Allenyl ethers, rearrangements with

The reaction mechanism of photocyclization of aryl vinyl ethers was derived from results obtained by means of flash photolysis. The ground state intermediate rearranges by mono-or bi-molecular 1,4-hydrogen shifts to yield the products (Scheme 62) (81JOC978). The photocyclization of 2-aryloxyenones was used in the total synthesis of ( )-lycoramine (77JA8065). The formation of dihydrobenzo[6 ]furans by radical cyclization from o-allenyl-oxyarenediazonium salts with tri-n-butyltin(IV) hydride was successful (81CC136). 

The addition of an allyl alcohol to racemic allenyl sulfoxides results in vinyl ethers with the sulfinyl moiety at C-1 that undergo sigmatropic rearrangements upon distillation to produce 2,4-dienones after ehmination of sulfenic acid. In one example, an isomeric vinyl ether was obtained with a sulfinyl methyl substituent at C-2 that gave rise to a sulfinyl enone upon rearrangement [138]. In related work, the addition-elimination of an allyl alkoxide to a functionalized vinyl sulfoxide results in a sulfinyl enol ether that rearranges with loss of sulfenic acid to the unsaturated ester [139-141] (Scheme 21). 

Claisen rearrangement. Allyl allenyl ethers resulting from isomerization of allyl propargyl ethers with t-BuOK are ready to undergo Claisen rearrangement. A route to a-substituted acroleins is established. 

Norpseudoephedrine-derived amino ether 81 was also used as a chiral coordinating agent for the enantioselective [2,3]-Wittig rearrangement. The rearrangement of propargyl ether 82 induced by n-BuLi/81 provided allenyl alcohol (5 )-83 in 62% ee (equation 45). In contrast, a similar reaction with (-)-24 provided only 9% ee of (S)-S3. 

Allenyl trialkylsilyl ethers 771 can be a-deprotonated with f-BuLi in THF at — 78 °C to give the allenyllithiums 77210901091. They underwent reverse Brook rearrangement to afford the silaacrolein enolates 773, which react with aldehydes and ketones to yield the a,/9-unsaturated acyl silanes 774 (Scheme 200). For enolizable aldehydes transmetallation with ZnCl2-TMEDA, and MgBr2 for ketones, provided better yields. 

By analogy with the mechanistic pathway described for the enol ether 119 (Scheme 44), we believe that the transformation of 123a-c also occurs via the formation of the /J-metalated allenyl intermediate, generated from the -elimination of the corresponding zirconacyclopropane and subsequent rearrange-... 

Brook rearrangement. a-Trimethylsilylpropargyl alcohols undergo Brook rearrangement to afford allenyl silyl ethers, which can be used to condense with aldehydes. Silyl group transfer from the ether four bonds apart is preferred after the Sn/Li exchange from silyl ethers of l-tributylstannyl-l,3-alkanediols. °... 

DienonesJ A new synthesis of 2,4-dienones (4) involves reaction of an allylic alcohol (1) with an allenyl phenyl sulfoxide (2) in benzene (ice bath). The resulting enol ether (3) is treated with this base, which induces Claisen rearrangement and elimination of benzenesulfenic acid, with formation of the dienone 4 (equation I). 

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

A metal-mediated rearrangement of 3-alkynyl flavone ethers has been found to involve 5-endo enyne cyclization to a platinum-containing spiro-oxocarbenium intermediate which may be trapped with methanol to give spirodihydrofurans or further rearranged to allenyl chromanediones or benzofuranones (Scheme 150). ... 

Allenylation Reactions. The propargyltrimethylsilanes undergo Lewis acid-catalyzed reactions with electrophiles, with regiospecific rearrangement, to give substituted aUenes (eq 10). The catalysts usually employed are aluminum chloride, boron trifluoride etherate, and titanium(IV) chloride. 

The reaction of lithium diisopropylamide with propargyl acetate 3.559 followed by treatment of the enolate formed with trimethylchlorosilane at —78°C affords ketene acetal 3.561. After heating the reaction mixture to 40°C, the intermediate vinylpropargyl ether 3.560 undergoes the Claisen rearrangement to produce silyl ester 3.561, which after hydrolysis forms allenyl-acetic acid 3.562 with a 50% overall yield (Scheme 3.42) [281]. This reaction is of interest as a synthetic strategy for obtaining the benzoannelated enyne-allene that by radical cyclization forms polycyclic aromatic compounds with an embedded fluorene section [281]. (Scheme 3.43)... 

---