Partial stereochemistry

Yessotoxin (214) is a polyether from the scallop Patinopecten yessoensis and has been implicated in diarrhetic shellfish poisoning (DSP). The structure and partial stereochemistry of yessotoxin were deduced from spectral data [219]. The relative stereochemistry of yessotoxin and the structures of two new analogues, 45-hydroxyyessotoxin (215) and 45,46,47-trinoryessotoxin (216) were also established [220]. The absolute stereochemistry of yessotoxin (214) was determined by NMR spectroscopy using a chiral anisotropic reagent [221]. The absolute configuration at C45 in 45-hydroxyyessotoxin (215), isolated from P. yessoensis, was determined by the use of a modified Mosher s method [222]. 

AND CINACHYROLIDE A (3), SHOWING THAT NO STEREOCHEMISTRY WAS DETAILED IN THE ORIGINAL REPRESENTATION OF 1, THE PARTIAL STEREOCHEMISTRY PROPOSED FOR 3 CONFLICTED WITH THAT FOR 2 AND LATER REPRESENTATIONS OF 1 BY PETTIT. 

The fungus Claviceps paspali Stevens et Hall elaborates a series of metabolites derived from indole and a diterpenoid unit. The structures of paspaline (5) and paspalicine (6) were proposed earlier, together with the partial stereochemistry of paspaline. These conclusions have now been confirmed, and the complete stereochemistry of paspaline and paspalicine has been elucidated, by X-ray... 

A total structure including partial stereochemistry has been proposed for MTX on the basis of extensive spectroscopic analysis (Mnrata et al. 1992, 1993, 1994). The 2D NMR analysis of MTX led to the structnral elncidation of some partial stmctnres snch as the C1-C36 moiety. However, overlap and poor resolntion of H and NMR signals hampered the elncidation of the H- H cotmectivity,... 

CAS now registers and names substances with partially defined stereochemistry. Previously, partial stereochemistry was generally ignored. The presence of unknown chiral centers is indicated by the addition of the term [partial]- to the end of the normal stereochemical descriptor. When the reference ring or chain has incompletely defined chiral atoms/bonds, the format cites the stereochemistry using R and S terms with their nomenclature locants for all known centers. If this method is used to describe a substance for which only relative stereochemistry is known, rel is added to the stereochemical descriptor. Any stereochemical descriptor marked as rel always cites the first centre as R-. 

Securinol C, isomeric Avith both securinol A and B 57), exhibited IR, UV, and NMR spectra consistent with a hydroxylated dihydro-securinine formulation (5S). Treatment of securinol C with methanesul-fonyl chloride in pyridine gave allosecurinine (Section II, D). Thus securinol C is a 14,15-dihydroallosecurinine-type alkaloid with a hydroxyl group located at C-14 or C-15. The latter possibility (C-15-OH) was excluded by showing that securinol C was not enantiomeric with either of the two possible 14,15-dihydroviroallosecurinine alkaloids, securinol A and B. Further examination of the NMR and mass spectra of securinol C provided evidence for the assignment of structure and partial stereochemistry 163 to securinol C. 

Cradwick, P.D., and G.A. Sim Crystallographic Determinations of Partial Stereochemistries of the Sesquiterpenoids Illudol and Marasmic Acid. J. Chem. Soc., Chem. Commun., 431 (1971). 

Partial Synthesis of a-Amyrin Acetate Proof of the Structure and Stereochemistry... 

Ion pair (Section 11.5) A loose complex between two ions in solution. Ion pairs are implicated as intermediates in S l reactions to account for the partial retention of stereochemistry that is often observed. 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

We have previously discussed the possibilities of racemization or inversion of the product RS of a solvolysis reaction. However, the formation of an ion pair followed by internal return can also affect the stereochemistry of the substrate molecule RX. Cases have been found where internal return racemizes an original optically active RX, an example being solvolysis in aqueous acetone of a-p-anisylethyl p-nitrobenzoate, while in other cases partial or complete retention is found, for example, solvolysis in aqueous acetone of p-chloro benzhydryl p-nitrobenzoate. the pathway RX R+X some cases where internal return involves racemization, it has been shown that such racemization is faster than solvolysis. For example, optically active p-chlorobenzhydryl chloride racemizes 30 times faster than it solvolyzes in acetic acid. ... 

The nucleophile OH uses its lone pair electrons to attack the alkyl halide carbon 180° away from the departing halogen. This leads to a transition state with a partially formed C-O bond and a partially broken C-X bond. The stereochemistry is inverted as the C-O bond fully forms, and the halide ion departs with the electron pair from the former C-X bond. 

In accord with the current interest in stereochemistry at phosphorus a number of optical studies on phosphonous derivatives have been carried out. Benschop and his group have achieved a partial resolution of alkyl alkylphosphinates (133) by stereospecific inclusion in cycloamyloses. Optical purities in the range 20—80% were obtained. 

The stereochemistry of the nonconcerted reaction has been a topic of considerable study. Frequently, there is partial stereorandomization, indicating a short-lived diradical intermediate. The details vary from case to case, and both preferential inversion and retention of relative stereochemistry have been observed. 

In view of the results obtained for attack of acetone singlets on 1-methoxy-butene to yield singlet biradicals (partial loss of stereochemistry due to bond rotation in the biradical), acetone attack on dicyanoethylene to yield oxetane stereospecifically via a similar biradical intermediate is difficult to envision. Thus a new mechanism must be developed to account for these results. 

C NMR of linear cross-linked PS. The proton decoupled 13C NMR spectra of linear and 1% cross-linked PS at 75 MHz in chloroform are illustrated in Figure 3. These spectra are similar to those for linear and cross-linked chloromethylated PS previously reported at lower field (14), although we have been able to resolve more structure in tHe" aliphatic and aromatic regions here. The quarternary and methylene carbon resonances at about 146 ppm and between 40 and 50 ppm respectively, are the most strongly affected by stereochemistry (20). The ortho and meta resonances at 128.4 ppm show partially resolved structure in the linear PS, as does the para carbon at 126.1 ppm. The methine resonance at... 

The configurational differences between 104 and 105 with the partial cis-fused junctions and those of the all trans-fused 95 and 101 (and related compounds) were pointed out [22] and resolved [62] with the determination of the absolute stereochemistry of diisocyanoadociane (95) (vide supra). Selective conversion to, and preparation of, an appropriate formamido-(p-bromobenzamide) derivative for X-ray determination [62], coupled with total synthesis [63], support the lR,2f ,3aS,5S,6/ ,8S,8aS, lOaS, lObS, lOcS absolute configuration. 

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