Diketomorpholines: Synthetic Accessibility and Utilization.

Diketomorpholines (DKMs; morpholine-2,5-diones) possess a six-membered ring with a lactone and lactam moiety and belong to the family of cyclodepsipeptides. In this review, the synthetic accessibility of DKMs is summarized and their utilization, in particular, for ring-opening polymerization reactions, is highlighted. The occurrence of the DKM scaffold in natural products encompasses small monocyclic compounds but also complex, polycyclic representatives with a fused DKM ring.

Introduction

Diketomorpholines (DKMs; morpholine-2,5-diones) constitute derivatives of the heterocyclic structure 1 (Figure ). DKMs bear one lactam and one lactone moiety, which occur each two-fold in highly abundant diketopiperazines 2 and glycolides 3, respectively (Figure ). With the lactam and lactone group in the same six-membered ring, DKMs can be regarded as the simplest members of the large family of cyclodepsipeptides and are also referred to as cyclodepsidipeptides.[1] In this review, a comprehensive overview on the synthetic access to DKMs is given, and some remarkable applications and the occurrence of the DKM scaffold in natural products are described.

Figure 1

General structures of diketomorpholines (1), diketopiperazines (2), and substituted glycolides (3).

Solution-Phase Syntheses

In the following, recent and representative examples showing the synthetic access to DKMs will be summarized (Schemes and 2). In both schemes, only one possible configuration for the two chiral carbons is shown. However, not only the depicted (3S,6S)-configured products have been prepared by the different outlined methods, but also diastereomers with other defined configurations, racemic products, as well as achiral compounds 1 and 10, in which R1 and R2 are hydrogens. Further protocols employed for DKM synthesis are summarized elsewhere.[1]

Scheme 1

Synthetic Entries to DKMs from Hydroxyacyl or Haloacyl Derivatives

Reagents and conditions: (a) DMAP, BOP reagent, CH2Cl2,[2] or MeSO3H, CHCl3, Δ;[3] (b) DEAD, THF;[2] (c) Amberlyst 15, toluene, Δ,[2] or p-TsOH, toluene, Δ;[4,5] (d) p-TsOH, toluene, Δ,[4] or DBU, MS 4 Å, toluene;[6] (e) NaHCO3, DMF, Δ,[7,8] or TEA, DMF, Δ,[2,9] or DIPEA, CHCl3, Δ,[10] or (1) NaOH, EtOH, H2O, (2) H2SO4;[2] (f) DIPEA, DMSO,[11] or (1) K2CO3, H2O, (2) HCl,[12] or (1) NaOH, EtOH, H2O, (2) HCl.[2]

Scheme 2

Synthetic Entries to DKMs from Aminoacyl Derivatives

Reagents and conditions: (a) 2-chloro-1-methylpyridinium iodide, DIPEA, CH2Cl2,[13] or 2-chloro-1-methylpyridinium iodide, TEA, CH2Cl2;[13] (b) piperidine, DMF,[13] DMAP, pyridine, Δ.[13]

DKMs are available through cyclative lactonizations (Scheme ). For the ring-closure reactions, precursors 4–9 have been utilized, all of which already contain a central amide bond. In the resulting product structures 1 and 10, it appears as an unsubstituted or substituted lactam moiety, respectively. The formation of the lactone bond from free acids 4 or 5 was accomplished either by proton-catalyzed condensations or in the presence of coupling reagents or under Mitsunobu conditions. Acid- or base-promoted conversions of the esters 6 or 7 produced products 1 and 10 in the course of interesterifications. Furthermore, halogen-substituted educts 8 or 9 were employed for lactone generation under basic conditions. In some cases, ester progenitor compounds were saponified to the corresponding acids 8 or 9 prior to the cyclocondensation.

As a second opportunity, educts 11 and 12 with a preformed central ester group have been subjected to lactamization, leading to cyclized products 1 and 10 (Scheme ). The conversions include the successful application of the Mukaiyama reagent. Typically, the terminal amino group was deprotected before the cyclization occurred.

Macrocyclic analogues of DKMs have been prepared from lactams bearing an ε-hydroxyacyl residue at the cyclic nitrogen atom. Attack of the terminal oxygen at the ring carbonyl led to side-chain insertion to bicyclic cyclols, and the subsequent ring expansion gave monocyclic products which represent macrocyclic counterparts of DKMs 1.[14]

Recently, in a study on macrocyclo-oligomerizations, it was observed that the tetradepsipeptide 13 underwent an intramolecular attack of the carboxylate at the central ester, leading to an anhydride isomer 14, followed by fragmentation due to the nucleophilic attack of the alcoholic group and generation of the protected compound 15 and DKM 16 (Scheme ).[15]

Scheme 3

Proposed Mechanism for the Self-Cleavage of a Tetradepsipeptide

Scheme shows ester-based prodrugs (17) of glucagon-like peptide 1 (GLP) in its biologically active form GLP(7–36) and fused to peptide CEX, a nine-amino-acid C-terminal extension.[16] The N-terminal phenylalanine was replaced with phenyllactic acid, and the prodrugs dissociate under physiological conditions through formation of DKMs 1 and liberate the active peptide 18.

Scheme 4

Hydroxy-Mediated Ester Cleavage of GLP Prodrugs (pH 7.4, 37 °C)

Solid-Phase Syntheses

DKMs are accessible through various polymer-supported methods.[17] Cyclization of resin-bound bromides 19 (Scheme ) was induced by treatment with TFA, initially leading to cleavage from the Wang resin, followed by ring closure to DKMs (R1 = H or alkyl, R2 = alkyl).[18] A resin which consisted of polyethylene glycol attached to cross-linked polystyrene through an ether linkage was employed for DKM synthesis.[18] Here, the resin-bound structure 20 (R1 = alkyl, R2 = alkyl) was assembled by Ugi reaction, and the NR3 portion of the products corresponded to the structure of amino acid amides with R3 = CH(alkyl)CONH(cyclohexyl).

Scheme 5

General Entries to DKMs via Solid-Phase Synthesis

Reagents and conditions: (a) TFA (Wang resin);[18] (b) TEA, CH2Cl2 (TentaGel resin).[18] Racemic mixtures were applied,[18] and only single (S,S)-configured stereoisomers are depicted here.

DKM precursors were coupled to the polymer matrix via a photolabile 5-bromo-7-nitroindoline moiety (21), and alanine, leucine, and phenylalanine-based DKMs 22 were produced in the course of an intramolecular photoinduced cyclorelease (Scheme ).[19]

Scheme 6

Photoinduced Cleavage of DKMs from the Resin

The resin-bound structure 23 was produced by loading tert-butyl serine, deprotection, alkylation with bromoketones, and acylation (Scheme ).[11] TFA-mediated liberation from the solid support triggered a cyclization to the 3,4-dihydro-2H-1,4-oxazine scaffold, followed by based-catalyzed lactonization to the fused second ring. At prolonged reaction time, eliminative cleavage occurred and monocyclic DKMs 25 were formed, for example, from 24. An analogous protocol could be used for the solid-phase-supported generation of tricyclic DKMs 29 (Scheme ).[20]

Scheme 7

Formation of Bi- and Monocylic DKMs

Scheme 8

Formation of Tricylic DKMs

Reagents and conditions: (a) halocarboxylic acid, DIC, CH2Cl2; (b) TFA, CH2Cl2; (c) DIPEA, DMSO.

An N-terminal degradation of peptoid oligomers through sequential cleavage of N-substituted glycine units was accomplished on a solid phase.[21] The protocol relied on the treatment of resin-bound bromoacetylated peptoids 31 with silver perchlorate, leading to an intramolecular lactonization, releasing the terminal residue as part of an N-substituted DKM 34, and resulting in the truncated peptoid 33 (Scheme ).

Scheme 9

Iterative Peptoid Sequencing through DKM Liberation

Reagents and conditions: (a) BrCH2CO2H, DIC, DMF; (b) AgClO4, THF, (c) H2O.

Chemical Reactivity and Utilization in Ring-Opening Polymerizations

The chemical reactivity of DKMs has mainly been explored to perform ring-opening polymerization reactions (see below). In a model transformation, N-substituted, racemic DKMs 35 were treated with ethanolic ammonia to undergo ring opening to diamides 36 (Scheme ). The second step leading to intermediates 37 occurred slower and involved the intramolecular participation of the primary carboxamide moiety. The imides 37 were susceptible to subsequent conversions with nucleophiles.[22] The regioselective course of the ring opening of 35 reflected the expected higher electrophilic reactivity of the lactone group compared to that of the lactam group in DKMs. The cleavage of a defined DKM with a functionalized benzylamine was utilized for the preparation of a thrombin inhibitor.[22]

Scheme 10

Ammonolysis of DKMs to Diamides and Acyl Transfer to Imides

Reagents and conditions: (a) 5 M NH3/EtOH, room temperature.

Overall, the synthetic potential of DKMs to generate defined low-molecular weight compounds with tailored properties has not yet been fully exploited. In contrast, DKMs have been frequently utilized as monomers for polymerization reactions.

Polydepsipeptides are alternating copolymers of an α-amino and an α-hydroxy acid. They are valued for their nontoxic properties and their degradability and are suitable for numerous applications, such as tissue engineering and drug delivery. Compared to polypeptides, polydepsipeptides do not necessarily require enzymes for their degradation because of the hydrolytic susceptibility of the ester groups. The presence of the carboxamide portions in polydepsipeptides allows for strong intramolecular hydrogen bond interactions, in contrast to polyesters. These hydrogen bonds influence their mechanical and thermal properties, which can be fine-tuned by variations of the amino acid moieties. In particular, telechelic oligodepsipeptides, capable of entering into further polymerization or other reactions through their reactive terminal groups, serve as valuable building blocks for biomedical applications.[3,23]

Polydepsipeptides (38) are generally accessibly by employing DKMs (1) in ring-opening polymerization reactions (Scheme ). Several attempts have been made to control copolymer compositions, molecular weights, crystallinity, and degradability by using different polymerization conditions.[10,23]

Scheme 11

Ring-Opening Polymerization to Produce Polydepsipeptides from DKMs

Reagents and conditions: (a) Sn(Oct)2, CHCl3,[10] or BnOH, TBD or DBU, THF or CHCl3.[7,8]

As a catalyst, stannous octoate (tin(II) 2-ethylhexanoate, Sn(Oct)2) has frequently been used. Following the “coordination–insertion” mechanism, a tin alkoxide is formed from Sn(Oct)2 and a hydroxyl group of the initiator molecule, the carbonyl oxygen coordinates the metal center, followed by the nucleophilic attack of the alkoxide ligand and subsequent lactone bond cleavage, thus generating an analogous, active species.[23] 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD) was a particularly active catalyst for the generation of polymers 38 (R1 = H, R2 = H or alkyl) from corresponding DKMs 1 using benzyl alcohol as an initiator.[7] Several bases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), were investigated in polymerizations of methionine-derived DKMs 1 (R1 = H or alkyl, R2 = CH2CH2SCH3).[8] The resulting poly(ester amides) 38 were applicable for postpolymerization modifications via “methionine click” chemistry.[8,24]

DKMs could be copolymerized with substituted glycolides (3, Figure ) or lactones such as ε-caprolactone to achieve copolymers with tailored properties.[10,23] A detailed description has been provided elsewhere.[23] Instead of Sn(Oct)2 as the catalyst and ethylene glycol as the initiator,[23] a single Sn(IV) organotin compound could perform both tasks. 2,2-Dibutyl-1,3,2-dioxastannolane, prepared from dibutyltin oxide and ethylene glycol, was employed in the ring-opening polymerization to prepare 39 (Scheme ).[3] The softness of the metal catalyst influenced the outcome of the polymerization, and Fe(OAc)2 performed best among non-Sn(Oct)2 catalysts in ring-opening polymerizations with different DKMs and 1,8-octanediol as an initiator.[9] Block copolymers 41 were synthesized by means of ring-opening polymerization with 3-methylmorpholine-2,5-dione (40) and an amino-terminated polyethylene glycol (PEG) as an initiator.[23]

Scheme 12

Generation of Block Polymers by Ring-Opening Polymerization Reactions of DKMs

Reagents and conditions: (a) Sn(Oct)2, ethylene glycol, 140 °C;[23] or (1) dibutyltin oxide, ethylene glycol, (2) bulk, 140 °C;[3] (b) Jeffamines ED-600, ED-900, or ED-2001, THF, 140 °C.[23]

DKMs are expected to be involved in the chemical evolution from amino acids to peptides. Model prebiotic molecules were generated from monomers by wet–dry cycling experiments and DKMs such as 61 identified in the obtained mixtures, in addition to depsipeptides, polyesters, peptides, and yet unreacted hydroxy and amino acids (Scheme ).[25]

Scheme 13

DKM 61 as an Intermediate in a Proposed Pathway to Trimeric Prebiotic Compounds

Natural Products

Monocyclic DKMs have been reported as natural products. Examples are shown in Figure . Enniatins are mixtures of cyclic hexadepsipeptides found in Fusarium fungi. Enniatin B forms an 18-membered ring composed of each three alternating N-methyl-(S)-valine and (R)-2-hydroxy-3-methylbutanoic acid building blocks. The DKM congeners 42–44, supposed side products of the nonribosomal enniatin B biosynthesis, were isolated from Fusarium sporotrichioides (Figure ).[12] DKM 42, which contained the two enniatin B building blocks, was present in prevailing amounts in the broth and mycelium of Fusarium sporotrichioides, whereas the production of 43 and 44 was presumably due to low substrate specificity of the enniatin synthetase for (S)-amino acids.[12] Compound 42 exhibited inhibitory properties against xanthine oxidase and anti-inflammatory activity in human peripheral blood mononuclear cells.[26] Aliphatic DKM derivatives such as 42 and 43 have been evaluated with respect to their antimicrobial, antioxidant, immunomodulatory, and antiproliferative activities.[1,26]

Figure 2

Monocyclic DKMs from natural sources.

Bassiatin (45) was isolated from the cultured broth of the entomopathogenic sac fungus Beauveria bassiana, in addition to depsipeptides of a higher oligomerization state such as the cyclic hexadepsipeptide beauvericin.[27] Bassiatin possessed insecticidal activities against Bemisia tabaci, a whitefly and important agricultural pest, in contact and feeding assays. Bassiatin inhibited the ADP-induced platelet aggregation.[27] A diastereomer (46) from the sac fungus Isaria japonica induced apoptotic cell death in human leukemia cells in the micromolar range.[27] DKM 47 was identified as a constituent of the traditional Chinese medicine Bombyx batryticatus, the dried dead larva of silkworms after infection by Beauveria bassiana.[27] DKM 48 was isolated from the sea hare Bursatella leachii.[27] Other DKMs bearing N-benzyl substituents have been synthesized and investigated as inhibitors of glucosidases, as summarized elsewhere.[1]

The greater propensity of proline and N-methyl-substituted amino acids to adopt a cis conformation and the consequent preferred DKM cyclization might account for the more frequent appearance of bicyclic DKMs such as 61 (Scheme )[25] and N-methyl-substituted DKMs such as 42–48 (Figure ).

DKMs are substructures of naturally occurring polycyclic indole alkaloids. Respective natural products mainly contain a diketopiperazine ring fused to the terminal five-membered ring of the pyrrolidinoindoline system.[5] However, there are also natural products in which a DKM unit replaced the diketopiperazine core. Such tetracyclic compounds (49–58) are depicted in Figure . Mollenines A (49) and B (50) were isolated from the sclerotioid ascostromata of Eupenicillium molle.[28] The total synthesis of 49 was realized either by the preparation of a prenylated pyrrolidinoindoline ester from tryptophan and the final connection with leucic acid[6] or by a one-pot reaction of the DKM composed of tryptophan and leucic acid with a vinyl cyclopropane reagent.[28] Mollenine A possessed cytotoxic and antibacterial activity against Bacillus subtilis.[28] Javanicunines A (51) and B (52) are isoprenylated DKM alkaloids isolated from the extract of Eupenicillium javanicum.[28] The key step of the synthetic access to these DKMs was the linkage of the prenylated pyrrolidinoindoline ester with leucic acid followed by lactonization.[5] Recently, similar DKM natural products, such as deacetyl-javanicunine A (54), javanicunine C (53), and javanicunine D (55), were identified from a Penicillium species.[28]

Figure 3

Polycyclic natural products with a fused DKM ring.

The DKM alkaloids PF1233 B, also referred to as shornephine A (56) and 9-deoxy-PF1233 A (57) and B (58), were identified from marine-sediment-derived Aspergillus species.[4] Methanolysis of 56 occurred at the lactone moiety and led to the opening of the DKM cycle.[4] The total synthesis of 57 and 58 was accomplished via an epoxidation of the intermediate DKM containing a tryptophan and leucic acid moiety. Subsequent intramolecular epoxide opening led to the tetracyclic scaffold.[5] PF1233 B (56) was reported as a noncytotoxic inhibitor of P-glycoprotein transporters, key mediators of drug efflux in multi-drug-resistant human cancer cells.[4] Clonorosin A (59) with the DKM ring fused to a tetracyclic isoindolo[4,5,6-cd]indole system was isolated from the soil-derived fungus Clonostachys rosea. It showed activity against Fusarium oxysporum, an ascomycete fungus which is pathogenic to plants.[29]

Further DKM alkaloids such as acu-dioxomorpholines from the fungus Aspergillus aculeatus were identified using a fungal artificial chromosome and metabolomic scoring platform. Due to this technology, the DKM biosynthetic pathway was elucidated, and nonribosomal peptide synthetase gene clusters responsible for the production of DKM alkaloids were characterized.[30]

Conclusions

Despite the striking simplicity of the diketomorpholine structure, this chemotype has received less general attention than expected. This review highlights the various synthetic routes to DKMs and their chemical reactivity, in particular, in the application of ring-opening reactions leading to tailored polymers. Furthermore, compounds bearing the DKM scaffold are represented among natural products, particularly fungal metabolites. Naturally occurring DKMs comprise small, monocyclic compounds but also complex alkaloids with a fused DKM substructure. Obviously, the class of DKMs still has not been fully explored and gives space for further research and application. Hence, this review might encourage scientists to increasingly take account of several aspects of diketomorpholine synthetic and natural product chemistry for their own research.