Incorrect stereochemistry

Scheme 12). For this purpose, acid 68 was esterified with alcohol 64 to give the linear epothilone precursor 69. After selective deprotection of the primary TBS ether (CSA, CH2Cl2/MeOH) and oxidation, precursor 51 (Fig. 13), suitable for ring-closing chromium-Reformatsky reaction, was obtained. After treatment of 51 with CrCl2 and Lil in THF, the desired 6RJS isomer was obtained exclusively, whereas the diastereomeric precursor with the incorrect stereochemistry at C8 did not cyclize with the same efficiency, rate and selectivity. 

While scission of the three bonds a to the ester in 2 provides a very rich bounty of stabilised anion/electrophile partnerships, a detailed evaluation of each respective forward reaction reveals that only one is tactically viable. For example, the cleavage of bond a in 2 would not be strategic for it would require enolate 11 to undergo methylation from its more hindered top-face (Scheme 12.2), which would be most unlikely. Such a plan would almost certainly lead to a product with incorrect stereochemistry at the newly introduced quaternary carbon centre. In similar... 

The structures (68) and (69) were inadvertently drawn with incorrect stereochemistry in ref. 60 personal communication with Professor H. G. Floss. 

The bicyclic core of the targets was constructed using a Sml2-mediated Reformatsky reaction (Scheme 7.26). Shiina found that the use of HMPA in the Reformatsky reaction had a dramatic effect on its outcome. When a-bromo ester 58 was treated with Sml2 in the absence of HMPA, alcohol 60 (containing the incorrect stereochemistry required for the natural products) was obtained... 

In some cases, the natural selectivity of a reaction leads to a product that has the incorrect stereochemistry required for a given target. The synthetic chemist is not always forced to accept that stereochemistry. There are at least two options (1) change the synthetic pathway to generate a compatible intermediate that can produce the desired stereochemistry, or (2) change the stereocenter in the final product. 

CA refers to Yersiniose and Yersiniose A as different compds. and assigns two incorrect stereochemistries, (correct stereochem. is o-xylo-). Stereochemis-... 

By means of the key intermediates indicated in Scheme 13, D-ribose has been built up into the unit (60) representing the C(l) - C(15) portion of the halichondrins, incorporating rings A and B and the intramolecular ketal (halichondrin numbering circled). The initially incorrect stereochemistry at C-12 was inverted by an elimination-addition process.47... 

According to this mechanism, a molecule of bromine becomes complexed to the double bond of the alkene, and reorganization of the bonding electrons gives the product. This mechanism can be shown to be incorrect for most alkenes on the basis of stereochemistry. Most alkenes give bromination products in which the two added bromines are on opposite sides of the former carbon-carbon double bond. The above mechanism does not account for this and therefore must be incorrect... 

The 20-acetoxy derivatives of clavulones I, II and III (92-94) were reported shortly afterward with their structures being determined by spectroscopic methods [120]. A 4R and 12S (again, the incorrect 12R stereodescriptor was applied) stereochemistry in these three new compounds was deduced by the close comparability of their CD spectra with those obtained for clavulones I, II, and III (88-90). 

The stereochemistry at C7 was introduced by the hydrogenation of the ketone. Although this hydrogenation was very selective for the desired stereoisomer, the process provided no information to confirm or refute the tentative stereochemical assignment at this center, which remained incorrectly assigned until Weinreb completed a stereocontrolled synthesis of 7-epicylindrospermopsin in 2001.7a... 

The title compounds, isolated from Berberis buxifolia Lam. (Berberidaceae), are the most recently reported of the eight known bisbenzylisoquinoline /V-oxide alkaloids. They are noteworthy in being the first N-oxides of this group to have their complete stereochemistry assigned by NMR NOEDS studies (see Section VI,A,2,b). Unfortunately, the first report (59) was based on an incorrect assignment due to overlap of the NMe and aliphatic signals of calafatine (88, Section... 

It is clear that attempts to rationalise the axial stereochemistry of coupling reactions of pyranosyl radicals in terms of configurationally stable a-radicals are incorrect. The preferences for formation of axial C-glycosides by attack of radicals on olefins (Adlington et al., 1983 Albrecht and ScheflFold, 1985 ... 

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