Synthesis and fungicidal activity of quinolin-6-yloxyacetamides, a novel class of tubulin polymerization inhibitors.

A novel class of experimental fungicides has been discovered, which consists of special quinolin-6-yloxyacetamides. They are highly active against important phytopathogens, such as Phytophthora infestans (potato and tomato late blight), Mycosphaerella graminicola (wheat leaf blotch) and Uncinula necator (grape powdery mildew). Their fungicidal activity is due to their ability to inhibit fungal tubulin polymerization, leading to microtubule destabilization. An efficient synthesis route has been worked out, which allows the diverse substitution of four identified key positions across the molecular scaffold.

New ventures in the chemistry of avermectins.

An overview is given on recent work towards new avermectin derivatives of extremely high insecticidal and acaricidal activity. These compounds were prepared from commercially available abamectin (avermectin B1) 1. For the synthesis, many novel entries have been opened up, making use of modern synthetic methods and applying them, for the first time, to the chemistry of avermectins. Several types of avermectin derivatives can be regarded as key innovations in the field. These are, in particular, 4''-deoxy-4''-(S)-amino avermectins 3, 4'-O-alkoxyalkyl avermectin monosaccharides 5, 4''-deoxy-4''-C-substituted 4''-amino avermectins 6 and 2''-substituted avermectins 7. 4''-Deoxy-4''-(S)-amino avermectins 3 were obtained by the consecutive application of the Staudinger and Aza-Wittig reaction. 4'-O-Alkoxyalkyl avermectin monosaccharides 5 were prepared by alkoxyalkylation of 5-O-protected avermectin monosaccharide. For the synthesis of 4''-deoxy-4''-C-substituted 4''-amino avermectins 6, several methods were used to construct the fully substituted 4''-carbon centre, such as a modified Strecker synthesis, the addition of organometallics to a 4''-sulfinimine and a modified Ugi approach. In order to prepare 2''-substituted avermectins 7, 5-O-protected avermectin monosaccharide was coupled with carbohydrate building blocks. An alternative synthesis involved the hitherto unknown enol ether chemistry of 4''-oxo-avermectin and the conjugate addition of a cuprate to an avermectin 2'',3''-en-4''-one. In addition, a number of other highly potent derivatives were synthesised. Examples are 4''-O-amino avermectins 8, as well as products arising from intramolecular rhodium catalysed amidations and carbene insertions. A radical cyclisation led to an intriguing rearrangement of the avermectin skeleton. Many of the new avermectins surpassed the activity of abamectin 1 against insects and mites.

Rearrangement of a cyclohexyl radical to a cyclopentylmethyl radical on the avermectin skeleton.

The rearrangement of a substituted cyclohexyl radical to a cyclopentylmethyl radical on the skeleton of avermectin B1 was observed experimentally and explored computationally. The Stork-Nishiyama methodology was applied to the macrocycle of interest followed by a Tamao oxidation. The expected 5-6 fused ring product was observed in minor amounts. The major product was a 5-5 fused ring resulting from apparent conversion of the initially formed cyclohexyl radical to a cyclopentylmethyl radical. Preliminary computational results indicate that substituents in the macrocycle induce the rearrangement.