An alternative approach could involve an alkene cross metathesis reaction between the vinylphosphonate and a styrene (5 to 7).
Since substituted vinylphosphonates are reluctant to participate in cross metathesis reactions (Scheme 3), this approach to the synthesis of cetrolobine appeared to have little merit. reported the concept of “relay ring closing metathesis (RRCM)”, wherein typically unreactive α,ω-dienes bearing 1,1-disubstituted ethylene moieties 9 would react via the intermediacy of an additional terminal alkene 11 (Scheme 3) [19-21].
However, we have observed that β-substituted vinylphosphonates are unreactive towards cross metathesis and are therefore type IV substrates.
Since alkene cross metathesis is a powerful method of combing organic fragments in natural product synthesis, the value of vinylphosphonates as synthetic intermediates would increase if their reactivity could be enhanced to a level where they would participate in cross metathesis reactions.
Carbonyls and alkenes, two of the most common functional groups in organic chemistry, generally do not react with one another.
Now, a simple Lewis acid has been shown to catalyse metathesis between alkenes and ketones in a new carbonyl olefination reaction.
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) .
As expected, the reactions generally proceed with complete chirality transfer.
Various carbon, nitrogen, and oxygen nucleophiles participate in the palladium-catalyzed substitution reactions of phosphono allylic carbonates 1.