Rearrangements homopropargylic

The homopropargyl rearrangement is of preparative interest for the synthesis of cyclobu-tanone and substituted cyclobutanones (Table 3). For example, the rearrangement of 4-(triflu-oromethanesulfonyloxy)but-l-yne (4) takes place almost quantitatively (76% conversion) in the presence of trifluoroacetic acid and sodium trifluoroacetate to produce cyclobutanone.6... 

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

Benzothiepines 69 have been synthesised by a gold-catalysed rearrangement of homopropargyl arylsulfoxides 68 <07JACS4160>. The dibenzothiepine 71 was synthesised by intramolecular Friedel-Crafts acylation of a diarylsulfide 70 which was formed by the copper mediated substitution of an appropriately substituted aryl derivative with thiophenol... 

The work of Harding and Hanack on this system has confirmed the view that the reaction is observed only in solvents of low nucleophilicity in highly nucleophilic solvents, a bimolecular reaction proceeds without rearrangement. A series of isotope effect measurements confirms that in solvents of low nucleophilicity and good ionizing power, the homopropargylic assistance is of importance in the solvolysis reaction. 

The [2,3] Wittig rearrangement is a useful reaction involving a [2,3] sigmatropic shift, and has been used to obtain homopropargylic alcohols from allylic propargylic ethers with high stereoselectivity. The reaction has been modified by Marshall to provide a useful route to optically active allenic alcohols. 

The proposed intermediate (308) in the solvolytic rearrangement of homopropargyl derivatives (307) to cyclopropyl ketones and cyclobu tan ones has now been trapped. Thus, the cyclobutene (309) has been isolated from treatment of (307 R = Me, Y = OTf) with trifluoroethanol and sodium carbonate. ... 

An analogous Au(I)-catalyzed synthesis of furan 86 was recently described by Toste and coworkers [141]. Thus, sulfoxide 84 undergoes an Au(I)-catalyzed rearrangement into the homopropargylic ketone 85, which subsequently cycloisomerizes into the furan product (Scheme 8.33). 

The smooth addition to aldehydes with 25 occurs at —78 °C to provide /3-TMS homopropargylic alcohols (26, 80-96%) in extremely high dr (99 1) and ee (94—99%). When the enynylborane 27 (285 906650-65-5 28/ 906650-64-4) undergoes the TMSCH insertion and rearranges to 28 (285 929082-50-8 28/ 929082-62-2), allylboration is faster than allenylboration. Thus, 28 adds to benzaldehyde to form the 1,2,3-trienyl carbinol 29 in chemically and optically pure form (eq 11). 

Water was supposed to limit rearrangements between allenyltributylstannane and dibutylstannyl dichloride while these reagents were allowed to react with aldehydes. Nevertheless, complete a-allenic stereoconvergence was not observed. Homopropargylic alcohols could be obtained as well, especially with formaldehyde or a,p-unsaturated aldehydes (Boaretto et al., 1985b). 

These results may be interpreted in terms of the proposed mechanism shown in Scheme 23. Dissociation of Cl from the starting Ru precatalyst is followed by coordination of the alkyne and subsequent rearrangement leads to Ru vinylidene intermediate I. This key intermediate may undergo nucleophilic attack by the pendant amine or amide with concurrent removal of a proton by pyridine to provide the alkenyl Ru species II. Finally, protonolysis by the pyridinium salt formed could provide the final indoles or dihydroisoquinolines, with regeneration of the active catalytic species. A similar mechanistic proposal could be invoked for the preparation of dihydroquinolines from bis-homopropargylic amide. 

The gold(I)-catalysed oxidative rearrangement of propargyl alcohols has been reported as a selective route to 1,3-diketones (Scheme 123). a,j0-Unsaturated carbonyl compounds have been reported to be formed from gold-catalysed oxidative rearrangement of homopropargylic ether with a gold carbenoid and oxonium ylide proposed as intermediates (Scheme 124). ... 

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