Stereo centers

Stereochemical strategies The transform selection is guided by stereocenters that have to be removed in retrosynthesis. The user has to select strategic stereo-centers. 

Wlien an alkyl and an alkoxy moiety ate ptesenl al tlie 4-position of tlie dienone fSdieme 7.17fb)), desymmetrizalion during tlie 1,4-addition produces two stereo-centers in a single step. Hie diital coppet-pbospbotamidile calalyst derived from... 

It should be noted that the sense of asymmetric induction in the lithiation/ rearrangement of aziridines 274, 276, and 279 by treatment with s-butyllithium/ (-)-sparteine is opposite to that observed for the corresponding epoxides (i.e. removal of the proton occurs at the (S)-stereocenter) [102], If one accepts the proposed model to explain the selective abstraction of the proton at the (R) -stereo-center of an epoxide (Figure 5.1), then, from the large difference in steric bulk (and Lewis basicity) between an oxygen atom and a tosyl-protected nitrogen atom, it is obvious that this model cannot be applied to the analogous aziridines. 

The spiro compound 206 was prepared in five steps from (S)-l-naphthyl-ethylamine and was composed of a mixture of imine and enamine tautomers. Reduction of the imine function by sodium borohydride occurred on the less hindered si face, leading to the diamine with the R configuration of the newly formed stereo center, then the N-benzyl substituent was removed by hydrogenolysis to give 207 with good overall yield [98] (Scheme 30). 

Fig. 7.19 (a) Ambiguous 2D drawings of stereo centers, (b) Recommended encoding of tetrahedral atoms and double bonds. 

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

The present homoallylation with isoprene under Ni-Et3B catalysis shows marginal success for the reaction with aliphatic aldehydes. Results are summarized in Table 6. Primary alkyl aldehydes (bearing no a-substituents) and sterically less-hindered secondary alkyl aldehydes undergo the homoallylation successfully to provide the expected products in good yields with excellent stereoselectivity (runs 1-5). The results in runs 3-5 indicate that the present reaction shows almost no diastereofacial selectivity with respect to the a-stereo centers of secondary alkyl aldehydes. Sterically demanding aldehydes, such as cyclohexanecarbaldehye and pivalaldehyde, provide the... 

The configurations of the stereo centers for ervatamine alkaloids were established by chemical correlations (189, 190) with the vobasine series and supported further by an X-ray diffraction study of 44 (191). 

Instead of chirality center we use the more recently introduced stereo center terminology (70a). According to the IUPAC 1957 Rules we should have depicted our compounds upside down, but for the convenience of the reader we adhered to the traditional formulas. 

The alkaloid normelionine-B (25) has been isolated from the root bark of Strychnos nux-vomica L. (30) however, its structure has not been established completely. The relative steric positions of hydrogens attached to the C-3, C-15, and C-20 stereo centers are assumed to be a,a,(3 or a,a,a, respectively. 

Ahond et al. recently reported the isolation of two epimeric /V-oxides of 10-methoxydihydrocorynantheol from the trunk bark of Ochrosia moorei (26). The absolute configuration of the stereo centers at the annellation of rings C and D are 35,4/ in 26 and 35,45 in 27. 

Similarly to the corynantheine-type alkaloids, yohimbines could also be divided into four groups according to the relative configurations of the stereo centers C-3, C-15, and C-20. 

Dihydrocorynantheol (21), first isolated from Aspidosperma marcgravianum (147), is the simplest corynanthe alkaloid. The members of this type of alkaloid have three stereo centers in the D ring of the indolo[2,3-a]quinolizine skeleton. This substitution pattern allows four possible relative arrangements for the C-3, C-15, and C-20 stereo centers, the names of which are normal, pseudo, alio, and epiallo, respectively. 

The utilization of the Robinson annellation method for the synthesis of cory-nanthe-type alkaloids has been thoroughly investigated by Kametani and coworkers (149-152). The tetracyclic ring system was efficiently formed via the Michael addition of dimethyl 3-methoxyallylidenemalonate (247) to the enamine derived from 3,4-dihydro-1 -methyl-(3-carboline (150). Alkylation of 248, followed by hydrolysis and decarboxylation, resulted in a mixture of stereosiomeric enamides 250 and 251. Hydrogenation of 250 afforded two lactams in a ratio of 2 1 in favor of the pseudo stereoisomer 253 over the normal isomer 252. On the other hand, catalytic reduction of 251 gave 254 as the sole product in nearly quantitative yield. Deprotection of 254, followed by lithium aluminum hydride reduction, yielded ( )-corynantheidol (255) with alio relative configuration of stereo centers at C-3, C-15 and C-20. Similar transformations of 252 and 253 lead to ( )-dihydrocorynantheol and ( )-hirsutinol (238), respectively, from which the latter is identical with ( )-3-epidihydrocorynantheol (149-151.). 

The stereospecific preparation of aldehyde ester 462, already comprising five of the six stereo centers of the molecule and, furthermore, the stereoselective construction of the epialloyohimbane skeleton with Ec2 conformation are regarded the remarkable features of Woodward s approach. 

Water elimination from both 486 and 487 stereoisomers resulted in 18(3-hydroxyapoalloyohimbine (489). By this step, stereo centers at C-16 and C-17... 

It has been shown that CD measurement is a proper tool to determine the absolute configuration of the C-3 stereo center in corynantheine and yohimbine alkaloids (300). The chiroptical properties of stereoisomeric yohimbanes and 17-ketoyohimbanes also have been studied. Cotton effects due to aromatic and ketone absorptions have been considered in terms of the appropriate sector and... 

Structures of alkaloids 25, 26, and 27 were established by spectroscopic means (UV, IR, NMR, and mass) and by chemical correlations. All three compounds had a zero optical rotation, which might be attributed to an easy racemization of their unique stereo centers this was also observed for hygrine. 

The structure of hypercratine was deduced from spectroscopic data, including 200 MHz H NMR and I3C NMR. The mass spectrum of the acylated derivative of 28 was dominated by fragments at m/z 70 and 112, explained by an a cleavage of the pyrrolidine ring. The complexity of the NMR spectra and the distance between the C-2 and C-2 stereo centers have not permitted establishing the configuration of these centers. The nature of the substituents on atoms C-6 or C-7 remains yet to be settled [alternative structures 28 and 28 ]. 

The main lines of this approach were later embodied in an enantioselective synthesis of (—)-a-allokainic acid (Scheme 34) (179). The sole stereo center of die ene reaction starting material was derived from a glutamic acid derivative (132) to avoid loss of optical activity via double bond migration (see Scheme 33), the a acid function of kainic acid had to be reduced before the pyrolysis step... 

Z)-Isositsirikine (602), with (R) absolute configuration at stereo center C-16, has been isolated from the leaves, fruit, and stem bark of Alstonia sphaerocapi-tata Boit (390). 

C2 symmetry is a common feature in chiral bidentate ligands (119). If two identical enantiopure donor groups Y are connected by a tetrahedral bridging atom, any additional group Z at this atom will cause no additional stereo center but a prochiral center (Fig. 18a) (120). (S,S)- and (i2,i )-Trihydroxyglutaric acid are textbook examples for such compounds (120). Inversion of configuration at C3 in (S,S)-trihydroxyglutaric acid (Fig. 18b) yields the identical compound. 

I don t know the word "Stereo-center Please choose one of the following... 

The absolute configuration of the stereo centers of vinblastine (1) was determined from the X-ray crystal structure of vincristine (2) methiodide (79,80) in view of the known relationship between 1 and 2. The absolute stereochemistry at C-I8 in vinblastine (1) and related derivatives can also be deduced by means of ORD and CD spectroscopy (81,82). The determination was made possible by the synthesis and structure elucidation of several compounds possessing the unnatural configuration at C-18 (82,84). Because this stereo center controls the relative geometry of the... 

Instead of chirality center we use the more recently introduced stereo center terminology (78). 

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