Medium-sized cyclic alkenes

Wharton fragmentation Base-induced formation of medium-sized cyclic alkenes from 1,3-diol monosulfonates. 480... 

Several groups tried to access medium-sized rings (ca. 8-12 membered) with peptide-like moieties. Most prominent is the I-MCR/RCM strategy [15, 16], As shown in Scheme 11. 12-Memhered cyclic peptidomimetics were reported hy Oikawa and co-workers who used initial Ugi-4CRs performed in excellent yields, followed hy functionalization of the resulting Ugi-platform to introduce the desired alkene functionalities [65]. RCM with the second generation Hoveyda-Gruhhs... 

Trans -substituted diarylalkenes undergo oxidative cleavage upon treatment by potassium permanganate in the presence of moist alumina as a solid support (equation 32)158. Under the same conditions, cyclic alkenes, with medium-sized rings, give acyclic dialdehydes (equation 33). 

An equivalent reaction has been achieved via the treatment of hydroborated bisalkenes with alkaline silver nitrate solution (Table 1.4).22,23 This method has been used to synthesize a number of small and medium-size carbocyclic rings in moderate to good yield. The selectivity for terminal cyclization observed for 1,6-heptadiene and 1,7-octadiene indicates that, in these cases, hydroboration of each of the alkenes occurs independently to yield acyclic boranes. It has, however, been found that both cyclic and acyclic boranes react under these conditions to yield the ring-closed products (Scheme 1.3). 

This process is an important tool in synthetic organic photochemistry for the preparation of small rings. The difficulties in the preparation of medium-size and larger cyclic compounds, caused by entropic factors, have been circumvented by the presence of a rigid molecular scaffold (cyclopropanes, aromatic rings, cyclohexenes and cis-alkenes), which prevents hydrogen abstraction from positions close to the carbonyl group (Scheme 9.39) [64—66]. 

The equilibrium in this reversible process may be driven by removal of one of the four components (Le Chatelier s principle). This idea has been employed to form medium- and large-sized rings that are otherwise very difficult to constmct, because of unfavorable strain and entropy factors. The example below illustrates ring closure of an acyclic starting compound with terminal double bonds. The products are a cyclic alkene and ethene, which, being a gas, evolves rapidly out of the reaction mixture, driving the equilibrium toward product. 

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