1.3.2- Oxazaphospholidines isomerization

A similar reaction of (-)-ephedrine (42) was reported to give 2-chloro-l,3, 2-oxazaphospholidine 2-oxide either as a single diastereomer (25,45,5R)-73a [51-53] or as the isomeric pair 73a,b, which could be separated by chromatography over silica gel (Scheme 23) [32, 54],... 

Treatment of (-)-ephedrine (42) with a variety of aIkyl(aryl)phosphonic dichlorides 106a-d afforded the isomeric pairs of the corresponding 2-alkyl(aryl)-2-oxo-l,3,2 oxazaphospholidines (Scheme 31 and Table 1) [32, 62-65],... 

Treatment of (-)-ephedrine (42) with methylthiophosphonic dichloride afforded a mixture of the isomeric 2-methyl-l,3,2 oxazaphospholidine-2-thiones 120a,b, from which the pure isomers could be separated in approximately equal quantities by chromatography over silica (Scheme 34) [32],... 

Oxazaphospholidines of type 140 have been advocated as alternatives to the more normal phosphoramidite units used in solid-phase synthesis. The unit 141, as an isomeric mixture, has been developed as an all-purpose adaptor for synthesis of oligonucleotides on any of the commercial supports. The unit 141 is attached to the support through 0-5. After removal of the Dmtr group, the first nucleoside is attached via a 2 (3 )-3 link using a phosphoramidite. The desired oligonucleotide is then conventionally assembled, and disconnected by removal of the 0-benzoyl group from the adaptor, which is followed by cyclic phospho-diester formation. ... 

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