Transannular Diels-Alder/1,3-Dipolar Cycloaddition Cascade of 1,3,4-Oxadiazoles: Total Synthesis of a Unique Set of Vinblastine Analogues

Abstract

A powerful tandem [4+2]/[3+2] cycloaddition cascade of 1,3,4-oxadiazoles initiated by a transannular [4+2] cycloaddition is detailed. An impressive four rings, four carbon-carbon bonds, and six stereocenters are set on each site of the newly formed central six-membered ring in a cascade thermal reaction that proceeds at temperatures as low as 80 °C. The resulting cycloadducts provide the basis for the synthesis of unique analogues of vinblastine containing metabolically benign deep-seated cyclic modifications at the C3/C4 centers of the vindoline-derived subunit of the natural product.

Vinblastine (1) and vincristine (2) are the most widely recognized members of the Vinca alkaloid family and have been extensively used as antitumor drugs (Figure 1).¹ Originally isolated in trace quantities from the leaves of Catharanthus roseus (L.) G. Don.,²1 and 2 were among the first chemotherapeutic agents shown to block mitosis through inhibition of microtubule formation that is still regarded as an especially attractive oncology drug target.^1,3 They share an identical upper velbanamine subunit and contain nearly identical vindoline-derived lower subunits that differ only in the nature of the indoline N-substituent. Despite this minor structural difference, vinblastine and vincristine display distinct clinical and toxicological profiles.^1,3 Both compounds have enjoyed extensive use and success as antitumor drugs, however the major limitation to their continued clinical use is the emergence of resistance mediated primarily by overexpression of the drug efflux pump phosphoglycoprotein (Pgp).⁴ The identification of structural analogues that might address such resistance would constitute a major advance, but has largely remained an elusive goal. The exception to this generalization is the recent disclosure of a series of key C20’ urea derivatives that simultaneously improve potency (ca. 30-fold) and reduce the difference in activity against a vinblastine sensitive versus resistant cell line from 100-fold to 10-fold,⁵ indicating that such key modifications to vinblastine may be possible.

Figure 1.

Vinca alkaloids and key natural products.

Recently, we reported concise total syntheses of vindoline⁶ enlisting two variations on a tandem intramolecular [4+2]/[3+2] cycloaddition cascade of 1,3,4-oxadiazoles⁷ that form the fused pentacyclic core of the natural product in one step and as a single diastereomer. The development of a single flask Fe(III)-promoted coupling with catharanthine⁸ and subsequent in situ Fe(III)/NaBH₄ air oxidation⁹ provided the basis for a 12-step total synthesis of vinblastine and related natural products.^10,11 Significantly, this approach has proved sufficiently concise and general as to allow the preparation of numerous analogues¹² that systematically explore many of the key structural features of vinblastine (substituents and core redesign) that are otherwise inaccessible.³ Moreover, it assures that efficacious analogues can be accessed as needed for preclinical or clinical investigation.

In our efforts to examine such key analogues of vinblastine and in extensions of such cycloaddition methodology¹³ to additional natural products,¹⁴ we have conducted studies directed at expanding the substrate scope of the key [4+2]/[3+2] cycloaddition cascade leading to vindoline and related structures. Herein, we report the examination of cycloaddition substrates in which the initiating dienophile is constrained within a macrocycle, rendering the initiating [4+2] reaction of the cycloaddition cascade transannular in nature (Figure 2).¹⁵ The expectation was that this would not only provide a cycloaddition reaction that proceeds at relatively low reaction temperatures, but also that the cycloadducts would provide the basis for unique vinblastine analogues that incorporate an additional fused ring at the C3/C4 positions.

Figure 2.

Transannular [4+2]/[3+2] cycloaddition cascade.

In these initial efforts, cycloaddition cascade precursors exemplified by 6 were examined (Scheme 1), in which the initiating dienophile is linked to the 1,3,4-oxadiazole by two amide groups (X = NMe). It was anticipated that the requisite macrocycles could be accessed by ring closing metathesis (RCM) of diene 7, although several alternative approaches can be envisioned, and 7 would be derived from the oxadiazole 8 available from our prior studies.⁶

Scheme 1.

Addition of 4-amino-1-butene (9) to 8 provided amide 10 (Scheme 1). Acylation of 10 with 4-pentenoic acid (11) followed by N-methylation of the secondary amide 12 provided the RCM precursor 13 in excellent yield. Treating compound 13 with Grubbs second generation catalyst¹⁶ in refluxing CH₂Cl₂ (0.002 M) provided the desired macrocyclic product 14 as a mixture of isomers favoring Z-14 by a factor of ca. 3:1. The modest yield of 14 results from competitive formation of dimeric cyclic olefin metathesis products. Interestingly, alternative attractive RCM substrates in which the linking N-methyl amide was replaced by a secondary amide (X = NH), ester (X = O) or ketone (X = CH₂) were found to undergo preferential dimerization nearly exclusively under these conditions.

Using a defined reaction time (48 h), compound 14 was found to undergo the key cascade cycloaddition reaction at temperatures as low as 78 °C (refluxing benzene, 44%, 48 h) to provide 15, whose structure and relative stereochemistry were confirmed by X-ray^17a (Scheme 2). The rates of reaction were faster and the yields of product were higher when the 48 h reaction was conducted in refluxing toluene (110 °C, 73%; 73-91%) or xylenes (138 °C, 85%; 75-87%), incrementally improving with the higher reaction temperatures. Even at the higher reaction temperatures, the conditions are milder than those required for the initiating intramolecular [4+2] cycloaddition reactions. Compound 16, where the upper amide carbonyl is moved to the dipolarophile tether (Supporting Information), participated in the cascade cycloaddition reaction with near equal facility. Remarkably, 18, which was prepared by a route analogous to 14 (Supporting Information) and features a trisubstituted alkene dienophile expected to participate more slowly in the initiating [4+2] reaction, undergoes the cycloaddition cascade to provide 19 (77%) with similar ease. Impressively, 20 provided the cascade cycloadduct 21 in 75% yield in refluxing benzene (78 °C, 48 h).⁷

Scheme 2.

In each instance, a single diastereomer of the product is generated in which the C8/C9 relative stereochemistry is inherent in the dienophile alkene cis-geometry with the remaining four stereocenters arising from a subsequent indole endo [3+2] cycloaddition directed to the face of the intermediate 1,3-dipole opposite the newly formed six-membered lactams. An impressive four rings, four carbon-carbon bonds, and six stereocenters, of which three or four are quaternary, are set in a single reaction that proceeds at temperatures as low as 80 °C.

With the cycloaddition products 15 and 19 in hand, their incorporation into novel vinblastine analogues was examined. An X-ray crystal structure of vinblastine bound to tubulin revealed that this region of the natural product extends into and interfaces largely with solvent, indicating that alterations made to the functionality at C3/C4 may be well tolerated (Figure S1, Supporting Information).¹⁸

The enantiomers of 15 were separated by chiral phase HPLC (Daicel ChiralCel OD column, 2 × 25 cm, 40% EtOH/hexanes, 7 mL/min, α = 1.20) (Scheme 3). Their absolute configuration was unambiguously assigned based on a single crystal X-ray structure determination of the unnatural enantiomer of the thioamide derivative 22,^17b accessed by treating 15 with Lawesson's reagent (0.6 equiv, toluene, 75 °C). The thionation was completely selective for the C12 amide carbonyl, however a small amount of the side product 23^17c was isolated in which sulfur replaces oxygen as the bridging atom. S-Methylation of 22 with Me₃OBF₄ followed by reduction of the methylthioiminium ion with NaBH₄ and subsequent in situ oxido bridge cleavage and hydride reduction of the intermediate iminium ion from the convex face provided the vindoline analogue 24 as a single diastereomer in an unoptimized 66% yield. The same reaction sequence was applied to cycloadduct 19, which incorporates the C5 ethyl substituent. Resolution of the enantiomers of 19 was achieved using conditions identical to those described for 15 (α = 1.23). Thionation of the natural enantiomer of 19 (1 equiv Lawesson's reagent, toluene, 110 °C) provided 25 as the sole product in superb yield. A single crystal X-ray crystal structure determination of 25 conducted on the unnatural enantiomer allowed the unambiguous assignment of the absolute configuration.^17d Desulfurization and oxido bridge opening furnished the vindoline analogue 26.

Scheme 3.

Analogues 24 and 26 were coupled with catharanthine and subjected to a subsequent in situ oxidation in a one-pot reaction mediated by Fe(III) to provide the vinblastine analogues 27 and 28 in a single step (Scheme 4), completing their preparation in an 8-step total syntheses from 8 (available in 2-steps from 6-methoxytryptamine).^6,7 The analogues were examined for cell growth inhibitory activity against L1210 (mouse leukemia), HCT116 (human colon cancer), and HCT116/VM46 (resistant human colon cancer) tumor cell lines, the latter of which exhibits resistance (100-fold) to vinblastine through overexpression of Pgp.¹⁹ The compound most comparable to 27 and 28 is 6,7-dihydrovinblastine. Prior studies have shown that reduction of the vinblastine C6-C7 double bond produces a 100-fold loss in activity.^10b Compound 27, which lacks the C5 ethyl substituent found in vinblastine, proved to be 10-fold more potent than 28. Importantly, compound 27 was found to be equipotent with 6,7-dihydrovinblastine, indicating that vinblastine analogues incorporating the core structure of the vindoline analogue 24 produce analogues of significant interest. Notably, the vinblastine C3 and C4 substituents (CO₂Me and OAc) are metabolically labile and undergo in vivo hydrolysis that results in significant reductions in activity,^10b whereas their replacement with a benign fused N-methyl lactam may provide equally effective, but metabolically stable analogues.

Scheme 4.

The macrocyclic oxadiazoles 14, 16, 18 and 20 undergo a tandem [4+2]/[3+2] cycloaddition cascade initiated by a transannular [4+2] cycloaddition at temperatures substantially lower than those required for related intramolecular reactions. An impressive four rings, four carbon-carbon bonds, and six stereocenters are set on each site of the newly formed central six-membered ring in a single thermal reaction that proceeds at temperatures as low as 80 °C. The resulting cycloadducts provide the basis for the synthesis of unique analogues of vinblastine containing metabolically benign deep-seated modifications at the C3/C4 centers of the natural product. Such studies are in progress and will be reported in due course.