Cope rearrangement acetylenic

This finding confirms an opinion that, at least in some cases, diradicals such as 18 can be the actual intermediates in the non-concerted Cope rearrangement, so-called stepwise cyclization-then-cleavage mechanism. Berson and coworkers who previously excluded diyl intermediate in the acetylenic Cope rearrangement20 designed in their next work24... 

The Cope rearrangement and its variants are described very thoroughly in numerous reviews (see, e.g., References 11, 207 and 208). In general, these reactions can be represented by an extremely laconic scheme (equation 152). This system can also include cumulenes24 25 and acetylene fragments209 as well as various substituents and heteroatoms (see Sections IV.D and E). 

Whereas, cii-divinyl epoxides are reactive and well known to undergo thermal Cope rearrangement, the trans isomers are significantly more stable. White and co-worker (24) showed that thermolysis of divinyl epoxides such as 88 could generate the corresponding carbonyl ylide and that it could be intercepted by the addition of an activated acetylene to give the corresponding dihydrofuran 89, albeit in modest yield. 

Acetylenic oxy-Cope rearrangement (12, 51). The oxy-Cope rearrangement of 5-hexen-l-yn-3-ols is accelerated in refluxing N-methyl-2-pyrrolidone (9, 316), but the required temperature (165°) can result in rearranged products. The rearrangement can proceed at 20-60° in the presence of silver trifiate (1 equiv.), which is known to complex with triple bonds.2. ... 

The fact that Cope rearrangements of alkynes proceed as readily as those of alkenes53 has been puzzling, since it would appear that the transition state would be more easily attained in the case of alkenes. However, our results indicate that considerable bending of the acetylenic units occurs easily in the transition state, since the energy required is counteracted by increased HOMO-LUMO interaction. Thus,... 

The selective synthesis of pseudoionone is achieved by the acetylenic oxy-Cope rearrangement of 4-isopropenyl-3,7-dimethyl-6-octen-l-yn-3-ol (37) in A-methylpyrrolidone (NMP) as solvent containing a trace of hydrobromic acid, or iodine772,1127. 

Oxy-Cope rearrangement of acetylene derivatives 42 gives cyclooctadienones1012. The rearrangement of enynol 45 is applied in the synthesis of poitediol. The synthesis of ( )-phorcan-tholide I is achieved from substrate 471128. 

Bis-acetylenic alcohol 1 undergoes a facile oxy-Cope rearrangement to afford mixtures of ( )-and (Z)-enynones. The latter compounds react further to give spiroannulated methylenecy-clopentenone 4 upon enolization and clectrocyclic ring closure1134. 

Other Degraded Carotenoids. Several syntheses of ionones and related compounds have been described. Some of these may prove useful for end-group construction in carotenoid synthesis. Thus 4-keto-j8-ionone (126) and 3,4-dehydro-jS-ionone (127) have been prepared by condensation of the sulphone (128) with propylene oxide, followed by elimination of phenylsulphinic acid and either oxidation or dehydration. A novel method utilizes a thermal acetylenic oxy-Cope rearrangement process to prepare the ionone compounds (129—131 R = Me) and analogues (R = Et or Pr ) from cyclohex-2-enylprop-2rynols (132). 

The power of pericyclic reactions for forming new carbon-carbon bonds is nicely illustrated by the use of the Claisen orthoester rearrangement to prepare y-ketoesters with a quaternary centre next to the ketone (Scheme 9)The use of silver salts to catalyse the acetylenic oxy- Cope rearrangement for the preparation of a,0-6,e-unsaturated ketones has also been reported [equation (10)].57... 

Reaction with benzene follows a similar pathway, yielding a bicyclo[3.2.2]nonatriene structure. Vinylcarbenoids also react with pyrroles to give tropanes via a cyclopropanation-Cope rearrangement route. The direct addition of carbenes to acetylenes does not give satisfactory yields of cyclopropanes, but the rhodium carboxylate catalysed reaction of diazo compounds with acetylenes is a useful source of cyclopro-panes. Carbenoids can also attack a carbonyl oxygen atom, giving rise to a zwitterion (249). An excellent review of intramolecular carbenoid reactions has appeared. ... 

Their attempted total synthesis of geigerin started from the acetylenic ketone 197, which was generated in 76% yield from the oxy-Cope rearrangement of... 

The 9-membered cyclic ethers 116 have been prepared via oxy-Cope rearrangement of 115, ultimately derived from D-glucose. The 10-membered bis-ether 118 was prepared via Bergman cyclization of the bis-acetylene 117 (R=2,3,4,6-tetra-0-benzyl-p-D-glucosyl). Solution conformational analysis was reported from NMR experiments. Dialkyne 117 was prepared by Pd-catalysed coupling of the P-D-glucosyl-l-alkyne and 1,2-diiodobenzene, and a mechanism was proposed for... 

Many monoterpenes are desired fragrances in perfumery and flavors in food. They are produced on a larger scale from acetone (C3) and ethyne (acetylene C2) involving repetitive synthetic steps (Fig. 5). Initially, acetone is ethynylated by acetylene in the presence of a base (sodium hydroxide, amines with sodium carbonate) yielding 3-butyn-2-ol (C5) which is partially hydrogenated in the presence of deactivated catalysts (Lindlar catalysts) to 2-methyl-3-buten-2-ol. This can be converted to the key intermediate 6-methyl-5-hepten-2-one (Cg) via two pathways, either by transetherification with methylpropenylether and subsequent oxa-CoPE rearrangement, or by transesterification with methyl acetoacetate and subsequent Carroll decarboxylation. 

Nucleophilic Additions to Acetylenes.— Arylhydroxylamine addition to DMAD results in 3-amino-4-methoxycarbonylcoumarins (151), the initial nucleophilic addition being followed by a hetero-Cope rearrangement (Scheme 7). The reaction proceeds extraordinarily readily, the product... 

A selective one-pot synthesis of pseudoionone (76) from the unsaturated alcohol (77) has been achieved by an acetylenic oxy-Cope rearrangement followed by a double-bond migration (Scheme 123). The process, designed for large-scale industrial use, was also successfully applied to related systems. 

Among Cope rearrangements involving acetylenes is the thermolysis of hex-5-en-l-yn-3-ol. The products were all considered to be derived by further... 

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