Divinyl ethers, cyclization

Geometric considerations would seem to dictate that 1,4- and 1,5-dicncs should not undergo cyclopolymerization readily. However, in the case of 1,4-dienes, a 5-hexenyl system is formed after one propagation step. Cyclization via 1,5-backbiling generates a second 5-hexenyl system. Homopolymerization of divinyl ether (22) is thought to involve such a bicyclization. The polymer contains a mixture of structures including that formed by the pathway shown in Scheme 4.18. 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

The presence of an oxygen atom in the chain linking the two alkene moieties does not appear to affect the efficiency of the cyclizations encountered. Thus, the (2 + 2)-intramolecular cycloaddition of the divinyl ether 101a in ether solution with CuOTf... 

The so-called trimerization of propynal in the presence of piperidine acetate provides a synthesis of 4-ethynyI-4//-pyran-3,5-dicarbaldehyde (149) (50LA(568)34> it should be noted that the structure proposed for the product in the original work has been corrected (64CB1959). In the absence of moisture, the reaction fails and it seems likely that the synthesis involves hydration of the alkyne to the divinyl ether. Finally, condensation with the third molecule of the aldehyde results in cyclization to the product (Scheme 20). 

Radical cyclization of divinyl ethers prepared from the reaction of 1,3-dicarbonyl compounds and ethyl propynoate gave rise to trisubstituted furans as shown in Ae following example <03TL2125>. 

Intermolecular cyclization reactions are also possible, e.g., in the cyclopolymerization of divinyl ether with maleic anhydride ... 

Similar intramolecular cycloadditions are encountered where an ether linkage has been incorporated into the met a or para linking groups 123. In these cyclizations the better yields were obtained from the para-attached systems. The yields obtained are again dependent on the chain length of the separator and are indicated below the appropriate structures (124)57. Other hetero-atom-substituted cyclophanes (125) can be obtained by irradiation of the divinyl compounds (126)58,59. The use of tin and germanium derivatives... 

A similar coupling with a,p-unsaturated acid chlorides provides p-silyl divinyl ketones (Nazarov reagents). These ketones cyclize in the presence of a Lewis acid, particularly BF3 etherate, to cyclopentenones, generally with retention of the silyl group. 

Therefore, the aim of this work was to synthesize new silacyclic and oligomeric organosilicon compounds containing easily modifiable Si-O-R bonds, via competitive silylative coupling cyclization and polycondensation of divinyl-substituted silyl ethers in the presence of ruthenium hydride complex. 

In 2004, Frontier et al. found that iridium complex (154) is very reactive as a Lewis acidic promoter of Nazarov cyclization, especially relative to Cu(OTf)2 [48]. Recently, they have found that Ir (III) catalyst (154) is a strong Lewis acid capable of catalyzing Nazarov cyclization with great efficiency for various divinyl ketones (Scheme 16.41) [49]. The efficiency of the cyclization is attributed to the electrophilicity of the Ir(III) complex and substrate activation via 0,0 -chelation that employs two substrate carbonyl groups or one carbonyl and an ether function, and encourages the s-trans/s-trans conformation required for cyclization. 

---