Ethers, propargylic rearrangement

Thermal propargyl rearrangement-Claisen rearrangement of propargyl vinyl ethers and Cope-type rearrangements of propargyl malonates [93]. 

The TMS-propargyl ether (104) rearranges to an 80 20 mixture of the ( )- and (Z)-isomers (106) and (108) in an apparent violation of this transition state proposal (equation 26). (Note that because of the stereochemical descriptor rules the sense of ( ) and (Z) is inverted by the TMS substituent.) However, replacement of the vinylic TMS grouping by hydrogen led almost exclusively to the ( )-product (109). Evidently, the normally favored transition state (E) is destabilized by steric interactions between the vinylic TMS substituent and the n-pentyl grouping (R ). The alternative conformation (F) lacks this interaction. The situation is analogous to that noted by Still in the rearrangement of lithiomethyl allyl ethers (Scheme 4). 

Nonracemic (Z)-allylic propargyl ethers likewise rearrange with excellent enantioselectivity (Table 7). In these examples the configuration of the starred center in (114) and (115) should be opposite for ( )-and (Z)-products owing to the concerted suprafacial nature of the rearrangement. The stereochemistry at the carbinyl center is related to the equatorial/axial preference of the alkynyl grouping (Scheme 7). It should be noted that the ( )-products (114) and (115) are greatly favored because of repulsive 5yn-l,3 in-... 

It is well known that alk-2-ynones can be prepared from a copper(i) alkyne and an acyl halide. Now the troublesome metallation step can be avoided by direct reaction of the terminal alkyne with the acyl chloride in triethylamine containing copper(l) iodide and bis(triphenylphosphine)palladium(ii) chloride as catalysts. Cyanohydrins and propargyl bromides form alkynyl ethers which rearrange on treatment with lithium di-isopropylamide to form ar-allenic ketones (Scheme 59). ... 

Scheme 10.24 Propargyl vinyl ether Claisen rearrangement.

To a mixture of 65 ml of dry benzene and 0.10 mol of freshly distilled NN-di-ethylamino-l-propyne were added 3 drops of BFa.ether and 0.12 mol of dry propargyl alcohol was added to the reddish solution in 5 min. The temperature rose in 5-10 min to about 45°C, remained at this level for about 10 min and then began to drop. The mixture was warmed to 60°C, whereupon the exothermic reaction made the temperature rise in a few minutes to B5 c. This level was maintained by occasional cooling. After the exothermic reaction (3,3-sigmatropic rearrangement) had subsided, the mixture was heated for an additional 10 min at 80°C and the benzene was then removed in a water-pump vacuum. The red residue was practically pure acid amide... 

Cleavage of propargyl ethers by Grignard reagents 12-2 Rearrangement of alkynes... 

The cyclisation of naphthyl propargyl ethers occurs efficiently under microwave irradiation leading to naphthopyrans, but naphthofurans are formed in the presence of base <96JCR(S)338>. The thermal rearrangement of naphthyl 3-trimethylsilylprop-2-ynyl ethers yields the 4-trimethylsilyl derivatives of naphthopyrans <96H(43)751>. 

Shortly after Trost s works, two investigations demonstrated the high reactivity of platinum halide salts for this type of reactions. Blum reported a PtCU-catalyzed rearrangement of allyl propargyl ethers to 3-oxabicyclo[4.1.0]-heptenes (Scheme 79).294 This series of reactions also represented the premiere entry into the versatile formation of cyclopropyl products based on skeletal rearrangements of enynes.295 This intriguing aspect is discussed further. 

Another abnormal Claisen rearrangement is the product formed on heating phenyl propargyl ether. The normal product O-allenyl phenol rearranges by a [1, 5] hydrogen shift and then there is an electrocylic ring closure to give chromene which is the observed product. 

The Claisen rearrangement of propargyl vinyl ethers directly delivers the allene no equilibrium is observed. This reaction was also successful with complex substrates in order to show this, of numerous examples [375, 513-536], the compounds 159 [537] and 161 [538] are depicted (Scheme 1.71). 

Two groups independently demonstrated that the strategy for the preparation of 55 could be used for the synthesis of 2-substituted-3-allenylbenzo[b]furans 59 (Scheme 3.31). The synthesis of 59 was achieved by intermolecular cyclization between alkynylphenols 56 and propargyl carbonates 57 [67] or by intramolecular rearrangement of (2-alkynylphenyl)propargyl ethers 58 [67, 68]. 

Hydroxycoumarins are alkylated under extremely mild basic liquiddiquid phase-transfer catalytic conditions to produce the ethers and 4-alkylated derivatives 141 ]. The major product tends to be the ether (50-60%) but the yield of the C-alkyl-ated product is significant with allyl bromide and with propargyl bromide, where rearrangement of the allenic derivative occurs. 

The bis(oxazoline) S, 5)-(115) has been used as an external chiral ligand to induce asymmetric diastereoselective lithiation by r-BuLi during [2,3]-Wittig rearrangement of achiral substrates, (fj-crotyl propargylic ethers.It is believed that the enantios-electivity is determined predominantly at the lithiation step. 

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