William Guy Whittingham

Directing Group Enhanced Carbonylative Ring Expansions of Amino-Substituted Cyclopropanes: Rhodium-Catalyzed Multicomponent Synthesis of N-Heterobicyclic Enones

Aminocyclopropanes equipped with suitable N-directing groups undergo efficient and regioselective Rh-catalyzed carbonylative C-C bond activation. Trapping of the resultant metallacycles with tethered alkynes provides an atom-economic entry to diverse N-heterobicyclic enones. These studies provide a blueprint for myriad N-heterocyclic methodologies.

Reversible C-C bond activation enables stereocontrol in Rh-catalyzed carbonylative cycloadditions of aminocyclopropanes

Upon exposure to neutral or cationic Rh(I)-catalyst systems, amino-substituted cyclopropanes undergo carbonylative cycloaddition with tethered alkenes to provide stereochemically complex N-heterocyclic scaffolds. These processes rely upon the generation and trapping of rhodacyclopentanone intermediates, which arise by regioselective, Cbz-directed insertion of Rh and CO into one of the two proximal aminocyclopropane C-C bonds. For cyclizations using cationic Rh(I)-systems, synthetic and mechanistic studies indicate that rhodacyclopentanone formation is reversible and that the alkene insertion step determines product diastereoselectivity. This regime facilitates high levels of stereocontrol with respect to substituents on the alkene tether. The option of generating rhodacyclopentanones dynamically provides a new facet to a growing area of catalysis and may find use as a (stereo)control strategy in other processes.

Studies Towards the Total Synthesis of Cycloaraneosene and Ophiobolin M: A General Strategy for the Construction of the 5−8 Bicyclic Ring System

The synthesis of the core unit of cycloaraneosene and ophiobolin M has been investigated, following a general strategy applicable to both 5-8 bicyclic systems. The synthetic strategy includes a ring-closing metathesis reaction to generate the central eight-membered ring as well as a palladium-mediated coupling of a Grignard reagent to introduce the exocyclic side-chain. The stereochemistry of the ring junction is also discussed and moderate diastereoselectivity has been achieved

Modular Access to Substituted Azocanes via a Rhodium-Catalyzed Cycloaddition–Fragmentation Strategy

A short entry to substituted azocanes by a Rh-catalyzed cycloaddition–fragmentation process is described. Specifically, exposure of diverse N-cyclopropylacrylamides to phosphine-ligated cationic Rh(I) catalyst systems under a CO atmosphere enables the directed generation of rhodacyclopentanone intermediates. Subsequent insertion of the alkene component is followed by fragmentation to give the heterocyclic target. Stereochemical studies show, for the first time, that alkene insertion into rhodacyclopentanones can be reversible.

A new reactivity pattern for vinyl bromides: cine-substitution via palladium catalysed C–N coupling/Michael addition reactions

Palladium catalysed C-N bond formation can be used to convert vinyl bromides to the corresponding enamines, which are reacted in situ with alkylidene malonates to provide Michael adducts. The overall transformation results in cine-substitution of the starting vinyl bromide.

Efficient synthesis of protected cyclopropyl β-aspartylphosphates

The in situ reaction of protected dehydroamino acids with derivatives of vinyldiazomethane leads to good to excellent yields of vinyl cyclopropanes via 3 + 2 dipolar cycloaddition followed by N2 extrusion. Chromatographic separation of the cyclopropane diastereomeric products, followed by characterisation by 1H NMR and X-ray crystallography allowed the cis and trans diastereomers to be easily identified. Oxidative cleavage of the vinyl moiety then led directly to protected cyclopropane aspartic acid derivatives in three steps from commercially available materials. These compounds were converted to protected methylenephosphonate, difluoromethylenephosphonate and phosphoramidate analogues of β-aspartyl phosphate.

New Initiation Modes for Directed Carbonylative C–C Bond Activation: Rhodium-Catalyzed (3 + 1 + 2) Cycloadditions of Aminomethylcyclopropanes

Under carbonylative conditions, neutral Rh(I)-systems modified with weak donor ligands (AsPh3 or 1,4-oxathiane) undergo N-Cbz, N-benzoyl, or N-Ts directed insertion into the proximal C–C bond of aminomethylcyclopropanes to generate rhodacyclopentanone intermediates. These are trapped by N-tethered alkenes to provide complex perhydroisoindoles.

Fast and Flexible Synthesis of Pantothenic Acid and CJ-15,801

The fast and efficient syntheses of pantothenic acid and the antiparasitic agent CJ-15,801 have been achieved starting from a common imide unit through the selective manipulation of enamide intermediates.

Diastereoselective SmI2 mediated cascade radical cyclisations of methylenecyclopropane derivatives—syntheses of paeonilactone B and 6-epi-paeonilactone A

The SmI2 mediated cascade cyclisations of several methylenecyclopropyl ketones have been examined and found to proceed with high diastereoselectivity, which is critically dependent on the presence of HMPA in the reaction. In one case the radical species at the end of the cascade sequence underwent an unexpected and highly stereoselective dimerisation. The cascade methodology has been applied to a short synthesis of (±)-paeonilactone B and of (±)-6-epi-paeonilactone A.

Synthesis of (+)-crocacin D and simplified bioactive analogues

The total synthesis of (+)-crocacin D has been achieved in 15 steps (9 isolated intermediates) and 14% overall yield from commercially available starting materials and using (+)-crocacin C as a key intermediate. A number of simplified analogues and their biological activities are also reported.