Isomerization, under basic conditions

The chiral 1,3-divinylallene (1,3,4,6-heptatetraene) (3) is obtained when the vinylace-tylene Grignard reagent 198 is first coupled to allyl bromide (199) and the resulting skipped enyne is subsequently isomerized under basic conditions (Scheme 5.29) [74]. 

The isolation of flavonoids from the methanol extract of G. uralensis was carried out under non-basic conditions, because some flavonoids isomerize under basic conditions, e.g. racemization of flavanones and isoflavanones, ring-open reaction of flavanones etc. Bioactive fractions were separated by some chromatographic methods and each step was monitored with anti-H. pylori activity with the paper disk method. Eighteen compounds were isolated from these bioactive fractions and... 

Olefins are susceptible to isomerization. The API ivermectin undergoes isomerization under basic conditions due to the weak acidity of the allylic proton (which is also alpha to a carbonyl) on C2. The two resulting degradants can be derived from the delocalized carbanion formed from dissociation of the proton at C2. Reprotonation at C2 generates the epimeric product and reprotonation at C4 generates the structural isomer (Fig. 103) (147). 

Oxidation of N-arylthioureas (261) with acidic hydrogen peroxide produces Hector s bases (73) which readily isomerize under basic conditions to afford 3,5-diamino-l,2,4-thiadiazoles (84 Scheme 146) (65AHC(5)119) (also see Scheme 92). A variety of 3-amino-and 3,5-diamino-l,2,4-thiadiazoles (325) also have been obtained by the treatment of iminocarbonates (322) with chloramine at low temperature, as illustrated in Scheme 147 (also see Scheme 115) (76EGP119791). 

Predict which carbohydrates mutarotate, which reduce Tollens reagent, and which undergo epimerization and isomerization under basic conditions. (Those with free hemiacetals will, but glycosides with full acetals will not.) Problems 23-58, 62, 63, and 66... 

Information on the stereochemistry of the isomerization under basic conditions is provided by Eq. 118(fl) and 1... 

Honda et al. reported the formal synthesis of 1 based upon the interception of intermediate 82. 2-Acetyl pyridine (84) representing the cycle A was chosen as the starting material (Scheme 12). O-Acetylation with sorbic anhydride generated the corresponding vinyl ester, which was isomerized under basic conditions to 85 featuring both a diene and a dienophile system. A key intramolecular Diels—Alder reaction under thermal conditions in a... 

Contamination with other double-bond isomers occurs in some cases due to isomerization under basic conditions or under severe thermal conditions such as in refluxing THF. Some modiflcations have been developed to allow the elimination process to be accomplished under mild conditions. For example, in situ treatment of )8-hydroxyalkylsilane 40 with acetyl chloride or thionyl chloride gives the corresponding terminal alkene 41 without isomerization (Scheme 2.26) [50, 59, 60). 

Furan formation through 5-exo-dig type of cyclization involving functionalization of the alkyne moiety was observed by Luo et al. during their study of PdCb-catalyzed stereocontrolled syntheses of ( )-2-arylidene-2,3-dihydrobenzofurans 265 (Table 19.1) [130], The reaction of 2-propargylphenol 264 with BuLi followed by aryl iodides in the presence of 10 mol % PdCb produced a small amount of the benzofuran 266, formed by double-bond isomerization under basic conditions. The reaction using Pd(PPh3)4 gave only the benzofuran 266 in low yield (28%). 

Conjugated nkro ilkenes are isomerized to illylic nitro compounds under basic conditions Reactions of a,fi-unsarurated nitro compounds with aldehydes under basic conditions lead directly to y, 5-unsaturated fi-nitro alcohols fEq 3 24 "This reaction is very useful for preparing illyhc nitro compounds... 

Addition of benzoyl chloride to 2.5-dimethyl-3,4,6-triphenyl-3//-azepine (13) in benzene in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) leads to elimination of hydrogen chloride and formation of the 2-methylene-l//-azcpine 14.117 All attempts to isomerize the methylene derivative to 1-benzoyl-2,5-dimethyl-3,4,6-triphenyl-l//-azepine under basic conditions failed. Analogous reactions can occur with 2,5-dielhyl-3,4,6-triphenyl-3//-azepine. 

The reaction between cellulose and acrylamide was studied by quantitative, chromatographic separation of the substituted D-glucoses obtained on acid hydrolysis of the reaction product,320 followed by an analysis by Spurlin s method.249 Although, apparently, no check was made on the stability of the ethers to the conditions of hydrolysis, it might be expected that the ethers would isomerize only under basic conditions. The ratios of the relative equilibrium-constants for reaction at 0-2, 0-3, and 0-6 were 9 1 19, and these are attributable to the high, relative stability of the primary ether, together with the low reactivity of 0-3, also observed in rate-controlled reactions. 

Under basic conditions the D-erythro-L-gluco-octonate 88a underwent racemization at C-2 and anomerization at C-3 to give a mixture of four isomeric products. The configurations at C-2 and C-3 of these isomers were established by chemical and physical methods (82). Furthermore, alcohols and thiols, in the presence of a base, attack the double bond of the oct-2-en-onates (87b) affording isomeric oct-3-ulofuranosidonates. 

Under basic conditions, obviously only one isomerization step takes place and thus a terminal alkyne will deliver 1,2-dienes selectively. With internal alkynes, on the other hand, selectivity can only be achieved when the alkyne is either symmetrical as in 14 [34] (Scheme 1.6) or has a tertiary center on one side as in 16 [35, 36] (Scheme 1.7). So, unlike potassium 3-aminopropylamide in 1,3-diaminopropane, where the Jt-bonds can migrate over a long distance by a sequence of deprotonations and reprotonations, here the stoichiometric deprotonation delivers one specific anion which is then reprotonated (in 16 after transmetalation). 

Still, occasionally the other fuctional groups react as well, for example in 38 under basic conditions the propargylic alcohol isomerizes to the a,/3-unsaturated ketone [73] (Scheme 1.15), whereas in a closely related substrate from the synthesis of a subunit of compactin an allylic alcohol remains unchanged [74],... 

When the reaction was conducted under basic conditions, in boiling ethanol overnight in the presence of sodium ethoxide, diethyl 4-pyrimidi-nylaminomethylenemalonate (229) was obtained in 99% yield. Compound 229 could by cyclized to the isomeric 5-oxopyrido[2,3- /]pyrimidine (230) in 89% yield on boiling in diphenyl ether for I hr (85JHCI469). 

The spiro compound 15 is obtained in excellent yield by the cycloaddition of 3-(4-fluorophenylimino)indolin-2-one with mercaptopropionic acid under microwave irradiation <2003SUL201>. Treatment under basic conditions of 2,3-dihalopropylamines with carbon disulfide results in the formation of two isomeric products 5-halotetrahydro-l,3-thiazine-2-thione 204 and 5-(halomethyl)thiazolidine-2-thione 205 <2002CHE1533>. 

On the other hand, vV-aryl-//-nitrosoaminonitriles undergo interesting secondary reactions. Under acidic conditions, salts of the sydnonimes form (Eq. 13) under basic conditions, a rearrangement to an isomeric oxime takes place, which, on treatment with phosgene in pyridine leads to 4-substituted-3-cyano-l,2,4-oxadiazolinones (Eq. 14) [26b],... 

Reaction of triazolopyrimidinium ylides (256) with active acetylenes gave the 1 2 adducts 260. The formation of 260 may occur in two ways the shortest pathway consists of the double 1,3-dipolar cycloaddition of the diylide 256A with two molecules of the acetylene at two different sites to form the tetracyclic adduct 257, followed by ring opening under basic conditions to give 260. The second pathway consists of cycloaddition between the ylide carbanion of 256 and the bridged carbon C-4 to form the 1 1 adduct 258, which isomerizes to the more stable compound 259, which may be formed directly by the cycloaddition at the ylide carbanion and C-2. The second cycloaddition afforded the 1 2 adduct 257 [87JCS(P1)2531] (Scheme 49). 

The j8 hydrogens of the pyrroline rings are substituted with deuterium under basic conditions (BulOK-BulOD/THF, r.t) without isomerization to other hydroporphyrins. The acidic proton-... 

Table 2 Isomerization of cis Isomers to trans Isomers Under Basic Conditions...

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