The Pauson-Khand (-type) reaction is a [2+2+1] cycloaddition of an alkyne, alkene and carbon monoxde, which gives synthetically useful cyclopentenones. The carbonylative coupling is mediated or catalyzed by various transition metal complexes. This review summarizes the recent advances in the catalytic Pauson-Khand (-type) reaction.
Optically active β-ketoiminato cobalt complexes were recently developed as high performance catalysts for eanantioselective borohydride reduction of aromatic ketones or imines to afford the optically active alcohols or amines with high enantiomeric excesses. This enantioselective reduction is carried out using a precisely pre-modified borohydride with alcohols such as tetrahydrofurfuryl alcohol, ethanol and methanol, and high optical yields are realized by choosing the appropriate alcohol as modifiers and a suitable β-ketiominato ligand of the catalyst. The enantioselective borohydride reduction has been successfully applied to the efficient preparations of optically active 1, 3-diaryl-1, 3-propanediols, the stereoselective reduction of diacylferrocenes, and the dynamic and/or kinetic resolution of 1, 3-dicarbonyl compounds to afford the corresponding optically active anti-aldol compounds. In the presence of a catalytic amount of the cobalt complex and modified borohydride, the asymmetric 1, 4-reduction of β, β-disubstituted α, β-unsaturated carboxamides proceeded with high enantioselectivity, and the 1, 4-reduction of the α-substituted α, β-unsaturated carboxamides followed by enantioselective in situ protonation smoothly proceeded to afford the optically active α-substituted carboxamides with good enantiomeric excesses.
We have recently found the novel character of twisted 1, 3-dienes in two types of reactions; one is photosensitized oxygenation and another is anion formation with base. In the former reaction, the selective allene formation resulting from the ene reaction of the vinyl hydrogen rather than the allyl hydrogen was disclosed. On the basis of MM and MO calculations, the abnormally higher reactivity of the vinyl hydrogen would be rationalized by considering the σ-π orbital interaction between the vinyl C-H bond and another double bond. Diastereoselective allene formation resulting from the chiral projection from sp3 carbon into an allene was also realized. In the latter reaction, the selective vinyl anion formation rather than benzylic anion formation was observed in the twisted styrene derivatives. In this paper, we will describe our recent study on twisted 1, 3-dienes in detail, in addition to the recent topics by another research groups.
Synthesis and reaction of cyclic N, O-acetals can be of potential use and versatility in construction of polycyclic nitrogen heterocycles and alkaloid synthesis, as demonstrated in this review. The cyclic N-acyl-N, O-acetals, readily accessible by condensation of hydroxy-amines with ketocarboxylic acids, was transformed into the 2-azaspirocycles via intramolecular olefin-iminium cyclization mediated by TiCl4. Construction of the 6, 6-disubstituted 2-piperidone based on Lewis acid-mediated allylation of the cyclic N-acyl-N, O-acetals with allyltrimethylsilane led to a ladybird alkaloid adalinine in racemic form. This allylation reaction was performed with the cyclic N-acyl-N, O-acetal incorporating (R) -2- (1-aminoethyl) phenol as a chiral auxiliary to afford the (R) -6, 6-disubstituted 2-piperidone with high diastereoselectivity, which, on cleavage of the chiral auxiliary, provided (-) -adalinine. The enantioselective Lewis acid-mediated allylation of the cyclic N-acyl-N, O-acetals developed herein was successfully applied to the asymmetric syntheses of Dendrobatid alkaloids (-) -indolizidine 195B and 209D, and marine alkaloids (+) -stellettamide A and (-) -stellettamide B. On the other hand, intramolecular N, 0-acetalization of the keto-hydroxy-amines led to the one-step construction of the azabicyclononanes, which was then converted to the morphans. The azabicyclononane was subjected to bridgehead C-C bond formation with organometallic reagents via “anti-Bredt iminium ion” generated by treatment of aluminum reagents as Lewis acids. The 1-alkyl- and 1-vinylazabicyclononanes thus obtained were converted to the azatricyclododecane which is the core ring system of the novel immunosupressant FR 901483. The azabicyclononane formed by the N, O-acetalization of the keto-hydroxyamines was also elaborated into the diazatricyclododecane core of marine alkaloids madangamines.
The ring transformation is a powerful procedure for the preparation of polyfunctionalized compounds, which are not easily available by reported methods. 3-Methyl-5-nitropyrimidin4 (3H) -one (3) has a suitable structure for this purpose to cause three types of ring transformations. When the reaction proceeds at 2- and 6-positions of 3, 3, 5-difunctionalized 4-pyridones 7, 4, 5-disubstituted pyrimidines 8, functionalized 4-aminopyridines 17 can be obtained. The reaction at 4- and 6-positions of 3 affords 5, 6-disubstituted 3-nitro-2-pyridones 9. Pyrimidinone 3 behaves as a synthetic equivalent of activated diformylamine 4 in the former case, and as that of α-nitroformylacetic acid 10 in the latter case. The control of the selectivity of the above reactions is also discussed. Since all of the reactions are performed with simple experimental manipulations, the present ring transformations are useful methods for the preparation of functionalized azaheterocyclic compounds.