectra of the advanced C9−C34 intermediate 3 with the reported data for the C15−C28 region of chamuvarinin showed them to be essentially in complete agreement,[1,2] providing us with the first clear indication of the correct stereochemical assignment of the relative configuration of the C15−C28 region of chamuvarinin.

With the advanced intermediate 3 in hand, attachment of a suitable C1−C8 subunit at C9 was required to facilitate the completion of chamuvarinin. In considering a suitable C8−C9 coupling reaction, we initially focused on using a C9−C34 aldehyde for olefination with either a C1−C8 Wittig salt or sulfone derived from 9; however this approach proved unsuccessful. As a result we employed the reversed coupling strategy detailed in Scheme 5. Thus, the C15-OH in 3 was readily protected as its TBS ether (TBSOTf, 2,6-lutidine, 88%) and subsequent hydrogenolysis of 28 provided alcohol 29 in excellent yield. Treatment of 29 with 1H-mercaptophenyltetrazole under Mitsunobu conditions (99%) and subsequent oxidation of the intermediate sulfide (H2O2, cat Mo(VI), 77%) provided the corresponding sulfone 30, in readiness for the final C8−C9 Julia−Kocienski olefination.[11,12,22] In the event, treatment of sulfone 30 with NaHMDS in THF at −78 °C followed by the addition of aldehyde 2 and warming to −20 °C over 4 h gave the advanced intermediate 31 in 41% yield. Diimide reduction of the C8−C9 alkene (TsNHNH2/NaOAc in DME/H2O),(21) followed by deprotection of the C15-TBS ether (3 N HCl, MeOH), provided the 15R, 16R, 19R, 20R, 23S, 24S, 28S, 36S-diastereomer 32 in excellent yield over two steps. Gratifyingly, the spectroscopic data obtained for 32 (1H and 13C NMR, IR, and MS) correlated fully with that of natural chamuvarinin.(1) In the absence of an authentic sample for direct specific rotation and HPLC comparison, the measured specific rotation [α]D(20) +9.9 (c 0.1, CHCl3) [lit.1 +25 (c 0.026, CHCl3)] was consistent with that of the natural material. Further convincing evidence for 32 being the correct stereostructure of 1 is provided by the comparable levels of biological activity displayed by the synthetic material. In screening assays against the HeLa cervix cancer cell line and the bloodstream form of the parasite Trypanasoma brucei, the synthetic material displayed ED50 values of 2.88 ± 0.66 and 1.37 ± 0.08 μM, respectively,(23) providing unambiguous proof of the relative and absolute configuration of (+)-chamuvarinin (1), as that being indicated in structure 32.

Scheme 5

Completion of Chamuvarinin

PTSH = 1H-mercaptophenyltetrazole; Mo(VI) = (NH4)6Mo7O24·4H2O.

In conclusion, we have resolved the stereochemical ambiguity surrounding the structure of chamuvarinin by completing the first total synthesis (20 longest linear steps, 1.4% overall yield) and enabling further biological studies. The present work demonstrates the versatility of our modular alkyne coupling/cyclization strategy to construct adjacently linked acetogenin frameworks in an efficient manner to provide synthetic access to these rare bioactive metabolites and for establishing their full stereochemistry.