Subject ring-closing metathesis

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

Substituted malonates and (5-keto-esters have also been successfully used as pronucleophiles (Scheme 9.11) [25a, 36]. From P-ketoesters, approximately 1 1 mixtures of epimers are generally formed. Products derived from 2-alkenylmalo-nates have been subjected to Ru-catalyzed ring-closing metathesis to give cyclo-pentene derivatives in good yield [25a, 36]. With the ester-amide displayed in Scheme 9.11 as pronucleophile, 1 1 mixtures of epimers were also formed [44]. 

The combination of allylic amination, ring-closing metathesis, and a free radical cyclization provides a convenient approach to the dihydrobenzo[b]indoline skeleton, as illustrated in Scheme 10.10. The rhodium-catalyzed aUylic amination of 43 with the lithium anion of 2-iodo-(N-4-methoxybenzenesulfonyl)arrihne furnished the corresponding N-(arylsulfonyl)aniline 44. The diene 44 was then subjected to ring-closing metathesis and subsequently treated with tris(trimethylsilyl)silane and triethylborane to afford the dihydrobenzojhjindole derivative 46a in 85% yield [14, 43]. 

Construction of organic nanotubes starting from porphyrin dendrimers with core/shell architecture is also feasible. Figure 8.29 also shows how covalent nanotubes can be produced by removal of the dendritic component of the molecule. A coordination polymer is first synthesised from a dendritic metallopor-phyrin with alkene end groups. This is subjected to intramolecular and intermo-lecular crosslinking by ring-closing metathesis at the periphery. 

Another cyclic system was also constructed from the 4-hydroxyproHne derivative 29, which was subjected to the decarboxylation-iodination protocol (Scheme 17). Allylation at C-1 followed by a radical-mediated allylation at C-2 and ring-closing metathesis provided the bicyclic ring system 30 known from many naturally occurring alkaloids. 

Azocino[3,2-6]indoles were obtained when seleno ester precursors bearing 3-butenylamino and allylaminomethyl chains on the C-3 position of the indole were subjected to the same reductive radical conditions used previously with indole 237. Inclusion of a bromine atom on the alkene acceptor gave the most rewarding result (shown below) at a hydride concentration of 0.02 M yielding 75% of the %-endo product 241, without the detection of any products, formed as a result of reductirm or the alternative 1-exo cyclization. This method proved to be a nice complement to the ring-closing metathesis protocol used in this laboratory to effect similar cyclizations and recently resulted in the total synthesis of apparicine [126, 127]. 

Martin and co-workers applied their ring-closing metathesis strategy for the preparation of 5,6-fused-Y-butyrolactones to 5,7-fused-y-butyrolactones. Diene precursors 238 and 240 were subjected to ring-... 

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