Organostannanes, cyclization

The intramolecular coupling of organostannanes is applied to macrolide synthesis. In the zearalenone synthesis, no cyclization was observed between arylstannane and alkenyl iodide. However, intramolecular coupling take.s place between the alkenylstannane and aryl iodide in 706. A similar cyclization is possible by the reaction of the alkenylstannane 707 with enol triflate[579]. The coupling was applied to the preparation of the bicyclic 1,3-diene system 708[580]. 

Scheme 2.49. Cyclization of alkenylcopper compounds generated from organostannanes.

Barrett and Kohrt" ° and Kelly and Lang" independently reported the first examples of oxazole triflates (Scheme 6.17). In both cases, the requisite 2-aryl-4(5/7)-oxazolone, 56 or 59, was treated with trifluoromethanesulfonic anhydride (Tf20) to afford 60a or 60b, respectively, which were then coupled successfully with a variety of organostannanes. Kelly and Lang" attempted to extend this methodology to prepare the key oxazole triflates 63 in their approach to sulfomycin I. However, they were unexpectedly thwarted when 61 could not be cyclized to the requisite 4(57/)-oxazolone precursors 62. Schaus and Panek described an unproved procedure to prepare 56 in 90% yield very recently. 

The addition processes are characterized as Type II allylation reactions and featnre open transition states. Denmark and Keck have described mechanistic studies, which have provided basic information shaping current views regarding the Type II allylations of organostannanes. Denmark s models are briefly summarized by the cyclization of deuterium-labeled 4 (Scheme 5.2.2). Intra-molecular allylation proceeds with Lewis and Brpnsted acids to yield 6 with high selectivity. The alcohol 6 is rationalized throngh the synclinal transition state 5 via anti-Su substitution. To a lesser degree, various Lewis acids also produce the... 

Yamamoto has reported the desymmetrizing intra-molecular cyclization of the 1,3-diketone 97 for diastereoselective formation of cis-cis 98 vs. cis-trans 99. Optimized conditions with SnCU led to the jS-chelation in the synclinal arrangement 100 to give 98, whereas the use of TiCU facilitated rapid transmetalation of the organostannane yielding 99 (Scheme 5.2.22).22... 

Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Pre- and In Sitn-Formed Organostannanes... 

Scheme 5.6.18 Mechanistic interpretation of the tris-cyclization-organostannane capture cascade forming 57...

Scheme 5.6.19 One-pot sequential process interfacing oxypyridinium betaine cycloaddition with cyclization-carbonylation-organostannane capture...

The use of allene as a relay switch introduces the benefits/challenges of regio- and stereo-selectivity. Our initial studies of cyclization-anion capture with allene as a relay switch generated allylic amines in excellent yield. Reactions were conducted in toluene (90 °C, 20 h) in a Schlenk tube, and employed 0.5-1 atm of allene and various amines, using alkenes/alkynes as relay species (Scheme 5.6.23)." The use of organostannanes as anion-capture agents in this cascade has yet to be explored. 

In a similar manner to alkynes, arynes accept the addition of alkynylstannanes in the presence of a catalytic amount of a palladium-iminophosphine complex (Scheme 5.7.15). Here, a characteristic feature is the participation of a vinylstannane. The usual catalytic cycle (Scheme 5.7.16, left), starting with oxidative addition of a carbon-tin bond, is possible, but rather improbable, because there are no signs of oxidative addition of alkenylstannanes that can participate in the present carbostannylation. Consequently, the palladium(O) complex is likely to undergo oxidative cyclization with an aryne to form a palladacyclopropene (Scheme 5.7.16, right), which then reacts with an organostannane. 

Because tin-lifhium exchange and anionic cyclization are known to occur with retention of configuration at the metal-bearing carbon center [126], enantiomerically enriched organostannanes can serve as promising precursors for access to chiral heterocycles. On fhe basis of fhis idea Coldham prepared fhe optically active hexahydro-IH-pyrrolizine alkaloid (-i-)-pseudohehotridane with complete diastereoselectivity and enantiospecificity (Scheme 12.56) [127]. In sharp contrast with this success, six-membered ring formation by the same strategy resulted in a diastereomeric mixture of racemates (Scheme 12.57) [128]. 

Another palladium-catalysed process that has proven utility for cyclic structures is the Stille coupling between organostannanes and aiyl or vinyl halides/triflates. Specifically for the macrocyclic framework, this chemistry was applied to the preparation of cyclic trienes (102) with defined geometry (Scheme 11.11) that were intermediates for transannular Diels-Alder reactions. The intramolecular Stille reaction also has been efficiently conducted on solid phase to form 104, an advanced intermediate in the synthesis of (S)-zearalenone, from 103 by a cyclization-release strategy. ... 

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