Isomerization substrate

In a similar way, 6-chloro-2-(/V-methylhydrazino)quinoxaline 4-oxide gave dimethyl 7-chloro- (279, Q = C1, R = H) (61%)460 463 and the isomeric substrate, 6-chloro-3-(lV-methyIhydrazino)quinoxaline 1-oxide, gave dimethyl 8 -chloro-1 -methyl-1,2-dihydropyridazino [3,4-b] quinoxaline-3,4-dicarboxylate (279, Q = H, R = Cl) (50% after separation from a byproduct).489... 

In contrast to the high enantioselectivity achieved for the Z-isomeric substrates, hydrogenation of the S-isomeric substrates usually proceeds at a much lower rate and gives poor enantioselectivities [86]. With the Rh-BINAP system as the catalyst and tetrahydrofuran (THF) as solvent, hydrogenation of the Z-and S-isomeric substrates generates products with different configurations [2]. 

A recent study of group selectivity in iodocyclizations which could form either ds-fused tetrahydrofu-ran or 7-lactone products (equation IS) has shown that the observed selectivity correlates with the conformational bias of each isomeric substrate.62 One isomer, with no significant conformational bias, produced a mixture of the products upon cyclization of the ester, but gave only the 7-lactone upon cyclization of the carboxylic acid. 

Like the foregoing isomeric substrates (Section 2.2.7), these isoxazolopyrazines were frequently made from pyrazines. Thus 3-(/V-hydroxyamidino)-2(l//)-pyrazi-none (102) was converted in two stages into isoxazolo[4,5-b]pyrazin-3-aminc (103), which on vigorous treatment with acetic anhydride afforded 2-acetoxy-3-(5-methyl-l,2,4-oxadiazol-3-yl)pyrazine (104) in 78% yield the same substrate (103) in hot formic acid for 5 min gave mainly 3-(l,2,4-oxadiazol-3-yl)-2(l/7)-pyrazi-none (105) (50%) but if heating was prolonged for 3 h only 3-oxo-3,4-dihydro-2-pyrazinecarbonitrile (106) was obtained, presumably via the oxadiazolopyrazine (105).1115... 

Methyl 3-oxo-3,4-dihydro-2-pyrazinecarboxylate (22) gave methyl 3-chloro-2-pyrazinecarboxylate (23) (S02C1, trace Me2NCHO, PhMe, 80°C, N2, 3 h 80%) 54 the isomeric substrate, methyl 5-oxo-4,5-dihydro-2-pyrazinecarboxy-late, gave methyl 5-chloro-2-pyrazinecarboxylate (24, R = H) (neat POCl3, trace Me2NCHO, reflux, 2 h 68%) 85 and the homologous methyl 5-chloro-6-methyl-2-pyrazinecarboxylate (24, R = Me) (77%) was made similarly.85... 

The unique antagonistic features of the (butadiene)zirconocene isomers 3a/5a have been used as a probe for the elucidation of organometaUic reaction mechanisms. In some cases it was possible to distinguish between mechanistic alternatives by simply allowing the isomeric substrates 3 and 5 to compete for a reagent. An example is as follows. Transition metal-induced C—C coupling between a conjugated diene and an olefin can occur by two basic ly different types of reaction sequence. Either a new C— C bond can be formed by olefin insertion into a metal-carbon bond of a (o--allyl)M-type intermediate (24) (95), or, alternatively, the alkene may... 

Z)-2-Butenyl]phenol, on the other hand, gives only 0.7 % ee of the (R)-2,3-dihydro-2-vinyl-benzofuran. The difference in optical yields between the (E)- and (Z)-isomeric substrates must be due to the geometric difference, since no Z-> isomerization of the double bond occurs during the reaction. 

Horse and human plasma cholinesterases have been found to have similar substrate specificity profiles (M25), for example, in respect to the relative rates of hydrolysis of some isomeric substrates (B14), as summarized in Table 4. It can be assumed that the areas in the regions of the... 

Like a-melhyislyrene (2-phenyl-1-propene) the isomeric alkenes a-ethylstyrene (2-phenyl-1-butene), (Z)- and ( )-2-phenyl-2-bulene are hydrocarboxylated under similar conditions19. However, different optical rotations are observed for methyl 3-phenylbutanoate, the common reaction product (values between 8.1 to 10.5). This supports the assumption of individual induction steps for each isomer and excludes the isomerized substrate being released from the complex. 

The isomeric substrate, l,2-dihydro-Ar-(l-oxo-3-butenyl-l)-benzocyclobutadienamine was found to be more reactive, but the cyclization was less stereoselective32. 

When the acetonide and the bicyclic ring system are removed, as in the substrate triene 33, it becomes apparent that the origin of the stereoselectivity lies in the influence of the remote alkyl substituents. Cyclization of the hexatriene 33 (R1 = Et R2 = CH3), a substrate that retains the substitution pattern of those described previously (i.e., 27 and 30), affords the same sense and degree of selectivity [d.r. (34/35) 4 1]. The isomeric substrate 33 (R1 = CH3 R2 = Et). a substrate in which the alkyl substituents are reversed, affords a 1 1 mixture of diastereomers. 

Acetyl-2-chlorocycloheptatrien-l-one (136) or the isomeric substrate, 2-acetyl-7-chlorocycloheptatrien-l-one (138), underwent condensation and rearrangement with hydrazine to give 4-methyl-l(27/)-phthalazinone (137) (H2NNH2 H2O, MeOH, reflux, 2h 29 or %, respectively) substrate (138) with methylhydrazine gave 2,4-dimethyl-l(2fl)-phthalazinone (139) (MeOH,... 

A 2,5-disubstituted C g tetrahydrofuran fatty ester [13] was obtained from methyl ricinoleate by addition of bromine to the isomerized substrate, followed by hydrogenation over palladium on charcoal (68). Free radical and Lewis acid-induced reactions involving flie double bonds of unsaturated fetty esters have been conducted by Metzger et al. (69-74) these have resulted in the production of a large number of functionalized, cyclic, and branched fetty ester derivatives (e.g., [14], [15]). The synthesis of methyl rac-2-dodecyl-cyclopentane carboxylate from methyl 2-iodo-18 1(6Z) is presented in Scheme 5. 

Payne further investigated the scope and limitations of the epoxy alcohol rearrangement under the standard conditions depicted in Scheme 10.1. A major quantity of 3 was found to be readily transformed to the isomeric 4, attaining an equilibrium ratio of 3 4 = 8 92. The position of equilibrium for this structural transposition was further probed by using representative isomeric substrates thus, only a small amount of anti-6 was obtained from (E)-5, while the diastereomeric syn-6 was formed from (Z)-5 as a mixture of syn-6 (Z)-5 = 42 58 after 1 h at ambient temperature. The same situation was noticed for (E)-7, which afforded anti-S in a ratio of E)-7 anti-6 = 44 56, and (Z)-7, which was preferentially converted to syn-8, leaving only 5% of (Z)-7 in the mixture. 

As mentioned above, in comparison to dihydrofurans, dihydropyrans require more severe conditions than those used for the five-membered congener [17]. Moreover, cychc allyl ethers easily undergo isomerization to produce the corresponding thermodynamically more stable vinyl ethers (Scheme 4.36). Nevertheless, 2,5-dihydrofuran could be converted with PPhj as a hgand with an almost quantitative yield into 3-formyl tetrahydrofuran (a, n = 1). With 3,4-dihydro-2//-pyran as a substrate, at elevated syngas pressure and temperature mainly tetrahydropyran-2-carbaldehyde (b, = 2) was formed. Almost the same regiochemical outcome was stated with the isomeric substrate 3,6-dihydro-2//-pyran. This effect can best be explained by the prior isomerization of the latter into the more stable 3,4-dihydro-2H-pyran. In this case, the corresponding P Carbaldehyde (a, n = 2) was formed only in minor quantities and the 4-formyl isomer c was not observed at all. 

A group at Chirotech showed that a cyclic acetal of crotonaldehyde undergoes highly n-regioselective hydroformylation via a previously isomerized substrate... 

Under the same catalytic conditions, even the isomeric substrate methyl 2-pentenoate reacted at 69 bar into the desired w-aldehyde with 76% chemose-lectivity (Scheme 5.26) [131]. 

The Rh-catalyzed asymmetric hydrogenation of prochiral p-A-acet-ylamino-vinylphosphonates (145) allowed preparation of chiral p-A-acet-ylaminoethyl-phosphonates (146) with excellent yields (up to 100%) and high enantioselectivities (up to 92% ee) (Scheme 48). The reaction was strongly dependent on the structure of a chiral bidentate phosphorus ligand (PL ) and the solvent employed (THF, CH2CI2). In several cases an inversion of the induced chirality was observed by using the corresponding E- or Z-isomeric substrates. 

Homofulvenes 78 undergo a [6+2] cycloaddition reaction with CSI, and only one cycloadduct 79 is obtained from the isomeric substrates... 

WTiat you are being asked to do is to explain a result. So, you need to consider mechanisms. How to begin Draw the structures of the two isomeric substrates (even betto-, buUd models ), so you can visualize their diffoences ... 

Dienes can be readily accessed by treating allylic ethers with LIDA in the presence of catalytic or stoichiometric amounts of potassium tert-butoxide. If there is a choice between methyl and alkyl at the 5-center, the former is deprotonated. Thus, both geranyl and neryl methyl ether afford myrcene (7-methyl-3-methylene-l,6-octadiene). (Z)-Alk-2-enyl ethers (Z-254) give mainly or solely ( )-dienes as the ( )-isomeric substrates ( -254) give (Z)-dienes (Scheme 1-201). ... 

---