Total Synthesis of Sphingofungin F Based on Chiral Tricyclic Iminolactone

A Modular Approach to the Antifungal Sphingofungin Family: Concise Total Synthesis of Sphingofungin A and C

Sphingofungins are fungal natural products known to inhibit the biosynthesis of sphingolipids which play pivotal roles in various cell functions. Here, we report a short and flexible synthetic approach towards the sphingofungin family. Key step of the synthesis was a decarboxylative cross-coupling reaction of chiral sulfinyl imines with a functionalized tartaric acid derivative, which yielded the core motif of sphingofungins carrying four consecutive stereocenters and a terminal double bond. Subsequent metathesis reaction allowed for the introduction of different side chains of choice resulting in a total of eight sphingofungins, including for the first time sphingofungin C (eight steps from commercially available protected tartaric acid with an overall yield of 6 %) and sphingofungin A (ten steps). All newly synthesized derivatives were tested for their antifungal, cell-proliferative and antiparasitic activity unraveling their structure-activity relations.

Asymmetric Synthesis of Oxindole-Derived Vicinal Tetrasubstituted Acyclic Amino Acid Derivatives by the Mannich-Type Reaction

The catalytic asymmetric Mannich-type reaction of 3-hydroxy/3-aminooxindoles with 2-aminoacrylates to afford oxindole-derived acyclic amino acid derivatives bearing vicinal tetrasubstituted stereocenters is reported. (DHQ)₂PHAL (4g) and quinine-derived squaramide (4d) were identified as efficient catalysts. Transformations of the Mannich-type reaction products highlight the utility of this synthetic strategy.

Synthesis and anticancer profile of novel sphingoid base-like compounds with a quaternary stereocentre

The synthesis of novel sphingoid base-like compounds with a quaternary stereocentre was achieved in a sequence featuring [3,3]-sigmatropic rearrangements and olefin cross-metathesis transformation as the key reaction steps, which were accompanied by the rational selection of suitable functional group transformations. The stereochemistry of the desired tetra-substituted carbon bearing nitrogen functionality was determined via NOESY experiments of the advanced oxazolidine-2-thiones. Cell viability experiments revealed significant antiproliferative/cytotoxic activity of the target compounds 7, ent-7 and 29 against the Jurkat cell line, with the IC₅₀ values of 6.6 μM, 5.6 μM and 6.1 μM, respectively.

Total Synthesis of Sphingofungin E and 4,5-Di-epi-sphingofungin E

Total synthesis of sphingofungin E and 4,5-di-epi-sphingofungin E was achieved from an intermediate same as that of myriocin and mycestericin D via antipodal stereoselective dihydroxylations.

Copper-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines

A Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines was developed, providing the corresponding chiral α-tertiary amines with up to 98% ee.

Enantioselective Synthesis of Quaternary α-Amino Acids via l-tert-Leucine-Derived Squaramide-Catalyzed Conjugate Addition of α-Nitrocarboxylates to Enones

Enantioselective Michael addition of tertiary α-nitroesters to β-unsubstituted vinyl ketones has been carried out in the presence of an l-tert-leucine-derived squaramide as organocatalyst. The products, quaternary α-nitroesters, were formed in excellent yield and moderate to good ee's in most cases. Scale-up of the reaction and synthetic applications of the products, including transformation to representative quaternary α-amino acids, have also been demonstrated.

Chirality extension of an oxazine building block en route to total syntheses of (+)-hyacinthacine A2 and sphingofungin B

Concise total syntheses of (+)-hyacinthacine A₂ and sphingofungin B were achieved by palladium(0)-catalyzed intramolecular oxazine formation and diastereoselective nucleophilic additions.

DIBAL-mediated reductive transformation of trans-dimethyl tartrate acetonide into ε-hydroxy α,β-unsaturated ester and its derivatives

Stepwise, selective DIBAL reduction of the acetonide diester derived from tartaric acid followed by the Horner-Emmons reaction effectively provided desymmetrized hydroxy mono-olefination products in a one-pot operation.

Synthesis of α-amino acids based on chiral tricycloiminolactone derived from natural (+)-camphor

Amino acids are one of the most important classes of the building blocks of life: they are the structural subunits of proteins, peptides, and many secondary metabolites. In addition to the 20 α-amino acids that constitute the backbone of proteins, hundreds of other natural α-amino acids have been discovered either in free form or as components in natural products. The difference between these molecules is the substituents at the chiral carbon situated between the amino and carboxyl moieties; this carbon (and any atom along a chain attached to it) is thus an important synthetic target. Because tailor-made α-amino acids are increasingly popular in biochemistry and organic synthesis, further refinement in synthetic methods to generate both natural (L-configuration) and unnatural (D-configuration) amino acids is a very active area of current research. In this Account, we examine the tricycloiminolactones, which are versatile glycine equivalents derived from natural camphor. We have developed the tricycloiminolactones in our laboratory and used them in the synthesis of several kinds of enantiopure α-amino acids. As nucleophiles, enolated tricycloiminolactones were shown to successfully participate in alkylations, Aldol reactions, Michael additions, and Mannich reactions. These reactions all gave excellent stereoselectivities and high yields. Simple conversion of the products offered α-alkyl-α-amino acids, α,α-dialkyl-α-amino acids, β-hydroxy-α-amino acids, α,γ-diamino acids, and α,β-diamino acids. One particular advantage is that the same electrophile can react with two chiral templates in the same way, thus affording access to both enantiomeric amino acids. In other words, some natural (L-configuration) α-amino acids and their unnatural (D-configuration) counterparts can be prepared very conveniently. The relation between substrate structures and product stereoconformations derived from our investigations serves as a convenient guide in the synthesis of useful chiral amino acids. In addition, highly stereoselective 1,3-diploar cycloadditions between alkenes and chiral nitrones derived from tricycloiminolactones provide a potential method for the synthesis of γ-hydroxy-α-amino acids. We also discuss applications of our methods in the synthesis of complex natural products, including conagenin, polyoxamic acid, lactacystin, and sphingofungin F. The preparation of some clinically important drug molecules, such as thiaphenicol, florfenicol, and chloramphenicol, was greatly simplified with our methods. The tricycloiminolactones offer a number of advantages in the synthesis of both natural and unnatural α-amino acids and provide many useful building blocks in the synthetic pursuit of complex molecules.