1.3.2- Oxazaphospholidine

Applications of oxazaphospholidine-borane complexes, cyclic phosphoramides, compounds with N—P=0 function, and other P-heterocycles in enantio-selective catalysis 99SL377. 

Scheme 20 Preparation of isocyanides using PS-oxazaphospholidine and their subsequent use in Ugi reaction to give substituted isoindolinones...

Ionic liquid supports, functional 115 Isocyanides, PS-oxazaphospholidine 148 Isoquinolines 252 Isoxazoles 95... 

The stereochemistry of the first step was ascertained by an X-ray analysis [8] of an isolated oxazaphospholidine 3 (R = Ph). The overall sequence from oxi-rane to aziridine takes place with an excellent retention of chiral integrity. As the stereochemistry of the oxirane esters is determined by the chiral inductor during the Sharpless epoxidation, both enantiomers of aziridine esters can be readily obtained by choosing the desired antipodal tartrate inductor during the epoxidation reaction. It is relevant to note that the required starting allylic alcohols are conveniently prepared by chain elongation of propargyl alcohol as a C3 synthon followed by an appropriate reduction of the triple bond, e. g., with lithium aluminum hydride [6b]. 

The stereochemistries of the reactions between 0-aryl 0-methyl phosphonochloridothioates and nucleophiles have been studied in relation to the synthesis of 1,3,2-oxazaphospholidines. No displacement of chlorine takes place on treatment of O-methyl 0-4-nitrophenyl phosphonochloridothioate with 2-methoxyethanol, and in the presence of 1-phenylethylamine, it is only the latter which reacts. In addition, when the same phosphonochloridothioate is treated with sodium ethoxide, it is the 4-nitrophenoxy group, rather than chlorine, which is displaced. Both displacements were shown to occur with inversion of configuration at phosphorus. The use of such an acid chloride as a two-step 1cyclophosphorylating1 agent of 2-aminoalcohols to give 1,3,2-oxazaphospholidines (209), is illustrated. ... 

The asymmetric synthesis of 2-aryl(alkyl)-l,3,2-oxazaphospholidines 52 was based on the reaction of achiral organophosphonous diamides 51 with L-ephedrine (42) (Scheme 19) [44], The diastereomeric excess ranges from 0% (R=Ph) to 95%... 

A modification of this procedure allowed the isolation of 1,3,2-oxazaphospholidine 52a as a single diastereomer [41] and its application to asymmetric synthesis of enantiomerically and diastereomerically pure phosphinic acid derivatives 53 and 54 and tertiary phosphine oxides 55 (Scheme 20) [45], A few years later, a similar approach for the synthesis of enantiomerically pure tertiary phosphine oxides 55... 

Ethoxy-l,3,2-oxazaphospholidine 59 was prepared as a single diastereomer from (-)ephedrine (42) and ethyl dichlorophosphite 57. Its Arbusov reaction with allyl bromide gave the corresponding allyl phosphonates 61a,b as a diastereomeric mixture which could be separated by flash column chromatography and crystallization (Scheme 21) [48], On applying a similar protocol, starting from... 

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],... 

The first report on the reaction of D-pseudoephedrine 66 with phosphoryl chloride appeared as early as 1962 [49], More recently it was found that this condensation gave 2-chloro-l,3,2-oxazaphospholidine 2-oxides 67 as a single diastereomer which was subsequently esterified with racemic aldehyde cyanohydrins 68 without racemization at the phosphorus atom. The prepared diastereomeric esters 69 were used as substrates for the asymmetric synthesis of optically active cyanohydrins 72, which involves the intermediate formation of the tertiary esters 70, as shown in Scheme 22 [50],... 

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],... 

Scheme 29 Synthesis of 2-chloro-1,3,2-oxazaphospholidine 2-thiones 101a,b...

Table 1 Synthesis of 2-alkyl(aryl)-2-oxo-1,3,2-oxazaphospholidines (25,45,5/J)-107a-l 13a and (2/t,4S,5fl)-107b-113b...

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],... 

Scheme 32 Reaction of the 2-oxo-1,3,2-oxazaphospholidine (2/J,45,5/J)-109b with 4-nitroben-zenesulfonyl azide...

It is interesting to note that [(E)-2-phenylethenyl]-[018]phosphonic dichloride (116) upon treatment with (-)-ephedrine (42) in the presence of triethylamine afforded an equimolar mixture of (2S,4S,5R)- and (2/ ,4S, 5/ )-2,3-dimethyl-5-phenyl-2-[(E)-2-phenylethenyl]-l,3,2-oxazaphospholidin-2-[180]one 117a and 117b. The two diastereomers were completely separated by flash chromatography and the diastereomer 117b was hydrolyzed with water H20-170 to the corresponding monoester 118 which was finally converted to dihydro-(l,2-dibromo-2-phenyl- 1-ethyl) (R)-160,170,180 phosphonic acid 119 (Scheme 33) [66],... 

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],... 

A similar condensation of (+)-norephedrine (121) gave the corresponding 2-methyl-1,3,2 oxazaphospholidine-2-thiones 122a,b which were separated by rapid, medium pressure chromatography (Scheme 35) [67],... 

Synthesis of 1,3,2 oxazaphospholidines 128a-c derived from (S)-prolinol 127 was based either on the thermal aminoalcoholysis of the latter with prochiral alkyl(aryl) phosphonousdiamides 51a-c or its condensation reaction with /-butylphosphonous dichloride carried out in the presence of triethylamine (Scheme 37) [68], The diastereomeric excesses of the prepared derivatives ranged from 80 to 95%. 

A series of 2-chloro-l,3,2-oxazaphospholidine derivatives 131a-f were prepared by reaction of six enantio-pure alcohols 129a-f with phosphorus trichloride carried out in the presence of an organic base as HC1 scavenger (Scheme 38) [69], The 31P and II-NMR spectra of crude 131a, d, e containing a small amount of the HC1 salt produced during the synthesis of 131, as well as the distilled samples, indicated that the formed chloro derivatives were ca. 1 1 mixtures of the cis and trans isomers. 

Scheme 38 Synthesis of the nucleoside 3 -0-oxazaphospholidine derivatives 132a-f...

The reaction of 2-deoxy-3,4,6-tri-0-methyl-2-methylamino-D-glucopyranose 149 with methyl(phenyl)phosphonic dichlorides (106a-b) or methylthiophosphonic dichloride in benzene in the presence of triethylamine afforded mixtures of four isomers of the corresponding 1,3,2 oxazaphospholidine-2-ones 150, 151 and... 

Scheme 43 Synthesis of 1,3,2 oxazaphospholidine-2-one 155 a and b and their conversion to the pyridinium salts 156a and 156b...

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