Allene reactions

Alkylallenes are obtained by the reaction of 1-ethynylcycloalkanol acetates with organocopper reagents, lithium dimethyl- and dibutylcuprates643 (see Section B.l). Even in the case of the presence of a substituent at the acetylenic terminus, SN2 displacement takes place, giving tetra-substituted allenes. Reaction of the steroidal 17-acetoxy-17-ethynyl derivative la shows that the... 

Cyclopropylidenes undergo ring opening to give allenes. Reactions that would be expected to generate a cyclopropylidene therefore lead to allene, often in preparatively useful yields. 

Burger2 has shown that alkynes undergo both Lewis acid-catalyzed and thermal carbonyl-yne reactions with 3,3,3-trifluoropyruvates to give allenes. Reaction of 1 (Equation (2)) occurs to give a 1 1 mixture of diastereomeric allenyl carbinols 2. Alternatively, reaction of hexyne 1 and methyl trifluoropyruvate with MgBr2-OEt2 at low temperature afforded 2 as an 8 1 mixture of diastereomers. The thermal reaction does not suffer from allylic alcohol byproducts arising from reaction of the substrate with the Lewis acid.3... 

A closer survey revealed that in the period elapsed since 1984 an average of 400 publications on allenes have appeared each year, amounting to close to 8000 in total. This made it obvious that one mortal author could not cover the whole field alone. As a consequence, Norbert Krause, both a renowned expert in allene chemistry and experienced in editing books, was commissioned as an additional editor. The crucial part then was to identify those fields of allene chemistry displaying the new and significant developments since 1984. We decided to divide the project into four parts, Synthesis of Allenes , Special Classes of Allenes , Reactions of Allenes , and Applications . 

Beginning with allene (1) itself, its thermal dimerization to 1,2-bismethylenecy-clobutanes (126) constitutes one of the oldest allene reactions known. Although the yield is only moderate (in the region of 30%, depending on the pyrolysis conditions), the dimerization generates a very useful diene from a readily available starting material (Scheme 5.43) [117]. 

The perfect diastereoselectivity observed in these reactions lead to the assumption that they do not proceed via a simple anionic intramolecular cyclization (i.e. carbozincation) as was proposed in earlier61 publications. Cyclizations of non-propargylic substrates15,62 were shown to proceed with significantly lower diastereoselectivity (cis/trans typically 75/25). Thus, in the case of propargylic compounds, e.g. 114, a simple carbozincation process is unlikely to be operative. It was therefore reasoned that the active species in these reactions is not 115-ZnBr but rather its allenic isomer 120, which undergoes a metallo-ene-allene reaction in a chair-like transition state as depicted in equation 5659a. 

The intermediate methylenetriazolines from azide-allene reactions also give isomeric methyltriazoles.185,186,254,301 The triazole formed in the... 

The synthesis of polysubstituted pyrrolidines has been achieved344 in a diastereo-selective and enantioselective manner via the zinca-ene-allene cyclization. In an extension of this work,345 the zinca-ene-allene reaction of polysubstituted enynes lithiated on the propargylic position has been used to prepare polysubstituted... 

Cyclopropylidene reaction Neither the barrier nor the equilibrium constant for the cyclopro-pylidene/allene reaction have been measured. The only direct experimental information of these species come from the failure to observe cyclopropylidene at 77 K (Chapman OL (1974) Pure and Applied Chemistry 40 511). This and other experiments (references in Bettinger HF, Schleyer PvR, Schreiner PR, Schaefer HF (1997) J Org Chem 62 9267 and in Bettinger HF, Schreiner PR, Schleyer PvR, Schaefer HF (1996) J Phys Chem 100 16147) show that the carbene is much higher in energy than allene and rearranges very rapidly to the... 

Hydroxy(tosyloxy)iodo]benzene and Its Analogues Reactionswith Alkenes and Allenes. Reactions with Alkynes and Alcohols. Reactions with Keto Compounds. Reactions with Nitrogen, Sulfur, and Other compounds. 

Another facet of the cyclization of secondary organometallic derivatives is the use of d-ethenic allylmetals [61] this has been abundantly reviewed [58] and is considered as a metallo-ene reaction. However, although the ene-allene reaction was disclosed 10 years ago [59] to interpret the formation of a cyclic product by flash pyrolysis of a 2,3,8-nonatriene, the corresponding metallo-ene-allene reaction has received attention only veiy recently in organic synthesis [60]. 

The previously described zinca-ene-allene reaction was successfully applied to the diastereoselective synthesis of tetrahydrofurans [68] and pyrrolidines [69] as described in Scheme 7-62. 

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