Inversion double

In this aforementioned Heine reaction the initial ring opening takes place by iodide ions. Subsequent ring closure by S 2 displacement of iodide by reaction with the negative oxygen center then leads to the products. This process proceeds with double inversion at the same carbon atom, thus with net retention. Hydrolysis of these oxazolines gives j9-hydroxy-a-amino acids (Scheme 31) [1,38]. The stereochemical course of ring expansion is the same as that observed in Scheme 29. 

The dienone intermediate (53a), as well as enolising to the phenol (52a), is itself capable of undergoing a Cope rearrangement to yield a second dienone (cf. 56a), whose enol is the p-substituted phenol (c/ 57a). Enolisation normally predominates, but where (51) has o-substituents, i.e. (54a), o-enolisation cannot take place, and only the p-phenol (57a) is then obtained. That this product is indeed formed not by direct migration of the allyl group, but by two successive shifts, is suggested by the double inversion of the position of the, 4C label in the allyl group that is found to occur ... 

Further confirmation of the two-fold shift, and of the double inversion of the position of the 14C label, is provided by trapping (cf. p. 50) the first dienone intermediate (55a) with maleic anhydride in a Diels-Alder reaction. An exactly analogous rearrangement is found to occur in allyl ethers of aliphatic enols, e.g (58) ... 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

However, it is often observed that the correlation of addn/rearr vs. [alkene] is not linear, but curved.17-20,25-28 On the other hand, a double inverse plot of [4]/[5] (rearr/addn) vs. l/[alkene] is linear These results can be understood if a second intermediate, besides the carbene, contributes to the formation of product 4. The kinetic and mechanistic formulation of this scenario appears below (Section III.B). 

An alternative approach, in which the anomeric centre of the sugar is reacted with a carbon nucleophile, but the amino group is subsequently inserted at the other side or in both sides at the same time, is reported in Fig. 41 for the synthesis of a- and (3-homogalactonojirimycin.72 Reaction of 2,3,4,6-tetra-O-benzyl-p-D-galactopyranose with methylenetriphenylphosphorane generated an enitol on which the amino group was inserted as phtalimide by a double inversion process of the 5-OH, under Mitsunobu s conditions. 

A Hydrolysis mechanism of retaining glycosidase (R = saccharide or alcohol) - double inversion at the anomeric centre. 

Treatment of the enantiomerically enriched acyclic allylic carbonate (S)-l (97% ee) under the standard reaction conditions furnished the allylic alkylation product (S)-14 (95% ee) in 86% yield, with net retention of absolute configuration (Eq. 3). This result implies that the displacement occurs via a classical double inversion process, albeit through a configurationally stable distorted u-allyl or enyl ff+n) organorhodium intermediate. This is supported by the fact that the achiral ff-spedes iii would undoubtedly have afforded the racemate of 14 (Scheme 10.3). Additionally, the enyl (a+n) organo-metallic intermediate provides a model for the regio- and enantiospedfidty observed in the reaction. 

A thermal solid-state N-N double inversion between kinetically stable in-vertomers (exo/endo isomerism) was studied by high-temperature X-ray dif-... 

The fluorinase enzyme mediates a substitution reaction whereby fluoride ion displaces L-methionine with formation of a C-F bond and cleavage of a C-S bond. In order to explore the mechanism in detail, the process has been explored both experimentally and theoretically. The stereochemical course of the reaction was of immediate interest, particularly to delineate between direct inversion or double inversion during the substitution process. A direct substitution will proceed with inversion of configuration at the C5 carbon, whereas a double inversion process will proceed with an overall retention of configuration at the C-5 carbon. To explore this, it was important to prepare SAM 8 carrying a stereospecifically... 

Epoxides also undergo the Ritter reaction in good yields with retention of configuration via a episulfonium intermediate 190a (double-inversion process). For monosubstituted epoxides, the yields of oxazolines are lower due to nondis-criminatory attack of the nitrile on both the primary and the secondary carbon atom of the episulfonium intermediate. Complete retention of configuration is still observed despite the lower yield (Scheme 8.54). 

Appropriately substituted azolyldieneamines were found to undergo double inverse electron demand Diels-Alder reactions with tetrazine derivatives, yielding azolylpyridazines and... 

In a synthetic application of this double inversion sequence, tris(benzyloxy)bromo boronic ester 6 obtained in the ribose sequence (Section 1.1.2.1.3.2.) is converted to the 4-methoxyphenyl-methoxy derivative 7 in the usual way. 2,3-Dichloro-5,6-dicyano-l,4-benzoquinone cleaves the protecting group to furnish the a-hydroxy boronic ester 8. Conversion of the a-hydroxy boronic ester 8 to the methanesulfonate 9 followed by displacement of the sulfonate by phenylmethoxide yields a-benzyloxy boronic ester 10, which is a diastereomer of one of the ribose intermediates37. 

The double inversion sequence provides unequivocal evidence that the nucleophilic displacements assisted by neighboring boron proceed with inversion of configuration at carbon. 

The mechanism proposed by Foster49 for this deamination postulates the formation of an unstable, intermediate a-lactone (22a), which subsequently undergoes attack by the hydroxyl group on C-5 this process would explain, by a double inversion, the net retention of configuration that is observed. 

---