Synthesis of High-Order and High-Oxidation State Securinega Alkaloids

Securinega alkaloids, composed of more than 100 members characterized by the compact tetracyclic scaffold, have fascinated the synthetic community with their structural diversity and notable bioactivities. On the basis of the structural phenotype, oligomerizations and oxidations are major biosynthetic diversification modes of the basic Securinega framework. Despite the rich history of synthesis of basic monomeric Securinega alkaloids, the synthesis of oligomeric Securinega alkaloids, as well as oxidized derivatives, has remained relatively unexplored because of their extra structural complexity. In the first half of this Account, our synthetic studies toward high-order Securinega alkaloids are described. We aimed to establish a reliable synthetic method to form C14-C15' and C12-C15' bonds, which are prevalent connection modes between monomers. During our total synthesis of flueggenine C (9), we have invented an accelerated Rauhut-Currier reaction capable of forming the C14-C15' bond stereoselectively. Installation of the nucleophilic functionality to the Michael acceptor, which ushers the C-C bond forming conjugate addition to follow the intramolecular pathway, was the key to success. The C12-C15' linkage, which was inaccessible via an accelerated Rauhut-Currier reaction, was established by devising a complementary cross-coupling/conjugate reduction-based dimerization strategy that enabled the total synthesis of flueggenines D (11) and I (14). In this approach, the C12-C15' linkage was established via a Stille cross-coupling, and the stereochemistry of the C15' position was controlled during the following conjugate reduction step. In the later half of this Account, our achievements in the field of high-oxidation state Securinega alkaloids synthesis are depicted. We have developed oxidative transformations at the N1 and C2-C4 positions, where the biosynthetic oxidation event occurs most frequently. The discovery of a VO(acac)₂-mediated regioselective Polonovski reaction allowed us to access the key 2,3-dehydroallosecurinine (112). Divergent synthesis of secu'amamine A (62) and fluvirosaones A (60) and B (61) was accomplished by exploiting the versatile reactivities of the C2/C3 enamine moiety in 112. We have also employed a fragment-coupling strategy between menisdaurilide and piperidine units, which allowed the installation of various oxygen-containing functionality on the piperidine ring. Combined with the late-stage, light-mediated epimerization and well-orchestrated oxidative modifications, collective total synthesis of seven C4-oxygenated securinine-type natural products was achieved. Lastly, the synthesis of flueggeacosine B (70) via two synthetic routes from allosecurinine (103) was illustrated. The first-generation synthesis (seven overall steps) employing Pd-catalyzed cross-coupling between stannane and thioester to form the key C3-C15' bond enabled the structural revision of the natural product. In the second-generation synthesis, we have invented visible-light-mediated, Cu-catalyzed cross-dehydrogenative coupling (CDC) between an aldehyde and electron-deficient olefin, which streamlined the synthetic pathway into four overall steps. Organisms frequently utilize dimerization (oligomerization) and oxidations during the biosynthesis as a means to expand the chemical space of their secondary metabolites. Therefore, methods and strategies for dimerizations and oxidations that we have developed using the Securinega alkaloids as a platform would be broadly applicable to other alkaloids. It is our sincere hope that lessons we have learned during our synthetic journey would benefit other chemists working on organic synthesis.

N-Hydroxy pipecolic acid methyl ester is involved in Arabidopsis immunity

The biosynthesis of N-hydroxy pipecolic acid (NHP) has been intensively studied, though knowledge on its metabolic turnover is still scarce. To close this gap, we discovered three novel metabolites via metabolite fingerprinting in Arabidopsis thaliana leaves after Pseudomonas infection and UV-C treatment. Exact mass information and fragmentation by tandem mass spectrometry (MS/MS) suggest a methylated derivative of NHP (MeNHP), an NHP-OGlc-hexosyl conjugate (NHP-OGlc-Hex), and an additional NHP-OGlc-derivative. All three compounds were formed in wild-type leaves but were not present in the NHP-deficient mutant fmo1-1. The identification of these novel NHP-based molecules was possible by a dual-infiltration experiment using a mixture of authentic NHP and D9-NHP standards for leaf infiltration followed by UV-C treatment. Interestingly, the signal intensity of MeNHP and other NHP-derived metabolites increased in ugt76b1-1 mutant plants. For MeNHP, we unequivocally determined the site of methylation at the carboxylic acid moiety. MeNHP application by leaf infiltration leads to the detection of a MeNHP-OGlc as well as NHP, suggesting MeNHP hydrolysis to NHP. This is in line with the observation that MeNHP infiltration is able to rescue the fmo1-1 susceptible phenotype against Hyaloperonospora arabidopsidis Noco 2. Together, these data suggest MeNHP as an additional storage or transport form of NHP.

Collective total synthesis of C4-oxygenated securinine-type alkaloids via stereocontrolled diversifications on the piperidine core

Securinega alkaloids have fascinated the synthetic chemical community for over six decades. Historically, major research foci in securinega alkaloid synthesis have been on the efficient construction of the fused tetracyclic framework that bears a butenolide moiety and tertiary amine-based heterocycles. These "basic" securinega alkaloids have evolved to undergo biosynthetic oxidative diversifications, especially on the piperidine core. However, a general synthetic solution to access these high-oxidation state securinega alkaloids is lacking. In this study, we have completed the total synthesis of various C4-oxygenated securinine-type alkaloids including securingines A, C, D, securitinine, secu'amamine D, phyllanthine, and 4-epi-phyllanthine. Our synthetic strategy features stereocontrolled oxidation, rearrangement, and epimerization at N1 and C2-C4 positions of the piperidine core within (neo)securinane scaffolds. Our discoveries provide a fundamental synthetic solution to all known securinine-type natural products with various oxidative and stereochemical variations around the central piperidine ring.

A Convenient On-Site Oxidation Strategy for the N-Hydroxylation of Melanostatin Neuropeptide Using Cope Elimination

A convenient synthetic protocol for the unprecedented N-hydroxylation of proline residue in Melanostatin (MIF-1) neuropeptide is reported. This methodology is grounded on the incorporation of N-(cyanoethyl)prolyl residue followed by on-site oxidation by Cope elimination with m-chloroperbenzoic acid, exploring the unrecognized dual role of the cyanoethyl group as an effective N-protecting group under peptide synthesis conditions and as a suitable leaving group during the chemoselective on-site N-oxidation. Following this protocol N-hydroxy-MIF-1 is obtained in 78% global yield from N-(cyanoethyl)-l-proline. This synthetic approach opens a new avenue for access to N-hydroxylated Melanostatin analogues with direct application in neurochemistry and Parkinson’s research.

Mechanistic insights for the transprotection of tertiary amines with Boc2O via charged carbamates: access to both enantiomers of 2-azanorbornane-3-exo-carboxylic acids

Development of a synthetic methodology for the selective transprotection in hindered tertiary amines using Boc₂O under N–C hydrogenolysis catalyzed by Pd/C: access to both enantiomers of 2-azanorbornane-3-exo-carboxylates.

Diastereoselective synthesis of fluoroisosteric analogues of antiparasitic pyrrolobenzoxazine alkaloids from tryptophan by successive fluorination–cyclization and a Meisenheimer-type rearrangement

Fluoroisosteric analogues of an antiparasitic pyrrolobenzoxazine alkaloid CJ-12663 were designed and diastereoselectively synthesized.