First Reference to the Alkoxymetallation of Diisobutylaluminium and Bromomagnesium-2-vinyloxy Ethoxide to the Corresponding (1,3-Dioxolan-2-yl)methyl Organometallics

Abstract

Diisobutylaluminium and bromomagnesium-2-vinyloxy-ethoxide add at carbonyl compounds in the manner of (1,3-dioxolan-2-yl)methyl organometallics, giving the carbon-carbon addition products. It is assumed that the aluminium and magnesium compounds of 2-vinyloxy-ethoxide are in equilibrium with the corresponding (1,3-dioxolan-2-yl)methide.Organometallic compounds, bearing a functional group at the β-position, undergo facile β-elimination. 1 This elimination is only prevented by generating the carbanion at low temperature, with a very poor leaving group X at the β-position, or in the case of a perpendicular arrangement of lone electron pair and C-X bond. Earlier studies established that the action of sodium on 2-bromomethyl-1,3-dioxolane (1) in ether proceeds according to Scheme 1. 2 Similar results were obtained by Feugeas for the Grignard reagent from 2-bromomethyl-2-methyl-1,3-dioxolane. Refluxing in tetrahydrofuran yielded the corresponding β-elimination compound as the sole reaction product. 3 On the other hand, there are a few examples in the literature on the successful addition of the Grignard reagent of 2-bromomethyl-1,3-dioxolane (1) to carbonyl compounds. Kibayashi et al. reported the generation of 3 by addition of the organomagnesium compound of 1 to the aldehyde group of a threo-configurated carbohydrate derivative. 4 Reagents and conditions: (i): Na, Et 2 O, 35°C, H 2 O, 64%; (ii): Mg, THF, r.t., RCHO, NH 4 Cl/H 2 O, 70% (100% de). Scheme 1In a preceding paper we described the preparation of the Grignard reagent from (1) and magnesium and its reaction with less reactive carbohydrate ketones in refluxing tetrahydrofuran. These uloses gave the corresponding branched chain carbohydrate derivatives in good to excellent yields (Scheme 2, path A). 5 The discrepancy in both findings caused us to approach the problem from the opposite side. Bromomagnesium-2-vinyloxy-ethoxide (4a), the product of β-elimination from (1,3-dioxolan-2-yl)-methylmagnesium bromide, was generated at room temperature (Scheme 2, path B), by addition of ethylmagnesium bromide in tetrahydrofuran to a solution of 2-vinyloxy-ethanol (2a) in tetrahydrofuran. The mixture was allowed to warm to room temperature and then a solution of ulose 6 in tetrahydrofuran was added. After refluxing for four hours the reaction was quenched. Reagents and conditions: (i): Mg, 1.0 equiv, THF, r.t.; (ii): EtMgBr, 1.0 equiv, THF, r.t.; (iii):1,2;5,6-Di-O-isopropyliden-α-D-ribo-hexofuranos-3-ulose (6), 0.2 equiv, 4h, 65°C, NH 4 Cl/H 2 O; A:64%, B:67%.

Five‐Membered Heterocycles with Two Heteroatoms: O and S Derivatives

“…A similar preparation involving enol ethers starts with 45, which reacts with either EtMgBr or Bu 2 i AlH followed by benzaldehyde to give hydroxyalkyldioxolanes 46 (Scheme 11.11) [40]. Treatment of glycidic esters like 47 with cationic zirconium species and AgClO 4 affords the dioxolane 48 [41].…”