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.

Synthesis of γ-Lactams by Formal Cycloadditions with Ketenes

The synthesis of γ-lactams is reported by a formal (3+2) cycloaddition between readily available ketenes and aziridines or a one-pot formal (2+1+2) cycloaddition using imines as aziridine precursors. The method is practical, is scalable, and affords high yields. It also offers a high level of regio- and diastereoselectivity on a wide range of substrates as well as a high stereoselectivity in the case of enantiopure aziridines.

Stereo- and regioselective hydroboration of 1-exo-methylene pyranoses: discovery of aryltriazolylmethyl C-galactopyranosides as selective galectin-1 inhibitors

Galectins are carbohydrate recognition proteins that bind carbohydrates containing galactose and are involved in cell signaling and cellular interactions, involving them in several diseases. We present the synthesis of (aryltriazolyl)methyl galactopyranoside galectin inhibitors using a highly diastereoselective hydroboration of C1-exo-methylene pyranosides giving inhibitors with fourfold or better selectivity for galectin-1 over galectin-3, -4C (C-terminal CRD), -4N (N-terminal CRD), -7, -8C, -8N, -9C, and -9N and dissociation constants down to 170 µM.

A ring closing metathesis strategy for carbapyranosides of xylose and arabinose

The synthesis of β-carba-xylo and arabino pyranosides of cholestanol is described. The synthetic strategy, which is analogous to the Postema approach to C-glycosides, centers on the ring closing metathesis of an enol ether-alkene precursor to give a cyclic enol ether that is elaborated to a carba-pyranoside via hydroboration-oxidation on the olefin. The method, which is attractive for its modularity and stereoselectivity, may find wider applications to carba-hexopyranosides and other complex cycloalkyl ether frameworks.

A ring closing metathesis strategy for carbapyranosides of xylose and arabinose

The synthesis of β-carba-xylo and arabino pyranosides of cholestanol is described. The synthetic strategy, which is analogous to the Postema approach to C-glycosides, centers on the ring closing metathesis of an enol ether-alkene precursor to give a cyclic enol ether that is elaborated to a carba-pyranoside via hydroboration-oxidation on the olefin. The method, which is attractive for its modularity and stereoselectivity, may find wider applications to carba-hexopyranosides and other complex cycloalkyl ether frameworks.

Diverse Natural Products from Dichlorocyclobutanones: An Evolutionary Tale

11-Nor PGE2 was prepared in our laboratory several years ago and used to obtain the corresponding ring-expanded γ-butyrolactam, γ-butyrolactone, and cyclopentanone derivatives. The conversion of a cyclobutanone into a cyclopentanone had relatively little precedent and merited further study. It was soon found that the presence of a single chlorine adjacent to the carbonyl not only greatly accelerated the reaction with ethereal diazomethane, but also substantially enhanced its regioselectivity; not surprisingly, a second chlorine further increased both. The confluence of this finding and the discovery by Krepski and Hassner that the presence of phosphorus oxychloride significantly improved the Zn-mediated dehalogenation procedure for the preparation of α,α-dichlorocyclobutanones from olefins provided the starting point for decades' worth of exciting adventures in natural product synthesis. A wide variety of naturally occurring 5-membered carbocycles (e.g., hirsutanes, cuparenones, bakkanes, guaianolides, azulenes) could thus be prepared by using dichloroketene-olefin cycloaddition, followed by regioselective one-carbon ring expansion with diazomethane. Importantly, it was also found that natural γ-butyrolactones (e.g., β-oxygenated γ-butyrolactones, lactone fatty acids) could be secured through regioselective Baeyer-Villiger oxidation of cycloadducts with m-CPBA and that naturally occurring γ-butyrolactam derivatives (e.g., amino acids, pyrrolidines, pyrrolizidines, indolizidines) could be efficiently obtained by regioselective Beckmann ring expansion of the adducts with O-(mesitylenesulfonyl)hydroxylamine (Tamura's reagent). These 5-membered carbocycles, γ-butyrolactones, and γ-butyrolactam derivatives were generally secured in enantiopure form through the use of either intrinsically chiral olefins or olefins bearing Stericol, a highly effective chiral auxiliary developed specifically for this "three-atom olefin annelation" approach. In addition, considerable useful chemistry has been developed in the context of this synthesis program. This includes new methods for olefin vicinal dicarboxylation, β-methylene-γ-butyrolactonization, γ-butyrolactone and δ-valerolactone α-methylenations, transesterification, angelic ester synthesis, chiral enol and ynol ether preparations, dichloroacetylene synthesis, and trans, trans hydroxy triad introduction. This versatile dichlorocyclobutanone-centered approach to natural product synthesis, together with the attendant new methods that have been developed, forms the basis of this Account, which is presented as an evolutionary tale. It is hoped that the Account will stimulate other research groups to seek to exploit the rich chemistry of dichlorocyclobutanones for possible solutions to problems in organic synthesis.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Strategies towards the synthesis of calyciphylline A-type Daphniphyllum alkaloids

The Daphniphyllum alkaloids are a diverse family of natural products rich in number and structural diversity that have been known for many decades. However, the structurally unique subclass of calyciphylline A-type alkaloids has only recently been discovered and is relatively unexplored. Several noteworthy core syntheses and the development of a wide range of novel synthetic strategies have been achieved. This includes strategies based on intramolecular Michael addition, Pd-catalysis, cycloaddition, and Mannich-type reactions. This review will provide an overview of these synthetic studies.

Diastereoselective nitrenium ion-mediated cyclofunctionalization: total synthesis of (+)-castanospermine

The asymmetric total synthesis of the α-glucosidase inhibitor (+)-castanospermine is reported. The central theme in our approach to this polyhydroxylated alkaloid is the simultaneous generation of the piperidine ring and the C-1/8a erythro stereodiad through the diastereoselective, oxamidation of an unsaturated O-alkyl hydroxamate. This process is believed to proceed sequentially via singlet acylnitrenium and aziridinium ion intermediates.

Asymmetric total synthesis of 1-deoxy-7,8-di-epi-castanospermine

An efficient, stereocontrolled synthesis of (6S,7S,8S,8aR)-6,7,8-trihydroxyindolizidine (alias 1-deoxy-7,8-di-epi-castanospermine) (14) has been developed, which exploits an asymmetric vinylogous Mukaiyama aldol reaction (VMAR) between N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsilyloxy)pyrrole (1) and 2,3-O-isopropylidene-D-glyceraldehyde (2) to construct the initial pyrrolidine building block 3, and an ene-ene ring closing metathesis reaction (RCM) (9 to 10) to install the indolizidine skeleton. The synthetic sequence was 13 steps, proceeding in 19.5% overall yield. The configurational and conformational structure of 14 was ascertained unambiguously and confronted to previously published assignments of rac-14 and ent-14.