Propargylic acetals synthesis

Nantz, M. H. Bender, D. M. Janaki, S. A convenient terminal allene synthesis from propargylic acetates. Synthesis 1993, 577-578. 

Scheme 2.21 Postulated mechanism for the synthesis of substituted indenes by intramolecular hydroarylation of propargylic acetates...

Scheme 2.2 Allene synthesis via SN2 substitution of propargylic acetates 7 with cuprates.

In the area of allenic non-natural product chemistry, the synthesis of the [34]alle-nophane 14 (Scheme 2.4) is particularly noteworthy, with all four of its allenic bridges being formed through subsequent SN2 substitution reactions of propargylic acetates with a methyl magnesium cuprate [14] (see Section 2.5 for an alternative synthesis of macrocyclic allenes). 

Scheme 2.14 Synthesis of allenes 38 and 40 by reduction of propargyl acetates with lithium dimethylcuprate. THP = tetrahydropyranyl.

Ley et al. also applied this method to the synthesis of sesquiterpenes through a strategy involving a Diels-Alder reaction. Taking into account the effect of conditions and silver counterion on allene isomerization (see Scheme 33), they obtained the diene partner via isomerization of the acetoxyallene produced on treatment of a propargyl acetate with catalytic amount of silver hexafluoroantimonate (Scheme 3.36).57... 

Another, albeit less-frequently employed option for a copper-mediated reduction of propargylic electrophiles to allenes relies upon the use of a copper hydride, for example, Stryker s reagent [(Ph3P)CuH]6. This reagent was applied by Brummond and Lu147 to the synthesis of the structurally complex precursor 197 for a potent antitumor agent, ( )-hydroxymethylacylfulvalene, from propargyl acetate 196 (Equation (11)). 

CRABBE AlleneSynthesis Synthesis of terminal allenes from propargylic acetates. 

Propargyl acetates undergo reductive isomerization to give terminal allenes. Conjugate additions. Interesting preparations pertain to the synthesis of 3-tri-fluoromethyl-3-hydroxyalkanoic acids, 3-aminomethylcycloalkanones, and 4-vinyl-2-acetoxy-2-cyclobutenones. Compounds of the last type give catechol derivatives on thermolysis. 

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... 

Synthesis of Hydroxy-acids.— The allylborane (15) (Scheme 10) is readily obtained from an optically active propargyl acetate by hydroboration and rearrangement (with sodium hydroxide). Its reaction with an aldehyde then gives (16) in 63% yield and 80% enantiomeric excess subsequent ozonolysis and oxidation then... 

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