Chemistry
Central to our strategy of examining ketone-based thiophiles was the preparation of an array of intermediates containing diverse methylketone moieties and protecting groups. The preparation of halomethylketone intermediates was accomplished in a two-step procedure by the generation of a diazoketone intermediate31 or directly by a modified Kowalski–Haque reaction,32 as depicted in Scheme 1.
Scheme 1  Preparation of P1 Synthetic Intermediates
(i) 3 M NaOH, MeOH (95%), (ii) isobutylchloroformate (IBCF), triethylamine (TEA), tetrahydrofuran (THF), (iii) CH2N2/ether (92%), (iv) 4 M HCl/dioxane (83%), (v) 1.0 equiv 48% HBr (83%), (vi) HNCH3(OCH3), EDC, HOBt, NMM, dichloromethane (DCM) (79%), (vii) Mg, HgCl2, BOM-Cl, THF, −78°C (50%), and (viii) lithium diisopropylamide (LDA), ClCH2I, THF (54%). Saponification of amino ester 5(33) afforded the corresponding amino acid 6 in nearly quantitative yield. Acid 6 was treated with isobutylchloroformate and triethylamine, and the resulting mixed anhydride was reacted with diazomethane34 to provide high isolated yields of 7. Diazoketone 7 was treated with hydrochloric acid for simultaneous nitrogen deprotection and conversion to the chloromethylketone (CMK) intermediate 10. Alternatively, the N-Boc-protected bromomethylketone was prepared by treatment of 7 with stoichiometric quantities of 48% HBr in dichloromethane to provide 11. Another preparation of CMK intermediates that avoids the use of diazomethane was accomplished by the reaction of 5 with excess LDA and chloroiodomethane to afford 8 in moderate yields. A direct approach to oxymethylketone intermediate 9 was achieved by conversion of acid 6 to the corresponding Weinreb amide followed by treatment with the Grignard generated from benzyl chloromethyl ether.35
A key element for rapid access to substituted methylketone derivatives was the generation of the fully elaborated CMK 12 (Scheme 2). The utility of the CMK moiety to prepare acyloxy- and hydroxy-substituted methylketones is well established. Elaboration of 10 in a sequence of deprotection/1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU)-mediated peptide couplings provided CMK derivative 12 in moderate yield with no epimerization observed. Reaction of 12 with various carboxylic acids in the presence of cesium fluoride at 60 °C provided acyloxymethylketones 13–28. Using this synthetic approach with substitution of P2 and amine capping structural elements provided inhibitors 3 and 29–31.
Scheme 2  Preparation of Substituted Acyloxymethylketones and Aminomethylketones
(i) Boc-Leu-OH, HATU, NMM, N,N-dimethylformamide (DMF), 0 °C (64%), (ii) 4 M HCl/dioxane, (iii) HATU, 4-MeO-2-indolecarboxylic acid, NMM, DMF, 0 °C (61%), and (iv) CsF, DMF, carboxylic acid, 60 °C (43–91%). Hydroxymethylketone (HMK) derivatives were accessed by two complimentary approaches illustrated in Scheme 3. Elaboration of the P1′-protected oxygen intermediate 9 by sequential amide bond coupling reactions provided benzylether derivatives such as 34 in moderate yields with a low propensity for epimerization at the P1 center. Hydrogenolytic debenzylation readily afforded the final HMK inhibitors exemplified by 36. Alternatively, fully elaborated CMK derivatives such as 35 were reacted with benzoylformic acid followed by methanolysis to generate HMK final products such as 36.36 Alkylation of the terminal hydroxyl was achieved under microwave-assisted conditions in the presence of silver (I) oxide to afford alkoxymethylketone inhibitors, as illustrated by 37. Using these two synthetic approaches with substitution of P2 and amine capping structural elements provided inhibitors 4 and 38–50.
Scheme 3  Preparation of Hydroxymethylketone Derivatives
(i) 4 M HCl/dioxane, (ii) Boc-Leu-OH, HATU, NMM, DMF, 0 °C, (iii) HATU, 4-MeO-2-indolecarboxylic acid, NMM, 0 °C, (iv) Pd/C, H2, EtOH (77%), (v) CsF, DMF, benzoylformic acid, 60 °C, and then MeOH, cat. K2CO3 (53%), and (vi) CH3I, Ag2O, DCE (9%).