Stereoisomers, Preparation

The epoxidation procedures described here are not general ones they are used in special cases to prepare stereoisomers that are difficult to obtain by other means. 2,4-Disubstituted-6-hydroxymethylphenols have been oxidized with good yields to spirooxirane derivatives with sodium periodate in the case of bulky substituents, for example, when R = R = ferf-butyl (Eq. 47). ... 

Several research groups have, however, prepared stereoisomers and positional isomers of LTB4 to better understand the structural requirements of biological activity. Two groups have reported total syntheses of LTB [55,125-6,10 ,8,14Z)-dihydroxyeicosatetraenoic acid] 107, a naturally occurring LTB4 isomer derived from a double lipoxygenation of arachidonic acid (see Scheme... 

The Wittig reaction with dialdose derivatives has also been employed to synthesize polyols of long-chain hydrocarbons, as illustrated in Scheme 2 lyxose and xylose derivatives were similarly used to prepare stereoisomers which are relatives of naturally occurring plant products.- ... 

A strategy for preparing stereoisomers of castanospermine has appeared in which the key step involves asymmetric allylation of the v>-ribo-, L-arabino- or D-jcy/o-aldehydes 54 as shown in Scheme 9. ... 

The 9 — 15 fragment was prepared by a similar route. Once again Sharpless kinetic resolution method was applied, but in the opposite sense, i.e., at 29% conversion a mixture of the racemic olefin educt with the virtually pure epoxide stereoisomer was obtained. On acid-catalysed epoxide opening and lactonization the stereocentre C-12 was inverted, and the pure dihydroxy lactone was isolated. This was methylated, protected as the acetonide, reduced to the lactol, protected by Wittig olefination and silylation, and finally ozonolysed to give the desired aldehyde. 

Allylic amines can be cleaved. Hydrogenolysis of allylic amines of different stereochemistry with NaBH CN was applied to the preparation of both dia-stereoisomers 655 and 657 of cyclopentenylglycine from the cyclic amines 654 and 656 of different stereochemistry[405]. 

A second isomer of [lOJannulene (the cis trans cis cis trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydro gens directed toward the interior of the ring To minimize the van der Waals strain between these hydrogens the nng adopts a nonplanar geometry which limits its ability to be stabilized by tt electron delocalization It too has been prepared and is not very stable Similarly the next higher (4n + 2) system [14]annulene is also somewhat desta bilized by van der Waals strain and is nonplanar... 

The preparation of cis 4 tert butylcyclohexanol from its trans stereoisomer was earned out by the following sequence of steps Write structural formulas including stereochemistry for com pounds A and B... 

The structure of the bicychc monoterpene borneol is shown in Figure 26 7 Isoborneol a stereoisomer of borneol can be prepared in the labora tory by a two step sequence In the first step borneol is oxidized to camphor by treatment with chromic acid In the second step camphor is reduced with sodium borohydride to a mixture of 85% isoborneol and 15% borneol On the basis of these transformations deduce structural formulas for isoborneol and camphor... 

If you synthesized the tnpeptide Leu Phe Ser from amino acids prepared by the Strecker synthesis how many stereoisomers would you expect to be formed ... 

Long-chain primary alcohols, eg, triacontanol, can be prepared by the hydroboration, isomerization, and oxidation of the corresponding internal alkenes (437). The less thermodynamically stable stereoisomer can be transformed into the more stable one by heating, eg, i j -into /ra/ j -myrtanjiborane (204). 

Methylenetetrahydrofohc acid (5,10-CH2-H4 folate) (5) is a coen2yme in thymidylate biosynthesis the natural (6R)-stereoisomer is prepared by en2ymatic reduction of H2 folate (2), foUowed by condensation with formaldehyde (54). 

Annulene was first prepared in 1960. ° Its NMR spectrum has been investigated and shows that two stereoisomers are in equilibrium ... 

Since the stereochemistry of the triene system of LTB4 had not been determined prior to synthesis, a number of stereoisomers of LTB4 were prepared for purposes of definitive comparison of physical properties and bioactivity with biologically produced LTB4. The various stereoisomers of LTB4 were much less active biologically than LTB4 itself. 

From a mixture of two different aldehydes, each with a-hydrogens, four different aldols can be formed—two aldols from reaction of molecules of the same aldehyde -I- two crossed aldol products not even considering possible stereoisomers (see below). By taking into account the unsaturated carbonyl compounds which could be formed by dehydration from the aldols, eight different reaction products might be obtained, thus indicating that the aldol reaction may have preparative limitations. 

Saturation of the aromatic ring of pentopril analogues is also consistent with ACE inhibition as demonstrated by the oral activity of indolapril (23). The necessary heterocyclic component (21) can in principle be prepared by catalytic perhydrogenation (Rh/C, HOAc) of the corresponding indole. A single isomer predominates. The product is condensed by amide bond formation with the appropriate alanylhomophenylalanyl dipeptide ester 20 to give 22. Selective saponification to 23 could be accomplished by treatment with HCl gas. Use of the appropriate stereoisomers (prepared by resolution processes) produces chiral indolapril [8]. 

By the dry distillation of trimethyl - thujylammonium hydroxide, Tschugaeff obtained a thujene quite similar to the above, but of considerably higher optical rotation. He therefore considers that two stereoisomers may result from different methods of preparation from thujone. 

A hydrocarbon was also prepared by Kondakoff, to which he gave the name tbujene, which may have been a crude mixture of the two stereoisomers. The characters of these bodies are summarised in the following table —... 

A Preparation of a-Ethyl-m-Nitrocinnamic Acid This acid is prepared from 100 g of m-nitrobenzaldehyde, 210 g of butyric anhydride and 73 g of sodium butyrate. The crude a-ethyl-m-nitrocinnamic acid is crystallized from ethanol giving about 105 g, MP 140° to 142°C. From the filtrates there may be isolated a small amount of a stereoisomer, which when pure melts at 105° to 106°C. 

Alkylamines have a variety of applications in the chemical industry as starting materials for the preparation of insecticides and pharmaceuticals. Labetalol, for instance, a so-called /3-blocker used for the treatment of hi h blood pressure, is prepared by SN2 reaction of an epoxide with a primary amine. The substance marketed for drug use is a mixture of all four possible stereoisomers, but the biological activity derives primarily from the (R,R) isomer. 

The key intermediate 25 was prepared efficiently from aldehyde 23, obtained by reduction of nitrile 22 with Dibal-H. Treatment of 23 with the lithium salt of frans-diethyl cinnamylphosphonate furnishes compound 24 in 75 % yield and with a 20 1 ratio of E Z olefin stereoisomers. The stage is now set for the final and crucial operations to complete the molecular skeletons of endiandric acids A and B. 

Other important porphyrins which can be derived from hemin are hematoporphyrin (5) and mesoporphyrin (6). Hematoporphyrin (5) which is commercially available at a relatively low price is sensitive towards acid due to the 1-hydroxyethyl groups, so commercial samples contain only 60 to 70% of hematoporphyrin. Pure hematoporphyrin dimethyl ester which is a racemic and diastereomeric mixture of four stereoisomers can be obtained by esterification with diazomethane and subsequent chromatography on neutral alumina.84 The pure stereoisomers can be prepared by enantioselective reduction of diacetyldeuteroporphyrin dimethyl ester.85a b The... 

Single isobacteriochlorin stereoisomers even in enantiomcrically pure form can be obtained230 when the Claisen rearrangement is performed with the pure hematoporphyrin stereoisomers23d which can be prepared by stereogenic enantioselective reduction from diacetyl deuteroporphyrin dimethyl ester. 

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