Synthetic studies towards batrachotoxin 1. A furan-based intramolecular diels-alder route to construct the a-d ring system

Synthesis of 3-Alkyl Oxazolidines, Derived from 2-Hydroxymethyl Piperidine, as Analytical Standards for the Analysis of Volatile Aldehydes in the Workplace

Hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-15 for the quantitative analysis of various aldehydes were obtained in good yield via the condensation reaction of 2-hydroxymethylpiperidine (2-HMP) with aldehydes under mild conditions. When acrolein was used, the bicyclic 17 was obtained. This novel compound has suitable physical characteristics for an analytical standard. The hexahydro-3-vinyl-3H-oxazolo[3,4-a]pyridine 16 can be obtained at higher temperatures using an excess of acrolein (3 equiv). Following the same procedure as for 16, but with an excess of 2-HMP (2 equiv), a diastereomeric mixture of 18/19, which are both bisadducts of 2-HMP with acrolein, was obtained. The latter mixture can be easily converted into pure 18. Mechanistically, a thorough 1H-NMR study did not show any evidence that the condensation reaction proceeded via an enamine. The reaction probably proceeded through an elusive hemiaminal and fleeting iminium ion, which underwent subsequent cyclization to give hexahydro-3-alkyl-3H-oxazolo[3,4-a]pyridines 4-16. The reaction pathways for the preparation of 4-18 are described.

Conceptual design and cost-efficient environmentally Benign synthesis of beta-lactams

Stereoselective preparation of diverse trans and cis β-lactams following different experimental conditions are executed. A variety of circumstances are critically analyzed. It has been found that the stereochemistry of the products depends on a number of parameters including the conditions of the procedures, composition of the Schiff bases and acid chlorides or equivalents, method of addition of the reactants, temperature of the process and nature of the media. Using some of the compounds and methods as described herein, a number of useful chemical transformations for the preparation of heterocycles are achieved. These methods include indium-catalyzed glycosylation of amino β-lactams, preparation of pyrrole-substituted β-lactams, cycloaddition with sterically congested Schiff bases towards β-lactams, Michael reaction for the preparation of polycyclic oxazepenes and synthesis of two chiral isomers of the thienamycin side chain. Most of the products are obtained stereospecifically and in optically active forms. Many reactions described here are catalytic and therefore, these are environmentally friendly.

Expeditious diastereoselective synthesis of medium ring heterocycle-fused chromenes via tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition

The first examples of highly diastereoselective tandem 8/9-endo-dig and 8-exo-dig hydroalkoxylation-formal-[4 + 2] cycloaddition are described for the synthesis of medium ring heterocycle-fused chromenes. TMS-alkynols preferred the exo-dig mode of hydroalkoxylation over the endo-dig mode leading to spiro-cyclic chromenes. The method could be used for the synthesis of linearly-fused ladder-like polyethers. A thia-heterocycle-fused chromene could be transformed into a complex bridged tricyclic ketal by a tandem carbene-insertion-[2,3]-sigmatropic shift.

Copper catalysed [3 + 2] cycloaddition with concomitant annulation: formation of 2,4-diaryl-1,4-oxazepan-7-ones via a ketenimine route

A novel strategy of copper catalyzed cascade reaction involving intramolecular nucleophilic addition to N-sulfonylketenimine gratifyingly furnishing 1,4-diaryl oxazepan-7-one has been described.

Chemical synthesis of the cardiotonic steroid glycosides and related natural products

The active components from the extracts of Digitalis, cardiotonic steroid glycosides, have been ingested by humans for more than 200 years as a medicinal therapy for heart failure and abnormal heart rhythms. The positive inotropic activity of the cardiotonic steroids that mediates clinically useful physiological effects in patients has been attributed largely to a high affinity inhibitory interaction with the extracellular surface of the membrane-bound sodium pump (Na(+)/K(+)-ATPase). However, previously unrecognized intracellular signaling pathways continue to be uncovered. This Review examines both partial and de novo synthetic approaches to the medicinally important and structurally captivating cardenolide and bufadienolide steroid families, with an emphasis on the stereocontrolled construction of the pharmacophoric aglycone (genin) framework.

Terpenoid-Alkaloids: Their Biosynthetic Twist of Fate and Total Synthesis

Terpenes and alkaloids are ever-growing classes of natural products that provide new molecular structures which inspire chemists and possess a broad range of biological activity. Terpenoid-alkaloids originate from the same prenyl units that construct terpene skeletons. However, during biosynthesis, a nitrogen atom (or atoms) is introduced in the form of β-aminoethanol, ethylamine, or methylamine. Nitrogen incorporation can occur either before, during, or after the cyclase phase. The outcome of this unique biosynthesis is the formation of natural products containing unprecedented structures. These complex structural motifs expose current limitations in organic chemistry, thus providing opportunities for invention. This review focuses on total syntheses of terpenoid-alkaloids and unique issues presented by this class of natural products. More specifically, it examines how these syntheses relate to the way terpenoid-alkaloids are made in Nature. Developments in chemistry that have facilitated these syntheses are emphasized, as well as chemical technology needed to conquer those that evade synthesis.

Synthesis of Polycyclic Oxanorbornanes via a Sequential Epoxyhexopyranoside Ring Contraction-Intramolecular Diels-Alder Reaction

Ring contraction of methyl 2,3-anhydro-6-deoxy-6-(furfurylthio)-alpha-D-allopyranoside with LiBr/TMU in boiling toluene gave the corresponding alpha,beta-unsaturated furanosidic aldehyde 1 together with a small amount of the corresponding oxanorbornene Diels-Alder adduct 2. Pumping a mixture of heptane and Et(3)N slowly through a SiO(2) column containing 1 and 2 shifted the ratio strongly toward 2. The products were eluted from the column by EtOAc/EtOH and chromatographed to give pure 2 and 1 in 79% and 15% yield, respectively. Compound 2 was submitted to a number of oxidations and reductions, which gave the oxanorbornanes 4-15. The structures were determined by 2D (1)H and (13)C NMR techniques and by a single-crystal X-ray investigation of compound 5.

Stereocontrolled Assembly of Cis or Trans Angularly Substituted Hydrindenes by the Unactivated Intramolecular Diels-Alder Reaction

The unactivated intramolecular Diels-Alder reaction has become a powerful tool for the construction of polycyclic natural products. Nevertheless, the factors that govern diastereoselectivity in these cyclizations have not been fully understood. We here report that through the choice of the proper substituents, it is possible to make the unactivated intramolecular Diels-Alder reaction proceed to give either the cis angularly-substituted hydrindene 12b or the trans product 19a.