Stereogenic atoms, absolute phosphorus

A-trityl-(l/ ,2S)-norephedrine (58), the corresponding allyl phosphonates 62a,b were obtained via the Arbusov rearrangement of 2-ethoxy-1,3,2-oxazaphospholidine 60. The absolute configuration of the major diastereomer, 62a was determined by X-ray as (2S1,45,51 ).The reaction of the major diastereomer of allyl phosphonates 61a and 62a with DBU afforded the corresponding vinylphosphonates 63a,b (Scheme 21) [48], Nucleophilic addition to these resulted in induction of chirality at the [1-position of the stereogenic phosphorus atom in the initially produced diastereomeric phosphonates 64 or 65 (Scheme 21) [48],... 

For (-) castoramine (56) Rule 6 allows a priority determination that assigns the stereogenic nitrogen center as (R). Just as a nitrogen atom can be stereogenic, a phosphorus atom in a phosphine such as 53 can be considered chiral since the rate of inversion is negligible at temperatures up to = 130°C, as noted in Table 1.4. The lone pair electrons have the lowest priority by Rule 6 and the absolute configuration is (R). 

Juge developed a powerful method (Juge-Stephan method) [1, 51] for the preparation of / -stereogenic phosphines based on the use of ephedrine as a chiral auxiliary. The key reactants in this methodology are 1,3,2-oxazaphospholidine boranes 78, prepared by a one-pot reaction from bis(diethylamino)phenylphosphine and (—)-ephedrine, followed by protection with BH3. The cyclization of the (—)-ephedrine takes place stereoselectively, with preferential formation of the (/ p)-diastereoisomer in 90% de [52, 53]. The absolute configuration at the phosphorus atom has been determined by chemical correlations and NMR analysis, and proved by X-ray analysis [54]. Oxazaphospholidines react readily with electrophiles or nucleophiles to provide various chiral phosphorus compounds. Enantiomeric antipodes of tertiary phosphines (Sp)-79 and (Rp)-81 were obtained from (-1-)- or (-)-ephedrine, as shown in Scheme 25. The configuration at the E-atom is controlled by the configuration at the Ph-substituted Cj of (-i-)-pseudoephedrine or ( )-ephedrine, respectively. This was confirmed by X-ray crystal-structure analyses of two intermediate compounds in the synthetic route to the chiral triarylborane-phosphine adducts [54]. 

Scheme 7 Absolute structure of boranophosphate and phosphorothioate diastereomers and their R/S designations. The substituents at the phosphorus atom are numbered from 1 (highest priraity) to 4 (lowest priority) in decreasing order of the atomic number of the atrans attached to the stereogenic center according to the Cahn-Ingold-Prelog priority mles. Note that boranophosphate and phosphorothioate diastereomers with the same absolute ctmiiguiation in space have opposite RjS designations [92]...

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