Stereospecific reactions derivatives

The thermal solid-to-solid cyclization reaction of diallene derivatives also proceeds stereospecifically. Reaction of 1,6-diphenyl-1,6-di(p-tolyl)hexa-2,4-diyne-l,6-diol (113) with HBr gave meso- (114) and rac-3,4-dibromo-l,6-di-... 

Ajtai, K. (1992) Stereospecific reaction of muscle fiber proteins with the 5 or 6 iodoacetamido derivative of tetramethylrhodamine Only the 6 isomer is mobile on the surface of SI. Biophys. J. 61, A278, Abstract 1647. 

Kahn and Chen (87TL1623) performed the regio- and stereospecific reaction of 1,3-cyclooctadiene and acyliminium derivative 79, which resulted exclusively in compounds 80 in 37% yield after chromatographic purification. 79 was prepared by treatment of the A-methylimine of benzal-dehyde with methyl chloroformate in the presence of titanium(IV) chloride. 

Numerous examples of stereospecific reactions in the gas phase are reported in the mass spectrometric literature [1,2]. Many if not most of them, however, deal with relatively rigid systems, e.g., 1, or polyfunctional molecules such as derivatives of tartaric acid [6] the latter gave rise to the first chirality effect observed in mass spectrometry [7]. For stereogenic centers linked by flexible alkyl chains, however, diastereoisomeric differentiation in ion fragmentation is often poor. Two epimers of the aminoalkanol 2, for example, show quite small differences in their mass spectra whereas these differences increase if the two centers are linked by cyclization upon formation of 3 as indicated in Scheme 2 with the epimeric center being marked by an asterisk [8]. 

Crystalline traras-cinnamic acid forms cyclobutane derivatives upon irradiation of the crystal when C=C double bonds in neighboring molecules interact with each other to give cyclobutane derivatives, as illustrated in Figure 18.1. In solution or in the molten state, the only reaction observed is isomerization from trans- to cfs-cinnamic acid, but in the crystalline state the stereospecific reaction to form cyclobutane derivatives may occur. 

In this stereospecific reaction, the D(—) isomer is converted to a /ra 5-a, S-unsaturated acyl-ACP derivative. 

Mention should also be made of the addition of tervalent phosphorus compounds to alkynylphosphonic and to (alka-l,2-dienyl)phosphonic derivatives [in which addition occurs across the C(i)—C(2) bond]. The main product is of type 521 (X = Ph, OR or NR2), but is normally accompanied by smaller amounts of the isomers 522. Although this process is formally of a [3 + 2] format, a P NMR study of the kinetics suggests that the addition is not a synchronous addition but rather occurs in a stepwise manner . On the other hand, the phosphorylated Schiff base 523 undergoes regioselective and stereospecific reaction with a,j -unsaturated esters which, a kinetic study has shown, appears to be concerted. ... 

Two reports of Simmons-Smith methylene addition (iodomethylzinc iodide) on to A -olefinic steroids contain several points of disagreement. The earlier paper states that cholest-5-en-3a-ol (epicholesterol) reacts readily to give the 5a,6a-methano-derivative (165). A stereospecific reaction in this sense would accord with earlier indications that a suitably placed hydroxy-group directs the approach of the reagent towards the same side of the olefinic bond. The 3/ -alcohol (cholesterol), as well as the methyl ethers and acetates of both alcohols, apparently failed to react. The later paper, by different authors, states that both unsaturated alcohols react, each giving a similar mixture of the 5a,6a- and 5) ,6/ -methano-adducts. with no evidence for control by the oxygen function. 

Ethylene derivatives from acetals Stereospecific reaction... 

A new synthesis of chiral cyclopentanoid intermediates involves the stereospecific reaction of the carbanion (30) with the tartaric acid derived epoxide (31). An intramolecular enolate anion alkylation, induced by a Michael reaction, has been utilized in the preparation of cyclopentanes (Scheme 6). Additionally, variation... 

Stereospecific reaction—Walden inversion s. 18, 431 reduction with Raney-Ni, purine derivatives, s. W. Jahn, B. 98, 1705 (1965)... 

Further syntheses of KDO (26) have appeared. Dondoni and coworkers have used 2-acetyl thiazole as a pyruvate synthon in a stereospecific reaction with an a Idehydo-D-arahinose derivative (Scheme 7),31 whilst in another... 

Some electrophiles require a separate activation step, as in the stereospecific reaction of alkenes with benzenesulfenyl chloride (eq 15). The intermediate that actually reacts with the aUylsilane is presumably the episulfonium ion. There is a corresponding reaction of epioxonium ions derived from 2-bromoethyl ethers. Intramolecular hydrosilylation of an ester generates an acetal, which reacts with allyltrimethylsilane in the usual way and with high stereocontrol to make an anti 1,3-diol derivative (eq 16). ... 

Stereospecific reactions acoxy-2-ethylenes from ethylene derivatives 14, 211... 

Stereospecific reactions alcohols from ethylene derivatives 19, 188 amines, axial, from sulfonic acid esters 19, 418 ethylene derivatives from glycols 19, 962... 

You may already have realized that we are back Through the Looking Glass, with Alice. The regiochemistry of the reaction derives from looking at it as an electrophilic process, while the stereochemistry comes from considering it as concerted. The real reaction is both regio- and stereospecific. 

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