Enol phosphonates, rearrangement

In conclusion, C-C bond formation on carbohydrates has absorbed lots of widely used methods from general synthetic organic chemistry. Typical among these methods are intramolecular alkylation and intramolecular condensation of aldehyde with enolates, phosphonates, and nitro-stabilized anions. Metal-mediated radical reactions, cycloadditions, and rearrangements have also been applied frequently. 

Two reports are available on the rearrangement of mixed vinyl phosphite esters to produce phosphonate diesters in moderate yield.94 95 In both instances, the vinyl phosphite esters were prepared by reaction of the dialkyl phosphorous chloride with highly enolized carbonyl compounds. The mixed ester products undergo thermal rearrangement to the phosphonate diesters (Figure 6.25). 

Reaction of enol ether 1 a with sodium methoxidc or ethoxide. or 1 b with sodium methoxide, gives ketene acetals, which rearrange in the presence of triethylamine to give esters 3.3 The phosphorus ylide trimethoxy[2,2,2-trilluoro-l-(trifluoromethyl)cthylidene]-A ,-phosphane f(CF3)2C = P(OMc),] rearranges readily to dimethyl f2.2,2-trifluoro-1-mcthyl-l-(trifluoro-mcthyi)ethyl]phosphonate [(CF,)2C(Me)PO(OMe)2] in a similar 1.3-shift.5-0... 

The principally practicable route to allyl vinyl ethers by alkenation of allyl formates, has not yet been exploited in this context. In another approach, the nucleophilic addition of allylic alcohols to alkynic esters, one observes the exclusive formation of franr-enol ethers 2 the stereochemical information is lost, however, after the rearrangement, due to enolization of the formed formylacetic acid derivatives. On the other hand the nucleophilic addition of allyl alcohols to acceptor-substituted allenes like allenic sul-fones or phosphonates offers a novel route with fascinating potential. " Such systems that are readily... 

The simplest mechanism consistent with the experimental facts is depicted for 2-cyclopentenone in equation 12. While initial attack by the phosphorus reagent at the carbonyl (eqs. 13 and 14) is conceivable, subsequent rearrangement of phosphorus from oxygen to carbon via a phosphorane intermediate is not possible, due to the restricted geometry of the intermediate in equation 13. Since only the 7-ketocyclopentyl-phosphonate ester (72%) and none of the enol phosphate ester (eq. 13) or hydroxyphosphonate ester (eq. 14) were detected in the product, attack by phosphorus is exclusively at the terminal carbon atom of the conjugated system. It is reasonable to assume that a similar mechanism holds for the majority of dienophiles. 

When the preparation of alkali metal enolates derived from alkanoylphosphonates was attempted by treatment with strong anhydrous bases such as lithium diisopropylamide or sodium hydride, the formation of phosphate phosphonate-type products was observed. This was interpreted in terms of fragmentation of the enolate formed in the first step to ketene and dialkyl phosphite anion (equation 75), and addition of the latter to the carbonyl group of an unreacted acylphosphonate molecules to form a bisphosphonate. Such molecules are known to rearrange to phosphate phosphonates ... 

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... 

Fig. 3 (A) [33] Sigrnatropic rearrangement of chorismate to prephenate catalyzed by antibody 11F1-2E11. The hapten is a shape mimic of the cyclic transition state. (B) Antibody 1E9 catalyzes a Diels-Alder reaction of tetrachlorothiophene dioxide and yV-ethylmaleimide. The hapten is comprised of the endo hexachloronorbornene unit. (C) Antibody 21H3A catalyzes the transesterification reaction between iec-phenethyl alcohol and an enolic ester to form a chiral ester. The hapten here is racemic phosphonate.

A DFT study of the origins of stereoselectivity in the aldol reaction of bicyclic amino ketones (20) with aromatic aldehydes has been reported (Scheme 18). ° Base-catalysed direct aldolization of a-alkyl-a-hydroxy trialkyl phosphonoacetates with aldehydes proceeds via a fully substituted glycolate enolate intermediate formed by a [l,2]-phosphonate-phosphate rearrangement. High enantioselectivity can be achieved by the application of chiral iminophosphorane catalysts. 

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