Organocatalytic Asymmetric Conjugate Additions to Cyclopent-1-enecarbaldehyde: A Critical Assessment of Organocatalytic Approaches towards the Telaprevir Bicyclic Core.

In the context of a programme directed at the manufacture of telaprevir, eight possible approaches to its bicyclic α-amino acid core, based on organocatalytic enantioselective conjugate additions to cyclopent-1-enecarbaldehyde, were identified and preliminarily explored. Four reactions, delivering advanced intermediates en route to the target amino acid, were selected for a thorough optimisation. Three of this reactions involved iminium ion catalysis with a prolinol catalyst (addition of nitromethane, nitroacetate and acetamidomalonate) and one was based on a Cinchona-derived phase-transfer catalyst (addition of glycine imines). A careful choice of additives allowed lowering of the catalyst loading to 0.5 mol% in some cases. The preparation of intermediates that would give access to the core of telaprevir in good yields and enantioselectivities by exploiting readily available substrates and catalysts, highlights the potential of organocatalytic technology for a cost-effective preparation of pharmaceuticals.

An enantioselective formal synthesis of montelukast sodium.

A formal synthesis of the antiasthma drug montelukast sodium is described, wherein the key chiral diol intermediate was accessed with greater convergence of the C-C bond-forming steps as compared to previous routes. Improved synthetic efficiency was achieved by deploying homogeneous metal-based catalysis in two pivotal steps. In the first, a tandem Mizoroki-Heck reaction and double-bond isomerization between a previously known allyl alcohol intermediate and a hindered 2-(2-halophenyl)propan-2-ol secured direct access to the 3-(2-(2-hydroxypropan-2-yl)phenyl)-1-phenylpropan-1-one moiety in the product. In the second step, asymmetric hydrogenation of the ketone functionality in the Mizoroki-Heck reaction product provided a convenient method to introduce the benzylic alcohol chiral center and obtain the desired chiral diol precursor of montelukast sodium. A detailed catalyst screening led to the identification of ((R)-Xyl-BINAP)((R,R)-DPEN)RuCl2 as a catalyst that afforded an enantioselectivity of 99% ee in the hydrogenation step on a multigram lab scale at a molar substrate:catalyst loading of 5000:1.

Dynamic kinetic resolution of dehydrocoronamic acid.

Dehydrocoronamic acid can be racemised by dehydration of an N-acyl derivative to an azlactone, which undergoes facile racemisation. For the N-trifluoroacetyl derivative, the racemisation process was combined with an enzymatic resolution, to achieve a dynamic kinetic resolution process by which the racemate can be converted to either enantiomer.

Bis-(2,5-diphenylphospholanes) with sp2 carbon linkers: synthesis and application in asymmetric hydrogenation.

Four chiral diphosphine ligands consisting of bis(2,5-diphenylphospholan-1-yl) groups connected by the sp(2) carbon linkers 2,3-quinoxaline ((S,S)-Ph-Quinox), 2,3-pyrazine ((S,S)-Ph-Pyrazine), maleic anhydride ((S,S)-Ph-MalPhos), and 1,1'-ferrocene ((S,S)-Ph-5-Fc) were synthesized, and their cationic [rhodium(I)(COD)] complexes were prepared. These complexes were tested in asymmetric hydrogenation of functionalized olefins. [((S,S)-Ph-Quinox)Rh(COD)]BF4 showed high activity and selectivity against itaconate and dehydroamino acid substrates. The corresponding (S,S)-Ph-Pyrazine and (S,S)-Ph-MalPhos complexes exhibited lower activities and selectivities. [((S,S)-Ph-5-Fc)Rh(COD)]BF4 showed high activity with low selectivity for these substrates, but high activity and selectivity against 2-C-substituted cinnamate salts, whereas rhodium complexes of (S,S)-Ph-Quinox and (R,R)-Ph-BPE showed low activity and selectivity against 2-C-substituted cinnamate salts.

Identifying stereoisomers of the asymmetric hydrogenation catalyst [Me-BPE-Rh(COD)]+BF4 -.

The performance of a catalyst used in asymmetric synthesis is likely to be dependent upon its stereoisomeric purity. An impurity was detectable by (31)P NMR in early development batches of the asymmetric hydrogenation catalyst [(S,S)-Me-BPE-Rh(COD)](+)BF(4) (-) made from the ligand bis((2S,5S)-2,5-dimethylphospholano)ethane [(S,S)-Me-BPE]. Its identity as a stereoisomer with one chiral and one meso-phospholane ring was deduced by comparison of the (31)P NMR spectra and GC traces of the ligand with a deliberately synthesized mixture of isomers. Interestingly, the impurity corresponded to a trans-meso isomer formed by thermal (200 degrees C) pyramidal inversion at phosphorus of the initially synthesized cis-meso-phospholane when the ligand was purified by distillation. Low levels of this trans-meso/chiral impurity do not significantly impair the enantioselectivity of the rhodium complex as an asymmetric hydrogenation catalyst, but high levels of stereochemical impurities resulted in a loss of both enantioselectivity and activity. Therefore it is indeed important to establish that a catalyst used in asymmetric catalysis is sufficiently stereoisomerically pure. Owing to strict control of the stereochemical purity of the key hexane-2,5-diol intermediate, the impurity is not detected in production batches.

A convergent synthesis of the 11-oxa prostaglandin analogue AL-12182.

[reaction: see text] The 11-oxa prostaglandin analogue AL-12182 1 has potent topical ocular hypotensive activity. A convergent and concise general synthesis of this class of prostanoid was developed employing a stereoselective coupling reaction between a tetrahydrofuran core electrophile and a nucleophilic omega side chain component, providing a route that should be suitable for commercial scale production. The tetrahydrofuran core was assembled from dimethyl d-malate using a stereoselective beta-hydroxy ester dianion alkylation reaction.