Mark A. Semones

Ligand effects in the rhodium(II) catalysed reactions of diazoamides and diazoimides

Abstract A range of substituted α-diazoamides 1–8 and diazoimides 9–12 was prepared from the corresponding amines or amides. Rhodium(II) catalysed decomposition of the diazoamides resulted in attack on the aromatic ring to give oxindoles or attack on the alkyl group to give either β-lactams or cycloheptapyrrolones. The chemoselectivity of the rhodium carbenoid intermediate was dependent on the metal ligands, fluorinated carboxamides strongly promoting attack on aromatic rings in preference to other processes. Decomposition of the diazoimides resulted in intramolecular attack on the carbonyl group to give an ylide which could be trapped inter- or intramolecularly. X-Ray crystal structures are reported for the diazo compounds 2 and 4, the indoles 17 and 25, the β-lactam 20, the cycloheptapyrrolones 24 and 28, the dimer 29 and the Pictet Spengler product 39.

Rhodium carbenoid mediated cyclizations. Intramolecular cyclopropanation and C-H insertion reactions derived from type II o-alkynyl substituted α-diazoacetophenones

Abstract The Rh(II) catalyzed reaction of alkynyl substituted diazo ketones gives rise to products derived from migration of the metal to the remote acetylenic carbon atom.

Details associated with the bimolecular 1,4-dipolar cycloaddition reaction of cross-conjugated heteroaromatic betaines

Abstract A series of 3,3-disubstituted bicyclic anhydro-4-hydroxy-2-oxo-1,3-thiazinium hydroxides are easily prepared from the reaction of 3H-thiolactams with 1,3-bielectrophiles. These cross-conjugated heteroaromatic betaines undergo regio- and diastereospecific 1,4-dipolar cycloaddition with electron-rich and electron-deficient π-bonds to produce 1,4-cycloadducts containing a carbonyl sulfide bridge. A representative betaine dipole and a 1,4-cycloadduct were characterized by single crystal X-ray determinations. In certain cases, the initially formed cycloadduct can be induced to lose COS on further heating. The frontier orbital coefficients of the thiazinium betaine were determined by semi-empirical MOPAC calculations with the PM3 Hamiltonian. The HOMO of the 1,4-dipole is dominant for reactions with electron-deficient dipolarophiles such as N-phenylmaleimide, while the LUMO becomes important for cycloaddition to more electron-rich species such as ynamines or vinyl ethers.

An expedient synthesis of epi-eburnamenine via an intramolecular 1,4-dipolar cycloaddition reaction

Abstract The intramolecular 1,4-dipolar cycloaddition of an anhydro-4-hydroxy-2-oxo-1,3-thiazium hydroxide across a tethered indole π-bond has been used for the construction of the pentacyclic skeleton of epi -16,17-dihydroeburnamenine.

Synthesis of oxa-bicyclic ring systems via a tandem Rh(II) catalyzed cyclization-cycloaddition sequence

Abstract A new annulation sequence leading to oxabicyclic ring systems is effected by treating o-alkynyl substituted α-diazoacetophenones containing tethered carbonyl groups with Rh(II) carboxylates in the presence of CC π-bonds.

Bimolecular 4+2-cycloaddition reactions of cross conjugated betaines with Electron rich π-systems

Abstract Bicyclic anhydro-2-oxo-4-hydroxy-1,3-thiazinium hydroxides undergo 1,4-dipolar cycloadditions with various electron rich π-systems to give 4+2-cycloadducts which on further heating, extrude carbonyl sulfide producing substituted α-pyridones.