Total Syntheses of (+)-Ricinelaidic Acid Lactone and of (−)-Gloeosporone Based on Transition-Metal-Catalyzed C−C Bond Formations

Metátese de olefinas aplicada ao fechamento de anéis: uma ferramenta poderosa para a síntese de macrociclos naturais

“…Em conformidade com as conclusões de Fürstner e Langemann, é possível resumir os casos favoráveis e desfavoráveis segundo a representação mostrada na Figura 3 39 . A fim de desfavorecer a formação de tais quelantes não reativos durante a síntese de macrociclos, os autores recomendam a adição de uma quantidade catalítica de um ácido de Lewis no meio reacional da RCM, geralmente o isopropóxido de titânio (IV) [Ti(Oi-Pr) 4 ].…”

“…A formação do macrolido inibidor de germinação (-)-gloeosporona 21 39 , assim como da macrolactama 6-nor-fluvirucina B 1 22…”

Towards novel difluorinated sugar mimetics; syntheses and conformational analyses of highly-functionalised difluorinated cyclooctenones

“…First and second generation Grubbs' catalysts 1 and 2 were compared for the RCM of syn15b (Table 1); RCM failed completely when 1 w a su s e da st h e catalyst in the absence of the Ti(IV)c o -c a t a l y s t . 23 The reaction time shortened (from 7 days to 3 days) when 2 was used as the catalyst in lower loading (2.5 mol%) in the absence of the Ti(IV) co-catalyst, whereas the reaction time shortened to 18 hours (an overall ten-fold reduction) in the presence of the co-catalyst (Table 1).…”

Diene, Enyne, and Diyne Metathesis in Natural Product Synthesis

“…Attempts to convert acrylate 43 to lactone 44 using Grubbs' catalyst A or Schrock's molybdenum catalyst B resulted in very low yields of the Diene, Enyne, and Diyne Metathesis in Natural Product Synthesis 281 Comparison of substrate reactivity and catalyst activity in total synthesis of ricciocarpin A (50) and B (51) [32] desired product. With Ti(OiPr) 4 as an additive, which is suggested to reduce deactivation of catalytic intermediates by the Lewis basic carbonyl oxygen [33], the yield was improved to 65%. With second-generation catalysts C and E the catalyst loading could be reduced, and the reaction led -without additive -to comparable yields of 44.…”

“…When various natural 16-membered macrolide antibiotics with a 1,3-diene unit in the marocyclic core (e.g., josamycin (398)) were exposed to catalyst E (20 mol%) in the presence of 1-hexene (2 equiv), a ROM-RCM sequence occurred with excision of ethylene and ring contraction to 14-membered-ring lactones 400 (Scheme 78) [165]. The reaction did not occur with catalyst A, and demanded -without additives -a stoichiometric amount of catalyst E. 1-Hexene, which was added instead of ethylene to initiate and propagate the catalytic cycle by generating the highly active L n Ru=CH 2 species, could also be replaced by titanium isopropoxide, which is known to destabilize catalyst-deactivating chelates between the catalyst and hydrogen bond acceptors in the metathesis substrate [33].…”