Crotyl enol ethers

An orf/io-directed lithiation allows the conversion of 25 to aryl iodide 40. Reductive ether formation of aldehyde 40 with crotyl alcohol yields compound 41. Intramolecular Heck reaction of 41 affords a mixture of the olefins 42 and 43. The undesired alkene 42 can be isomer-ized quantitatively to the desired enol ether 43 with Wilkinson s catalyst. Sharpless dihydroxylation ee 94 %) of the enol ether 43 provides lactol 44, which is oxidized directly to lactone 45. Finally, the pyridone-O-methyl ester is cleaved under acid conditions (45 — 7). 

As a typical example, the enol ether of acetoacetic ester 3, when treated with anhydrous crotyl alcohol in hot xylene in the presence of dinitrophenol/potassium hydrogen sulfate (4 1, 2g/100g 3), undergoes transetherification and rearrangement to give a 1 1 mixture of diastereomers of the alkylated /J-oxo ester 4452. 

Scheme 30. Transition state geometry and stereoselectivity in the Claisen rearrangement of a crotyl sugar enol ether [61]...

Nucleophile = Allyl/crotyl silanes, Allyl/crotyl stannanes, Silyl enol ethers, Silyl ketene acetals, Silyl hydrides. Metal cyanides. Alkyl zincs... 

Addition of an T -allyl-Fp complex to this compound affords an T -aIlyl-Fp-substituted cycloheptatriene system. Two double bonds are involved in an (T -diene)iron complex. The remaining free double bond of the silyl enol ether attacks as a nucleophile at the cationic r -alkene-Fp moiety to form an (Tj -diene)iron complexed cyclopentane annulated cycloheptadienone. Treatment with CAN in methanol under carbon monoxide atmosphere releases the methoxycarbonyl-substituted free ligand (Scheme 4-25). Reaction of the Ti -dienyliumiron intermediate of Scheme 4-25 with an ( , Z)-isomeric mixture of ri -crotyl-Fp proceeds with high diastereoselectivity. Four new stereogenic centers are formed in the course of this formal [3+2] cycloaddition. A hetero [3+2] cycloaddition is also feasible between T -ailyl-Fp complexes and aromatic aldehydes in the presence of zinc chloride or titanium(IV) chloride to provide tetrahydrofuran derivatives (Scheme 4-26). A 1,2-shift of the iron complex fragment occurs in the course of this reaction. Employment of imines affords the corresponding pyrrolidines. ... 

The O-protected and unprotected ( )-crotyl lactates 5a-d require longer enolate generation times and higher temperatures for the rearrangement486. Best conditions for the rearrangement of the benzyl ether 5b involves deprotonation with lithium N-isopropylcyclohexylamide in tetrahydrofuran at —78 °C followed 30 minutes later by chlorotrimethylsilane and slow warming to reflux. 

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