Diastereomers, alkyl halides groups

Dehydrohalogenation of alkyl halides is stereospecific, requiring an anti arrangement between the hydrogen being lost and the leaving group in the transition state. (Z)-l,2-Diphenylpropene must therefore be formed from the diastereomer shown. 

A useful mnemonic for deriving the preferred diastereomer formed in the alkylation reaction of pseudoephedrine amide eno-lates with alkyl halides is as follows the alkyl halide enters from the same face as the methyl group of the pseudoephedrine auxiliary when the (putative) ( -enolate is drawn in a planar, extended conformation (eq 1). ... 

Given that an E2 reaction proceeds with anti periplanar stereochemistry, draw the products of each elimination. The alkyl halides in (a) and (b) are diastereomers of each other. How are the products of these two reactions related Recall from Section 3.2A that CsHs- is a phenyl group, a benzene ring bonded to another group. 

C-Alkylations of l,4-dihydro-27/-pyrazino[2,l-A]quinazoline-3,6-diones at positions C-l and CM were studied in detail. Compounds of type 57 could be alkylated diastereoselectively at C-l, owing to the geometry of the piperazine ring, which is locked in a flat boat conformation with the R4 or R1 substituent in a pseudoaxial position to avoid steric interaction with the nearly coplanar C(6)-carbonyl group. Alkylation of 57 (R2 = Me, Bn, R4 = Me) in the presence of lithium hexamethyldisilazide (LHMDS) with benzyl and allyl halides resulted, under kinetic control, in the 1,4-trans-diastereomer 59 as the major product, with retention of the stereocenter at CM (Scheme 5). 

There is considerable evidence for this type of reaction in organoarsenic chemistry and, indeed dissociation-equilibrium was believed to be the cause of racemization of arsonium halides which, for synthetic reasons, usually contained at least one alkyl substituent (often" the benzyl group) on the arsenic atom. In 1939, fractional crystallization was achieved of the diastereomers of the diarsonium picrate 3 . The individual diastereo-mers of the salt, racemic (R, R ) and meso (R, S ), were stable in boiling ethanol, a fact that should have dispelled concerns of dissociation equilibria in halide-free arsonium salts. In 1940, the tetrahedral structure of [AsPh4]I was established by X-ray crystallography. ... 

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