Base induced reactions of 1‐alkoxy‐ or l‐methylthio‐l‐(α‐ or β‐hydroxyalkyl)allenes. Formation of 3‐alkoxy‐ and 3‐methylthiodihydrofurans

Abstract

l‐Alkoxy‐l‐(α‐hydroxyalkyl)allenes R1CH=C=C(OR2)C(OH)R3R4 (Ia) have been converted with high yields into 3‐alkoxy‐2,5‐dihydrofurans (II) using potassium tert‐butoxide in dimethyl sulfoxide. Hydrolysis of II with dilute sulfuric acid afforded 3‐oxotetrahydrofuran derivatives III. l‐(β‐Hydroxyalkyl)‐l‐methylthioallenes R1CH=C=C(SCH3)CH2CH2OH (IVb) and the analogous oxygen compounds IVa yielded 2‐alkyl‐3‐methylthio‐4,5‐dihydrofurans (Vb) and a mixture of 3‐methoxy‐2‐methyl‐4,5‐dihydrofurans (Va) and 3‐methoxy‐2‐methylenetetrahydrofuran VI, respectively. The preparation of the hydroxyallenes is described.

Behavior of dialkylallyl(4-hydroxybut-2-ynyl)ammonium bromides toward powdered sodium hydroxide and aqueous alkali

“…The proposed scheme for the cyclization of amino alcohols with an allene group corresponds to published data, according to which the cyclization of many acetylene compounds to heterocycles takes place through intermediate allene compounds [5] and the allene amines and alcohols undergo intramolecular cyclization in the presence of silver ions, forming 3-pyrrolines, substituted 2,5-dihydrofurans, and pyrans [6][7][8][9][10][11][12][13][14][15][16].…”

Alkoxyallene‐Based De Novo Synthesis of Rare Deoxy Sugars: New Routes to L‐Cymarose, L‐Sarmentose, L‐Diginose and L‐Oleandrose

“…We could earlier demonstrate the utility of lithiated alkoxyallenes as versatile C 3 building blocks in numerous applications from heterocyclic to natural product synthesis. [5] Thus, we also planned to use alkoxyallene methodology for the synthesis of carbohydrates 1-4 and two synthetic methods previously explored by our group emerged as key transformations for this objective (Scheme 1): (i) the preparation of 3-alkoxy-2,5-dihydrofurans 8 from lithiated methoxyallene 6 and aldehydes 7 [6] and (ii) the oxidative ring opening of these intermediates to α,β-unsaturated γ-keto aldehydes 9. [7] For α-functionalized dihydrofuran substrates like 8, the latter transformation resembles a parallel to the Achmatowicz re-reductions as well as face-selective hydrogenation protocols.…”

3-Alkoxy-2,5-dihydrofurans by Gold-Catalyzed Allenyl Cyclizations and Their Transformation into 1,4-Dicarbonyl Compounds, Cyclopentenones, and Butenolides

“…This method for the preparation of 2,5-dihydrofurans is not new, since the 5-endo-trig cyclization of the key intermediates, alkoxyallenyl alcohols 3, has first been described by Brandsma and Arens in 1969. 8 However, the investigation of different reagent systems suitable to promote this transformation has been ongoing, and we have just recently found a general, chemoselective, and widely applicable set of conditions for this important reaction employing a gold(I) catalyst. 9 Scheme 1 Preparation and oxidation reactions of alkoxyallenederived 3-alkoxy-2,5-dihydrofurans 4 and their subsequent transformations into g-keto aldehydes 5, aldol products 7, and butenolides 6…”

“…Brandsma and co-workers have described the first 5-endo-trig cyclizations of these substrates using a substoichiometric amount of potassium tert-butoxide (0.1 to 0.5 equiv) in DMSO at 50-60°C. 8 These basic reaction conditions often provide good yields of dihydrofurans 4 in the range of 40-80%. In a subsequent report, it has been suggested by Magnus et al that the reaction proceeds via a radical mechanism.…”