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Exomethylene groups

Reaction of the aldehyde-tethered furanone 244 with pipecolinic acid results in the formation of the oxazolopyr-idine derivative 245, which undergoes spontaneous decarboxylation to give the ylide 246. This in turn undergoes an intramolecular cycloaddition with the tethered exomethylene group to give 247, or with the endocyclic alkene to give the furoindolizine 248 <1997T10633> (Scheme 66). [Pg.814]

Does the 10 kJmol 1 stabilization for adjacent exomethylene groups in cyclobutane arise from conjugative interactions Is the 5 kJmol-1 destabilization for nonadjacent exomethylenes in cyclobutane general for other cycloalkane derivatives ... [Pg.84]

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). [Pg.71]

These reagents are usually prepared by treatment of a phosphine with an alkyl halide and successive addition of base, like BuLi or NaHMDS. Reaction with the ketone takes place by attack of the carbanionoid carbon of the ylide form on the electrophilic carbon of the carbonyl group via a four-membered heterocyclic transition state, the oxaphosphetane 35. The driving force for this transformation is provided by formation of the very strong phosphorus-oxygen bond. Subsequent collapse of the oxaphosphetane furnishes the desired exomethylene group under retention of geometry. [Pg.218]

In the first step of the catalytic cycle a coordinatively unsaturated Pd(0) species - which is formed in situ from Pd(OAc)2 and PPh3 -inserts into the alkenyl-I bond of 8 to give 42 (syn addition). Next an insertion of the terminal olefin into the cr-alkenyl-C-Pd bond forms the six-membered ring in 43. The stereochemistry can be explained by 41 reaction of the si-face of the exomethylene group involves a nearly coplanar orientation of the Pd-C bond and the C-C-zrbond. The siloxy substituent is placed pseudo-equatorially. [Pg.220]

Murofushi et al. (8) protected the 16,17 double bond of GA5 methyl ester by forming the epoxide with metachloroperben-zoic acid. After catalytic reduction of the 2,3 double bond they restored the exomethylene group by treatment with a mixture of sodium iodide, sodium acetate and zinc and hydrolyzed the methyl ester to obtain [2,3-3h2]GA2o Yakota et al. (81) prepared [2,3-3h2]GASelective catalytic reduction of the 3-methane-... [Pg.46]

The intramolecular interactions in l,3,3-trimethyl-2-methyleneindolines 182, containing various groups with tri- and tetracoordinated phosphorus atom in the exomethylene group, were studied by IR and UV spectroscopy [201]. [Pg.33]

Using methylene-triphenylphosphorane 205, in many cases carbonyl functions have been converted into exomethylene groups in cyclic terpene chemistry. This technique was used, among others in the preparation of (+)-steviol methyl ether 244), dihydro-5,6-norcaryophyllene 245), ( )-nootkatone 246 (-)-phyllodadene 247), and (+)-e-cadinene 248). In the synthesis of the latter the trans-decalin derivative 462 was formed from the cis-decalone 461 the epimerization of which proceeded via the enol form during the methylenation 248 (Scheme 80). [Pg.142]

In this second example, the trisubstituted double bonds inside the five-membered rings of the product are more stable than the exomethylene groups in the starting material. [Pg.949]

Instead, H NMR chemical shifts values for the C-4 methylene protons in the C-5 unsaturated bicyclomycins 116a-j were sensitive to modifications of the C(5)-C(5a) exomethylene group (X). It was observed that there was a... [Pg.193]

Michael addition of phenylmercaptan to the exomethylene group gives the proteinase inbitor tanomastat (34). ... [Pg.47]


See other pages where Exomethylene groups is mentioned: [Pg.100]    [Pg.65]    [Pg.73]    [Pg.66]    [Pg.68]    [Pg.53]    [Pg.92]    [Pg.93]    [Pg.276]    [Pg.288]    [Pg.214]    [Pg.452]    [Pg.22]    [Pg.65]    [Pg.326]    [Pg.90]    [Pg.134]    [Pg.215]    [Pg.644]    [Pg.182]    [Pg.239]    [Pg.94]    [Pg.131]    [Pg.143]    [Pg.143]    [Pg.100]    [Pg.297]    [Pg.297]    [Pg.298]    [Pg.51]    [Pg.52]    [Pg.192]    [Pg.194]    [Pg.195]    [Pg.210]    [Pg.226]    [Pg.375]    [Pg.644]    [Pg.289]    [Pg.5]   
See also in sourсe #XX -- [ Pg.214 ]




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