Iodine(V) reagents in organic synthesis. Part 3. New routes to heterocyclic compounds via o-iodoxybenzoic acid-mediated cyclizations: generality, scope, and mechanism

The discovery and development of the o-iodoxybenzoic acid (IBX) reaction with certain unsaturated N-aryl amides (anilides) to form heterocycles are described. The application of the method to the synthesis of delta-lactams, cyclic urethanes, hydroxy amines, and amino sugars among other important building blocks and intermediates is detailed. In addition to the generality and scope of this cyclization reaction, this article describes a number of mechanistic investigations suggesting a single electron transfer from the anilide functionality to IBX and implicating a radical-based mechanism for the reaction.     

N-phenylselenophthalimide (NPSP) : A valuable selenenylating agent

The phenylseleno group (PhSe) has evolved in recent years as a very useful and versatile functionality. Its facile introduction into organic molecules and its subsequent oxidative or reductive removal, has allowed many important synthetic transformations.^1–7 Due to the fact that, similarly to halogens, it can exist either as an electrophilic species (PhSe⁺) or as a nucleophilic one (PhSe^-), this group can be introduced either via nucleophilic substrates (e.g. carbanions, olefins), or via electrophilic ones (e.g. epoxides, halides), as illustrated in Scheme 1. Another valuable aspect of the phenylseleno group is that it can be readily oxidized to the corresponding selenoxide(PhSe(O)—), which undergoes β-hydrogen abstraction and syn-elimination to form olefins, under relatively mild conditions (Scheme 2(a)). Furthermore, this group can be substituted with hydrogen, upon the action of an appropriate reducing agent (Scheme 2(b)).The great synthetic utility of the phenylseleno group is apparent from its extensive utilization in numerous natural products syntheses,¹ as well as many other synthetic studies.^2–7     

Organoselenium-induced cyclizations in organic synthesis

A number of organoselenium reagents are introduced as efficient initiators of ring closures leading from unsaturated substrates to lactones, cyclic ethers, cyclic thioethers, N-heterocycles and carbocycles. These cyclizations often proceed with high ring selectivity and stereoselectivity and are accompanied by the incorporation of the phenylseleno group (PhSe) into the final product. Methods are described for the effective removal of this group (PhSe) by oxidation or reduction achieving unsaturation or saturation. Finally the successful application of this Se-based methodology to the synthesis of stable and biologically active prostacyclins is outlined. Representative experimental procedures are included.     

The importance of the geometrical factor in nuclear activation analysis

When a beam of radiation is used as a probe in order to study the elemental composition of an object, the factors involved in obtaining maximum detection sensitivity include target homogeneity, beam uniformity and the solid angle subtended between target and detector. Here we have investigated, both theoretically and experimentally, the significance of these factors in an experimental facility for in-vitro prompt gamma-ray neutron activation analysis and an arrangement used in in-vivo activation analysis. The correction factor to the solid angle, to account for non-uniformity, and the optimisation of reaction rate and solid angle are considered.     

Total synthesis and biological evaluation of (-)apicularen A and analogues thereof

Apicularen A (1) and related benzolactone acylenamines belong to a growing class of novel natural products possessing highly cytotoxic properties. The challenging structure of 1 includes a 10-membered macrolactone ring, a tetrahydropyran system, an o,m-substituted phenol and a doubly unsaturated acyl group attached on the side chain enamine functionality. The total synthesis of apicularen A described herein involves a strategy equivalent to its proposed biosynthesis and entails a reiterative two-step procedure featuring allylation and ozonolytic cleavage to grow the molecule's chain by one acetate unit at a time. The developed synthetic technology was applied to the construction of a series of apicularen A analogues whose biological evaluation established a set of structure-activity relationships in this new area of potential importance in cancer chemotherapy.     

Total synthesis of everninomicin 13,384-1--Part 1: retrosynthetic analysis and synthesis of the A1B(A)C fragment

In this first of a series of four articles we introduce everninomicin 13,384-1 (1), a powerful antibiotic effective against drug resistant bacteria, as a target for total synthesis and discuss its retrosynthetic analysis. From the three defined fragments required for the synthesis (2: A1B(A)C fragment; 4: DE fragment; 5: FGHA2 fragment), we describe herein two approaches to the A1B(A)C block. The first strategy relied on an olefin metathesis reaction to construct a common intermediate for rings B and C, but was faced with final protecting group problems. The second, and successful approach, involved a 1,2-phenylsulfeno migration and a sulfur directed glycosidation procedure to link rings B and C, as well as an acyl fluoride intermediate to install the sterically hindered aryl ester moiety (ring A1). The final stages of the synthesis of the required 2-phenylseleno glycosyl fluoride 2 required introduction of a phenylseleno group at C-1 of ring C followed by a novel, DAST-promoted 1,2-migration to produce the desired 2-beta-phenylseleno glycosyl fluoride moiety.