Preparation of α,β‐Unsaturated γ‐Keto Aldehydes and New Tetronic Acid and Pyridazine Derivatives by Oxidative Transformations of Alkoxyallene‐Based Dihydrofurans

Abstract

Oxidation of 3-alkoxy-substituted dihydrofuran derivatives 6 and 11 with DDQ unexpectedly provided α,β-unsaturated γ-keto aldehydes 10 and 12. A mechanism for this new oxidative ring-cleavage is presented. Since α,β-unsaturated γ-keto aldehydes are versatile intermediates, other 3-methoxy-substituted dihydrofuran derivatives 24, 26, and 28 were prepared from lithiated methoxyallene and the corresponding aldehydes. Oxidation of dihydrofuran derivatives with DDQ and subsequent treatment with sodium chlorite furnished hydroxy-substituted tetronic acid derivatives, such as 30 and 31. Condensation of 30 with hydrazine provided the unsatur-

Regioselectivity Control in the Metal‐Catalyzed OC Functionalization of γ‐Allenols, Part 1: Experimental Study

“…[PtCl 2 A C H T U N G T R E N N U N G (CH 2 =CH 2 )] 2 and AgNO 3 afforded rather low yields or disappointing diastereomeric mixtures of bicycle 5 a. [12,13] Although AgNO 3 was less diastereoselective than [PtCl 2 A C H T U N G T R E N N U N G (CH 2 =CH 2 )] 2 (60:40 vs 100:0), it was a more efficient catalyst, affording adduct 5 a in reasonable yield. Gratifyingly, we found that Au salts were effective as 5-exo-selective hydroalkoxylation catalysts.…”

Unusual Enantiopure Heterocyclic Skeletons by Lewis Acid Promoted Rearrangements of 1,3‐Dioxolanyl‐Substituted 1,2‐Oxazines

“…Solvents were dried using standard procedures. Starting materials ( p ‐methoxybenzyl)oxyallene,17 N ‐benzyl‐2,3‐isopropylidene‐ D ‐glyceraldehyde nitrone18 and syn ‐ 1b – d 1b were prepared according to literature procedures. Reagents were purchased and were used as received without further purification unless otherwise stated.…”

Oxidative Cleavage of 3‐Alkoxy‐2,5‐dihydrofurans and its Application to the De Novo Synthesis of Rare Monosaccharides as Exemplified by L‐Cymarose

“…Reactions of lithiated alkoxyallenes 2 with aldehydes 1 yield primary adducts 3 which are cyclized with potassium tert ‐butoxide to afford 3‐alkoxy‐2,5‐dihydrofurans 4 (Scheme 1) 2a. The one‐step oxidative cleavage of these substrates can be performed under various conditions: Reaction of compounds 4 with two equivalents of DDQ3 at room temperature in the presence of a proton source (water or alcohol) or by treatment with an excess of the complex formed between chromium trioxide and 3,5‐dimethylpyrazole ( 7 )4 at −20 °C leads to the α,β‐unsaturated γ‐keto aldehydes 6 . These products can also be accessed by the oxidation of the corresponding 2‐substituted 3‐alkoxyfurans 5 , but preparation of compounds with this particular substitution pattern is quite difficult 5.…”