Triethyl phosphite, reaction with epoxides

F-17(20)-dehydro-20-cyanopregnene, which may be isomerized in base to the Z-isomer. Elaboration of the side-chain by successive Grignard reaction, reduction, and removal of the 22-hydroxy-group followed. Key steps in two stereospecific syntheses of Z-20(22)-dehydrocholesterol (234) from pregnenolone were (a) stereospecific removal of the 20- and 22-oxygen atoms of (20i ,22S)-20,22-dihydroxycholesterol by conversion into the thiocarbonate (232) and treatment with triethyl phosphite and (b) selective epoxidation of iE -20(22)-dehydrocholes-teryl benzoate to the epoxides (233), which were allowed to react with hexamcthyIdisilane-KOMe in HMPA (see also ref. 179). Syntheses of the 24-... 

Alkylidene derivatives of phthalic thioanhydride are formed as shown in Scheme 160. Reaction of phthalic thioanhydride with hot triethyl phosphite produces trafts-S -bithioph-thalide (457), probably via the carbene and phosphorane (Scheme 161) (72AHC(14)331>. Support for this mechanism stems from the fact that brief treatment of phthalic thioanhydride with triethyl phosphite in the presence of phthalic anhydride gives (458) in the presence of benzaldehyde the same reaction leads to the benzylidene derivative (456). An alternative mechanism has also been suggested, in which the penultimate step is the formation of an epoxide, which is deoxygenated to yield the product (72AHC(14)331>. 

The reagent is prepared by heating triethyl phosphite with ethyl bromoacetate and is used in the modified Wittig reaction. Sodium hydride abstracts an activated a-hydrogen atom to give an anion salt (2) which functions Hke an ylide in reacting with an aldehyde or ketone to give an a,j3-un aturated ester (3). 1,2-Dimethoxy-ethane is used as solvent. The anion salt (2) reacts with diphenylketene to form the allene (5) and with styrene epoxide to form the cyclopropane (6). 

Although ethylene and propylene oxide quantitatively release their oxygen when treated with triethyl phosphite at 150-175° (286), the 2-buteue epoxides undergo this reaction much less satisfactorily (240). 

Sobhani S, Vafaee A. Efficient one-pot synthesis of P-hydrox3fphosphonates regioselective nucleophilic ring opening reaction of epoxides with triethyl phosphite catalyzed by Al(OTf)3. Tetrahedron 2009 65 7691-7695. 

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