Allenols, carbonylation

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

With NIS and aqueous NaHC03, the carbamates of tertiary 2,3-allenols 404 reacted to give 4,4-dialkyl-l-iodo-3-buten-2-ones 405 via the sequential intramolecular attack of the carbonyl oxygen or intermolecular attack of OH in 406 and iodina-tion [181]. 

Insertion of carbon monoxide can also allow the formation of the C-C bond. For example, y-butyrolactones were produced by carbonylative cyclization of allenols [125] (Eq. 94). 

For instance, Choi and Chung described for the first time the rhodium-catalyzed carbonylative arylation of allenols using 53 (Scheme 10.2) [127]. 

Allenols may be carbonylated under quite different conditions. Using Rus (CO)i 2 as the catalyst, unsaturated lactones can be synthesized efficiently." The reaction may also be extended to lactams." This cyclization, combined with an intramolecular propargylic Barbier reaction has been used in syntheses of mintlactone 4.111 from allenol 4.110 (Scheme 4.43)" " and its diastereoisomer, isomintlactone," as well as the stemona alkaloid, stemoamide 4.115 from allenol 4.113 (Scheme 4.44)," and in the skeleton of stenine." A different synthesis of stemoamide can be found in Chapter 8, Scheme 8.112. 

The iron(III) chloride-catalyzed ring expansion reaction of 2-azetidinone-tethered alkynols and allenols provides pyrroles or y-lactones, respectively (Scheme 4-351). The proposed mechanism for the pyrrole formation includes a Meyer-Schuster rearrangement of the propargyl alcohol to the corresponding allenyl alcohol, P-lactam ring opening, tautomerization of the allenyl alcohol to the a,P-unsaturated carbonyl compound, and cyclization by attack of the amino group to the ketone under dehydration. ... 

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