Cycloalkynes, cycloaddition with

In a more recent and improved approach to cyclopropa-radicicol (228) [ 110], also outlined in Scheme 48, the synthesis was achieved via ynolide 231 which was transformed to the stable cobalt complex 232. RCM of 232 mediated by catalyst C led to cyclization product 233 as a 2 1 mixture of isomers in 57% yield. Oxidative removal of cobalt from this mixture followed by cycloaddition of the resulting cycloalkyne 234 with the cyclic diene 235 led to the benzofused macrolactone 236, which was converted to cyclopropa-radicicol (228). 

The 1,3-dipolar cycloaddition reactions between 7- and 10-membered cycloalkynes and triazoles (R-N3, R = H, Me, CgHj) have been investigated theoretically using the B3LYP/6-3/G level method. The cycloadditions with alkynes occur by concerted mechanisms. ... 

In addition to 534, further [4+2]-cycloadducts of 5 were prepared by using 1,3-dienes, some of which are well known as trapping reagents of short-lived cyclic allenes and cycloalkynes. Further, cycloadditions could be achieved with tropone and several 2-substituted tropones, 8,8-dicyanoheptafulvene, 1,3,5-cycloheptatriene and a few of its 7-substituted derivatives. The products of these reactions are represented in Scheme 6.108. 

Cycloaddition reactions, particularly the Diels-Alder and 1,3-dipolar cycloaddition, have been used extensively in trapping unstable cycloalkynes. In addition, some typical cycloaddition reactions leading to interesting products are observed only with angle strained cycloalkynes. 

Typical reactions of strained cycloalkynes are the [2 + 1] cycloaddition reaction with isonitriles and the [3 + 2] cycloaddition reaction with CS2 (Schemes 8-22 and 8-23) [lb]. As the latter... 

The stoichiometric reaction of platinum complex 184 with the cycloalkyne 185 also gives rise to cycloaddition products, 186 and 187 (Scheme 47), but these products were not formed in the reaction of 185 with l,2-bis(di-methylsilyl)benzene catalyzed by [Pt(CH2=CH2)(PPh3)2] <1997ICA(265)1>. 

McGrath NA, Raines RT (2012) Diazo compounds as highly tunable reactants in 1,3-dipolar cycloaddition reactions with cycloalkynes. Chem Sci 3(11) 3237-3240. doi 10.1039/c2sc20806g... 

The successful preparation of cycloalkynes also opened up the possibility to explore their unique chemical reactivity. In fact, the transient existence of the cycloalkyne species could initially only be indirectly corroborated by fast in situ trapping of the smaller-sized rings (seven carbons and below) before decomposition [31]. While not strictly applicable to cyclooctyne, which is the smallest cyclic alkyne that can be isolated and stored in pure form, Blomquist already noted that nevertheless careful exclusion of air was requisite to avoid rapid decomposition. More importantly, he was also the first to observe that cyclooctyne reacts explosively when treated with phenyl azide, forming a viscous liquid product [8]. This remark is in fact the first historic administration of a process that has now become known as strain-promoted azide-alkyne cycloaddition (SPAAC). 

Since these first reports on the reactions of cycloalkynes with isobenzofuran and phenyl azide, a wide range of other alkynophile cycloaddition partners have become known, as conceptually illustrated with model compounds in reactions with cyclooctyne in Fig. 4. It becomes immediately clear that cycloalkynes can undergo... 

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