J.L. van der Baan

Palladium(II)-catalyzed claisen rearrangement of allyl vinyl ethers

Abstract The Claisen rearrangement of allyl vinyl ethers is catalyzed by PdCl2(CH3CN)2, provided that alkyl substituents protect the vinyl ether double bond from coordination by the metal catalyst.

THE TOTAL SYNTHESIS OF THE ANTIBIOTIC MALONOMICIN (K16)

Abstract The total synthesis of the antibiotic malonomicin has been accomplished by a convergent synthesis in which the crucial step is a very mild coupling of the dipeptide sidechain to the 3-position of the pyrrolidin-2,4-dione ring.

3-Methylenetetrahydrofurans and 3-methylenepyrrolidines by addition of 2-bromozincmethyl-2-propenyl ethers to aldehydes, ketones and imines followed by pd(O)-catalyzed cyclization

Abstract Reaction of 2-(benzyloxymethyl)- and 2-(phenoxymethyl)allylzinc bromides 1a and 1b with aldehydes, ketones and imines afforded the addition products 3 and 6 , which underwent Pd(O)-catalyzed cyclization to the tetrahydrofurans 4 and pyrrolidines 7 .

Oxygen and nitrogen heterocycles by intramolecular magnesium- and zinc-ene reactions; Methylenecyclopentanes by Pd(0) - catalyzed isomerization of 5-(bromozincmethyl)-3-methyleneoxacycloalkanes

Abstract 2-(Alkenyloxymethyl)-2-propenylzinc bromides 2a - e and 2-(allylmethylaminomethyl)-2-propenylzinc (or magnesium) halides 2g rearrange thermally to 5-(l-bromozincalkyl)-3-methyleneoxacycloalkanes 1a - e and 5-(bromozinc[or chloromagnesium]methyl)-3-methylene- N -methylpiperidine 1g ; some of the former can be isomerized by Pd(PPh 3 ) 4 to methylenecyclopentanes 3 .

1,5-annelated 4-methylenecyclopentenes by intramolecular type I zinc-ene reactions followed by Pd(0)-catalyzed cyclization

3-(Alk-m-ynyl)-2-(methoxymethyl)-2-propenylzinc bromides 1 (m = 4,5,6) undergo intramolecular carbometallation. The products 2 were converted by Pd(0)-catalyzed cyclization to 1,5-annelated 4-methylenecyclopentenes 3.

Total synthesis of Cyclotheonamide B, a facile route towards analogues.

A flexible, convergent synthesis of Cyclotheonamide B (1b) was developed, starting from the constituent amino acids, using conventional benzyl-, t-butyl- and allyl-based protecting groups. By modification of the key intermediates, this approach allows the preparation of cyclotheonamide analogues.

The knoevenagel reaction of malononitrile with some cyclic β-keto-esters—II: Mechanism of formation of heterocyclic reaction products

Abstract The formation of alkoxypyridinols 3a and 3b from the Knoevenagel reaction products 2a and 2b was shown to proceed by a mechanism in which the ester CO group initiates ring closure by intramolecular nucleophilic attack on the cyano group, possibly assisted by an acidic catalyst. Opening of the oxygen ring after nucleophilic attack by a basic catalyst on the former ester carbonyl C atom results in the formation of an amide, which in turn affords the alkoxypyridinols by a conventional cyclization mechanism.

Total synthesis of c19-diterpene alkaloids: construction of a functionalized bcd-ring system

Abstract The construction of the BCD-ring system of C 19 -diterpene alkaloids was initiated by development of a ring expansion reaction of cyclic enamino-esters with propiolic acid esters, leading from the 5-membered ring ketone 3 to the 7-membered ring ketone 10. Epoxidation and stereospecific reductive epoxide ring opening to give hydroxy ester 18 were subsequent key-steps which eventually furnished cyclic β-keto ester 22. This versatile intermediate has a full potential of functional groups suited for further elaboration into the A-, E- and F-rings and substituents of a variety of C 19 -diterpene alkaloids.

Intramolecularly alkylated Costa complexes: New models for coenzyme B12 with a cobalt-to-ligand carbon bridge

Abstract The synthesis of Costa-type B 12 models 1 with a carbon bridge between the equatorial ligand and cobalt has been accomplished by condensation of butanedione monoxime and 2-(ω-functionalized)alkyl-1,3-diaminopropanes 8 followed by complexation with Co(II), introduction of a leaving group and intramolecular alkylation via Co(I) intermediates. The solution structure of intramolecularly alkylated Costa complexes with a bridge of two ( 1a ) or of three ( 1b ) methylene groups was investigated by NMR spectroscopy and compared with that of propyl(iodo) Costa complex 13 .

Intramolecularly alkylated Cosalen complexes: thermolysis and photolysis of coenzyme B12 models with a cobalt-to-ligand carbon bridge.

Abstract Product and kinetic studies on the anaerobic thermolysis and photolysis of the cobalt-carbon bond in intramolecularly bridged organoCosalen compounds are presented. NMR, UV-Vis and electron spin resonance spectroscopic investigations reveal the formation of paramagnetic Co(II) complexes upon thermolysis or photolysis in solution. Solid state thermolysis (studied by differential thermal analysis, thermogravimetry, mass spectrometry and magnetochemistry) also leads to cobalt (II) products. The fate of the carbon radical produced by homolytic cleavage of the Co C bond in solution depends on the reaction conditions used. In the presence of a radical trap (tempo, pbn), carbon radical spin trapping is the major pathway. On the other hand, a Co(II) salen complex carrying an ω-alkenyl side chain was shown to be the major product when Cosalen(CH 2 ) 4 ( 2 ) was subjected to prolonged photolysis in the absence of a radical trap. While the chemistries of intramolecularly bridged organoCosalen and open chain alkylCosalen complexes are qualitatively quite similar, kinetic studies using the radical trap method reveal large quantitative differences. The bridged complex Cosalen(CH 2 ) 3 ( 1 ) is quite unreactive towards thermolysis and photolysis, even at high trap concentration. Quantum yields (Φ) of Cosalen(CH 2 ) 4 ( 2 ) and butylCosalen ( 4 ) were determined in toluene as a function of the concentration of tempo. For 2 , Φ max is reached at 0.5 M of tempo, while for 4 Φ is already maximal at a very small excess of tempo (≈0.0005 M); in both cases the complex concentration is ca. 10 4 M. These results are discussed in terms of the solvent-caged radical pair concept.