Propargylic acetals cyclization

Reaction of the chloro-substituted propargyl acetate 132 with aniline gave the pyrrolidine derivative 133 that was cyclized through treatment with... 

A palladium-catalyzed cyclization of 2-iodobenzyl 2-butynyl ethers 562 with ethynyltrimethylsilane leads to 4-substituted isochromans 563 (Equation 234) <2002T9007>. Isochromans 564 can be accessed using a similar palladium-catalyzed cyclization of a propargylic acetal 565 with (Z)-selectivity observed in the exocyclic double bond (Equation 235) <2005JOC489>. 

Early examples of catalytic radical cyclizations were provided by Okabe and coworkers, who subjected 2-bromoethyl propargyl ethers or bromoacetaldehyde propargyl acetals 257 to complex 255 as a catalyst and sodium borohydride as the stoichiometric reducing agent (Fig. 63, Table 5, entry 1) [307, 308]. 

An interesting cyclization of the aUyl-homoallyl-propargylic acetate in eqnation (82) has been shown to afford a mixture of three componnds by nsing an An(I) complex with an NHC as ligand. The first prodnct is that expected from a 1,6-enyne cyclization, whereas the formation of the other two prodncts by cyclization of the 1,5-enyne suggests that alternative mechanisms take place in this case. 

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... 

A higher oxidation state product is obtained by the analogous cyclization of propargylic acetals.Unfortunately, the location of the additional unsaturation cannot be controlled. 

A new oxidative cyclization of propargylic acetates to 1,3-dioxolanes has been achieved using a palladium(ll) catalyst with carbon monoxide in methanol (Equation 48) <2002TL6587, 2006T2545>. 

This section covers cyclizations to the pyrrole nucleus catalyzed by other metals (Ti, Mn, Ru, Pd, Pt, Zn, In). Dembinski and co-workers used zinc(II) chloride as ligand-free catalyst for the microwave-assisted cyclization of homopropargyl azides 26 to afford substituted pyrroles 27 (Scheme 8) [62]. A similar methodology for the synthesis of 2,4,5-trisubstituted pyrroles was described by Driver et al. employing substituted 1-azidobuta-l,3-dienes in a cyclization reaction using catalytic amounts of zinc(ll) iodide [63]. A three-component zinc-catalyzed one-pot cyclization of aromatic and aliphatic propargylic acetates, silyl enol ethers, and primary amines to substituted pyrroles has been described by Zhan et al. The reaction sequence includes propargylation of the silyl enol ether, amination, 5-exo-(7ig-cyclization, and isomerization [64]. Hiroya and co-workers have shown... 

Kato, K., Yamamoto, Y. and Akita, H. (2002) Unusual formation of cyclic-orthoesters by Pd(II)-mediated cyclization-carbonylation of propargylic acetates. Tetrahedron Letters,... 

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)... 

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