George A. O'Doherty

Syntheses of four D- and L-hexoses via diastereoselective and enantioselective dihydroxylation reactions.

An expeditious approach to various protected hexoses has been developed by the use of the Sharpless catalytic asymmetric dihydroxylation reaction. Applying the Sharpless catalytic asymmetric dihydroxylation reaction on vinylfuran, diols with high enantioexcess are produced. The resulting diols can be stereoselectively transformed into either protected D- or L-mannose in five steps and approximately 39% yield from furfural. Similarly, both D- and L-talose and gulose have been synthesized in 19% overall yields, respectively. Using a modified strategy, both protected D- and L-gulo- and allo-sugar-delta-lactones were synthesized in eight steps and approximately 20%, overall yield from furfural.

De Novo Asymmetric Synthesis of Anthrax Tetrasaccharide and Related Tetrasaccharide

A de novo asymmetric approach to the natural product anthrax tetrasaccharide 1 and an analogue 2 with an anomeric hexyl azide group has been developed from acetylfuran. The construction of the tetrasaccharide was achieved by a traditional [3 + 1] glycosylation strategy. An iterative diastereoselective palladium-catalyzed glycosylation, Luche reduction, diastereoselective dihydroxylation, and regioselective acylation were employed for the assembly of the L-rhamno-trisaccharide building block. The anthrose building block also required a palladium-catalyzed azide allylation and a triflate inversion to set the gluco-stereochemistry in addition to Luche reduction and dihydroxylation.

De Novo Asymmetric Synthesis of 8a-epi-Swainsonine

An enantioselective and diastereocontrolled approach to 8a-epi-d-swainsonine has been developed from achiral furfural. The key step to this synthesis was a one-pot procedure for the hydrogenolytic removal of two protecting groups and two intramolecular reductive amination reactions. The absolute stereochemistry was introduced by asymmetric Noyori reduction of furfuryl ketone. This route relies on diastereoselective palladium-catalyzed glycosylation to install the anomeric bond, and Luche reduction, diastereoselective dihydroxylation to set up the manno-stereochemistry of the indolizidine precursor.

Total synthesis of jadomycin A and a carbasugar analogue of jadomycin B.

The first syntheses of Jadomycin A and the carbosugar analogue of Jadomycin B have been achieved in 6 and 20 longest linear steps respectively. The key ring system of the aglycone was prepared by a 6π-electron electrocyclic ring closure and subsequent hemi-aminal ring closure. Acid sensitivity of the glycosidic bond in Jadomycin B precluded its synthesis but led to the synthesis of the carbasugar analogue.

An olefination approach to the enantioselective syntheses of several styryllactones

A flexible enantioselective route to highly functionalized α,β-unsaturated δ-lactones, has allowed for the syntheses of the styryllactones: altholactone, isoaltholactone, 3-epi-altholactone, 2-epi-altholactone and 5-hydroxy goniothalamin in 2.5, 10, 5, 1 and 13% overall yields from furfural, respectively. This approach derives its asymmetry by applying the Sharpless catalytic asymmetric dihydroxylation to vinylfuran. The resulting diols are produced in high enantioexcess and can be stereoselectively transformed into α,β-unsaturated-δ-lactones via a short highly diastereoselective oxidation and reduction sequence. Wittig olefination or Julia olefination reactions were used to introduce the phenyl group side chain either cis or trans selectively and these intermediates were further elaborated into the altholactone isomers via selective epoxidation reactions.

Digitoxin and a synthetic monosaccharide analog inhibit cell viability in lung cancer cells

Mechanisms of digitoxin-inhibited cell growth and induced apoptosis in human non-small cell lung cancer (NCI-H460) cells remain unclear. Understanding how digitoxin or derivate analogs induce their cytotoxic effect below therapeutically relevant concentrations will help in designing and developing novel, safer and more effective anti-cancer drugs. In this study, NCI-H460 cells were treated with digitoxin and a synthetic analog D6-MA to determine their anti-cancer activity. Different concentrations of digitoxin and D6-MA were used and the subsequent changes in cell morphology, viability, cell cycle, and protein expressions were determined. Digitoxin and D6-MA induced dose-dependent apoptotic morphologic changes in NCI-H460 cells via caspase-9 cleavage, with D6-MA possessing 5-fold greater potency than digitoxin. In comparison, non-tumorigenic immortalized bronchial and small airway epithelial cells displayed significantly less apoptotic sensitivity compared to NCI-H460 cells suggesting that both digitoxin and D6-MA were selective for NSCLC. Furthermore, NCI-H460 cells arrested in G(2)/M phase following digitoxin and D6-MA treatment. Post-treatment evaluation of key G2/M checkpoint regulatory proteins identified down-regulation of cyclin B1/cdc2 complex and survivin. Additionally, Chk1/2 and p53 related proteins experienced down-regulation suggesting a p53-independent cell cycle arrest mechanism. In summary, digitoxin and D6-MA exert anti-cancer effects on NCI-H460 cells through apoptosis or cell cycle arrest, with D6-MA showing at least 5-fold greater potency relative to digitoxin.

De Novo Asymmetric Synthesis of an α-6-Deoxyaltropyranoside as Well as its 2-/3-Deoxy and 2,3-Dideoxy Congeners

A highly divergent de novo asymmetric synthesis of benzyl alpha-6-deoxyaltropyranoside, benzyl alpha-ascarylopyranoside, benzyl alpha-amicetopyranoside, and benzyl alpha-digitoxopyranoside has been achieved via a common pyranone intermediate. The routes rely upon a palladium(0)-catalyzed glycosylation reaction and corresponding post-glycosylation transformations. The control of the absolute and relative stereochemical configuration came from a Noyori reduction of 2-acylfuran and subsequent diastereoselective introduction of other stereogenic centers.

Palladium‐Catalyzed Glycosylation Reaction: De‐Novo Synthesis of Trehalose Analogues

The unnatural L‐1,1‐α‐disaccharides were synthesized from acylfurans using a palladium‐catalyzed glycosylation reaction. The 1,1‐α‐manno‐disaccharides were achieved in six total steps starting from an acylfuran. The overall efficiency of this protocol was the use of highly diastereoselective palladium‐catalyzed glycosylation, reduction, and dihydroxylation. This paper is dedicated to Prof. Denis W. H. MacDowell during the year of his 80th birthday.

De novo asymmetric synthesis of cladospolide B-D: structural reassignment of cladospolide D via the synthesis of its enantiomer.

The enantioselective synthesis of cladospolide B, C, and (ent)-cladospolide D has been achieved in 11-15 steps from 1-nonyne. The route relies upon an alkyne zipper reaction to relay an ynone and dienoate functional groups across a nine carbon fragment, which enables a highly enantioselective Noyori ynone reduction and a diastereo- and regioselective Sharpless dihydroxylation of a dienoate. In addition to being a flexible approach to three members of the cladospolide natural products, this route for the first time correctly established the structure for cladospolide D.

Synthesis of SL0101 Carbasugar Analogues: Carbasugars via Pd-Catalyzed Cyclitolization and Post-Cyclitolization Transformations

A general approach to the stereoselective synthesis of 5a-carbasugars has been developed. The route mimics our palladium-catalyzed glycosylation/postglycosylation approach to carbohydrates in that it also utilizes a highly regio- and stereospecific palladium-catalyzed allylation and postglycosylation reaction sequence for the installation of either D- or L-cyclitols. This cyclitolization/postcyclitolization sequence was used for the enantioselective synthesis of a cyclitol analogue of SL0101, its D-sugar enantiomer, as well as several acetylation pattern analogues.