Trans relationship

The relative configurations of vicinal protons follow from the characteristic values of their coupling constants. Thus 16.1 Hz confirms the trans relationship of the protons on C-8 and C-9, 10.8 Hz confirms the cis relationship of the protons on C-6 and C-1. The 2.0 Hz coupling is common to the oxirane protons at = 3.00 and i.27 this value fixes the trans relationship of the protons at C-4 and C-5 following a comparison with the corresponding coupling in the methyloxirane (2.6 Hz). The anti relationship of the protons A-H and h-H can be recognised from their 8.7 Hz coup-... 

These experiments verified that cleavage of the C—H bond is occurring in the rate-limiting step, but proof of the necessity of a trans relationship of the C—H to the nitrogen-mereury complex was lacking. Indications that such a relation was necessary are found in studies of the mercuric acetate oxidation of alkaloids, which will be discussed subsequently. 

The dehydration reaction leads by an Ea process to 8 and is promoted by the tertiary, benzylic nature of the OH group at Ce and its antiperiplanar trans relationship to the H atom at Csg. Furthermore, one of the cannonical forms of the enolizable 0-dicarbonyl system present at Cn and Cia has a double bond in the C ring. Thus, dehydration leads to aromatization of the C ring, and this factor must provide some of the driving force for the reaction. 

A Assume that you have a variety of cvclohexanes substituted in the positions indicated. Identify the substituents as either axial or equatorial. For example, a 1,2-cis relationship means that one substituent must be axial and one equatorial, whereas a 1,2-trans relationship means that both substituents are axial or both are equatorial. 

Scheme 8 presents the sequence of reactions that led to the synthesis of the B-ring of vitamin B12 by the Eschenmoser group. An important virtue of the Diels-Alder reaction is that it is a stereospecific process wherein relative stereochemical relationships present in the diene and/or the dienophile are preserved throughout the course of the reaction.8 Thus, when the doubly activated dienophile 12 (Scheme 8) is exposed to butadiene 11 in the presence of stannic chloride, a stereospecific reaction takes place to give compound 27 in racemic form. As expected, the trans relationship between... 

Standard retrosynthetic manipulation of PGA2 (1) converts it to 5 (see Scheme 2). A conspicuous feature of the five-membered ring of intermediate 5 is the /(-keto ester moiety. Retrosynthetic cleavage of the indicated bond in 5 furnishes triester 6 as a potential precursor. Under basic conditions and in the synthetic direction, a Dieck-mann condensation4 could accomplish the formation of a bond between carbon atoms 9 and 10 in 6 to give intermediate 5. The action of sodium hydroxide on intermediate 5 could then accomplish saponification of both methyl esters, decarboxylation, and epi-merization adjacent to the ketone carbonyl to establish the necessary, and thermodynamically most stable, trans relationship between the two unsaturated side-chain appendages. 

The trans relationship between the ring nitrogen appended to C-16 (strychnine numbering) and the C-8 tertiary hydroxyl group in 14 could conceivably be secured through an intramolecular Sn2... 

Thus the trans relationship between the hydroxymethyl group and the C-l hydroxy group in a-D-glucopyranose, and the cis relationship between the methyl group and the C-l hydroxy group in P-L-fucopyranose, are clearly shown. Note that representation of ketoses may require a different modification of the Fischer projection, as shown in the fructofuranose example above. Here C-2 is rotated about the bond with C-3 to accommodate the long bond to C-2 from the oxygen at C-5. 

Formation of trans isomers in overwhelming predominance in the ISOC reaction leading to five-membered rings (Entries a-d) has been ascribed to the orientation in which H% H , and R are on the exo face of TS 182b (this avoids a possible strain between R and NO or between H and [48b] that is presumably present in TS 182 a). Since elimination of silanol involving H in no way interferes with the orientation of H and R, a trans relationship between H and is abundantly clear. This fully accords with the widely accepted view that approach of the dipole and dipolarophile takes place in two parallel planes [49] and that the endo TS is preferred in the absence of obvious steric effects [50]. Formation of approximately 5% cis isomer when the dipolarophile terminus is disubstituted is accountable in terms of the cycloaddition taking place via TS 182a. 

Entries 3 to 6 are examples of ester enolate alkylations. These reactions show stereoselectivity consistent with cyclic TSs in which the hydrogen is eclipsed with the enolate and the larger substituent is pseudoequatorial. Entries 4 and 5 involve SN2 substitutions of allylic halides. The formation of the six- and five-membered rings, respectively, is the result of ring size preferences with 5 > 7 and 6 > 8. In Entry 4, reaction occurs through a chairlike TS with the tertiary C(5) substituent controlling the conformation. The cyclic TS results in a trans relationship between the ester and vinylic substituents. 

A peptide chain consists of a succession of -C(=0)-N-C - atoms that are coplanar, with the C-N bond being shorter than that of a normal amide. There is a partial sharing of the 7t-electrons between the C=0 and the C-N bond, giving the latter double-bond character (-40%), so that it is unable to rotate freely. The NH-proton and the oxygen atom are in the same plane but in a trans relationship (Figure 6.24). 

Highly functionalized 3,4-dihydroqumolin-2(l//)-ones 189 have been prepared via sequential Ugi/acrylamide [6 7t] photocyclization (PhC) reaction with a preferential trans relationship between the substituents R and (Fig. 39) [151]. 

ZjE-Isomers - These are considered here configurational rather than conformational isomers (see Section 1.1.3.6.). The descriptors Z and E must not be applied to characterize cis or trans relationships of substituents of rings. 

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