Propargyl allyl ethers, rearrangement

Scheme 6 Envelope transition states for 2,3-Wittig rearrangements of propargyl allyl ethers...

Thus, for example, the cyclic propargylic allylic ether (242 equation 49) rearranges completely within 30 min upon treatment with Bu"Li in THF-hexane at -20 C, whereas the acyclic analog (244 equation SO) requires 12 h under comparable conditions. Alcohol (243) serves as a useful intermediate for the synthesis of cembranoid diterpenes. ... 

ACYCLIC r2.31-WITTIG REARRANGEMENT OF PROPARGYL-ALLYL ETHERS... 

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. 

If X in Scheme 41 represents a cyanide group (X = CN) the rearrangement of the anion leads to a cyanohydrin anion as the product, which can yield a carbonyl function directly.The reaction is not only known for allylic ethers but also for propargylic ethers, giving access to allenic carbonyl compounds in the latter case (equation 39). ... 

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). 

Unsymmetrical diallyl ethers, like their propargyl allyl counterparts, can theoretically yield isomeric 2,3-rearrangement products depending upon the site of deprotonation. In the first systematic study of... 

Wittig rearrangement of a-allyloxycarboxylic acid dianions and allyl propargylic dianions (Section 3.11.3.3) might be expected to proceed analogously. In fact, the same high preference for ( )-pro-ducts is observed, but the diastereoselectivity is reversed (Table 17, entries 5 versus 6). A chelated bicyclo [3.3.0] transition state readily explains the anti selectivity of (Z)-allylic ethers [Scheme 12, compare (R) with (T)]. The basis for syn selectivity observed with ( )-allylic ethers (Q) versus (S) is less clear. 

Intramolecular a-diazo ketone initiated 1,2- and 2,3-rearrangements of ethers, allylic ethers and propargylic ethers E. J. Roskamp and C. R. Johnson,/. Am. Chem. Soc. 1986,108, 6062. 

Pregnane Series. Steroid allyl and propargyl enol ethers tindergo the Claisen rearrangement. - Surprisingly, cyanogen bromide reacts with 18-hydroxy-20-dimethylamino steroids to form the corresponding 18,20-epoxy deri-vatives. Additional work has been reported on the synthesis of l8-methyl steroids. ... 

Entry 1 was the first example, reported in 1970 [40], of a highly stereoselective [2,3]-Wittig rearrangement, but comparison with entry 5 shows that only the Z isomer is selective. Entries 2-4 illustrate substituted propargyl Z allyl ethers. 

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