Stereoselective total synthesis of (+)-myriocin from D-mannose

Recent advances in the total synthesis of polyhydroxylated alkaloids via chiral oxazines

This review summarizes recently established methodologies developed for the enantioselective and diastereoselective synthesis of chiral 1,3-oxazines. These compounds are of interest as advanced synthetic intermediates in the total synthesis of structurally complex and biologically active polyhydroxylated alkaloids such as (+)-1-deoxynojirimycin, (-)-anisomycin, (+)-castanospermine, (+)-casuarine, (-)-conduramine F-1, (-)-sphingofungin B, Neu5Ac methyl ester, and other natural products. The devised synthetic approach aims to offer a direct, efficient, and adaptable method for obtaining both pure enantiomers and pure diastereomers. It revolves around utilizing chiral building blocks like syn,syn-, syn,syn,anti-, syn,anti-, syn,anti,syn-, anti,syn-, anti,syn,syn-, and anti,syn,anti-oxazines. By integrating oxazine chemistry with established and innovative transformations, this approach enabled the synthesis of 30 polyhydroxylated amines across various studies conducted between 2007 and 2022.

Synthetic Approaches to Phosphasugars (2-oxo-1,2-oxaphosphacyclanes) Using the Anomeric Alkoxyl Radical β-Fragmentation Reaction as the Key Step

The anomeric alkoxyl radical β-fragmentation (ARF) of carbohydrates possessing an electron-withdrawing group (EWG) at C2, promoted by PhI(OAc)₂/I₂, gives rise to an acyclic iodide through which a pentavalent atom of phosphorus can be introduced via the Arbuzov reaction. After selective hydrolysis and subsequent cyclization, the phosphonate or phosphinate intermediates can be converted into 2-deoxy-1-phosphahexopyranose and 2-deoxy-1-phosphapentopyranose sugars. The ARF of carbohydrates with an electron-donor group (EDG) at C2 proceeds by a radical-polar crossover mechanism, and the cyclization occurs by nucleophilic attack of a conveniently positioned phosphonate or phosphinate group to the transient oxocarbenium ion. This alternative methodology leads to 5-phosphasugars with a 4-deoxy-5-phosphapentopyranose framework. The structure and conformation of the 2-oxo-1,2-oxaphosphinane and 2-oxo-1,2-oxaphospholane ring systems in different carbohydrate models have been studied by NMR and X-ray crystallography.

The synthesis of unnatural α-alkyl- and α-aryl-substituted serine derivatives

The synthesis of α-aryl- and α-alkyl-substituted serine derivatives via [3,3]-sigmatropic rearrangement of allyl carbamates as a key step is reported. Allyl carbamates were obtained from the corresponding allyl alcohols. The former were prepared through three approaches. Aryl-substituted ones were synthesized via the Stille coupling reaction of aryl iodides with enantiomerically enriched vinyl stannanes. Conversely, alkyl-substituted allyl alcohols were prepared by an analogous strategy involving the Negishi coupling reaction of enantiomerically enriched vinyl iodides or by enzymatic kineric resolution of the corresponding racemic alcohols.

Construction of Quaternary Carbon Stereocenter of α-Tertiary Amine through Remote C-H Functionalization of Tris Derivatives: Enantioselective Total Synthesis of Myriocin

We describe the development of a strategy for the construction of the quaternary carbon stereocenter of α-tertiary amines. This strategy highlights a site-selective C-H functionalization involving an alkoxy-radical-triggered 1,5-hydrogen transfer (1,5-HAT) reaction of a conformationally fixed spiro-compound derived from trishydroxymethylaminomethane (Tris). The utilization of this strategy enabled an enantioselective total synthesis of myriocin, a naturally occurring sphingosine analog that displays potent immunosuppressive activity.

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.

The evolution of comprehensive strategies for furanoid glycal synthesis and their applications

Cyclic enol ether frameworks, especially stereochemically pure furanoid and pyranoid glycals are well known highly functionalized chiral building blocks.

2-Vinyl Threoninol Derivatives via Acid-Catalyzed Allylic Substitution of Bisimidates

A diastereoselective synthesis of 4-vinyl oxazolines syn-2 was developed based on an acid-catalyzed cyclization of bistrichloroacetimidates (E)-1. The reaction likely involves an allyl carbenium ion intermediate in which the adjacent stereocenter directs the stereoselectivity for C-N bond formation. Oxazolines syn-2 were transformed to C-quaternary threoninol, threoninal, and threonine derivatives which can be further incorporated into complex natural compounds.

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.