Synthesis and Structure Verification of the Vaccine Adjuvant QS-7-Api. Synthetic Access to Homogeneous <em>Quillaja saponaria</em> Immunostimulants

The clinical success of conjugate anticancer and antiviral vaccines critically depends on the identification of, and access to, novel potent adjuvants with attenuated toxicity. In this context, specific fractions from extracts of the bark of Quillaja saponaria (QS) have proven to be exceedingly powerful adjuvants in immunotherapy. The QS-21 fraction,¹3 comprising isomeric forms of a complex triterpene saponin, is currently the most promising immuno-potentiator⁴ in several antitumor (melanoma, breast, SCLC, prostate)⁵ and infectious-disease (HIV, malaria)⁶9 vaccine therapies. However, the tolerated dose of QS-21 in patients typically does not exceed 100 μg, above which significant local erythema and systemic flu-like symptoms arise. On the other hand, QS-7, an additional QS extract fraction, was found not only to possess significant stand-alone adjuvant activity,¹10 but also to induce remarkable synergistic immune response augmentation¹¹ when coadministered with QS-21. Importantly, QS-7, unlike QS-21, exhibited negligible toxicity in mice.¹1011 That QS-7 has not been advanced to the clinic likely stems from (1) a more difficult purification protocol compared to that of QS-21 and (2) uncertainty in its structural constitution, postulated to be QS-7-Api (1, Figure 1).¹⁰11 We now report the synthesis and structural verification of QS-7-Api (1).¹² Furthermore, a novel semisynthetic sequence has been developed, enabling procurement of homogeneous QS-7-Api (1) for imminent preclinical evaluation.

Figure 1

The synthesis of the hexasaccharide fragment within QS-7-Api (“R”, Figure 1) required initial preparation of the selectively protected monosaccharides 2–4, 6, and 8 (Scheme 1). While the xylo-, gluco-, and apio-derived monosaccharides 2–4 (Scheme 1A) were obtained in multistep sequences by previously reported procedures and modifications thereof,¹³14 the novel sugars 6 and 8 were prepared from rhamnopyranose 5 and fucopyranoside 7, respectively. Silylation of the selectively protected rhamnopyranose 5 (Scheme 1B) with TIPSOTf provided the α-TIPS glycoside (96%), which subsequently underwent C4-O-debenzylation to furnish the rhamnopyranoside 6 (98%). Synthesis of the fucosyl residue within QS-7 commenced with selective C3-O-alkylation of the allyl fucopyranoside 7 (Scheme 1C) with PMBCl (56%) via its transient stannylene acetal. This allowed for sequential selective silylation of the equatorial C2-OH (97%) and acetylation of the axial C4-OH (>99%). Finally, oxidative removal of the PMB ether with DDQ provided the selectively protected fucopyranoside 8 (86%).

Scheme 1^a

^a Reagents and conditions: (a) TIPSOTf, 2,6-lutidine, CH₂Cl₂, 0→23 °C, 96%; (b) H₂, Pd–C, MeOH, 23 °C, 98%; (c) n-Bu₂SnO, PhMe, reflux; CsF; PMBCl, DMF, 23 °C, 56%; (d) TBSCl, imidazole, DMAP, CH₂Cl₂, 23 °C, 97%; (e) Ac₂O, Et₃N, DMAP, CH₂Cl₂, 23 °C, >99%; (f) DDQ, MeOH, H₂O, 0→23 °C, 86%.

Convergent assembly of the branched hexasaccharide (Scheme 2) involved dehydrative glycosylation (Ph₂SO·Tf₂O)¹⁵ of fucopyranoside 8 with rhamnopyranose 5 (84%). The resulting α-disaccharide 9 then underwent a series of protective group exchanges, including TBS removal (95%), a novel Et₂Zn/Pd(PPh₃)₄-mediated anomeric deallylation (68%),¹⁶ and selective anomeric silylation (75%) to afford the disaccharide 10 as a suitable glycosyl acceptor. Its glycosyl donor coupling partner was prepared by chemo- and stereoselective dehydrative glycosylation of rhamnopyranoside 6 with xylopyranose 2 to afford the β-disaccharide 11 (75%), which directly underwent glycosylation¹⁷ with the apiose-derived donor 4 to afford trisaccharide 12 (86%). The acetate esters in 12 were then exchanged for a benzylidene acetal protective group (94%), followed by selective acid hydrolysis of the rhamno-derived isopropylidene ketal to afford the corresponding vicinal diol (71%). Selective alkylation of the resulting axial rhamno-C2-OH with BnBr could then be accomplished (84%), allowing for Schmidt glycosylation¹⁸ of the C3-OH with the glucosyl imidate 3 to afford the tetrasaccharide 13 (86%). Exchange of the benzoate ester for a TES ether (91%, two steps) and conversion of the anomeric TIPS group to its α-trichloroacetimidate counterpart 14 (92%, two steps) secured a suitable donor for glycosylation of disaccharide 10. This was accomplished by treating the two components with TMSOTf to afford hexasaccharide 15 (62%), whose fucosyl-TIPS-acetal was then transformed to the α-trichloroacetimidate 16 (84%, two steps).

Scheme 2^a

^a Reagents and conditions: (a) Ph₂SO, Tf₂O, TBP, CH₂Cl₂, −78→23 °C, 84%; (b) TBAF, THF, 0→23 °C, 95%; (c) Et₂Zn, Pd(PPh₃)₄, Et₂O, 23 °C; 68%; (d) TIPSCl, imidazole, DMAP, DMF, 23 °C, 75%; (e) Ph₂SO, Tf₂O, TBP, CH₂Cl₂, −78→23 °C, 75%; (f) 4, TBSOTf, CH₂Cl₂, 0 °C, 86%; (g) K₂CO₃, H₂O, MeOH, 23 °C; (h) PhCH(OMe)₂, p-TsOH, 23 °C, 94% (two steps); (i) p-TsOH, H₂O, MeOH, 23 °C, 71%; (j) BnBr, Bu₄NBr, NaOH, H₂O, CH₂Cl₂, 23 °C, 84%; (k) 3, TMSOTf, Et₂O, −45 °C, 86%; (l) DIBAL-H, CH₂Cl₂, −78 °C, 92%; (m) TESOTf, 2,6-lutidine, CH₂Cl₂, 0→23 °C, 99%; (n) TBAF, THF, 0 °C, >99%; (o) CCl₃CN, DBU, CH₂Cl₂, 0 °C, 92%; (p) TMSOTf, 4Å ms, CH₂Cl₂, −15 °C, 62%; (q) TBAF, THF, 0 °C; (r) CCl₃CN, DBU, CH₂Cl₂, 0→23 °C, 84% (two steps).

Late stage construction of the full QS-7-Api skeleton involved the elaborately protected triterpene–trisaccharide conjugate 17 (Scheme 3A), previously prepared in 18 steps from glucuronolactone during the course of the synthesis of QS-21.¹³ This C28-carboxylic acid glycosyl acceptor 17 responded well to glycosylation with trichloroacetimidate glycosyl donor 16 (BF₃·OEt₂) to afford fully protected QS-7-Api (71%), which underwent global deprotection under carefully managed conditions (TFA; H₂, Pd–C). The resulting product (71%) was found to be identical to naturally derived QS-7-Api (1).¹⁹

Scheme 3^a

^a Reagents and conditions: (a) 16, BF₃·OEt₂, 4Å ms, CH₂Cl₂, −78→23 °C, 71%; (b) TFA, H₂O, CH₂Cl₂, 0 °C; H₂, Pd–C, EtOH, THF, 23 °C, 71%; (c) KOH, EtOH, H₂O, 80 °C; (d) TESOTf, Py, 40 °C; (e) CbzCl, Py, TBP, CH₂Cl₂, 23 °C, 68%; (f) 16, BF₃·OEt₂, 4Å ms, CH₂Cl₂, −78→23 °C, 80%; (g) H₂, Pd–C, EtOH, THF, 23 °C; TFA, H₂O, 0 °C, 77%.

This synthesis of 1 (Scheme 3A) from de novo construction of all oligosaccharide fragments confirms the structure of QS-7-Api and provides significantly more dependable access to homogeneous samples of 1 than isolation from natural sources. This notwithstanding, the synthesis of 1 can be further augmented. Quil-A (18, Scheme 3B) is a commercially available semipurified extract from Quillaja saponaria and contains variable quantities of >50 distinct saponins,²⁰ many of which incorporate the triterpene–trisaccharide substructure within QS-7 (and QS-21). This monodesmoside saponin 19 (Scheme 3B) can be isolated in semipure form via direct base hydrolysis of the Quil-A mixture.²¹ Subsequent poly(silylation) of 19 with excess TESOTf afforded the corresponding nonakis(triethylsilyl ether) (257 mg from 1.15 g of 18), whose glucuronic acid functionality could be selectively derivatized to the benzyl ester 20 (CbzCl, 68%). This triterpene–trisaccharide conjugate, obtained in only a three-step protocol from Quil-A (18), was an effective acceptor in a C28-carboxylate glycosylation (80%) with hexasaccharide 16 to provide, after global deprotection, QS-7-Api (1) (77%). The evolution of the first synthesis of 1 to this semisynthetic variant furnishes complex QS-saponin adjuvants (and likely non-natural analogues) with markedly enhanced facility, enabling heretofore untapped opportunities for novel adjuvant discovery in antitumor and antiviral vaccine development.