Cycloaddition to Alkynes

Attempts to add nitrenes derived by heating azides to alkynes are usually thwarted by the facile addition of the azide 1,3-dipole to the triple bond with the formation 

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Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

Cycloaddition to alkynes, cyclobutenones. This ketene when formed in situ from CCI3COCI and Zn/Cu, reacts with alkynes to form 4,4-dichlorocyclobuten-ones,3 which can rearrange in part to 2,4-dichlorocyclobutenones.4 Both products are reduced to the same cyclobutenone by Zn(Cu) in HOAc/pyridine (4 1) or by zinc and acetic acid/TMEDA.5... 

Numerous organic and inorganic azides undergo 1,3-dipolar cycloaddition to alkynes to give... 

In constrast with intermolecular nitrone cycloadditions to alkynes and allenes, very little work has been done on the corresponding intramolecular cycloadditions. The bicyclic isoxazolidines (65a-b) were reported as products from reaction of an alkynone with methylhydroxylamine in ethanol.26b Presumably the initial strained bridgehead C—C double bond of the AMsoxazoline added ethanol under the reaction conditions. Cyclization of an allenyl ketone with methylhydroxylamine in ethanol solution also led to isoxazolidines (65a-b) as the major products and isoxazolidine (66) as a minor product.266 Thus, preferential cyclization to the internal C—-C double bond of the allene occurred followed by addition of ethanol to the exocyclic C—C double bond of the methyleneisoxazolidine intermediate. 

The cycloaddition to alkynes affords a straightforward synthc of cyclo-propencs. 

Cycloaddition to alkynes. Dichlorokelene, generated from trichloroacetyl chloride by zinc (8,156), adds to alkynes to give cyclobutenones. The reaction occurs preferentially with the triple bond of enynes. 

Yet another rather general approach towards highly substituted cyclopenta-dienones has been developed via /1-aminosubstituted a,/ -unsaturated chromi-umcarbene complexes such as 32. Their cycloaddition to alkynes proceeds without carbonyl insertion to yield 1,2,3-trisubstituted 5-dimethylamino-3-ethoxy-cyclopentadienes which are readily hydrolyzed to the correspondingly substituted cyclopentenones [26]. After quaternization with methyl iodide the ammonium salt 33 is obtained. Treatment of the latter with a base such as NaOMe or NaOH results in deprotonation and elimination of trimethylamine to yield a trisubstituted cyclopentadienone which immediately dimerizes by [2 + 2] cycloaddition probably via a 1,4-diradical intermediate like 35 to yield the bis(cyclo-pentenone)-annelated cyclobutane derivative 34 (Scheme 8) [27]. 

V-Sulfonylcarboxamides 16, which are prepared in situ, can react with multiple-bond systems.56 58 From the cycloaddition to alkynes, oxathiazine derivatives 17 can be obtained.59-61... 

A previous observation concerning the misbehavior of sulfonyl azides in cycloaddition to alkynes prompted a study that eventually identifies the optimal condition of the reaction The photoinitiated cyclization involving an allene and an azido group is improved by Cul, in terms of regioselectivity in the cycUzation ° Imidocopper species are thought to undergo electrocyclization prior to demetallation. 

Miscellaneous. Acylation of unprotected amino acids, desilylation of TBS-ethers, alkylation of ketone enolates with (iso)/quinolinium salts, Wolff rearrangement, bromination of active arenes with NBS, and dichloroketene formation from Zn and CI3CCOCI (for cycloaddition to alkynes) have all been promoted with ultrasound. 

When carbene 21 is generated from the diazo precursor photochemically in solution, it reacts with added alcohols by 0,H insertion (equation 55), in contrast to the gas-phase copyrolysis where the silene intermediate is trapped . Similarly, photolysis of 20 in the presence of 2,3-dimethylbutadiene gives mainly vinylcyclopropane 184, while after copyrolysis of 20 and the same diene one finds that most of the vinylcyclopropane rearranged to the cyclopentene, together with the [4-1-2] cycloaddition product of diene and silene 22 °. Furthermore carbene 21, generated photochemically or thermally (at 117-148 °C) in solution, undergoes [1 4-2] cycloaddition to alkynes to give cyclopropenes 185. 

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