Synthesis of a Series of Enantiopure Polyhydroxylated Bicyclic N-Heterocycles from anL-Erythrose-Derived Nitrone and Alkoxyallenes

[3+3]-Annulation of Cyclic Nitronates with Vinyl Diazoacetates: Diastereoselective Synthesis of Partially Saturated [1,2]Oxazino[2,3-b][1,2]oxazines and Their Base-Promoted Ring Contraction to Pyrrolo[1,2-b][1,2]oxazine Derivatives

A rhodium(II)-catalyzed reaction of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with vinyl diazoacetates proceeds as a [3+3]-annulation producing bicyclic unsaturated nitroso acetals (4a,5,6,7-tetrahydro-2H-[1,2]oxazino[2,3-b][1,2]oxazines). Optimization of reaction conditions revealed the use of Rh(II) octanoate as the preferred catalyst in THF at room temperature, which allows the preparation of target products in good yields and excellent diastereoselectivity. Under basic conditions, namely, the combined action of DBU and alcohol, these nitroso acetals undergo ring contraction of an unsaturated oxazine ring into the corresponding pyrrole. Both transformations can be performed in a one-pot fashion, thus constituting a quick approach to oxazine-annulated pyrroles from available starting materials, such as nitroalkenes, olefins, and diazo compounds.

Smart and concise entry to chiral spiro[cyclopentane-indolizidine]-tetraol diastereomers as a new aza-spirocyclic framework

Synthesis of chiral oxacarba-spiroindolizidine tetraols was achieved by alkene cis-dihydroxylation, diol protection, and lactam carbonyl reduction followed ultimately by acetonide deprotection.

New total synthesis and structure confirmation of putative (+)-hyacinthacine C3 and (+)-5-epi-hyacinthacine C3

A unique synthesis of polyhydroxylated pyrrolizidine alkaloids, namely (+)-hyacinthacine C₃ and (+)-5-epi-hyacinthacine C₃ is presented. The strategy relies on a 1,3-dipolar cycloaddition of an l-mannose derived nitrone, which owing to its great syn-stereoselectivity builds up the majority of the required stereocenters. The following key steps include Wittig olefination and iodine-mediated aminocyclisation, that provide two epimeric pyrrolizidines with the appropriate configuration. As a result, structure and steric arrangement of the first synthetically prepared (+)-hyacinthacine C₃ are proved to be correct, clearly confirming the inconsistency with the stereochemistry assigned to the natural sample. With respect to the previously proven glycosidase inhibitory activities, the antiproliferative effect of (+)-hyacinthacine C₃ and (+)-5-epi-hyacinthacine C₃ was evaluated using several cell line models.

A second total synthesis of (+)-hyacinthacine C₃ is reported. As a result, structure of the first synthetically prepared alkaloid is proved to be correct, clearly confirming the inconsistency with the stereochemistry assigned to the natural sample.

The History of the Glycosidase Inhibiting Hyacinthacine C-type Alkaloids: From Discovery to Synthesis

Background:

The inherent glycosidase inhibitory activity and potentially therapeutic value of the polyhydroxylated pyrrolizidine alkaloids containing a hydroxymethyl substituent at the C-3 position have been well documented. Belonging to this class, the naturally occurring hyacinthacine C-type alkaloids are of general interest among iminosugar researchers. Their selective micromolar α-glycosidase inhibitory ranges (10 – 100 µM) suggest that these azasugars are potential leads for treating type II diabetes. However, the structures of hyacinthacine C1, C3 and C4 are insecure with hyacinthacine C5 being recently corrected.

Objective:

This review presents the hyacinthacine C-type alkaloids: their first discovery to the most recent advancements on the structures, biological activities and total synthesis.

Conclusion:

The hyacinthacine C-type alkaloids are of exponentially increasing interest and will undoubtedly continue to be reported as synthetic targets. They represent a challenging but rewarding synthetic feat for the community of those interested in accessing biologically active iminosugars. Since 2009, ten total syntheses have been employed towards accessing similarly related products but only three have assessed the glycosidase inhibitory activity of the final products. This suggests the need for an accessible and universal glycosidase inhibitory assay so to accurately determine the structure-activity relationship of how the hyacinthacine C-type alkaloids inhibit specific glycosidases. Confirming the correct structures of the hyacinthacine C-type alkaloids as well as accessing various analogues continues to strengthen the foundation towards a marketable treatment for type II diabetes and other glycosidase related illnesses.

Copper(i)-catalyzed highly enantioselective [3 + 3]-cycloaddition of γ-alkyl enoldiazoacetates with nitrones

Chiral copper(i) catalysts are preferred over chiral dirhodium(ii) catalysts for [3 + 3]-cycloaddition reactions of γ-alkyl-substituted enoldiazoacetates compounds with nitrones.

Facile Synthesis of Azetidine Nitrones and Diastereoselective Conversion into Densely Substituted Azetidines

An electrocyclization route to azetidine nitrones from N-alkenylnitrones was discovered that provides facile access to these unsaturated strained heterocycles. Reactivity studies showed that these compounds undergo a variety of reduction, cycloaddition, and nucleophilic addition reactions to form highly substituted azetidines with excellent diastereoselectivity. Taken together, these transformations provide a fundamentally different approach to azetidine synthesis than traditional cyclization by nucleophilic displacement and provide novel access to a variety of underexplored strained heterocyclic compounds.

Pyrrolizidine alkaloids: occurrence, biology, and chemical synthesis

Covering: 2013 up to the end of 2015This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

Iodination of carbohydrate-derived 1,2-oxazines to enantiopure 5-iodo-3,6-dihydro-2H-1,2-oxazines and subsequent palladium-catalyzed cross-coupling reactions

Iodination of carbohydrate-derived 3,6-dihydro-2H-1,2-oxazines of type 3 using iodine and pyridine in DMF furnished 5-iodo-substituted 1,2-oxazine derivatives 4 with high efficacy. The alkenyl iodide moiety of 1,2-oxazine derivatives syn-4 and anti-4 was subsequently exploited for the introduction of new functionalities at the C-5 position by applying palladium-catalyzed carbon–carbon bond-forming reactions such as Sonogashira, Heck, or Suzuki coupling reactions as well as a cyanation reaction. These cross-coupling reactions led to a series of 5-alkynyl-, 5-alkenyl-, 5-aryl- and 5-cyano-substituted 1,2-oxazine derivatives being of considerable interest for further synthetic elaborations. This was exemplarily demonstrated by the hydrogenation of syn-21 and anti-24 and by a click reaction of a 5-alkynyl-substituted precursor.

Square sugars: challenges and synthetic strategies

The synthesis of square sugars requires innovative strategies based on efficient stereoselective methodologies, from organocatalysis to metal carbene insertion.

[1,4]-sigmatropic rearrangement of chiral nitrones and their utilization in the synthesis of new iminosugars

A new mechanism of nitrone epimerization via [1,4]-sigmatropic rearrangement was proposed and a set of epimeric iminosugars was synthesized.

Enantioselective synthesis of polyhydroxyindolizidinone and quinolizidinone derivatives from a common precursor

A concise asymmetric synthetic route to two new tetrahydroxyindolizidinone and quinolizidinone derivatives has been developed from a common intermediate which featured a highly selective dihydroxylation reaction and a RCM reaction as key steps.