Modern methods for the radical deoxygenation of alcohols

Total syntheses of (+)- and (-)-pestalotiopsin A

An enantioselective total synthesis of both enantiomers of caryophyllene-type sesquiterpenoid pestalotiopsin A has been achieved, thereby establishing the absolute stereochemistry of natural (+)-pestalotiopsin A. Highlights of the synthesis include a [2 + 2] cycloaddition of N-propioloyl Oppolzer's camphorsultam and ketene dialkyl acetal and subsequent highly stereoselective 1,4-hydride addition/protonation, an aldol reaction of functionalized bicyclic lactone with aldehyde, an efficient intramolecular Nozaki-Hiyama-Kishi (NHK) reaction for the construction of the highly strained (E)-cyclononene ring, and a palladium-catalyzed reduction of allylic mesylate with retention of the E configuration.

Facile synthesis of D-lividosamine

A simple, four-step synthesis of D-lividosamine starting from N-acetyl-D-glucosamine via a furanosyl oxazoline intermediate is described.

Cycloaddition and one-carbon homologation studies in the synthesis of advanced iridoid precursors

A Diels-Alder cycloaddition approach to the sweroside aglycone intermediate of iridoids was explored using silylated butenolides and levoglucosenone as dienophiles under both Lewis acid and thermal conditions. Results of this study reveal no evidence that using less sterically demanding derivatives compromise the diastereofacial selectivity of the cycloaddition using silylated butenolides. Further chemistry performed on cycloadducts concentrated on the identification and management of methodologies suitable for its conversion into sweroside aglycone. During the course of these studies, a dehydrative cyclisation onto a preformed tetrahydrofuran ring to a bis-tetrahydrofuranoid moiety was unravelled. In addition studies on levoglucosenone-derived cycloadducts provide extensive insight into the conformational behaviour and reactivity. Further, the X-ray crystal structure of an alcohol intermediate from one-carbon homologation studies provided the first structural evidence confirming the diastereoselectivity of the cycloaddition procedure.

Organometallic enantiomeric scaffolding. Sequential semipinacol/1,5-"Michael-like" reactions as a strategic approach to bridgehead-quaternary center aza[3.3.1]bicyclics: application to the total synthesis of (-)-adaline

A nontraditional approach to the enantiocontrolled construction of quaternary center-bearing heteroatom-bridged bicyclo[3.3.1]nonanes (homotropanes) is reported that is based on organometallic enantiomeric scaffolding. This strategy takes advantage of the unique reactivity profiles of TpMo(CO)(2)(5-oxo-eta(3)-pyranyl) and TpMo(CO)(2)(5-oxo-eta(3)-pyridinyl) scaffolds, and features a molybdenum-mediated semipinacol/1,5-"Michael-like" reaction sequence to establish the quaternary center and synthesize the bridged bicyclic structure. An asymmetric total synthesis of (-)-adaline highlights this methodology.

Fine tuning of electrostatics around the internucleotidic phosphate through incorporation of modified 2',4'-carbocyclic-LNAs and -ENAs leads to significant modulation of antisense properties

In the antisense (AS) and RNA interference (RNAi) technologies, the native single-stranded 2'-deoxyoligonucleotides (for AS) or double-stranded RNA (for RNAi) are chemically modified to bind to the target RNA in order to give improved downregulation of gene expression through inhibition of RNA translation. It is shown here how the fine adjustment of the electrostatic interaction by alteration of the substituents as well as their stereochemical environment around the internucleotidic phosphodiester moiety near the edge of the minor grove of the antisense oligonucleotides (AON)-RNA heteroduplex can lead to the modulation of the antisense properties. This was demonstrated through the synthesis of various modified carbocyclic-locked nucleic acids (LNAs) and -ethylene-bridged nucleic acids (ENAs) with hydroxyl and/or methyl substituents attached at the carbocyclic part and their integration into AONs by solid-phase DNA synthesis. The target affinity toward the complementary RNA and DNA, nuclease resistance, and RNase H elicitation by these modified AONs showed that both the nature of the modification (-OH versus -CH(3)) and their respective stereochemical orientations vis-a-vis vicinal phosphate play a very important role in modulating the AON properties. Whereas the affinity to the target RNA and the enzymatic stability of AONs were not favored by the hydrophobic and sterically bulky modifications in the center of the minor groove, their positioning at the edge of the minor groove near the phosphate linkage resulted in significantly improved nuclease resistance without loss of target affinity. On the other hand, hydrophilic modification, such as a hydroxyl group, close to the phosphate linkage made the internucleotidic phosphodiester especially nucleolytically unstable, and hence was not recommended. The substitutions on the carbocyclic moiety of the carba-LNA and -ENA did not affect significantly the choice of the cleavage sites of RNase H mediated RNA cleavage in the AON/RNA hybrid duplex, but the cleavage rate depended on the modification site in the AON sequence. If the original preferred cleavage site by RNase H was included in the 4-5nt stretch from the 3'-end of the modification site in the AON, decreased cleavage rate was observed. Upon screening of 52 modified AONs, containing 13 differently modified derivatives at C6' and C7' (or C8') of the carba-LNAs and -ENAs, two excellent modifications in the carba-LNA series were identified, which synergistically gave outstanding antisense properties such as the target RNA affinity, nuclease resistance, and RNase H activity and were deemed to be ideal candidates as potential antisense or siRNA therapeutic agents.

Total synthesis studies on macrocyclic pipecolic acid natural products: FK506, the antascomicins and rapamycin

This chapter derives its inspiration from the challenges presented to total synthesis chemists, by a particular group of macrocyclic pipecolic acid natural products. Although there is considerable emphasis on the completed syntheses of the main characters (FK506 (1), the antascomycins (4 and 5) and rapamycin (7)), the overall complexity of the molecular problem has stimulated a wealth of new knowledge, including the development of novel strategies and the invention of new synthetic methods. The ingenious and innovative approaches to these targets have enabled new generations of analogues, and provided material to further probe the biology of these fascinating molecules. With pharmaceutical application as an immunosuppressant, as well as potential use for the treatment of cancer and neurodegenerative diseases, this family of natural products continues to inspire new and interesting science while providing solutions to healthcare problems of the world.

Site-selective catalysis of phenyl thionoformate transfer as a tool for regioselective deoxygenation of polyols

We report the application of peptide-embedded imidazoles as catalysts for the site-selective delivery of the phenyl thionoformate unit as a prelude to deoxygenation reactions of polyols. Methodology was developed that allows for the synthesis of thiocarbonyl derivatives based on a combination of additives that include N-alkylimidazoles and FeCl3 as co-catalysts. The use of this reagent combination leads to increased reaction rates and efficient yields relative to those of simple base-mediated reactions. In terms of controlling regioselectivity during the course of polyol modification, we found that histidine-containing peptides, in combination with FeCl3, could lead to modulation of the product distribution. Through screening of peptides and control of reaction conditions, products could be observed that reflected both the inherent preference of substrates and also reversal of inherent selectivity.

Studies on the synthesis of quartromicins a(3) and d(3): synthesis of the vertical and horizontal bis-spirotetronate fragments

Syntheses of the vertical (3) and horizontal (4) bis-spirotetronate units of quartromicins A3 and D3 are described, along with an efficient synthesis of alpha-hydroxy aldehyde exo-8b, a precursor to the exo-spirotetronate fragments 19 and 21.

The Genesis of the Reversible Radical Addition–Fragmentation–Transfer of Thiocarbonylthio Derivatives from the Barton–McCombie Deoxygenation: A Brief Account and Some Mechanistic Observations

The observations and reasoning leading to the discovery of the degenerative transfer of xanthates and related thiocarbonylthio derivatives are briefly described. A few synthetic applications are presented, and the consequences on the emergence of the RAFT and MADIX polymerization technologies as well as some mechanistic aspects are briefly discussed.