Ring-opening epoxide carbonylation

Finally, the conversion of epoxides to p-hydroxy amides has been accomplished by the reaction of epoxides with CO and silylamines (Equation 17.51). This reaction was originally reported with COj(CO)j as catalyst to form the p-silyloxyamide products in 60 to 84% yields using 1 atm of CO at room temperature for 24-50 h. Optically active p-hydroxy 

The ring-opening copolymerization of propylene oxide and carbon monoxide forms the polyester poly(p-hydroxybutyrate) (PHA, Equation 17.52). The physical and mechanical properties of some PHAs are similar to those of isotactic polypropylene. This polymerization was first reported by Furukawa and co-workers in 1965, and more recent studies have been reported by Osakada, Rieger, and Alper. These polymerizations have been conducted with Co2(CO)j and additives. The combination of Co2(CO)j, a 1,10-phenan-throline derivative, and benzyl bromide afforded polyester with an value of 19.4 kg/ mol and M /M of 1.41. In addition to propylene oxide, 1,2-epoxybutane was successfully copolymerized with CO to yield the corresponding poly(3-hydrox5 entanoate) with an value of 16.7 kg/mol and a M /M of 1.28. The role of benzyl bromide is unclear. Related copolymerizations of aziridines and CO to form polyamides have also been reported. Polymer values as high as 27.5 kg/mol have been reported, and tjqjical M /M values varied from 1.11 to 1.64. 

These polymerizations could occur either by direct enchainment of the epoxide or by carbonylation of the epoxide to the 3-lactone, followed by ring-opening polymerization of the lactone. Rieger and co-workers and Alper and co-workers showed that these polymerizations occur by direct enchainment of the epoxide. - A variety of mechanisms for epoxide enchainment have been proposed most involve reaction of flie epoxide with an acylcobalt species and a nucleophile, either a nitrogen base or [Co(CO)J, in a transition state that is not well defined. 

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The suggested reaction mechanism involves a nucleophilic attack of the imine nitrogen at the activated triple bond, followed by a proton exchange, to give a benzimidazolinium system which, by intramolecular attack at the carbonyl group, leads to an epoxide that ring opens to the observed product. For the ethyl derivative (R = Et) a tub conformation could be established by X-ray crystallographic analysis.33... 

The regioselective ring opening of the silyl epoxides is facilitated by the stabilizing effect that silicon has on a positive charge in the -position. This facile transformation permits vinylsilanes to serve as the equivalent of carbonyl groups in multistep synthesis.149... 

Finally, it is important to mention that there are other related publications in which porphyrin macrocycles are not directly used as dipolarophiles but are transformed into new derivatives that can react with carbonyl ylides via ACE (alkene cyclobutene epoxide) reactions. This idea arose in 1997, when Russell and co-workers found that fused ester-activated cyclobutene epoxides 86 can be ring-opened to give carbonyl ylides 87, and that these can be trapped stereospecifically by ring-strained alicyclic dipolarophiles, such as 2,5-norbomadiene, to form hetero-bridged norbomanes 88 in good yields, through ACE transformations (Scheme 31) <97CC1023>. 

Treatment of the /3-keto ester 220 with sodium ethoxide at elevated temperature triggered off an epoxide ring opening by / -elimination that was followed by the desired Knoevenagel condensation to afford the tricyclic product 206 (Scheme 34). The enone moiety in the intermediate 221 did not show a propensity for deprotonation and, therefore, the ketone carbonyl function of the enone moiety was available for a Knoevenagel condensation. The reduction of the p-keto ester (206) to the corresponding diol was the next objective. Treatment of the TES-protected -keto ester (TES-206) with DIBAH afforded the diastereomeric diols 222 and 223 in a moderate diastereoselec-tivity in favour of the undesired diastereomer 222. The diastereomers were separated and the undesired diastereomer 222 was epimerized to 223 by a sequence that consists of Mitsunobu inversion and benzoate ester reduction [98, 99]. 

P8 Thermal conrotatory 4tt ring opening of aziridines and epoxides is a well-recognized method of generating azomethine and carbonyl ylidesd (From Johnson, 2003)... 

Condensation of the carbonyl group with the yUde from trimethylsulfonium iodide leads initially to an addition product. The anion formed on the carbonyl oxygen then internally displaces dimethyl sulfide to give an oxirane yielding epoxide (114-3). Reaction of that intermediate with a 1,2,4-triazine leads to an epoxide ring opening with the consequent incorporation of the second heterocyclic moiety. There is thus obtained fluconazole (114-4) [125]. 

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