Fragmentation of Homoallylic Alkoxides. Thermolysis of potassium 2‐substituted bicyclo [2.2.2]oct‐5‐en‐2‐alkoxides

Abstract

SummaryThermolysis of the potassium 2-substituted bicyclo[2.2.2]oct-5-en-2-alkoxides derived from alcohols 2-17 at 90-120" in hexamethylphosphoric triamide affords unsaturated ketones resulting from allylic bond cleavage. The mechanistic and synthetic aspects of this anionic fragmentation are discussed with reference to the formation of 1-(3'-cyclohexenyl)-2-alkanones 18-28 via initial heterolytic C (l), C (2)-bond cleavage in the substrate alkoxide and regioselective, intramolecular protonation of the resultant transient allylic anion.Introduction. -The thermolysis of a homoallylic alcohol to give an alkene and a carbonyl compound (Scheme I , M =H) is well documented'). Synthetic use of this reaction is, however, limited by the generally high temperatures required ( > 200 ") and the stereochemical necessity for a syn-relationship between the hydroxyl group and the olefinic double bond. Conversely, the fragmentation of a homoallylic metal alkoxide occurs at a substantially lower temperature and leads to the formation of an alkene and a metal enolate (Scheme 1, M=metal)2). This reaction involves reversible cleavage of the allylic bond followed by enolatization of the resulting ketone by either the transient organometallic species or the alkoxide substrate3). Although the first step is believed to be concerted when the metal is magnesium (M=MgCl, MgBr) 151 little is known about the reaction mechanism for more electropositive metals such as sodium or potassium (M=Na, K). In addition, the synthetic aspects of this anionic fragmentation reaction have never been systematically studied4). We now describe the thermolysis of a series of potassium 2-substituted bicyclo [2.2.2]oct-5-en-2-alkoxides.

The regioselective outcome of ring rearrangement metathesis transformations performed on bicyclo[2.2.2]oct-2-ene derivatives

“…methylmagnesium and allylmagnesium bromide) to 4 has been reported previously 5,9 the product distributions shown in scheme 3 indicate a moderate to good degree of diastereoselectivity in favour of the exoaddition product, and is presumably due to the approach of the nucleophile to the less hindered face of the carbonyl, i.e. over the alkene.…”

“…While the allylation of 4 has been described by Snowdon and co-workers the diastereoselectivity of the addition was not reported. 5 Scheme 2. Organometallic addition and subsequent RRM outcome.…”

The (Schiff base)vanadium(V) Complex Catalyzed Oxidation of Bromide − A New Method for the in situ Generation of Bromine and Its Application in the Synthesis of Functionalized Cyclic Ethers

“…Treatment of 2-(cyclohex-2-en-1-yl)-1-phenylethanol (22, 50:50 mixture of like/unlike stereoisomers) [32] with the reagent combination of TBHP, pyHBr, and vanadium() catalyst 5c (5 mol %) furnished a 33% yield of bicyclic tetrahydrofuran 23 [33] and a 30% yield of a 77:23 diastereomeric mixture of two dibromohydrins 24 of hitherto unknown [33] whereas dibromide 24, for reasons of mass balance, is considered to originate mainly from unlike-22.…”

Bicyclo[2.2.2]octane-2-spirocyclohexanes, Part 5 [1] The Action of Grignard Reagents on Spirodiisophora-3′,6-dione

“…The doublets o f C-2' and C-3' were erroneously entered as singlets in that Table. Unauthenticated Download Date | 5/11/18 5:35 PM to derivatisation [21], reduction by several m e thods [10], and brom ination [11] is seen to persist in the G rignard reaction now described. In con trast, G rignard reagents interact readily with the detached bicyclo [2.2.2]octanone molecule, both saturated [9,25] and olefmic [26,27], including ex amples bearing additional substituents [25,27]. The inertness of bicyclo [2.2.2]octanone when part of the extended structure (e.g.…”