Enol phosphates, oxidation

Step c of Eq. 24-34 may occur by ring opening to an enol phosphate which ketonizes to the observed product, but step e is a more complex multistep oxidative process.314a,b The last step is transamination to methionine with a glutamine-specific aminotransferase. Another enzyme from Klebsiella converts the same intermediate anion to methylthiopropionate, formate, and CO (Eq. 24-34, step/).315... 

A convenient enantioselective catalytic oxidation of a variety of differently substituted, cyclic (E) and acyclic (Z)-enol phosphates with (salen)manganese(III) complex has been reported. The influence of electronic and steric effects of the enol phosphate substituents on the stereoselectivity of oxidation has been studied.50... 

Thiochromen 4-yl enol phosphates have been obtained from 2 -fluoroacetophenone through initial reaction with prop-2-enethiol followed by conversion to the enol phosphate. Ring closing metathesis (RCM) using a second generation Grubbs catalyst delivered the. V-hctcrocycle. Oxidation at sulfur prior to RCM provided access to the thiochromen-4-yl 1,1-dioxide enol phosphate (Scheme 120) <2003TL4275>. 

The phosphoric acid ester of an enol ether is even more readily hydrogenolyzed than an enol acylate. When the (Z)-enol phosphate 51 was subjected to hydrogenation in ethyl acetate at 0.3 MPa H2, the hydrogenolysis products were isolated in 95% yield not only over platinum oxide but also over 5% Pd-C and 5% Rh-Al203 (eq. 13.50).94 The hydrogenolysis of an enol phosphate has been utilized in conversion of one of two oxo groups to the methylene as shown in eq. 13.51 95... 

The synthesis of the limondd azadiradione, utiiizing a Hg° cyclization-oxidative demercuration sequence with an enol phosphate derived from trans,trans-famesol has been reported. Azadiradione, a te-tracarbocyclic memter of the iimonoid group isoiated from the neem tree, Azadirachta indica, has been converted to other tetracyclic limonoids, and is thus a key intermediate. The sequence is shown in Scheme 33. 

Phosphine Chalcogenides as Ligands. - The complexation of lanthanide and actinide ions by phosphine oxide ligands remains an active area, and a theoretical assessment of the coordination of phosphine oxides (and phosphate esters) by trivalent lanthanide ions has appeared." Trivalent lanthanide complexes of the functionalised enol phosphine oxide (265), (and a related phosphonate), have been describedComplexes of thorium(iv) with bis(di-phenylphosphino) ethane dioxide and bis(diphenylphosphinoyl)amide have also been characterised." Calixarene systems which bear phosphine oxide... 

Enol phosphates of a-ketosulfones provide the key intermediate to alternate schemes leading to acetylenes. Acylation of phenyl sulfones with esters affords the ketone derivatives directly. Alternately, the ketones can be obtained by Moffat oxidation of the condensation products from aldehydes with sulfone anions. Conversion to the enol derivatives followed by treatment with Na/NH or Na(Hg) gives the acetylenes. 

Phosphine oxides remain untouched by LAH [NW2] or borohydrides [EWl, HJ5]. They are reduced to corresponding phosphines by LAH-CeClg in THF [G02] or in a few cases by LAH in hot THF [TA4] (Figure 5.9). Phosphates are cleaved by LAH in THF to corresponding alcohols [JFl], while enol phosphates 5.18 are converted into carbonyl compounds. However, by using LAH-CuBr2 or DIB AH [IK3], it is... 

Scheme 3 presents substrates which carry both electron-donating and electron-withdrawing substituents. These systems, in comparison with the previous set of substrates, were considerably less reactive so that longer reaction times and excess oxidant were necessary for complete epoxidations. In the rosette are displayed the oxidation of p-oxo enol phosphates [13] 13, dihydrofuranone [17] 14, p-alkoxycyc-lohexenones [17] 15, alkoxymethylenecyclohexanones [17] 16, benzalphthalide [12a] 17, aurones [18] 18, flavones [19] 19, and isoflavones [18] 20. Many of these epoxides, which have become available for the first time, constitute valuable building blocks for natural product chemistry. 

Yet another rearrangement involving phosphate esters is that of oxiranyl phosphates following their formation by oxidation of enol phosphates (134) by peroxy acids or dimethyidioxiran the products are (2-oxoalkyl)phosphonic diesters (135). A preliminary communication concerning the reaction sequence was included in last year s... 

Catalytic hydrogenation of carbonyl compounds to alkanes is a difficult proposition under normal conditions, although limited success is attainable with aromatic ketones. However, certain enolates derived from ketones have been shown to undergo catalytic reduction to alkanes quite efficiently. For example, enol triflates of ketones are reduced over platinum oxide catalyst to alkanes (equation 56) . Similarly, enol phosphates, conveniently prepared from ketones, can be quantitatively hydrogenated to alkanes (equation 57) . ... 

The asymmetric oxidation of a variety of differently substituted, acyclic and cyclic enol phosphates using the Sharpless AD (asymmetric dihydroxylation)-reagents, AD-mix-a and AD-mix- 0, and a fructose-derived chiral ketone as a catalyst, with PMS was a terminal oxidant, afforded the corresponding a-hydroxy ketones in good yield and with high enantioselectivity. The influence of substrate steric and electronic factors on the facial stereoselectivity has been studied. Kinetic and activation parameters for copper(II)-catalysed and -uncatalysed oxidation of ornithine with PMS have been determined. Cyclic voltammetric and absorption studies confirmed the formation of a copper-ornithine-PMS complex and ESR spectral studies ruled out the participation of free radical intermediates. Kinetic and activation parameters for the oxidation of aspartic acid and nicotinic acid with PMS have been determined and plausible mechanisms have been proposed. 

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). ... 

The Pd-catalysed P-C cross-coupling reaction between a-amido enol phosphates (67) and secondaiy phosphine-borane complexes (70) or phosphine oxides (71) afforded under mild condition hindered tertiary a-enamido phosphine derivatives (68) or (69) with up to 99.4% ee and in up to 70% chemical yields (Scheme 22). ... 

The asymmetric oxidation of a variety of acyclic (89) and cyclic (90) substituted enol phosphates using commercially available Sharpless reagent (93), and a fructose derived chiral (94) as a catalyst, afforded the corresponding a-hydroxy ketones (91) and (92) in high enantioselectivily and good yields (Scheme 30). The influence of steric and electronic... 

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