Metathesis And Substitution

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Vinylphosphonates formed in this way, for example 2a–e (Figure 1), have been used in the synthesis of the natural products turmerone [4] and enterolactone [5], the phosphonate derivatives of the natural product cyclophostin [6], the C18–C34 fragment of amphidinolide C [7], and the oxylipids from Australian brown algae [8].

The potential of vinylphosphonates as intermediates in organic synthesis is limited by their chemistry.

More highly substituted vinylphosphonates (5 and 19) failed to react at all with methyl acrylate under similar conditions, even with higher catalyst loading and extended reaction times.

Initially, the synthesis of the allyl vinylphosphonate esters was achieved using a transesterification reaction catalysed by tetra n-butylammonium iodide (TBAI) (Scheme 5) [28].

Not surprisingly, the terminal vinylphosphonate 15 underwent smooth cross metathesis with either 1-hexene or 1-heptene using our standard conditions (2% Grubbs II, 4% Cu I, CH reflux) [2,26,27] to give the substituted vinylphosphonates 12a or 12b in good yield.

In contrast, when vinylphosphonate 12a was subjected to a cross metathesis reaction with methyl acrylate, the cross metathesis product 16a was formed in low yield (~11%) as part of a complex mixture.A solution of the vinylphosphonate 12b, 1.1 equivalents of allyl bromide and 5 mol % TBAI in toluene was heated at reflux for 7 hours to give a 53% conversion to both the mono- and diallyl vinylphosphonates 14a and 14b in a 10:1 ratio.The overall conversion could be improved with excess allyl bromide, increasing the amount of TBAI and prolonged heating times, either at reflux or in a microwave reactor.As an example, we recently described a method for the formal synthesis of centrolobine (8) [9], an antileishmananial compound isolated from the heartwood of various Centrolobium species [12-16] (Scheme 2).The cis-THP substituted vinylphosphonate 5, formed by a stereospecific palladium-catalyzed cyclization of phosphono allylic carbonate 4, was cleaved via ozonolysis to the aldehyde 6, a known intermediate [17,18] on route to (−)-centrolobine (8).Dimethyl (β-substituted) vinylphosphonates do not readily undergo cross metathesis reactions with Grubbs catalyst and terminal alkenes.However, the corresponding mono- or diallyl vinylphosphonate esters undergo facile cross metathesis reactions.Grubbs and co-workers classified terminal vinylphosphonates as type III substrates [11].Type III alkenes do not homodimerize, but will engage in alkene cross metathesis reactions.The products were isolated by silica gel chromatography to give 31% yield of mono-allyl and 25% yield of diallyl phosphonate esters.Similarly, THF-substituted vinylphosphonate 19 was treated with 5 equivalents of allyl bromide and 5 mol % TBAI in refluxing toluene for 36 hours to give a 76% conversion with 1.8:1 ratio of mono- to diallylated vinyl phosphonates 20a and 20b (Scheme 5).


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