Diastereoisomer

Menthol can also be synthesized from optically active terpenoids such as (+)-citroneUal, (-)- P-pheUandrene, and (+)-3-carene. The synthesis from (+)-3-carene has already been discussed in the section on hydrocarbons. Such methods must avoid any racemization during the course of a usually multiple-step synthesis. One disadvantage of such methods is that the other menthol diastereoisomers must be equilibrated and recycled. 

Vitamin K compounds ate yellow solids or viscous liquids. The natural form of vitamin is a single diastereoisomer with 2 (E), 7 (R), ll (R) stereochemistry. The predominant commercial form of vitamin is the racemate and a 2 (E)j (Z) mixture. Table 1 fists some physical and spectral properties of vitamin K. ... 

The [2 + 2] cycloaddition reaction of A -benzyl-l,4-dihydropyridine 34b with acrylonitrile, followed by catalytic reduction gave two pairs of diastereoisomeric amides 36 and 37 with a low diastereomeric excess, probably due to the large distance between the asymmetric center and the site of acrylonitrile attack. Compounds 36 and 37 were resolved into the four individual diastereoisomers (ca 5% for compound 36 and 15% for 37) [97JCR(M)321], Irradiation of 1,4-dibenzyl-1,4,5,6-tetrahydropyridine 38 in the presence of 29 gave two stereoisomers. 

The validity of the model was demonstrated by reacting 35 under the same reaction conditions as expected, only one diastereoisomer 41 was formed, the structure of which was confirmed by X-ray analysis. When the vinylation was carried out on the isothiazolinone 42 followed by oxidation to 40, the dimeric compound 43 was obtained, showing that the endo-anti transition state is the preferred one. To confirm the result, the vinyl derivative 42 was oxidized and the intermediate 40 trapped in situ with N-phenylmaleimide. The reaction appeared to be completely diastereoselective and a single diastereomer endo-anti 44 was obtained. In addition, calculations modelling the reactivity of the dienes indicated that the stereochemistry of the cycloaddition may be altered by variation of the reaction solvent. 

Reaction of 2-[A -(rra -crotyl)-A -benzylamino]-3-formyl-4/f-pyrido[l,2-n]pyrimidin-4-one (269) with chiral primary amines 270 and 271 gave mixtures of diastereoisomers of tetracyclic compounds 273 and tricyclic 275 (96T131]]). The chiral centers in 272 and 274 did not provide any stereocontrol for the formation of diastereomers 273 and 275, respectively. 

Thermal cyclization of 2-vinyl-N,N-dialkylanilines 138 afforded 139 with creation of a new chiral center in 98% purity (89JOC199). In case of pyrrolidine with methyl or methoxymethyl substituent, cyclization with ZnCl2 occurs via an irreversible 1,5-hydrogen shift in boiling acetonitrile (87JA3136) or BuOH (91RTC115) to afford the diastereoisomers 140 (33%), 141 (35%) and 142 (6%) (87JA3136) (Scheme 27). 

OOJA11009). Similarly, aryl acyl selenide 111 gave a mixture of diastereoisomers 112 and 113. 

An important stage in the synthesis has been reached. It was anticipated that cleavage of the trimethylsilyl enol ether in 18 using the procedure of Binkley and Heathcock18 would regiospecifically furnish the thermodynamic (more substituted) cyclopentanone enolate, a nucleophilic species that could then be alkylated with iodo-diyne 17. To secure what is to become the trans CD ring junction of the steroid nucleus, the diastereoisomer in which the vinyl and methyl substituents have a cis relationship must be formed. In the... 

An important task remaining is the stereocontrolled introduction of a methyl group at C-8. When a cold (-78 °C) solution of 14 in THF is treated successively with LDA and methyl iodide and then warmed to -45 °C, intermediate 24 admixed with minor amounts of the C-8 epimer is formed in a yield of 95 %. The action of LDA on 14 generates a lactone enolate which is alkylated on carbon in a diastereoselective fashion with methyl iodide to give 24. It is of no consequence that 24 is contaminated with small amounts of the unwanted C-8 epimer because hydrolysis of the mixture with lithium hydroxide affords, after Jones oxidation of the secondary alcohol, a single keto acid (13) in an overall yield of 80%. Apparently, the undesired diastereoisomer is epimerized to the desired one under the basic conditions of the saponification step. 

Still s synthesis of monensin (1) is based on the assembly and union of three advanced, optically active intermediates 2, 7, and 8. It was anticipated that substrate-stereocontrolled processes could secure vicinal stereochemical relationships and that the coupling of the above intermediates would establish remote stereorelationships. Scheme 3 describes Still s synthesis of the left wing of monensin, intermediate 2. This construction commences with an aldol reaction between the (Z) magnesium bromide enolate derived from 2-methyl-2-trimethylsilyloxy-3-pentanone (21) and benzyloxymethyl-protected (/ )-/ -hydroxyisobutyraldehyde (10).2° The use of intermediate 21 in aldol reactions was first reported by Heathcock21 and, in this particular application, a 5 1 mixture of syn aldol diastereoisomers is formed in favor of the desired aldol adduct 22 (85% yield). The action of lithium diisopropylamide (LDA) and magnesium(n) bromide on 21 affords a (Z) magnesium enolate that... 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

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