Alkynes enol phosphate

I-Alkynes from methyl ketones This reaction can be effected by conversion to Ihe enol phosphate followed by -elimination with LDA (equation I). In the case of a simple ketone such as 2-octanone the yield is low because of formation also of an allene. In such cases lithium tetramethylpiperidide is recommended as base. 

Trapping the kinetic enolate of a methyl ketone with diethyl phosphochloridate provides an enol phosphate, that can, in turn, be converted to an alkyne ... 

Enol phosphates may be reduced to alkenes,79 eliminated to alkynes,80,81 employed as synthetic intermediates in organophosphorus chemistry and are useful in their own right, for example, as insecticides. For reviews covering enol phosphates and the Perkow reaction, see Refs 17, 82, and 83. 

Dialkyl alkynes. A recently reported route to these substances involves reductive elimination of enol phosphates of /8-oxo sulfones with sodium amalgam in DMSO-THF at 0°. 

A similar synthesis of alkynes by reductive elimination of enol phosphates of /3-oxosulfones with sodium in liquid ammonia has been reported. ... 

A mild and efficient synthesis of terminal alkynes starts with readily accessible methyl ketones and converts them to the corresponding enolates with LDA. The eno-lates produced are trapped with diethyl chlorophosphate to give enol phosphates, which possess a good leaving group for elimination. Subsequent treatment of the enol phosphates with LDA furnishes the corresponding lithium alkynylides and on protonation of these, the corresponding terminal acetylenes. 

Enol phosphates treated with NaNH2 in liquid ammonia or with lithium dialkylamides give the alkynes in good yields (Scheme 49). Vinyl selenoxides can also be used as a starting material for the preparation of alkynes. Thermolysis at 85-95 C takes place in the presence of DABCO if a syn elimination to the alkyne is possible (Scheme 50). If a syn elimination to an alkyne is blocked by a substituent, allene formation can occur. ... 

A connective synthesis of alkynes inspired by the Julia alkenation was developed by Lythgoe and coworkers for the synthesis of la-hydroxy vitamin D3, as shown in Scheme 34. The P-keto sulfone (101) derived by condensation of the the metalated sulfone (99) with the ester (100) was converted to the enol phosphate (102), which on reductive elimination gave the enynene (103). 

The precise conditions for the reductive elimination are important. Use of Na(Hg) in THF-MeOH or THF alone gives messy reactions, owing to substantial cleavage of the P—O bond of the enol phosphate but this can be avoided with Na(Hg) in THF-DMSO. Alternatively, Na in liquid ammonia can be used to effect the reductive elimination but the reaction must be very carefully controlled in order to avoid reduction of the alkyne to a tra/ii-alkene. A deliberate over-reduction of the initial alkyne product to an alkene was used in a synthesis of the intermediate (104) in a synthesis of brefeldin (Scheme 35). ... 

Allenes. Substituted allenes are prepared from ketones via elimination of the enol phosphates. Interestingly, enol Inflates tend to give alkynes. The method can be applied to protected aminoalkenyl phosphates. Silyl enol ethers" also undergo elimination. 

P-Ketophosphonates are valuable intermediates in the realm of Homer-Emmons alkenation methodology. Acyclic variants are difficult to obtain from enol phosphates due to competing alkyne and allene formation. One solution utilized the dianion derived from a-bromo ketones and trapping with diethyl phosphorochloridate however, only moderate yields of P-keto phosphonates were reported. The most efficient procedure utilizes the anion derived from dialkyl methylphosphonate, addition to an aldehyde, followed by oxidation (eq 8). ... 

Alkynes are available from methyl ketones by elimination of the enol phosphate (eq 16). When the ketone contains o-branching, lithium tetramethylpiperidide has been recommended to circumvent allene formation. 

---