Alkynes ring-closure reactions

A versatile approach to the 5-ary ldibenzo[7>,/]oxcpin system is the electrophilic ring-closure reaction of 2-phenoxy-substituted diphenylacetylenes or 1-bromostilbenes. Cyclization of the alkynes 4 occurs after addition of a suitable electrophile such as H +, PhS+, Br+, or I+ to give products 5.100-101 Addition across the triple bond gives stilbenes as byproducts. 

As a combined reaction of (3-cleavage of a cyclopropylcarbinyl radical and intermolecular addition, treatment of vinylcyclopropane (266) and activated alkyne in the presence of PhSSPh and AIBN forms a cyclopentene skeleton (267), through the initial addition of a thiyl radical to the vinyl group, P-cleavage of the cyclopropylcarbinyl radical, addition of the carbon-centered radical to the alkyne, ring closure of a vinyl radical via 5-exo-trig manner, and finally subsequent P-elimination of the thiyl radical, as shown in eq. 3.107 [272-276]. Here, PhSSPh acts as a catalyst, since the thiyl radical is regenerated. Aliphatic disulfides such as... 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Cyclic Alkynes from Ring-Closure Reactions... 

Diederich and coworkers have developed a new class of push-pull chro-mophores resulting from the formal [2-1-2] cycloaddition-retroelectrocyclization (CA-REC) reaction between electron-rich alkynes and electron-poor olefins such as tetracyanoethylene (TCNE) [134]. A highly functionalized 6,6-dicyanopentafulvene 226 with an intense, low-energy charge-transfer band is obtained by a novel ring-closure reaction of a push-pull chromophore 227 (Scheme 6.61a) [135]. Later, they also reported the synthesis of new fulvene-based push-pull chromophores 228 and 229 via the formal [2-1-2] CA-REC reaction of dicyanopentafulvenes 222 and 230 with electron-rich alkynes (Scheme 6.61b,c) [136]. 

As shown in Scheme 6.37, some transition-metal-assisted ring-closure reactions of 1,2-diethynylbenzenes afford dibenzopentalene derivatives unexpectedly. Diederich s group designed a carbopalladation cascade reaction of easily accessible ew-dibromoolefins 286 and alkynes to furnish monobenzo-287 and mononaphthopentalenes 288 (Scheme 6.72). This successful methodology allows the preparation of asymmetrically substituted pentalene derivatives with only one fused aromatic system [167]. 

Recent studies related to Pd-catalyzed competitive exo- vs e electronic factors on the regioselectivity in ring closure reaction. The reactions in general are influenced by the electronic nature of the substituent on the alkyne unit. A preference for endo-dig cyclization over exo-dig is noted, if the alkynyl substituent is not sufficiently electron withdrawing. In the... 

Dotz reaction, since both an alkyne and CO are inserted. However, the additional double bond present in the starting complex participates in the subsequent electrocyclic ring closure, giving rise to eight-membered carbocycles. 

Entries 20 to 23 involve additions to C=N double bonds in oxime ethers and hydrazones. These reactions result in installation of a nitrogen substituent on the newly formed rings. Entry 20 involves the addition of the triphenylstannyl radical to the terminal alkyne followed by cyclization of the resulting vinyl radical. The product can be proto-destannylated in good yield. The ring closure generates an anti relationship for the amino substituent, which is consistent with the TS shown below. 

The reaction of Ba[P(SiMe3)2]2(THF)2 with diphenylbutadiyne in toluene for 12 days induces a m-addition of the diyne to the phosphide, followed by a 1,3-silyl group shift and ring closure. The dinuclear complex 132 is then isolated in good yield.283 Its complex structure contains Ba-C a bonds (2.881(5), 2.899(5) A), side-on Ba-alkyne (3.003(6), 3.363(6) A) and arene interactions, and Ba-phospholide bonds (Ba-P = 3.487(2) A) (Figure 65). 

Alder/retrograde Diels-Alder reaction sequence of a diaryl alkyne with a 3,6-dicarbomethoxy tetrazine. The resulting diazine (14) is then reduced, cleaved and cyclized with Zn/acetic acid to the 2,3,4,5-tetrasubstituted pyrrole (15), which is then N-alkylated with a-bromo-4-methoxyacetophenone to give a pentasubstituted pyrrole (16). The synthesis of lukianol A is completed by ester hydrolysis, decarboxylation, ring closure and deprotection. 

Among the synthetically useful reactions of Fischer carbenes, the benzannulations are certainly the most prominent. In particular, the so-called Dotz reaction, first reported by Dotz in 1975 [3], is an efficient synthetic method that starting from aryl- or alkenyl-substituted alkoxycarbene complexes of chromium affords p-alkoxyphcnol derivatives by successive insertion of the alkyne and one CO ligand in an a,/Tunsaturated carbene, and subsequent electrocyclic ring closure (see Figure 1). 

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