Current Organic Chemistry - Volume 6, Issue 7, 2002

Oxabicyclic compounds are valuable intermediates for the synthesis of a large array of interesting structures on the biological or pharmaceutical point of view.In this way a key transformation in many synthesis using oxabicyclic intermediates is the cleavage of the oxigen bridge to produce functionalized cyclohexane or cycloheptane derivatives. In order to make this transformation synthetically useful, the introduction of an element of regio- and stereocontrol in the rigid skeleton has been proved to be necessary. A phenylsulfonyl functionality attached to the double bond of the oxabicyclic derivatives render this transformation totally regio- and stereoselective through two different methodologies: a) Michael addition-ring opening sequence and b) strain directed ring opening reaction.Synthetic applications of both methods including the synthesis of carbasugars, cyclitols, alcaloids and polypropionate fragments will be presented and discused.

Copper-catalyzed O- and N-arylation reactions involving triarylbismuth diacetates or aryllead triacetates are ligand coupling reactions which were discovered in the eighties independently by Barton's and Dodonov's groups. These reagents lead generally to efficient arylation under mild neutral conditions (room temperature or 40 dgree C, no added basic reagent). Recently, the scope of these main group metal mediated reactions was broadened when Chan and Evans reported that organoboron compounds can be used as the source of the organic aryl ligand. Subsequently, organosiloxanes, organostannanes and diaryliodonium salts proved also to be efficient reagents for these mild copper catalyzed O- and N-arylation reactions. Organoantimony compounds were also used but appeared to be less efficient reagents.

The Baylis-Hillman reaction is one of the most powerful carbon-carbon bond-forming methods in organic synthesis. The Baylis-Hillman adducts have been widely used as intermediates for the synthesis of useful acyclic compounds as well as many cyclic compounds including heterocyclic ones. In this review the synthetic applications of the Baylis-Hillman adducts toward cyclic compounds have been categorized according to the key reaction type.

The extension of efficient cation radical chain cycloaddition chemistry to the difunctional context has recently provided a novel and efficient mechanism for the polymerization of monomers possessing readily ionizable, electron-rich alkene moieties. The new mechanism, cation radical chain cycloaddition polymerization, appears to provide the most efficient method presently available for cycloaddition polymerization and is unique in affording cyclobutapolmers, i.e., polymers in which the monomer units are interconnected via cyclobutane linkages. In addition to cyclobutapolymerization, the new mechanism is also capable of yielding Diels-Alder polymers when one functionality is an ionizable alkene moiety and the other is a conjugated diene moiety. Further Diels-Alder cation radical chain cycloaddition polymerization can proceed either directly or via cyclobutapolymerization followed by facile cation radical vinylcyclobutane rearrangement. The new method is also exceptional in that propenyl functionalities are usually preferred over vinyl moieties. Reactions are typically initiated via catalytic amounts of a stable cation radical salt, tris(4-bromophenyl)aminium hexachloroantimonate in dichloromethane solvent, and are extremely fast, reaching high (often >100 000)molecular weights in 2-5 minutes. In several cases, polymerizations have also been initiated by electrochemical oxidation and photosensitized electron transfer. Although homopolymerization is usually by far the most efficient approach to cation radical cycloaddition polymerization, a number of copolymerizations have also been investigated and will be discussed in the present review.The recently established use of cation radical Diels-Alder cycloadditions of monofunctional, highly electron-rich substrates such as propenylcarbazoles to generate monomers of interest in conection with ring-opening metathesis polymerization will also be presented. The cycloadditions of various electron-rich monomers to 1,3- cyclopentadiene efficiently generate norbornene monomers , which readily undergo ring-opening metathesis polymerization (ROMP) to yield electron rich polymers.