Cycloaddition of acetylenes

The cycloaddition of acetylenes to 3,6-di(pyridin-2-yl)-l,2,4,5-tetrazines to give the corresponding di(pyridin-2-yl) pyridazines was considerably accelerated under microwave assisted conditions <06JOC4903>. 

Reactions of salts of 1,2,3-triazole with electrophiles provide an easy access to 1,2,3-triazol-jV-yl derivatives although, usually mixtures of N-l and N-2 substituted triazoles are obtained that have to be separated (see Section 5.01.5). Another simple method for synthesis of such derivatives is addition of 1,2,3-triazole to carbon-carbon multiple bonds (Section 5.01.5). N-l Substituted 1,2,3-triazoles can be selectively prepared by 1,3-dipolar cycloaddition of acetylene or (trimethylsilyl)acetylene to alkyl or aryl azides (Section 5.01.9). 

The cyclobutene (268) was constructed through the photo[2 + 2]cycloaddition of acetylene to anhydromevalonolactone, which was followed by DIBAL reduction 92). 

Anderson and co-workers86 used the cycloaddition of acetylenic esters to 19a and 19 (R = COPh) in the synthesis of the antimitotic agent Verrucarin E (28). The pyrrole 19a and DMAD gave 35% of 21a, whereas 19 (R = COPh) gave 67% of 21 (R = COPh) and 23% of recovered starting material 1-benzoylpyrrole gave 54% of 29 with DEAD. Further transformations produced the natural product. 

The cycloaddition of acetylenic esters (e.g., DMAD) to betaines (e.g., 58) derived from 3-hydroxypyridine gives 1 1-molar adducts (e.g., 59), and has been reviewed.266... 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

Fig. 15.9. Frontier orbital interactions in the transition state of the one-step [4+21-cycloaddition of acetylene and butadiene.

The most general methods for preparing seven- or eight-membered rings from enamines are by ring expansion of the cyclobutene, cyclobutanone or chlorocyclopro-pane adducts formed by cycloaddition of acetylene carboxylates, ketenes or chlor-ocarbenes, respectively, to enamines of cyclopentanone or cyclohexanone. These are two-carbon or one-carbon ring expansions. Three-carbon ring expansions can also be carried out by cycloaddition of activated cyclopropenes or cyclopropenones. 

Katritzky AR, Zhang Y, Yuming S, Sandeep K (2003) 1,2,3-triazole formation under mild conditions via 1,3-dipolar cycloaddition of acetylenes with azides. Heterocycles 60 1225-1239... 

A review on the 1,2,3-triazole formation via 1,3-dipolar cycloaddition of acetylenes with azides under mild conditions has been published <03H(60)1225>. The synthesis of a benzotriazole azo dye phosphoramidite and the subsequent use in solid phase synthesis of oligonucleotides has been reported <03TL1339>. The chemical reactivity of [l,2,3]triazolo[l,5-a]- and [l,5-c]-pyrimidinium salts has been published <03T4297>. A review on the use of benzotriazole as an ideal synthetic auxiliary has been disclosed <03CEJ4586>. 

This unique addition-transformation process has also been found in cycloaddition of acetylenic esters to dienes and azadienes containing the 5,6-double bond of uracil as part of the chromophore. Activated and polarized dienes of this type afford, with olefines, pyridol[2,3-Michael adducts, which are thermally converted into 8-(di-methylamino)theoph-ylline. 

Many ring systems have been prepared by cycloaddition of acetylenic compounds, following concerted or multi-step intra- or intermolecular reaction mechanisms. In-particular, the placing in close proximity of triple bonds or a triple bond and another unsaturated system, such that intramolecular cycloaddition might lead to four-seven-membered rings, would seem of interest. This section deals with transannular carbon-carbon bond formation of triple bonds in acyclic and cyclic systems. 

The cycloaddition of the allenes (63) to cyclopentenone results in the formation of the two adducts (64) and (65) in a ratio of 4 1. The addition reaction occurs primarily across the less substituted double bond of the aliens. Cycloaddition of acetylene and but-l-yne to the enones (66) can be brought about using left-circularly polarised light. The resultant cyclobutane derivatives are readily converted to the optically active enones (67). The photoaddition of hex-1-yne to the lactone (68) affords the two isomeric adducts... 

The starting material can be made by a photochemical [2 + 2] cycloaddition of acetylene and rr. anhydride. Treatment with butanol and base leads to the monoester because, after the butane attacked once, the product is the anion of the carboxyhc acid and cannot be attacked again b. nucleophile. DMAP is a base whose structure appears in the margin. 

Triazole formation under mild conditions via 1,3-dipolar cycloaddition of acetylenes with azides 03H(60)1225. 

Cycloaddition of acetylenes to pyrroles gave 7-azabicyclo[2.2.1]hepta-2,5-diene derivatives 16 in high yields. Direct and sensitized light excitation led in good to excellent yields to 3-azaquadricyclanes 17. ... 

Acetylenes can undergo a number of thermal and transition metal promoted cycloaddition reactions. Besides the [2 + 2 + 2] cycloaddition (see Sect. 5) the reaction of acetylenes with late transition metal (so-called Fischer ) carbenes is noteworthy for the synthesis of highly and regioselectively functionalized naphthalene derivatives (Dotz reaction), while the co-cycloaddition of acetylenes with alkenes and carbon monoxide gives cyclopentenones (Pauson-Khand reaction) [159,160]. 

Scheme 1-4 Transition structure of the cycloaddition of acetylene with fulminic acid (MCSCF/4-31G, McDouall et al. [104]).

While the number of transition-metal-promoted cycloadditions of acetylenes is quite large... 

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