Propargylic Acetal Substrates

Propargyl halides [56] and propargyl acetates [57] have also been employed for Pd-catalyzed carbonylation (Scheme 3.22). From the latter substrates, allenylcarboxylic acids were obtained under phase-transfer conditions (with nBu4NBr in aqueous NaOH and 4-methyl-2-pentanone) [56],... 

Propargylic acetates in equation (80) react differently in a process that also takes place via a 1,3-acyl migration. Thus, the reaction of such substrates gives bicyclo[3.1.0]hexenes, which can be transformed into cyclohexenones on treatment with K2CO3 in MeOH. As shown in equation (81),... 

The cyclization of other propargylic acetates as in equations (83) and (84) gives tricyclic derivatives with high stereoselectivity by an endocyclic pathway. " In the cyclization of this type of substrates, better yields were usually... 

Other Reactions. Other substrates that have been found to react with silyl-copper reagents and with silyl cuprates are allyl chlorides, an alkyl bromide, epoxides, acid chlorides, propargyl acetates and sulfides, a vinyl iodide, an amino-methyl acetate, ethyl tetrolate, and some strained allylic ethers. ... 

Coupling of (1) with propargylic tosylates, mesylates, or acetates in the presence of copper(I) leads to a-trimethylsilyl-allenes, while the use of a propargyl alcohol substrate leads to the substituted allylsilane (eq 12). ... 

Cuprates react rapidly witli allylic halides for acetates) [17, 23], propargyl halides for acetates) [ 106-108], and vinyloxiraties, often witli S 2 regioselectivity iSclieme 10.9) [17]. Hie reaction takes place witli emit stereodieniistry iwitli respect to tlie leaving group), while syn substitution occurs when an allylic carbamate is employed as tlie substrate [109]. 

Initial attempts to perform the 1,5-substitution enantioselectively with chiral enyne acetates proceeded disappointingly. For example, treatment of the enantio-merically pure substrate 51 with the cyano-Gilman cuprate tBu2CuLi LiCN at -90 °C provided vinylallene 52 as a 1 3 mixture of E and Z isomers with 20 and 74% ee, respectively (Scheme 2.19) [28], As previously described for the corresponding Sn2 substitution of propargylic electrophiles, this unsatisfactory stereoselection may be attributed to a racemization of the allene by the cuprate or other organome-... 

---