Partial racemization, reactions occurring

If the product and the starting material have opposite configurations, the reaction occurred with inversion. If the product has the same configuration, the reaction went with retention of configuration. If both process occurred to the same extent, the product would be completely racemized (100% racemization) and if the two processes were not equal, partial racemization would occur. 

A further advantage of the new process lies in the fact that the reaction proceeds under relatively mild conditions with fewer side reactions occurring concurrently. A reaction temperature of 80 to 90° is as a rule most suitable. The time of reaction at this temperature is in the range of one to a few hours, whereas at e.g. 135° it is only a few minutes. Temperature and time conditions are of course chosen as mild as possible, as with unnecessarily long reaction times and high temperatures partial racemization may occur. 

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Preparation of the appropriate optically active sulfmate ester is initially required for reaction with a Grignard or other organometallic reagent. If the method is to produce homochiral sulfoxides, the precursor sulfmate ester must be optically pure. An exception to this statement occurs if the reaction yields a partially racemic sulfoxide which can be recrystallized to complete optical purity. 

On the other hand, it is found that only partial racemization occurs on alkaline hydrolysis of optical active 198 in aqueous methanol136) and no racemization takes place in the hydrolysis of 199 in dioxane/water137). Moreover, the latter reaction is only ca. 80 times faster at 29 °C than that of the analogous morpholide 200, for which a metaphosphorimidate mechanism is precluded a priori by the absence of an NH function and whose hydrolysis is likewise stereospecific,37). Clearly a free metaphosphorimidothioate of type 191 cannot be involved in this case. The experimental findings are compatible, however, with the hypothesis that the nucleophile water attacks a metaphosphorothioimidate/phenolate associate 201. The question of how free metaphosphates occur in solution is of a general nature it has also been considered in the previous Section. 

The hydrogenation of 109 over palladium on charcoal in acidic methanol produced (l/ ,7aA)-l,3-diphenyl-5,6,7,7a-tetrahydro-1 //-pyrrolo[ 1,2-r]imidazolc 110 in 61% isolated yield (Equation 14). This reaction was totally unexpected and the authors proposed a mechanism that explains this transformation <2005JOC2368>. The mechanism is shown in Scheme 13. Many of the steps are interchangeable, but the end product is the same. The partial racemization observed could occur by epimerization of the phenyl-substituted carbon adjacent to the imine produced after the cleavage of the bicyclo-adduct. 

The behavior of chiral phenyl /-butyl sulfoxide 219 and a-phenyl-ethyl phenyl sulfoxide 220 is completely different in strongly acidic media and in the presence of halide ions. Two reactions were found (266) to occur in parallel. One results in the loss of optical activity, and the second leads to the decomposition of the sulfoxide. It was observed that the racemization process is not accompanied by [ 0] oxygen exchange. In the case of sulfoxide 220 the complete loss of optical activity at chiral sulfur is accompanied by partial racemization at the chiral carbon center. These results are consistent with a sulfenic acid-ion-pair mechanism formulated by Modena and co-workers (266) as follows (it is obvious that the formation of achiral sulfenic acid is responsible for racemization). 

Replacement of a group or atom on a chiral center can occur with retention or inversion of configuration or with a mixture of the two (complete or partial racemization), depending on the mechanism of the reaction. 

Carbon-Nitrogen Bonds. Several groups have studied the synthesis of optically active a-amino acids from inexpensive and readily available a-haloesters by displacement with phthalimide in the presence of chiral cinchona catalysts [1 le,24h,24i,47e,60d,77]. Early studies, with chiral, non-racemic starting material, showed that this reaction occurs with partial... 

In 1961 it was reported that the reaction of chiral l-bromo-l-methyl-2,2-diphenylcyclopropane (51) with magnesium metal produced a partially optically active Grignard reagent . It was suggested that the racemization observed occurred in the Grignard formation step. In 1964 it was also established that the racemization occurred... 

When the carboxyl moiety of the CDA is thus activated, often to the acid chloride, (partial) racemization of the reagent may be a possibility, depending on the nature of the add used and reaction conditions of activation. In some of the reports using such activation, it was ascertained that no significant racemization occurred, but in many other studies no such check was made. 

Reaction of the intermediate radicals with rm-butyl mercaptan or carbon tetrachloride, however, is not fast enough and partial racemization occurs. Trapping with samarium(II) iodide, is faster and the reduced product finally obtained is isolated with almost complete retention of configuration5. 

A reaction of the dinitrile (83) with hydrogen chloride in ether yielded the enaminonitrile (84). From the unsaturated (85), a dark blue immonium salt (86) was obtained <80JCR(S)233>. A new metabolite from Streptomyces olivaceus has been shown to be (25)-l-oxo-2,3-dihydropyrrolizin-3-carboxylic acid (88b) by total synthesis. Compound (87) was cyclized stereoselectively with phosphorus pentoxide in toluene. Partial racemization occurred during the hydrolysis of (88a) <81TL3293>. 

The (S)-lactone acid 1, obtained from L-glutamic acid by nitrous acid deamination, was converted to the acid chloride, then treated with excess diazomethane followed by hydrogen iodide to yield the keto-lactone 2. Amidation occurred quantitatively to give the partially racemized amide 3, which was purified by repeated recrystallizations. The vicinal diol resulting from reaction with excess methylmagnesium iodide was protected as the acetonide 4. An isomeric mixture of olefins (Z , 26 74) was obtained from the subsequent Wittig reaction. Reduction followed by separation on silver nitrate coated silica gel gave the (Z)-and ( )-alcohols in 20% (6) and 61% (5) yield, respectively. Conversion of the (S)-( )-alcohol (5) to the chloride then afforded the thioether (7) on reaction with sodium phenylsulfide. The thio ether anion was formed by treatment with n-butyllithium. Alkylation with the allylic chloride" (8), followed by removal of sulfur, then yielded the diene 9, which was converted in several steps to (/ ) (-t-)-10,11 -epoxy famesol. 

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