A novel deprotection of trichloroacetamide

Synthesis of Type-I and Type-II LacNAc-Repeating Oligosaccharides as the Backbones of Tumor-Associated Lewis Antigens

Type-I and Type-II LacNAc are Gal-GlcNAc disaccharides bearing a β1,3- or β1,4-linkage respectively. They exist as the backbones of Lewis antigens that are highly expressed in several cancers. Owing to the promise of developing carbohydrate-based anti-cancer vaccines, glycan synthesis at a large scale is indeed an important task. Synthesis of Type-I and Type-II tandem repeat oligomers has been hampered by the presence of GlcNAc residues. Particularly, N-protecting group plays a determining role in affecting glycosyl donor’s reactivity and acceptor’s nucleophilicity. This review discusses several representative studies that assembled desirable glycans in an efficient manner, such as chemoselective one-pot synthesis and chemoenzymatic methods. Additionally, we also highlight solutions that have been offered to tackle long-lasting problems, e.g., prevention of the oxazoline formation and change of donor/acceptor reactivity. In retrospect of scientific achievements, we present the current restrictions and remaining challenges in this less explored frontier.

Synthesis of orthogonally protected and functionalized bacillosamines

2,4-Diamino-2,4,6-trideoxyglucose (bacillosamine) is a monosaccharide found in many pathogenic bacteria, variation in the functionalities appended to the amino groups occurs depending on the species the sugar is derived from. We here report the first synthesis of bacillosamine synthons that allow for the incorporation of two different functionalities at the C-2-N-acetyl and C-4-amines. We have developed chemistry to assemble a set of conjugation ready Neisseria meningitidis C-2-N-acetyl bacillosamine saccharides, carrying either an acetyl or (R)- or (S)-glyceroyl at the C-4 amine. The glyceroyl bacillosamines have been further extended at the C-3-OH with an α-d-galactopyranose to provide structures that occur as post-translational modifications of N. meningitidis PilE proteins, which make up the bacterial pili.

Total Synthesis of (-)-Himeradine A

(-)-Himeradine A is a complex lycopodium alkaloid with seven rings and ten stereogenic centers that shows anticancer activity against lymphoma L1210 cells. A total synthesis has been developed that builds off prior work on (+)-fastigiatine. A 2,4,6-trisubstitited piperidine ring forms the core of the quinolizidine segment, and was prepared by diastereoselective reduction of a pyridine and classic resolution of an intermediate. The remaining secondary amine was introduced with a catalyst-controlled Overman rearrangement. The piperidine segment was coupled in a B-alkyl Suzuki reaction with a bicyclic bromoenone, which was a key intermediate for the synthesis of (+)-fastigiatine. The final transformation featured a transannular Mannich reaction and cyclization to complete the quinolizidine. Five bonds and four new rings were generated in this one-pot procedure. (-)-Himeradine A was prepared in 17 steps in the longest linear sequence.

One-Step Transformation of Trichloroacetamide into Isonitrile

A one-step transformation of trichloroacetamide, a protective group for the amine function, into isonitrile was successfully developed. The substrate scope and functional group tolerance of this procedure are also described.

Investigations of scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent, atom-economical, one-step conversion to α-urea-glycosides

The development and mechanistic investigation of a highly stereoselective methodology for preparing α-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the selective nickel-catalyzed conversion of glycosyl trichloroacetimidates to the corresponding α-trichloroacetamides. The α-selective nature of the conversion is controlled with a cationic nickel(II) catalyst, [Ni(dppe)(OTf)2 ] (dppe=1,2-bis(diphenylphosphino)ethane, OTf=triflate). Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2 -ether functionality of the α-glycosyl trichloroacetimidate to be paramount for achieving an α-stereoselective transformation. A cross-over experiment has indicated that the reaction does not proceed in an exclusively intramolecular fashion. The second step in this sequence is the direct conversion of α-glycosyl trichloroacetamide products into the corresponding α-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only α-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric CN bond.

An improved synthesis of (-)-5,11-dideoxytetrodotoxin

We describe an improved synthesis of (-)-5,11-dideoxytetrodotoxin from an enone, which was used for synthesis of tetrodotoxin and its analogues in this laboratory. One of the major modifications was to establish a two-step guanidinylation of trichloroacetamide of a highly functionalized intermediate, which allowed us to prepare (15)N(2)-labeled 5,11-dideoxytetrodotoxin for biosynthetic investigations.

Total synthesis of (+/-)-agelastatin A, a potent inhibitor of osteopontin-mediated neoplastic transformations

A stereoselective synthesis of agelastatin A, a potent cytotoxin and inhibitor of osteopontin (OPN)-mediated neoplastic transformations, has been accomplished in 14 steps (12 operations) with an approximate overall yield of 8%. Notable features of this route include the direct manner in which the pyrroloketopiperazine A-ring of the target is generated and the efficient employment of a trichloroacetamide, introduced through Overman rearrangement, as a protecting group, pendant nucleophile, and latent urea.

Palladium(II)-catalyzed rearrangement of glycal trichloroacetimidates: application to the stereoselective synthesis of glycosyl ureas

The research on the area of glycosyl urea derivatives, in which the O- and N-glycosidic bonds are replaced with the urea-glycosidic linkages, has recently emerged with applications in the field of aminoglycoside antibiotics. We have developed a novel method for the stereoselective synthesis of alpha- and beta-glycosyl ureas via Pd(II)-catalyzed rearrangement of glycal trichloroacetimidates. In our approach, the alpha- and beta-selectivity at the anomeric carbon of N-glycosyl trichloroacetamides depends on the nature of the palladium-ligand catalyst. While the cationic Pd(II)-L-4 (2-trifluoroacetylphenol) complex promotes alpha-selectivity, the neutral Pd(II)-TTMPP-L-5 (4-chloro-2-trifluoroacetylphenol) complex favors beta-selectivity. The resulting alpha- and beta-N-glycosyl trichloroacetamides were further coupled with a diverse array of primary and hindered secondary nitrogen nucleophiles to provide the corresponding glycosyl ureas in moderate to good yields and with no loss of stereochemical integrity at the anomeric carbon. We have further demonstrated the utility of N-glycosyl trichloroacetamides as robust and versatile intermediates in the synthesis of unsymmetrical urea-linked disaccharides and trisaccharide.

Palladium-catalyzed glycal imidate rearrangement: formation of alpha- and beta-N-glycosyl trichloroacetamides

A novel palladium(II)-catalyzed stereoselective synthesis of alpha- and beta-N-glycosyl trichloroacetamides has been developed. The alpha- and beta-selectivity at the anomeric carbon depends on the nature of the palladium-ligand catalyst. While the cationic palladium(II) promotes the alpha-selectivity, the neutral palladium(II) favors the beta-selectivity.

An efficient total synthesis of optically active tetrodotoxin from levoglucosenone

Tetrodotoxin, a toxic principal of puffer-fish poisoning, is one of the most famous marine natural products, and has been known as a formidable synthetic target in synthesis owing to its multifunctional structure and unusual chemical properties. From the perspective of supplying tetrodotoxin derivatives such as labeled molecules for biochemical research, we have completed our second total synthesis of tetrodotoxin from a synthetic intermediate for 11-deoxytetrodotoxin, which was previously prepared from levoglucosenone as a chiral starting material in this laboratory. This paper discloses the details of the total synthesis with special reference to significant influences on the neighboring functional groups found in the installation of guanidine. The established route should allow us to prepare the tetrodotoxin-related compounds required for biochemical studies.