Retention of configuration reactions

Chloroacetoxylation of 6-benzyloxycyclohepta-1,3-diene was highly diastereoselective and produced only the diastereoisomer 65 shown. Transformation of the chloroacetate 65 to 66 was realized by a Pd(0)-catalyzed substitution of the chloride by a sulfonamide, which occurs with retention of configuration. Reaction of the allylic chloride with the sulfonamide... 

Intermediates (40) react at -110 C with TMS-Cl to give compounds (42), which are stable, readily purified products. The H NMR of (42) is in accord with the assigned stereochemistry and shows that the oxidation of (40) had indeed proceeded with retention of configuration. Reaction of (42) with HF/MeCN gives ( )-alkenes in excellent yields (Scheme 6) with none of the many by-products seen in the original reaction. The ( ) (Z) ratios range from 100 0 to 95 5 and the reaction tolerates NO2, Cl, OMe and alkyl groups in the aromatic aldehyde. ... 

The chloroacetoxylation approach was also used for the stereoselective synthesis of tropine and pseudotropine employing a sulfonamide as the nucleophile [91]. Using the same approach, scopine and pseudoscopine were synthesized (Scheme 11.26) [92]. The chloroacetoxylation of 6-benzyloxy-l,3-cycloheptadiene was highly diastereoselective, and produced only the diastereoisomer 73 shown. Transformation of the chloroacetate 73 to 74 was realized by a Pd(0)-catalyzed substitution of the chloride by a sulfonamide, which occurred with retention of configuration. Reaction of the aUyUc chloride with the sulfonamide salt in DMSO-water at 80 °C afforded the inversion product 75. Subsequent stereoselective epoxidation, cyclization, and deprotection afforded the target molecules scopine and pseudoscopine. 

The trichloroacetimidates are readily displaced to give substitution at C-1. They react with hydrogen halides to give chlorides, bromides, and fluorides, with retention of configurations (reaction 4.64). Carboxylic acids, being relatively weak acids, react to give inversion of configuration (reaction 4.65). 

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... 

The reaction of phenylzinc reagent proceeds with opposite stereochemistry, namely by retention of configuration at the final step via transmetallation. Both the (S)-( )- and (i )-(Z)-allylic acetates 4 and 9 afford the (/ )-( )-phe-nylated product II by overall inversion[23]. 

Imidazole can be A -allylated. The A -glycosylimidazole 299 is prepared by regiospecific amination at the anomeric center with retention of configuration. Phenoxy is a good leaving group in this reaction[181]. Heterocyclic amines such as the purine base 300 are easily allylatedfl 82]. 

The reaction is stereospecific the alkyl group migrates with retention of configuration O O... 

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

These reactions can be cataly2ed by bases, eg, pyridine, or by Lewis acids, eg, 2inc chloride. In the case of asymmetric alcohols, steric control, ie, inversion, racemi2ation, or retention of configuration at the reaction site, can be achieved by the choice of reaction conditions (173,174). Some alcohols dehydrate to olefins when treated with thionyl chloride and pyridine. 

Each reaction in the sequence shown is reported to proceed with retention of configuration, yet the starting material has the R configuration and the product has the S configuration. Reconcile this apparent contradiction. 

On the other hand, as a result of participation of a neighboring group, complete or predominant retention of configuration takes place in many reactions of hy-droxylic compounds with DAST A number of examples have been reported in the field of steroids [727, 729], m the conversion of vitamins D into fluoro vitamins D [745], and in the fluormation of liquid crystals [146] (equation 72]... 

The reaction of diethyl tartrate with sulfur tetrafluonde at 25 °C results in replacement of one hydroxyl group, whereas at 100 °C, both hydroxyl groups are replaced by fluonne to form a,a -difluorosuccinate [762] The stereochemical outcome of the fluonnation of tartrate esters is retention of configuration at one of the chiral carbon atoms and inversion of configuration at the second chiral center [163,164, 165] Thus, treatment ofdimethyl(+)-L-tartrate with sulfur tetrafluonde gives dimethyl meso-a,a difluorosuccinate as the final product [163, 164], whereas dimethyl meso tartrate is converted into a racemic mixture of D- and L-a,a -difluorosuccmates [765] (equation 80)... 

The alkylboranes obtained by the hydroboration reaction are versatile intermediates for further transformations. The most important transformation is the oxidation to yield alcohols 17 it is usually carried out by treatment with hydroperoxide in alkaline solution. The group R migrates from boron to oxygen with retention of configuration ... 

---