Showing papers by "Christian V. Stevens published in 2006"

Synthesis and activity for ROMP of bidentate Schiff base substituted second generation Grubbs catalysts

Abstract: The synthesis and characterisation of Schiff base substituted second generation Grubbs catalysts is described using the pyridine functionalised second generation catalyst and a Schiff base-Tl salt. The complexes are less active for the ROMP of COD (cycloocadiene) than their second generation analogues though their activity for the ROMP of DCPD (dicyclopentadiene) at high temperatures shows great potential due to the thermal stability of the catalysts.

Enhancement of fungicidal and insecticidal activity by reductive alkylation of chitosan

Abstract: A series of N-alkyl chitosan (NAC) derivatives were synthesized using a reductive alkylation reaction to examine their fungicidal and insecticidal activity The chemical structures were characterized by IR and 1H NMR spectroscopy, and the degree of substitution (DS) ranged from 002 to 037 Their fungicidal activity was evaluated against the grey mould Botrytis cinerea Pers ex Fr (Leotiales: Sclerotiniaceae) and the rice leaf blast pathogen Pyricularia grisea Sacc [Teleomorph: Magnaporthe grisea (Hebert) Barr] by a radial growth bioassay It was of interest that most of the NAC derivatives were more active against both fungi than chitosan itself The most active derivative was N-(2,2-diphenylethyl)chitosan with EC50 values of 0031 and 023 g L−1 against B cinerea and P grisea respectively In addition, some derivatives, at higher concentrations up to 10 g L−1, inhibited the mycelial growth and spore formation of P grisea Bioassays against larvae of the cotton leafworm, Spodoptera littoralis (Boisd) (Lepidoptera: Noctuidae) with the NAC derivatives at a rate of 50 g kg−1 in artificial diet demonstrated that N-(3-phenylbutyl)chitosan was the most active compound In addition, N-propylchitosan, N-undecanylchitosan and N-(3-phenylpropyl)chitosan derivatives strongly inhibited larval weight gain in S littoralis, with respective reductions of 76, 66 and 65% after 4 days of feeding on treated diet Copyright © 2006 Society of Chemical Industry

Synthesis of 2-phosphonopyrroles via a one-pot RCM/oxidation sequence

Abstract: A four-step synthesis of 2-phosphonopyrroles is presented starting from suitable aldehydes. The key step in the synthesis involves a one-pot ring-closing metathesis/oxidation sequence of a functionalized α-aminoalkenyl phosphonate. Notwithstanding the presence of a nucleophilic nitrogen atom and high substitution patterns in the substrate, the results of the RCM reaction are excellent using mild reaction conditions. Furthermore, a synergism is observed between the RCM catalyst and the oxidizing agent, causing higher oxidation rates and allowing reaction for substrates that normally fail to ring close under standard RCM conditions.

Unexpected four-membered over six-membered ring formation during the synthesis of azaheterocyclic phosphonates : Experimental and theoretical evaluation

Abstract: The cyclization of functionalized aminophosphonates is studied on both experimental and theoretical grounds. In a recently described route to phosphono-β-lactams [Stevens C. V.; Vekemans, W.; Moonen, K.; Rammeloo, T. Tetrahedron Lett. 2003, 44, 1619], it was found that starting from an ambident allylic anion only four-membered rings were formed without any trace of six-membered lactams. New anion trapping experiments revealed that the γ-anion is highly reactive in intermolecular reactions. Ab initio calculations predict higher reaction barriers for the γ-anion due to restricted rotation about the C−N bond and due to highly strained transition states during ring closure. The sodium or lithium counterion, explicit dimethyl ether solvent molecules, and bulk solvent effects were properly taken into account at various levels of theory.

Continuous Synthesis of Tri- and Tetrasubstituted Imidazoles via a Multicomponent Reaction under Microreactor Conditions

Abstract: Using a modified Radziszewski reaction, a procedure has been optimized for the generation of tri- and tetrasubstituted imidazoles via microreactor technology. Optimization included the search for a suitable solvent mixture, temperature and reaction time. Finally, the generality of the optimized reaction was tested using different starting materials. It was possible to create a variety of tri- and tetrasubstituted imidazoles in moderate to good yields via a continuous procedure.

Tandem addition of trialkyl phosphites to α,β-unsaturated imines : A comparison with silylated phosphites

Abstract: Trialkyl phosphites were evaluated for addition reactions to α,β-unsaturated imines. An acidic medium is required to allow consecutive 1,4- and 1,2-addition to occur. In this manner, 3-phosphonyl-1-aminophosphonates, phosphonic acid analogues of glutamate, are obtained in good yields (32−90%). The reaction is mainly influenced by the steric bulk of the nitrogen substituent: less steric N-substituents lead to better yields of the tandem adducts.

Synthesis of N(3),N '(3)-polymethylene-bis-hydantoins and their macrocyclic derivatives

Abstract: An efficient and straightforward two-step approach toward N3,N'3-polymethylene-bis-hydantoins was developed. As a first step, a pyroglutamate is reacted with a diisocyanate to produce a bis-carbamoyllactam. The second step is a double-ring transformation by treatment of this bis-carbamoyllactam with KOtBu in ethanol. In this fashion N3,N'3-polymethylene-bis-hydantoins are produced in two quantitative steps and under very mild conditions. When properly derivatized, these compounds can be converted to their macrocyclic derivatives upon treatment with 5 mol % of second-generation Grubbs' catalyst. These macrocyclic derivatives are so far not described in the literature. It was proven that exclusively (E)-isomers are formed.

One-Pot Synthesis of N-Chloroacetyl 1-Aminoalkyl Phosphonates — Precursors of 4-Phosphono-β-lactams.

Abstract: 4-Phosphono-β-lactams are synthesized via a three-step sequence, including final formation of the C3-C4 bond through a phosphorus-stabilized carbanion. The chlorinated precursors can be synthesized via two different methods: a one-pot N-acylation of an aromatic imine followed by addition of a trialkyl phosphite or phosphonylation of a suitable imine followed by N-acylation in a separate reaction step. The former method was preferred because of the ease of the reaction and the good yields obtained.

Domino ring-closing enyne-metathesis-cross-metathesis approach to 1-phosphonylated benzazepines

Abstract: A short approach was developed towards phosphonylated benzazepines. Aminophosphonates containing an 'yne' and an 'ene' moiety were synthesised via phosphonylation of the corresponding imines. The key step in the reaction sequence is a domino enyne-metathesis-cross-metathesis.

Synthesis of 2‐Phosphonopyrroles via a One‐Pot RCM/Oxidation Sequence.

Abstract: A four-step synthesis of 2-phosphonopyrroles is presented starting from suitable aldehydes. The key step in the synthesis involves a one-pot ring-closing metathesis/oxidation sequence of a functionalized α-aminoalkenyl phosphonate. Notwithstanding the presence of a nucleophilic nitrogen atom and high substitution patterns in the substrate, the results of the RCM reaction are excellent using mild reaction conditions. Furthermore, a synergism is observed between the RCM catalyst and the oxidizing agent, causing higher oxidation rates and allowing reaction for substrates that normally fail to ring close under standard RCM conditions.

The Pyroglutamate Hydantoin Rearrangement.

Abstract: When a mixture of a pyroglutamate and an isocyanate in THF is treated with NaH, a ring transformation occurs leading to functionalised hydantoins. The novel reaction involves a ring-closing ring-opening sequence providing a new and straightforward access to an interesting class of heterocyclic compounds. Furthermore, starting from pyroglutamates allows the synthesis of highly substituted hydantoins under very mild conditions. This ring transformation in combination with ring-closing metathesis is used in a four-step reaction sequence for the synthesis of multi-functionalised bicyclic hydantoin derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis of 1,3‐Dioxo‐hexahydropyrido[1,2‐c][1,3]diazepine Carboxylates, a New Bicyclic Skeleton Formed by Ring Expansion—RCM Methodology.

Abstract: A short and elegant synthetic pathway was developed for the synthesis of 1,3-dioxo-hexahydropyrido[1,2-c][1,3]diazepine carboxylates, a new 1,3-diazepan-2,4-dione containing bicyclic moiety, starting from pyroglutamate esters.