Asymmetric [2 + 2] Cycloaddition: Total Synthesis of (−)-Swainsonine and (+)-6-Epicastanospermine

γ-Lactam synthesis from cyclobutanone via transoximation and the Beckmann rearrangement

This manuscript describes the synthesis of γ-lactam from the nitrogen insertion reaction of cyclobutanones using an oxime as an aminating reagent with a catalytic amount of Brønsted acid. This method was employed with a more stable oxime reagent, which is a precursor analog of hydroxylamine derivatives with explosive properties. The reaction was tolerated by various substituted cyclobutanones and less strained five- or six-membered ketones. The obtained γ-lactam products could be transformed into γ-aminobutyric acid derivatives via ring-opening hydrolysis. The reaction mechanism is discussed from the perspective of the isotope effect, etc.

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.

Enantioselective synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters: application to the total synthesis of (+)-alexine

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed. This method gives moderate to excellent yields with high chemo-, diastereo- and enantioselectivities for a broad range of substrates. In order to highlight the versatility of the methodology it was applied in an efficient asymmetric synthesis of the polyhydroxylated pyrrolizidine alkaloid (+)-alexine.

A straightforward synthesis of anti-3-alkenyl-2-amido-3-hydroxy esters from the corresponding racemic α-amino-β-keto esters by using a ATH/DKR protocol has been developed.

A novel piperazine linked β-amino alcohols bearing a benzosuberone scaffolds as anti-proliferative agents

A new series of 1-((9-chloro-2,3-dimethyl-6,7-dihydro-5H-benzo[7]annulen-8-yl)methoxy)-3-(4-phenylpiperzin-1-yl) propan-2-ols (6a-k) have been designed, synthesized and their structures were established by spectroscopic data (FT-IR, ¹H NMR, ¹³C NMR, HRMS) and further confirmed by X-ray analysis. The newly synthesized compounds 6a-k were evaluated for their in vitro anti-proliferative activity against four cancer cell lines such as HeLa (cervical), MDA-MB-231 (breast), A549 (lung) and MIAPACA (pancreatic). Among the compounds tested, the compound 6e displayed most potent activity against four cancer cell lines with GI₅₀ values ranging from 0.010 to 0.097μM. The structure and anti-proliferative activity relationship was further supported by in silico molecular docking study of the active compounds against Colchicine binding site of β-tubulin.

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.

Trihaloethenes as versatile building blocks for organic synthesis

Trihaloethenes are versatile C₂-building blocks that can be simply modified via addition, elimination and transition metal-catalyzed reactions.

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.

De novo asymmetric synthesis of the pyranoses: from monosaccharides to oligosaccharides

The various methods for the de novo asymmetric synthesis of the pyranose sugars are surveyed. The presentation begins with the work of Masamune and Sharpless with the use of the Sharpless asymmetric epoxidation for the synthesis of all eight l-hexoses. The development of other asymmetric reactions and their application for the synthesis of specific hexopyranoses are further discussed. The broad application of the Achmatowicz rearrangement with asymmetric catalysis, for the synthesis of various pyranones and imino sugars, is also presented. Finally, the use of a diastereoselective palladium-catalyzed glycosylation with the Achmatowicz approach for the synthesis of oligosaccharides and applications to medicinal chemistry are discussed.

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.

Total synthesis of (+)-swainsonine and (+)-8-epi-swainsonine

Enantioselective total synthesis of (+)-swaisonine that hinges on a combination of organocatalyzed aldolization and reductive amination, affords the title compound in 9 steps, with 24% overall yield.

Nitrenium ion-mediated alkene bis-cyclofunctionalization: total synthesis of (-)-swainsonine

The total synthesis of (-)-swainsonine from 2,3-O-isopropylidene-D-erythrose in 12 steps and an overall yield of 28% is reported. The pivotal transformation in our route to this indolizidine alkaloid is the formation of the pyrrolidine ring and C-8a/8 stereodiad through the diastereoselective, bis-cyclofunctionalization of an γ,δ-unsaturated O-alkyl hydroxamate. This transformation is believed to proceed via the intramolecular capture of an N-acyl-N-alkoxyaziridinium ion generated by the diastereoselective addition of a singlet acylnitrenium ion to the pendant alkene.

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.