Reactions with stereochemistry

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

It was claimed that the Z-form of the allylic acetate 430 was retained in homoallylic ketone 431 obtained by reaction with the potassium enolate of 3-vinylcyclopentanone (429), after treatment with triethylborane[282]. Usually this is not possible. The reaction of a (Z)-allylic chloride with an alkenylaluminum reagent to give 1,4-dienes proceeds with retention of the stereochemistry to a considerable extent when it is carried out at -70 C[283]. 

In the olivanic acid series of carbapenems the ( )-acetamidoethenyl grouping can be isomerised to the (Z)-isomer (19) (22) and reaction with hypobromous acid provides a bromohydrin that fragments to give a thiol of type (20) when R = H, SO H, or COCH. The thiol is not isolated but can react to provide new alkyl or alkenyl C-2 substituents (28). In the case of the nonsulfated olivanic acids, inversion of the stereochemistry at the 8(3)-hydroxyl group by way of a Mitsunobu reaction affords an entry to the 8(R)-thienamycin series (29). An alternative method for introducing new sulfur substituents makes use of a displacement reaction of a carbapenem (3)-oxide with a thiol (30). Microbial deacylation of the acylamino group in PS-5 (5) has... 

At Smith Kline French a general approach to cephalosporin and penicillin nuclear analogs was developed that utilizes a monocyclic /3-lactam (59) with the correct cis stereochemistry as a key intermediate. This is prepared by reaction of the mixed anhydride of azidoacetic acid and trifluoroacetic acid with imine (58) followed by oxidative removal of the dimethoxybenzyl group. This product could be further elaborated to intermediate (60) which, on reaction with a -bromoketones, provides isocephalosporins (61). These nuclear analogs displayed antibacterial properties similar to cephalosporins (b-79MI51000). 

In summary, modem synthetic methodology allows the stereoselective generation of one, two, or even more stereocenters in a single reaction with or without spatial control by the substrate. The application of transforms to retrosynthetic simplification of stereochemistry requires the selection of transforms on the basis of both structural and stereochemical information in the target and also validation of the corresponding synthetic processes by analysis for both chemical feasibility and stereoselectivity. 

Other sulfonate derivatives are obtained by the use of trifluoromethanesulfonyl hypochlorite and hypobromite (CF3SO2OQ and CF3S020Br) in reactions with petfluoroalkyl halides and their derivatives [30. These reactions lead to the corresponding trifluoromethanesulfonate derivatives of alkanes (equation 28) (Table 11). The reaction proceeds with complete retention of stereochemistry at the carbon center [30]. 

Several factors influence the diastereoselectivity of the Pictet-Spengler condensation to form 1,3-disubstituted and 1,2,3-trisubstituted tetrahydro-P-carbolines (39 and 40, respectively). The presence or absence of an alkyl substituent on the nitrogen of tryptophan has a large influence on the relative stereochemistry of the tetrahydro-P-carboline products formed from a condensation reaction with an aldehyde under various reaction conditions. 

The desilylacetylated qrcloadducts, produced from the reactions of trimethylsilyl-diazomethane with 3-crotonoyl-2-oxazolidinone or 3-crotonoyl-4,4-dimethyl-2-oxa-zolidinone, were transformed to methyl traws-l-acetyl-4-methyl-l-pyrazoline-5-car-boxylate through the reactions with dimethoxymagnesium at -20 °C. When the optical rotations and chiral HPLC data were compared between these two esters, it was found that these two products had opposite absolute stereochemistry (Scheme 7.39). The absolute configuration was identified on the basis of the X-ray-determined structure of the major diastereomer of cycloadduct derived from the reaction of trimethylsilyldiazomethane to (S)-3-crotonoyl-4-methyl-2-oxazolidi-none. 

The stereoisomers of olefin saturation are often those derived by cis addition of hydrogen to the least hindered side of the molecule (99). But there are many exceptions and complications (97), among which is the difficulty of determining which side of the molecule is the least hindered. Double-bond isomerization frequently occurs, and the hydrogenation product is the resultant of a number of competing reactions. Experimentally, stereochemistry has been found to vary, sometimes to a marked degree, with olefin purity, reaction parameters, solvent, and catalyst 30,100). Generalizing, it is expedient, when unwanted products arise as a result of prior isomerization, to avoid those catalysts and conditions that are known to favor isomerization. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

Assign configuration to the following substrate, and show the stereochemistry and identity of the product you would obtain by SK1 reaction with water (reddish brown = Br) ... 

A Dimethyl butynedioate undergoes a Diels-Alder reaction with (2 ,4 )-hexadiene. Show the structure and stereochemistry of the product. 

Write the mechanism of the hydrolysis of t/s-5,6-epoxydecane by reaction with aqueous acid. What is the stereochemistry of the product, assuming norma] backside SK2 attack ... 

Name the following amine, including R,S stereochemistry, and draw the product of its reaction with excess iodomethane followed by heating with Ag20 (Hofmann elimination). Is the stereochemistry of the alkene product Z or E Explain. 

Adenosine triphosphate, coupled reactions and. 1128-1129 function of, 157, 1127-1128 reaction with glucose, 1129 structure of, 157, 1044 S-Adenosylmethionine, from methionine, 669 function of, 382-383 stereochemistry of, 315 structure of, 1045 Adipic acid, structure of, 753 ADP, sec Adenosine diphosphate Adrenaline, biosynthesis of, 382-383 molecular model of, 323 slructure of, 24... 

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

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