Cocyclization

Cocyclic vicinal exendo bonds, especially in centrally-located rings may be selected for topologically strategic disconnection. Structures 124 and 125 are provided for illustration. One reason for the effectiveness of such disconnections is that it can signal the application of various annulation transforms. The broken bonds may involve heteroatoms such as N, O and S. 

The highest priority ring disconnective T-goals for 272 are those which disconnect a cocyclic 5,5-fusion bond and offexendo bond pair. The internal ketene-olefin cycloaddition in tactical combination with the Baeyer-Villiger transform is well suited to the double disconnection of such a cyclopentane-y-lactone ring pair. 

Cocyclic Bonds. Endo bonds which are within the same ring. 

Scheme 4. Presumed mechanism of the CpCo(CO)2-catalyzed cocyclization of 7 with 8.

Keywords Fischer carbenes Template synthesis Cocyclization Cycloaddition Cyclopentadienes Cyclopentenones Domino reactions... 

Cocyclizations of internal alkynes and carbene complexes 57 with larger substituents R1 (e.g., R z Pr) not only lead to formation of an increased proportion of the regioisomers 60b, but also to that of the isomeric cyclopentadi-enes 61, which would result from 60a by 1,2-migration of the dimethylamino... 

Scheme 19 Formation of cyclopenta[b]pyrans 91 and 92 by a [3+2+2+1] cocyclization [41,80]. For further details see Table 3...

The novel highly substituted spiro[4.4]nonatrienes 98 and 99 are produced by a [3+2+2+2] cocyclization with participation of three alkyne molecules and the (2 -dimethylamino-2 -trimethylsilyl)ethenylcarbene complex 96 (Scheme 20). This transformation is the first one ever observed involving threefold insertion of an alkyne and was first reported in 1999 by de Meijere et al. [81]. The structure of the product was eventually determined by X-ray crystal structure analysis of the quaternary ammonium iodide prepared from the regioisomer 98 (Ar=Ph) with methyl iodide. Interestingly, these formal [3+2+2+2] cycloaddition products are formed only from terminal arylacetylenes. In a control experiment with the complex 96 13C-labeled at the carbene carbon, the 13C label was found only at the spiro carbon atom of the products 98 and 99 [42]. 

Scheme 26 Cocyclizations of 3-phenyl-substituted propenylidenechromium complexes 124 with isocyanides [39,89,90]...

Scheme 27 Cocyclization of the 1-naphthylcarbene complex 128 with ferf-butylphospha-alkyne 129 [91]...

Scheme 31 Formation of cyclopentenones 143 and 144 by a formal [4+2+1-2] cocyclization from the cyclopropylmethoxycarbenechromium complex 142 and alkynes [100-103]...

Under the conditions of the cobalt-mediated carbonylative A-oxide-promoted cocyclization (Pauson-Khand reaction) at room temperature, compound 547 provides exocyclic 1,3-diene 548 as the major product (>98%) together with only traces of the corresponding carbonylative product 549. Owing to the relative instability of the diene, it is more efficient to perform a one-pot cobalt cyclization/Diels-Alder process after A-oxide-promoted cyclization of the cobalt complexes. Compound 550 is obtained as a single diastereomer in 39% overall yield if MTAD is used as a dienophile (Scheme 90) <2003JOC2975>. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Also, cocyclizations of butadiene with olefins and acetylenes are possible to form the 10-membered rings 7 and 8 (10) ... 

Furthermore, azines as a diene system were used for cocyclization with butadiene to form 12-membered heterocyclic compounds as 9 (/1) ... 

The second characteristic reaction catalyzed by palladium catalysts is cocyclization of butadiene with the C=0 bonds of aldehydes and the C=N bonds of isocyanates and Schiff bases to form six-membered heterocyclic compounds (19) with two vinyl groups, as expressed by the following general scheme ... 

These cocyclization reactions of butadiene are possible only with palladium catalysts. 

Unlike nickel Catalysts, palladium complexes do not catalyze the homo-cyclization reaction to give CDT or COD. The difference seems to be due to a different degree of hydride shift and atomic volume. With palladium catalysts, the hydride shift is easier, and hence linear oligomers are formed. The characteristic reaction catalyzed by palladium is the cocyclization of two moles of butadiene with one-hetero atom double bonds such as C=N and C=0 bonds to give six-membered rings with two vinyl groups (19) ... 

Another molecule which takes part in the cocyclization [see Eq. (67)] is arylisocyanate. The C=N double bond, rather than the C=0 double bond in phenylisocyanate, reacts with butadiene to give 3-ethylidene-l-phenyl-6-vinyl-2-piperidone (114) in 75% yield. In this reaction, the double bond migration to the conjugated position took place (101). With isoprene, the selective head-to-head dimerization-cyclization took place at 100°C to give 3,6-diisopropenyl-l-phenyl-2-piperidone (115). 

With nickel complexes, these cocyclizations are not possible. A related reaction is the cocyclization of butadiene with azines to give 12-mem-bered heterocyclic compounds 9 (11) [see Eq. (3)]. 

Molybdenum-promoted cyclodimerization of cyclooctatetraene262 and its cocyclization with alkynes263,2633 also appear to proceed via three- and five-membered molybdenacycles followed by further cyclization reactions that may or may not involve promotion by Mo (Scheme 56). 

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