Separation tagging with cyclodextrin-binding groups: Mitsunobu reactions with bis-(2-(1-adamantyl)ethyl) azodicarboxylate (BadEAD) and bis-(1-adamantylmethyl) azodicarboxylate (BadMAD)

A new method for separation tagging with cyclodextrin-binding groups is introduced and is exemplified in the context of the Mitsunobu reaction with adamantyl tags. HPLC experiments showed that molecules containing adamantyl groups were especially well retained on Sumichiral OA7500 β-methylated cyclodextrin bonded silica columns relative to many other types of molecules. Two new Mitsunobu reagents, bis-(1-adamantylmethyl) azodicarboxylate (BadMAD) and bis-(2-(1-adamantyl)ethyl) azodicarboxylate (BadEAD), were prepared, used in typical Mitsunobu reactions and separated with both β-methylated cyclodextrin bonded silica and standard silica.     

Separation-friendly Mitsunobu reactions: a microcosm of recent developments in separation strategies

The Mitsunobu reaction is famous for its scope and power, but infamous for its separation headaches. Typically, the target product is enticed away from the reagent-derived byproducts by careful chromatography. The use of polymer-bound Mitsunobu reagents solves only half of the problem, because polymer-bound diethyl azodicarboxylate (DEAD) and phosphine reagents cannot be employed simultaneously. This article classifies, compares, and contrasts various emerging strategies for product isolation in Mitsunobu reactions. Because so many different strategies have been used, the Mitsunobu reaction is a microcosm for the new field of strategy level separations.     

An aldol-based build/couple/pair strategy for the synthesis of medium- and large-sized rings: discovery of macrocyclic histone deacetylase inhibitors

An aldol-based build/couple/pair (B/C/P) strategy was applied to generate a collection of stereochemically and skeletally diverse small molecules. In the build phase, a series of asymmetric syn- and anti-aldol reactions were performed to produce four stereoisomers of a Boc-protected γ-amino acid. In addition, both stereoisomers of O-PMB-protected alaninol were generated to provide a chiral amine coupling partner. In the couple step, eight stereoisomeric amides were synthesized by coupling the chiral acid and amine building blocks. The amides were subsequently reduced to generate the corresponding secondary amines. In the pair phase, three different reactions were employed to enable intramolecular ring-forming processes: nucleophilic aromatic substitution (S(N)Ar), Huisgen [3+2] cycloaddition, and ring-closing metathesis (RCM). Despite some stereochemical dependencies, the ring-forming reactions were optimized to proceed with good to excellent yields, providing a variety of skeletons ranging in size from 8- to 14-membered rings. Scaffolds resulting from the RCM pairing reaction were diversified on the solid phase to yield a 14?400-membered library of macrolactams. Screening of this library led to the discovery of a novel class of histone deacetylase inhibitors, which display mixed enzyme inhibition, and led to increased levels of acetylation in a primary mouse neuron culture. The development of stereo-structure/activity relationships was made possible by screening all 16 stereoisomers of the macrolactams produced through the aldol-based B/C/P strategy.     

Convergent synthesis of complex diketopiperazines derived from pipecolic acid scaffolds and parallel screening against GPCR targets

A convergent approach to highly functionalized diketopiperazines (DKPs) using enantioenriched pipecolic acids is described. Scandium triflate-catalyzed [4 + 2] aza-annulation was employed to produce stereochemically well-defined building blocks. A resin "catch and release" strategy was devised to convert annulation products to pipecolic acid monomers. Complex diketopiperazines were efficiently assembled utilizing one-pot cyclodimerization of pipecolic acids. Massively parallel screening of the complex DKPs against a panel of molecular targets identified novel ligands for a number of G-protein-coupled receptors (GPCRs).     

Burn-up measurements on nuclear reactor fuels using high performance liquid chromatography

Burn-up measurements on thermal as well as fast reactor fuels were carried out using high performance liquid chromatography (HPLC). A column chromatographic technique using di-(2-ethylhexyl) phosphoric acid (HDEHP) coated column was employed for the isolation of lanthanides from uranium, plutonium and other fission products. Ion-pair HPLC was used for the separation of individual lanthanides. The atom percent fissions were calculated from the concentrations of the lanthanide (neodymium in the case of thermal reactor and lanthanum for the fast reactor fuels) and from uranium and plutonium contents of the dissolver solutions. The HPLC method was also used for determining the fractional fissions from uranium and plutonium for the thermal reactor fuel.     

Carrier-free separation of 22Na from magnesium by chromatographic techniques

Carrier-free ²²Na was separated from bulk quantities of magnesium by both ion exchange and extraction chromatographic techniques. An extraction chromatographic procedure based on di-(2-ethylhexyl) phosphoric acid (HDEHP) coated on to an inert support (Amberlite XAD-7) was developed for the first time for separation of sodium from magnesium. The sorption behavior for sodium and magnesium was studied as a function of percentage of HDEHP loaded on to the inert support as well as pH of aqueous phase. These data were used to arrive at the optimum conditions of separation. In addition, carrier free ²²Na was also separated from magnesium using ion exchange chromatographic technique.     

Diversity-oriented synthesis of 13- to 18-membered macrolactams via ring-closing metathesis

An efficient build/couple/pair approach to diversity-oriented synthesis was employed to access several structurally complex macrolactams. In this paper, we describe the successful evaluation of ring-closing metathesis toward the systematic generation of skeletal diversity. By appropriately varying the nature and chain length of the alkenol fragment, a diverse collection of 13- to 18-membered macrolactams were obtained.