Erythro diastereoisomerism

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

The sulfone derivative is metallated 12 and attached to the carbonyl 13, followed by functionalization 15, and reductive elimination, to produce the alkene 3. The diastereoisomeric erythro- and f/irco-acetoxy sulfones 15 could be separated and both isomers were converted to the /rans-alkene. It was hypothesized that the (jE)-selectivity is derived from the reductive removal of the arylsulfonyl group, generating an anion 17 that assumes the low energy tra/is-configuration before loss of the acylate anion. As demonstrated by numerous examples, the mechanism for reductive elimination is consistent with the frnding that the alkenes obtained are the thermodynamic mixture and that increased branching at the site of elimination should, for steric reasons, increase the trans selectivity. 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

Dihydromuscimol (49) is a conformationally restricted analogue of the physiologically important neurotransmitter y-aminobutyric acid (GABA) and has been prepared using the cycloaddition of dibromoformaldoxime to A-Boc-allylamine followed by N-deprotection with sodium hydroxide (Scheme 6.52) (278). The individual enantiomers of dihydromuscimol were obtained by reaction of the bromonitrile oxide with (5)-( + )-l,2-0-isopropylidene-3-butene-l,2-diol, followed by separation of the diastereoisomeric mixture (erythro/threo 76 24), hydrolysis of respective isomers, and transformation of the glycol moiety into an amino group (279). 

Taking a simple general example to explain the usefulness of this reaction, treatment with base (usually butyllithium) of the alkyldiphenylphosphine oxide gives the anion which then reacts with an aldehyde to form the corresponding-erythro threo diastereoisomeric mixture of alcohols (cf. p. 517) in unequal proportions only one enantiomer of each diastereoisomeric pair is formulated [(8)... 

An alternative route to the erythro.threo diastereoisomeric mixture of alcohols results from the acylation of the alkyldiphenylphosphine oxide with an ester or lactone to yield the /J-ketophosphine oxide, followed by reduction with sodium borohydride. This reduction shows f/ireo-selectivity, so that, following separation of the diastereoisomers, a preparatively useful route to ( )-alkenes is achieved. 

Reaction with aldehydes.1 The reagent (1) reacts with propionaldehyde to form one diastereoisomeric form of a cyclic saturated oxyphosphorane structure with the 1,3-dioxaphospholane ring system (2). The product is hydrolyzed by water in benzene solution to give an erythro-a,/8-dihydroxy ketone (3). Compare 1,1233. 

The diastereoisomeric configuration of the sugar-free cerebrosides (ceramides) was determined by Carter and coworkers,80 and found to be erythro, in accordance with the structure of sphingosine, which is isolated after drastic hydrolysis of sphingolipid mixtures or purified cerebrosides with acid. 

Comparison of diastereoisomeric pairs has been used to demonstrate the "syn" nature of pyrolytic elimations. Both acetates of 2-deutero-l, 2-diphenyl-ethanol undergo pyrolysis to give tra/i -stilbene but the product from the erythro substrate is labelled with deuterium (135) while that from the threo substrate only contains hydrogen (136), viz. 

Steric interactions in the cyclic transition structures for a hydrogen transfer result in different abundance of the butene radical cation fragment, obtained from the diastereoisomeric unsaturated alcohols erythro-10 and threo-10. The higher energy of the transition state of erythro-10 due to the steric interaction of the methyl and R groups, results in a lower abundance of the fragment, as compared with the stereoisomeric threo-9 (see Scheme 9). 

Four different patterns of adsorption regarding the retention of the diastereoisomeric compounds of type I (Figure 8.6) are discussed by Palamareva et al. [23]. The data and the conclusions of the study supported the previously elaborated criteria for TLC assessment of the relative configurations of other diastereoisomeric pairs of tetrasubstituted ethanes (type I) based on the relation RB erythro) > Rv(threo) The main requirement is that the groups X and Y to be adsorbed more strongly, and to have smaller effective volumes, than the groups Ar and Ar. The results also provide a further development of the criteria in the case where X is smaller and Y is bulkier than Ar and Ar. ... 

Several general observations of retention behaviour of isomeric and diastereoisomeric compounds are evident. Acids with the erythro configuration at C-2 and C-3 are eluted before acids of the threo configuration,... 

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