DP4+-Based Stereochemical Reassignment and Total Synthesis of Polyenic Macrolactam Muanlactam

The total synthesis of the stereoisomer of muanlactam predicted by DP4+ calculations, which differed from that reported for the natural product on the relative configuration at C19, was completed, and the structure of the polyenic macrolactam was fully confirmed. Construction of the stereocenters involved the iterative enantio- and diastereoselective Krische's allylation reaction for the formal syn-1,3-diol and the addition of a propargylic Grignard reagent to Ellman's chiral nonracemic tert-butylsulfinamide for the enantiopure amine fragment. The conjugated triene and diene units were constructed by Suzuki-Miyaura cross-coupling reactions of the corresponding alkenylboronates and alkenyl iodides. Formation of the conjugated tetraene by Horner-Wadsworth-Emmons condensation of the functionalized partners was followed by challenging macrolactamization using hexafluorophosphate azabenzotriazole tetramethyluronium and N,N-diisopropylethylamine. The NMR data of the synthetic polyenic macrolactam matched those of the natural product, thus correcting the relative configuration of muanlactam at C19, which had previously been assigned by DP4.

Stereocontrolled Hydrogenation of Conjugated Enones to Alcohols via Dual Iridium-Catalysis

The concept of dual catalysis is an emerging area holding high potential in terms of preparative efficiency, yet faces severe challenges in compatibility of reaction conditions and interference of catalysts. The transition-metal catalyzed stereoselective hydrogenation of olefins and ketones typically proceeds under different reaction conditions and/or uses a different reductant. As a result, these two types of hydrogenations can normally not be performed in the same pot. Herein, the stereocontrolled hydrogenation of enones to saturated alcohols is described, enabled by orthogonal dual iridium catalysis, using molecular hydrogen for both reductions. In this one-pot procedure, N,P-iridium catalysts (hydrogenation active towards olefins) and NHC,P-iridium catalysts (hydrogenation active towards ketones) operated independently of one another allowing the construction of two contiguous stereogenic centers up to 99 % ee, 99/1 d.r. Ultimately, by simple selection of the chirality of either ligands, the enone could be efficiently reduced to all four stereoisomers of the saturated alcohol in equally high stereopurity. This degree of stereocontrol for the synthesis of different stereoisomers by dual transition-metal catalyzed hydrogenation was previously not attained. The generality in substituted enones (alkyl, aryl, heteroaryl) demonstrate the wide applicability of this concept.

Iriomoteolide-1a and -1b: Structure Elucidation by Integrating NMR Spectroscopic Analysis, Theoretical Calculation, and Total Synthesis

The structure of iriomoteolide-1a, a marine macrolide with potent cytotoxic activity against human cancer cells, has been under scrutiny for more than a decade since the first total synthesis of the proposed structure was achieved by Horne. Here we disclose the correct structure of iriomoteolide-1a. Given a huge number of possible stereoisomers, we adopted an integrated strategy toward the structure elucidation of iriomoteolide-1a: (1) NMR spectroscopic analysis/molecular mechanics-based conformational analysis for configurational reassignment of the macrolactone domain; (2) model synthesis for validating the reassigned configuration of the macrolactone domain; (3) GIAO NMR calculation/DP4+ analysis of side chain stereoisomers; and (4) total synthesis of the most likely structure. Moreover, the correct structure of iriomoteolide-1b, a natural congener, was also determined by an integration of NMR spectroscopic analysis, GIAO NMR calculation/DP4+ analysis, and total synthesis.

Synthesis of Stereochemical Library of a Potent Antimalarial Monocerin Derivative and Its Stereochemical Revision

This study presents a total synthesis and revision of the stereochemical configuration of a potent antimalarial lead compound 2 possessing a benzo-pyranone framework, which was derived from the (+)-monocerin natural product of marine fungi, Exserohilum sp. Chiral hypervalent iodine(III)-catalyzed oxylactonization and late-stage O-methylation were highlights of the synthesis, which enabled access to the library of all possible eight stereoisomers of 2 for further understanding of stereochemical structure activity relationships (S-SARs).

Total Syntheses and Stereochemical Assignment of Acremolides A and B

The absolute stereochemical configurations of acremolides A and B were predicted by a biochemistry-based rule and unambiguously confirmed through their total syntheses. The features of the total syntheses include sequential Krische's Ir-catalyzed crotylation, Brown's borane-mediated crotylation, Mitsunobu esterification reaction, and cross-metathesis reaction. The efficient total synthesis enabled clear validation of the predicted stereochemistry for acremolides A and B.

Total Synthesis and Revision of Stereochemistry of a Natural Benzo[g]isochromene Stereodiad Isolated from Rubia philippinensis

This study presents a total synthesis and revision of the stereochemical configuration of the conformationally flexible natural product benzo[g]isochromene stereodiad alongside its diastereomeric counterparts. The highlights of the synthesis are the TiCl4-mediated diastereoselective aldol reaction, Pd-catalyzed lactonization, and Schmidt glycosidation. Our efforts using total synthesis disclosed herein proved that a previously assigned structure required revision.

Total Synthesis and Determination of the Absolute Configuration of Berkeleylactone I

Herein, we report the first total synthesis of berkeleylactone I and its 12S diastereomer. Consequently, this chemical synthesis allowed us to establish the unknown absolute stereochemistry at the C-12 center as 12R, which was unidentified by the isolation group. This synthetic approach includes several critical reactions, such as the Sharpless asymmetric dihydroxylation, Baran's Ni-catalyzed alkyl-alkyl cross-coupling reaction, Brown allylation, Mitsunobu reaction, and ring-closing metathesis, as key steps.

Total Synthesis and Absolute Configuration Determination of the α-Glycosidase Inhibitor (3S,4R)-6-Acetyl-3-hydroxy-2,2-dimethylchroman-4-yl (Z)-2-Methylbut-2-enoate from Ageratina grandifolia

Herein, we report the first total synthesis of α-glycosidase inhibitor (3R, 4S)-6-acetyl-3-hydroxy-2,2-dimethylchroman-4-yl (Z)-2-methylbut-2-enoate as well as its enantiomer. Our synthesis confirms the chromane structure separately proposed by Navarro-Vazquez and Mata, on the basis of DFT computations. Furthermore, our synthesis allowed us to determine the absolute configuration of the natural compound as (3S, 4R) and not (3R, 4S).

Epimeric Mixture Analysis and Absolute Configuration Determination Using an Integrated Spectroscopic and Computational Approach-A Case Study of Two Epimers of 6-Hydroxyhippeastidine

Structural elucidation has always been challenging, and misassignment remains a stringent issue in the field of natural products. The growing interest in discovering unknown, complex natural structures accompanies the increasing awareness concerning misassignments in the community. The combination of various spectroscopic methods with molecular modeling has gained popularity in recent years. In this work, we demonstrated, for the first time, its power to fully elucidate the 2-dimensional and 3-dimensional structures of two epimers in an epimeric mixture of 6-hydroxyhippeastidine. DFT calculation of chemical shifts was first performed to assist the assignment of planar structures. Furthermore, relative and absolute configurations were established by three different ways of computer-assisted structure elucidation (CASE) coupled with ORD/ECD/VCD spectroscopies. In addition, the significant added value of OR/ORD computations to relative and absolute configuration determination was also revealed. Remarkably, the differentiation of two enantiomeric scaffolds (crinine and haemanthamine) was accomplished via OR/ORD calculations with cross-validation by ECD and VCD.

Synthesis of the C50 diastereomers of the C33–C51 fragment of stambomycin D

The preparation of two C50 diastereomers of the C33–C51 region of stambomycin D is described. In addition to excellent correlation with the natural product, this synthesis establishes conditions for eventual global deprotection.

Synthesis of the C1-C27 Fragment of Stambomycin D Validates Modular Polyketide Synthase-Based Stereochemical Assignments

The stambomycins are a family of bioactive macrolides isolated from Streptomyces ambofaciens. Aside from two stereocenters installed through cytochrome P450 oxidations, their stereochemistry has been predicted by sequence analysis of the polyketide synthase. We report a synthesis of the C1–C27 fragment of stambomycin D, the spectroscopic data of which correlates well with that of the natural product, further validating predictive sequence analysis as a powerful tool for stereochemical assignment of complex polyketide natural products.

Asymmetric Total Synthesis of Amphirionin-2

A convergent route for the asymmetric total synthesis of potent anticancer polyketide natural product amphirionin-2 has been developed. Our initial synthetic trials revealed that the proposed structures of amphirionin-2 need to be revised consistent with a recent report of Fuwa et al., where the actual structure of amphirionin-2 was established. The key features of our synthesis comprised Sharpless asymmetric dihydroxylation, followed by cycloetherification, Wittig olefination, Julia-Kocienski olefination, and Crimmins propionate aldol reaction.

18 O/16 O-Encoding Strategy for Microscale Stereochemical Determination of Peptidic Natural Products

The demand for more efficient methods of establishing the undetermined stereochemistries of peptidic natural products continues unabated. A new method for microscale stereochemical determination was devised by integrating solid-phase synthesis, split-and-mix randomization, ¹⁸ O/¹⁶ O-encoding of d/l-configurations, tandem mass spectrometry, and biological evaluation. Here we applied gramicidin A as the molecule for a blind test. Gramicidin A and its 31 diastereomers were randomly prepared in microgram scale with ¹⁸ O/¹⁶ O-stereochemical encoding and subjected to MS/MS-structural determination and cytotoxicity assay. Only the parent gramicidin A was selected from among the 32 stereoisomers, validating the high reliability of the present strategy.

Structure determination, correction, and disproof of marine macrolide natural products by chemical synthesis

Integration of chemical synthesis, NMR spectroscopy, and various analytical means is key to success in the structure elucidation of stereochemically complex marine macrolide natural products.